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Title: The Ancient Volcanoes of Great Britain, Volume II (of 2)
Author: Archibald Geikie
Release Date: January 4, 2022 [eBook #66493]
Language: English
Produced by: T Cosmas, MWS and the Online Distributed Proofreading Team
at https://www.pgdp.net (This file was produced from images
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*** START OF THE PROJECT GUTENBERG EBOOK THE ANCIENT VOLCANOES OF
GREAT BRITAIN, VOLUME II (OF 2) ***
Transcriber Note
Text emphasis denoted by _Italics_ and =Bold.= Whole and fractional parts
of numbers as 123-4/5.
THE ANCIENT VOLCANOES OF GREAT BRITAIN
[Illustration]
THE
ANCIENT VOLCANOES
OF
GREAT BRITAIN
BY
SIR ARCHIBALD GEIKIE, F.R.S.
D.C.L. Oxf., D.Sc. Camb., Dubl.; LL.D. St. And. Edinb.
DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN AND
IRELAND; CORRESPONDENT OF THE INSTITUTE OF FRANCE; OF THE
ACADEMIES OF BERLIN, VIENNA, MUNICH, TURIN, BELGIUM,
STOCKHOLM, GÖTTINGEN, NEW YORK; OF THE IMPERIAL MINERALOGICAL
SOCIETY AND SOCIETY OF NATURALISTS ST. PETERSBURG; NATURAL
HISTORY SOCIETY, MOSCOW; SCIENTIFIC SOCIETY, CHRISTIANIA;
AMERICAN PHILOSOPHICAL SOCIETY; OF THE GEOLOGICAL
SOCIETIES OF LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC.
WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS
IN TWO VOLUMES
VOL. II
London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1897
_All rights reserved_
CONTENTS
CHAPTER XXIX
The Carboniferous Volcanoes of England
PAGE
The North of England: Dykes, Great Whin Sill--The Derbyshire
Toadstones--The Isle of Man--East Somerset--Devonshire 1
CHAPTER XXX
The Carboniferous Volcanoes of Ireland
King's County--The Limerick Basin--The Volcanic Breccias of
Doubtful Age in County Cork 37
BOOK VII
THE PERMIAN VOLCANOES
CHAPTER XXXI
The Permian Volcanoes of Scotland
Geographical Changes at the Close of the Carboniferous Period--Land
and Inland-Seas of Permian time--General Characteristics and
Nature of the Materials erupted--Structure of the several
Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2.
Basin of the Firth of Forth 53
CHAPTER XXXII
Permian Volcanoes of England
The Devonshire Centre--Eruptive Rocks of the Midland Coal-fields 94
BOOK VIII
THE VOLCANOES OF TERTIARY TIME
CHAPTER XXXIII
Vast lapse of time between the close of the Palæozoic and beginning
of the Tertiary Volcanic Eruptions--Prolonged Volcanic
Quiescence--Progress of Investigation among the Tertiary
Volcanic Series of Britain 107
CHAPTER XXXIV
The System of Dykes in the Tertiary Volcanic Series
Geographical Distribution--Two Types of Protrusion--Nature of
component Rocks--Hade--Breadth--Interruptions of Lateral
Continuity--Length--Persistence of Mineral Characters 118
CHAPTER XXXV
The System of Dykes--_continued_
Direction--Termination upward--Known vertical extension--Evidence
as to the movement of the Molten Rock in the
Fissures--Branches and Veins--Connection of Dykes with
Intrusive Sheets--Intersection of Dykes--Dykes of more than
one infilling--Contact metamorphism of the Dykes--Relation of
the Dykes to the Geological Structure of the Districts which
they traverse--Data for estimating the Geological Age of the
Dykes--Origin and History of the Dykes 145
CHAPTER XXXVI
The Plateaux
Nature and Arrangement of the Rocks: 1. Lavas.--Basalts, Dolerites,
Andesites--Structure of the Lavas in the Field--2. Fragmental
Rocks.--Agglomerates, Conglomerates, and Breccias--Tuffs and
their accompaniments 181
CHAPTER XXXVII
The Several Basalt-Plateaux and their Geological History--Antrim,
Mull, Morven and Ardnamurchan 199
CHAPTER XXXVIII
The Basalt-Plateau of the Parish of Small Isles--Rivers of the
Volcanic Period 215
CHAPTER XXXIX
The Basalt-Plateaux of Skye and of the Faroe Isles 249
CHAPTER XL
The Modern Volcanoes of Iceland as illustrative of the Tertiary
Volcanic History of North-Western Europe 260
CHAPTER XLI
The Eruptive Vents of the Basalt-Plateaux
Vents filled with Basalt or other Lava-form Rock--Vents filled
with Agglomerate 270
CHAPTER XLII
The Basic Sills of the Basalt-Plateaux 298
CHAPTER XLIII
The Bosses and Sheets of Gabbro
Petrography of the Rocks--Relations of the Gabbros to the other
members of the Volcanic series--Description of the Gabbro
districts--Skye 327
CHAPTER XLIV
The Bosses and Sheets of Gabbro in the Districts of Rum,
Ardnamurchan, Mull, St. Kilda and North-East Ireland.
History of the Gabbro Intrusions 349
CHAPTER XLV
The Acid Rocks
Their Petrography--Their Stratigraphical Position and its
Analogies in Central France 364
CHAPTER XLVI
Types of Structure in the Acid Rocks--Bosses 378
CHAPTER XLVII
The Acid Bosses of Mull, Small Isles, St. Kilda, Arran, and
the North-East of Ireland 395
CHAPTER XLVIII
The Acid Sills, Dykes and Veins 430
CHAPTER XLIX
The Subsidences and Dislocations of the Plateaux 447
CHAPTER L
Effects of Denudation 455
CHAPTER LI
Summary and General Deductions 466
LIST OF ILLUSTRATIONS
FIG. PAGE
176. Section from the great Limestone escarpment on the west to
the Millstone Grit hills east of Teesdale 4
177. Sections of the Carboniferous Limestone series of
Northumberland showing the variations in the position of the
Whin Sill. By Messrs. Topley and Lebour 6
178. View of two volcanic necks in the Carboniferous Limestone
series, at Grange Mill, five miles west of Matlock Bath,
from the north 14
179. Plan of necks and bedded tuff at Grange Mill, five miles
west of Matlock Bath 15
180. Section across the smaller volcanic neck and the stratified
tuff in Carboniferous Limestone, Grange Mill 15
181. Section of vesicular and amygdaloidal diabase resting on
Carboniferous Limestone, Peak Forest Limeworks, Great Rocks
Quarry 19
182. View of the superposition of Carboniferous Limestone upon
toadstone, Raven's Tor, Millersdale (length about 100 feet) 19
183. Section at lime-kiln, south of Viaduct, Millersdale Station 20
184. Limestones passing under stratified tuffs, Poyll Vaaish,
Isle of Man 24
185. Section of tuff, showing intercalations of black impure
chert, west of Closenychollagh Point, near Castletown, Isle
of Man 25
186. Section of intercalated dark limestone, shale and chert in
the tuff south of Poyll Vaaish Bay, Isle of Man 26
187. Section of part of a volcanic neck on shore to the
south-east of Poyll Vaaish Bay, Isle of Man 29
188. Section of successive discharges and disturbances within a
volcanic vent. Scarlet Point, Isle of Man 29
189. Section of dyke and sill in the tuffs west of Scarlet Point,
Isle of Man 30
190. Section on south side of vesicular sill west of Scarlet
Point 31
191. Bands of vesicles in the same sill 31
192. Croghan Hill, King's County, from S.S.W. 38
193. Section in quarry on roadside east of Limerick, close to
viaduct of the Limerick and Erris Railway 44
194. Section of the volcanic escarpment, east of Shehan's
Cross-roads, south of Limerick 45
195. View of Derk Hill, a volcanic neck on the south side of the
Limerick basin 47
196. Section across the Limerick volcanic basin 48
197. Section of a bed of volcanic breccia in the Carboniferous
Slate; White Bull Head, County Cork 50
198. Volcanic breccia invading and enclosing Carboniferous slate,
White Bull Head 50
199. General section across the Permian basin of Ayrshire 59
200. Section of lavas, east side of Mauchline Hill 60
201. Section of the top of the volcanic series near Eastside
Cottage, Carron Water, Nithsdale 60
202. Section of two outliers of the Permian volcanic series at
the foot of Windyhill Burn, Water of Ae, Dumfriesshire 61
203. The Green Hill, Waterside, Dalmellington, from the south; a
tuff-neck of Permian age 62
204. Patna Hill from the Doon Bridge, Ayrshire; a tuff-neck of
Permian age 63
205. Ground plans of Permian volcanic vents from the Ayrshire
Coal-field. On the scale of six inches to a mile 64
206. Section of sills traversing the Permian volcanic series.
River Ayr, Ballochmyle 66
207. Section showing the relations of the later rocks of Arthur
Seat 68
208. Section in brooks between Bonny town and Baldastard, Largo 70
209. View of Largo Law from the east 71
210. View of small neck in Calciferous Sandstones, on the shore,
three miles east from St. Andrews 72
211. Ground-plan of Permian volcanic vents 73
212. Small neck in Calciferous Sandstones a little east from the
"Rock and Spindle," two and a half miles east from St.
Andrews 74
213. Plan of volcanic necks at Kellie Law, East of Fife, on the
scale of three inches to one mile 75
214. Plan of the craters in Volcanello, Lipari Islands 75
215. Section of the strata at the edge of the volcanic vent on
the east side of Elie Harbour 76
216. Agglomerate of neck on shore at Ardross, two miles east from
Elie 77
217. Ground-plan of volcanic neck, Elie Harbour, showing circular
disposition of the stratification 80
218. Section across the great vent of Kincraig, Elie, on a true
scale, vertical and horizontal, of six inches to a mile 81
219. Dyke in volcanic neck, on the beach, St. Monans 82
220. Section of part of crater rim, Island of Volcano 83
221. Dyke rising through the agglomerate of a volcanic vent;
Kincraig, Elie 84
222. Radiating columnar dyke in the tuff of a volcanic vent. Rock
and Spindle, two and a half miles east from St. Andrews 86
223. View of part of the shore front of the great vent at
Kincraig, looking westward, with the columnar basalt in
front 88
224. Plan of volcanic neck on beach near St. Monans 89
225. Columnar basalt in the neck of Kincraig, Elie, seen from the
west 90
226. Section across Largo Law 91
227. Vein of "white-trap" cutting black carbonaceous shales, a
little west from St. Monans Church 92
228. Section at Belvedere, S. W. of Exeter 97
229. Diagram to show the unconformability and overlap of the
Permian rocks in the Crediton Valley 97
230. Section of the volcanic series at Kellerton, Devonshire 98
231. Section of agglomerate overlain with sandstone and andesite,
Posbury, Crediton 99
232. Diagrammatic section across Titterstone Clee Hill 102
233. Dyke on the south-east coast of the Island of Mull 119
234. Fissure left by the weathering out of a dyke 120
235. Plan of basalt-veins with selvages of black basalt-glass,
east side of Beinn Tighe, Isle of Eigg 126
236. Arrangement of lines of amygdales in a dyke, Strathmore,
Skye 130
237. Systems of joints in the dykes 132
238. Section of cylindrical vein or dyke, cutting the bedded
lavas, east side of Fuglö, Faroe Islands 133
239. Joint-structures in the central vitreous portion of the
Eskdale Dyke (B. N. Peach) 133
240. Microscopic structure of the vitreous part of the Eskdale
Dyke 136
241. Section along the line of the Cleveland Dyke at Cliff Ridge,
Guisbrough (G. Barrow), scale, 12 inches to 1 mile 147
242. Section along the course of the Cleveland Dyke, at the head
of Lonsdale, Yorkshire (G. Barrow, in the _Memoirs of the
Geol. Survey_, Geology of Cleveland, p. 61) 148
243. Section across the extreme upper limit of Cleveland Dyke, on
the scale of 20 feet to one inch (Mr. G. Barrow) 149
244. Upper limit of Cleveland Dyke in quarry near Cockfield
(after Mr. Teall) 149
245. Section along the course of the Cleveland Dyke across the
Cross Fell escarpment (scale of one inch to one mile) 150
246. Branching portion of the great dyke near Hawick (length
about one mile) 153
247. Branching dyke at foot of Glen Artney (length about four
miles) 153
248. Basic veins traversing Secondary limestone and sandstone on
the coast cliffs, Aidnamurchan 155
249. Section showing the connection of a Dyke with an Intrusive
Sheet, Point of Suisnish, Skye 156
250. Section to show the connection of a Dyke with an Intrusive
Sheet, Stirlingshire Coal-field 157
251. Intersection of dykes in bedded basalt, Calliach Point, Mull 158
252. Basalt veins traversing bedded dolerites, Kildonan, Eigg 159
253. Ground-plan of intersecting dykes in Lias limestone, Shore,
Harrabol, East of Broadford, Skye 159
254. Compound dyke, Market Stance, Broadford, Skye 162
255. Section of coal rendered columnar by intrusive basalt,
shore, Saltcoats, Ayrshire 164
256. Dolerite dyke with marginal bands of "white trap," in black
shale, Lower Lias, Pabba 166
257. Map of the chief dykes between Lochs Ridden and Striven (C.
T. Clough, Geological Survey, Sheet 29) 170
258. Basalt-veins traversing granophyre, St. Kilda 173
259. Section of scoriaceous and prismatic basalt, Camas
Tharbernish, north shore of Canna Island 187
260. Banded amygdaloidal basalt showing layers of elongated and
steeply inclined vesicles, Macleod's Maidens, Skye 191
261. Termination of basalt-beds, Carsaig, Mull 193
262. Breccia and blocks of mica-schist, quartzite, etc., lying
between bedded basalts, Isle of Mull 197
263. Section of Knocklayd, an outlier of the Antrim
basalt-plateau lying on Chalk 202
264. Diagram-Section of the Antrim Plateau 203
265. View Of Basalt escarpment, Giant's Causeway, with the
Amphitheatre and Chimneys. (From a photograph by Mr. R.
Welch) 207
266. Basalt-capping on the top of Ben Iadain, Morven 209
266_a_. View of the south side of Staffa, showing the bedded and
columnar structure of the basalt 210
267. View of Rum from the harbour of Canna 216
268. Section of the cliffs below Compass Hill, Isle of Canna 218
269. Lava cutting out conglomerate and shale. Shore below Canna
House 224
270. Section of shales and tuffs, with a coniferous stump lying
between two basalt-sheets, Cùl nam Marbh, Canna 225
271. Dùn Mòr, Sanday. (From a photograph by Miss Thom) 226
272. View of the Dùn Beag, Sanday, seen from the south. (From a
Photograph by Miss Thom) 230
273. View of Dùn Beag, Sanday, from the north. The island of Rum
in the distance. (From a Photograph by Miss Thom) 231
274. Section of eastern front of Dùn Beag 232
275. Enlarged section on the western side of Dùn Beag 233
276. Geological map of the Island of Eigg 235
277. Section of the geological structure of the Island of Eigg 236
278. View of the Scuir of Eigg from the east 237
279. Natural section at the cliff of Bideann Boidheach,
north-west end of the Scuir of Eigg 239
280. View of the Scuir of Eigg from the south 242
281. View of the Scuir of Eigg from the south-west of the Loch a
Bhealaich, showing the bedded character of the mass 243
282. Section at the base of the Scuir of Eigg (east end) 244
283. Terraced hills of basalt plateau (Macleod's Tables), Skye 250
284. "Macleod's Maidens" and part of basalt cliffs of Skye 251
285. Intercalated group of strata between Basalts, An Ceannaich,
western side of Skye 252
286. Escarpment of Plateau-basalts, Cliffs of Talisker, Skye 253
287. Section of the largest of Macleod's Maidens 254
288. Dying out of lava-beds, east side of Sandö, Faroe Isles 257
289. Lenticular lavas, western front of Hestö, Faroe Isles 257
290. Lenticular lavas, east side of Svinö, Faroe Isles 258
291. Section at Frodbonyp, Suderö, Faroe 258
292. Fissure (gjá) in a lava-field, Iceland. (From a photograph
by Dr. Tempest Anderson) 262
293. Cones on the great Laki fissure, Iceland. (From a photograph
by Dr. Tempest Anderson) 263
293_a_. Plan of small craters along the line of great Laki
fissure, Iceland. (After Mr. Helland, reduced) 264
294. Slemish, a volcanic neck or vent on the Antrim plateau, seen
from the north 272
295. Section of volcanic vent at Carnmony Hill (E. Hull) 272
296. Section of the east side of Scawt Hill, near Glenarm 273
297. Section of Neck of basalt, Bendoo, Ballintoy 273
298. Volcanic Neck of dolerite near Cushendall 274
299. Section of Volcanic Neck at 'S Airde Beinne, near Tobermory,
Mull 274
300. Interior of the Volcanic Neck of 'S Airde Beinne, near
Tobermory, Mull 275
301. Diagram to show the probable relation of the Neck at
Carrick-a-raide, Antrim, to an adjacent group of tuffs 277
302. Section of agglomerate Neck at Maclean's Nose, Ardnamurchan 279
303. Diagram to show the probable relations of the rocks on the
southern flank of Beinn Dearg Bheag 282
304. Section of Volcanic Vent and connected lavas and tuffs,
Scorr, Camas Garbh, Portree Bay, Skye 284
305. Section of the Volcanic Series at Ach na Hannait, south of
Portree, Skye 288
306. View of part of a Volcanic Neck at the eastern end of the
island of Canna. (From a photograph by Miss Thom) 289
307. Columnar Basalt invading agglomerate of Volcanic Vent,
Coroghon Mòr, Isle of Canna. (Height above 20 feet) 291
308. Columnar Basalt invading Volcanic conglomerate, north side
of Alman Islet, Canna 291
309. View of neck-like mass of breccia, Brochel, Raasay 292
310. View of Volcanic Neck piercing and overlain by the
Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe
Islands. (From a photograph by Colonel Evans) 294
311. Section of the same Neck as that shown in Fig. 310 295
312. Volcanic Neck close to that shown in Figs. 310 and 311 296
313. Section of wall of another Neck of agglomerate in the same
group with those represented in Figs. 310, 311, and 312 296
314. View of "Segregation-Veins" in a dolerite sill, Portrush,
Antrim 300
315. View of Fair Head, from the east, showing the main upper
sill and a thinner sheet cropping out along the talus 301
316. View of Fair Head from the shore. (From a photograph by Mr.
R. Welch) 302
317. Section at Farragandoo cliff, west end of Fair Head, showing
the rapid splitting up and dying out of an Intrusive Sheet
slope 304
318. View of the Trotternish Coast, showing the position of the
band of Sills 305
319. Columnar Sill intrusive in Jurassic Strata east of
Kilmartin, Trotternish, Skye 306
320. View of the northern precipice (500 feet high) of the
largest of the Shiant Isles. (From a photograph by Colonel
Evans) 308
321. Section of thin Intrusive Sheets and Veins in carbonaceous
shales lying among the Plateau-basalts, cliffs north of
Ach na Hannait, between Portree Bay and Loch Sligachan 311
322. Upper part of Sill, Moonen Bay, Waternish, Skye, showing the
divergence of veins 313
323. Section of the base of the Basalt-plateau with sill and
dykes, Sound of Soa, Skye 314
324. Section of Dolerite Sill cut by another sill, both being
traversed by dykes, Rudh' an Iasgaich, western side of
Sleat, Skye 316
325. Section to show Bedded and Intrusive Sheets, Eigg 318
326. Ground plan of Sills at Ben Hiant, Ardnamurchan. 321
327. Section of two Sills in schistose grits, west end of Beinn
na h-Urchrach, Ardnamurchan 322
328. Sill traversing bedded Basalts, cliffs of Stromö, at
entrance of Vaagöfjord 323
329. View of the same Sill seen from the channel opposite the
island of Kolter 324
330. Granulitic and coarsely foliated Gabbro traversed by later
veins of felspathic Gabbro, Druim an Eidhne, Cuillin
Hills, Skye 331
331. Scuir na Gillean, Cuillin Hills, showing the characteristic
craggy forms of the Gabbro. (From a photograph by Mr.
Abraham, Keswick) 335
332. Section across Glen Brittle, to show the general relations
of the Bedded Basalts and the Gabbros 336
333. View of the crest of the Cuillin Hills, showing the
weathering of the Gabbro along its joints and of a
compound basic dyke which rises through it. (From a
photograph by Mr. Abraham, Keswick) 338
334. Section across the Coire Uaigneich, Skye 341
335. Banded and puckered gabbro, Druim an Eidhne, Glen Sligachan,
Skye 342
336. Banded structure in the Gabbro, from the ridge of Druim an
Eidhne, between Loch Coruisk and Glen Sligachan 343
337. Banded and doubly folded Gabbro, Druim an Eidhne, 10 feet
broad 345
338. Sketch of banded structure in the Gabbros of the hills at
the head of Loch Scavaig 347
339. Outline of the hills of the Island of Rum, sketched from
near the Isle of Eigg 350
340. View of Allival, Rum, sketched from the base of the
north-east side of the cone 352
341. Section of foliated Gabbros in the Tertiary volcanic series
of Allival, Rum 353
342. Altered Plateau-Basalts invaded by Gabbro, and with a Dyke
of prismatic Basalt cutting both rocks, north slope of
Ben Buy, Mull 357
343. Theoretical representation of the structure of one of the
Gabbro bosses of the Inner Hebrides 362
344. Section through the Puy de la Goutte and Puy de Chopine 374
345. View of the Huche Pointue and Huche Platte west of Le
Pertuis 376
346. View of Glamich, 2537 feet, Glen Sligachan. (From a
photograph by R. J. A. Berry, M.D., lent by the Scottish
Mountaineering Club) 380
347. Section across the north slope of Beinn an Dubhaich, Skye 383
348. Section from Beinn Dearg to Beinn an Dubhaich, Skye 385
349. Section at north end of Beinn na Cro, Skye 388
350. Ground-plan of basic dyke in Cambrian limestones truncated
by granophyre which encloses large blocks of the dyke,
Torrin, Skye 393
351. Section on south side of Beinn an Dubhaich, Skye, showing
the truncation of a basalt-dyke 394
352. View of the hills on the south side of the head of Loch na
Keal, showing the junction of the Granophyre and the
bedded basalts 396
353. Section on south side of Cruach Tòrr an Lochain, Mull 398
354. Section at head of Allt na Searmoin, Mull 398
355. Section on south side of Beinn Fhada, Mull 399
356. Section to south of Loch na Dàiridh, Mull 400
357. Section of junction of south side of Loch Ba' Granophyre
boss, with the bedded basalts, Mull 401
358. Mass of dark gabbro about two feet in diameter traversed by
pale veins of Granophyre, lying on north slope of Creag
na h-Iolaire, Mull 402
359. Section at Creag na h-Iolaire, Glen More, Mull, showing
basalts and gabbros resting on and pierced by Granophyre 402
360. Section on north side of Orval, Rum 404
361. Junction of Quartz-porphyry (Microgranite) and basic rocks,
south-east side of Orval, Rum 404
362. Junction of Granophyre and gabbro, north side of St. Kilda 410
363. Veins of Granophyre traversing gabbro and splitting up into
thin threads, north side of St. Kilda 411
364. Pale Granophyre injected into dark basalt, South Bay, St.
Kilda 412
365. Veins of Granophyre traversing a fine-grained gabbro and
scarcely entering a coarse-grained sheet, west side of
Rueval, St. Kilda 413
366. View of sills and veins of pale Granophyre traversing sheets
of gabbro, west side of St. Kilda. (From a photograph by
Colonel Evans) 414
367. Section of the sea-cliff below Conacher, St. Kilda, showing
basic dykes in Granophyre 417
368. Triple basic dyke, sea-cliff, east side of St. Kilda 417
369. Jointed structure of the Granite near the top of Goatfell,
Arran. (From a photograph by Mr. W. Douglas, lent by the
Scottish Mountaineering Club) 419
370. Intrusive Rhyolite in the lower basalt group of Antrim,
Templepatrick 427
371. Section across the southern slope of Carnearny Hill, Antrim 427
372. Section across the Granophyre Sills at Loch a' Mhullaich,
above Skulamus, Skye 433
373. Section to show the connection of a sill of Granophyre with
its probable funnel of supply, Raasay 436
374. Granophyre sill resting on Lower Lias shales with a dyke of
basalt passing laterally into a sill, Suisnish Point, Isle
of Raasay 436
375. Weathered surface of spherulitic Granophyre from dyke
in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size 438
376. Plan of portion of the ridge north of Druim an Eidhne, Glen
Sligachan, Skye, showing three dykes issuing from a mass
of Granophyre 439
377. Weathered surface of spherulitic Granophyre, from dyke
in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size 440
378. Plan of pale Granophyric dyke, with spherulitic and
flow-structure, cutting and enclosing dark gabbro, Druim
an Eidhne 441
379. Dyke (six to ten feet broad) proceeding from a large body of
Granophyre and traversing gabbro, from the same locality
as Figs. 375 and 377 442
380. Section of intruded veins of various acid rocks above River
Clachaig, Mull 443
381. Pitchstone vein traversing the bedded basalts, Rudh an
Tangairt, Eigg 445
382. Reversed fault on the eastern side of Svinö, Faroe Isles 454
383. Reversed fault on the north-east headland of Sandö, Faroe
Isle 454
MAPS
V. Map of the Permian volcanic districts of Scotland _To face p._ 106
VI. Map of the Tertiary volcanic region of the West
of Scotland _To face p._ 296
VII. Map of the Tertiary volcanic district of the
North-East of Ireland _To face p._ 446
CHAPTER XXIX
THE CARBONIFEROUS VOLCANOES OF ENGLAND
The North of England: Dykes, The Great Whin Sill--The Derbyshire
Toadstones--The Isle of Man--East Somerset--Devonshire
1. THE NORTH OF ENGLAND
The volcanic intercalations which diversify the Lower Carboniferous
formations of Southern Scotland extend but a short way across the
English Border, and although, over the moors and hills of the north of
Cumberland and Northumberland, the Carboniferous sandstones, limestones
and shales are well exposed, they present no continuation of either the
plateau or puy-eruptions which play so prominent a part in the geology
of Roxburghshire and Dumfriesshire. This deficiency is all the more
noticeable seeing that the Carboniferous system is exposed down to its
very base, in the deep dales of the North of England. Had any truly
interstratified volcanic material existed in the system there, it could
hardly fail to have been detected.
But while contemporaneous volcanic rocks are absent, the northern
English counties contain many intrusive masses of dolerite, diabase,
andesite or other eruptive rocks, which may be found traversing all
the subdivisions of the Carboniferous system. These eruptive materials
have taken two forms: in some cases they rise as Dykes, in others they
appear as Sills.
Dykes.--With regard to the dykes, some are probably much later than
the Carboniferous period, and consequently will be more appropriately
considered in Chapters xxxiv. and xxxv. The great Cleveland dyke,
for example, which runs across the Carboniferous, Permian, Triassic
and Jurassic formations, is probably referable to the Older Tertiary
volcanic period. One dyke known as the Hett Dyke, has been plausibly
claimed as possibly of Carboniferous age. It runs in a W.S.W. direction
from the Magnesian Limestone escarpment at Quarrington Hill, a few
miles to the east of Durham, through the great Coal-field, across the
Millstone Grit and Carboniferous Limestone, disappearing near Middleton
in Teesdale. Its total length is thus about 23 miles. It varies in
breadth from about 6 to about 15 feet, and appears to increase in
dimensions as it goes westward.[1]
[Footnote 1: Sedgwick, _Trans. Geol. Soc._ 2nd series, iii. part 1
(1826-28), p. 63; _Trans. Cambridge Phil. Soc._ ii. (1822), p. 21. Sir
J. Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p. 543.]
The age of this dyke cannot at present be satisfactorily fixed.
It must be later than the Coal-measures through which it rises.
Sedgwick long ago pointed out that though it reaches the escarpment
of the Magnesian Limestone, it does not cut it; yet it is found in
coal-mining to traverse the Coal-measures underlying the Limestone. He
was accordingly inclined to believe it to be of older date than the
Magnesian Limestone. At its western extremity it approaches close to
the Great Whin Sill of Teesdale, though no absolute connection between
the two has been established. Mr. Teall, however, has called attention
to the similarity between the microscopic structure of the rock forming
the Hett Dyke and that of the mass of the Whin Sill, and he is strongly
inclined to regard them as belonging to the same period of intrusion.[2]
[Footnote 2: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.]
It is especially worthy of remark that in the course of its nearly
rectilinear course across the Durham Coal-field, the Hett Dyke, where
it crosses the Wear, is flanked on the north at a distance of a little
more than two miles by a second parallel dyke of nearly identical
composition. Between the two dykes, during mining operations, a sill
about 20 feet thick has been met with, lying between two well-known
coal-seams at a depth of about 60 fathoms from the surface, and
extending over an area of at least 15 acres.[3] Microscopic examination
of this sill by Mr. Teall proved that the rock presents the closest
resemblance to that of the Hett Dyke.[4] In this case, it may be
regarded as probable that the two dykes and the intermediate sill form
one related series of intrusions, and the conjecture that the Hett Dyke
may be connected with the Whin Sill thus receives corroboration. The
age of the Whin Sill itself will be discussed a few pages further on.
[Footnote 3: Sir Lowthian Bell, _Proc. Roy. Soc._ xxiii. (1875), p.
544.]
[Footnote 4: _Quart. Journ. Geol. Soc._ xl. (1884), p. 230.]
Of the other dykes which may possibly be coeval with the Hett Dyke we
may specially note those which follow the same W.S.W. trend, for that
strike differs from the general W.N.W. direction of most of the dykes.
Two conspicuous examples of the south-westerly trend may be seen,
one near Morpeth, the other north of Bellingham. The former dyke, as
regards microscopic structure, is more nearly related to the majority
of the series in the North of England. But that north of Bellingham
(High Green) presents affinities both in structure and composition
with the Hett Dyke,[5] and may perhaps belong to the same period of
intrusion.
[Footnote 5: Mr. Teall, _op. cit._ p. 244. _Quart. Journ. Geol. Soc._
xxxix. (1884), p. 656, and _Proc. Geol. Assoc._ (1886). See also Prof.
Lebour, _Geology of Northumberland and Durham_, chap. xi.]
The Great Whin Sill.--The geologist who, after making himself
acquainted with the abundant sills among the Carboniferous rocks in the
centre of Scotland, finds his way into Northumberland, meets there with
geological features that have become familiar to him further north.
The sea-cliffs of Bamborough and Dunstanborough, the rocky islets of
Farne, the long lines of brown crag and green slope that strike inland
through the Kyloe Hills and wind across the cultivated lowlands and the
moorlands beyond, remind him at every turn of the scenery in the basin
of the Forth. But not until he has traced these ridges for many miles
southwards and found their component rocks to form there an almost
continuous sheet does he realize that nothing of the kind among the
Scottish Carboniferous rocks can be compared for extent to this display
in the North of England.[6]
[Footnote 6: The Whin Sill has been the subject of much discussion,
and a good deal of geological literature has been devoted to its
consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips
and Tate are especially deserving of recognition. The intrusive
character of the Sill, maintained by some of these writers, was finally
established by the mapping of the Geological Survey, and was discussed
and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in
the 33rd volume of the _Quart. Journ. Geol. Soc._ (1877), in which
references to the earlier observers will be found. See also Prof.
Lebour's _Outlines of the Geology of Northumberland_, 2nd edit. (1886),
p. 92. The petrography of the Whin Sill is fully treated by Mr. Teall
in _Quart. Journ. Geol. Soc._ xl. (1884), p. 640, where a bibliography
of the subject is also given.]
From the furthest skerries of the Farne Islands southwards to Burton
Fell on the great Pennine escarpment, a distance in a straight line of
about 80 miles, this intrusive sheet may be traced in the Carboniferous
Limestone series (Map I.). There are intervals where its continuity
cannot be actually followed at the surface, but that it really runs
unbroken from one end to the other underground cannot be doubted by any
one who has examined the region. This singular feature in the geology
and scenery of the North of England is known locally as the Great
Whin Sill.[7] From the rocky islets and castle-crowned crags of the
coast-line it maintains its characteristic topography, structure and
composition throughout its long course in the interior. So regularly
parallel with the sedimentary strata does it appear to lie, that it was
formerly regarded by many observers as a true lava-sheet, poured out
upon the sea-floor over which the limestones and shales were laid down.
But its really intrusive character has now been clearly demonstrated.
Not a vestige of any tuff has been detected associated with it, nor
does it ever present the usual characters of a true lava-stream.[8] Its
internal structure and the wonderful uniformity in its character mark
it out as a typical intrusive sheet.
[Footnote 7: "Whin" is a common term in Scotland and the North of
England for any hard kind of stone, especially such as can be used for
making and mending roads. "Sill" denotes a flat course or bed of stone,
and was evidently applied to this intrusive sheet from its persistent
flat-bedded position and its prominence among the other gently inclined
strata among which it lies. It is from this example in the North of
England that the word "sill" has passed into geological literature.]
[Footnote 8: On the coast at Bamborough and the Harkess Rocks the
usual petrographical characters of the Whin Sill are exchanged for
those of fine-grained amygdaloidal diabases arranged in distinct
sheets, which in their upper parts are highly vesicular and show ropy
surfaces--peculiarities suggestive of true lava-streams. But according
to Professor Lebour the rocks are intrusive into limestone and shale
(_Geology of Northumberland and Durham_, p. 98). Mr. Teall has
expressed the suspicion that these rocks must have consolidated under
conditions somewhat different from those which characterized the normal
Whin Sill (_Quart. Journ. Geol. Soc._ xl. p. 643). They seem to be
the only parts of the sill which present features that might possibly
indicate superficial outflow.]
Among the manifestations of the subterranean intrusion of igneous rocks
in the British Isles the Great Whin Sill, next after the Dalradian
sills of Scotland, is the most extensive. Its striking continuity for
so great a distance, and the absence around it of any other trace of
igneous action, save a few dykes, place it in marked contrast to the
ordinary type of Carboniferous sills. The occasional gaps on its line
of outcrop in the northern part of its course do not really affect
our impression of the persistence of the sheet. They not improbably
indicate merely that in its protrusion it had a wavy irregular limit,
which in the progress of denudation has occasionally been not yet
reached. For mile after mile the sill has been mapped by the Geological
Survey in lines of crag across the moorlands, and as a conspicuous
band among the limestones and shales that form the steep front of the
Pennine escarpment, where it has long been known in the fine sections
exposed among the gullies by which that noble rock-face has been
furrowed.
[Illustration: Fig. 176.--Section from the great Limestone escarpment
on the west to the Millstone Grit hills east of Teesdale.
1. Silurian strata; 2. Carboniferous Limestone series; 3. The Great
Whin Sill, which gradually rises to higher stratigraphical position as
it goes westward; 4. Millstone Grit.]
Along its main outcrop, the sill dips gently eastwards below the
portion of the Carboniferous Limestone series which overlies it. But
so slight are the inclinations, so gentle the undulations of the rocks
in this part of the country, that far to the east of that outcrop
the sill has been laid bare by the streams which in the larger dales
have cut their way through the overlying cake of Carboniferous strata
down to the Silurian platform on which they rest (Fig. 176). Among
these inland revelations of the eastward continuation of the sill
under Carboniferous Limestone strata, the most striking and best known
are those which have been made by the River Tees, and of which the
famous waterfalls of the High Force and Cauldron Snout are the most
picturesque features. The distance of the remotest of these denuded
outcrops or "inliers" from the main escarpment is not less than 20
miles.
It is not possible to form an accurate estimate of the total
underground area of the Whin Sill. In the southern half of the
district, south of the line of the Roman Wall, where, the inclination
of the strata being generally low, the same stratigraphical horizons
are exposed by denudation far to the east of the main outcrops of
the rocks, we know that the sill must have a subterranean extent of
more than 400 square miles. Yet this is probably only a small part of
the total area over which the molten material was injected. In the
northern part of the district, the Carboniferous Limestone series is
not exposed over so broad a stretch of country, and denudation has not
there revealed the eastward extension of the sill. But there is no
reason to suppose the sheet to be less continuous and massive there.
We must remember also that the present escarpment has been produced
by denudation, and that the intrusive sheet must have once extended
westwards beyond its present limits at the surface. If, therefore, we
were to state broadly that the Great Whin Sill has been intruded into
the Carboniferous Limestone series over an area of 1000 square miles
we should probably be still below the truth.
The rock composing this vast intrusive sheet is a dolerite or diabase,
which maintains throughout its wide extent a remarkable uniformity of
petrographical characters. In this and other respects it illustrates
the typical features of sills. Thus it is coarsest in texture where
it is thickest, and somewhat finer in grain towards its upper and
lower surfaces than in the centre. Among the coarser varieties the
component crystals of augite are not infrequently an inch in length and
occur in irregular patches.[9] Occasional amygdaloidal portions are
observable, but these are not more marked than those to be found in
the "whin-dykes" of the same region.[10] The amygdaloidal and vesicular
fine-grained rocks of the Bamborough district may possibly be quite
distinct from the main body of the Whin Sill.
[Footnote 9: Sedgwick, _Cambridge Phil. Trans._ ii. p. 166. Mr. Teall,
_Quart. Journ. Geol. Soc._ xl. p. 643.]
[Footnote 10: Messrs. Topley and Lebour, _Quart. Journ. Geol. Soc._
xxxiii. p. 418.]
Under the microscope the rock is seen to consist essentially of the
usual minerals--plagioclase, augite and titaniferous magnetic iron-ore.
An ophitic intergrowth of the augite and felspar is observable,
likewise a certain quantity of micropegmatite which plays the part of
groundmass between the interstices of the lath-shaped felspars. Full
details of the characteristics of the component minerals and their
arrangement are given by Mr. Teall in the paper already cited.
The main body of the sill is a sheet which sometimes diminishes to
less than 20 feet in thickness and sometimes expands to 150 feet,
but averages from 80 to 100 feet. It occasionally divides, as near
Great Bavington, where it appears at the surface in two distinct
beds separated by an intervening group of limestones and shales.
Occasionally, as at Elf's Hill Quarry, it gives out branches which send
strings into the adjacent limestone.[11]
[Footnote 11: Messrs. Topley and Lebour, _op. cit._ p. 413.]
Although in most natural sections it seems to lie quite parallel with
the strata above and below, yet a number of examples of its actual
intrusion have been observed. When traced across the country, it is
found not to remain on a definite horizon, but to pass transgressively
across considerable thicknesses of strata. Its variations in this
respect are well shown in the accompanying table of comparative
sections constructed by Messrs. Topley and Lebour.[12] It will be
seen that while at Harlow Hill the sill is found overlying the Great
Limestone of Alston Moor, at Rugley, five miles off it lies about 1000
feet lower down, far below the position of the Tyne-bottom Limestone.
Still farther north, however, the sill west of Holy Island is said to
lie 800 feet above the Great Limestone and to come among the higher
beds of the Carboniferous Limestone series.[13]
[Footnote 12: _Op. cit._ plate xviii.]
[Footnote 13: _Op. cit._ p. 414.]
The Whin Sill appears generally to thicken in an easterly or
north-easterly direction. There are further indications that it was
intruded from east to west. Thus, at Shepherd's Gap, on the Great Roman
Wall, the dolerite, coming evidently from an easterly quarter, has
broken up and thrust itself beneath a bed of limestone. Again, when the
sill bifurcates the branches unite towards the east or north-east.[14]
The sill can be proved to thin away to the west from Teesdale to the
Pennine escarpment, and in Weardale the "Little Whin Sill" diminishes
from 20 feet, till in three miles it disappears.[15]
[Footnote 14: _Op. cit._ p. 415.]
[Footnote 15: _Op. cit._ p. 419.]
[Illustration: _Walker & Bontall sc._
Fig. 177.--Sections of the Carboniferous Limestone series of
Northumberland showing the variations in the position of the Whin Sill.
By Messrs. Topley and Lebour.]
The strata in contact with the Whin Sill, both above and below, have
been more or less altered. Sandstones have been least affected;
shales have suffered most, passing into a kind of porcellanite, with
development of garnet and other minerals.[16] Limestone often shows
only slight traces of change, though here and there it has become
crystalline.
[Footnote 16: Mr. Teall, _op. cit._ xxxix. (1884), p. 642, and authors
cited by him.]
No trace of any boss or neck has been detected in the whole region
which might be supposed to mark a funnel of ascent for the material of
the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed,
may, however, have been possibly connected with the injection of this
great intrusive sheet. No other visible mass of igneous rock in the
region has been even plausibly conjectured to indicate a point or line
of emission for the sill.
It is certainly singular that in so wide a territory, where the whole
succession of strata has been so admirably laid bare by denudation in
thousands of natural sections, and where, moreover, much additional
information has been obtained from lead-mining as to the nature of
the rocks below ground, not a single vestige of tuff, agglomerate or
interstratified lava has been up to the present time recorded, unless
the Harkess rocks already alluded to can be so regarded.
Judging, however, from the analogy of the other districts of igneous
rocks in Britain, we can hardly resist the conclusion that the Great
Whin Sill is essentially a manifestation of volcanic action, that it
was connected with the uprise of basic lava in volcanic orifices, and
that the subterranean energy may quite probably have succeeded in
reaching the surface and ejecting there both lavas and tuffs.
It appears to be certain that any vents which existed cannot have lain
to the west of the present escarpment of the sill, for no trace of them
can be found there piercing the Carboniferous or older formations. They
must have lain somewhere to the east in the area now overspread with
Millstone Grit and Coal-measures, or still farther east in the tract
now concealed under the North Sea. The evidence of the sill itself, as
we have seen, corroborates this view of the probable situation of the
centre of disturbance.
The question of the geological age of the sill is one of considerable
difficulty, to which no confident answer can be given.[17] The injection
of the diabase must obviously be considerably later than the highest
strata through which it has risen; that is, it must be younger than
some of the higher members of the Carboniferous Limestone series. But
here our positive evidence fails.
[Footnote 17: See Messrs. Topley and Lebour, _op. cit._ p. 418.]
The Sill is traversed by the same faults which disrupt the surrounding
Carboniferous rocks. It is therefore of older date than these
dislocations. Its striking general parallelism with the shales and
limestones probably proves that it was intruded before the rocks were
much disturbed from their original horizontal position. But the manner
in which the intrusive rock has been thrust into and has involved the
shales and limestones seems to indicate that these strata had already
become consolidated and lay under the pressure of a great thickness of
superincumbent Carboniferous strata.
In the absence of all certainty on the subject it seems most natural
to place the Whin Sill provisionally among the Carboniferous volcanic
series with which petrographically and structurally it has so much
in common. In Scotland the puy-eruptions continued till the time of
the Coal-measures. If, before the close of the Carboniferous period,
volcanic vents were opened somewhere to the east of the coal-fields of
Northumberland and Durham, they might be accompanied with basic sills
injected into the Carboniferous Limestone series, which was then lying
still approximately horizontal under a thickness of from 3500 to 5000
feet of Carboniferous sedimentary deposits. These still undiscovered
volcanoes seem to have been endowed with even more energy than those
of Central and Southern Scotland, at least nowhere else among the
Carboniferous records of Britain is there such a colossal manifestation
of subterranean intrusion as the Great Whin Sill.
2. THE DERBYSHIRE TOADSTONES
In the absence of any certain evidence that the Whin Sill belongs to
the Carboniferous period, we must advance southward into the very heart
of England before any clear vestiges can be found of contemporaneous
volcanic eruptions among the members of the Carboniferous system.
After quitting the lavas and tuffs of Roxburghshire and their brief
continuations across the English border, we do not again meet with any
truly bedded volcanic rocks in that system until we reach the middle
of Derbyshire. In this picturesque district, famous for its lead-mines
and its mineral waters, a feebly developed but interesting group of
intercalated lavas, locally called "toadstones," has long been known.
There is thus a space of some 150 miles across which, though the
formations are there so fully developed and so abundantly trenched by
valleys from the top to the bottom of the system, no volcanic vents nor
any trace of Carboniferous volcanic ejections has yet been found. On
the other hand, after the district of the "toadstones" is passed, the
Carboniferous rocks are again destitute of any volcanic intercalations
across the centre and south-west of England and over Wales, until after
a space of about 150 miles they reappear in Somerset.
The volcanic group of Derbyshire thus stands out entirely isolated.
Lying in the Carboniferous Limestone, where that formation is typically
developed, it presents an admirable example of a thoroughly marine
phase of volcanic action (Map I.).
One of the most prominent features in the geology of the centre of
England is the broad anticlinal fold which brings up the lower portion
of the Carboniferous system to form the long ridge of the Pennine chain
that runs from Yorkshire to the Midland plain, and separates the
eastern from the western coal-fields. This fold widens southwards until
not only the Millstone Grit and Yoredale rocks, but the underlying
Mountain Limestone is laid bare. A broad limestone district is thus
exposed in the very heart of the country, ranging as a green fertile
undulating tableland, deeply cut by winding valleys, which expose
admirable sections of the strata, but nowhere reach the base of the
system. The total visible depth of the limestone series is computed to
be about 1500 feet; the Yoredale shales and limestones may be 500 feet
more; so that the calcareous formations in which the volcanic phenomena
are exhibited reach a thickness of at least 2000 feet.
It is not yet definitely known through what vertical extent of this
thickness of sedimentary material the volcanic platforms extend, but
where most fully developed they perhaps range through 1000 feet, lying
chiefly in the Carboniferous Limestone, but apparently in at least one
locality extending up into the lower division of the Yoredale group.
The area within which they can be studied corresponds nearly with that
in which the limestone forms the surface of the country, or a district
measuring about 20 miles from north to south, with an extreme breadth
of 10 miles in an east and west direction.
A special historical interest belongs to the Derbyshire
"toadstones."[18] They furnished Whitehurst with material for his
speculations, and were believed by him to be as truly igneous rocks
as the lava which flows from Hecla, Vesuvius or Etna. But he thought
that they had been introduced among the strata and "did not overflow
the surface of the earth, according to the usual operations of
volcanoes."[19]
[Footnote 18: This word has by some writers been supposed to be
corrupted from _tod-stein_, dead-stone, in allusion to the dying out of
the lead veins there; by others the name has been thought to be derived
from the peculiar green speckled aspect of much of the rock, resembling
the back of a toad.]
[Footnote 19: _An Enquiry into the Original State and Formation of the
Earth_, 1778, Appendix, pp. 149, _et seq._]
His views were published as far back as 1778, three years after
Hutton read the first outline of his theory of the earth and made
known his observations regarding the igneous origin of whinstones.[20]
The first detailed account of the Derbyshire eruptive rocks was that
given by Fairey,[21] which has served as the basis of all subsequent
descriptions. Conybeare, in particular, prepared a succinct narrative
from Fairey's more diffuse statements, and thus placed clearly
before geologists the nature and distribution of these volcanic
intercalations.[22] Subsequently the district was mapped by De la Beche
and the officers of the Geological Survey, and the areas occupied by
the several outcrops of igneous rock could then be readily seen.[23]
[Footnote 20: _Trans. Roy. Soc. Edin._ i. p. 275, _et seq._ Hutton
specially mentions the toadstone of Derbyshire as one of the rocks
produced by fusion, p. 277.]
[Footnote 21: _General View of the Agriculture and Minerals of
Derbyshire_ (1811).]
[Footnote 22: _Outlines of the Geology of England and Wales_ (1822), p.
448.]
[Footnote 23: See Sheets 71 N.W., 72 N.E., 81 N.E. and S.E. and 82 S.W.
of the Geological Survey of England and Wales.]
Though the "toadstones" were believed to form definite platforms among
the limestone strata, and thus to be capable of being used as reliable
horizons in the mineral fields of Derbyshire, they appear to have
been generally regarded as intrusive sheets like the Whin Sill of the
north. Thus De la Beche in his _Manual of Geology_, giving a summary
of what was known at the time regarding intercalated igneous rocks,
remarks with regard to the Derbyshire toadstones that they may from
all analogy be considered to have been injected among the limestones
which would be easily separated by the force of the intruded igneous
material.[24] But the same observer, after his experience among the
ancient volcanic rocks of Devonshire, came fully to recognize the
proofs of contemporaneous outflow among the Derbyshire toadstones. In
his subsequently published _Geological Observer_, he described the
toadstones as submarine lavas that had been poured out over the floor
of the sea in which the Carboniferous Limestone was deposited, and had
been afterwards covered up under fresh deposits of limestone.[25] It is
remarkable, however, that he specially comments on the absence, as he
believed, of any contemporaneously ejected ashes and lapilli, such as
occur in Devonshire. That true tuffs or volcanic ashes are associated
with the toadstones was noticed by Jukes in 1861,[26] and afterwards by
the Geological Survey.[27] Since that time geologists have generally
recognized these Derbyshire igneous rocks as truly contemporaneous
intercalations. But very little has recently been written on the
structure of the district, our information regarding it being still
based mainly on the early observations of Fairey and the mapping of the
Geological Survey.
[Footnote 24: _Manual_, 3rd edit. 1833, p. 462.]
[Footnote 25: _Geological Observer_ (1851), pp. 642-645.]
[Footnote 26: _Student's Manual of Geology_, 2nd edit. (1863), p. 523.
For a general _résumé_ of the proofs of contemporaneity furnished by
the toadstones, see "The Geology of North Derbyshire," by Messrs. A.
H. Green and A. Strahan (_Memoirs of the Geological Survey_, 2nd edit.
(1887), p. 123).]
[Footnote 27: In the first edition of the _Memoir on the Geology of
North Derbyshire_, published in 1859, the authors of which were Messrs.
A. H. Green, C. le Neve Foster and J. R. Dakyns.]
The subject, however, has now been resumed by Mr. H. Arnold Bemrose,
who in 1894, after a prolonged study of the petrography of the
rocks, communicated the results of his researches to the Geological
Society.[28] In his excellent paper, to which I shall immediately make
fuller reference, he mentions the localities at which lava-form and
fragmental rocks may be observed, but does not enter on the discussion
of the geological structure of the region or of the history of the
volcanic eruptions. Before the announcement of his paper, hearing that
I proposed to make for the first time a rapid traverse of the toadstone
district, for the purpose of acquainting myself with the rocks on the
ground, he kindly offered to conduct me over it. My chief object,
besides that of seeing the general nature of the volcanic phenomena of
the region, was to examine more particularly the areas of the volcanic
fragmental rocks, with the view of discovering whether among them some
remains might not be found of the actual vents of discharge. In this
search I was entirely successful. Aided by Mr. Bemrose's intimate
knowledge of the ground, I was enabled to visit in rapid succession
those tracts which seemed most likely to furnish the required evidence,
and in a few days was fortunate enough to obtain proofs of six or seven
distinct vents, ranging from the extreme northern to the furthest
southern boundary of the volcanic district. Mr. Bemrose has undertaken
to continue the investigation, and will, I trust, work out the detailed
stratigraphy of the Carboniferous Limestone so as eventually to furnish
an exhaustive narrative of the whole volcanic history of Derbyshire.
Meanwhile no adequate account of the area can be given. But I will here
state all the essential facts which up to the present time have been
ascertained.
[Footnote 28: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 603.]
1. THE ROCKS ERUPTED.--Mr. Allport has described the microscopic
character of some of the toadstones,[29] and further details have
been supplied by Mr. Teall.[30] The fullest account of the subject,
however, is that given by Mr. Bemrose in the paper above referred to.
This observer distinguishes the lava-form from the fragmental rocks,
and gives the minute characters of each series. He does not, however,
separate true interstratified lavas from injected sills, nor the bedded
tuffs from the coarse agglomerates which fill up the vents. These
distinctions are obviously required in order that the true nature and
sequence of the materials in the volcanic eruptions may be traced,
and that the phenomena exhibited in Derbyshire may be brought into
comparison with those found in other Carboniferous districts. But
to establish them satisfactorily the whole region must be carefully
re-examined and even to some extent re-mapped.
[Footnote 29: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 529.]
[Footnote 30: _British Petrography_, p. 209.]
The lavas (including, in the meantime, sheets which there can be
little doubt are sills) show three main types of minute structure
and composition, which are discriminated by Mr. Bemrose as--(_a_)
Olivine-dolerites; these, the most abundant of the series, consist
of augite in grains, olivine in idiomorphic crystals, plagioclase
giving lath-shaped and tabular sections, and magnetite or ilmenite
in rods and grains; (_b_) Ophitic olivine-dolerites, consisting of
augite in ophitic plates forming the groundmass, in which are imbedded
idiomorphic olivine, plagioclase (often giving large lath-shaped
sections and magnetite or ilmenite); (_c_) Olivine-basalts; these rocks
are distinguished by containing crystals of augite and olivine in a
groundmass of small felspar-laths, granular augite and magnetite or
ilmenite, with very little interstitial matter. They have been noticed
only in two of the outcrops of toadstone.
The fragmental rocks have been shown by Mr. Bemrose to cover a much
more extensive space than had been previously supposed. He has found
them to be distinguished by an abundance of lapilli varying from minute
fragments up to pieces about the size of a pea, and composed of a
material that differs in structure from the dolerites and basalts with
which the tuffs are associated. These lapilli consist largely of a
glassy base more or less altered, which is generally finely vesicular
and encloses abundant skeleton crystals and crystallites. The tuffs
thus very closely resemble some of the Carboniferous basic tuffs of
Fife, already referred to (vol. i. p. 422), and like these they include
abundant blocks of dolerite and basalt.
2. GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT.--As the volcanic
rocks of Derbyshire lie among the Carboniferous Limestones of a broad
anticlinal dome, they are only exposed where these limestones have been
sufficiently denuded, and as the base of the limestones is nowhere laid
bare, the lowest parts of the volcanic series may be concealed. Over
the tract where the toadstones can be examined they appear as bands
regularly intercalated with the limestones, but varying in thickness in
the course of their outcrops. As they are prone to decay, they usually
form smooth grassy slopes between the limestone scarps, though isolated
blocks of the dull brown igneous rocks may often be seen protruding
from the surface. Now and then a harder bed of toadstone caps a hill,
and thus forms a prominent feature in the landscape, but as a rule
these igneous bands play no distinguishing part in the scenery, and are
indeed less conspicuous than the white escarpments of limestone which
overlie them.
It was the opinion of the older geologists that three distinct
platforms of toadstone extend without break throughout the district,
and subdivide the limestones into four portions. But this opinion
does not seem to have been based on good evidence either of sequence
or of continuity. Various facts were brought forward by the officers
of the Geological Survey to show that the supposed persistence of the
three platforms of toadstone did not really exist, but that these
sheets of igneous material are found at different spots on very
different horizons, and are of limited horizontal range.[31] So far as
my own limited observations go, they entirely corroborate this view.
There can be little doubt, I think, that the identity of certain
outcrops of toadstone has been assumed, and the assumption has been
carried throughout the district. The truth is that the number of
successive platforms on which igneous materials appear will never be
satisfactorily determined until the stratigraphy of the Derbyshire
Carboniferous Limestone is worked out in detail. When the successive
members of this great calcareous formation have been identified by
lithological and palæontological characters over the district, it will
be easy to allocate each outcrop of toadstone to its true geological
horizon. When this labour has been completed, it will probably be found
that instead of three, there have been many discharges of volcanic
material during the deposition of the limestone series; that these
have proceeded from numerous small vents, and that they are all of
comparatively restricted horizontal extent. Such a detailed examination
will also determine how far the toadstones include veritable sills, and
on what horizons these intrusive sheets have been injected.
[Footnote 31: _Geol. Surv. Mem. on North Derbyshire_, by Messrs. Green
and Strahan (1887), p. 104.]
In the meantime, we know that the lowest visible bands of toadstone
are underlain by several hundred feet of limestone, thus proving that
the earliest known volcanic explosions took place over the floor
of the Carboniferous Limestone sea, after at least 700 or 800 feet
of calcareous sediment had accumulated there. The latest traces of
volcanic activity are found in a part of the Yoredale group of shales
and limestones which form the uppermost member of the Carboniferous
Limestone of this region. But it is not quite clear whether the
vesicular diabase found there is interstratified or intrusive.
Certainly no contemporaneous tuffs have yet been found among the
Yoredale rocks, nor in any higher subdivision of the Carboniferous
system, though coarse agglomerates marking the position of vents do
traverse the Yoredale group at Kniveton.
It may be remarked that in the district over which the toadstones can
be seen, two areas are recognizable, in each of which the exposures of
the igneous rocks are numerous, while between them lies an intervening
tract wherein there is hardly any visible outcrop of these rocks. The
northern and much the more extensive area stretches from Castleton
to Sheldon, while the southern spreads from Winster to Kniveton.
This distribution not improbably points to the original position of
the vents, and indicates a northern more numerous group of volcanic
orifices, and a southern tract where the vents were fewer, or at least
spread their discharges over a more limited space.
3. THE VENTS.--It had always appeared to me singular that, in ground
so deeply trenched by valleys as the toadstone district of Derbyshire,
no trace had been recognized of any bosses or necks from which these
volcanic sheets might have been erupted. It is true that in mining
operations masses of toadstone had been penetrated to a considerable
depth without their bottom being reached, and the suggestion had been
made that in such cases a shaft may actually have been sunk on one
of the vents through which the toadstone came up.[32] One instance in
particular was cited where, at Black Hillock, on Tideswell Moor, close
to Peak Forest Village, a mass of toadstone was not cut through, though
pierced to a depth of 100 fathoms. In that neighbourhood, however,
several of the sheets of eruptive material are probably sills, and the
shaft at Black Hillock may have been sunk upon the pipe or vein that
supplied one or more of these intrusive sheets.
[Footnote 32: _Geol. Surv. Mem. on North Derbyshire_, p. 134.]
It was therefore with no little interest that I detected a series of
vents at four separate localities, viz. Castleton, Grange Mill, Hopton,
and Kniveton Wood. I have no doubt that a more extended search will
bring others to light. Those observed by me are all filled with coarse
agglomerate, the blocks in which are mostly composed of different
lavas, sometimes with the addition of blocks of limestone, while the
matrix consists mainly of lapilli of basic devitrified glass.
The most typical examples form a group of two, possibly three, vents
which rise into two isolated, smooth, grassy dome-shaped hills at
Grange Mill, five miles west from Matlock Bath.[33] In external form
and colour, these eminences present a contrast to the scarped slopes
of limestone around them. They at once recall the contours of many of
the volcanic necks in Central Scotland. On examination it is found that
the material composing them is a dull green agglomerate, the matrix
of which is a compact substance weathering spheroidally, and full of
small lapilli of minutely vesicular diabase. The larger stones consist,
for the most part, of various vesicular dolerites or diabases, together
with some pieces of limestone and occasionally large blocks of the
latter rock, altered into a saccharoid condition. Two dykes of dolerite
or basalt traverse the margin of the larger vent.
[Footnote 33: This is Mr. Bemrose's outcrop, No. 46, _op. cit._ p. 633.]
The steep sides of these agglomerate domes rise from the low ground
around them to a height of 100 to 180 feet, their summits being a
little more than 900 feet above the sea. The smaller neck is nearly
circular, and measures about 1000 feet in diameter. The larger mass
is less regular in shape, and is prolonged into such a bulge on the
south-east as to suggest that its prolongation in that direction may
really mark the position of a third and much smaller vent contiguous to
it. The longer diameter of the larger mass is 2300 and the shorter 1300
feet.
[Illustration: Fig. 178.--View of two volcanic necks in the
Carboniferous Limestone series, at Grange Mill, five miles west of
Matlock Bath, from the north.]
On the south and west sides, the surrounding limestone can be traced
up to within a few feet of the edge of the agglomerate, and its strata
are there found to be much jumbled and broken, while their texture is
rather more crystalline than usual, though not saccharoid. The two
necks are separated by a narrow valley in which no rock is visible.
Their opposite declivities meet at the bottom of this hollow, and are
so definitely marked off that, even in the absence of proof that they
are disjoined by intervening limestone, there can be little hesitation
in regarding each hill as marking a distinct vent. A wider valley
extends along the eastern base of the necks, and slopes upward on its
east side until it is crowned by a long escarpment of limestone, which
reaches a height of 1000 feet above the sea, or about 100 feet above
the valley from which it rises. Unfortunately, the bottom and slopes
of this depression are thickly covered with soil, but at one or two
places debris of fine tuff may be observed, and at the northern and
southern ends of the hollow well-bedded green and reddish tuff appears,
dipping gently below the limestone escarpment. This band of volcanic
detritus evidently underlies the limestone, and forms most of the
gentle slope on the east side of the valley. It may be from 70 to 100
feet thick. That it was discharged from one or both of the necks seems
tolerably clear. Its material resembles that forming the matrix of the
agglomerate. The general arrangement of the rocks at this interesting
locality is represented in Fig. 179, which is reduced from my survey on
the scale of six inches to a mile. A section across the smaller vent
would show the structure represented in Fig. 180.
[Illustration: Fig. 179.--Plan of necks and bedded tuff at Grange Mill,
five miles west of Matlock Bath.]
[Illustration: Fig. 180.--Section across the smaller volcanic neck and
the stratified tuff in Carboniferous Limestone, Grange Mill.
1. Limestone; 2. Stratified tuff intercalated among the limestones; 3.
Agglomerate.]
This group of vents lies in the southern of the two tracts of the
volcanic district. In the northern tract a mass of agglomerate pierces
the base of the limestone escarpment about a quarter of a mile west
from the entrance to the Peak Cavern at Castleton.[34] It is rudely
semicircular in area, stretching down the slope until its northern
extension is lost under the lower ground. The agglomerate is not well
exposed, but it can be seen to be a green, granular crumbling rock,
made up in great part of minutely vesicular lapilli, enclosing blocks
of various diabases two feet long or more. From the abrupt way in which
this agglomerate rises through the limestone, there can be little doubt
that it marks the position of one of the volcanic vents of the time.
As it stands on the extreme northern verge of the limestone area,
the ground further north being covered with the Yoredale rocks and
Millstone Grit, it is the most northerly of the whole volcanic district.
[Footnote 34: This is outcrop No. 1 of Mr. Bemrose's paper, p. 625.]
Along the southern margin of the limestone country a group of
agglomerate masses probably marks another chain of vents. These are
specially interesting, inasmuch as they abut on the Yoredale series,
and may thus be looked upon as among the latest of the volcanic
chimneys. One of them is seen at Hopton,[35] where along the side of the
road a good section is exposed of coarse tumultuous agglomerate, having
a dull green matrix, composed of green, brown, and black, minutely
cellular, basic, devitrified, glassy lapilli, showing under the
microscope abundant microlites and crystals or calcareous pseudomorphs
of olivine, augite, and felspar, and much magnetite dust. Through
this matrix are distributed blocks of slaggy basalt and dolerite. An
interesting feature of this mass is the occurrence in it of some veins,
two or three inches broad, of a compact black porphyritic basalt. I
did not trace the relations of this agglomerate to the stratified
rocks around it. But its internal structure and composition mark it
out as a true neck. It extends, according to the Geological Survey
map, for about half a mile along the edge of the limestone, and is
represented as being separated by two faults from the Yoredale series
immediately to the south. So long as the belief is entertained that
the toadstones are contemporaneous outflows of lava lying on certain
definite horizons, far below the summit of the limestones, the position
of the Hopton agglomerate is only explicable on the assumption of some
dislocation by which the Yoredale shales have been brought down against
it. But when we realize that the rock is an unstratified agglomerate,
probably marking the place of a volcanic vent, and therefore rising
transgressively through the surrounding strata, the necessity for a
fault is removed, or if a fault is inserted its existence should be
justified on other evidence than the relations of the igneous rock to
the surrounding strata.
[Footnote 35: _Geol. Surv. Mem. North Derbyshire_, p. 24. This is
outcrop No. 53 of Mr. Bemrose's paper, p. 635.]
Four miles to the south-west of Hopton, on the slope of the hill
at Kniveton Wood, another remarkable mass of agglomerate forms a
rounded ridge between the two forks of a small stream.[36] Its granular
matrix, like that of the other necks, consists of lapilli of minutely
vesicular basic glassy lava or pumice, and encloses large and small
rounded blocks of finely cellular basalt and pieces of limestone. The
rock is unstratified, and in all respects resembles that of ordinary
Carboniferous necks in Scotland. Its relations to the Yoredale rocks
are laid bare in the channels of the streamlets. There the shales and
thin limestones may be seen much broken and plicated, their curved
and fractured ends striking directly at the agglomerate. They may be
traced to within a yard of the agglomerate. On the Geological Survey
map the igneous rock is represented as bounded by two parallel faults.
But I hardly think that this explanation suffices to account for the
relations of the rocks and their remarkable boundary-line, which seems
to me to be undoubtedly the wall of a volcanic vent. To the east of
the streams, another mass of agglomerate may mark another neck, while
to the north, a third detached area of the same kind of rock, rising
among the limestones, may be regarded as likewise a distinct mass. At
this locality, therefore, there are two, possibly three, vents. One of
these, from the way in which it cuts across the Yoredale shales and
limestones, is to be assigned to a time later than the older part of
the Yoredale series, and thus, like the Hopton mass, it indicates that
in the south of the volcanic area eruptions did not cease with the
close of the deposition of the thick limestones, but were prolonged
even into the time of the Yoredale rocks.
[Footnote 36: Outcrop No. 56, p. 638 of Mr. Bemrose's paper.]
A further proof of the late age of these southern patches of volcanic
material is shown by two bands of vesicular toadstone in the Yoredale
series, a little south from the village of Kniveton. These rocks are
traced on the Survey Map, and are shown in a diagram in the Memoir,
where their position is sought to be explained by a system of parallel
faulting.[37] I was able to trace the actual contact of the western band
with the strata underneath it, and satisfied myself that there is no
fault at the junction. The igneous material is regularly bedded with
the Yoredale shales and limestones. Either, therefore, these bands
are intercalated lava-streams or intrusive sills. If mere vesicular
structure were enough to distinguish true outflowing lavas, then there
could be no doubt about these Kniveton rocks. But this structure is
found in so many Carboniferous sills, particularly in those thin sheets
which have been injected into coals and black shales, that its presence
is far from decisive. The vesicles in the Kniveton rocks are small and
pea-like, tolerably uniform in size and shape, and crowded together.
They are thus not at all like the irregular cavities in the ordinary
cellular and scoriaceous lavas of the toadstone series.
[Footnote 37: _Op. cit._ p. 87.]
Whether or not the question of their true relations be ever
satisfactorily settled, these Kniveton bands are certainly younger than
the lower portion of the Yoredale group. Their evidence thus agrees
with that of the southern agglomerates in showing that the volcanic
activity of this region was continued even after the thick calcareous
masses of the Carboniferous Limestone series had ceased to be deposited.
Besides the six necks to which I have referred, a rock in Ember Lane,
above Bonsall, probably belongs to another vent.[38] It is particularly
interesting from the great preponderance of limestone fragments in
it. The volcanic explosions at this locality broke up the already
solidified limestones on the floor of the Carboniferous Limestone sea,
and strewed them around, mingled with volcanic blocks and dust of the
prevailing type.
[Footnote 38: This is outcrop No. 39 of Mr. Bemrose's paper, p. 632.]
When the district has been more carefully searched, other centres of
eruption will no doubt be discovered. It may then be possible to depict
the distribution of the active vents, and to connect with them the
outflow of the bedded lavas. So far as I have been able to ascertain,
there are no necks of dolerite or basalt, though, as I have shown,
dykes or veins of molten rock are occasionally to be found in the
agglomerates of the necks.
4. THE LAVAS AND TUFFS.--I have referred to the opinion of De la Beche
that the toadstones of Derbyshire were poured out as lava-streams
without any accompanying fragmentary discharges, and to the correction
of this opinion by the subsequent observations of Jukes and of the
Geological Survey. But though the existence of interbedded tuffs
has long been known, it was not until Mr. Bemrose's more careful
scrutiny that the relative importance of the tuffs among the lavas was
first indicated. He has shown that a number of the bands mapped as
"toadstone" are tuffs, and he has discovered other bands of tuff which
have not yet been placed on any published map.
In examining the outcrops of the various toadstones of Derbyshire we
learn that some of them are lavas without tuffs, probably including a
number of bands, which are really sills; that others are formed of both
lavas and tuffs, and that a third type shows only bedded tuff. Each
of these developments will deserve separate description. But before
entering into details, we may take note of the varying thicknesses of
the different toadstones which have been determined by observation at
the surface or by measurement underneath in mining operations. In some
cases a distinct band of toadstone, separated by many feet or yards of
limestone from the next band, and therefore serving to mark a separate
volcanic discharge, may not exceed a yard or two in total thickness,
and from that minimum may swell out to 100 feet. The majority of the
bands probably range between 50 and 100 feet in thickness. In one
exceptional case at Snitterton, a mass of "blackstone" is said to have
been proved to be 240 feet thick, but this rock may not improbably have
been a sill.[39] The true contemporaneous intercalations seem to be
generally less than 100 feet in thickness.
[Footnote 39: A difference is made by the mining community between
"toadstone" and what is called "blackstone." The former name appears
to be restricted to the amygdaloidal green and generally more or less
decayed lavas; the latter, so far as I can learn, is applied to the
dark, more solid and crystalline rocks. If this distinction be well
founded the one name may perhaps serve to mark the open cellular lavas,
the other the more compact, dark, and heavy intrusive sheets.]
(_a_) Lavas without Tuffs.--Examples occur of sheets of toadstone
which consist entirely of contemporaneously ejected diabase, basalt
or dolerite. This rock is then dull green or brown in colour, more or
less earthy in texture, and irregularly amygdaloidal. The vesicles are
extremely varied in size, form and distribution, sometimes expanding
until the rock becomes a slaggy mass. A central more solid portion
between a scoriaceous bottom and top may sometimes be observed, as
at the Great Rocks Quarry, Peak Forest Limeworks (Fig. 181). In this,
as in other examples, a remarkably hummocky and uneven surface of
limestone lies below the igneous band, the calcareous rock presenting
knobs and ridges, separated by cauldron-shaped cavities and clefts,
some of which are several yards deep. These inequalities are filled
in and covered over with a soft yellow and brown clay, varying up to
three or four feet thickness, and passing upwards into the more solid
toadstone. There can hardly be any doubt that this singularly uneven
limestone surface is due to the solvent action of water lying between
the limestone and the somewhat impervious toadstone above, and that the
clay represents partly the insoluble residue of the calcareous rock,
but chiefly the result of the action of the infiltrating water on the
bottom of the igneous band.[40]
[Footnote 40: _Geological Survey Memoir on North Derbyshire_, p. 20 and
footnote.]
[Illustration:
Fig. 181.--Section of vesicular and amygdaloidal diabase resting on
Carboniferous limestone, Peak Forest Limeworks, Great Rocks Quarry.
1. Limestone with a surface dissolved into cauldron-like
hollows; 2. Rotten yellow and brown clay resulting from
decomposition of toadstone and white clay from the solution of the
limestone--sometimes three or four feet thick; 3. Toadstone, a
diabase with highly slaggy base.
]
[Illustration: Fig. 182.--View of the superposition of Carboniferous
limestone upon toadstone, Raven's Tor, Millersdale (length about 100
feet).
1. Toadstone; 2. Limestone; _f_, Fault.]
Junctions of the upper surfaces of the lava-sheets with the overlying
limestone show that the igneous material sometimes assumed hummocky
forms, which the calcareous deposits gradually overspread and
covered.[41] A good example of this kind may be observed by the roadside
at the foot of Raven's Tor, Millersdale. As shown in the subjoined
figure, the limestone has here been worn into a cave, the floor of
which is formed by the toadstone. The latter rock, of the usual dull
green, slaggy and amygdaloidal character, is covered immediately by the
limestone, but I did not observe any fragments of the toadstone, nor
any trace of ashy materials in the overlying calcareous strata. This
section shows that after the outflow of the lava, the sedimentation of
the limestone was quietly resumed, and the igneous interruption was
entirely buried.
[Footnote 41: Compare De la Beche, _Geological Observer_, pp. 559, 560,
and _North Derbyshire Memoir_, p. 123.]
In some cases there is evidence of more than one outflow of lava in the
same band of toadstone. Jukes believed that each band "was the result,
not of one simultaneous ejection of igneous matter, but of several,
proceeding from different foci uniting together to form one band," and
he found that near Buxton, two solid beds of toadstone could be seen
to have proceeded from opposite quarters towards each other without
overlapping.[42]
[Footnote 42: _Student's Manual of Geology_, 2d edit. (1862), p. 523.]
In Millersdale the authors of the _Geological Survey Memoir on North
Derbyshire_ observed that a band of toadstone about 100 feet thick
showed six distinct divisions, which they were disposed to regard as
marking so many separate beds.[43] In Tideswell Dale, on the west side
of the valley, immediately to the south of the old toadstone quarry,
two bands of toadstone are seen to be separated by a few yards of
limestone.
[Footnote 43: _Op. cit._ p. 19.]
(_b_) Lavas with Tuffs.--It will probably be found that in many, if
not in most cases, the outflow of lava was preceded, accompanied or
followed by fragmental discharges. As far back as 1861, Jukes noticed
that a toadstone band, about 50 feet thick, near Buxton consisted of
two solid beds of lava "with beds of purple and green ash, greatly
decomposed into clay, both above and below each bed and between the
two."[44]
[Footnote 44: _Op. cit._ p. 523.]
[Illustration:
Fig. 183.--Section at lime-kiln, south of Viaduct, Millersdale
Station.
]
An interesting section, showing this intercalation of the two kinds
of material is exposed at the lime-kilns beyond the southern end of
the railway viaduct at Millersdale Station. Over a mass of solid blue
limestone (1 in Fig. 183) lies a band of bright yellow and brown
clay (2), varying from six inches to two feet in thickness. This may
be compared with the clay found above the limestone at Peak Forest
(Fig. 181). But it is probably a layer of highly decomposed tuff.
It is succeeded by a thin band of greenish limestone (3) containing
an admixture of fine volcanic detritus, and partially cut out by an
irregular bed, four to eight feet thick, of a highly slaggy, greenish,
decomposing, spheroidal and amygdaloidal diabase (4). This unmistakable
lava-sheet is followed by a bed of green granular tuff (5), which
in some places reaches a thickness of three feet, but rapidly dies
out. Over a space several yards in breadth, the succeeding strata are
concealed, and the next visible rock is a dark, compact dolerite which
weathers spheroidally (6).
(_c_) Tuffs without Lavas.--Mr. Bemrose has shown that some of the
bands of toadstone consist entirely of bedded tuff. In these cases, so
far as the present visible outcrops allow us to judge, no outflow of
lava accompanied the eruption of fragmentary materials. But that the
ejection of these materials was not the result of a sudden spasmodic
explosion, but of a continued series of discharges varying in duration
and intensity, is indicated by the well-bedded character of the tuff
and the alternation of finer and coarser layers. Large blocks of
lava, two feet or more in diameter, may mark some of the more vigorous
paroxysms of the vents, while the usual fine granular nature of the
tuff may point to the prevailing uniformity and less violent character
of the eruptions. Bands of tuff 70 feet or more in thickness, without
the intercalation of any limestone or other non-volcanic intercalation,
point to episodes of such continued volcanic activity that the ordinary
sedimentation of the sea-bottom was interrupted, or at least masked, by
the abundant fall of dust and stones.
One of the best exposures of such intercalations of bedded tuffs was
pointed out to me by Mr. Bemrose, immediately to the east of the
village of Litton. The matrix is crowded with the usual minutely
vesicular glassy lapilli, and encloses fragments of diabase of all
sizes, up to blocks more than a foot in diameter. The rock is well
stratified, and the layers of coarse and fine detritus pass beneath
a group of limestone beds. The actual junction is concealed under
the roadway, but only two or three feet of rock cannot be seen. The
lowest visible layer of limestone is nodular and contains decayed
bluish fragments which may be volcanic lapilli. Immediately above the
lower limestones the calcareous bands become richly fossiliferous.
Some of their layers consist mainly of large bunches of coral; others
are crowded with cup-corals, or are made up mainly of crinoids with
abundant brachiopods, polyzoa, lamellibranchs, gasteropods and
occasional fish-teeth. This remarkable profusion of marine life is
interesting inasmuch as it succeeds immediately the band of volcanic
ash.
Another well-marked zone of tuff, with no traceable accompaniment of
lava, has already been referred to as connected with the Grangemill
vents. In this case also, the limestone that lies directly upon the
volcanic material is rather impure and nodular in character. The tuff
itself is well bedded, perhaps from 70 to 100 feet thick and dips
underneath an overlying series of marine limestones.
I did not observe thin partings of tuff and disseminated volcanic
lapilli among the limestones, such as are so marked in the Lower
Carboniferous formations of West Lothian, and in the Limerick basin,
to be described in the following chapter. But a diligent search might
discover examples of them, and thus prove that, besides the more
prolonged and continuous eruptions that produced the thick bands of
tuff, there were occasional feeble and intermittent explosions during
the accumulation of the thick sheets of limestone. Some of the layers
of "red clay" observed in shafts sunk for mining purposes may perhaps
represent such spasmodic discharges of fine fragmental material.
5. THE SILLS.--No attempt has yet been made to determine whether and
to what extent the toadstone bands include true intrusive sheets. My
own brief examination of the ground does not warrant me in making
any positive statement on this subject. I can hardly doubt, however,
that some, perhaps not a few, of the toadstone bands are really
sills. In the accounts of these rocks contained in the mining records
a distinction, as already remarked, appears to have been generally
drawn between "toadstone" and "blackstone." The latter term is applied
to the black, fresh, more coarsely crystalline, and generally
non-amygdaloidal rocks, which, so far as I have been able to examine
them, have the general external and many of the internal characters of
the Carboniferous sills of Central Scotland. At Snitterton near Matlock
one of these "blackstones," as already mentioned, is said to have been
found to be 240 feet thick.[45]
[Footnote 45: _North Derbyshire Memoir_, p. 23.]
It is stated that the toadstones, though subject to great variations
in thickness, are never seen to cut across the limestones.[46] But
I suspect that proofs of intrusion and transgression will be found
when diligently sought for. It appeared to me that the dark, compact,
crystalline dolerite, which was formerly quarried in the middle of
Tideswell Dale, may be separated from the vesicular toadstone of
that valley, which is undoubtedly a true lava-flow, and that it does
not always occupy the same horizon there, being sometimes below and
sometimes above the amygdaloid. Where it rests on a band of red clay
the latter rock has been made columnar to a depth of nine feet.[47]
Alteration of this kind is very rare among the Carboniferous bedded
lavas, but is by no means infrequent in the case of sills. But the
most important proof of alteration which I have myself observed occurs
at Dale Farm near the village of Peak Forest, where the limestone
above a coarsely crystalline dolerite has been converted into a white
saccharoid marble for about two yards from the junction.
[Footnote 46: _Op. cit._ p. 123.]
[Footnote 47: J. M. Mello, _Quart. Journ. Geol. Soc._ vol. xxvi. (1871),
p. 701.]
3. THE ISLE OF MAN
Rising from the middle of the Irish Sea, within sight of each of the
three kingdoms, with a history and associations so distinct, yet so
intimately linked with those of the rest of Britain, this interesting
island presents in its geological structure features that connect it
alike with England, Scotland and Ireland, while at the same time it
retains a marked individuality in regard to some of the rocks that
form its framework. Its great central ridge of grits and slates, which
still rises 2000 feet above the sea in the summit of Snaefell, must
have formed a tract of dry land in Carboniferous time, until it sank
under sea-level, and was buried beneath the Carboniferous and later
formations. Along the southern margin of this ancient land, a relic of
the floor of the Carboniferous sea has been preserved in a small basin
of Carboniferous Limestone which covers about seven or eight square
miles. This remnant has a special interest in geological history, for
it has preserved the records of a series of volcanic eruptions which
took place contemporaneously with the deposition of the Carboniferous
Limestone.
The geology of the Isle of Man was sketched in outline by J. F.
Berger,[48] J. Macculloch,[49] and J. S. Henslow,[50] and was afterwards
more fully illustrated by J. G. Cumming.[51] To the last-named observer
we owe the recognition of true intercalated volcanic rocks among
the calcareous formations of the southern end of the island. These
rocks have subsequently been studied in greater detail by a number of
geologists. An excellent general account of them was published in 1874
by Mr. John Horne, of the Geological Survey.[52] A few years later some
further observations on them were prepared by J. Clifton Ward.[53] More
recently their petrography has been studied by Messrs. E. Dickson, P.
Holland and F. Rutley,[54] and in more detail by Mr. B. Hobson.[55] To
some of the observations of these writers reference will be made in
the succeeding pages. During the progress of the Geological Survey in
the Isle of Man, the rocks in question have been mapped in detail by
Mr. A. Strahan and Mr. G. W. Lamplugh, and I have had an opportunity
of examining the coast-sections with the last-named geologist. The
following description of these sections is taken mainly from my field
note-book. The full details will appear in the official _Memoirs_.
[Footnote 48: _Trans. Geol. Soc._ 1st ser. vol. ii. (1814), p. 29.]
[Footnote 49: _Western Islands of Scotland_ (1819), vol. ii. p. 571.]
[Footnote 50: _Trans. Geol. Soc._ 1st ser. vol. v. (1821), p. 482.]
[Footnote 51: _The Isle of Man_ (1848), chap. x.]
[Footnote 52: _Trans. Geol. Soc. Edin._ ii. (1874), p. 332.]
[Footnote 53: _Geol. Mag._ 1880, p. 4.]
[Footnote 54: _Proc. Liverpool Geol. Soc._ vol. vi. (1888-89), p. 123.]
[Footnote 55: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 432. This
paper was reprinted with additions and corrections in _Yn Lioar
Manninagh_, Douglas, Isle of Man, vol. i. No. 10, April 1892.]
It may be remarked at the outset that the last outcrop of the
plateau-lavas of the Solway basin occurs only 60 miles from the south
end of the Isle of Man, at the foot of the hills of Galloway, the blue
outline of which can be seen from that island. The distance from the
Manx volcanoes to the nearest of the puys of Liddesdale is about 100
miles. Though the fragment which has been left of the ejections is too
small to warrant any confident parallelism, there appears to be reason
to believe that, alike in geological age and in manner of activity, the
Manx volcanoes may be classed with the type of the puys.
The Carboniferous strata of the Isle of Man lie in a small trough at
the south end of the island. The lowest members of the series consist
of red conglomerates and sandstones, which pass upward into dark
limestones full of the characteristic fossils of the Carboniferous
Limestone. As the bottom of the basin is on the whole inclined
seawards, the highest strata occur along the extreme southern coast. It
is there that the volcanic rocks are displayed. They occupy a narrow
strip less than two miles in length, which is almost entirely confined
to the range of cliffs and the ledges of the foreshore. Yet though
thus extremely limited in area, they have been so admirably dissected
along the coast, that they furnish a singularly ample body of evidence
bearing on the history of Carboniferous volcanic action.
Unfortunately the bottom of the volcanic group is nowhere visible.
At the east or lower end of the series, exposed on the shore, an
agglomerate with its dykes appears to truncate the Castletown
Limestones. No trace of any tuff has been noticed among these lower
limestones. We may infer that the volcanic activity began after they
were deposited. The highest accessible portions of the volcanic group,
as Mr. Horne showed, are clearly exposed on the coast at Poyll Vaaish,
intercalated in and overlying the dark limestones of that locality
(Fig. 184), which have been assigned, from their fossil contents, to
the upper part of the Carboniferous Limestone series.[56] The Manx
volcanoes may therefore be regarded as having probably been in eruption
during the later portion of the Carboniferous Limestone period.
[Footnote 56: R. Etheridge jun., in Mr. Horne's paper above cited.]
[Illustration: Fig. 184.--Limestones passing under stratified tuffs,
Poyll Vaaish, Isle of Man.]
Owing to irregularities of inclination, the thickness of the volcanic
group can only be approximately estimated. It is probably not less
than 200 or 300 feet. But as merely the edge of the group lies on the
land, the volcanic rocks may reach a considerably greater extent and
thickness under the sea.
The volcanic materials consist mainly of bedded tuffs, but include
also several necks of agglomerate and a number of dykes and sills. So
far as I have observed, they comprise no true lava-streams.[57] These
Manx tuffs present many of the familiar features of those belonging
to the puy-eruptions of Central Scotland, but with some peculiarities
worthy of attention. They are on the whole distinctly bedded, and as
their inclination is generally in a westerly direction, an ascending
order can be traced in them from the eastern end of the section to the
highest parts of the group associated with the Poyll Vaaish limestones.
Their colour is the usual dull yellowish-green, varying slightly in
tint with changes in the texture of the materials, the palest bands
consisting of the finest dust or volcanic mud. Great differences in the
size of their fragmentary constituents may be observed in successive
beds, coarse and fine bands rapidly alternating, with no admixture
of non-volcanic sediment, though occasional layers of fine ash or
mudstone, showing distinct current-bedding, may be noticed.
[Footnote 57: The occurrence of intercalated lavas has been described
in this series, but, as I shall show in the sequel, they are probably
intrusive masses.]
Pauses in the succession of eruptions are marked by the intercalation
of seams of limestone or groups of limestone, shale and black impure
chert. Such interstratifications are sometimes curiously local and
interrupted. They may be observed to die out rapidly, thereby allowing
the tuff above and below them to unite into one continuous mass. They
seem to have been accumulated in hollows of the tuff during somewhat
prolonged intervals of volcanic quiescence, and to have been suddenly
brought to an end by a renewal of the eruptions. There are some four or
five such intercalated groups of calcareous strata in the thick series
of tuffs, and we may regard them as marking the chief pauses in the
continuity or energy of the volcanic explosions.
An attentive examination of these interpolated sedimentary deposits
affords some interesting information as to the submarine conditions in
which the eruptions took place. The intercalations, sometimes 12 feet
or more in thickness, consist mainly of dark limestones, enclosing
the usual Carboniferous Limestone fossils; black shales, sometimes
showing very fragmentary and much macerated remains of ferns and other
land-plants; and black impure argillaceous chert or flint, arranged
in bands interposed between the other strata, and also in detached
lumps and strings. The dark flaggy limestones and black shales may
be paralleled lithologically with those of Castletown and Poyll
Vaaish. Indeed, there seems to be little doubt that they represent
the contemporaneous type of marine sediment that was gathering on
the sea-floor outside the volcanic area, and which during intervals
of quiescence or feeble eruptivity spread more or less continuously
into that area. The thick mass of tuff must thus have been strictly
contemporaneous with a group of calcareous muddy and siliceous deposits
which gathered over the bottom beyond the limits of the showers of
ashes.
[Illustration: Fig. 185.--Section of tuff, showing intercalations of
black impure chert, west of Closenychollagh Point, near Castletown,
Isle of Man.]
One of the most singular features of these sedimentary intercalations
is the occurrence of the black cherty material. It may generally be
observed best developed at the bottom and top of each group of included
strata. Looking at the lumps of this substance scattered through the
adjoining tuffs, we might at first take them for ejected fragments, and
such no doubt may have been the derivation of some of them. But further
examination will show that, as a rule, they are of a concretionary
nature, and were formed _in situ_ contemporaneously with or subsequent
to the deposition of the tuffs. The accompanying section (Fig. 185)
represents the manner in which the chert is distributed through two
or three square yards of tuff overlying one of the calcareous groups.
The material has been segregated not only into lumps, but into veins
and bands, which, though on the whole parallel with the general
stratification-planes of the deposits, sometimes run irregularly in
tongues or strings across these planes, as shown in Fig. 186, where the
dark chert band which overlies the limestones and shales sends a tongue
upwards for several inches into the overlying tuff.
That these interstratified calcareous and muddy strata were laid down
in water of some considerable depth may be inferred from their general
lithological characters. The dark carbonaceous aspect of the limestones
points to the probable intermingling of much decayed vegetation with
the remains of the calcareous organisms of which these strata chiefly
consist. The thin unimportant bands or partings of dark shale show that
only the finest muddy sediment reached the quiet depths in which the
strata were deposited, while the macerated fern-fragments suggest a
long flotation and ultimate entombment of terrestrial vegetation borne
seawards from some neighbouring land.
[Illustration:
Fig. 186.--Section of intercalated dark limestone, shale and chert
in the tuff south of Poyll Vaaish Bay, Isle of Man.
1. Limestones and shales; 2. Chert; 3. Tuff.]
The cherty bands and nodules, like the flints of the chalk, bear their
testimony to the quiet character of the sedimentation in rather deep
water beyond the limits within which the sediment from the land was
mainly accumulated on the sea-bottom. The origin of these siliceous
parts of the series of deposits has still to be investigated. Whether
or not they are to be referred to organic causes like chalk-flints, and
the radiolarian cherts of the Lower Silurian system, they furnish a
fresh example of the remarkable association of such siliceous material
with volcanic phenomena, which has now been observed in many widely
separated areas all over the world.
If we next turn to the stratification of the tuffs, we obtain further
evidence of undisturbed conditions of deposition on the sea-floor.
The bedding of these volcanic masses, though distinct, appears for
the most part to be due rather to the eruption and settlement of
alternately finer and coarser detritus than to any marked drifting
and rearrangement of these materials by current-action into different
layers. Throughout the series of tuffs, indeed, there is, on the whole,
a notable absence of any structure suggestive of strong currents or
of wave-action in the dispersal and reassortment of the volcanic
detritus. The ashes and stones were discharged in such a way as to
gather irregularly over the sea-floor into ridges and hollows. There
does not seem to have been sufficient movement in the bottom water
to level down these inequalities of surface, for we find that they
remained long enough to allow twelve feet or more of calcareous and
siliceous ooze to gather in the hollows, while the intervening ridges
still stood uneffaced until buried under the next fall of ashes.
At rare intervals some transient current or deeper wave may have
reached the bottom and spread out the volcanic detritus lying there.
Such exceptional disturbances of the still water are not improbably
indicated by occasional well-defined stratification, and even by
distinct false-bedding, in certain finer layers of tuff.
The materials of the tuffs are remarkably uniform in character and
conspicuously volcanic in origin. With the exception of occasional
blocks of limestone, which range up to masses several feet, and
occasionally several yards, in diameter, the dust, lapilli and included
stones consist entirely of fragmentary basic lava, so persistent in
its lithological features that we may regard its slightly different
varieties as merely marking different conditions of the same rock. The
accumulation of pumiceous ash in this southern coast of the Isle of Man
is one of the most remarkable in Britain. As Mr. Hobson has well shown,
the matrix of this tuff consists of irregular lapilli, representing
what may have been various conditions of solidification in one original
volcanic magma. This magma he has described as an "augite-porphyrite"
or olivine-basalt. Some of the lapilli, as he noted, consist of a
pumice "crowded with vesicles which occupy more space than the solid
part"; others show nearly as many vesicles, but the glass is made brown
by the number of its fine dust-like inclusions; a third type presents
the cells and cell-walls in nearly equal proportions. The same observer
found that where the substance is most cellular the vesicles, fairly
uniform in size, measure about a tenth of a millimetre in longest
diameter.
An interesting feature of the tuffs is the abundant occurrence of loose
felspar crystals throughout the whole group up to the highest visible
strata. These crystals, sometimes nearly an inch in length, appear
conspicuously as white spots on weathered surfaces of the rock. They
are so much decayed, however, that it is difficult to extract them
entire. On the most cursory inspection they are observed to enclose
blebs of a greenish substance like the material that fills up the
vesicles in the pumiceous fragments and in the pieces of cellular lava.
I have not ascertained the original source of these scattered felspars.
In one of the dykes on the north side of the agglomerate at Scarlet
Point, as was pointed out by Mr. Hobson, large crystals of plagioclase
occur in the melaphyre, but the felspars in the tuffs and agglomerates
differ so much from these that we cannot suppose them to have come from
the explosion of such a rock. I failed to detect any other mineral in
detached crystals in the tuffs, but a more diligent search might reveal
such, and afford some grounds for speculating on the probable nature
of the magma from the explosion of which the scattered crystals were
derived. It is at least certain that this magma must have included a
large proportion of plagioclase crystals.
Between the lapilli and the minute pumice-dust that constitute the
matrix of this tuff much calcite may be detected. Though this mineral
may have been partly derived from the decay of the felspar in the
lava-fragments, I believe that it is mainly to be attributed to the
intermingling of fine calcareous ooze with the ash accumulated on the
sea-floor. A more remarkable association of the same kind will be
described in later pages from King's County in Ireland. That abundant
calcareous organisms peopled the sea in which the Manx Carboniferous
volcanoes were active is shown by the contemporaneously deposited
limestones. The tuffs themselves are occasionally fossiliferous.
Species of _Spirifer_, _Productus_ and other brachiopods, together
with broken stems of encrinites, may be found in them, and doubtless
the diffused calcite, though now crystalline, as in the limestones, and
showing no organic structure, owes its presence to the detritus of once
living organisms.
The stones imbedded in the tuff consist almost exclusively of slightly
different varieties of the same pale, always vesicular rock, and
sometimes pass into a coarse slag. They vary up to six feet or more
in length. In many cases, they appear to have been derived from the
disruption of already solidified lava, for their vesicles are not
elongated or arranged with reference to the form of the block, but have
been broken across and appear in section on the outer surface. In other
instances, however, the cavities are large and irregular in the centre
of the block, while on the outside they are smaller and are drawn out
round the rudely spherical shape of the mass, as in true volcanic bombs.
The limestone fragments enclosed in the tuff include pieces of the
dark carbonaceous and of the pale encrinal varieties. In no case did I
observe any sensible alteration of these fragments. They seem to have
been derived from material disrupted and ejected during the opening of
successive vents, and not to have been exposed for any considerable
time to the metamorphic influence of volcanic heat and vapours.
Narrow though the strip of volcanic material is along the south coast
of the Isle of Man, it has fortunately preserved for us some of the
vents from which the tuffs were ejected. A group of these vents, three
or four in number, may be traced along the shore in a general W.N.W.
and E.S.E. line from Scarlet Point for rather more than a mile. Their
margins are in some places exceedingly well defined. The most striking
example of this feature occurs in the most westerly vent, where a neck
of remarkably coarse volcanic agglomerate rises vertically through
well-bedded, westerly-dipping tuff (Fig. 187). In other portions of
their boundaries no sharp line can be drawn between the material
filling the vent and that of the surrounding tuffs. Hence it is
difficult to define precisely the form and size of the vents. I am
inclined to believe from this indefiniteness of outline, and from the
remarkable structure of the dykes, to which I shall afterwards refer,
that the presently visible parts of these necks must lie close to the
mouths of the original vents, if indeed they do not actually contain
parts of the craters and of their surrounding walls.
The materials that have filled up the eruptive vents consist
chiefly of agglomerate, but partly also of intrusive portions of
vesicular lava. The agglomerate is composed of similar materials to
the tuffs. Its matrix shows the same extraordinarily abundant fine
greenish-grey basic pumiceous lapilli, with the same kind of plentiful
loose felspar-crystals. The large blocks of lava, too, resemble in
composition and structure those of the bedded tuffs, but greatly exceed
them in size and abundance.
Besides the fragments of vesicular lava, there occur also occasional
blocks of limestone. Some of these are several yards in length.
Messrs. Strahan and Lamplugh have mapped a large mass of limestone
at the Scarlet vent, which, so far as can be observed, lies in the
agglomerate--a large cake of white limestone with pebbles of quartz,
which has probably been broken off from some underlying bed and carried
up in the chimney of the volcano.
As a rule the agglomerate is a tumultuous, unstratified mass. But in
many places it shows lines of bedding and, as already stated, passes
outward into ordinary bedded tuff, the number and size of the ejected
blocks rapidly diminishing. Where this transition occurs we seem to
see a remnant of the base of the actual volcanic cone. Thus, in the
most westerly vent already cited, while the wall of the vent has been
laid bare on the side next the sea, so that the agglomerate on the
beach descends vertically through the surrounding bedded tuffs, on the
western side the cliffs have preserved a portion of the material that
accumulated outside the orifice (Fig. 187). In this section we observe
that the coarse agglomerate which fills up the main part of the vent
has been left with a hummocky, uneven surface, and that a subsequent
and perhaps feebler eruption of finer material has covered over these
inequalities, and has extended to the left above the fine tuffs through
which the agglomerate has been drilled.
[Illustration: Fig. 187.--Section of part of a volcanic neck on shore
to the south-east of Poyll Vaaish Bay, Isle of Man.]
[Illustration: Fig. 188.--Section of successive discharges and
disturbances within a volcanic vent. Scarlet Point, Isle of Man.]
Again, in the largest of the vents, that near Scarlet Point, still
clearer proof of successive eruptions and dislocations within a
volcanic chimney may be noticed. At one point the accompanying section
(Fig. 188) has been laid bare by the waves. The oldest accumulation is
a fine green granular tuff (_a_), rudely and faintly arranged in layers
inclined at high angles, like the fine materials in many of the vents
of the basin of the Firth of Forth. This peculiar stratification, due
not to the assortment of materials in water, but to the deposition of
coarser and finer detritus by successive explosions, and to subsequent
slipping or tilting, is a characteristic feature of the detritus which
has filled up ancient volcanic funnels. A later explosion from some
adjacent part of the same vent has given rise to the discharge of a
coarse agglomerate (_b_), which with blocks sometimes six feet long,
overspreads the earlier material. A third detrital accumulation in the
same vent, consisting of a firm brecciated tuff (_c_) with much calcite
in its matrix, has been brought down by a slip (_f_) which cuts across
both of the previous deposits. A broad dyke (_d_) of vesicular diabase
(augite-porphyry) traverses the vent, and is probably later than any of
the other rocks in the section.
I will conclude this account of the Manx Carboniferous volcanic rocks
with a brief reference to the intrusive masses which form a prominent
feature of the coast-line. From the picturesque headland of Scarlet
Point the broad dyke which forms that promontory may be traced for
some distance westwards. Several other parallel dykes run in the
same direction which, it will be observed, is also that of the chain
of vents. It might be said that the vents are, as it were, strung
together by a line of dykes. These eruptive masses traverse both the
agglomerates and the bedded tuffs. They probably belong, therefore,
to a comparatively late part of the volcanic history. That they are
truly intrusive and not lava-flows is, I think, clearly shown by their
vertical walls which descend through the surrounding rocks, and by
the greater closeness of their texture, as well as the diminution in
the size of their vesicles along the contact surfaces. But it must be
admitted that in their remarkably developed vesicular structure they
look more like streams of lava than ordinary dykes.
It is this structure which gives to these dykes their peculiar
interest. Bands of vesicles, from an inch or less to several inches in
breadth, run along the dykes parallel to the outer walls. Unlike the
familiar rows of little amygdaloidal cells in ordinary basalt dykes,
such as those of the Tertiary series in Scotland, these vesicles,
though small and pea-like in the narrower bands towards the margins of
the dykes, became so large, numerous, and irregular in the broader and
more central bands, that the rock passes there into a rough slag.
[Illustration: Fig. 189.--Section of dyke and sill in the tuffs west of
Scarlet Point, Isle of Man.]
While the intrusive material has for the most part risen in the form
of dykes, in one part of the coast-section, a little to the west
of Scarlet Point, it has been injected as a sill among the bedded
tuffs.[58] A section taken at this locality gives the structure
represented in Fig. 189. On the north side of the great dyke, the
strata of tuff which dip under it, roll over and support an outlying
sheet of the same material. The slaggy structure of parts of this sill
give it some resemblance to a true lava-flow. But it is the same
structure which can be seen in the dykes, while the closer grain along
the contact-surface further connects it with these intrusions.
[Footnote 58: It is this sheet which has been described as a
lava-stream.]
[Illustration:
Fig. 190.--Section on south side of vesicular sill west of Scarlet
Point.
]
[Illustration: Fig. 191.--Bands of vesicles in the same sill.]
There is, however, a peculiarity about the development of the
vesicular structure in this sill which I have not observed anywhere
else. If we examine the southern side of the crag near its eastern
end we observe that the successive bands of vesicles are arranged
in the same direction as the surface of contact with the underlying
tuffs, precisely as they are ranged in dykes parallel to the bounding
walls. So far the structure is quite normal. But, moving a few yards
westwards, we find that the bands begin to curve, and, instead of
following the contact surface, strike it first obliquely and then
at right angles, until we have the structure shown in Fig. 191. The
bands here vary from less than an inch to more than a foot in breadth,
and where broadest assume a slaggy texture. I sought in vain for any
evidence of subsequent disturbance such as might have truncated these
parallel rows of vesicles and pushed the rock bodily over the tuffs.
The perfect parallelism of the bands with the surface of the tuff at
the east end, and the absence of all trace of a thrust-plane at the
base of the sill, seem to show that, though the rows of vesicles were
undoubtedly at first arranged parallel to the surfaces between which
the intrusion took place, the mass, before completely consolidating and
coming to rest, was ruptured, and a portion of it was driven onwards at
right angles to its previous line of movement.
A consideration of the singularly slag-like structure of the injected
masses in the tuffs and agglomerates leads to the conclusion that
though what we now see of these rocks did not actually flow out at
the sea-bottom in streams of lava, it was intruded so close to the
surface that the imprisoned vapours had opportunity to expand, as in
superficial outflows.[59] This inference is in accord with that derived
from an examination of the necks, wherein we find evidence of the
probable survival of parts of the actual craters and volcanic cones.
[Footnote 59: As illustrative of the occurrence of the vesicular
structure in superficial intrusions, I may again cite the dyke which
cuts the ash of the outer crater-wall of the Puy de Pariou in Auvergne.
The andesite of this dyke is in places as vesicular as the lava-stream
with which it was doubtless connected, but the vesicles have been
flattened and drawn out parallel to the walls of the dyke. In this
instance it is quite certain that there could never have been any great
depth of detrital material above the fissure into which the material of
the dyke was injected (see vol. i. p. 66).]
As the records of the earliest eruptions during the Carboniferous
Limestone period in the district of the Isle of Man are concealed,
so also those of the last of the series lie under the sea. Where the
highest visible tuffs overlie the Poyll Vaaish limestones they show
no change in the nature of the materials ejected, or in the energy of
eruption. They lie so abruptly on the dark calcareous deposits as to
show that a considerable pause in volcanic activity was followed by a
violent explosion. The same abundant grey-green pumice, the same kind
of loose crystals of felspar, the same type of lava-blocks and bombs
as had characterized the foregoing eruptions remained as marked at the
end. But the further volcanic records cannot be perused, and we are
left to speculate whether the coast-sections reveal almost the whole
chronicle, or if they merely lay before us the early chapters of a
great volcanic history of which the main records lie buried under the
waves of the Irish Sea.
4. EAST SOMERSET
Various limited outcrops of igneous rocks have long been known to
occur in the eastern part of Somerset. The largest of these lies in
the midst of the Old Red Sandstone, on the crest of the axis of the
Mendip Hills, between Downhead and Beacon Hill. Smaller patches occur
in the Carboniferous Limestone near Wrington Warren, on the north
side of Middle Hope, on Worle Hill and at Uphill. These rocks have
been mapped as intrusive, though some of them have been described as
conglomeratic or as volcanic breccias. While some of the masses are
probably intrusive, others appear to be truly contemporaneous with
the deposition of the Carboniferous Limestone. The highly vesicular
basalt of Middle Hope looks much more like a superficial lava than
an intrusion. Mr. Aveline gave a section showing three alternations
of limestone and "igneous rock" at Middle Hope. A recent examination
of that coast-line by Mr. A. Strahan shows that there are undoubted
tuffs interstratified with the calcareous strata. There is thus
proof that one or more small volcanic vents were in eruption on the
floor of the Carboniferous Limestone sea in the neighbourhood of
Weston-super-Mare.[60]
[Footnote 60: See _Geological Survey Memoir_ "On East Somerset," by H.
B. Woodward, 1876, and authorities there cited. Mr. Aveline's section
above referred to will be found on p. 22.]
5. DEVONSHIRE
The change from the typical Old Red Sandstone of South Wales
to the Devonian system of Devonshire, to which I have already
referred, is hardly more striking than the contrast between the
Carboniferous formations of these two areas.[61] The well-marked
threefold subdivisions of Carboniferous Limestone, Millstone Grit and
Coal-measures, so persistent throughout Britain, and nowhere more
typically developed than in South Wales, are replaced in a distance of
less than forty miles by the peculiar "Culm-measures" of Devonshire--a
series of black shales, grey sandstones and thin limestones and
lenticular seams of impure coal (culm), which are not only singularly
unlike in original characters to the ordinary Carboniferous formations,
but have been made still more unlike by the extensive and severe
cleavage to which the Palæozoic rocks of Devon and Cornwall have been
subjected. That these Culm-measures are truly Carboniferous is made
abundantly clear by their fossil contents, though it has not yet been
possible to determine how far they include representatives of the great
stratigraphical subdivisions in other parts of the country.
[Footnote 61: In the centre of England numerous outlying areas of
igneous rocks are found in the Carboniferous Limestone, Millstone Grit
and Coal-measures. These will be considered by themselves in Chap.
xxxii.]
It is to De la Beche that geology owes the first intimation of the
occurrence of interstratified igneous rocks in the Carboniferous
series of Devonshire. As far back as the year 1834, in his singularly
suggestive treatise, _Researches in Theoretical Geology_, this eminent
geologist expressed his opinion that not only were the "trappean" bands
regularly intercalated in the sedimentary series and continuously
traceable with the general stratification, but that they occurred at
various localities in such a manner as to raise the suspicion that
these points may mark some of the centres of eruption. He particularly
cited the example of Brent Tor as a remarkable volcanic-looking hill,
composed in part of a conglomerate "having every appearance of volcanic
cinders."[62]
[Footnote 62: _Op. Cit._ p. 384.]
In his subsequently published _Report on the Geology of Cornwall,
Devonshire and West Somerset_, De la Beche dwelt in more detail on
the results of his study of these rocks, which he had traced out on
the ground and expressed upon the maps of the Ordnance Geological
Survey.[63] Hardly any additions have since been made to our knowledge
of the field-relations of the rocks. It is to the maps and Report of
De la Beche that we must turn for nearly all the published information
on the subject. I shall therefore give here a summary of what can be
gathered from these publications.
[Footnote 63: Sheets 22, 23, 24, 25, 30, 31, 32 and 33.]
In tracing the limits of the Culm-measures, De la Beche found that
no well-defined line could be drawn between these strata and the
"grauwacke" or Devonian formations underneath. The Carboniferous
series lies in a great trough, of which the axis runs nearly east and
west, so that the lowest members of the series rise along the northern
and southern margins. But De la Beche was struck with one remarkable
contrast between the two opposite sides of the trough--a contrast
which marks the Devonian as well as the Carboniferous formations of
this region. On the south side an abundant and persistent group of
intercalated bands of igneous, or as he called them, "trappean,"
materials can be followed along the whole line of boundary, while
no such group occurs on the north side. He found these bands to be
lenticular, traceable sometimes for a number of miles, then dying out
and reappearing on the same or other horizons. He mapped them the
whole way from Boscastle on the west to near Exeter on the east, and
found that though the individual sheets might be short, the trappean
zone was continuous as far as the southern margin of the Carboniferous
series could be seen, except where it had been broken through by the
great granitic mass of Dartmoor. He ascertained that the intercalated
trappean rocks are not confined to the Culm-measures, but occur also in
the contiguous portions of the "grauwacke" or Devonian system.
But further, he clearly recognized that the bands of igneous material
which he mapped included both "greenstones," together with other
varieties of massive eruptive rocks, and also volcanic ash or tuff,
though he did not attempt to separate these out upon the maps, but
contented himself with representing them all under the same colour.
He admitted that some doubt might be entertained as to the age of the
greenstones, for some of them might be intrusive and therefore later
than the sedimentary deposits between which they lie. But he contended
that there could be no uncertainty with regard to the trappean ash
or tuff, which being regularly interstratified in the Carboniferous
series, must be contemporaneous with it. He pointed out that many of
the greenstones, as well as fragments in the conglomerates or ashes,
were highly vesicular and must originally have been in the condition of
pumice.
As an illustration of the centres of eruption from which these
materials were ejected, De la Beche drew special attention once
more to the conspicuous eminence of Brent Tor and the rocks in its
neighbourhood. His remarks on this subject are well worthy of being
quoted--"The idea that in the vicinity of Brent Tor a volcano has
been in action, producing effects similar to those produced by active
volcanoes, forcibly presents itself. That this volcano projected ashes,
which, falling into adjacent water, became interstratified with the
mud, silt and sand there depositing, seems probable. That greenstones
and other solid trappean rocks constituted the lavas of that period
and locality, here and there intermingled with the ash, appears also a
reasonable hypothesis. Upon the whole there seems as good evidence as
could be expected that to the north and north-west of Tavistock, ash,
cinders and liquid melted rocks were ejected and became intermingled
with mud, silt and sand during this ancient geological epoch,
corresponding with the phenomena exhibited in connection with volcanoes
of the present day, more particularly when they adjoin or are situated
in the sea, or other waters where ejected ashes, cinders and lava can
be intermingled with ordinary mud, silt and sand."[64]
[Footnote 64: _Op. cit._ p. 122.]
It remains for some future observer to fill up the outlines thus
sketched by De la Beche, by tracing the respective areas of lavas and
tuffs, distinguishing the various petrographical types, separating the
intrusive from the interstratified sheets, identifying the necks and
bosses that may mark centres of eruption, and expressing these various
details upon maps on a sufficiently large scale.
A serious difficulty in this research arises from the effect of the
profound alteration which has been produced on the igneous rocks by
the cleavage of the region. Many of the "greenstones" have been so
cleaved as to become slaty or almost schistose. De la Beche recognized
this change and wrote of the "schistose trappean ash." A result of this
metamorphism has been to impart to rocks originally massive the same
fissile structure as the adjacent slates possess; and in this condition
it is often hardly possible to distinguish between "greenstone" and
fine-grained "ash." There can indeed be little doubt that among these
Carboniferous volcanic rocks, as we have seen to be the case with those
of the Devonian system in the same region, many lavas or sills have
been mapped as tuffs.
The chief additions to our knowledge of the Carboniferous volcanic
group of Devonshire since the time of De la Beche have been made by
Mr. F. Rutley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley[65] has
endeavoured to trace the respective areas occupied by the different
varieties of volcanic rocks in the district around Brent Tor, near
Tavistock, and to show the probable connection of the successive bands
of lavas and tuffs with a central vent of discharge situated at that
hill. He believes that these bands occur on four different horizons in
the sedimentary series. He has studied the microscopic structure of
the rocks, which in his view include "amphibolites, gabbros, basalts,
pitchstones and schistose ashes, or clastic rocks of a doubtful
nature."[66]
[Footnote 65: "The Eruptive Rocks of Brent Tor and its Neighbourhood,"
_Mem. Geol. Surv._ 1878. "On the Schistose Volcanic Rocks occurring on
the west of Dartmoor, with some Notes on the Structure of the Brent Tor
Volcano," _Quart. Journ. Geol. Soc._ xxxvi. (1880), p. 286.]
[Footnote 66: "The Eruptive Rocks of Brent Tor," p. 45.]
Mr. Ussher has re-mapped the tract of Culm-measures on the east side
of the Dartmoor granite, besides visiting some of the other areas
outside of the granite mass. While confirming the general accuracy
of De la Beche's survey, he has been able to improve the mapping
by inserting more detail, separating especially the tuffs from
the "greenstones." The latter have been found by him to be mostly
dolerites, some of which, from their parallelism the bands of tuff,
may be in his opinion contemporaneous lavas, though the majority of
them are evidently intrusive. The tuffs are regularly interstratified
among the Culm-measures, their most important band in this district
having an average breadth of about 100 yards, and being traceable for
at least two miles, possibly considerably further.[67] In going over
this tract with Mr. Ussher I was led to regard many of the sheets of
diabase (dolerite) or gabbro as true sills and bosses. Most of them
occur as short lenticular or oval patches tolerably numerous, but not
traceable for more than a short distance, though a connection may
often exist which cannot be detected by the scanty evidence on the
surface. One sheet which has been followed by Mr. Ussher from Combe
to beyond Ashton, a distance of nearly two miles, presents in the
centre a somewhat coarsely crystalline texture which rapidly gives way
to a much closer grain, and the rock then becomes highly vesicular.
It is overlain with dark Culm-shales and bands of fine shaly tuff,
passing upward into a granular tuff. Some layers of this tuff assume
a finely foliated appearance by the development of pale leek-green
folia, which show slickensided surfaces parallel with the bedding. The
rock then presents one of the usual appearances of schalstein. This
structure seems obviously due to mechanical movement along the planes
of stratification.
[Footnote 67: "The British Culm-measures," _Proc. Somerset Archæol. and
Nat. His. Soc._ xxxviii. (1892), p. 161.]
Bands of black chert and cherty shale are interpolated among the tuffs,
which also contain here and there nodular lumps of similar black impure
earthy chert--an interesting association like that alluded to as
occurring in the Carboniferous volcanic series of the Isle of Man, and
like the occurrence of the radiolarian cherts with the Lower Silurian
volcanic series already described.[68]
[Footnote 68: Cherts containing numerous species of radiolaria have
recently been found by Dr. Hinde and Mr. Howard Fox to form an
important part of the Lower Culm-measures of Devonshire, _Quart. Journ.
Geol. Soc._ vol. li. (1895), p. 609.]
The volcanic belt in the valley of the Teign can be followed for
about two miles. It is undoubtedly interstratified among the dark
Culm-measures, which are distinctly seen dipping under and overlying it.
General M'Mahon has recently shown what may be done by careful and
detailed examination of the ground broadly sketched in by De la Beche.
He chose for study a strip of "greenstone" shown on the Geological
Survey Map to extend for about three and a half miles along the
north-west margin of the Dartmoor granite. He has found that what is
represented under one wash of colour on that map includes both tuffs
and lavas. The tuffs, in spite of the alteration which they appear
to have undergone from the proximity of the great granite mass, are
found by microscopic investigation to be made up of fine volcanic
dust containing minute lapilli of various lavas. Sometimes as many as
six or seven different kinds of lava may be represented in the same
microscopic slide. These include felsitic or rhyolitic and trachytic
rocks together with fragments of dark glassy lava full of magnetite
dust. With the tuffs are intercalated sheets of felsite and trachyte.
In the same district coarse volcanic agglomerate occur, made up of
blocks of different lavas and pieces of different sedimentary rocks.[69]
[Footnote 69: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 338.]
These observations are of special interest, inasmuch as they point to
the eruption of a much more acid series of volcanic lavas and tuffs
than had previously been known to exist in the Culm-measures. Until the
ground has been more accurately mapped, it is impossible to say whether
these rocks are older or younger than those that lie around Brent Tor,
a few miles to the south-west. General M'Mahon has noted the presence
of more basic eruptive rocks in the same district. He specially cites
the occurrence of mica-diorite, of basaltic lavas altered into a
serpentinous mass, and of a dolerite which may possibly mark the actual
vent of the old Brent Tor volcano. His observations on the influence
of the Dartmoor granite in inducing new mineral rearrangements in the
igneous rocks of the Culm-measure series are full of interest.
CHAPTER XXX
THE CARBONIFEROUS VOLCANOES OF IRELAND
King's County--The Limerick Basin--The Volcanic Breccias of
Doubtful Age in County Cork.
Although the Carboniferous system spreads over by far the larger
part of the surface of Ireland, and is laid bare in many thousands
of natural and artificial sections, it displays undoubtedly
contemporaneous igneous rocks, so far as at present known, at only one
locality--the region around Limerick. A second district, however, lies
in King's County, where some vents occur which may be of Carboniferous
age, and of which a description will be given in the following
pages. That the relics of volcanic action should be so few, while
the exposures of the Carboniferous formations are so numerous and so
completely disclose the geological history of the whole system, must be
regarded as good evidence that while volcanoes abounded and continued
long active in Scotland and in parts of the Centre and South-west
of England, they hardly appeared at all in Ireland. It is worthy of
remark, also, that the Irish eruptions belong to the time of the
Carboniferous Limestone--a period distinguished by volcanic activity in
Scotland and England--that the nature of the materials erupted bears a
close resemblance to that of the lavas and tuffs of the sister island,
and that the manner of their eruption finds a close counterpart in the
Puy-eruptions, already described.
1. KING'S COUNTY
In the progress of the Geological Survey several small tracts of
"greenstone ash" and "greenstone" were mapped within an area of a few
square miles lying to the north of Philipstown. These igneous rocks
were shown to form Croghan Hill, which, rising into a conical eminence
769 feet above the sea, and some 450 feet above the general level of
the great limestone plain around it, forms the only conspicuous feature
in the landscape for many miles. In the maps and their accompanying
Explanations, the "greenstones" are treated as intrusive masses, but
the "greenstone ash" or breccia appears to have been regarded as
interstratified in the Carboniferous Limestone, though the admission
is made that "from the scanty exposures of the rocks and the total
absence of any connected section, it has been found impossible to
arrive at any definite conclusion as to the relations existing between
these traps and ashes with regard to each other or to the surrounding
limestone."[70]
[Footnote 70: See Sheets 109 and 110 of the Geological Survey of Ireland
and Explanation to accompany Sheets 98, 99, 108 and 109, by F. J. Foote
and J. O'Kelly (1865), pp. 7-18.]
In the course of a brief visit to this locality I did not succeed in
obtaining any certain proof of the age of the igneous rocks, but I
found their structures to be more varied and interesting than would be
inferred from the way in which they have been mapped, and I came to
the conclusion that the strong balance of probability was in favour of
regarding them as of the age of the Carboniferous Limestone.
[Illustration: Fig. 192.--Croghan Hill, King's County, from S.S.W.]
The first and most important fact to be announced regarding the
district is that it includes a group of volcanic necks which rise
through the Carboniferous Limestones. The chief of these forms Croghan
Hill. It is nearly circular in ground-plan, and measures about 4000
feet in diameter from the limestone on one side to that on the other.
It rises with steep grassy slopes out of the plain, the naked rock
projecting here and there in crags and low cliffs. Its general outward
resemblance to the Carboniferous necks of Scotland strikes the eye of
the geologist as he approaches it (Fig. 192).
But Croghan Hill, though the chief, is not the only vent of the
district. It forms the centre round which a group of subsidiary vents
has been opened. These form smaller and lower eminences, the most
distant being one and a half miles E.S.E. from the summit of Croghan
Hill, and measuring approximately 1200 feet in its longest and 800 feet
in its shortest diameter.
That the igneous materials of these necks really break through the
limestones may be clearly seen in several sections. Thus by the
roadside at Gorteen, on the south-western side of Croghan Hill, the
limestones have been thrown into a highly inclined position, dipping
towards the east at 60° or more, and their truncated ends abut against
the side of the neck. Again, on the eastern side of the same hill the
limestones have been much disturbed close to the margin of the neck,
sometimes dipping towards the volcanic centre, and sometimes striking
at it. Among these strata a small neck of breccia, of which only a
few square yards are visible, rises close to the edge of the bog that
covers the adjacent part of the great plain.
The material which chiefly forms these necks is one of the most
remarkable breccias anywhere to be found in the volcanic records
of the British Isles. The first feature noticeable in it is the
pumiceous character of its component fragments. These consist of a
pale bluish-grey basic pumice, and are generally about the size of
a hazel-nut, but descend to mere microscopic dust, while sometimes
exceeding a foot in length. They are angular, subangular and rounded.
Occasionally they stand out as hollow shells on weathered surfaces, and
in one instance I noted that the vesicles were flattened and drawn out
parallel to the surfaces of the shell, as if deformed by gyration, like
a true bomb.
The breccia remains singularly uniform in character throughout all
the necks. Its basic pumice presents much resemblance to that so
characteristic of the Carboniferous necks of Scotland, Derbyshire and
the Isle of Man. The abundant vesicles are generally spherical, and as
they have been filled with calcite or chlorite, they look like small
seeds scattered through a grey paste. Though I broke hundreds of the
lapilli, I did not notice among them any volcanic rock other than this
pumice. I am not aware of any other neck so homogeneously filled up
with one type of pyroclastic material, and certainly there is no other
example known in the British Isles of so large and uniform a mass of
fragmentary pumice.
Limestone fragments are not uncommon in this breccia. They resemble
the strata around the vents. Pieces of the adjacent cherts may also be
observed. In one or two cases, the limestone fragments were found by
me to have an exceptionally crystalline texture, which may possibly
indicate a certain degree of marmarosis, but on the whole there is
little trace of alteration.
The fragments of pumice in the breccia are bound together by a cement
of calcite. In fact the rock is, so to speak, saturated with calcareous
material, which, besides filling up the interstices between the
lapilli, has permeated the pumice and filled up such of its vesicles as
are not occupied by some chloritic infiltration.
I did not observe unmistakable evidence that any part of the breccia is
stratified and intercalated among the limestones, nor any vestige of
ashy material in these limestones. But it is possible that traces of
such interstratification may occur in the low ground to the north-west
of Croghan Hill, which I did not examine.
In only two places did I notice even a semblance of the intercalation
of limestone in the breccia. One of these is at Gorteen, where a
band of limestone strata a few feet thick is underlain and overlain
by breccia. But though the superposition of the layers of finely
stratified dark limestone and chert on the breccia is well seen and
thoroughly defined, no lapilli or ashy material are to be seen in
the limestone. Detached pieces of similar limestone and chert occur
in the breccia. The band of stratified rock, if _in situ_, may be a
tongue projecting from the wall into the body of the neck, like some
instances already cited from Scotland, but more probably it is really a
large included mass lying within the vent itself. The breccia here as
elsewhere is entirely without any trace of stratification. The second
locality occurs at the most easterly neck north of Coole House, where
the limestones, rapidly undulating, seem at last to plunge below the
breccia, which shows a series of parallel divisional planes suggestive
of bedding. But these may be only joint-structures, for there is no
stratification of the component materials of the rock.
In the necks, and also through the limestone surrounding them, masses
of eruptive rock have been intruded as irregular bosses and veins.
The material of these intrusions presents little variety, and, so far
as I could note, gives no indication of the successive protrusion
of progressively different lava. It varies from a deep blue-black
fine-grained basalt to a dolerite where the plagioclase is distinct.
Some portions, however, are more basic and pass into limburgite.
Externally there is nothing worthy of special remark in these rocks
unless it be their prevalent amygdaloidal structure. The amygdales,
generally of calcite, vary from small pea-like forms in the basalts
up to kernels half an inch long or more in the dolerites. From a
microscopic examination Mr. Watts found that some of the basalts
have a base of felspar and augite rich in brown mica, and that their
porphyritic felspars enclose idiomorphic crystals of augite.
Perhaps the most noticeable feature in these later parts of the
volcanic series is the occurrence in them at one locality in Croghan
Demesne of lumps of a highly crystalline material quite distinct from
the surrounding rock. These enclosures vary from an inch or two to a
foot or more in diameter. They must be regarded as blocks which have
been carried up in the ascent of the basic lava. Their composition
has been ascertained by Mr. Watts from microscopic examination to
be somewhat singular. One specimen "contains relics of garnets,
surrounded by rings of kelyphite, imbedded in a mosaic of felspar,
with a mineral which may possibly be idocrase." Another specimen from
the same locality (south-east from Gorteen) "contains the relics of
garnets preserved as kelyphite, set in a matrix of quartz-grains, much
strained, and containing a profusion of crystals of greenish-yellow or
red sillimanite. This appears to be a metamorphic rock, and may be a
fragment of some sediment enclosed in the igneous rocks."[71]
[Footnote 71: _Guide to the Collections of Rocks and Fossils belonging
to the Geological Survey, in the Museum of Science and Art, Dublin_
(1895), pp. 38, 39.]
As regards the history of volcanic action in Britain one of the
chief points of interest connected with these Irish breccias and
lavas relates to their geological age. As no proof has been produced
that any portion of them is contemporaneously interstratified in the
Carboniferous Limestone which surrounds them, we cannot definitely
affirm that the volcanic eruptions which they record took place during
the accumulation of that formation. The vents must, of course, be
later than that portion of the limestone which they pierce. But the
evidence seems to me to be on the whole most favourable to the view
that they are of Carboniferous Limestone age, for the following
reasons:--
1. The breccias of Croghan Hill do not present a resemblance to any of
those belonging to the Tertiary volcanic series in Antrim or the Inner
Hebrides. The possibility of their being of Tertiary age may therefore
be dismissed from consideration.
2. There are no known Permian volcanic rocks in Ireland. Nor does the
Croghan Hill breccia show any resemblance to the ordinary material of
the breccias in the Permian necks of Scotland. It is thus not likely to
be of Permian age.
3. The peculiar basic pumice of these Croghan Hill vents has many
points in common with the palagonite fragments so abundant among
the volcanic breccias and tuffs of Carboniferous age in Scotland,
Derbyshire, and the Isle of Man, and which occurs also among the
Carboniferous tuffs of the Limerick basin. It differs from the
general type of the material in its pale colour, in its uniformity
of character, in its calcareous cement, and above all in its vast
preponderance over all the other materials in the breccia.
4. The saturation of the Croghan Hill breccia with calcite is a
singular feature in the composition of the rock. Had the vents
been opened long subsequent to the deposition of the Carboniferous
Limestone, it is difficult to understand how this calcite could
have been introduced. Mere percolation of meteoric water from the
adjacent limestone does not seem adequate to account for the scale
and thoroughness of the permeation. But if the vents were opened on
the floor of the Carboniferous Limestone sea, it is intelligible that
much fine calcareous silt should have found its way down among the
interstices of the breccia and into the pores of the pumice which,
being caked together within the vent, did not all float away when the
sea gained access to the volcanic funnel. The effect of subsequent
percolation would doubtless be to carry the lime into still unfilled
crevices, and to impart to the cement a crystalline structure similar
to that which has been developed in the ordinary limestones.
2. THE LIMERICK BASIN
About 70 miles to the south-west of the area just described lies the
most compact, and, for its size, one of the most varied and complete,
of all the Carboniferous volcanic districts of Britain (Map I.). It
takes the form of an oval basin in the Carboniferous Limestone series
near the town of Limerick, about twelve miles long from east to west
and six miles broad from north to south. Round this basin the volcanic
rocks extend as a rim about a mile broad. A portion of a second or
inner rim, marking a second and higher volcanic group, partially
encloses a patch of Millstone Grit or Coal-measures, which lies in the
heart of the limestone basin. (See the section in Fig. 196.)
But it is evident that, as the denuded edges of the volcanic sheets
emerge at the surface all round the basin, the present area over
which these rocks extend must be considerably less than that which
they originally covered. Some indication of their greater extension
is supplied by outliers of the bedded lavas and tuffs, as well as by
bosses which doubtless indicate the position of some of the eruptive
vents. The distance between the furthest remaining patches is 24 miles.
The original tract over which the volcanic materials were spread cannot
have been less than 24 miles long by 10 miles broad. If we assume its
area to have been between 250 and 300 square miles we shall probably be
under the truth.
This volcanic centre made its appearance on the floor of the
Carboniferous Sea in the same district which had witnessed the
eruptions of Upper Old Red Sandstone time. The two visible vents that
crown the Knockfeerina and Ballinleeny anticlines (Chapter xxii.), are
only some ten miles distant, and there may be others of the same age
even under the Limerick basin. This district thus supplies another
instance of that recurrence of volcanic energy in the same area,
after a longer or shorter geological interval, which stands out as a
conspicuous feature in the history of volcanic action in Britain. That
a prolonged interval elapsed between the extinction of the Old Red
Sandstone volcanoes and the outbreak of their successors during the
accumulation of the Carboniferous limestone series, may be inferred
from the thickness of strata which separate their respective tuffs.
From the published sections of the Geological Survey there would appear
to be about 500 feet of Old Red Sandstone above the volcanic series of
that formation. Then comes the Lower Limestone shale, which is computed
to be about the same thickness. From the scarcity of observable dip
among the Lower Limestones and their variable inclination, it is not
easy to form any satisfactory estimate of the depth of this group up
to the base of the volcanic series. It may be as much as 800 feet,[72]
and if so there would thus intervene a mass of sedimentary material
nearly 2000 feet in thickness between the two volcanic platforms.
Throughout this thick accumulation of stratified deposits no trace
of contemporaneous volcanic activity has been detected. From the
descriptions published more than thirty years ago by Jukes and his
colleagues in the Geological Survey of Ireland, geologists learnt
how full and interesting are the proofs of great volcanic activity
contemporaneous with the deposition of the Carboniferous Limestone
series in the Limerick district.[73] Nowhere, indeed, is the evidence
more complete for the occurrence of a long succession of volcanic
eruptions during a definite period of geological time. The officers of
the Survey showed that two epochs of activity during the older part
of the Carboniferous period were each marked by a group of tuffs and
lavas, while the interval of quiescence between them is represented
by a thousand feet of limestone. The same observers likewise mapped
outside the volcanic ring a number of eruptive bosses, which they
regarded as probably marking some of the actual vents of that time.
[Footnote 72: This is the thickness given in the Explanation to Sheet
144 of the Geological Survey of Ireland, p. 8. A still greater
thickness is claimed in Explanation to Sheet 154, p. 8.]
[Footnote 73: See especially Explanations of Sheets 143, 144, 153 and
154, Geol. Surv. Ireland (1860, 1861). The geology of the district
had been previously noticed by earlier observers, to whose writings
reference is made on p. 26 of the Explanation of Sheet 144. See also
Jas. Apjohn, _Journ. Geol. Soc. Dublin_, vol. i. (1832), p. 24;
Prof. Hull, _Geol. Mag. for 1874_, p. 205. Jukes (_Student's Manual
of Geology_, 2nd edit. 1862, p. 325) gave subsequently an excellent
epitome of the volcanic history. The microscopic structure of some of
the Limerick volcanic rocks has been described by Mr. Allport, _Quart.
Journ. Geol. Soc._ vol. xxx. (1874), p. 552, and by Prof. Hull, _Geol.
Mag. for 1873_, p. 153. See also Mr. Watts' account of these rocks in
the _Guide to the Collections of Rocks and Fossils_ (Dublin, 1895), p.
93.]
The lower volcanic group, which forms a complete ring round the Upper
Limestones of the Limerick basin, is estimated to reach a thickness of
1000 feet in some parts of its course.[74] Its base appears to coincide
generally with the upward termination of the Lower Limestone group of
this district, though here and there small patches of volcanic rocks in
that group have been regarded as interstratified and contemporaneous
bands.[75] It consists of a series of lavas and tuffs, the alternations
and rapid incoming and dying out of which were well made out by the
Geological Survey.
[Footnote 74: Explanation of Sheet 144, p. 27.]
[Footnote 75: Some of them, however, have characters that rather seem to
place them with the intrusive materials of the district, and therefore
not necessarily earlier than the bedded lavas and tuffs. The boundary
line of the volcanic series is not consistently followed along the
same horizon on the Survey maps. Thus to the east of Caherconlish, a
strip of the Upper Limestone is inserted below the base of the tuffs
for a distance of about four miles. Unless a different horizon has
been in some places taken for the boundary between the two groups of
limestones, it would appear that the eruptions had not extended over
the north and north-east of the district until some time after the
deposition of the Upper Limestone had begun. The division between the
two limestone groups is taken at a set of chert-bands, but as these are
not constant it is sometimes difficult to draw a satisfactory line of
division.]
_Tuffs._--The base of the volcanic series is generally formed by a band
of tuff sometimes as much as 350 feet thick,[76] which may be traced
nearly continuously round the basin as well as in detached outliers
even as far as Carrigogunnel overlooking the alluvial plain of the
Shannon. The manner in which the bottom of this tuff is interstratified
with the limestone below it may be instructively examined in many
quarries around the town of Limerick. Striking evidence is there
supplied that the first eruptions were comparatively feeble and
spasmodic, and were separated by intervals of longer and shorter
duration, during which the limestone with its fragmentary organisms
was deposited, little or no volcanic detritus falling at that time.
Yet even in some of the limestones the microscope reveals fine broken
needles of felspar, representing doubtless the finest ejected dust.[77]
[Footnote 76: Explanation of Sheet 154, p. 21.]
[Footnote 77: For the details of the microscopic structure of the
Limerick volcanic rocks I am mainly indebted to the examination of them
made for me by my Survey colleague, Mr. W. W. Watts.]
As an illustration of the way in which the volcanic and organic
detritus alternated over the sea-floor, the following section from a
quarry in the townland of Loch Gur on the southern side of the basin is
here given:[78]--
[Footnote 78: Explanation of Sheet 154, pp. 21, 22.]
Cherty limestone more than 20 feet 0 in.
Decomposed green tuff 2 " 6 "
Bluish-green, calcareous laminated tuff 4 " 0 "
Limestone, slightly ashy 1 " 8 "
Green tuff 0 " 2 "
Fine-grained decomposed tuff 0 " 4 "
Green tuff, obliquely laminated 1 " 7 "
Fine laminated tuff 0 " 8 "
Green compact tuff 1 " 8 "
Obliquely laminated shaly tuff 0 " 10 "
Concretionary ashy limestone 1 " 4 "
Compact ashy limestone 2 " 0 "
Green shaly tuff, much weathered 0 " 5 "
Ashy limestone 0 " 7 "
Compact green tuff more than 4 " 0 "
--------------
41 feet 9 in.
The tuffs which in the southern part of the basin underlie the less
basic lavas differ in some respects from those which further north
are associated with the Upper Limestones. They are green, sometimes
dull purplish-red, finely granular rocks, made up in large part of
andesitic debris. They are full of loose felspar crystals, minute,
somewhat rounded and subangular lapilli of andesite or some less basic
lava, together with bits of grit and baked shale. Though generally
much decomposed, they are sometimes compact enough to be used for
building-stone. Under the microscope these tuffs are seen to abound in
andesite-lapilli, with a few pieces of felsitic rocks enclosed in an
opaque base, through which are scattered broken felspars and occasional
vesicular lapilli.
[Illustration: Fig. 193.--Section in quarry on roadside east of
Limerick close to viaduct of the Limerick and Erris Railway.
1. Limestone; 2. Calcareous tuff; 3. Ashy limestone or calcareous tuff.]
The tuffs around Limerick, interbedded with the Black (Upper)
Limestone, are distinguished by a scarcity of andesitic debris, by
their persistent dull greenish-grey colour, and more particularly
by the abundance of minute lapilli and larger fragments of an
epidote-green, finely vesicular, easily sectile basic pumice. Under
the microscope much of this material is found to be an altered basic
glass of the nature of palagonite. These tuffs are in evident relation
with the more basic lavas that accompany them. The manner in which they
alternate with the black limestone shows that the conditions for the
eruption of this more basic detritus continued to be very similar to
those that existed when the andesitic tuffs were ejected. As a good
illustration of this feature the accompanying section (Fig. 193) is
given from a quarry on the side of the high-road between Limerick and
Annacotty. The total depth of strata here represented is about 15 feet.
The black limestone at the bottom is a tolerably pure calcareous rock.
It is divided into bands by thin partings of a fine greenish calcareous
tuff, each marking a brief discharge of ashes from some neighbouring
vent. Half-way up the succession of strata, the ashy material rapidly
increases until it usurps the place of the limestone, though its
calcareous composition shows that the accumulation of calcareous
sediment had not been entirely suspended during the eruption of ash.
Among these tuffs I have noticed fragments of fine, dark, flinty
felsite, grit and other rocks. The stones are for the most part small,
but vary up to blocks occasionally a foot in diameter.
_Lavas._--The lavas occur in numerous sheets, sometimes separated by
thin partings or thicker beds of tuff and volcanic conglomerate. On the
northern rim of the basin Mr. G. H. Kinahan has described the volcanic
series east of Shehan's Cross-roads as composed of six zones of tuff,
each bed varying from about 50 to 250 feet in thickness, alternating
with as many sheets of lava ranging from 27 to 180 feet in thickness,
the total depth of tuff being estimated at nearly 500 feet and that
of the lavas at about 800 feet.[79] Some of these tuffs are coarse
conglomerates or agglomerates, with blocks of lava occasionally 10 feet
long.
[Footnote 79: Explanation of Sheet 144, p. 28.]
Some of the lavas in the lower volcanic group are andesites quite like
those of the plateau series in the Carboniferous system of Scotland.
Externally they appear as dull reddish-brown or purplish-red compact
rocks, with abundant porphyritic felspars scattered through the
fine-grained base. They are generally much decomposed, showing on a
fresh fracture pseudomorphs of chlorite, hæmatite and calcite after
some of the minerals, with abundant hæmatitic staining through the body
of the rock. Amygdaloidal structure is commonly developed.
These andesites, when examined microscopically, were found by Mr.
Watts to present the characteristic base of minute felspar-laths with
magnetite and enstatite, and with porphyritic crystals, often large, of
zoned plagioclase, as well as of ilmenite and hæmatite.
[Illustration: Fig. 194.--Section of the volcanic escarpment, east of
Shehan's Cross-roads, south of Limerick.
1. Limestone; 2 2. Tuffs; 3 3. Lavas.]
But besides the andesites there occur also, and, so far as I have
observed, in larger number, sheets of true basalt. This rock is
typically black, exceedingly close-grained in the central portion of
each sheet, but becoming highly slaggy and vesicular along the upper
and lower parts. Under the microscope it is found to contain granular
augite and magnetite, set in a more or less devitrified glass, with
microlites of felspar, porphyritic plagioclase, serpentinized olivine,
and some well-marked augite. These rocks form distinct escarpments
along the northern rim of the basin as in the foregoing section east
from Shehan's Cross-roads (Fig. 194). From the summit of this ridge,
which is about 600 feet above the sea, the eye looks northward over the
plain, across which low outliers of the volcanic series are scattered,
and southwards across the basin to the corresponding line of volcanic
heights forming the southern rim.
The upper volcanic group has been estimated by the officers of the
Geological Survey to lie about 1000 feet higher in the Carboniferous
system than the lower, the intervening strata consisting of the Upper
Limestone.[80] It is possible that the interval is greater in some parts
of the district than in others, and if so, the difference may be due
either to greater local accumulation of volcanic materials, or to local
prolongation of the eruptions into higher stratigraphical horizons. The
outcrop of the upper volcanic band forms about half of a ring round the
little cup of Millstone Grit or Coal-measures which lies within the
volcanic basin. On the north-west side of the cup the volcanic rocks
disappear. Hence the upper band has a much more restricted area than
the lower. But if the tuffs immediately around Limerick are assigned
to the upper group, its extent will be proportionately increased.
There can be little doubt, however, that neither in thickness nor in
superficial area did the lavas and tuffs of the second group equal
those of the first. The volcanic energy was gradually dying out.
[Footnote 80: Explanation of Sheet 154, p. 24.]
The lavas of the second period are characteristic dull, black, compact
basalts, like those of the first period, becoming here and there
strongly amygdaloidal, and being occasionally separated by slaggy or
conglomeratic partings. But they include also certain rocks wherein
the felspar diminishes in quantity, while augite and olivine become
conspicuous, together with a little enstatite. The augite occurs in
large porphyritic forms, as well as of medium size and in small prisms.
The olivine, as usual, is now in the condition of serpentine. These
rocks are more basic than the ordinary basalts, containing only 38·66
per cent of silica, and thus approaching the limburgites. With these
basic lavas are associated dull green tuffs and conglomerates, made
up largely of basalt-debris, together with abundant pieces of finely
vesicular basic pumice and lapilli of a palagonitic material.
The manner in which the lavas and tuffs have alternated with each
other, and also with the limestones, is well seen on Nicker Hill
above Pallas Grean.[81] The Survey sections show eight sheets of lava,
separated by six bands of tuff and eight intercalations of limestone,
the whole passing under the Coal-measures.
[Footnote 81: See Explanation of Sheet 144, p. 30, where a description
with detailed map and sections of this ground will be found.]
The upper volcanic group may be as much as 600 or 800 feet thick. It
appears to have been left, at the close of the eruptions, with a very
uneven surface, some portions being so low as to be overspread with the
Upper Limestones, other parts so high as not to be covered until the
Coal-measure shales and flagstones came to be deposited.[82]
[Footnote 82: Explanation of Sheet 154, pp. 24, 35.]
_Vents._--All round the edges of the Limerick basin, where the
escarpments of the volcanic groups, rising abruptly above the plain,
show that these rocks once extended beyond their present limits, the
progress of denudation has revealed a number of bosses which, as above
stated, Jukes and his associates looked upon as marking some of the
vents from which the lavas and tuffs were erupted. Especially striking
is the line of these vents along the southern margin. The rocks now
filling them present some unusual and rather anomalous features. They
are decidedly more acid than the lavas of the basin, some of them even
containing free quartz. Mr. Watts remarks that "though they have a good
deal in common with the trachytes, they are crystalline throughout.
They are red granite-looking rocks, which are made up chiefly of
stumpy idiomorphic prisms of felspar which is mainly orthoclase.
Some plagioclase also occurs, and the two felspars are imbedded in
interstitial quartz. A trace of hornblende or mica is frequently
present, and the rocks contain about 65 per cent of silica." These
characters are specially observable in the necks furthest removed from
the basin, which may possibly have been connected with the andesitic
outflows. Nearer to the basin the necks "contain about 60 per cent
of silica, seldom show any interstitial quartz, and stand between
trachytes and porphyrites, some perhaps being bostonites."[83]
[Footnote 83: _Guide to the Collections of Rocks, etc., Geol. Survey,
Ireland_, p. 93, Dublin 1895.]
[Illustration: Fig. 195.--View of Derk Hill, a volcanic neck on the
south side of the Limerick basin.]
A geologist, familiar with the Carboniferous and Permian necks of
Scotland, has no hesitation in confirming the surmise of Jukes and
his colleagues that the cones and domes around the Limerick basin
mark the sites of eruptive vents. On the south side of the basin, at
least nine such necks rise into view, partly from among the lavas and
tuffs, but chiefly through the limestones that emerge from below
these volcanic sheets. One of the most conspicuous of them, Derk Hill
(Fig. 195), rises to a height of 781 feet above the sea, and comes
through the bedded andesites, as represented in Fig. 196, which gives,
in diagrammatic form, the general structure of the Limerick volcanic
basin. Around the northern side of the basin a smaller number of necks
has been observed, consisting of similar acid rocks.
[Illustration: Fig. 196.--Section across the Limerick volcanic basin.
1. Lower limestone; 2. Lower series of lavas and tuffs; 3. Middle and
Upper Limestone; 4. Upper series of lavas and tuffs; 5 5. Two volcanic
necks; 6. Millstone Grit series.]
A few of the necks appear to be filled with volcanic agglomerate. Here
and there detached patches of fragmental volcanic material have been
shown on the Survey maps, and referred to in the Explanations, as
if they were outliers of the bedded tuffs; though in some cases the
coarseness of their materials and the want of any distinct bedding,
together with the absence of any indication of their relation to the
nearest limestones, have evidently offered considerable difficulty in
their mapping. One of the best examples occurs about two miles to the
south-east of the village of Oola. The boundaries of this patch, as put
on the map, are confessed to be "entirely speculative." It was only
seen on the side of the railway where it appeared as "a very coarse
brecciated purple ash."[84]
[Footnote 84: Explanation of Sheet 154, p. 25.]
On comparing the maps of the Limerick basin with those of the
Carboniferous districts of Scotland, the main difference will probably
be acknowledged to be the absence of any recognizable sills in the
Irish ground. That no sills actually occur, I am not prepared to
affirm. Indeed some of the more acid rocks, both outside the basin and
among the rocks of the older volcanic group, appeared to me during
my traverses of the ground to have much of the character of sills.
A more critical examination of the area would not improbably detect
some truly intrusive sheets which have hitherto been mapped among the
interstratified lavas. Some appear to exist among the surrounding Lower
Limestones.
An intrusive mass, like a sill or dyke, is represented on the
Geological Survey Map as traversing the Coal-measures in the inner
basin south of Ballybrood. But as the strata are on end along its
southern margin, it may possibly be only a portion of the upper
volcanic series which has been thrown into its present position by one
or more faults.[85]
[Footnote 85: Sheet 154 and Explanation to the same, p. 24.]
3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK
In the south-western headlands of Ireland, from Bear Island to
Dursey Island, various igneous rocks have been traced on the maps
of the Geological Survey. They have been described as consisting of
"greenstone," "felstone," and "ash" or "breccia," and as including
both interstratified and intrusive masses.[86] If contemporaneous with
the strata in which they occur, they would prove the existence of a
group of volcanic rocks in the Carboniferous slate, or lowest division
of the Carboniferous system. After an examination of the coast-line
I came to the conclusion that while there is undoubtedly evidence of
former volcanic activity in this part of Ireland, no proof has been
obtained that the eruptions occurred in the Carboniferous period. The
felsites and dolerites appeared to me to be all intrusive, the former
having certainly been injected before the terrestrial movements that
have disturbed the rocks, for some of them share very markedly in the
cleavage of the region. The dolerites and diabases, on the other hand,
so far as I observed, are not cleaved, and are thus probably of later
date.
[Footnote 86: See Sheets 197 and 198 of the Geological Survey of
Ireland, and the Explanation of these Sheets by Messrs. Jukes, Kinahan,
Wilson, and O'Kelly, 1860.]
The most interesting rocks are undoubtedly the "ash" and "breccia," for
they are obviously of volcanic as distinguished from plutonic origin.
On the coast north of White Bull Head, a bed of volcanic breccia may be
seen made up of rounded and angular fragments of different sandstones,
shales and limestones, with pieces of felsite and andesite wrapped up
in a dull-grey fine-grained sandy felspathic matrix. The rock weathers
with a rough or rugged surface, owing to the dropping out of the more
decomposable stones. This bed, about five feet thick, runs with the
bedding of the strata around it, and like these dips S.S.W. at an
angle of 70°. If no other evidence were obtainable, this breccia would
be naturally set down as a truly interstratified deposit of volcanic
detritus. A short distance from it, a second, rather thicker band of
similar material occurs, specially distinguished by its abundant worn
crystals of hornblende, sometimes three inches in diameter, as well as
large crystals of muscovite. These minerals are not unknown elsewhere
in volcanic agglomerates. The occurrence of lumps of augite in the
vents of Upper Old Red Sandstone age in Caithness has been already
alluded to, and a still larger series of ejected minerals will be
shown in a later chapter to characterize the younger necks of Central
Scotland.
In parts of its course, this second band appears to run so perfectly
parallel with the bedding of the strata between which it lies that the
observer would readily believe it to be a part of the same series of
deposits, and might therefore regard it as affording good evidence of
volcanic action contemporaneous with the formation of these deposits.
A transverse section of the bed, where thus apparently conformable, is
shown in Fig. 197.
[Illustration: Fig. 197.--Section of a bed of Volcanic Breccia in the
Carboniferous Slate; White Bull Head, County Cork.
1 1. Sandstones and shales; 2. Breccia.]
[Illustration: Fig. 198.--Volcanic Breccia invading and enclosing
Carboniferous Slate, White Bull Head.]
Further examination, however,reveals that this seemingly regular
sequence is entirely deceptive. At various points the breccia abruptly
truncates the sandstones, and involves large pieces of them, as shown
in Fig. 198 A. At other places, the lower side of the breccia, or what
would be its base if it were a regular bed, cuts out the strata and
sends veins into them (B). And the same structure is visible, on its
upper side, or what would be its top (C).
It is clear that these highly-inclined bands of breccia are not
contemporaneous with the deposition of the Carboniferous Slate, but
have been introduced into their position at some time subsequent
not only to the deposition, but to the disturbance and elevation of
the strata. The peninsula of White Bull Head is crossed by several
other similar bands. On Black Bull Head, also, together with abundant
felsitic and doleritic intrusions, a similar breccia or agglomerate
is to be seen. In some parts it is compact in texture with spheroidal
flinty lumps, and weathers somewhat like a nodular felsite. This
variety ends off rather abruptly to the north, but swells out
southward, and then runs out into a high, narrow headland, in which
it contains asbestos, as well as rounded crystals of hornblende. It
has here disrupted the shales and sandstones, and near the junction
is largely composed of fragments of them, the strata themselves being
jumbled, bent, and broken up.
The only semblance of a neck-like mass of this volcanic fragmental
material occurs on White Bull Head, where one of the bands expands
about the centre of the ridge, and is there full of large blocks of
grey sandstone. The breccia appears to have filled fissures which have
been opened between the bedding planes of the highly tilted strata,
giving rise to long narrow dyke-like intercalations. We have seen that
among the Carboniferous volcanic phenomena such dyke-like masses of
agglomerate occasionally present themselves in the vents both of the
plateaux and the puys.
In one or two places I noticed what may be traces of cleavage in
the breccia. The rock is not one that would yield easily to the
rearrangements required for the production of this structure, and the
doubtful cleavage may be deceptive. If we are justified in regarding
the introduction of this volcanic material as having necessarily taken
place after the tilting of the strata, we may not unreasonably infer
further that the eruptions could only have been effected at no great
distance from the surface. But the Carboniferous Slate in which these
agglomerates lie is the lowest member of the Carboniferous system. As
there is no known unconformability throughout this system in the south
of Ireland, the whole of the rest of the pile of Carboniferous strata,
amounting to a depth of several thousand feet, once probably extended
over this region. It must, therefore, have been not only after the
plication, but after extensive denudation of the formations that the
fissures were filled with agglomerate. These geological changes no
doubt occupied a vast period of time. While, therefore, no positive
evidence has yet been gathered to fix the age of these volcanic
eruptions of the south-west of Ireland, it is tolerably clear that they
cannot be assigned to the Carboniferous period, but must belong to some
later volcanic epoch. They may be of Permian age, perhaps even as late
as the Tertiary volcanic series.
BOOK VII
THE PERMIAN VOLCANOES
CHAPTER XXXI
THE PERMIAN VOLCANOES OF SCOTLAND
Geographical Changes at the Close of the Carboniferous
Period--Land- and Inland-Seas of Permian time--General
Characteristics and Nature of the Materials erupted--Structure of
the several Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale;
2. Basin of the Firth of Forth.
The close of the Carboniferous portion of the geological record in
Britain is marked by another of those great gaps which so seriously
affect the continuity of geological history. No transitional formation,
such as in other countries marks the gradation from the Carboniferous
into the succeeding period, has been definitely recognized in this
country. The highest Carboniferous strata are here separated from all
younger deposits by an unconformability, indicating the lapse of vast
periods of time whereof, within the British area, no chronicle has been
preserved.
When we pass from the Carboniferous system to that which comes next
to it in order of time, we soon become sensible that great changes in
geography, betokening an immense interval, took place between them. The
prolonged subsidence during which the Coal-measures were accumulated,
not only carried down below sea-level all the tracts over which the
Carboniferous system was deposited, but possibly submerged the last
of the islets, which, like those of Charnwood Forest, had survived so
many geological changes. Eventually, however, and after what may have
been a vast period of quiescence, underground movements began anew, and
the tracts of Coal-measures were unequally ridged up into land. The
topography thus produced appears to have resulted in the formation of
a series of inland seas somewhat like those of the Old Red Sandstone,
but probably less in area and in depth. In these basins the water seems
to have been on the whole unfavourable to life, for the red sand and
mud deposited in them are generally unfossiliferous, though, when
the conditions became more suitable, calcareous or dolomitic sediment
accumulated on the bottom, to form what is now known as the "Magnesian
Limestone," and muddy sediment was deposited which is now the "Marl
Slate." In these less ferruginous strata, betokening a less noxious
condition of water, various marine organisms are met with.[87]
[Footnote 87: In some recent borings around Hartlepool the Magnesian
Limestone has been found to be interstratified with thick bands of
gypsum and anhydrite, and to be overlain by more than 250 feet of the
latter substance. Nothing could show more forcibly the exceedingly
saline and insalubrious character of the Permian lakes or inland seas.]
The vegetation of the land surrounding these basins was still
essentially Palæozoic in character. It presented a general resemblance
to that of Carboniferous time, but with some notable differences. The
jungles of _Sigillaria_ seem to have disappeared, while on the other
hand, conifers increased in number and variety. The sediments of the
water-basins have handed down only a scanty remnant of the animal
life of the time. Along the sandy shores walked various amphibians
which have left their footprints on the sand. A few genera of ganoid
fishes have been found in some of the shales, and a comparatively
poor assemblage of crinoids and molluscs has been obtained from the
Magnesian Limestone. To the geological period distinguished by these
geographical and biological characters the name of Permian is assigned.
In his survey of the progress of volcanic history in the area of
Britain, the geologist finds that the long period of quiescence
indicated by the deposition of the Coal-measures, and probably also
by the unconformability between the Coal-measures and the Permian
formations, was at length terminated by a renewed volcanic outbreak,
but on a singularly diminished scale and for a comparatively brief
period of time. Whether, had the Permo-Carboniferous strata which
connect the Coal-measures with the Permian formations on the Continent
been found in this country, they would have filled up the gap in the
geological record, and would have supplied any trace of contemporaneous
volcanic action, cannot even be surmised. All that we know is that,
after a vast interval, and during the deposition of the breccias and
red sandstones which unconformably overlie the Coal-measures, a few
scattered groups of little volcanoes appeared in the area of the
British Isles.
It is unfortunate that in those districts where these volcanic
relics have been preserved, the stratigraphical record is singularly
imperfect, and that on the eastern side of England, where this record
is tolerably complete, there are no intercalated volcanic rocks. The
latter occur in tracts where the strata are almost wholly destitute of
fossils, and where therefore no palæontological evidence is available
definitely to fix the geological age of the eruptions. Nevertheless
there is usually ample proof that the strata in question are much
later than the Coal-measures, while their geological position and
lithological characters link them with the undoubted Permian series of
the north-east of England. They may, however, belong to a comparatively
late part of the Permian period, if indeed some of them may not be
referable to the succeeding or Triassic period.
The comparatively feeble and short-lived volcanoes now to be described
are found in two regions wide apart from each other. The more important
of these lies in the south-west and centre of Scotland. A second
group rose in Devonshire. It is possible that a third group appeared
between these two regions, somewhere in the midlands. The evidence for
the history of each area will be given in a separate section in the
following pages.
i. GENERAL CHARACTERISTICS--NATURE OF MATERIALS ERUPTED
The chief district for the display of volcanic eruptions that may be
assigned to the Permian period lies in the centre of Ayrshire and the
valleys of the Nith and Annan. But, for reasons stated below, I shall
include within the same volcanic province a large part of the eastern
half of the basin of the Firth of Forth (see Map V.).
Unfortunately the interesting volcanic rocks now to be considered
have suffered severely from the effects of denudation. They have been
entirely removed from wide tracts over which they almost certainly
once extended. But this enormous waste has not been wholly without
compensations. The lavas and tuffs ejected at the surface, and once
widely spread over it, during the deposition of the red sandstones,
have been reduced to merely a few detached fragments. But, on the other
hand, their removal as a superficial covering has revealed the vents of
discharge to an extent unequalled in any older geological system, even
among the puys of the Carboniferous period. The Permian rocks, escaping
the effects of those great earth-movements which dislocated, plicated
and buried the older Palæozoic systems of deposits, still remain for
the most part approximately horizontal or only gently inclined. They
have thus been more liable to complete removal from wide tracts of
country than older formations which have been protected by having large
portions of their mass carried down by extensive faults and synclinal
folds, and by being buried under later sedimentary accumulations. We
ought not, therefore, to judge of the extent of the volcanic discharges
during Permian time merely from the small patches of lava and tuff
which have survived in one or two districts, but rather from the
number, size and distribution of the vents which the work of denudation
has laid bare.
The evidence for the geological age of the volcanic series now to be
described is less direct and obvious than most of that with which I
have been hitherto dealing. It consists of two kinds. (_a_) In the
first of these comes the series of lavas and tuffs just referred to
as regularly interstratified with the red sandstones, which, on the
grounds given in the next paragraph, it is agreed to regard as Permian.
(_b_) Connected with these rocks are necks which obviously served as
vents for the discharge of the volcanic materials. They pierce not only
the Coal-measures, but even parts of the overlying bedded lavas. So
far there is not much room for difference of opinion; but as we recede
northward from Ayrshire and Nithsdale, where the intercalation of
the volcanic series in the red sandstones is well displayed, we enter
extensive tracts where these interstratified rocks have disappeared and
only the necks remain. All that can be positively asserted regarding
the age of these necks is that they must be later than the rocks
which they pierce. But we may inferentially connect them with the
interstratified lavas and tuffs by showing that they can be followed
continuously outward from the latter as one prolonged group, having the
same distribution, structure and composition, and that here and there
they rise through the very highest part of the Coal-measures. It is
by reasoning of this kind that I include, as not improbably relics of
Permian volcanoes, a large number of vents scattered over the centre of
Scotland, in the East of Fife.
The red sandstones among which the volcanic series is intercalated
cover several detached areas in Ayrshire and Dumfriesshire.
Lithologically they present a close resemblance to the Penrith
sandstone and breccias of Cumberland, the Permian age of which is
generally admitted. They lie unconformably sometimes on Lower and Upper
Silurian rocks, sometimes on the lower parts of the Carboniferous
system, and sometimes on the red sandstones which form the highest
subdivision of that system. They are thus not only younger than the
latest Carboniferous strata, but are separated from them by the
interval represented by the unconformability. On these grounds they are
naturally looked upon as not older than the Permian period. The only
palæontological evidence yet obtained from them in Scotland is that
furnished by the well-known footprints of Annandale, which indicate the
existence of early forms of amphibians or reptiles during the time of
the deposition of the red sand. The precise zoological grade of these
animals, however, has never yet been determined, so that they furnish
little help towards fixing the stratigraphical position of the red
rocks in which the footprints occur.
The stratigraphical relations of the red sandstones of Ayrshire
and Nithsdale were discussed by Murchison, Binney and Harkness.[88]
These observers noticed certain igneous rocks near the base of the
sandstones, to which, however, as being supposed intrusive masses, they
did not attach importance. They regarded the volcanic tuffs of the
same district as ordinary breccias, which they classed with those of
Dumfries and Cumberland, though Binney noticed the resemblance of their
cementing paste to that of volcanic tuff, and in the end was doubtful
whether to regard the igneous rocks as intrusive or interstratified.
[Footnote 88: See Murchison's _Siluria_, 4th edit. p. 331; _Quart.
Journ. Geol. Soc._ vol. vii. (1851), p. 163, note; vol. xii. (1856), p.
267; Binney, _ibid._ vol. xii. (1856), p. 138; vol. xviii. (1862), p.
437; Harkness, _ibid._ vol. xii. (1856), p. 262.]
In the year 1862, on visiting the sections in the River Ayr, I
recognized the breccia as a true volcanic tuff. During the following
years, while mapping the district for the Geological Survey, I
established the existence of a series of contemporaneous lavas and
tuffs at the base of the Permian basin of Ayrshire, and of numerous
necks marking the vents from which these materials had been erupted.
An account of these observations was published in the year 1866.[89]
Since that time the progress of the Survey has extended the detailed
mapping into Nithsdale and Annandale, but without adding any new facts
of importance to the evidence furnished by the Ayrshire tract.[90]
[Footnote 89: _Geol. Mag. for 1866_, p. 243; and Murchison's _Siluria_,
4th edit. (1867), p. 332.]
[Footnote 90: The rocks are shown in Sheets 9, 14 and 15 of the
Geological Survey of Scotland, to which, and their accompanying
Explanations, reference is made. The Ayrshire basin was mapped by
me, the necks in the Dalmellington ground by Mr. James Geikie, the
Nithsdale area by Mr. R. L. Jack, Mr. H. Skae and myself.]
The materials erupted by the Scottish Permian volcanoes display a very
limited petrographical range, contrasting strongly in this respect with
the ejections of all the previous geological periods. They consist
of lavas generally more or less basic, and often much decayed at the
surface; and of agglomerates and tuffs derived from the explosion of
the same lavas.
The lavas are dull reddish or purplish-grey to brown or almost black
rocks; sometimes compact and porphyritic, but more usually strongly
amygdaloidal, the vesicles have been filled up with calcite, zeolites
or other infiltration. The porphyritic minerals are in large measure
dull red earthy pseudomorphs of hæmatite, in many cases after
olivine. These rocks have not yet been fully studied in regard to
their composition and microscopic structure. A few slides, prepared
from specimens collected in Ayrshire and Nithsdale, examined by Dr.
Hatch, were found to present remarkably basic characters. One from
Mauchline Hill is a picrite, composed chiefly of olivine and augite,
with a little striped felspar. Others from the Thornhill basin in
Dumfriesshire show an absence of olivine, and sometimes even of
augite. The rock of Morton Castle consists of large crystals of augite
and numerous grains of magnetite in a felspathic groundmass full of
magnetite. Around Thornhill are magnetite-felspar rocks, composed
sometimes of granular magnetite with interstitial felspar. Throughout
all the rocks there has been a prevalent oxidation of the magnetite,
with a consequent reddening of the masses.
The pyroclastic materials consist of unstratified agglomerates and
tuffs, generally found in necks, and of stratified tuffs, which more or
less mingled with non-volcanic material, especially red sandstone, are
intercalated among the bedded lavas or overlie them, and pass upward
into the ordinary Permian red sandstones.
The agglomerates, though sometimes coarse, never contain such large
blocks as are to be seen among the older Palæozoic volcanic groups.
Their composition bears reference to that of the bedded lavas
associated with them, pieces of the various basalts, andesites,
etc., which constitute these lavas being recognizable, together with
others, especially a green, finely-vesicular, palagonitic substance,
which has not been detected among the sheets of lava. In general the
agglomerates contain more matrix than blocks, and pass readily into
gravelly tuffs. A series of specimens collected by me from necks which
pierce the Dalmellington coal-field has been sliced and examined under
the microscope by Mr. Watts, who finds it to consist of basic tuffs, in
which the lapilli include various types of olivine-basalt, sometimes
glassy, sometimes palagonitic, and occasionally holocrystalline, also
pieces of grit, shale and limestone. In one case a crinoid joint
detached from its matrix was noticed. A specimen from Patna Hill
consists of "a clear irregularly cracked aggregate of carbonates and
quartz with hornblende, and its structure reminds one of that of
olivine. The hornblende is in small irregular patches surrounded by the
clear mineral, and is probably a replacement of a pyroxene, perhaps
diallage." If this stone was once an olivine nodule, the agglomerate
might in this respect be compared with some of the tuffs of the Eifel
so well known for their lumps of olivine.
The stratified tuffs are generally more or less gravelly deposits,
composed of lapilli varying in size from mere grains up to pea-like
fragments, but with numerous larger stones and occasional blocks of
still greater dimensions. They often pass into a tough dull compact
mudstone. In colour they are greenish or reddish. They have been
largely derived from the explosion of lavas generally similar to those
of which fragments occur in the agglomerates. They often contain
non-volcanic detritus, derived from the blowing up of the rocks through
which the vents were opened. Occasionally they include also various
minerals such as pyrope, black mica, sanidine, augite, and others which
appear to have been ejected as loose and often broken crystals. This
character is more fully described in regard to its occurrence among the
necks of the east part of Fife.
The intrusive rocks, probably referable to the same volcanic period,
consist chiefly of dolerites and basalts which occur as dykes, sills
and bosses, and are more particularly developed in the south-west of
Ayrshire.
ii. GEOLOGICAL STRUCTURE OF THE VOLCANIC DISTRICTS
1. Ayrshire, Nithsdale and Annandale
(1) _Interstratified Lavas and Tuffs._--It will be convenient to
consider first the volcanic chronicle as it has been preserved in
the south-west and south of Scotland, where the existence of Permian
volcanoes in Britain was first recognized. The volcanic rocks in the
middle of the Ayrshire coal-field rise from under a central basin of
red sandstone, which they completely enclose. Their outcrop at the
surface varies up to about a mile or rather more in breadth, and forms
a pear-shaped ring, measuring about nine miles across at its greatest
width (Map V.).[91]
[Footnote 91: Mr. Gunn has recently detected among the newest red
sandstones of Arran a small patch of volcanic rocks which may be of
this age. Mr. A. Macconochie has also found what may be traces of a
similar volcanic band below the Permian sandstones of Loch Ryan, in
Wigtonshire.]
This volcanic ring runs as a tract of higher ground encircling the
hollow in which the Permian red sandstones lie, and forming a marked
chain of heights above the Carboniferous country around. It is built up
of a succession of sheets of different lavas, with occasional partings
of tuff or volcanic breccia, which present their escarpments towards
the coal-field outside, and dip gently into the basin under the inner
trough of brick-red sandstones. Good sections of the rocks are exposed
in the ravines of the River Ayr, particularly at Ballochmyle, in the
Dippol Burn near Auchinleck House, and in the railway cutting near
Mossgiel.
That these are true lava-flows, and not intrusive sills, is
sufficiently obvious from their general outward lithological aspect,
some of them being essentially sheets of slag and scoriæ. Their
upper surfaces may be found with a fine indurated red sand wrapping
round the scoriform lumps and protuberances, and filling in the
rents and interspaces, as in the case of the Old Red Sandstone lavas
already referred to. As an example of these characteristics, I may
cite the section represented in Fig. 200. At the bottom lies a red
highly ferruginous and coarsely amygdaloidal basalt (_a_). Over it
comes a volcanic conglomerate three feet thick, made up of balls of
vesicular lava like that below, wrapped in a brick-red sandy matrix
(_b_). Lenticular bands of sandstone without blocks occur in the
conglomerate, and others lie in hollows of its upper surface (_c_).
This intercalation of detrital material is followed by another basic
lava (_d_), about six feet thick, highly amygdaloidal in its lower and
upper parts, more compact in the centre. The amygdales and joints are
largely filled with calcite. The slaggy bottom has caught up and now
encloses some of the red sand of the deposit below. Another lava from
three to six feet thick next appears (_e_), which is remarkable for its
slaggy structure, and is so decomposed that it crumbles away. Like the
others it is dull-red and ferruginous and full of calcite. It must have
been at the time of its outflow a sheet of rough slag that cracked into
open fissures. That it was poured out under water is again shown in the
same interesting way just referred to, by the red sand which has been
washed into the interspaces between the clinkers and has filled up the
fissures, in which it is stratified horizontally between the walls.
Above this band, and perhaps passing into it as its slaggy base, lies
another more compact lava (_f_) like the lower sheets.
[Illustration: Fig. 199.--General section across the Permian basin of
Ayrshire.
1. Highest group of the Coal-measures; 2. Volcanic tuffs and ashy
brick-red sandstones; 3. Lavas with interstratified tuffs and brick-red
sandstones; 4. Brick-red Permian sandstones; 5, 5. Necks of volcanic
agglomerate; 6. Boss of dolerite.]
Throughout the series of lavas, as indicated in the foregoing section,
traces of the pauses that elapsed between the separate outflows may
be seen in the form either of layers of red sandstone or of tuff and
volcanic breccia. Here and there, under the platform of bedded lavas,
the brick-red sandstone is full of fragments of slag and fine volcanic
dust. But the most abundant accumulation of such detritus is to be
seen at the top of the volcanic series, where it contains the records
of the closing phases of eruption. Thick beds of tuff and volcanic
breccia occur there, interleaved with seams of red sandstone, like the
chief mass of that rock, into which they gradually pass upward. Yet,
even among the sandstones above the main body of tuff, occasional nests
of volcanic lapilli, and even large bomb-like lumps of slag, point to
intermittent explosions before the volcanoes became finally extinct and
were buried under the thick mass of red Permian sandstone.
[Illustration: Fig. 200.--Section of lavas east side of Mauchline Hill.]
[Illustration: Fig. 201.--Section of the top of the volcanic series
near Eastside Cottage, Carron Water, Nithsdale.]
There is good reason to believe that both the volcanic sheets and
the red sandstones overlying them, instead of being restricted to an
area of only about 30 square miles, once stretched over the lowlands
of Ayrshire; and not only so, but that they ran down Nithsdale, and
extended into several of its tributary valleys, if indeed, they
were not continuous across into the valley of the Annan.[92] Traces
of the lavas and tuffs are to be found at intervals over the area
here indicated. The most important display of them, next to their
development in Ayrshire, occurs in the vale of the Nith at Thornhill,
whence they extend continuously up the floor of the Carron Valley
for six miles. They form here, as in Ayrshire, a band at the base of
the brick-red sandstones, and consist mainly of bedded lavas with
the basic characters above referred to. These lavas, however, are
followed here by a much thicker development of fragmental volcanic
materials. Abundant volcanic detritus is diffused through the overlying
sandstones, sometimes as a gravelly intermixture, sometimes in large
slaggy blocks or bombs, and sometimes in intercalated layers of tuff,
while an occasional sheet of one of the dull red lavas may also be
detected. The final dying-out of the volcanic energy in a series of
intermittent explosions, while the ordinary red sandy sediment was
accumulating, is here also admirably chronicled. As an illustration of
these features the accompanying section is given (Fig. 201). The last
of the lavas (_a_) presents an uneven surface against which the various
kinds of detritus have been laid down. First comes a coarse volcanic
breccia (_b_) made up of angular and subangular blocks of different
lavas imbedded in a matrix of red ashy sand. This deposit is succeeded
by a band of dull red tufaceous sandstone, evidently formed of ordinary
red sandy sediment, into which a quantity of volcanic dust and lapilli
fell at the time of its accumulation. Some of the ejected blocks which
lie inclosed in the finer sediment are upwards of a foot in length. A
more vigorous discharge of fragmental material is shown by the next bed
(_d_), which consists of a coarse nodular tuff, mingled with a little
red sandstone and crowded with blocks of the usual lavas. Beyond the
locality of this section these tuffs are found to pass up insensibly
into the ordinary Permian sandstone.
[Footnote 92: See _Memoirs of Geol. Surv. Scotland_, Sheet 15 (1871), p.
35; Sheet 9 (1877), p. 31.]
[Illustration: Fig. 202.--Section of two outliers of the Permian
volcanic series at the foot of Windyhill Burn, Water of Ae,
Dumfriesshire.]
But we can detect the edges of yet more distant streams of lava
emerging from under the red sandstones and breccias to the east of
the Nith. On the farther side of the Silurian ridge that forms the
eastern boundary of the Nith valley, above which it rises some 700
or 800 feet, there is preserved at the bottom of the valley of the
Capel Water, which flows into Annandale, another small outlier of a
similar volcanic band. Three miles to the south-east of it two little
fragments of the volcanic group lie on the sides of a small tributary
of the Water of Ae. Since these may serve as a good illustration of the
extent to which denudation has reduced the area of the Permian volcanic
series, a section of the locality is here given (Fig. 202). The general
foundation rocks of the country are the Silurian greywackes and shales
in highly inclined and contorted positions (_a_). Each outlier has, as
its basement material, a volcanic breccia (_bb_) in which, together
with the usual lava-fragments, are mingled pieces of the surrounding
Silurian strata. In the smaller outlier lying to the north-east, this
detrital layer is only about one foot thick. It is overlain by a slaggy
amygdaloid of the usual character (_cc_), which in the lower outlier is
covered with boulder clay (_d_). There can be little doubt that these
detached fragments were once united in a continuous sheet of lava which
filled the valley of the Water of Ae and that of its tributary. That
the lava stretched down the Ae valley for some distance is proved by
the occurrence of another outlier of it two miles below.
But there is still additional evidence for the wide extension of these
volcanic sheets. It appears to be certain that they stretch far to
the eastward, under the Permian sandstones of the Lochmaben basin of
Annandale, for breccias largely made up of pieces of the bedded lavas
are found close to the northern edge of the basin on the west side of
the River Annan. To this remarkable adherence of the lavas and tuffs
to the bottom of the Permian valleys I shall afterwards more specially
refer.
The thickness of the whole volcanic group cannot be very accurately
determined. It reaches a maximum in the Ayrshire basin, where, at its
greatest, it probably does not exceed 500 feet, but is generally much
less; while in the Nithsdale and Annandale ground the detached and
much denuded areas show a still thinner development.
[Illustration: Fig. 203.--The Green Hill, Waterside, Dalmellington,
from the south; a tuff-neck of Permian age.]
(2) _Vents._--One of the most interesting features in this
south-western district of Scotland is the admirable way in which the
volcanic vents of Permian time have been preserved. Their connection
with the lavas and tuffs can there be so clearly traced that they
serve as a guide in the interpretation of other groups of vents in
districts where no such connection now remains. In Ayrshire, the
lower part of the Permian volcanic band is pierced by several small
necks of agglomerate. There cannot, I think, be any doubt that these
necks mark the positions of some of the vents from which the later
eruptions took place. Immediately beyond them necks of precisely
similar character rise through the upper division of the Coal-measures.
There can be as little hesitation in placing these also among the
Permian vents. And thus step by step we are led away from the central
lavas, through groups of necks preserving still the same features,
external and internal, and rising indifferently through rocks of any
geological age from the Coal-measures backward. Thus, although if we
began the investigation at the outer limits of the chain of necks,
we might well hesitate as to their age, yet, when we can fix their
geological position in one central area, we are, I think, justified
in classing, as parts of one geologically synchronous series, all the
connected groups that retain the same general characteristics. It is to
denudation that we owe their having been laid bare to view; but at the
same time, denudation has removed the sheet of ejected materials which
may have originally connected most of these vents together.
In this regard, it is most instructive to follow the vents
south-eastwards from the Ayrshire basin into Nithsdale for a distance
of some eighteen miles. If we traced them down that valley to Sanquhar,
without meeting with any vestige of superficial outflows to mark their
stratigraphical position, we might possibly hesitate whether the age
of those which are so far removed from the evidence that would fix it
should not be left in doubt. But if we continued our traverse only a
few hundred yards farther, we should find some fragmentary outliers
of the Permian lavas capping the Upper Coal-measures; and if we merely
crossed from the Nith into the tributary valley of the Carron Water,
we should see preserved in that deep hollow a great series of Permian
lavas, tuffs and agglomerates. It is only by a happy accident that here
and there these superficial volcanic accumulations have not been swept
away. There was probably never any great thickness of them, but they no
doubt covered most, if not all, of the district within which the vents
are found.
The Permian necks are, on the whole, smaller than those of the
Carboniferous period. The largest of them in the Ayrshire and Nithsdale
region do not exceed 4000 feet in longest diameter; the great majority
are much less in size, while the smallest measure 20 yards, or even
less. Those of Fife, to be afterwards described, exhibit a wider range
of dimensions, and have the special advantage of being exposed in plan
along the shore.
[Illustration: Fig. 204.--Patna Hill from the Doon Bridge, Ayrshire; a
tuff-neck of Permian age.]
These necks, from their number and shapes, form a marked feature in the
scenery. They generally rise as prominent, rounded, dome-shaped, or
conical hills, which, as the rock comes close to the surface, remain
permanently covered with grass (Figs. 203 and 204). Such smooth green
puys are conspicuous in the heart of Ayrshire, and likewise further
south in the Dalmellington coal-field, where some of them are locally
known as "Green Hill," from their verdant slopes in contrast to the
browner vegetation of the poorer soil around them (Fig. 203).
As in those of older geological periods, the necks of this series
are, for the most part, irregularly circular or oval in ground-plan,
but sometimes, like those of the Carboniferous system, they take
curious oblong shapes, and occasionally look as if two vents had
coalesced (Fig. 205). Here and there also the material of the vents
has consolidated between the walls of a fissure or the planes of the
strata, so as to appear rather as a dyke than as a neck. Descending,
as usual, vertically through the rocks which they pierce, the necks
have the form of vertical columns of volcanic material, ending at the
surface in grassy rounded hillocks or hills.
In almost all cases, the necks of the Ayrshire region consist of a
gravelly tuff or agglomerate, reddish or greenish in colour, made up of
blocks of such lavas as form the bedded sheets, together with fragments
of the stratified rocks through which the chimneys have been blown out.
Thus, in some of the necks, pieces of black shale are abundant, as at
Patna. In other cases, there are proofs of the derivation of the stones
from much greater depths, as in the Green Hill of Waterside, where
fragments of fine greywacke are not infrequent, probably derived from
the Silurian formations which lie deep beneath the Carboniferous and
Old Red Sandstone series.
The fragmentary material of the necks is generally unstratified, but a
rude stratification may sometimes be noticed, the dip being irregularly
inward at high angles towards the middle of the vent. This structure,
best seen in the vents of the Fife coast, as will be shown in the
sequel, may be detected in some of the necks of the Dalmellington
district.
[Illustration: Fig. 205.--Ground plans of Permian volcanic vents from
the Ayrshire Coal-field. On the scale of six inches to a mile.
1. Neck half a mile north-west from Dalmellington; 2. Neck at
Auchengee, four miles north-east from Patna; 3. Neck at head of
Drumbowie Burn, five and a half miles due north from Dalmellington;
4. Patna Hill, 853 feet above sea-level (for outline of this hill
see the preceding Fig.); 5. Neck on Kiers Hill (1005 feet above the
sea), two miles south from Patna, with lava adhering to part of the
wall.
]
Occasionally some form of molten rock has risen in the funnel, and has
partially or wholly removed or concealed the agglomerate. This feature
is especially noticeable among the necks that pierce the Dalmellington
coal-field. Portions of basic lavas traverse the agglomerate or
intervene between it and the surrounding strata. These have probably in
most cases been forced up the wall of the funnel, while here and there
sills run outward from the necks into the surrounding Coal-measures.
Sometimes a thin sheet of lava, adhering to the wall of a funnel, may
be the remnant of a mass of rock that once filled up the orifice. In
one of the necks of the Muirkirk Coal-field, which was pierced by
a mine driven through it from side to side, fingers and sheets of
"white trap," or highly altered basalt, were found to run out from the
neck into the surrounding strata.[93] Dark heavy basalt, or some still
more basic rock, has here and there filled up a vent. As so many of
the necks rise through the coal-fields, opportunities are afforded of
studying the effects of volcanic action upon the coal-seams, which for
some distance from them have been destroyed.
[Footnote 93: Explanation of Sheet 23, Geol. Surv. Scotland, p. 39.]
Another feature, which can be recognized from the information obtained
in mining operations, is that, in the great majority of instances,
no connection is traceable between the positions of the vents and
such lines of dislocation as can be detected at the surface or in the
underground workings. Some vents, indeed, have evidently had their
positions determined by lines of fault, as, for instance, that of the
Green Hill below Dalmellington. Yet in the same neighbourhood a number
of other examples may be found where the volcanic funnels seem to have
avoided faults, though these exist close to them.
In this south-western district of Scotland upwards of sixty distinct
vents have been mapped in the course of the Geological Survey. They run
from the north of Ayrshire to the foot of the Southern Uplands, and
descend for some distance the vale of the Nith. The area over which
they are distributed measures roughly about forty miles from north-west
to south-east, and at its greatest breadth twenty miles from south-west
to north-east. Within this tract the vents are scattered somewhat
sporadically in groups, sometimes numbering twenty necks in a space of
sixteen square miles, as in the remarkable district of Dalmellington.
In considering their distribution we cannot but be impressed by the
striking manner in which these necks keep to the valleys and low
grounds. I have already alluded to this characteristic, as shown by the
volcanoes of the Old Red Sandstone and Carboniferous periods. But it
is displayed by the Permian volcanoes in a still more astonishing way.
Beginning at the northern end of the long chain of necks in the West
of Scotland, we find a row of them on the plains fronting the volcanic
plateau of the Ardrossan, Dunlop and Stewarton Hills. Thence we may
follow them, as single individuals or in small groups, across the broad
lowland of Ayrshire, southward to the very base of the great chain
of the Southern Uplands. There, a cluster of some two dozen of them
may be seen rising out of the Carboniferous rocks on the low grounds,
but they abruptly cease close to the base of the hills; not one has
been detected on the adjacent Silurian heights. Moreover, if we turn
into the valleys that lead away from the great Ayrshire plain to the
interior, we find necks of the same character in these depressions.
They ascend the valley of Muirkirk, and may be met with even at its
very head, near the base of the Hagshaw Hills. Again, on the floor
of the remarkable transverse valley trenched by the Nith across the
Southern Uplands, Permian necks pierce the Coal-measures, while the
outlying fragments of bedded lava show that these vents flooded the
bottom of that valley with molten rock. Turning out of Nithsdale into
the long narrow glen of the Carron Water, we observe its floor and
sides to be covered with the sheets of lava and tuff already noticed.
And so travelling onward from the vale of the Nith into that of the
Capel Water, thence into the Water of Ae and across into the great
strath of Annandale, we may detect, if not actual vents, at least the
beds of lava and layers of volcanic detritus that were ejected from
them.
All along these valleys, which were already valleys in Carboniferous
time, traces of the volcanic activity of this epoch may be detected.
But, so far as I am aware, in not a single case has any vent been
observed to have been opened on the high surrounding ridges. There has
obviously been a determining cause why the volcanic orifices should
have kept to the plains and the main valleys with their tributaries,
and should have avoided the hills which rise now to heights of 500 to
1000 feet or more above the bottoms of the valleys that traverse them.
It might be said that the valleys follow lines of fracture, and that
the vents have been opened along these lines. But my colleagues in the
Geological Survey, as well as myself, have failed, in most cases, to
find any evidence of such dislocations among the rocks that form the
surface of the country, while it is sometimes possible to prove that
they really do not exist there.
Though only a few scattered patches of the Permian bedded lavas
and tuffs have been preserved, enough is left to indicate that the
vents were active only in the early part of the period represented
by the Scottish Permian red sandstones, for it is entirely in the
lower part of these strata that volcanic rocks occur. The eruptions
gradually ceased, and the sheets of ejected material, probably also
the volcanic cones, were buried under at least several hundred feet of
red sandstone. Whether or not any portion of the erupted material was
for a time built up above the level of the water, there seems to be no
question that the vents were, on the whole, subaqueous.
[Illustration: Fig. 206.--Section of sills traversing the Permian
volcanic series. River Ayr, Ballochmyle.
_a_, Coal-measures; _b_ _b_, Basic lavas; _c_ _c_, Brick-red sandstones
with tuff; _d_, Red tuff and volcanic breccia; _e_ _e_, Dolerite sills.]
3. _Sills._--The phenomena of sills and dykes are less clearly
developed among the Permian volcanic rocks of the Ayrshire basin than
among those of older formations. In the section exposed in the course
of the River Ayr at Howford Bridge, a coarsely crystalline dolerite
which extends for nearly 300 yards up the stream, cuts the Permian
lavas, of which it encloses patches as well as pieces of sandstone.
At the contact, the rock becomes fine-grained (Fig. 206). Through the
coarsely crystalline material run long parallel "segregation veins" of
a paler, more acid substance, as among the Carboniferous sills. Similar
rocks are well seen in the Dippol Burn near Auchinleck House.
Passing outward into the Coal-measures, we encounter a much larger
display of similar intrusive sheets. The best district for the study
of these sills lies around Dalmellington. The Coal-measures are there
traversed by many intrusions, which have produced great destruction
among the coal-seams. Some of the rocks are extremely basic, including
a beautiful picrite like that of Inchcolm (Letham Hill, near
Waterside). The age of these sills must be later than the Coal-measures
into which they have been injected. Some of them are obviously
connected with the agglomerate-necks, and the whole or the greater
number should thus probably be assigned to the Permian period.[94]
The phenomena of intrusion presented by these rocks reproduce the
appearances already described in connection with the basic intrusive
sheets of Carboniferous age.
[Footnote 94: Explanation of Sheet 14, Geol. Surv. Scotland, p. 22.]
2. Basin of the Firth of Forth
The other district of Southern Scotland, where traces of volcanic
action later in age than the Coal-measures may be observed, lies in the
basin of the Firth of Forth (Map V.). They include no bedded lavas,
and only at one locality do any relics of a covering of stratified
tuffs overspread the Carboniferous formations. The evidence for the old
volcanoes consists almost entirely of necks of tuff, which mark the
position of vents of eruption.
(1) _Vents._--On the south side of the estuary of the Forth there is
only one neck which may be plausibly placed in this series. It forms
the upper part of Arthur Seat, at Edinburgh. This hill has already
been cited as consisting of two distinct portions. The lower, built up
of bedded tuffs, basalts and andesites, forms part of the Midlothian
volcanic plateau of Carboniferous time. The vent from which these
materials were ejected must lie at some little distance, and its site
has not been certainly ascertained. The upper part of the hill is
formed of a distinct group of rocks which has now to be described.
The geological structure of Arthur Seat has long been well known.
It served as a theme for discussion in the Neptunist and Plutonist
controversy, and was often referred to in the various mineralogical or
geognostical writings of the time. The first thorough examination of
it as a relic of ancient volcanic action was that of Charles Maclaren,
published in 1839.[95] This author clearly recognized the later age and
unconformable position of the coarse mass of agglomerate pierced by
the basalt of the apex, and pointed out the evidence of the upheaval
and denudation of the older volcanic series during a long interval of
repose before the latest eruptions took place. Subsequently Edward
Forbes suggested that the upper part of the hill might be of Tertiary
age.[96] Thereafter I mapped the ground in detail for the Geological
Survey, entirely confirming the observations of Maclaren.[97] In the end
it seemed to me that the interval between the two epochs of volcanic
activity might not be so great as Forbes had supposed; and after
tracing the Permian vents of Ayrshire, I came to the conclusion that
the younger unconformable agglomerate of Arthur Seat was not improbably
Permian.
[Footnote 95: _Geology of Fife and the Lothians_, p. 34. In a reprint of
this work, published in 1866, the venerable author briefly remarked in
a footnote that he no longer believed in the second period of volcanic
activity. This view was adopted in 1875 by Professor Judd, _Quart.
Journ. Geol. Soc._ xxxi. p. 131. For the reasons stated in the text I
believe Maclaren's original explanation of the structure of the hill to
be correct.]
[Footnote 96: Forbes never published his views regarding Arthur Seat,
but expounded them to his class, and explained them in diagrams, some
of which are preserved in the Edinburgh Museum of Science and Art, in
association with the specimens which he collected from the hill.]
[Footnote 97: Sheet 32, Geol. Survey of Scotland and descriptive Memoir.
See also _Rep. Brit. Assoc._ 1867, address Geol. Sect., and Murchison's
_Siluria_, 4th edit. p. 331.]
The older volcanic series of this hill has been broken through by the
agglomerate which occupies a true neck, and is abruptly marked off
from all the rocks older than itself. There is no trace of any of the
older lavas or tuffs thickening towards this vent. On the contrary
they are completely truncated by it, and their outcrops on the north
side reappear from under the agglomerate on the south side. Their
escarpments are wrapped round by the agglomerate which likewise fills
the head of the hollow that had been previously worn by denudation out
of the stratified deposits between the oldest lavas. There is thus a
violent unconformability between the later and the older volcanic rocks
of Arthur Seat.
The length of time indicated by this stratigraphical break must be
great. There is no known discordance in the Carboniferous system of
the Lothians, yet the Coal-measures, Millstone Grit, Carboniferous
Limestone series and much of the Calciferous Sandstones were stripped
from this hill before the eruption of the agglomerate. It will be shown
in the sequel that a nearly similar amount of denudation preceded some
of the probably Permian eruptions of Fife.
The agglomerate contains abundant fragments of the older volcanic
series. Its matrix is a dull red gravelly detritus, crowded with blocks
of all sizes up to a yard or more in diameter. It is pierced by a
column or plug of basalt, which sends veins into it, and rises to the
apex of the hill. A beautiful olivine-basalt forms the lateral mass of
the Lion's Haunch, which rests on the agglomerate.
[Illustration: Fig. 207.--Section showing the relations of the later
rocks of Arthur Seat.
1. Grey and reddish sandstones and shales (Calciferous Sandstones);
2. The lava of the Long Row: the oldest of the Carboniferous
volcanic series; 3. Tuffs of the Dry Dam; 4. Columnar basalts
overlying the tuffs; 5. Andesite lavas of the eastern half of
Arthur Seat; 6. Sill of Heriot Mount; 7. Sill of Salisbury Crags;
8. Sill of the Dasses. These complete the Lower Carboniferous
volcanic series (compare Fig. 112). 9. White sandstones and black
shales, upper division of the Calciferous Sandstones; 10. Younger
volcanic agglomerate resting on the denuded ends of the older
volcanic series; 11. Basalt of the summit sending veins into the
agglomerate; 12. Basalt of the Lion's Haunch.
]
In general characters the agglomerate of Arthur Seat resembles that of
some of the younger vents of Fife which pierce the Coal-measures and
are connected with tuffs that lie unconformably on the Carboniferous
Limestone. On these various grounds I think that it may be reasonably
assigned to the same geological period.
That a new vent should be opened, after the lapse of one or more
geological periods, on or near the site of more ancient volcanic
orifices is an incident of which, as we have seen, the geological
history of the British Isles furnishes a number of examples. It will
be remembered that little more than a mile to the south of Arthur Seat
lies the great vent of the Braid Hills, which in the time of the Lower
Old Red Sandstone gave forth such a huge pile of lavas and tuffs.
Volcanic energy thereafter entirely died away, and in this district
was succeeded by a prolonged period of quiescence, during which the
Lower Old Red Sandstone was upraised and extensively denuded, while
the Upper Old Red Sandstone was deposited. At length, in the immediate
neighbourhood, from one or more vents, the exact site of which is not
certainly known, the older lavas and tuffs of Arthur Seat, Calton
Hill and Craiglockhart Hill were erupted. Again, after another vast
interval, a new volcano appeared, and the agglomerate and younger
basalts of Arthur Seat were ejected from it. This is one of the most
striking examples in this country of the remarkable persistence of
volcanic energy in the same locality.
There is no evidence at Arthur Seat itself to fix the geological date
of the last volcanic activity of the hill. If the group of younger
rocks stood alone, with no other trace of post-Carboniferous eruptions
in the surrounding district, a plausible conjecture as to its age would
not be easily offered. But in reality it is not a solitary example
of such rocks; for within sight, on the opposite side of the Firth
of Forth, its counterparts may be seen. To the description of these
numerous and clearer illustrations I now proceed.
The East of Fife is remarkable for a large assemblage of volcanic
vents, which, unlike those in Ayrshire and Nithsdale, stand alone,
their superficial ejections having been removed by denudation, and no
connection being traceable between them and any Permian sandstones.
The vents filled up with agglomerate and pierced with plugs and veins
of basalt, rise through the Carboniferous rocks, but have left no
record for precisely defining their geological age. On the one hand,
it is quite certain that in this district volcanic eruptions took
place during the earlier half of the Carboniferous period. To the
north of Largo, and still more distinctly to the north-east of Leven,
sections occur to show the contemporaneous outpouring of volcanic rocks
during the time of the Carboniferous Limestone. The Leven section,
seen in a ravine a little to the north-east of the town, is specially
important. It presents a succession of red and green fine sandy tuffs,
interstratified with fire-clays and sandstones, and containing a zone
of basalt in the centre. These rocks lie not far from the top of the
Carboniferous Limestone series.
On the other hand, there is equally clear proof of far later eruptions.
From St. Andrews to Elie a chain of necks may be traced, having the
same general characters, and piercing alike the Calciferous Sandstones,
and the older part of the Carboniferous Limestone series. That these
vents must in many cases be long posterior to the rocks among which
they rise, is indicated by some curious and interesting kinds of
evidence. They are often replete with angular fragments of shale,
sandstone and limestone, of precisely the same mineral characters as
the surrounding strata, and containing the same organic remains in an
identical state of fossilization. It is clear that these strata must
have had very much their present lithological aspect before the vents
were opened through them. Again, the necks may often be observed to
rise among much contorted strata, as, for example, along the crest of a
sharp anticlinal arch, or across a synclinal basin. The Carboniferous
rocks must thus have been considerably plicated before the time of
the volcanic eruptions. In the next place, the vents often occur on
lines of dislocation without being affected thereby. They must be
posterior, however, not only to these dislocations, but also to much
subsequent denudation, inasmuch as their materials overspread the rocks
on each side of a fault without displacement. Hence we conclude with
confidence, that a great deal of volcanic activity in the East of Fife
must have been posterior to most, if not all, of the Carboniferous
period.
[Illustration: Fig. 208.--Section in brooks between Bonnytown and
Baldastard, Largo.
_a_, Sandstone shales and coals of Carboniferous Limestone series; _b_,
unconformable tuff.]
In the neighbourhood of Largo, further important evidence is presented,
confirming and extending this conclusion. The highest member of the
Upper Coal-measures, consisting of various red sandstones, with red
and purple clays, shales, thin coals and ironstones, is prolonged
from the Fife coal-field in a tongue which extends eastward beyond
the village of Lower Largo. It is well displayed on the shore, where
every bed may be followed in succession along the beach for a space
of nearly two miles. Two volcanic necks, presenting the same features
as those which pierce the older portions of the Carboniferous system
to the east, rise through these red rocks. We are thus carried not
only beyond the time of the Carboniferous Limestone, but beyond the
close of the very latest stage of the Carboniferous period in Central
Scotland. Connected with these and other vents farther north, there
is a large area of tuff which has been thrown out upon the faulted
and greatly denuded Carboniferous rocks. It may be traced passing
from the red Upper Coal-measures across the large fault which here
separates that formation from the Carboniferous Limestone, and
extending inland athwart different horizons of the latter series.
Outlying fragmentary cakes of it may be seen resting on the upturned
edges of the sandstones, shales and coal-seams, even at a distance
of some miles towards the north-west, proving that the fragmentary
materials discharged from the vents spread over a considerable area.
The accompanying section (Fig. 208) may serve as an illustration of the
relation between this sheet of bedded tuff and the underlying rocks.
Though interstratified volcanic rocks occur in the Carboniferous
system of the East of Fife, no connection has been traced between them
and any of the vents now referred to. While none of these vents can
be proved to be of Carboniferous age, it is of course possible that
such may be the true date of some of them. Others, nevertheless, and
probably much the largest number, judged from the data just given, may
be regarded as probably post-Carboniferous. Those which happen to rise
through the uppermost Coal-measures do not appear to be distinguishable
by any essential characters from those which pierce indifferently
the Carboniferous Limestone series and Calciferous Sandstones of the
East of Fife. They seem to be all one connected aggregate, resembling
each other alike in their external characters, internal structure and
component materials, and the limit of their age must be determined
by the geological horizon of the youngest formation which they
traverse. By this process of reasoning I reach the conclusion that
this remarkable series of old volcanoes in the East of Scotland not
improbably dates from the same time as that of Ayrshire and Nithsdale,
already described.
[Illustration: Fig. 209.--View of Largo Law from the east (the crag on
the left, at the base of the cone, is a portion of a basalt-stream. See
Fig. 226).]
Some idea of the importance and interest of the volcanic area of
Eastern Fife may be gathered from the fact that in a space of about
70 square miles no fewer than 60 necks may be counted, and others are
probably concealed below the drift-deposits which cover so much of the
interior of the country. The area of this remarkable display extends
from St. Andrews Bay and the Vale of the Eden southwards to the coast
of the Firth of Forth between Lundin Links and St. Monans. All over the
inland tract the necks form more or less marked eminences, of which the
largest are conspicuous landmarks from the southern side of the Firth.
But the distinguishing characteristic of the area is the display of the
necks along the coast, where, in a series of natural dissections, their
form, composition, internal structure and relations to the surrounding
rocks have been laid open in such clearness and variety as have been
met with in the volcanic records of no other geological period within
the compass of these islands. As this district thus possesses a
singular interest and value for the study of volcanic vents, I shall
enter in some detail into the description of the sections so admirably
laid bare.
[Illustration: Fig. 210.--View of small neck in Calciferous Sandstones,
on the shore, three miles east from St. Andrews.
(This illustration, likewise Figs. 212, 216, 219, 221, 222, 225 and 227
are from photographs taken for the Geological Survey by Mr. R. Lunn.)]
As in Ayrshire, the necks in the East of Fife generally rise as
isolated conical or dome-shaped hills, with smooth grassy slopes, but
where a dyke or boss of basalt occurs in them, it usually stands out
as a crag or knoll. Largo Law (Fig. 209) may be taken as a singularly
perfect example of the cone-shaped neck. This hill, however, comprises
more than one vent. The mass of tuff of which it consists probably
includes at least three distinct funnels of discharge, and surrounding
it there still remains a good deal of the fragmental material that
gathered around these vents and is now seen to lie unconformably upon
the Carboniferous formations (Fig. 208). There must be a total area
of not much less than four square miles over which tuff occupies the
surface of the ground.
While the Fife necks possess the great advantage of having been laid
bare by the sea, their frequent small size on the coast allows their
whole area to be examined. As illustrations of these little vents, two
plates are here given from the coast-line to the east of St. Andrews,
where a number of small necks of agglomerate have been planted among
the plicated Calciferous Sandstones. In Fig. 210 the abrupt truncation
of the sandstones by the volcanic rock is well shown. The strata on the
right have been broken through, and the sea has indented a small gully
along the wall of the old volcanic funnel. The sandstones in front,
however, still adhere firmly to the agglomerate, which rises above them
as a rugged mass of rock.
In Fig. 212 the edge of the vent can be traced partly in section and
partly in plan for about half of its circumference. On the right
hand, the actual wall of the funnel is visible where the false-bedded
sandstones are sharply cut off by the agglomerate. In front the strata
appear in plan on the beach, and their ledges can be seen to the left
striking at the margin of the neck.
[Illustration: Fig. 211.--Ground-plan of Permian volcanic vents.]
[Illustration: Fig. 212.--Small neck in Calciferous Sandstones a little
east from the "Rock and Spindle," two and a half miles east from St.
Andrews.]
The shape of the Fife vents is, as usual, generally circular or
oval; but is subject to considerable irregularity. The coast-section
between Largo and St. Monans exposes many ground-plans of them, and
permits their irregularities to be closely examined. The accompanying
figure (Fig. 211) exhibits some characteristic forms. Eccentricities
of outline no doubt arose from the irregular way in which the rocks
yielded to the forces of explosion during the piercing of a volcanic
orifice. This is often well shown by the veins and nests of tuff or
agglomerate which have been forced into the rents or sinuosities of
the orifices. In other cases, however, it is probable that, as among
the Ayrshire necks, and those of Carboniferous age already cited, what
appears now as one volcanic neck was the result of a shifting of the
actual funnel of discharge, so that the neck really represents several
closely adjacent vents. The case of Largo Law has been already noticed.
The necks at Kellie Law (Fig. 213) show clearly the same structure, the
Law itself (1) probably consisting of two contiguous vents, while a
third (2) forms a smaller cone immediately to the east. Such a slight
lateral displacement of the vent has been noticed at many Tertiary and
recent volcanic orifices. In the island or peninsula of Volcanello,
for example, three craters indicate successive shiftings of the vent,
the most perfect of them marking the latest and diminishing phase of
volcanic activity (Fig. 214, compare Fig. 29, vol. i., p. 70).
[Illustration: Fig. 213.--Plan of volcanic necks at Kellie Law, east of
Fife, on the scale of three inches to one mile.
1, Kellie Law (tuff); 2, Carnbee Law (tuff); 3, 4, 5, small tuff necks;
B B, basalt dykes and bosses; _c_ _c_, coal-seams; _l_, limestone; _f_,
fault. The arrows mark the dip of the strata through which the necks
have been drilled.]
[Illustration: Fig. 214.--Plan of the craters in Volcanello, Lipari
Islands.]
The Fife necks vary from only a few yards up to perhaps 4000 feet in
diameter. One of the smallest and most completely exposed occurs on the
shore at Newark Castle, near St. Monans. It measures only 60 yards in
length by about 37 yards in breadth. A ground-plan of it is given in
Fig 224. Still smaller is the neck at Buddo Ness, on the coast east of
St. Andrews, which measures only 20 yards across.
From the way in which the vents have been dissected by the sea along
the Fife coast, the geologist is enabled to study in minute detail
the effects of the volcanic operations upon the strata through which
the funnels have been drilled. Considerable variation may be observed
in the nature and amount of change. Sometimes the orifice has been
made without any noticeable alteration of the sandstones, shales and
limestones, which retain their dip and strike up to the very wall of
the chimney. Usually there is more or less jumbling and crushing of
the stratification, and often a considerable amount of induration. As
a typical example of these effects I give a section from the margin of
the neck of tuff on the east side of Elie Harbour (Fig. 215). Here the
sandstones and shales (_a_) have been doubled over and dragged down
against the tuff (_b_). They have likewise been hardened into a kind of
quartzite, and this alteration extends for about 20 to 30 feet from the
edge of the neck.
[Illustration: Fig. 215.--Section of the strata at the edge of the
volcanic vent on the east side of Elie Harbour.]
The material which has filled up the vents is almost entirely
fragmental, varying from a coarse agglomerate to a fine volcanic tuff.
Some minor necks have been completely or in great part filled with
angular debris of the ordinary rocks of the neighbourhood. In the
western neck on the Largo shore, for example, which rises through the
red rocks of the Upper Coal-measures, the material consists largely
of fragments of red sandstone, clay and shale. Between Elie and St.
Monans, some of the necks are filled almost wholly with debris of black
shale and encrinal limestone.
There does not appear to be any relation between the diameter of a
funnel and the size of the blocks that now fill it. Some of the larger
necks, for example, consist of comparatively fine tuff. The Buddo Ness,
on the other hand, though so small a vent, is packed with blocks of
shale six feet long, while the sandstone through which the orifice has
been drilled passes, as usual, into quartzite for several yards away
from the edge. As an example of the general aspect presented by one
of the coarse agglomerates in the necks of the Fife coast, a view is
given in Fig. 216 of a portion of the neck at Ardross, about two miles
east from Elie. This thoroughly volcanic accumulation is here shown to
consist of blocks of all sizes heaped together without any definite
arrangement.
[Illustration: Fig. 216.--Agglomerate of neck on shore at Ardross, two
miles east from Elie.]
Since the first stage in the history of the vents has been the
perforation of the solid crust by explosion, and the consequent
production of debris from the disrupted rocks, we may hope to detect
underneath the pile of thoroughly volcanic ejections, traces of the
first explosions. I have been much struck with the fact that in the
East of Fife such traces may frequently be found here and there within
the outer border of the vents. At Largo, and again between Elie and
St. Monans, it may be noticed that the mass of material adhering to
the wall of a neck, exposed in ground-plan upon the beach, often
consists largely, or even wholly, of debris of sandstone, shale and
limestone, while the central and chief mass is made up of green tuff or
agglomerate, with occasional pieces of the surrounding stratified rocks
scattered through it. It seems probable, therefore, that the sections
of these Fife necks, laid bare on the present shore, do not lie far
below the original crater-bottoms.
Some light might be expected to be thrown upon the phenomena in
an active volcanic chimney by the condition of the fragments of
recognizable sedimentary rocks imbedded in the ejected debris which has
filled up the orifice. But the assistance from this source is neither
so full nor so reliable as could be wished. In some of the Fife vents,
indeed, the fragments of shale, sandstone and other sedimentary strata
are so unchanged that they cannot on a fresh fracture be distinguished
from the adjacent parent strata. The _Spirifers_, _Lingulæ_, crinoids,
cyprid-cases, ganoid scales and other fossils are often as fresh and
perfect in the fragments of rock imbedded in tuff as they are in the
rock _in situ_. In some cases, however, distinct, and occasionally even
extreme, metamorphism may be detected, varying in intensity from mere
induration to the production of a crystalline texture. The amount of
alteration has depended not merely upon the heat of the volcanic vent,
but also in great measure upon the susceptibility of the fragments to
undergo change and the duration of their exposure to it.
Dr. Heddle has computed the temperature to which fragments of shale,
etc., in tuff-necks of the Fife coast have been subjected. He found
that the bituminous shales have lost all their illuminants, and of
organic matter have retained only some black carbonaceous particles;
that the encrinal limestones have become granular and crystalline;
that the sandstones present themselves as quartzite, and that black
carbonaceous clays show every stage of a passage into Lydian-stone. He
inferred from the slight depth to which the alteration has penetrated
the larger calcareous fragments, that the heat to which they were
exposed must have been but of short continuance. As the result of his
experiments, he concluded that the temperature at which the fragments
were finally ejected from the volcanic vents probably lay between 660°
and 900° Fahr.[98]
[Footnote 98: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 487.]
It may be perhaps legitimate to infer that, while the fragments that
fell back into the volcanic funnel, or which were detached from
the sides of the vent, after having been exposed for some time to
intense heat under considerable pressure, would suffer more or less
metamorphism, those, on the other hand, which were discharged by the
æriform explosions from the cool upper crust, on the first outburst of
a vent, would not exhibit any trace of such a change. Where, therefore,
we meet with a neck full of fragments of unaltered stratified rocks, we
may suppose it to have been that of a short-lived volcano; where, on
the other hand, the fragments are few and much altered, they may mark
the site of a vent which continued longer active. The metamorphism
of the fragmentary contents of a volcanic funnel by the action of
ascending vapours has already been described in the case of one of the
vents of the Carboniferous plateaux (vol. i. p. 404).
One of the most curious and puzzling features in the contents of the
tuff necks of the Fife coast is the occurrence there of crystals and
fragments of minerals, often of considerable size, which do not bear
evidence of having-been formed _in situ_, but have undoubtedly been
ejected with the other detritus. Dr. Heddle has noticed the fact,
and has described some of the minerals which occur in this way. The
following list comprises the species which he and I have found:--
Hornblende, in rounded fragments of a glassy black cleavable variety.
Augite, sometimes in small crystals, elsewhere in rounded fragments of
an augitic glass.
Orthoclase (Sanidine), abundant in worn twin crystals in the tuffs of
the east of Fife.
Plagioclase.
Biotite.
Pyrope, in the tuffs (and more rarely in the basalts) of Elie.
Nigrine, common in some of the dykes, more rarely in the tuffs of Elie.
Saponite, Delessite and other decomposition products.
Semi-opal, one specimen found in the later (Permian?) agglomerate of
Arthur's Seat.
Asphalt, abundant at Kincraig, near Elie.
Fragments of wood, with structure well preserved, may be included here.
Dr. Heddle has described from the neck of tuff at Kinkell, near
St. Andrews, large twin crystals of a glassy orthoclase, which are
invariably much worn, and preserve only rudely the form of crystals.
He justly remarks that they have no connection with drusy cavity,
exfiltration vein, or with any other mineral, and look as if a portion
of their substance has been dissolved away. Internally, however,
they are quite fresh and brilliant in lustre, though sometimes much
fissured.[99]
[Footnote 99: _Trans. Roy. Soc. Edin._ vol. xxviii. p. 223.]
The tuffs at Elie are full of similar crystals. I obtained from one
of the necks east of that village a specimen which measures 4 inches
in length, 3-1/2 in breadth, and 2-1/4 in thickness, and weighs about
2 lbs. It is, however, a well-striated felspar. From the same tuff
I procured an orthoclase twin in the Carlsbad form. All the felspar
pieces, though fresh and brilliant internally, have the same rounded
and abraded external appearance.
The fragments of hornblende form a characteristic feature in several of
the Elie dykes (to be afterwards described), and in the neighbourhood
of these intrusive rocks occur more sparingly in the tuff. It is a
glossy-black cleavable mineral, in rounded pieces of all sizes, up to
that of a small egg. Dr. Heddle obtained a cleavage angle of 124° 19',
and found on analysis that the mineral was hornblende.[100]
[Footnote 100: _Op. cit._ xxviii. p. 522.]
Augite occurs sparingly in two forms among the rocks. I have obtained
small crystals from the red agglomerate on the south side of Arthur
Seat, recalling in their general appearance those of Somma. Lumps of
an augitic glass have been found by Dr. Heddle, sometimes as large as
a pigeon's egg, in two of the dykes at Elie, and in the tuff at the
Kinkell neck, near St. Andrews. He observed the same substance at
the Giant's Causeway, both in the basalt and scattered through one of
the interstratified beds of red bole. Much larger rounded masses of
a similar augitic glass, but with a distinct trace of cleavage, have
already been referred to as occurring in a volcanic vent of Upper Old
Red Sandstone age at John o' Groat's House.[101]
[Footnote 101: _Op. cit._ xxviii. pp. 481 _et seq._, and _ante_, vol. i.
p. 352.]
Biotite is not a rare mineral in some tuffs. It may be obtained in
Lower Carboniferous tuffs of Dunbar, in plates nearly an inch broad;
but the largest specimen I have obtained is one from the same Elie
vent which yielded the large felspar fragment. It measures 2-1/2 × 2
× 1/2 inches. These mica tables, like the other minerals, are abraded
specimens.
That these various minerals were ejected as fragments, and have not
been formed _in situ_, is the conclusion forced upon the observer
who examines carefully their mode of occurrence. Some of them were
carried up to the surface by liquid volcanic mud, and appear in dykes
of that material like plums in a cake. But even there they present the
same evidence of attrition. They assuredly have not been formed in
the dykes any more than in the surrounding tuff. In both cases they
are extraneous objects which have been accidentally involved in the
volcanic rocks. Dr. Heddle remarks that the occurrence of the worn
pieces of orthoclase in the tuff is an enigma to him. I have been as
unable to frame any satisfactory explanation of it.[102]
[Footnote 102: Occasionally the crystals can be matched in some lava-form
rock of the same volcanic area; but many of them have no such
counterparts. See vol. i. p. 62 and _note_.]
[Illustration:
Fig. 217.--Ground-plan of volcanic neck, Elie Harbour, showing
circular deposition of the stratification.
T, Tuff of the neck, the arrows showing its inward dip; B B, Dykes;
S, Sandstones and shales, through which the neck has been opened.
]
It might have been thought that within the throat of a volcano, if in
any circumstances, loose materials should have taken an indefinite
amorphous aggregation. And, as has been shown in the foregoing
chapters, this is usually the case where the materials are coarse and
the vent small. Oblong blocks are found stuck on end, while small and
large are all mixed confusedly together. But in numerous cases where
the tuff is more gravelly in texture, and sometimes even where it is
coarse, traces of stratification may be observed. Layers of coarse
and fine material succeed each other, as they are seen to do among
the ordinary interstratified tuffs. The stratification is usually
at high angles of inclination, often vertical. So distinctly do the
lines of deposit appear amid the confused and jumbled masses, that
an observer may be tempted to explain the problem by supposing the
tuff to belong not to a neck, but to an interbedded deposit which
has somehow been broken up by dislocations. That the stratification,
however, belongs to the original volcanic vents themselves is made
exceedingly clear by some of the coast-sections in the East of Fife.
On both sides of Elie, examples occur in which a distinct circular
disposition of the bedding can be traced corresponding to the general
form of the neck. The accompanying ground-plan (Fig. 217) represents
this structure as seen in the neck which forms the headland at Elie
harbour. Alternations of coarse and fine tuff with bands of coarse
agglomerate, dipping at angles of 60° and upwards, may be traced round
about half of the circle. The incomplete part may have been destroyed
by the formation of another contiguous neck immediately to the east. To
the west of Earlsferry another large, but also imperfect, circle may
be traced in one of the shore necks. A quarter of a mile farther west
rises the great cliff-line of Kincraig, where a large neck has been
cut open into a range of precipices 200 feet high, as well as into a
tide-washed platform more than half a mile long. The inward dip and
high angles of the tuff are admirably laid bare along that portion of
the coast-line. The section in which almost every bed can be seen, and
where, therefore, there is no need for hypothetical restoration, is as
shown in Fig. 218.
I have already referred to the frequently abundant pieces of stratified
tuff, found as ejected blocks in vents filled with tuff, and to the
derivation of these blocks from tuff originally deposited within the
crater. There can, I think, be little hesitation in regarding the
stratification of these Fife vents as a record of successive deposits
of volcanic detritus inside the vents. The general dip inwards from the
outer rim of the vent strikingly recalls that of some modern volcanoes.
By way of illustration, I give here a section of part of the outer rim
of the crater of the Island of Volcano, sketched by myself in the year
1870 while ascending the mountain from the north side (Fig. 220). The
crater wall at this point consists of two distinct parts--an older tuff
(_a_), which may have been in great measure cleared out of the crater
before the ejection of the newer tuff (_b_). The latter lies on the
outer slope of the cone at the usual angle of 30°. It folds over the
crest of the rim, and dips down to the flat tuff-covered crater bottom,
at an angle of 37°. These are its natural angles of repose.
[Illustration: Fig. 218.--Section across the great vent of Kincraig,
Elie, on a true scale, vertical and horizontal, of six inches to a mile.
1, Sandstones, shale, etc., of Lower Carboniferous age, plunging down
toward the neck T; B, columnar basalt, shown also in Figs. 223 and
225.]
[Illustration: Fig. 219.--Dyke in volcanic neck, on the beach, St.
Monans.]
[Illustration: Fig. 220.--Section of part of crater rim, Island of
Volcano.]
Applying modern analogies of this kind, I have been led to conclude
that the stratification so conspicuous in the tuff of the vents in the
east of Fife and in the Carboniferous series of the Lothians belongs
to the interior of the crater and the upper part of the volcanic
funnel.[103] These stratified tuffs, on this view of their origin, must
be regarded as remains of the beds of dust and stones which gathered
within the crater and volcanic orifice, and which, on the cessation of
volcanic action, sometimes remained in their original position, or were
dislocated and slipped down into the cavity beneath. That the tuffs
consolidated on slopes, perhaps quite as steep as those of Volcano,
is now and then indicated by an interesting structure. The larger
stones imbedded in the layers of tuff may be observed to have on their
fronts in one direction a small heap of coarse gravelly debris, while
fine tuff is heaped up against their opposite side. This arrangement
doubtless points to deposit on a slope of loose debris, from which the
larger blocks protruded so as to arrest the smaller stones, and allow
the fine dust to gather behind.
[Footnote 103: Further illustrations of this characteristic structure of
some vents will be found in the account of the Tertiary vents of the
Faroe Isles in Chapter xli. See also the remarks in the introductory
chapters, vol. i. p. 63.]
If the inference be correct, that the stratification here described
belongs to the old craters or the upper parts of the funnels, it
furnishes additional evidence of the wide interval of time that elapsed
between the deposition of the Carboniferous strata and the outbreak
of these vents. During that interval prolonged denudation reduced
the upturned Carboniferous Limestone series to nearly its present
form of surface, and any materials discharged from the vents over the
surrounding ground would obviously lie with a violent unconformability
on the rocks below.
The frequent great disturbance in the bedding of the tuff within the
vents may be connected with some kind of collapse, subsidence or
shrinkage of the materials in the funnel below. That a movement of this
nature did take place is shown by the remarkable bending down of the
strata round the margins of the vents, which has been already described.
The minor vents for the most part contain only fragmentary materials;
but those of larger size usually present masses of lava in some
characteristic forms. In not a few cases, the lava has risen in the
central pipe and has hardened there into a column of solid rock.
Subsequent denudation, by removing most of the cone, has left the top
of this thick column projecting as a round knoll upon the hill-top.
Arthur Seat presents a good example of this structure. Where the
denudation has not proceeded so far, we may still meet with a remnant
of the cake of lava which sometimes overflowed the bottom of a crater.
The summit of Largo Law affords indications of this arrangement, the
cone of tuff being there capped with basalt, evidently the product of
successive streams, which welling out irregularly covered the crater
bottom with hummocks and hollows (Fig. 226). The knolls are beautifully
columnar, and sometimes show a divergent arrangement of the prisms.
[Illustration: Fig. 221.--Dyke rising through the agglomerate of a
volcanic vent; Kincraig, Elie.]
But the most frequent form assumed by the lava in the necks is that
of veins or dykes running as wall-like bands through the tuff or
agglomerate. Many admirable examples may be cited from the shore
between Largo and St. Monans. Two illustrations of them are given in
Figs. 219 and 221. In Fig. 219 the dyke is about four feet broad, and
is seen to present the common transverse jointing as it pursues its
way through the tuff. White veins of calcite along its margin serve to
define its limits. Its position in reference to the general body of the
neck is shown in the ground-plan Fig. 224. The second instance (Fig.
221) is that of a dyke of basalt only one foot wide, which runs like a
wall up the agglomerate of the Kincraig neck near Elie. It is seen at
the bottom of the cliff projecting from the agglomerate; but higher up
it has decayed, leaving its fissure as a gaping chasm. Here the general
character of the pyroclastic material is well brought out. One or two
large blocks may be seen imbedded in it, the largest lying above where
the dyke bends away to the left.
The intruded masses vary in breadth from mere threadlike veins up to
dykes several yards in breadth, which sometimes expand into large
irregular lumps. They generally consist of some form of basalt; now
and then, as at Ruddon Point, near Elie, they are amygdaloidal; and it
may be observed among them, as among dykes in general, that where the
amygdaloidal texture is developed, it is apt to occur most markedly in
the central part of the vein, the amygdales running there in one or
more lines parallel with the general trend of the mass.
That the basalt of these veins and dykes was sometimes injected in an
extremely liquid condition is shown by its frequently exceedingly close
homogeneous texture. Within the neck on the shore to the west of Largo,
the basalt assumes in places an almost flinty character, which here and
there passes into a thin external varnish of basalt-glass. A farther
indication of the liquidity of the original rock seems to be furnished
by the great number of included extraneous fragments here and there to
be observed in the basalt.
But besides basalt, other materials may more rarely be detected
assuming the form of dykes or veins within the necks. Thus, at
the Largo neck just referred to, strings of an exceedingly horny
quartz-felsite accompany the basalt--a remarkable conjunction of
acid and basic rock within the same volcanic chimney. To the east
of Elie some dykes, which stand out prominently on the beach from a
platform worn by the sea in a neck, consist of an extremely compact
volcanic mudstone, stuck full of the worn twin crystals of orthoclase
and pieces of hornblende and biotite already noticed. So like is
this rock to one of the decomposing basalts that its true fragmental
nature may easily escape notice, and it might be classed confidently
as a somewhat decayed basalt. A considerable amount of a similar fine
compact mudstone is to be seen round the edges of some of the Elie
vents. This material must have been injected into open fissures, where
it solidified. There is further evidence of the presence of "mud-lava"
in some of the vents of East Fife, where these orifices contain a
remarkable compact volcanic sandstone, composed of the usual detritus,
but weathering into spheroidal crusts, so as externally to be readily
mistaken for some form of basalt.
[Illustration: Fig. 222.--Radiating columnar dyke in the tuff of a
volcanic vent, Rock and Spindle, two and a half miles east from St.
Andrews.]
A columnar arrangement may often be observed among the basalt dykes.
When the vein or dyke is vertical, the columns of course seem piled
in horizontal layers one above the other. The exposed side of the
dyke then reveals a wall of rock, seemingly built up of hexagonal
or polygonal, neatly fitting blocks of masonry, as in the Lower
Carboniferous vent of the Binn of Burntisland (Figs. 166, 168). An
inclination of the dyke from the vertical throws up the columns to
a proportional departure from the horizontal. Sometimes a beautiful
fan-shaped grouping of the prisms has taken place. Of this structure
the Rock and Spindle, near St. Andrews, presents a familiar example
(Fig. 222). Much more striking, however, though less known, is the
magnificent basalt mass of Kincraig, to the west of Elie, where the
columns sweep from summit to base of the cliff, a height of fully 150
feet, like the Orgues d'Expailly, near Le Puy in Auvergne. The general
position of this basalt in the vent is represented in the section (B,
Fig. 218). The curvature of the basalt is shown in Fig. 223, which is
taken from the Elie side looking westward, beyond the intrusions, to
the picturesque cliffs of tuff. The details of the cliff are given in
Fig. 225.
That many of the dykes served as lines of escape for the basalt to the
outer slopes of the cones is highly probable, though denudation has
usually destroyed the proofs of such an outflow. A distinct radiation
of the dykes from the centre of a neck is still sometimes traceable.
This structure is most marked on the south cone of Largo Law, where
a number of hard ribs of basalt project from the slopes of the hill.
Their general trend is such that if prolonged they would meet somewhere
in the centre of the cone. On the south-east side of the hill a minor
eminence, termed the Craig Rock, stands out prominently (Fig. 209).
It is oblong in shape, and, like the dykes, points towards the centre
of the cone. It consists of a compact columnar basalt, the columns
converging from the sides towards the top of the ridge. It looks like
the fragment of a lava-current which flowed down a gully on the outer
slope of the cone (B' in Fig. 226).
Veins of basalt are not confined to the necks, but may be seen running
across the surrounding rocks. The shore at St. Monans furnishes some
instructive examples of this character. As the veins thin away from
the main mass of basalt they become more close-grained and lighter in
colour, and when they enter dark shales or other carbonaceous rocks
they pass, as usual, into the white earthy clay-like "white-trap." The
influence of carbonaceous strata in thus altering basic dykes and sills
may be instructively studied along the shore of the East of Fife. A
good instance occurs near St. Monans Church (Fig. 227), where a vein of
"white-trap" traverses black shales which have been somewhat jumbled.
[Illustration: Fig. 223.--View of part of the shore front of the great
vent at Kincraig, looking westward, with the columnar basalt in front.]
In a modern volcano no opportunity is afforded of examining the contact
of the erupted material with the rocks through which the vent has
been opened. But in the basin of the Firth of Forth, within the area
now under description, a numerous series of coast-sections lays bare
this relation in the most satisfactory manner. The superincumbent
cones of tuff have been swept away, and we can examine, as it were,
the very roots of the old volcanoes. The margin of a neck or volcanic
vent is thus found to be almost always sharply defined. The rocks
through which the funnel has been drilled have been cut across, as if
a huge auger had been sunk through them. This is well displayed in
the beautifully perfect neck already cited at Newark Castle, near St.
Monans (Fig. 224). The strata through which this neck rises consist
of shales, sandstones, thin coal and encrinal limestones, dipping
in a westerly direction at angles ranging from 25° to 60°. At the
south end of the neck they are sharply truncated, as if by a fault.
Elsewhere they are much jumbled, slender vein-like portions of the tuff
being insinuated among the projecting strata. A large vertical bed
of sandstone, 24 yards long by 7 yards broad, stands up as a sinuous
reef on the east side of the vent (_s_). It is a portion of some of
the surrounding strata, but, so far as can be seen at the surface, is
entirely surrounded with agglomerate. Here and there the shales have
been excessively crumpled, and at the north end have been invaded by
a vein of basalt which, where it runs through them, assumes the usual
clay-like character. The strata have been blown out, and their place
has been occupied by a corresponding mass of volcanic agglomerate. But
their remaining truncated edges round the margin of the orifice have
undergone comparatively little alteration. In some places they have
been hardened, but their usual texture and structure remain unaffected.
[Illustration: Fig. 224.--Plan of volcanic neck on beach near St.
Monans.
T, Neck of tuff enclosing a mass of sandstone (_s_), and piercing
sandstones and shales With beds of limestone, (_l_ _l_), and a thin
seam of coal (_c_); B, Basalt "white-trap" dyke. The arrows show
the dip of the strata.
]
In a few examples, the progress of denudation has not advanced so far
that the cone cannot still be partially made out amidst its surrounding
masses of tuff. One of the most interesting of these is Largo Law, of
which an outline has been given in Fig. 209. The accompanying section
(Fig. 226) represents what appears to me to be the structure of this
hill. Each of the two now conjoined cones was probably in succession
the vent of the volcano. The southern and rather lower eminence, as
already mentioned, is traversed by rib-like dykes of basalt, which
point towards its top, where there is a bed of the same rock underlying
a capping of tuff. On its eastern declivity lies the basalt stream
already described (p. 87). The higher cone is surmounted by a cake of
basalt which, as I have above suggested, may have solidified at the
bottom of the latest crater. Of course all trace of the crater has
disappeared, but the general conical form of the volcanic mass remains.
Doubtless, still more of the old volcano would have been removed by
denudation but for the protection afforded to the tuff by the intrusion
of the basalt. The upper dotted lines in the figure are inserted merely
to indicate hypothetically how the cone may originally have stood.
On the west side the sheets of tuff which were thrown out over the
surrounding country have been almost entirely removed, but on the east
and south they still cover an extensive area. (See Fig. 208).
[Illustration: Fig. 225.--Columnar basalt in the neck of Kincraig,
Elie, seen from the west.]
[Illustration: Fig. 226.--Section across Largo Law.
_l_ _l_, Lower Carboniferous strata; _t_, tuff of cones; _t'_, tuff
of plain beyond the cones; B B, basalt ascending vents and sending
out veins: B', basalt which has probably flowed out at the surface.
The dotted lines are suggestive of the original outline of the hill.
]
(2) _Sills._--In the Clyde coal-field and in the basin of the Firth
of Forth, among the vast number of sills which there traverse the
Carboniferous formations, it is possible that some belong to the
Permian volcanic period (see vol. i. p. 474). Where the sheets have
been intruded along horizons that lie below the upper stratigraphical
limit of the puy eruptions, they may not unnaturally be held to belong
to these manifestations of volcanic energy, though it is obviously
quite conceivable that some of them may be of much later date. But
where they lie above the highest platforms of Carboniferous lavas and
tuffs, they may be assigned to a younger volcanic period. We know as
yet of only two such periods after the deposition of the Carboniferous
Limestone series in Scotland--Permian and older Tertiary. Unless,
therefore, these higher sills formed part of some other display
of subterranean activity which is not known to have culminated in
eruptions at the surface, they must be looked upon as probably either
Permian or Tertiary.
In the great coal-field of Stirlingshire and Lanarkshire, among the
large sills that break into the Millstone Grit and the Coal-measures,
one lies entirely in the Coal-measures, and covers about six square
miles of ground, stretching from near Caldercruix Station, a little
east of Airdrie, to near Kirk of Shotts, a distance of about four
miles. A group of smaller sheets, possibly connected with the larger
mass, runs for four miles further west to beyond New Monkland. Another
chain of sills, which may also be part of the same great intrusion,
extends from the Cant Hills, near the Kirk of Shotts, for more than
eight miles in a north-easterly direction. The largest mass in this
chain stretches from Blackridge, west of Bathgate, for upwards of three
miles, covering an area of about three square miles and terminating on
the north at the line of dislocation which has been followed by one
of the east and west dykes. Another large sill, which appears nearly
two miles further east on the north side of that dyke, lies on a lower
stratigraphical horizon, for it cuts the Carboniferous Limestone
series, and does not reach the top of the Millstone Grit. This sill is
cut through by two of the later dykes.
[Illustration: Fig. 227.--Vein of "white-trap" cutting black
carbonaceous shales, a little west from St. Monans Church.]
That these great intrusions took place later than the deposition of
the Coal-measures is obvious. There is no satisfactory evidence to
enable us to decide to which of the two post-Carboniferous volcanic
periods they may with most probability be assigned. As one of them is
distinctly cut by dykes that have been referred to the Tertiary series,
it might be plausibly argued that it at least is of pre-Tertiary date,
and therefore probably Permian. On the other hand, as will be shown in
a later chapter, some portion of the sills appears to be connected with
the younger or Tertiary dykes. This problem must for the present remain
unsolved.
In Ayrshire where, as already described, basic sills traverse the
Permian volcanic series, other large intrusive sheets are found around
the Permian basin. On the north side an important group of them,
passing through the Coal-measures into the Carboniferous Limestone
series, runs from Troon eastward for more than eight miles to beyond
Craigie. On the south side a much more extensive series may be traced
from the River Ayr southwards into the Dalmellington coal-field, and
thence north-eastwards in a wide semicircular sweep into the coal-field
of New Cumnock and Airds Moss. That some of these sills proceed from
the Permian necks has been definitely ascertained, and this fact has
been already alluded to in connection with the vents. I have little
doubt that the great majority, if not the whole, of these intrusive
sheets are to be referred to the Permian period.
Some of the sills must be later than a part of the Permian volcanic
eruptions, for they are found in at least three places intercalated in
the zone of lavas and tuffs. But no instance has been observed of their
traversing the basin of Permian sandstone which overlies that zone,
though a few dykes, possibly of Tertiary age, do cut this sandstone.
CHAPTER XXXII
PERMIAN VOLCANOES OF ENGLAND
The Devonshire Centre--Eruptive Rocks of the Midland
Coal-fields.
From the south of Scotland we need to pass to the extreme south-west of
England before we again encounter a group of volcanic rocks which may
be referred with some confidence to the Permian period. An interesting
group of lavas and tuffs has been preserved in some of the valleys over
a limited area in the east of Devonshire. The Midland coal-fields,
however, are traversed by a series of basic eruptive rocks which are
younger than the Coal-measures, and may possibly be Permian. Their mode
of occurrence, and the arguments regarding their geological age, will
be given in the present chapter.
1. DEVONSHIRE
The counties of Devon and Cornwall furnish one of the most striking
examples to be met with in Britain of the persistence of volcanic
action over a limited area through a long succession of geological
periods. The extensive eruptions in Devonian time were followed after
a long interval by a diminished series in the Carboniferous period.
But the subterranean energy was not then wholly exhausted, for it
showed itself on a feeble scale in at least one limited tract of the
same region during the Permian period. Thus throughout the later half
of Palæozoic time the extreme south-west of England continued to be a
theatre of volcanic action.
The geological age of the igneous rocks now to be referred to depends
upon the particular place in the geological record to which we assign
the remarkable breccias and sandstones with which they are associated.
By many geologists who have been unable to recognize any true break
in the red rocks from their base up to the bottom of the Lias, these
strata have been grouped as one great series referable to the "New
Red Sandstone" or Trias. This is the classification adopted on the
one-inch maps of the Geological Survey. On the other hand, various able
observers have pointed out the close resemblance of the coarse and fine
breccias at the bottom of the series to recognized Permian deposits
in the centre of England and to parts of the typical Rothliegende
of Germany. I need only refer to the strongly expressed views of
Murchison, in which, as he stated in his _Siluria_, he "entirely agreed
with Conybeare and Buckland, who, after a journey in Germany in 1816,
distinctly identified the Heavytree conglomerate, near Exeter, with
the Rothliegende of the Germans."[104] In the absence of any fossil
evidence, we have only lithological characters and sequence to guide
us, and though the known facts hardly warrant a very positive opinion,
my inclination is to regard these red Devonshire breccias as probably
Permian, and to follow Murchison in looking upon their associated
igneous masses as furnishing additional reason for assigning them to
that particular geological platform.[105]
[Footnote 104: _Siluria_, 4th edit. (1867), p. 333. See also Berger,
_Trans. Geol. Soc._ vol. i. (1811), pp. 98-102; Conybeare and Phillips,
_Geology of England and Wales_, p. 313, footnote; De la Beche, _Report
on the Geology of Cornwall, Devon and West Somerset_ (1839), chap.
vii. p. 193. Messrs. Hull and Irving (_Quart. Journ. Geol. Soc._ vol.
xlviii. 1892, pp. 60, 68) have more recently discussed the subject, and
follow the view of Murchison.]
[Footnote 105: Murchison cogently argued that as no signs of volcanic
activity were known in the Trias, but were abundant in the Permian
system, the Devonshire rocks might be regarded as appertaining to the
older series, _op. cit._ Triassic volcanic rocks, however, are now well
known on the Continent.]
No proper account has yet been written of the volcanic group which
I now propose to describe.[106] De la Beche was, I think, the first
to recognize the true volcanic nature of the rocks and their
contemporaneous interstratification in the red sandstone series.[107] As
traced by him on the Geological Survey maps, these rocks lie at or near
the base of the red sedimentary deposits, resting sometimes directly
on the Culm-measures, sometimes on an intervening layer of red strata.
He found them in three separate districts in the neighbourhood of
Exeter, the most northerly lying near Tiverton, the central extending
from Kellerton for a few miles up the Yeo Valley, beyond Crediton,
and the third stretching from the City of Exeter to Pen Hill, about
five miles to the south-west. He recognized the amygdaloids as slaggy
lavas, and saw that the volcanic breccias and tuffs are interleaved
with the sandstones. With regard to the probable vents from which these
materials were ejected, he thought that the chief centre of activity
lay at Kellerton Park, while in other localities he believed the bosses
of igneous rock "to descend in mass downwards, as if filling up some
crater or fissure through which these rocks had been vomited."[108] He
speaks also of "quartziferous porphyries" occurring among them, a
statement which, if petrographically accurate, would suggest the uprise
of a later more acid lava in some of the vents.
[Footnote 106: An outline of some of their characters will be found in a
paper by Mr. W. Vicary in _Trans. Devonshire Assoc._ 1865, vol. i. part
iv. p. 43.]
[Footnote 107: See his "Report" cited in the note above. De la Beche
quotes J. J. Conybeare as pointing out the intimate connection of these
igneous and stratified rocks (_Annals of Philosophy_, 2nd series, vol.
ii. (1821) p. 165); but this author wrote at the time of the Plutonist
and Neptunist controversy, and does not commit himself to any distinct
expression of opinion on the subject.]
[Footnote 108: Report, p. 201.]
More recently the ground has been revised by Mr. W. A. E. Ussher of the
Geological Survey, who has ascertained that the volcanic rocks appear
in many more places than those where they were noted on the older
maps, and likewise extend for some miles further to the north and west.
It now appears that in the central and chief district the lavas can
be followed westward from Spray Down near Kellerton to Greenslade
near North Tawton, a distance of about twenty-one miles. Their most
northerly outcrop is at Thorn above Loxbere in the Tiverton district,
and their most southerly visible portion passes under the Cretaceous
rocks of Pen Hill. The distance between these extreme points is
likewise about twenty-one miles. The whole display of volcanic
phenomena is comprised within an area of less than 400 square miles.
One of the most obvious features in this volcanic tract is the way in
which the erupted materials lie along the lines of hollow or valley in
which the red rocks were deposited. This is most distinctly exhibited
in the central district. Here a belt of breccias and sandstones,
varying from one to three and a half miles in breadth, runs for about
five and twenty miles westward in a depression of the Culm-measures.
At intervals, the lavas which lie near the base of the red rocks crop
out along the margin of the belt throughout most of its extent. But
they do not spread out over the older rocks, and they have evidently
been erupted from orifices situated along the line of the valley.
It is another example of the relation between the trend of hollows
and the outbreak of volcanic vents, which I have referred to as so
strikingly displayed in the distribution of the Permian volcanic rocks
of south-western Scotland.
The volcanic materials of the Devonshire Permian district consist
mainly of lavas, but include also red sandy and gravelly tuffs. The
whole volcanic group is remarkably thin, never attaining even the
limited development of the Ayrshire series. No adequate petrographical
investigation of these rocks has yet been made. Externally, as seen
in the quarries and lanes, the lavas present the closest resemblance
to those of the Permian basins of Ayrshire and Nithsdale. They show
considerable differences of texture even within the same mass, some
portions being dull, fine-grained purplish-red rocks, with scattered
pseudomorphs of hæmatite and a few porphyritic felspars, other parts
passing into an exceedingly coarse amygdaloid or slaggy pumice. Dr.
Hatch, after a microscopical examination of a small collection of
specimens, found that while most are olivine-basalts, containing
ferruginous pseudomorphs after olivine (Raddon Court, Pocombe, and near
Budlake), others are true andesites (Ide, Kellerton Park) and even
mica-trachytes (Copplestone, near Knowle Hill).[109] As already remarked,
some of the older writers mention the existence of quartz-porphyries.[110]
[Footnote 109: _Geol. Mag._ 1892, p. 250. The rocks have been more
recently described by Mr. B. Hobson, _Quart. Jour. Geol. Soc._ vol.
xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr.
Hobson "mica-augite-andesite," and he gives a chemical analysis of it
by Mr. E. Haworth, _op. cit._ p. 507. Mr. Watts has lately found one of
the orthoclase rocks to be rich in olivine.]
[Footnote 110: See De la Beche, _Report_, pp. 203, 204. My colleague, Mr.
Ussher, found close to the Thurlestone outlier of conglomerate near
Kingsbridge, Devonshire, a small boss of quartz-porphyry which rises
through and alters the Devonian rocks. The actual junction of this
mass with the conglomerate is not seen, nor have any fragments of the
porphyry been noticed among the pebbles.
Mr. Ussher informs me that in the quarry the visible exposure of
the acid rock is surrounded an covered by mica-porphyrite, probably
andesite.]
The geographical conditions in which the red rocks of Devonshire
accumulated were those so characteristic of the Permian and Trias
formations throughout Britain. The red sandstones and sandy marls
gathered in inland basins, where the water seems to have become too
saline and bitter to support animal life. The strata are consequently
singularly devoid of organic remains. The climate was probably arid,
and the absence or scarcity of traces of terrestrial vegetation
indicates that the land around the water-basins stretched in wide
sandy and rocky wastes. In the dry atmosphere and under the influence
of rapid radiation the cliffs and crags of Culm-measures would
disintegrate into angular rubbish, and this material, slipping into the
lakes or washed down by occasional rain-storms, forms now the breccias
that constitute so typical a feature in the Permian system.
[Illustration: Fig. 228.--Section at Belvedere, S.W. of Exeter.
_a_, Culm-measures; _b_, breccia and marls; _c_, lavas; _d_, red pebbly
sandstones.]
[Illustration: Fig. 229.--Diagram to show the unconformability and
overlap of the Permian rocks in the Crediton Valley.
_a_, Culm-measures; _b_, breccias and sandstones; _c_, lava-group; _d_,
breccias with fragments of lava passing up into sandstones and marls
(_e_).]
It was while this geographical type continued in the South-west of
England that the volcanic eruptions took place which we are now
considering. De la Beche correctly referred these eruptions to the
early part of the red sandstone series. A brief examination of the
ground suffices to show that although, as he pointed out, the volcanic
rocks lie towards the base of that series, as shown in Fig. 228, they
do not all occupy the same platform. That in some cases the lavas lie
directly on the Culm-measures, while in others they are separated
from these strata by 100 feet or more of red sandstones and breccias
(Fig. 229), would not in itself be proof of any difference of age or
stratigraphical position in the igneous rocks, for the floor on which
the Permian formations were here laid down can be shown to have been
singularly uneven. Prominent hills of Culm grit, several hundred feet
high, rose above the basins in which the earliest Permian sediments
were deposited, and these eminences were gradually submerged and buried
under the detritus.
But that the volcanic zone includes in some places more than one
outflow of lava with layers of sandstone, breccia and tuff between the
successive sheets may be proved in different parts of the district.
Thus the two conspicuous hills at Kellerton are composed of several
sheets of highly slaggy lava, separated by breccia, and a third much
thinner sheet lies above these, intercalated in a mass of breccia,
sandstone and sandy tuff (Fig. 230). Again, at Budlake the sandstones
and fine breccias include a thin band of vesicular lava, while farther
to the east they are interrupted by a higher and thicker zone of
similar material.
[Illustration: _North Hill_ _South Hill_
Fig. 230.--Section of the volcanic series at Kellerton, Devonshire.
_a_, Breccias and sandstones; _b_, lavas.]
These igneous sheets can be shown by many interesting sections to have
been poured out contemporaneously with the deposit of the sedimentary
material among which they occur. At Crabtree, for instance, near
Kellerton, the uppermost lava is a thin sheet of highly slaggy texture,
which rests immediately on the gravelly red sandstone and catches
up parts of it, while the pebbles include fragments of some of the
andesites below. The dark lavas are occasionally traversed by veins of
fine hard sandstone, which descending from above, like those in the Old
Red Sandstone and Permian lavas of Scotland, have been produced by the
silting or drifting of fine sand into cracks in the lava, before the
igneous material was entirely buried. These features are well exposed
in the high ridge of the Belvedere near Exeter (Fig. 228), where, over
a thin and inconstant band of red breccia and marl which rests on the
upturned ends of the Culm-measures, a band of dull-red andesite may
be seen. This rock, partly compact and partly highly amygdaloidal, is
in some portions full of irregular fissures and cavities filled with
sandstone.
Nowhere among the Palæozoic volcanic rocks of Britain are more
remarkable examples of the slaggy structure to be found than in these
Devonshire lavas of probably Permian age. I would especially cite the
rock of Knowle Farm, a few miles to the west of Crediton, as in part a
mere spongy pumice, blocks of which would originally have floated in
water.
One of the best sections in the district for the exemplification of
the internal structures of these lavas is that in the large quarry at
the top of Posbury Hill. On the west side of this quarry the rock is
tolerably compact, but contains vesicles and irregular steam-holes. On
the east side it passes upward and laterally into a coarse agglomerate
of its own fragments, and in its mass it encloses similar agglomerate.
No sharp passage can be traced between the two rocks. So far as I could
judge, it seemed to me that the lava had broken up as it moved along,
possibly shattered by coming in contact with water. The agglomerate is
overlain by some reddish ashy sandstone, which fills up the interstices
between the slags, and is immediately covered by a bed of lilac
andesite, marking another distinct outflow.
[Illustration:
Fig. 231.--Section of agglomerate overlain with sandstone and
andesite, Posbury, Crediton.
]
As in Ayrshire, the lavas of Devonshire are not accompanied by any
thick accumulation of tuff. The fragmentary discharges consisted
in both areas of fine dust and gravelly detritus of small lapilli,
which were not ejected in such quantities as entirely to conceal the
ordinary non-volcanic sediment of the water-basin. The dust and cinders
mingled with the red sand and angular scree-material, so that we now
see a group of red, somewhat ashy sandstones and breccias. Among the
component fragments of the breccias, a considerable variety of igneous
material may be observed. While the most of the non-volcanic stones
may have been derived by ordinary processes of weathering from rocks
exposed at the surface, it is by no means improbable that some of them,
including even pieces of Culm grit, killas and baked slate, may have
been ejected from volcanic vents.[111]
[Footnote 111: On the composition of the Devonshire breccias see Mr. R.
N. Worth, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 69. This
author has adopted the view that the granite of Dartmoor represents the
neck of a great volcano from which these later volcanic materials were
ejected. But all the evidence seems to me in favour of numerous small
vents situated not far from the outcrops of the lavas, as stated in
the text. See Mr. B. Hobson, _Quart. Journ. Geol. Soc._ vol. xlviii.
(1892), p. 498. The Dartmoor granite is later than the surrounding
Carboniferous rocks, but no good evidence has been obtained to connect
it with the Permian volcanic phenomena of Devonshire.]
Taking the volcanic rocks of this district as a whole, I regard them
as the mere edges of sheets that have flowed from vents which not
improbably lie concealed somewhere along the centres of these old
Permian valleys. No visible necks have been described from any part
of the area, and though I have not examined the whole of it, nothing
of that nature was detected by me either in the Crediton Valley or
between Silverton and the Exeter neighbourhood. The Tiverton district,
which has not yet been searched, appears to be the only tract where any
chance remains of finding some of the vents.
No indication of any sills has been met with among the Devonshire
Permian rocks. None of the lavas which I have seen have the internal
characters of true sills, while in the field their association with the
sandstones and breccias in no observed case points to intrusion.
Though much remains to be done in this region before an adequate
account can be given of the interesting series of eruptions which
concludes the long volcanic history of the South-west of England,
enough is known to indicate the general character of the phenomena.
The eruptions were on even a feebler scale than those of the Permian
period in Scotland, but they seem to have resembled them in their
general character. Small puy-like vents were opened, from which
dark scoriaceous lavas and showers of gravelly tuff and stones were
discharged over the floor of the inland sea or lake-basin in which
the red sandstones and breccias were accumulated. These outflows and
explosions took place too, as in Scotland, towards the beginning of
the deposition of the red strata, and entirely ceased long before that
deposition came to an end. In each area the eruptions mark the close
of Palæozoic volcanic activity in Britain. The varied and recurrent
volcanic episodes which distinguished each successive geological period
from the Archæan onwards now definitely terminate, not to be resumed
until after the passing of the whole of the vast cycle of Mesozoic ages.
2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS
Between the thick and thoroughly marine development of the
Carboniferous Limestone in Derbyshire and in South Wales, there lies
the region, already referred to, wherein both the Carboniferous
Limestone and Millstone Grit die out against what must have been a
ridge of land or group of islands that stretched in a general east
and west direction from the high grounds of Wales through Shropshire,
Staffordshire and Leicestershire. On the slopes of this ridge the
limestone is gradually overlapped by the Millstone Grit, and both are
in turn overlapped by the Coal-measures, which are then found lying
immediately on the more ancient rocks of the region--Cambrian or
pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence
that led to the deposit of several thousand feet of Carboniferous
strata over the regions to north and south, before the beginning of
the Coal-measure period, does not seem to have sensibly affected the
persistence of this old terrestrial surface, which probably lay on an
axis of upward movement, so that, amidst the surrounding depression,
its position above water was on the whole maintained. But there are
indications that the inequality of movement in this part of the earth's
crust was of much older date than the Carboniferous period. The Old Red
Sandstone is conformably continuous below the base of the Carboniferous
system, and in Wales is estimated to be some 10,000 feet thick. No
break has yet been detected in this vast accumulation of sedimentary
material, though it is highly probable that some such unconformability
must exist in it as that between the Scottish Lower Old Red Sandstone,
which passes down into the Upper Silurian shales, and Upper Old Red
Sandstone, which graduates upward into the base of the Carboniferous
formations. But even if such a break should be discovered, it will
not account for the position of the Coal-measures on Cambrian or even
perhaps older rocks. It is hardly conceivable that, had these rocks
been covered with a full development of Old Red Sandstone, they could
have been stripped of it by denudation before the deposition of the
Coal-measures. It seems much more probable that the discrepancy in the
terrestrial movements had commenced in Old Red Sandstone time, and that
these ridges of ancient Palæozoic rocks never sank below the waters in
which the vast thickness of red sandstones, marls and conglomerates was
laid down.[112]
[Footnote 112: See a discussion of this subject in Jukes' Preface to his
_South Staffordshire Coal-field_.]
But apart from the question of its antiquity, this tract of persistent
land has a special interest in the history of volcanic action in
Britain, for it was the scene of some remarkable protrusions of
eruptive material which took place after a part, and possibly after
the whole, of the Coal-measures were accumulated. The date of these
protrusions cannot be fixed with greater precision; but there can be
no doubt that they belong to one of the later volcanic periods in the
geological history of Britain, and the account of them is therefore
included in the present Chapter of this work.
In the English Midlands south of Stafford, over a tract of country
about 700 square miles in extent, stretching from Birmingham on the
east, across the vale of the Severn, to the uplands of Shropshire
on the west, the Coal-measures, partly isolated into outliers by
denudation and partly separated by overlying younger formations, are
pierced by masses of intrusive igneous rocks. Many of these masses
have long been familiar to geologists. Those, for example, of the
Clee Hills of Shropshire, and the Rowley, Barrow and Pouk Hills of
Staffordshire and Worcestershire, have been frequently described, their
relations to the surrounding strata have been minutely sought out,
their composition has been chemically determined, and their microscopic
structure has been investigated. But they have been studied rather as
individual masses of local importance. No attempt has yet been made to
ascertain how far they are capable of being grouped together as one
connected series, linked with each other in chemical and mineralogical
characters, and containing a definite record in the volcanic history of
the country. This is a task which, it is to be hoped, some competent
inquirer will before long undertake.
In the meantime it is only possible to review here the already
published information, and to gather from it what may at present be
surmised to have been the history of these later eruptions of the
Midlands.
The areas where the igneous rocks now to be described are exhibited may
be conveniently placed in the following five groups:--1st, Titterstone
Clee Hill; 2nd, Brown Clee Hill; 3rd, The Forest of Wyre Coal-field;
4th, The Coalbrookdale Coal-field; and 5th, The South Staffordshire
Coal-field.
1. _The Titterstone Clee Hill_ forms a ridge about seven miles long
and a mile and a quarter broad, running in a north-easterly direction
over the Old Red Sandstone uplands of the south of Shropshire. The
ground rises gradually towards the south-west, until it reaches there a
height of 1754 feet (Fig. 232). On the north-western side of the ridge,
the last vanishing representative of the Carboniferous Limestone can
be seen to be overlapped the Millstone Grit, which, as it is traced
towards the south-west, is in turn overlapped by the Coal-measures,
and these, about 400 feet thick, then rest immediately on the Old
Red Sandstone. Two sheets of columnar olivine-dolerite, possibly
originally connected, lie as cakes on the summit and eastern slope
of the ridge, and cover in all a space of about a square mile and a
half. The larger sheet, which varies from 60 to 180 feet in thickness,
overlies the Coal-measures, and the coals of the Cornbrook coal-field
have been worked underneath it. The smaller mass, which may be 300
feet in thickness, forms the summit of the ridge. On its eastern side
it reposes on Coal-measures, which are there much disturbed; but on
the west side, where it forms a bold capping to the escarpment, it is
underlain at once by the Old Red Sandstone. There cannot be any doubt
that these masses of eruptive material are sills, which have been
injected into the Carboniferous strata, and partly between these strata
and the Old Red Sandstone. One or more dykes of eruptive rock have
been met with in mining, and the coal on approaching them undergoes
alteration.[113]
[Footnote 113: See J. R. Wright, _Trans. Geol. Soc._ (2nd ser.) iii.
(1832), p. 487. Titterstone Clee Hill is shown on Sheet 55 N.E. and
N.W. of the Geological Survey, and in Horizontal Sections, Sheets 33
and 36, from which Fig. 232 is reduced. The microscopic structure of
the dolerite has been described by Mr. Allport, _Geol. Mag._ 1870, p.
159; _Quart. Journ. Geol. Soc._ xxx. (1874), p. 550.]
[Illustration: Fig. 232.--Diagrammatic section across Titterstone Clee
Hill.
1. Old Red Sandstone; 2. Carboniferous Limestone; 3. Millstone Grit; 4.
Coal-measures; 5 5. Columnar olivine-dolerite.]
2. _Brown Clee Hill_ consists of two outliers of Coal-measures, each
about a mile long, placed on the summit of a broad ridge of Old Red
Sandstone, and rising to a height of 1800 feet above the sea. Both of
the outliers is capped with a cake of dolerite, and a third smaller
patch of the same material lies on the southern outlier between the
cappings. Neither at this locality nor around Titterstone Clee have any
eruptive rocks been observed rising through the older strata. It is
evident that in both cases the orifices or fissures up which the molten
material rose lie concealed under the surviving cakes of dolerite.[114]
[Footnote 114: Brown Clee Hill is mapped in Sheet 61 S.W. of the
Geological Survey, and its structure is shown in Sheet 36 of the
Horizontal Sections.]
3. _Forest of Wyre Coal-field._--On both sides of this extensive tract
of Coal-measures, the strata near the base of the series are traversed
by sills or dykes of olivine-dolerite like that of the Clee Hills. The
sandstones in contact with the eruptive rock have been indurated. In
this district, also, the evidence shows that the sheets are intrusive,
and later than the portion of the Coal-measures there visible.[115]
[Footnote 115: This district is represented in Sheets 55 N.E. and 61 S.E.
of the Geological Survey. The microscopic structure of the larger mass
on the west side of the coal-field, and the variations in the minute
structure of the intrusion which forms a long ridge on the east side,
are described by Mr. Allport, _Quart. Journ. Geol. Soc._ xxx. pp. 550,
551.]
4. _Coalbrookdale Coal-field._--In this interesting district a sill of
rather finely crystalline olivine-dolerite, which is estimated to be
nearly 200 feet thick, is traceable from near Little Wenlock for three
miles to the north, intercalated between the Carboniferous Limestone
and the Silurian rocks underneath. It appears to underlie the western
part of the Coal-field, for it is exposed by denudation in several
valleys between Little Wenlock and Great Dawley. Owing to the thinning
out of the Carboniferous Limestone in an easterly direction, the sill
gradually comes to have the Millstone Grit on its upper surface, and at
one point is represented on the Geological Survey map as even intruded
into the Coal-measures. Here again we have an intrusive sheet of later
date than at least the earlier part of the Coal-measures, and no
evidence of any superficial outflow of volcanic material.[116]
[Footnote 116: The Coalbrookdale coal-field has been described by Sir
Joseph Prestwich, _Trans. Geol. Soc._ (2) v. p. 428; and Prof. E. Hull,
_Quart. Jour. Geol. Soc._ xxxiii. (1877), p. 629. The minute structure
of the sill at Little Wenlock is referred to by Mr. Allport, _op. cit._
p. 550. The ground is mapped on Sheet 61 N.E. of the Geological Survey,
and its structure is shown on Sheet 54 of the Horizontal Sections.]
5. _South Staffordshire Coal-field._--This district, in respect to its
igneous intercalations, has been much more fully examined and described
than any of the others. It forms the subject of an exceedingly able
memoir by Jukes, who carefully studied its geology and delineated it
on the maps and sections of the Geological Survey. Since his time
the rocks have been studied microscopically, but no material facts
regarding the stratigraphy have been obtained in addition to those
which he patiently collected and generalized upon.[117]
[Footnote 117: Jukes, "South Staffordshire Coal-field," _Mem. Geol.
Surv._ 2nd edit. (1859). The area is embraced in Sheet 62 N.W. and S.W.
of the Geological Survey, and is illustrated in Sheets 23, 24 and 25 of
the Horizontal Sections.]
This coal-field is above 20 miles long and 5 miles broad. Its strata
rest unconformably on Upper Silurian strata, which, as part of the
ancient ridge or island already referred to, project here and there
from amidst the Coal-measures. The boundaries of the field on the east
and west sides are chiefly made by faults which bring down Permian and
Triassic formations against the Carboniferous strata.
Throughout this coal-field sheets of igneous rock are abundant. In
the detailed account of them given by Jukes in his admirable essay
on the South Staffordshire Coal-field,[118] he distinguished two kinds
of igneous material--"basalt," which comes out at the surface, and
sometimes overlies the Coal-measures in large cakes like that of
the Rowley Hills, which extends for two miles in one direction and
more than a mile in another; and "greenstone," which burrows among
the coal-bearing strata, and gives off dykes and veins of "white
rock-trap." There does not appear, however, to be any essential
difference in composition, age or origin between these contrasted kinds
of igneous material. They not improbably all belong to one series of
extrusions, their distinctions being due rather to the conditions
under which they were erupted, and in particular to their comparative
thickness, and the influence of adjacent coals and carbonaceous shales
upon them.
[Footnote 118: _Op. cit._ p. 117.]
The igneous rocks seen at the surface in this district form a series
of well-marked eminences. Of these the largest extends as a ridge from
Dudley to beyond Rowley Regis, a distance of more than two miles. To
the west of this tract, a number of small patches of the same material
crop out at the surface, the most important forming Barrow Hill. Six
miles farther north another group of similar patches may be seen. Of
these the largest occurs at Wednesfield, but the most noted forms the
Pouk Hill, which has long been noted for the beauty of its columnar
structure.
The sheets of "greenstone" met with in the coal-field are more
numerous and extensive than the detached areas of more compact rock
visible above ground, a single sheet being sometimes traceable in the
coal-workings for two miles in one direction.
The eruptive rocks of this district, when examined in their freshest
form, consist of well-preserved olivine-dolerite. An examination of
the "greenstone" and the "white rock-trap," which runs in fingers and
threads through the coal, shows that these are really the same dolerite
which has undergone alteration, the ferruginous silicates having
especially been decomposed.[119]
[Footnote 119: Allport, _Quart. Journ. Geol. Soc._ xxx. (1874), p. 547.
Chemical analysis also shows the identity of the rocks and the nature
of the alteration of the "white rock." See Jukes, "South Staffordshire
Coal-field," pp. 117, 118.]
The sills of greenish decomposed material that have been injected
amongst and alter the coals, vary from 15 feet to 80 or 90 feet in
thickness. The largest of the dolerite cakes on the surface, that of
the Rowley Hills, is somewhat irregular in its thickness, but may reach
as much as 100 feet.
That nearly the whole of the igneous material is intrusive is admitted
by all observers who have studied the ground. The manner in which the
"basalts" and "greenstones" send out veins into the Coal-measures shows
conclusively that they have been injected into the strata. The only
rock about which some doubt has been expressed is that of the Rowley
Hills, which Jukes was disposed, though not without some hesitation,
to consider as part of an actual lava-stream. He based this inference
chiefly on the occurrence, immediately under the dolerite, of what
he looked upon as a "trappean breccia or brecciated ash, containing
rounded and angular fragments of igneous rock lying in a brown
rather ferruginous paste, that looks like the debris of a basaltic
rock."[120] This breccia he regarded as belonging to and passing into the
Coal-measures, and he was thus inclined to regard the dolerite as a
lava of Coal-measure age.
[Footnote 120: _Op. cit._ p. 119.]
It is possible, however, that the "trappean breccia" may be of the
same nature as the "uncompressed balls of basalt bedded in a mass of
decomposed basalt or basaltic 'wacke' or clay"[121]--that is, a decayed
contact layer of the eruptive rock. But if it be regarded as the
fragmental accompaniment of a lava-stream, it can hardly belong to
the Coal-measures. If the dolerite had been a lava of that age, it
ought to be found lying conformably on the Coal-measures. But this it
does not appear to do. Making every allowance for the way in which an
advancing current of lava might plough up soft sediment on the bottom
of the sea or of a lake, we can hardly thus account for the very uneven
surface of Coal-measures on which the sheet of igneous rock rests.
If the Rowley rock be looked upon as a lava which flowed out at the
surface, it must, I think, be assigned to a time subsequent to that of
the Coal-measures, when these strata had been upraised and had suffered
some amount of denudation. I confess, however, that the petrographical
characters of the rock, the alteration of the coals which have been
worked underneath it, and the abundant veins of "white rock" which
there traverse the seams, induce me to regard this rock as forming
no exception to the general rule in the Midlands, but as having been
intruded as a sill, now laid bare by denudation. Its fresher condition
may arise from its thickness, or from some other circumstance which has
not been ascertained.
[Footnote 121: _Op. cit._ p. 126.]
We have now to consider the probable geological date of the various
intrusions of basic igneous material which can be traced over so wide
an area in the centre of England. In discussing the subject, Jukes
pointed out that in the surrounding district "no igneous rocks of any
kind are found in any formation newer than the Coal-measures."[122]
This statement is, with the exception of one locality, undoubtedly
true.[123] But on any view there must have been a long interval of time
between the formation of the highest strata of the South Staffordshire
coal-field and that of the lowest Permian deposits of the district. It
is quite conceivable, though at present incapable of proof, that the
extravasation of eruptive material took place after the close of the
Carboniferous period and during the earlier part of the Permian period.
[Footnote 122: _Op. cit._ p. 131.]
[Footnote 123: See note on next page.]
Jukes further shows that "at whatever period these igneous rocks were
produced, they were all existent before the production of the faults
and dislocations that have traversed the Coal-measures, and before any
great denudation had been effected on the country." This argument may
be readily granted. But, so far as we know, many, if not most, of the
faults traverse also the surrounding Permian and Triassic rocks, so
that igneous masses protruded during those periods would be affected by
the same dislocations.
When we consider the history of Palæozoic time in this country, and
especially the proof, obtainable everywhere else in Britain, that
volcanic energy became quiescent during the accumulation of the
Coal-measures, we may well demand better evidence than has hitherto
been forthcoming that any portion of the dolerites of the Midlands
is of Carboniferous age. It is important to notice that though the
dolerite sills and veins are so abundant in the South Staffordshire
coal-field, coming even in many places up to the present surface of
the ground, no single case has been observed where they rise into
the Permian rocks that overlie the Coal-measures unconformably. It
is difficult to believe that, had these intrusions taken place after
the deposition of the younger formation, they should not be found
penetrating it.[124] It seems almost certain that they must be of an age
intermediate between the Coal-measures of South Staffordshire and the
surrounding breccias and sandstones of the Permian series. And as there
is clear evidence of contemporaneous volcanic action in the lowest part
of the Permian system to the north in Scotland and to the south in
Devonshire, the inference seems not unreasonable that these intrusive
basalts of the Midlands are most probably of Permian age.
[Footnote 124: Only one instance is known where in Staffordshire
any igneous rock has been intruded into rocks younger than the
Coal-measures (Allport, _Quart. Journ. Geol. Soc._ vol. xxx. p. 551;
Sheet 72 S. W. of the Geological Survey, and Horizontal Sections,
Sheet 57). It forms a dyke which has been traced near Norton Bridge,
Swinnerton and Butterton, running for 8 miles in a N.N.W. direction,
and rising through Permian, Bunter and Keuper strata. It is a highly
basic olivine-basalt, and is unquestionably a dyke. Mr. J. Kirkby,
who has recently mapped and described it (_Trans. North Staffordshire
Naturalists' Field-Club_, xxviii. (1894), p. 129), suggests that it
may be connected with the igneous rocks of the South Staffordshire
coal-field. But of this idea there is no evidence. The last point to
which the dyke has been traced is some five-and-twenty miles from the
nearest known portion of the dolerites of the coal-field. I have little
doubt that this dyke is really an outlying member of the great system
of Tertiary dykes described in Book VIII. of the present work.]
No trace of vents has been met with in the Coal-measures of the
Midland district or among the surrounding older rocks, nor any proof
that the abundant sills and veins were connected with the eruption
of volcanic materials at the surface. Nevertheless, from the analogy
of the structure of these intrusive sheets to that of the sills in
such volcanic districts as the southern half of Scotland, we may well
believe that they were connected here and there with eruptive vents,
and thus that besides the northern and southern districts of Permian
volcanoes, there rose a central group among the lagoons of the heart
of England. Though no vestige of any such group has been detected, we
must remember that a large portion of the Midlands is overspread with
Permian and Triassic deposits, and that much more igneous rock may be
concealed than appears at the surface. Possibly there may be buried
under these younger sheets of red sandstone and marl, lavas and tuffs
with their connected vents, such as may be seen where the Permian
volcanic series has been laid bare by denudation in Ayrshire and
Devonshire. In this respect it would be interesting to make a thorough
examination of the Permian breccias of the district, with the view of
discovering whether, though the volcanic rocks _in situ_ may still lie
covered up, fragments of them may not be found in these deposits.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map V. MAP OF THE PERMIAN VOLCANIC DISTRICTS OF SCOTLAND
_The Edinburgh Geographical Institute_ Copyright J. G. Bartholomew.]
BOOK VIII
THE VOLCANOES OF TERTIARY TIME
CHAPTER XXXIII
Vast lapse of time between the close of the Palæozoic and
beginning of the Tertiary Volcanic Eruptions--Prolonged Volcanic
Quiescence--Progress of Investigation among the Tertiary Volcanic
Series of Britain.
From the evidence which has been led in the foregoing chapters it
is clear that during the later stages of the Palæozoic period there
was a gradual enfeeblement of volcanic vigour over the area of the
British Isles. When the last puys of the Permian series became extinct
a remarkable volcanic quiescence settled down on the region. This
interval of rest lasted throughout the whole of the long succession
of the Mesozoic ages. Though the geological record of this section of
geological time is singularly complete in Britain, not a single vestige
has yet been found in it of any contemporaneous eruption. And what is
true of this country is, on the whole, true of the entire European
continent. With some trifling exceptions there were no volcanoes in
Europe, so far as we know, during the enormous lapse of time between
the last of the Palæozoic and the earliest of the Tertiary eruptions.
When the geologist attempts to form an estimate of the chronological
value of this interval of time he is soon lost in bewilderment over its
obvious vastness, and the impossibility of discovering any standards of
measurement by which to reckon its duration. On the one hand, he sees
that it lasted long enough to admit of the gradual elaboration of many
thousands of feet of various sedimentary deposits, which, from their
remarkable diversities of character, were evidently accumulated, on the
whole, with extreme slowness and amidst many geographical vicissitudes.
On the other hand, he perceives that the interval sufficed to bring
about an entire change in the fauna and flora of the globe. Indeed, the
more he investigates the details of this biological transformation,
the more he is impressed with the length of time that it must have
required. For it is not merely one complete change, but a multifold
succession of changes. The stratigraphical records of the long array
of geological periods over which it was spread show that the biological
evolution advanced through a vast series of species, genera and orders
which one by one appeared and disappeared.
The ages that elapsed between the final dying out of the Palæozoic
volcanoes and the outburst of those of Tertiary time were so protracted
that many revolutions of the geography of Europe were comprised within
them. Land and sea changed places again and again. First came the
singular topography of the Trias, which prolonged and accentuated the
characteristics of the closing Palæozoic ages. Next arose the more
genial climate and more varied geography of the Jurassic period, when
comparatively shallow seas overspread the site of most of the European
continent, and tracts of old land stretched away to the west and north.
Another crowded succession of changes in the disposition of land and
sea filled the long Cretaceous period, at the close of which a more
rapid and complete transformation in European geography took place.
Yet during all these transitions and vicissitudes, so far as we know,
volcanic energy remained quiescent throughout Western Europe. It was
not until some time after the great terrestrial movements that raised
so much of the Cretaceous sea-floor into land, and laid the foundations
of the modern continent, that the subterranean fires once more awoke to
vigorous action.
The renewal of eruptions in the early ages of Tertiary time was as
widespread as it was energetic. Over many regions of the European
continent volcanoes broke out either in new areas or on old sites. For
the most part they appeared as scattered puys or as Vesuvian vents,
generally not of the first magnitude, like those of Central France,
Hungary, Würtemberg and Italy. But in the north-west they assumed more
colossal proportions, and took the form of fissure-eruptions by which
many thousands of square miles of country were deluged with lava.
From the South of Antrim all along the West of Scotland to the north
of the Inner Hebrides remains of these basalt-floods form striking
features in the existing scenery. The same kind of rocks reappear
in the Faroe Islands and in Iceland, so that an enormous tract of
North-western Europe, much of it now submerged under the sea, was the
scene of activity of the Tertiary volcanoes. In entering, therefore,
upon a consideration of the British Tertiary volcanic rocks, we are
brought face to face with the records of the most stupendous succession
of volcanic phenomena in the whole geological history of Europe.
Fortunately these records have been fully preserved in the British
Isles, so that ample materials remain there for the elucidation of this
last and most marvellous of all the volcanic epochs in the evolution of
the continent.
As the remains of the Tertiary series of volcanic eruptions are the
youngest of all the volcanic records of Britain, they are naturally
the freshest and most abundantly preserved. They consequently reveal
with singular clearness multitudes of volcanic phenomena that are
less distinctly recognizable, or not to be found at all, among the
Palæozoic systems. Hence they will be discussed in greater detail in
the following chapters.
As a consequence of their greater freshness and wider extent, and
largely also because of the way in which they have been exposed along
many leagues of picturesque sea-cliffs in the North of Ireland and the
West of Scotland, they attracted attention at an earlier time than the
less obvious volcanic memorials of older ages. The gradual development
of opinion regarding the nature and history of volcanic rocks is thus
in no small measure bound up with the progress of observation and
inference in regard to the Tertiary volcanic series. I shall therefore
begin this narrative by offering a rapid sketch of the history of
inquiry respecting the Tertiary volcanic areas of the British Isles.
The basaltic cliffs of Antrim and the Inner Hebrides had attracted the
notice of passing travellers, and their striking scenery had become
more or less familiar to the reading public, before any attention
was paid to their remarkable geological structure and history. In
particular, the wonders of the Giant's Causeway and the Antrim coast
had already begun to draw pilgrims, even from distant countries, at a
time when geology had not come into existence. The scientific tourist
of those days who might care to look at rocks was, in most cases, a
mineralogist, for whom their structural relations and origin were
subjects that lay outside of the range of his knowledge or habits of
thought. In the year 1772 Sir Joseph Banks, together with Solander and
a party, visited Staffa and brought back the earliest account of the
marvels of that isle as they appeared to the sober eyes of science.
His narrative was communicated to Pennant, together with a number of
drawings of the cliffs and of Fingal's Cave. These were inserted by
that geographer in his _Second Tour_, published in 1774, and from their
careful measurements of the basaltic pillars and their delineation
of the basaltic structure, are of special interest in the history of
volcanic geology.
An intelligent appreciation of some of the geological interest of the
region is to be found in the writings of Whitehurst,[125] who gave a
good account of the basalt-cliffs of Antrim, and regarded the basaltic
rocks as the results of successive outflows of lava from some centre
now submerged beneath the Atlantic. More important are the observations
contained in two letters of Abraham Mills.[126] This writer had been
struck with the dykes on the north coast of Ireland, and was led to
examine also those in some of the nearer Scottish islands. He believed
them to be of truly volcanic origin, and spoke of them as veins of
lava. A few years later, Faujas St. Fond made his well-known pilgrimage
to the Western Isles. Familiar with the volcanic rocks of Central
France, he at once recognized the volcanic origin of the basalts of
Mull, Staffa and the adjoining islands.[127] His account of the journey,
published in Paris in 1797, may be taken as the beginning of the
voluminous geological literature which has since gathered round the
subject. Three years afterwards (1800) appeared Jameson's _Outline
of the Mineralogy of the Scottish Isles_. Fresh from the teaching of
Werner at Freiberg, the future distinguished Professor of Natural
History in the Edinburgh University naturally saw everything in the
peculiar Wernerian light. He gave the first detailed enumeration of
some of the eruptive rocks of the Hebrides, but of course ridiculed
the idea of their igneous origin. Having heard of a reported "crater
of a volcano" near Portree, he ironically expressed a hope that "there
may be still sufficient heat to revive the spirits of some forlorn
fire-philosopher, as he wanders through this cold, bleak country."[128]
[Footnote 125: _Inquiry into the Original State and Formation of the
Earth_, 2nd edit. 1786.]
[Footnote 126: _Philosophical Transactions for 1790._]
[Footnote 127: _Voyage en Angleterre, en Écosse et aux Îles Hébrides._
Paris, 1797.]
[Footnote 128: It will be shown in a later chapter that there is a
remarkably perfect volcanic vent near Portree, but the supposed crater
referred to by Jameson was probably some little corry among the sheets
of basalt.]
The advent of Jameson to Edinburgh gave a fresh impetus to the warfare
of the Plutonists and Neptunists, for he brought to the ranks of the
latter a mineralogical skill such as none of their Scottish opponents
could boast. The igneous origin of basalt, which the Plutonists stoutly
maintained, was as strongly denied by the other side. For some years
one of the most telling arguments against the followers of Hutton was
derived from the alleged occurrence of fossil shells in the basalt of
the north coast of Ireland. Kirwan[129] quoted with evident satisfaction
Richardson's observation of "shells in the basalts of Ballycastle," and
Richardson[130] himself, though the true explanation, that the supposed
basalt is only Lias shale altered by basalt, had been stated in 1802
by Playfair,[131] continued for ten years afterwards to reiterate his
belief in the aqueous origin of basalt. Thus the Tertiary volcanic
rocks furnished effective weapons to the combatants on both sides.
The dispute regarding the black fossiliferous rocks of Portrush had
the effect of drawing special attention to the geology of the North
of Ireland. Among the more noted geologists who were led to examine
them, particular reference must be made to Conybeare and Buckland, who,
in the year 1813, studied the interesting coast-sections of Antrim.
The report of their observations gives an excellent summary of the
arguments for the truly igneous origin of basalt, and a statement of
opinion in favour of the view that the bedded basalts are the products
of submarine volcanoes. Berger also about the same time described in
fuller detail the geology of the Antrim district, and showed the rocks
of the basalt-plateau to be younger than the Chalk. He likewise made a
study of the basalt-dykes of the North of Ireland, and was the first
to point out their prevalent north-westerly direction. The memoirs
of these geologists[132] may justly be regarded, to quote the words
of Portlock, as "the first effectual step made in Irish geology."
Portlock's own description is still the most complete summary of the
geology of that interesting region.[133]
[Footnote 129: _Geological Essays_, 1799, p. 252, _footnote_.]
[Footnote 130: Richardson lived on the Antrim coast, and had daily
opportunities of examining the admirable rock-sections there exposed.
It was he who found the shells in supposed basalt, and led the
geologists of his day astray on this subject. He made a clever but
irrelevant reply to Playfair's plain statement of facts (_Trans. Roy.
Irish Acad._ vol. ix. 1803, p. 481). His elaborate attack on "the
Volcanic Theory" will be found in _Trans. Roy. Irish Acad._ vol. x.
(1806), pp. 35-107. Though lively enough as a specimen of controversial
writing, it forms, when seriously considered, rather a melancholy
chapter in geological literature.]
[Footnote 131: _Illustrations of the Huttonian Theory_, § 252.]
[Footnote 132: They are contained in the third volume of the
_Transactions of the Geological Society_.]
[Footnote 133: "Report on the Geology of the County of Londonderry and
parts of Tyrone and Fermanagh," _Mem. Geol. Survey_, 1843.]
While such advances were being made in the knowledge of the structure
of the volcanic rocks of the North of Ireland, the geologist had
already appeared who was the first to attempt a systematic examination
of the Western Islands, and whose published descriptions are still a
chief source of information regarding the geology of this extensive
region. Dr. Macculloch seems to have made his first explorations among
the Hebrides some time previous to the year 1814, for in that year he
published some remarks on specimens from that district transmitted to
the Geological Society.[134] For several years in succession he devoted
himself with great energy and enthusiasm to the self-imposed task of
geologically examining and mapping in a generalized way all the islands
that lie to the westward of Scotland, from the remote St. Kilda even
as far as the Isle of Man. From time to time, notices of parts of his
work were given in the _Transactions of the Geological Society_. But
eventually in 1819 he embodied the whole in his _Description of the
Western Islands of Scotland, including the Isle of Man_.
[Footnote 134: _Trans. Geol. Soc._ vol. ii. 1814.]
This great classic marks a notable epoch in British geology. Properly
to estimate its value, we should try to realize what was the state
of the science in this country at the time of its appearance. So
laborious a collection of facts, and so courageous a resolution
to avoid theorizing about them, gave to his volumes an altogether
unique character. His descriptions were at once adopted as part of
the familiar literature of geology. His sections and sketches were
reproduced in endless treatises and text-books. Few single works of
descriptive geology have ever done so much to advance the progress of
the science in this country. With regard to the special subject of the
present memoir, Macculloch showed that the basalts and other eruptive
rocks of the Inner Hebrides pierce and overlie the Secondary strata
of these islands, and must therefore be of younger date. But though
he distinguished the three great series of "trap-rocks," "syenites"
and "hypersthene-rocks" or "augite-rocks," and indicated approximately
their respective areas, he did not attempt to unravel their relations
to each other. Nor did he venture upon any speculations as to the
probable conditions under which these rocks were produced. He claimed
that those who might follow him would find a great deal which he
had not described, but little that he had not examined. Subsequent
observers have noted many important facts, of which, had he observed
them, he would at once have seen the meaning, and which he certainly
would not have passed over in silence. But as a first broad outline
of the subject, Macculloch's work possesses a great value, which is
not lessened by the subsequent discovery of details that escaped his
notice, and of important geological relations which he failed to detect.
It has already been pointed out that some of the earliest and ablest
observations among the volcanic rocks of this country, especially in
Scotland, were made by foreigners. Students who had repaired from
abroad to Edinburgh for education sometimes caught the geological
enthusiasm, then so marked in that city, and made numerous journeys
through the country in search of further knowledge of Scottish rocks
and minerals. In other instances, geologists of established reputation,
attracted by the interest which the published accounts of the geology
of Scotland had excited, were led to visit the country and to record
their impressions of its rock-structure. Of the first class of
observers the two most noted were Ami Boué and L. A. Necker; of the
second, special acknowledgment is due to Faujas St. Fond and to Von
Oyenhausen and Von Dechen.
The labours of Boué[135] have already been referred to in connection with
the literature of the Scottish Old Red Sandstone (vol. i. p. 269). In
his treatment of the Tertiary Volcanic series of Scotland he appears to
have relied mainly on the then recently published volumes of Macculloch.
[Footnote 135: _Essai géologique sur l'Écosse._ Paris, 1820.]
L. A. Necker, as the grandson of the illustrious De Saussure, had
strong claims on the friendly assistance of the School of Geology
at Edinburgh when he went thither in 1806, at the age of twenty, to
prosecute his studies. He was equally well received by the Plutonists
and Neptunists, and devoted some time to the exploration of the geology
not only of the Lowlands, but of the Highlands and the Inner Hebrides.
Most of his observations appear to have been made in the year 1807,
but it was not until fourteen years afterwards that he published the
account of them.[136] The geological part of this work must be admitted
to be somewhat disappointing. The author's caution not to commit
himself to either side of the geological controversy then waging makes
his descriptions and explanations rather colourless. He adds little to
what was previously known. Even as regards the origin of the basalts
of the Western Islands, he could not make up his mind whether or not
to regard them as volcanic, but contented himself by referring them to
"the trappean formation." Yet these islands had so fascinated him that
eventually he returned to them as his adopted home, passed the last
twenty years of his life among them, and died and was buried there.
Besides his _Voyage_, he published in French an account of the dykes of
the Island of Arran.[137]
[Footnote 136: _Voyage en Écosse et aux Îles Hébrides._ See also
biographical notice of L. A. Necker, by Principal J. D. Forbes, _Proc.
Roy. Soc. Edin._ v. (1862), p. 53.]
[Footnote 137: _Trans. Roy. Soc. Edin._ vol. xiv. (1840), p. 667.]
Among the foreign geologists who have been drawn to the Scottish
mountains and islands by the interest of their Tertiary volcanic rocks,
I have already spoken of Faujas St. Fond. Much more important, however,
were the observations made some thirty years later by two German men
of science, Von Oyenhausen and Von Dechen. Their careful descriptions
of the geology of Skye, Eigg and Arran added new materials to the
knowledge already acquired by native geologists.[138] To some of the more
interesting parts of their work reference will be made in later pages.
[Footnote 138: Karsten's _Archiv_ (1829), vol. i. p. 56.]
The numerous trap-dykes of Northumberland, Durham and Northern
Yorkshire at an early date attracted the attention of geologists.
As far back as 1817, they had been the subject of a memoir by N. J.
Winch,[139] who gave an account of their effects on the adjacent rocks.
More important were the subsequent papers on the same subject by
Sedgwick, who, discussing the lithological characters, probable origin
and geological age of the dykes, pointed out that while the Cleveland
dyke was undoubtedly younger than a large part of the Jurassic rocks,
there was no direct evidence to determine whether dykes farther north
were earlier or later than the time of the Magnesian Limestone.[140]
Subsequent accounts of the dykes of the same region were given by
Buddle,[141] M. Forster,[142] N. Wood,[143] H. T. M. Witham,[144] Tate
[145] and others, while in more recent years important additions to our
knowledge of these dykes and of their effects have been made by Sir J.
Lowthian Bell[146] and Mr. J. J. H. Teall.[147]
[Footnote 139: _Trans. Geol. Soc._ vol. iv. (1817), p. 21. See also
Tilloch's _Phil. Mag._ vols. xlix. and l.]
[Footnote 140: _Cambridge Phil. Trans._ vol. ii. (1827), pp. 21, 139.]
[Footnote 141: _Trans. Nat. Hist. Soc. Northumberland_, i. (1831), p. 9.]
[Footnote 142: _Op. cit._ i. p. 44.]
[Footnote 143: _Op. cit._ i. pp. 305, 306, 308, 309.]
[Footnote 144: _Op. cit._ ii. (1838), p. 343.]
[Footnote 145: _Trans. Northumberland and Durham_, ii. (1868), p. 30.]
[Footnote 146: _Proc. Roy. Soc._ xxiii. (1875), p. 543.]
[Footnote 147: _Quart. Journ. Geol. Soc._ xl. (1884), p. 209.]
The geological age of the great series of Tertiary volcanic rocks has
only been determined district by district, and at wide intervals.
That some part of the Antrim basalts is younger than the Chalk of
that region was clearly shown by Berger, Conybeare and Buckland.
Portlock, however, referred to the occurrence of detached blocks of
basalt which he supposed to be immersed in the Chalk near Portrush,
and which inclined him to believe that "the basaltic flows commenced
at a remote period of the Cretaceous system."[148] Macculloch showed
that the corresponding basaltic plateaux of the Inner Hebrides were
certainly younger than the Oolitic rocks of that region. But no
nearer approximation to their date had yet been made when in the year
1850 the Duke of Argyll announced the discovery of strata containing
fossiliferous chalk-flints and dicotyledonous leaves, lying between
the bedded basalts of Ardtun Head, in the Isle of Mull.[149] In the
following year these fossil leaves were described by Edward Forbes, who
regarded them as decidedly Tertiary, and most probably Miocene. This
was the first palæontological evidence for the determination of the
geological age of any portion of the basalt-plateaux, and it indicated
that the basalts of the south-west of Mull were of older Tertiary date.
Taken also in connection with the occurrence of lignite-beds between
the basalts of Antrim, it suggested that these volcanic plateaux
were not due to submarine eruptions, as the earlier geologists had
supposed, but were rather the result of the subærial outpouring of lava
at successive intervals, during which terrestrial vegetation sprang up
upon the older outflows.
[Footnote 148: _Report on the Geology of Londonderry_, p. 93. There
can be no doubt that this was an error of observation. The Antrim
basalts are all certainly younger than the Chalk. The supposed "lumps
of basalt" were probably the ends of veins intruded into the Chalk,
and perhaps partially disconnected from the main parts of the veins.
Such apparently detached masses of intrusive rock are not infrequent
occurrence in connection with the Tertiary intrusive sills. An example
will be found represented in Fig. 321.]
[Footnote 149: _Brit. Assoc. Report_, 1850, Sections, p. 70; and _Quart.
Jour. Geol. Soc._ vii. (1851), p. 87.]
While Forbes brought forward palæontological proofs of the Tertiary
age of the volcanic rocks of the south-west of Mull, he at the same
time laid before the Geological Society a paper on the Estuary Beds
and the Oxford Clay of Loch Staffin, in Skye, wherein, while admitting
the existence of appearances which might be regarded as favourable to
the view that the intercalated basalts of that region were of much
later date than the Oolitic strata between which they might have been
intrusively injected, he stated his own belief that they were really
contemporaneous with the associated stratified rocks, and thus marked
an outbreak of volcanic energy at the close of the Middle Oolitic
period.[150] The Duke of Argyll, in the paper which he on the same
occasion communicated to the Geological Society, adopted this view
of the probable age of most of the basalts of the Western Islands.
He looked upon the Tertiary volcanic rocks of Mull as occupying a
restricted area, the great mass of the basalt of that island, like that
of Skye, being regarded by him as probably not later than some part of
the Secondary period.
[Footnote 150: _Quart. Journ. Geol. Soc._ vol. vii. (1851), p. 104.]
It must be granted that the appearances of contemporaneous
intercalation of the basalt among the Secondary strata are singularly
deceptive. When, several years after the announcement of the Tertiary
age of the basalts of Ardtun, I began my geological work in the Inner
Hebrides, I was led to the same conclusion as Edward Forbes, and
expressed it in an early paper.[151] All over the north of Skye I traced
what appeared to be evidence of the contemporaneous interstratification
of basalts with the Jurassic rocks and I concluded (though with some
reservation) that the whole of the vast basaltic plateaux of that
island were not younger than some late part of the Jurassic period. In
that same paper the attention of geologists was called to the probable
connection of the great system of east-and-west dykes traversing
Scotland and the North of England, with the basalt-plateaux of the
Inner Hebrides, and as I believed the latter to be probably of the
age of the Oolitic rocks, I assigned the dykes to the same period in
geological history. But subsequent explorations enabled me to correct
the mistake into which, with other geologists, I had fallen regarding
the age of the volcanic phenomena of the Western Islands. In 1867 I
showed that instead of being confined to a mere corner of Mull, the
Tertiary basalts, with younger associated trachytic or granitic rocks,
covered nearly the whole of that island, and that in all likelihood
the long chain of basaltic masses, extending from the North of Ireland
along the west coast of Scotland to the Faroe Islands, and beyond
these to Iceland, was all erupted during the Tertiary period. At the
same time I drew special attention to the system of east-and-west
dykes as proofs of the vigour of volcanic action at that period, and
I furnished evidence that this action was prolonged through a vast
interval of time, during which great subærial denudation of the older
lavas took place before the outflow of the younger.[152] Later in the
same year, in an address to the Geological Section of the British
Association, I reiterated these views, and more particularly emphasized
the importance of the system of dykes, which in my opinion was possibly
the most striking manifestation of the vigour of Tertiary volcanic
action.[153] In 1871, after further explorations in the field, I gave a
detailed account of the structure which had led to the mistake as to
the age of the Tertiary volcanic rocks of the Western Islands; and in
a description of the island of Eigg, I brought forward data to show
the enormous duration of the Tertiary volcanic period in the west of
Britain.[154]
[Footnote 151: "On the Chronology of the Trap-rocks of Scotland,"
_Trans. Roy. Soc. Edin._ xxii. (1861), p. 649.]
[Footnote 152: _Proc. Roy. Soc. Edin._ vi. (1867), p. 71.]
[Footnote 153: _Brit. Assoc. Report_ (Dundee), 1867, Sections, p. 49.]
[Footnote 154: _Quart. Journ. Geol. Soc._ xxvii. (1871), p. 279.]
Three years later Mr. J. W. Judd read before the Geological Society
a paper "On the Ancient Volcanoes of the Highlands."[155] The most
novel feature of this paper was the announcement that the author had
recognized the basal wrecks of five great central volcanoes in the
Western Islands, among which that of Mull was inferred by him to have
been at least 14,500 feet high. He was led to the conclusion that the
volcanic period in these regions was divisible into three sections--the
first marked by the outburst of acid rocks (felspathic lavas and ashes,
connected with deeper and more central granitic masses); the second by
the extrusion of basic lavas and tuffs (the basaltic plateaux); the
third by the appearance of small sporadic volcanic cones ("felspathic,
basaltic, or intermediate in composition") after the great central
cones had become extinct. It will be seen in the following pages
that these conclusions of Professor Judd are not supported by a more
detailed study of the region.
[Footnote 155: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 220.]
In the year 1879, during a traverse of some portions of the volcanic
region of Wyoming, Montana and Utah, I was vividly impressed by
the identity of structure between the basaltic plateaux of these
territories and the youngest volcanic areas of Britain. It then
appeared to me that some of the puzzling features in the Tertiary
volcanic series of the Inner Hebrides might be explained by the
structures so admirably displayed in these lava-fields of the Far
West.[156] Riding over the great basalt-plains of the Snake River and
looking at the sections cut by the river through the thick series
of horizontal basalt-beds, I appreciated for the first time the
significance of Baron von Richthofen's views regarding "massive"
or "fissure" eruptions, as contradistinguished from those of great
central cones of the type of Etna or Vesuvius, and I gathered so
many suggestions from my examination of these American regions that
I renewed with increased interest the investigation of the Tertiary
volcanic tracts of Britain. At last, after another interval of nine
years, during which my weeks of leisure were given to the task, I was
able to complete a discussion of the whole history of Tertiary volcanic
action in this country, which was communicated to the Royal Society
of Edinburgh in the early summer of 1888.[157] Since that time I have
continued the research, and have from time to time communicated my
results to the Geological Society. These various memoirs are combined
with hitherto unpublished details in the following account of the
British Tertiary Volcanic Rocks.
[Footnote 156: _Geological Essays at Home and Abroad_ (1882), pp. 271,
274; _Nature_, November 1880.]
[Footnote 157: _Trans. Roy. Soc. Edin._ vol. xxxv. part ii. (1888), pp.
23-184.]
Professor Judd has also prosecuted the investigation of the petrography
of the rocks, and has published his observations in the _Quarterly
Journal of the Geological Society_.[158] To these papers by him more
detailed reference will be made in later Chapters.
[Footnote 158: _Quart. Journ. Geol. Soc._ vols. xlv. (1889), xlvi.
(1890), xlix. (1893). In the first of these volumes Professor Judd
offered a detailed criticism of my views as to the order of succession
and history of the volcanic rocks of the Inner Hebrides. Subsequent
investigation having entirely confirmed my main conclusions, it is
not necessary to enter here upon matters of controversy. Reference,
however, will be made in subsequent Chapters to some of the points in
dispute.]
In describing the geological history of a great series of rocks,
chronological order is usually the most convenient method of treatment.
Where, however, the rocks are of volcanic origin, and do not always
precisely indicate their relative age, and where moreover the same
kinds of rock may appear on widely-separated geological horizons, it
is not always possible or desirable to adhere to the strict order
of sequence. With this necessary latitude, I propose to follow the
chronological succession from the older to the newer portions of the
series. I shall treat first of the system of dykes, by which so large a
part of Scotland and of the north of England and Ireland is traversed.
Many of the dykes are undoubtedly among the youngest members of the
volcanic series, and in no case has their age been as yet determined
except relatively to the antiquity of the rocks which they traverse.
They must, of course, be posterior to these rocks, and hence it would
be quite logical to reserve them for discussion at the very end of the
whole volcanic phenomena. My reason for taking them at the beginning
will be apparent in the sequel. After the dykes, I shall describe the
great volcanic plateaux which, in spite of vast denudation, still
survive in extensive fragments in Antrim, the Inner Hebrides and the
Faroe Islands. The eruptive bosses of basic rocks that have broken
through the plateaux will next be discussed. An account will then be
given of the protrusions of acid rocks which have disrupted these basic
bosses. The last chapters will contain a sketch of the subsidences
and dislocations which the basalt-plateaux have suffered, and of the
denudation to which they have been subjected.
As has been explained in Chapter iii., the volcanic cycle of any
district, during a given geological period, embraces the whole range of
erupted products from the beginning to the end of a complete series of
eruptions. Reference was made in Book I. to the remarkable variation
in the character of the lavas successively poured out from the same
volcanic reservoir during the continuance of a single cycle, and it was
pointed out that Richthofen's law generally holds good that while the
first eruptions may be of a basic or average and intermediate nature,
those of succeeding intervals become progressively more acid, but are
often found to return again at the close to thoroughly basic compounds.
This law is well illustrated by the volcanic history of Tertiary time
in Britain. We shall find that the earliest eruptions of which the
relative date is known consisted generally of basic lavas (dolerites
and basalts), but including also more sparingly andesites, trachytes
and rhyolites; that the oldest intrusive masses consisted of bosses,
sills and dykes of dolerite and gabbro; that these intrusions
were followed by others of a much more acid character--felsites,
pitchstones, quartz-porphyries or rhyolites, granophyres and granites;
that the latest lava is a somewhat acid rock, being a vitreous form of
dacite; and that the most recent volcanic products of all are dykes of
a thoroughly basic nature, like some of the earlier intruded masses.
CHAPTER XXXIV
THE SYSTEM OF DYKES IN THE TERTIARY VOLCANIC SERIES
Geographical Distribution--Two Types of Protrusion--Nature
of Component Rocks--Hade--Breadth--Interruptions of Lateral
Continuity--Length--Persistence of Mineral Characters.
If a geologist were asked to select that feature in the volcanic
geology of the British Isles which, more than any other, marks this
region off from the rest of the European area, he would probably choose
the remarkable system of wall-like masses of erupted igneous rock, to
which the old Saxon word "dykes" has been affixed. From the moors of
eastern Yorkshire to the Perthshire Highlands, and from the basins of
the Forth and Tay to the west of Donegal and the far headlands of the
Hebrides, the country is ribbed across with these singular protrusions
to such an extent that it may be regarded as a typical region for the
study of the phenomena of dykes. That all the dykes in this wide tract
of country are of Tertiary age cannot be maintained. It has been shown
in previous Chapters that each of the great volcanic periods has had
its system of dykes, even as far back as the time of the Lewisian
Gneiss.
But when all the dykes which can reasonably be referred to older
geological periods are excluded, there remains a large series which
cannot be so referred, but which are connected together by various
kinds of evidence into one great system that must be of late geological
date, and can be assigned to no other than the Tertiary period in the
volcanic history of Britain. As far back as the year 1861, when I first
drew attention to this great system of dykes in connection with the
progress of volcanic action in the country, I pointed out the grounds
on which it seemed to me that these rocks belong to a comparatively
recent geological period.[159] My own subsequent experience and the full
details of structure collected by my colleagues of the Geological
Survey in all parts of the country, have amply confirmed this view.
The characters which link this great series of dykes together as one
connected system of late geological date are briefly enumerated in the
following list, and will be more fully discussed in later pages.
[Footnote 159: _Trans. Roy. Soc. Edin._ vol. xxii. (1861), p. 650.]
1. The prevalent tendency of the dykes to take a north-westerly
course. There are exceptions to this normal trend, especially where
the dykes are small and locally numerous; but it remains singularly
characteristic over the whole region.
2. The increasing abundance of the dykes as they are traced to the west
coast and the line of the great Tertiary volcanic plateaux of Antrim
and the Inner Hebrides.
3. The rectilinear direction so characteristic of them and so different
from the tortuous course of local groups of dykes. The exceptions to
this normal feature are as a rule confined to the same localities where
departures from the prevalent westerly trend occur.
[Illustration: Fig. 233.--Dyke on the south-east coast of the Island of
Mull.]
4. The great breadth of the larger dykes of the system and their
persistence for long distances. This is one of their most remarkable
and distinctive characters.
5. The posteriority of the dykes to the rest of the geological
structure of the regions which they traverse. They are not only younger
than the other rocks, but younger than nearly all the folds and faults
by which the rocks are affected.
6. The manner in which they cut the Jurassic, Cretaceous and older
Tertiary rocks in the districts through which they run. At the
south-eastern end of the region they rise through the Lias and Oolite
formations, in the west they intersect the Chalk and also the Tertiary
volcanic plateaux together with their later eruptive bosses.
7. Their petrographical characters, among which perhaps the most
distinctive is the frequent appearance of the original glass of the
plagioclase-pyroxene-magnetite (olivine) rock, of which they mostly
consist. This glass, or its more or less completely devitrified
representative, often still recognizable with the microscope among the
individualized microlites and crystals throughout the body of a dyke,
is also not infrequent as a black vitreous varnish-like coating on the
outer walls, and occasionally appears in strings and veins even in the
centre.
It is the assemblage of dykes presenting these features which I propose
to describe. Obviously, the age of each particular dyke can only be
fixed relatively for itself. But when this remarkable community of
characters is considered, and when the post-Mesozoic age of at least a
very large number of the dykes can be demonstrated, the inference is
reasonable that one great system of dykes was extravasated during a
time of marked volcanic disturbance, which could not have been earlier
than the beginning of the Tertiary period. And this inference may be
maintained even when we frankly admit that every dyke within the region
is by no means claimed as belonging to the Tertiary series.
[Illustration: Fig. 234.--Fissure left by the weathering out of a dyke.]
In spite of their number and the extraordinary volcanic activity to
which they bear witness, the dykes form a much less prominent feature
in the landscape than might have been anticipated. In the lowlands of
the interior, they have for the most part been concealed under a cover
of superficial accumulations, though in the water-courses they not
infrequently project as hard rocky barriers across the channels, and
occasionally form picturesque waterfalls. On the barer uplands, they
protrude in lines of broken crag and scattered boulders, which by their
decay give rise to a better soil covered by a greener vegetation than
that of the surrounding brown moorland. Among the Highland hills, they
are often traceable from a distance as long black ribs that project
from the naked faces of crag and corry. Along the sea-coast, their
peculiarities of scenery are effectively displayed. Where they consist
of a close-grained rock, they often rise from the beach as straight
walls which, with a strangely artificial look, mount into the face
of the cliffs on the one side, and project in long black reefs into
the sea on the other (Fig. 233). Every visitor to the islands of the
Clyde will remember how conspicuous such features are there. But it is
among the Inner Hebrides that this kind of scenery is to be found in
greatest perfection. The soft dark Lias shales of the island of Pabba,
for example, are ribbed across with scores of dykes which strike boldly
out to sea. Where, on the other hand, the material of the dykes is
coarse in grain, or is otherwise more susceptible to the disintegrating
influences of the weather, it has often rotted away and left yawning
clefts behind, the vertical walls of which are those of the fissures
up which the molten rock ascended (Fig. 234). Some good instances of
this kind are well known to summer visitors on the eastern shores of
Arran. Others, on a large scale, may be seen in the interior of the
same island along the crests of the granite ridges, and still more
conspicuously on the jagged summits of Blath Beinn and the Cuillin
Hills (Fig. 333), and intersecting the Jurassic strata along the cliffs
of Strathaird in Skye.
1. GEOGRAPHICAL DISTRIBUTION
The limits of the region within which the dykes occur cannot be very
precisely fixed. There can be no doubt, however, that on their southern
side they reach to the Cleveland Hills of Yorkshire and the southern
borders of Lancashire, perhaps even as far as North Staffordshire (p.
106), and on the northern side to the farther shores of the island of
Lewis--a direct distance of 360 miles. They stretch across the basin of
the Irish Sea, including the Isle of Man, and appear in Ireland north
of a line drawn from Dundalk Bay to the Bays of Sligo and Donegal.
Dykes are of frequent occurrence over the north of England and south
of Scotland, at least as far north as a line drawn from the coast of
Kincardineshire along the southern flank of the Grampian Hills, by
the head of Glen Shee and Loch Tay, to the north-western coast of
Argyleshire. They abound all along the line of the Inner Hebrides and
on parts of the adjacent coasts of the mainland, from the remoter
headlands of Skye to the shores of County Louth. They traverse also the
chain of the Long Island in the Outer Hebrides. So far as I am aware,
they are either absent or extremely rare in the Highlands north of the
line I have indicated. But a good many have been found by my colleagues
in the course of the Geological Survey of the northern lowlands of
Aberdeenshire and Banffshire. The longest of these has been traced by
Mr. L. Hinxman for rather more than two miles running in a nearly east
and west direction through the Old Red Sandstone of Strathbogie, with
an average width of about 35 feet. Another in the same district has a
width of from 45 to 90 feet, and has been followed for a third of a
mile. But far beyond these northern examples, I have found a number of
narrow basalt-veins traversing the Old Red flagstones of the Mainland
of Orkney, which I have little doubt are also a prolongation of the
same late series. Taking, however, only those western and southern
districts in which the younger dykes form a notable feature in the
geology, we find that the dyke-region embraces an area of upwards of
40,000 square miles--that is, a territory greater than either Scotland
or Ireland, and equal to more than a third of the total land-surface of
the British Isles (Map I.).
Of this extensive region the greater portion has now been mapped in
detail by the Geological Survey. Every known dyke has been traced,
and the appearances it presents at the surface have been recorded. We
are accordingly now in possession of a larger body of evidence than
has ever before been available for the discussion of this remarkable
feature in the geology of the British Isles. I have made use of this
detailed information, and besides the data accumulated in my own
note-books, I have availed myself of those of my colleagues in the
Survey, for which due acknowledgment is made where they are cited.
The Tertiary basalt-plateaux of Britain have their counterpart in
the Faroe Islands and in Iceland, and whether or not the lava-fields
stretched throughout North-western Europe from Antrim to the farthest
headlands of _Ultima Thule_, there can hardly be any doubt that,
if not continuous, these volcanic areas were at least geologically
contemporaneous in their activity. Their characteristic scenery and
structure are prolonged throughout the whole region, reappearing with
all their familiar aspects alike in Faroe and in Iceland. I have not
seen the latter island, but in the Faroe archipelago I have found the
dykes to be sufficiently common, and to cut the basalt-plateaux there
in the same way as they do those of the Inner Hebrides. On the whole,
however, dykes do not play, in these northern isles, the important part
which they take in the geology and scenery of the West of Scotland. I
have not had sufficient opportunity to ascertain whether there is a
general direction or system among the Faroe dykes. In the fjords north
of Thorshaven, and again along the west side of Stromö, many of them
show an E. and W. strike or one from E.N.E. to W.S.W.
2. TWO TYPES OF PROTRUSION
The dykes are far from being equally distributed over the wide region
within which they occur. In certain limited areas they are crowded
together, sometimes touching each other to the almost entire exclusion
of the rocks through which they ascend, while elsewhere they appear
only at intervals of several miles. Viewed in a broad way, they may
be conveniently grouped in two types, which, though no hard line can
be drawn between them, nevertheless probably point to two more or
less distinct phases of volcanic action and to more than one period
of intrusion. In the first, which for the sake of distinction we may
term the Solitary type, there is either a single dyke separated from
its nearest neighbours by miles of intervening and entirely dykeless
ground, or a group of two or more running parallel to each other, but
sometimes a mile or more apart. The rock of which they consist is,
on the whole, less basic than in the second type; it includes the
andesitic varieties. It is to this type that the great dykes of the
north of England and the south and centre of Scotland belong. The
Cleveland dyke, for example, at its eastern end has no known dyke near
it for many miles. The coal-field of Scotland is traversed by five
main dykes, which run in a general sense parallel to each other, with
intervals of from half a mile to nearly five miles between them. Dykes
of this type display most conspicuously the essential characters of
the dyke-structure, in particular the vertical marginal walls, the
parallelism of their sides, their great length, and their persistence
in the same line.
In the second, or what for brevity may be called the Gregarious type,
the dykes occur in great abundance within a particular district. They
are on the whole narrower, shorter, less strikingly rectilinear,
more frequently tortuous and vein-like, and generally more basic in
composition than those of the first type. They include the true basalts
and dolerites. Illustrative districts for dykes of this class are the
islands of Arran, Mull, Eigg and Skye.
The great single or solitary dykes may be observed to increase in
number, though very irregularly, from south to north, and also in
Central Scotland from east to west. They are specially abundant in the
tract stretching from the Firth of Clyde along a belt of country some
thirty miles broad on either side of the Highland line, as far at least
as the valley of the Tay. They form also a prominent feature in the
islands of Jura and Islay.
Dykes of the gregarious type are abundantly and characteristically
displayed in the basin of the Firth of Clyde. Their development in
Arran formed the subject of the interesting paper by Necker, already
mentioned, who catalogued and described 149 of them, and estimated
their total number in the whole island to be about 1500.[160] As the
area of Arran is 165 square miles, there would be, according to this
computation, about nine dykes to every square mile. But they are far
from being uniformly distributed. While appearing only rarely in many
inland tracts, they are crowded together along the shore, particularly
at the south end of the island, where the number in each square mile
must far exceed the average just given. The portion of Argyleshire,
between the hollow of Loch Long and the Firth of Clyde on the east
and Loch Fyne on the west, has been found by my colleague, Mr. C. T.
Clough, to contain an extraordinary number of dykes (see Fig. 257). The
coast line of Renfrewshire and Ayrshire shows that the same feature is
prolonged into the eastern side of the basin of the Clyde estuary. But
immediately to the westward of this area the crowded dykes disappear
from the basin of Loch Fyne. In Cantire their scarcity is as remarkable
as their abundance in Cowal.
[Footnote 160: _Trans. Roy. Soc. Edin._ xiv. (1840), p. 677.]
Both in the North of Ireland and through the Inner Hebrides, dykes
are singularly abundant in and around, but particularly beneath, the
great plateaux of basalt. Their profusion in Skye was described early
in this century by Macculloch, who called attention more especially to
their extraordinary development in the district of Strathaird. "They
nearly equal in some places," he says, "when collectively measured, the
stratified rock through which they pass. I have counted six or eight
in the space of fifty yards, of which the collective dimensions could
not be less than sixty or seventy feet." He supposed that it would not
be an excessive estimate to regard the igneous rock as amounting to
one-tenth of the breadth of the strata which it cuts.[161] This estimate,
however, falls much short of the truth in some parts of Strathaird,
where the dykes are almost or quite contiguous, and the Jurassic
strata, through which they rise, are hardly to be seen at all.
[Footnote 161: _Trans. Geol. Soc._ iii. (1815), p. 79. This locality is
further noticed on p. 164.]
Among the districts where dykes of the gregarious type abound at a
distance from any of the basalt-plateaux, reference should be made
to the curious isolated tract of the central granite core of Western
Donegal. In that area a considerable number of dykes rises through
the granite, to which they are almost wholly confined. Again, far to
the east another limited district, where dykes are crowded together,
lies among the Mourne Mountains. These granite hills are probably to
be classed with those of Arran, as portions of a series of granite
protrusions belonging to a late part of the Tertiary volcanic period
which will be treated of in Chapter xlvii.
Though the dykes may be conveniently grouped in two series or types,
which on the whole are tolerably well marked, it is not always
practicable to draw any line between them, or to say to which group
a particular dyke should be assigned. In some districts, however, in
which they are both developed, we can separate them without difficulty.
In the Argyleshire region above referred to, for example, which
Mr. Clough has mapped, he finds that the abundant dykes belonging
to the gregarious type run in a general N.W. or N.N.W. direction,
and distinctly intersect the much scarcer and less basic dykes of
the solitary type, which here run nearly E. and W. (Fig. 257).
Hence, besides their composition, distinction in number, breadth,
rectilinearity and persistence, the two series in that region
demonstrably belong to distinct periods of eruption.[162]
[Footnote 162: Mr. Clough is inclined to suspect that the E. and W. dykes
are older than the Tertiary series and may be later Palæozoic.]
The characteristic habit in gregarious dykes of occurring in crowded
groups which are separated from each other by intervals of variable
dimensions, marked by the presence of comparatively few dykes, is well
illustrated in the district of Strath in Skye, which indeed may be
taken as a typical area for this peculiarity of distribution. While the
dykes are there singularly abundant in the Cambrian Limestone and the
Liassic strata, they have been found by Mr. Clough and Mr. Harker to be
comparatively infrequent in the tracts of Torridon Sandstone. It is not
easy to understand this peculiar arrangement. As the Torridon Sandstone
is the most ancient rock of the district, it probably underlies all the
Cambrian and Jurassic formations, so that the dykes which penetrate
these younger strata must also rise through the Torridonian rocks.
Some formations appear to have been fissured more readily than others,
and thus to have provided more abundant openings for the uprise of the
basaltic magma from below. To the effect of such local differences in
the structure of the terrestrial crust we have to add the concentration
of the volcanic foci in certain areas, though there seems no means
of ascertaining what part each of these causes has played in the
distribution of the dykes of any particular district.
3. NATURE OF COMPONENT ROCKS
The Tertiary dykes of Britain include representatives of four distinct
groups of igneous rocks. 1st, The vast majority of them consist of
plagioclase-pyroxene-magnetite rocks with or without olivine. These are
the normal basalts and dolerites. 2nd, A number of large dykes have
a rather more acid composition and are classed as andesites. 3rd, A
few dykes of trachyte have been observed in Cowal and in Skye cutting
the dykes of basalt (p. 138). 4th, In some districts large numbers
of still more acid dykes occur. These are sometimes crystalline in
structure (granophyre), more frequently felsitic (felsite, spherulitic
quartz-porphyry), and often glassy (pitchstone). In some exceptional
cases the basic and acid materials are conjoined in the same dyke.
Such compound varieties are described at p. 161. The acid dykes,
connected as they so generally are with the large bodies of granophyre
or granite, are doubtless younger than the great majority of the basic
dykes. They will be treated in connection with the acid intrusions in
Chapter xlviii.
By far the greater number of the dykes of the Tertiary volcanic series
belong to the first group, and it is these more especially which will
be discussed in the present and the following Chapter. As, however, the
andesitic group is intimately linked with the basaltic it will be here
included with them.
1. Basalt, Dolerite and Andesite Dykes.--To the field-geologist, who
regards merely their external features, the Tertiary dykes present a
striking uniformity in general petrographical character. They vary
indeed in fineness or coarseness of texture, in the presence or absence
of porphyritic crystals, amygdales, glassy portions and other points
of structure. But there is seldom any difficulty in perceiving that
they generally belong to one or other of the types of the basalts,
dolerites, diabases or andesites. This sameness of composition,
traceable from Yorkshire to Skye and from Donegal to Perthshire, is
one of the strongest arguments for referring this system of dykes to
one geological period. At the same time, there are enough of minor
variations and local peculiarities to afford abundant exercise for the
observing faculties alike in the field and in the study, and to offer
materials for arriving at some positive conclusions regarding the
geological processes involved in the uprise of the dykes.
There appears to be reason to believe that, when the petrography of the
dykes is more minutely studied, marked differences of material will be
found to denote distinct periods of eruption. Already Mr. A. Harker of
the Geological Survey, who is engaged in mapping the interesting and
complicated district of Strath in Skye, has observed that the dykes
which are older than the great granophyre bosses of that tract may be
distinguished from those which are later than these protrusions. The
older basic dykes are not conspicuously porphyritic, are frequently
marked by a close-grained margin or even with a veneer of basalt-glass,
sometimes have an inclination of as much as 45°, are occasionally
discontinuous, and not infrequently branch or send out veins. The
younger dykes, on the other hand, as will be more particularly noticed
in the following chapter, are distinguished by the frequent and
remarkable character of their porphyritic inclusions, by the presence
of foreign fragments in them, by the greater perfection of their
jointing, and by their seldom departing much from the vertical.[163] They
are likewise often markedly acid in composition, including such rocks
as granophyre, felsite and pitchstone.
[Footnote 163: In the Blath Bheinn group of gabbro-hills, however, it is
the youngest dykes which have been found by Mr. Harker to possess the
lowest hade.]
(1) _External Characters._--As regards the grain of the rock, every
gradation may be found, from a coarsely crystalline mass, in which the
component minerals are distinctly traceable with the naked eye, to a
black lustrous basalt-glass. Each dyke generally preserves the same
character throughout its extent. As a rule, broad and long dykes are
coarser in grain than narrow and short ones. For the most part, there
runs along each side of a dyke a selvage of finer grain than the rest
of the mass. This marginal strip varies in breadth from an inch or less
up to a foot or more, and obviously owes its origin to the more rapid
chilling of the molten rock along the walls of the fissure. It usually
shades away inperceptibly into the larger-grained inner portion. Even
with the naked eye its component materials can be seen to be more
finely crystalline than the rest of the dyke, though where dispersed
porphyritic felspars occur they are as large in the marginal strip as
in any other part of a dyke, for they belong to an earlier period of
crystallization than the smaller felspars of the groundmass and were
already floating in the magma while it was still in a molten state.
This finer-grained external band, so distinctive of an eruptive and
injected rock, is of great service in enabling us to trace dykes
when they traverse other dykes or masses of igneous rock of similar
characters to their own. When one dyke crosses another, that which has
its marginal band of finer grain unbroken must obviously be the younger
of the two.
[Illustration:
Fig. 235.--Plan of basalt-veins with selvages of black
basalt-glass, east side of Beinn Tighe, Isle of Eigg.
]
But in many examples in the south of Scotland, Argyleshire and the
Inner Hebrides, the fineness of grain of the outer band culminates
in a perfect volcanic glass. Where this occurs, the glass is usually
jet black, more rarely greenish or bluish black in tint, and varies
in thickness from about a couple of inches to a mere varnish-like
film on the outer face of the dyke, the average width being probably
less than a quarter of an inch (Fig. 235). On their weathered surface
these external glassy layers generally present a pattern of rounded or
polygonal prominences, varying up to four or five lines or even more in
diameter, and separated by depressions or narrow ribs. The transition
from the glass to the crystalline part of the marginal fine-grained
strip is usually somewhat abrupt, insomuch that on weathered faces
it is often difficult to get good specimens, owing to the tendency
of the vitreous portion to fly off when struck with the hammer. The
glass doubtless represents the original condition of the rock of the
dyke. It was suddenly chilled and solidified by contact with the
cold walls of the fissure. Inside this external glassy coating, the
molten material could probably still move, and had time to assume a
more or less completely crystalline condition before solidification.
Not infrequently the glass shows spherulitic forms, visible to the
naked eye, and likewise a more or less distinctly developed perlitic
structure. These features, however, are best studied in thin sections
of the rock with the aid of the microscope, as will be subsequently
referred to.
In some dykes, the glass is not confined to the edges, but runs in
strings or broader bands along the central portions, or has been
squeezed into little cavities like steam-holes or into minute fissures.
One of the most remarkable examples of this peculiarity occurs in the
well-known dyke of Eskdale, which runs for so many miles across the
southern uplands of Scotland.[164] This dyke throughout most of its
course is a crystalline rock of the andesitic type. At Wat Carrick,
in Eskdale, it presents an arrangement into three parallel bands.
On either side, a zone about eight feet broad consists of the usual
crystalline material. Between these two marginal portions lies an
intercalated mass 16 to 18 feet broad, of a very compact and more or
less vitreous rock. The demarcation between this central band and the
more crystalline zones of the outside is quite sharp, and the two
kinds of rock show a totally distinct system of jointing. There can,
therefore, be little doubt that the glassy centre belongs to a later
uprise than the outer portions, though possibly it may still have been
included in the long process of solidification of one original injected
mass of molten material. If the marginal parts adhered firmly to the
walls, the centre, which with its band of vesicles seems often to have
been a line of weakness, might be ruptured and subsequent intrusions
would find their way along the rent. Examples of this splitting of
dykes with the intrusion of later eruptive Material will be cited in
later pages.
[Footnote 164: See _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 241.]
Mr. Clough, while mapping for the Geological Survey the extraordinarily
numerous dykes in the eastern part of Argyleshire between the Firth
of Clyde and Upper Loch Fyne, observed six or seven examples of dykes
showing glassy bands in their centres, with characters similar to those
of the Eskdale dyke. He found an absence of definite and regular joints
in the central glassy band, and on the other hand, an irregular set of
divisional planes by which the rock is traversed, and which he compared
to those seen in true perlitic structure.
While, as a general rule, the external portions of a dyke are
closer-grained than the centre, rare cases occur where the middle is
the most finely crystalline part. I am disposed to regard these cases
and the glassy centres as forming in reality no true exceptions to the
rule, that the outer portions of a dyke consolidated first, and are
therefore finest in texture. For the most part, each dyke appears to be
due to a single uprise of molten matter, though considerable movements
may have taken place within its mass before the whole stiffened into
stone. Some particulars regarding these movements will be given in
section 12 of the next Chapter. It has already been mentioned that in
large dykes which have served as volcanic pipes, it is conceivable
that while the material next the outside consolidated and adhered to
the walls, the central portion may have remained liquid, and may even
have been propelled upward and have been succeeded by a different kind
of magma, as has been suggested by Mr. Iddings. In such cases, which,
if they occur, are probably excessively rare, we may expect that the
earlier and later material will not be sharply marked off from each
other, unless we suppose that the whole of the earlier liquid magma was
so entirely ejected that only its congealed marginal selvage was left
as bounding walls for the newer injection.
Where, after more or less complete consolidation had taken place,
the fissure opened again, or from any other cause the dyke was split
along its centre, any lava which rose up the rent would tend to take a
finer grain than the material of the rest of the dyke, and might even
solidify as glass.
Large scattered crystals of felspar, of an earlier consolidation than
that of the minuter forms of the same mineral in the general groundmass
of the rock, give a porphyritic structure and andesitic character to
many dykes. Occasionally such crystals attain a considerable size. Mr.
Clough has observed them in some of the Argyleshire dykes reaching
a length of between three and four inches, with a thickness of two
inches. Sometimes they are distributed with tolerable uniformity
through the substance of the dyke. But not infrequently they may be
observed in more or less definite bands parallel with the boundary
walls. Unlike the younger lath-shaped and much smaller felspars of the
groundmass, they show no diminution either in size or abundance towards
the edge of the dyke. On the contrary, as already mentioned, they are
often conspicuous in the close-grained marginal strip, and may be found
even in the glassy selvage, or touching the very wall of the fissure.
Indeed, they are sometimes more abundant in the outer than in the
inner portions of a dyke, having travelled outwards to the surfaces of
earliest cooling and crystallization.
Mr. Clough has given me the details of an interesting case of this kind
observed by him in Glen Tarsan, Eastern Argyleshire:--"For an inch
or so from the edge of this dyke," he remarks, "porphyritic felspars
giving squarish sections, and ranging up to one-third of an inch in
length, are so abundant as nearly to equal in bulk the surrounding
groundmass. For the next inch and a half, they are decidedly fewer,
occupying perhaps hardly an eighth of the area exposed. Then for a
breadth of three inches they come in again nearly as abundantly as at
the sides; after which they diminish through a band 27 inches broad,
where they may form from 1/8 to 1/12 of the rock." He found another
case where, in a dyke several yards wide, porphyritic felspars,
sometimes an inch long, are common along the eastern margin of the
dyke in a band about two inches broad, but nearly absent from the rest
of the rock. Elsewhere the crystals are grouped rather in patches than
in bands. Among the dykes south of Oban some similar instances of
coarsely porphyritic felspars may be observed.
Not only are these porphyritic felspars apt to occur in bands parallel
with the outer margins of the dykes, but they tend to range themselves
with their longer axis in the same direction, thus even on a large
scale, visible at some distance, showing the flow-structure, which is
so often erroneously regarded as essentially a microscopic arrangement,
and as specially characteristic of superficial lava-streams.
Mr. Harker in his survey of Strath, Skye, has met with some remarkable
examples of the enclosure and incorporation of foreign materials
in the younger group of dykes which in that district traverse the
granophyres and gabbros. He remarks that the great majority of these
dykes are basic, and he has found them to be capable of convenient
division into two groups. 1st, Non-porphyritic basic dykes with a
specific gravity between 2·87 and 2·97, and an amygdaloidal structure
affording clear indication of flowing movement, either at the sides or
along a central band. These dykes do not greatly differ from those of
pre-granophyre eruption. 2nd, Porphyritic basic dykes which present
features of peculiar interest. The porphyritic (or pseudo-porphyritic)
elements, according to Mr. Harker's observations, are constantly
felspar, frequently subordinate augite, and exceptionally quartz. The
felspars have for the most part rounded outlines with a bordering
zone of glass cavities apparently of secondary origin. The augite, in
rounded composite crystal-grains, differs from that of the groundmass
and resembles the augite of the gabbros. The quartz-grains are likewise
rounded, and show sometimes a distinct corroded border.
These characters, Mr. Harker observes, are those of crystals derived
from some foreign source, and it can scarcely be doubted that this
is the explanation of their presence. He noticed that the dykes in
question frequently enclose fragments, varying up to several inches in
diameter, of gabbro, granite or granophyre, bedded lava, quartzite,
etc., which show clear evidence of having been rounded and corroded
by an enveloping magma, and recognizable crystals from some of the
fragments may be observed in the surrounding parts of the matrix
of the dykes. Most of the felspar and augite crystals disseminated
through these porphyritic basic dykes may be referred to the partial
reabsorption of enclosed fragments of gabbro. The same observer has
found that many of the dykes which rise through the basalt-plateau of
Strathaird are crowded with gabbro fragments.
Another megascopic character of the material composing the dykes is the
frequent presence of amygdales. It has sometimes been supposed that
amygdaloidal structure may be relied upon as a test to distinguish
a mass of molten rock which has reached the surface from one which
has consolidated under considerable pressure below ground. That this
supposition, however, is erroneous is demonstrated by hundreds of dykes
in the great system which I am now describing. But the amygdales of a
dyke offer certain peculiarities which serve in a general way to mark
them off from those of an outflowing lava. They are usually smaller
and more uniform in size than in the latter rock. They are also more
regularly spherical and less frequently elongated in the direction of
flow. Moreover, they are not usually distributed through the whole
breadth of a dyke, but tend to arrange themselves in lines especially
towards its centre (Fig. 236). In these central bands the cavities are
largest and depart farthest from the regular spherical form, so that
for short spaces they may equal in bulk the mass of enclosing rock.
In some rare instances, a whole dyke is composed of cellular basalt,
like one of the lava-sheets in the plateaux, as may be seen on the
north flank of Beinn Suardal, Skye. Mr. Harker has observed that an
amygdaloidal structure is more common among the earlier than among the
later dykes of that district.
[Illustration: Fig. 236.--Arrangement of lines of amygdales in a dyke,
Strathmore, Skye.]
Besides the common arrangement of fine-grained edges and a more
coarsely crystalline centre, instances are found where one of the
contrasted portions of a dyke traverses the other in the form of veins.
Of these, I think, there are two distinct kinds, probably originating
in entirely different conditions. In the first place, they may be of
coarser grain than the rest of the rock; but such a structure appears
to be of extremely rare occurrence. I have noticed some examples on the
coast of Renfrewshire, where strings of a more coarsely crystalline
texture traverse the finer-grained body of the rock. Veins of this kind
are probably of the same nature as the so-called "segregation-veins,"
to be afterwards referred to as of frequent occurrence among the
thicker Tertiary sills. They consist of the same minerals as the
rest of the rock, but in a different and more developed crystalline
arrangement, and they contain no glassy or devitrified material, except
such portions of that of the surrounding groundmass as may have been
caught between their crystalline constituents.
The second kind of veins, which, though not common, is of much more
frequent occurrence than the first, is more particularly to be met with
among the broader dykes, and is distinguished by a remarkable fineness
of grain, sometimes approaching the texture of felsite or jasper, and
occasionally taking the form of actual glass. Such veins vary from
half an inch or less, up to four or five inches in breadth. They run
sometimes parallel with the walls of the dyke, but often irregularly
in all directions, and for the most part avoid the marginal portions,
though now and then coming up to the edge. They never extend beyond
the body of the dyke itself into the surrounding rock. Though they
have obviously been injected after the solidification of the rock
which they traverse, they may quite possibly be extrusions of a deeper
unconsolidated portion of the same rock into rents of the already
stiffened overlying parts. The field-geologist cannot fail to be struck
with the much greater hardness of these fine-grained veins and strings
that ramify through the coarsely crystalline dolerite, andesite or
other variety of the broader dykes. He can readily perceive in many
cases their more siliceous composition, and the inferences he deduces
from the rough observations he can make in the field are confirmed by
the results of chemical analysis (see p. 137).
In connection with veins of finer material, that may belong to a late
stage of the consolidation of the general body of a dyke, reference may
be made here to the occasional occurrence of patches of an exceedingly
compact or homogeneous texture immersed in the usual finely crystalline
marginal material. They look like angular and subangular portions of
the more rapidly cooled outer edge, which have been broken off and
carried upward by the still moving mass in the fissure.[165]
[Footnote 165: See Mr. J. J. H. Teall, _Quart. Journ. Geol. Soc._ xl.
(1884), p. 214.]
In general, each dyke is composed of one kind of rock, and retains
its chemical and mineralogical characters with singular persistence.
The difference of texture between the fine-grained chilled margin,
with its occasional glassy coating, and the more coarsely crystalline
centre is due to cooling and crystalline segregation in what was no
doubt originally one tolerably uniform molten mass. The glassy central
bands, too, though they indicate a rupture of the dyke up the middle,
may at the same time quite conceivably be, as I have said, extrusions
from a lower portion of the dyke before the final solidification of the
whole. The ramifying veins of finer grain that now and then traverse
one of the large dykes are likewise explicable as parts of a stage
towards entire consolidation. All these vitreous portions, whether
still remaining as glass or having undergone devitrification, are
more acid than the surrounding crystalline parts of the rock. They
represent the siliceous "mother-liquor," so to speak, which was left
after the separation from it of the crystallized minerals, and which,
perhaps, entangled here and there in vesicles of the slowly cooling
and consolidating rock, was ready to be forced up into cracks of the
overlying mass during any renewal of terrestrial disturbance.
But examples occur where a dyke, instead of consisting of one rock, is
made up of two or more bands of rock which, even if they resemble each
other closely, can be shown to be the results of separate eruptions.
These, which are obviously not exceptions to the general rule of the
homogeneity of dykes, I will consider in the next Chapter.
Among the petrographical varieties observable in the field is the
occasional envelopment of portions of the surrounding rocks in the
body of a dyke. Angular fragments torn off from the fissure-walls
have been carried upwards in the ascending lava, and now appear more
or less metamorphosed, the amount of alteration seeming to depend
chiefly upon the susceptibility of the enclosed rock to change from
the effects of heat. Cases of such entanglement, however, are of less
common occurrence than those already referred to, where pieces of some
deep-seated rock, such as the gabbros of Skye, have been carried up in
the ascending magma. Occasionally, where the enclosed fragments are
oblong, they are arranged with their longer axes parallel to the walls
of the dyke, showing flow-structure on a large scale. Mr. Clough has
found some dykes near Dunoon which enclose fragments of schist nearly
three feet in length.
One of the most interesting of the megascopic features of the dykes is
the joints by which they are traversed. These divisional planes are no
doubt to be regarded as consequences of the contraction of the original
molten rock during cooling and consolidation between its fissure-walls.
They are of considerable interest and importance, inasmuch as they
furnish a ready means of tracing a dyke when it runs through rock of
the same nature as itself, and also help to throw some light on the
stages in the consolidation of the material of the dyke.
[Illustration: Fig. 237.--Systems of joints in the dykes.
_a_, parallel; _b_, transverse.]
Two distinct systems of joints are recognizable (Fig 237). Though
sometimes combined in the same dyke, they are most conspicuously
displayed when each occurs, as it generally does, by itself. The first
and less frequent system of joints (_a_) has been determined by lines
of retreat, which are parallel to the walls of the dyke. The joints
are then closest together at the margin, and may be few or altogether
absent in the centre. They are sometimes so numerous, parallel and
defined towards the borders of the dyke, as to split the rock up into
thin flags. Where transverse joints are also present these flags are
divided into irregular _tesseræ_.
In the second or transverse system of joints (_b_), which is the more
usual, the divisional lines pass across the breadth of the dyke,
either completely from side to side, or from one wall for a longer or
shorter distance towards the other. Where this series of joints is
most completely developed the dyke appears to be built up of prisms
piled horizontally, or nearly so, one above another. These prisms, in
rare instances, are as regular as the columns of a basalt-sheet (see
Fig. 166). Usually, however, they have irregularly defined faces, and
merge into each other. Where the prismatic structure is not displayed,
the joints, starting sharply at the wall of the dyke, strike inwards
in irregular curving lines. It is such transverse joints that enable
the eye, even from a distance, to distinguish readily the course of
a dyke up the face of a cliff of basalt-beds, for they belong to the
dyke itself, are often at right angles to those of the adjacent basalt,
and by their alternate projecting and re-entering angles seam the dyke
with parallel bars of light and shade (see the double dyke in Fig.
333). Where they traverse not only the general mass of a dyke, but
also the "contemporaneous veins" which cross it, it may be inferred
that these veins were injected before the final solidification and
contraction of the whole dyke.
[Illustration:
Fig. 238.--Section of cylindrical vein or dyke, cutting the bedded
lavas, east side of Fuglö, Faroe Islands.
]
An interesting modification of the transverse joints may sometimes be
observed, where, as in the case of the Palæozoic "Rock and Spindle,"
at St. Andrews (Fig. 222), the molten material has solidified in a
tubular or spherical cavity. The joints then radiate inwards from the
outer curved surface. The most remarkable instance of this structure
which I have found among the Tertiary volcanic plateaux occurs on the
east side of the island Fuglö, the most north-easterly of the group of
the Faroes. It is cut in section by the face of the precipice, where
it appears as a round mass about 40 or 50 feet in diameter piercing
the plateau-basalts. A selvage of finer material round its outer edge
shows the effect of rapid chilling, while the joints diverge from the
periphery and extend in fan-shape towards the centre (Fig. 238).
[Illustration: Fig. 239.--Joint-structures in the central vitreous
portion of the Eskdale Dyke (B. N. Peach).
A, View of a square yard of the outer wall of the vitreous central
band, showing the polygonal arrangement of the prisms and their
investing sheath of ribs.
B, View of a smaller portion of the same wall to show the detailed
structure of the ribs (_a_ _a_) and their vitreous cores (_b_ _b_).
C, Profile of a part of the weathered face of the wall, showing the
way in which the hard ribs or sheaths project at the surface.
]
One of the most remarkable exhibitions of joint-structure hitherto
noticed among the Tertiary dykes is that which occurs in the central
vitreous band of the Eskdale dyke already referred to. The rock is
divided into nearly horizontal prisms, each of which consists of an
inner more vitreous core and an outer more lithoid sheath. By the
coherence of their polygonal and irregular faces, and the greater
durability of their material, these sheaths project on the weathered
wall of the vitreous centre of the dyke in a curiously reticulated
grouping of prominent ribs each about two inches broad (Fig. 239, A),
while the vitreous cores, being more readily acted on by the weather,
are hollowed out into little cup-shaped depressions. Each rib is thus
composed of the sheaths or outer lithoid portions of two prisms, the
line of separation being marked by a suture along the centre (B).
Between this median suture and the inner glassy core the rib is further
cut into small segments by a set of close joints, which are placed
generally at right angles to the course of the rib (C). Examined with a
lens, the lithoid substance of these sheaths has a dull finely granular
aspect, like that of felsitic rocks, with scattered felspars. It is
obviously a more devitrified condition of the material which forms
the core of each prism. This material presents on a fresh fracture a
deep iron-black colour, dull resinous lustre and vitreous texture.
It at once recalls the aspect of many acid pitchstones, and in the
early days of petrography was naturally mistaken for one of these
rocks. Through its substance numerous kernels of more glassy lustre
are dispersed, each of which usually contains one or more amygdales
of dull white chalcedony, but sometimes only an empty black cavity.
These black glistening kernels of glass, of all sizes up to that of a
small bean, scattered through the dull resinous matrix, form with the
white amygdales the most prominent feature in the cores; but crystals
of felspars may also be observed. Some details of the microscopic
characters of this remarkable structure will be given in a subsequent
page. The relation of the cores and sheaths to the prismatic jointing
of the rock seems to show that devitrification had not been completed
when these joints were established, and that it proceeded from the
faces of each prism inwards.
(2) _Microscopic Characters._--Much information has now been obtained
regarding the microscopic structure of the basaltic, doleritic and
andesitic dykes. The crystalline characters of those in the North of
England have been studied by Mr. Teall,[166] and some of those from
the West of Scotland have been investigated by Professors Judd and
Cole.[167] Taken as a whole, the rocks composing the dykes are found,
when examined microscopically, to consist essentially of mixtures of a
plagioclase felspar, pyroxene and iron oxide, with or without olivine,
and usually with more or less interstitial matter.
[Footnote 166: _Quart. Journ. Geol. Soc._ vol. xl. (1884).]
[Footnote 167: _Op. cit._ vol. xxxix. (1883) p. 444 (basalt-glass); xlii.
(1886) p. 49, where Professor Judd discusses the gabbros, dolerites and
basalts as a whole.]
The felspar appears to be in some cases labradorite, in others
anorthite, but there may be a mingling of several species in many
of the dykes, as in the augite-andesite of the Santorin eruption in
1866, wherein Professor Fouqué found that the larger porphyritic
felspars were mainly labradorite, but partly anorthite, while those
of the groundmass were microlites of albite and oligoclase.[168] The
large felspars scattered porphyritically through the groundmass
are evidently the result of an early consolidation, unless where
they are survivals from fragments of older porphyritic rocks which
have been enveloped and partially dissolved in the dykes. They are
often cracked, penetrated by the groundmass, or even broken into
fragments, and have corroded borders. They sometimes include portions
of the groundmass, and present the zonal growth structure in great
perfection. The small felspars of the groundmass, on the other hand,
are as obviously the result of a later crystallization, for they vary
in size and crystallographic development according to their position
in the dyke. Those from the centre are often in well-formed crystals,
which sometimes pass round their borders into acicular microlites.
Those in the marginal parts of the dyke occur chiefly in the form
of these microlites, forming the felted aggregate so characteristic
of the andesites. Curious skeleton forms, composed of aggregates of
microlites, connect the latter with the more completely developed
crystals, and illustrate the mode of crystallization of the felspathic
constituents of the dykes.[169]
[Footnote 168: _Santorin et ses Éruptions_, 1879, p. 203.]
[Footnote 169: See Mr. Teall's excellent description of the Cleveland
dyke, in the paper above cited.]
The pyroxene is probably in most cases monoclinic (black or common
augite), but is sometimes rhombic (usually enstatite, less frequently
perhaps hypersthene). It occurs in (_a_) well-developed crystals,
(_b_) crystalline masses with some of the faces of the crystals
developed, (_c_) granular aggregates which polarise in one plane, (_d_)
separate granules and microscopic microlites, which may be spherical
(globulites) or oblong (longulites).
The black iron-oxide is sometimes magnetite, sometimes ilmenite, or
other titaniferous ore. Apatite not infrequently occurs among the
original constituents. Olivine is entirely absent from most of the
large solitary dykes, especially at a distance from the great volcanic
centres, and no serpentinous matter remains to indicate that it was
ever present in them. But it is to be met with in numerous basalt-dykes
in the volcanic areas, either in sparsely scattered or in tolerably
abundant crystals. Biotite occasionally appears. Among the secondary
products, calcite and pyrites are doubtless the most common. To these
must be added quartz, chalcedony and various zeolitic substances,
besides the aggregates which result from the decomposition of the
ferro-magnesian constituents and the oxidation of the ferrous oxides.
In many dykes there is little or no interstitial matter between the
crystalline constituents of the groundmass. In others this matter
amounts to a half or more of the whole composition, and from such cases
a series of gradations may be traced into a complete glass containing
only the rudimentary forms of crystals (globulites, longulites, etc.),
with scattered porphyritic crystals of an earlier consolidation. The
process of the disappearance of this original glass may be admirably
studied in many dykes. At the outer wall, the glass remains nearly
as it was when contact with the cold walls of the fissure solidified
it. From that external vitreous layer the successive devitrification
products and crystalline growths may be followed inwards until in the
central parts of a broad dyke little or no trace of the interstitial
matter may be left.
[Illustration: Fig. 240.--Microscopic structure of the vitreous part of
the Eskdale Dyke.
This section shows a crystal of augite, enclosing magnetite and
surrounded with microlites, each of which consists of a central
pale yellow rod crusted with pale yellow isotropic globulites. The
glass around this aggregation is clear, but at a little distance
globulites (many of them elongated and dichotomous) abound, with
here and there scattered microlites, some of which are curved and
spiral. (800 diameters.)[170]
]
[Footnote 170: _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 255.]
The most instructive example of the process of devitrification which
has come under my observation occurs in the Eskdale dyke. The central
"cores" already referred to present a true glass, which in thin
sections is perfectly transparent and almost colourless, but by streaks
and curving lines of darker tint shows beautiful flow-structure. The
devitrification of this glass has been accomplished by the development
of crystallites and crystals, which increase in number until all
the vitreous part of the rock disappears. What seems under a low
power to be a structureless or slightly dusty glass can be resolved
with a higher objective into an aggregate of minute globules or
granules (globulites), which average perhaps 1/20,000 of an inch in
diameter. Some of these bodies are elongated and even dichotomous at
the ends. These granules are especially crowded upon clear yellow
dart-shaped rods, which in turn are especially prominent upon crystals
and crystalline grains of augite that bristle with them, while the
immediately surrounding glass has become clear. There can be little
doubt that these rudimentary bodies are stages in the arrested
development of augite crystals. There occur also opaque grains, rods
and trichites, which no doubt consist in whole of magnetite (or other
iron oxide), or are crusted over with that mineral.
At least two broad types of microscopic structure may be recognized
among the basic and intermediate dykes. (1) Holocrystalline, or with
only a trifling proportion of interstitial matter. This type includes
the dolerites and basalts, as well as rocks which German petrographers
would class as diabases or diabase-porphyrites. The rocks are very
generally characterized by ophitic structure, where the lath-shaped
felspars penetrate the augite, and are therefore of an earlier
consolidation. In such cases there is a general absence of any true
interstitial matter. The rocks of this type are often rich in olivine,
and appear to be on the whole considerably more basic than those of
the second group. It is observable that they increase in numbers from
the centre of Scotland westwards, and throughout the region of the
basalt-plateaux they form the prevailing type. (2) In this type there
is a marked proportion of interstitial substance, which is inserted in
wedge-shaped portions among the crystallised constituents ("intersertal
structure" of Rosenbusch). The ophitic structure appears to be absent,
and olivine is either extremely rare or does not occur at all. The
rocks of this group are obviously less basic than those of the other.
They form the large dykes that rise so conspicuously through the South
of Scotland and North of England, and their general characters are well
described by Mr. Teall in the paper already cited. In some instances
they enclose abundant porphyritic felspars of earlier consolidation,
and then present most of the characters of andesites. Professor
Rosenbusch has extended the name of "Tholeiites" to rocks of this group
in the North of England.[171] The vitreous condition is found in both
types, but is perhaps more frequent in the second. The glass of the
basalts, however, even in thin slices, is characteristically opaque
from its crowded inclusions; while that of the andesitic forms, though
black in hand specimens, appears perfectly transparent and sometimes
even colourless in thin slices.
[Footnote 171: _Mikroskopische Physiographie_, 3rd edit. 1071 _et seq._]
(3) _Chemical Characters._--The only one of these to which reference
will be made here is the varying proportion of silica. While the dykes
as a whole are either intermediate or basic, some of them contain so
high a percentage of silica as to link them with the acid rocks. The
average proportions of this ingredient range from less than 50 to
nearly 60 per cent. The rocks with the lower percentage of acid are
richer in the heavy bases, and have a specific gravity which sometimes
rises above 3·0. They include the true dolerites and basalts. Those,
on the other hand, with the higher ratio of silica, are poorer in
the heavy bases, and have a specific gravity from 2·76 to 2·96. They
comprise the tholeiites, andesites and other more coarsely crystalline
rocks of the great eastern and south-eastern dykes.[172]
[Footnote 172: For analyses of dykes, see Sir I. L. Bell, _Proc. Roy.
Soc._ xxiii. p. 546; Mr. J. S. Grant Wilson, _Proc. Roy. Phys. Soc.
Edin._ v. p. 253; Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209;
Professors Judd and Cole, _Quart. Jour. Geol. Soc._ xxxix. p. 444.]
Not only do the dykes differ considerably from each other in their
relative proportions of silica, but even the same dyke may sometimes be
found to present a similar diversity in different parts of its mass. It
has long been a familiar fact that the glassy parts of such rocks are
more acid than the surrounding crystalline portions. The original magma
may be regarded as a natural glass or fused silicate, in which all the
elements of the rock were dissolved, and which necessarily became more
acid as the various basic minerals crystallised out of it.[173] In the
Eskdale dyke the silica percentage of this glassy portion is 58·67,
that of the little kernels of black glass dispersed through the rock
as much as 65·49.[174] In the Dunoon dyke observed by Mr. Clough the
siliceous finer-grained veins contain no less than 68·05 per cent of
silica, while the mass of the dyke itself shows on analysis only 47·36
per cent.[175] Similar red strings have been noticed by the same careful
observer in an east and west dyke near Lochgoilhead. From Mr. Teall's
examination a large part of the felspar in these veins is probably
orthoclase. It forms a much larger percentage of the entire rock than
the felspar does in normal dolerites.
[Footnote 173: On this subject see a paper by Dr. A. Lagorio, "Über die
Natur der Glasbasis sowie der Krystallisationsvorgänge im eruptiven
Magma," Tschermak's _Mineralog. Mittheil._ viii. (1887), p. 421.]
[Footnote 174: Mr. J. S. Grant Wilson, _Proc. Roy. Soc. Phys. Edin._ v.
(1880) p. 253.]
[Footnote 175: Unpublished analyses made by the late Professor Dittmar of
Glasgow, and communicated to me by Mr. Clough.]
2. Trachyte Dykes.--In the Cowal District of Argyleshire, and in
the south of Skye, Mr. Clough has encountered a limited number
of dykes of trachyte. On a hasty inspection these are not always
readily distinguished from the basalt-dykes with which they agree in
general external aspect and in direction. Where their relation to
these dykes, however, can be determined they are found to traverse
them, and thus to be on the whole later, though one case has been
observed where a trachytic dyke is in turn traversed by one of the
basic series. Mr. Clough has supplied me with the following notes
of his observations regarding the trachytic dykes. They are all
characterized by the possession of spherulitic structures near their
margins. These features, easily perceptible to the naked eye, afford
the readiest means of distinguishing the dykes of this group. So
abundant are the spherulites that they not infrequently impinge on
each other in long parallel rows forming rod-like aggregates. Thus
in a dyke near Craigendavie, at the head of Loch Striven, numerous
planes about a quarter of an inch apart, and composed of such close-set
rods, may be observed running parallel to the marginal wall for a
distance of several inches from the edge. Most of these planes show
on their surfaces that the rods are always parallel to each other,
but may run in different directions in the different layers, being
sometimes horizontal, sometimes vertical, or at any angle between. On
examination, each rod is found to be made up of polygonal bodies, the
angles of which are quite sharp, but with their sides often slightly
curved, as if they had assumed their forms from the mutual pressure of
original spherical bulbs. Further scrutiny shows that the polygonal
bodies often exhibit an internal radiate structure.
In the central parts of the dyke the spherulitic arrangement is not
traceable. About a foot from the margin it begins to be recognizable.
At a distance of three or four inches the spherulites are about the
size of peas, and gradually diminish towards the edge until they can no
longer be seen.
Another characteristic of the trachyte dykes has been found by
Mr. Clough to be a useful guide in discriminating them from the
basalt-group. While the amygdales in the latter are generally rudely
spherical, those in the trachytes are commonly elongated in the
direction of the length of the dyke, and are frequently three quarters
of an inch, sometimes even an inch and a half, in length, though less
than a quarter of an inch in breadth.
A good example of these trachytic dykes, which occurs at Dunans,
about the head of Glendaruel, has been examined microscopically and
chemically. The central better crystallised portion was found by
Mr. Teall to be composed mainly of small lath-shaped crystals of
orthoclase, together with scales of brown biotite, a few prismatic
crystals of pale somewhat altered pyroxene and scattered granules of
magnetite. The chemical analysis of this rock by Mr. J. H. Player gave
the following composition:--
Silica 56·4
Alumina 19·0
Ferric oxide 3·5
Ferrous oxide 4·8
Lime 2·6
Magnesia 1·5
Soda 4·5
Potash 5·0
Loss on ignition 2·6
----
99·9
====
4. HADE
In the majority of cases, especially among the great single dykes,
the intrusive rock has assumed a position nearly or quite vertical.
But occasionally, where one of these solitary examples crosses a deep
valley, a slight hade is perceptible by the deviation of the line of
the dyke from its normal course. Sedgwick long ago noticed that the
Cleveland dyke has, in places, an inclination of at least 80° to its
N.E. side.[176] In the coal-workings, also, a trifling deviation from
the vertical is sometimes perceptible, especially where a dyke has
found its way along a previously existing line of fault, as in several
examples in Stirlingshire. But in those districts where the dykes are
gregarious, departures from the vertical position are not infrequent,
more particularly near the great basalt-plateaux. It was noticed by
Necker, that even in such a dyke-filled region as Arran, almost all of
the dykes are vertical, though sometimes deviating from that position
to the extent of 20°.[177] Berger found that the angle of deviation
among those of the north of Ireland ranges from 9° to 20°, with a mean
of 13°.[178] The most oblique examples are probably those which occur
in the basalt-plateaux of the Inner Hebrides, where the same dyke in
some parts of its course runs horizontally between two beds, across
which it also descends vertically (see Figs. 251, 252, 374). But with
these minor exceptions, the verticality of the great system of dykes,
pointing to the perpendicular fissure-walls between which the molten
rock ascended, is one of the most notable features in their geological
structure. In the Strath district of Skye Mr. Harker has noticed that
while the earlier dykes have sometimes a hade of 45°, those younger
than the granophyre are generally vertical or nearly so. In the Blath
Bheinn group of hills, however, as already alluded to, he has observed
that it is the youngest dykes which are inclined in a north-westerly
direction, with a hade of as much as 40° from the horizon.
[Footnote 176: _Cambridge Phil. Trans._ ii. p. 28.]
[Footnote 177: _Trans. Roy. Soc. Edin._ xiv. p. 677.]
[Footnote 178: _Trans. Geol. Soc._ iii. p. 227.]
5. BREADTH
An obvious characteristic of most dykes is the apparent uniformity of
their breadth. Many of them, as exposed along shore-sections, vary as
little in dimensions as well-built walls of masonry do. Departures
from such uniformity may often indeed be noted, whether a dyke is
followed laterally or vertically. The largest amount of variation is,
of course, to be found among the dykes of the gregarious type, the
thinner examples of which may diminish to a width of only one inch or
less, while their average breadth is much smaller than in the case
of the great solitary dykes. In the district of Strathaird, in Skye,
Macculloch estimated that the remarkably abundant dykes there developed
vary from 5 to 20 feet in breadth, but with an average breadth of not
more than 10 feet.[179] In the isle of Arran, according to Necker's
careful measurements, most of the dykes range from 2 or 3 to 10 or 15
feet, but some diminish to a few inches, while others reach a width of
20, 30, or even 50 feet.[180] In the North of Ireland, Berger observed
that the average breadth of thirty-eight dykes traversing primitive
rocks (schist, granites, etc.) was 9 feet; and of twenty-four in
Secondary rocks, 24 feet.[181]
[Footnote 179: _Trans. Geol. Soc._ iii. p. 80.]
[Footnote 180: _Trans. Roy. Soc. Edin._ xiv. p. 690 et seq.]
[Footnote 181: _Trans. Geol. Soc._ iii. p. 226. He believed that dykes
in Secondary rocks reach a much greater thickness than in other
formations. My own observations do not confirm this generalisation.]
But when we pass to the great solitary dykes, that run so far and so
continuously across the country, we encounter much thicker masses of
igneous rock. Most of the measurements of these dykes have been made
at the surface, and the variations noted in their breadth occur along
their horizontal extension. The Cleveland dyke, which is the longest
in Britain, varies from 15 feet to more than 100 feet, with perhaps an
average width of between 70 and 90 feet.[182] Some of the great dykes
that cross Scotland are of larger dimensions. Most of them, however,
like that of Cleveland, are liable to considerable variations in
breadth when followed along their length. The dyke which runs from
the eastern coast across the Cheviot Hills and Teviotdale to the head
of the Ale Water, is in some places only 10 feet broad, but at its
widest parts is probably about 100 feet. The Eskdale and Moffat dyke
is in parts of its course 180 feet wide, but elsewhere it diminishes
to not more than 40 feet. These variations are repeated at irregular
intervals, so that the dyke alternately widens and contracts as its
course is traced across the hills. Some of the dykes further to the
north and west attain yet more gigantic proportions. That which crosses
Cantyre opposite Ardlamont Point has been measured by Mr. J. B. Hill,
of the Geological Survey, who finds it to be from 150 to 180 feet broad
on the shore of Loch Fyne, and to swell out beyond the west side of
Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo,
in Fife, is about 400 feet wide. The broadest dyke known to me is one
which I traced near Beith, in Ayrshire, traversing the Carboniferous
Limestone. Its maximum width is 640 feet.
[Footnote 182: At Cockfield, where it has long been quarried, it varies
from 15 to 66 feet; at Armathwatie, in the vale of the Eden, it is
about 54 feet (Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 211).]
Unfortunately, it is much less easy to get evidence of the width of
dykes at different levels in their vertical extension. Yet this is
obviously an important point in the theoretical discussion of their
origin. Two means are available of obtaining information on the
subject--(_a_) from mining operations, and (_b_) from observations at
precipices and between hill-crests and valley-bottoms.
(_a_) In the Central Scottish coal-field and in that of Ayrshire,
some large dykes have been cut through at depths of two or three
hundred feet beneath the surface. But there does not appear to be any
well-ascertained variation between their width so far below ground and
at the surface. In not a few cases, indeed, dykes are met with in the
lower workings of the coal-pits which do not reach the surface or even
the workings in the higher coals. Such upward terminations of dykes
will be afterwards considered, and it will be shown that towards its
upper limit a dyke may rapidly diminish in width.
(_b_) More definite information, and often from a wider vertical range,
is to be gathered on coast-cliffs and in hilly districts, where the
same dyke can be followed through a vertical range of many hundred
feet. But so far as my own observations go, no general rule can be
established that dykes sensibly vary in width as they are traced
upward. Every one who has visited the basalt-precipices of Antrim or
the Inner Hebrides, where dykes are so numerous, will remember how
uniform is their breadth as they run like ribbons up the faces of the
escarpments.[183] Now and then one of them may be observed to die out,
but in such cases (which are far from common) the normal width is
usually maintained up to within a few feet of the termination.
[Footnote 183: This point did not escape the attention of that excellent
observer, Berger, in his examination of the dykes in the North of
Ireland. We find him expressing himself thus:--"The depth to which the
dykes descend is unknown; and after having observed the sections of a
great many along the coast in cliffs from 50 to 400 feet in height, I
have not been able to ascertain (except in one or two cases) that their
sides converge or have a wedgeform tendency" (_Trans. Geol. Soc._ iii.
p. 227).]
All over the southern half of Scotland, where the dykes run along
the crests of the hills and also cross the valleys, a difference of
level amounting to several hundred feet may often be obtained between
adjacent parts of the same dyke. But the breadth of igneous rock
is not perceptibly greater in the valleys than on the ridges. The
depth of boulder clay and other superficial deposits on the valley
bottoms, however, too frequently conceals the dykes at their lowest
levels. Perhaps the best sections in the country for the study of this
interesting part of dyke-structure are to be found among the higher
hills of the Inner Hebrides, such as the quartzites of Jura and the
granophyres and gabbros of Skye. On these bare rocky declivities,
numerous dykes may be followed from almost the sea-level up to the
rugged and splintered crests, a vertical distance of between 2000 and
3000 feet. The dykes are certainly not as a rule sensibly less in width
on the hill-tops than in the glens. So far, therefore, as I have been
able to gather the evidence, there does not appear to me to be, as
a general rule, any appreciable variation in the width of dykes for
at least 2000 or 3000 feet of their descent. The fissures which they
filled must obviously have had nearly parallel walls for a long way
down.
6. INTERRUPTIONS OF LATERAL CONTINUITY
In tracing the great solitary dykes across the country, the geologist
is often surprised to meet with gaps, varying in extent from a few
hundred feet to several miles, in which no trace whatever of the
igneous rock can be detected at the surface. This disappearance
is not always explicable by the depth of the cover of superficial
accumulations; for it may be observed over ground where the naked
rocks come almost everywhere to the surface, and where, therefore, if
the conspicuous material of the dykes existed, it could not fail to
be found. No dyke supplies better illustrations of this discontinuity
than that of Cleveland. Traced north-westward across the Carboniferous
tracts that lie between the mouth of the Tees and the Yale of the Eden,
this dyke disappears sometimes for a distance of six or eight miles.
In the mining ground round the head of the South Tyne the rocks are
bare, so that the absence of the dyke among them can only be accounted
for by its not reaching the surface. Yet there can be no doubt that
the various separated exposures, which have the same distinctive
lithological characters and occur on the same persistent line, are all
portions of one dyke which is continuous at some depth below ground. We
have thus an indication of the exceedingly irregular upward limit of
the dykes, as will be more particularly discussed further on.
But there are also instances where the continuity is interrupted
and then resumed on a different line. One of the best illustrations
of this character is supplied by the large dyke which rises through
the hills about a mile south of Linlithgow and runs westward across
the coal-field. At Blackbraes it ends off in a point, and is not
found again to the westward in any of the coal-workings. But little
more than a quarter of a mile to the south a precisely similar dyke
begins, and strikes westward parallel to the line of the first one.
The two separated strips of igneous rock overlap each other for
about three-quarters of a mile. But that they are merely interrupted
portions of what is really a single dyke can hardly be questioned. A
second example is furnished by another of the great dykes of the same
district, which after running for about twelve miles in a nearly east
and west direction suddenly stops at Chryston, and begins again in the
same direction, but on a line about a third of a mile further north.
Such examples serve to mark out irregularities in the great fissures up
which the materials of the dykes rose.
7. LENGTH
In those districts where the small and crowded dykes of the gregarious
type are developed, one cannot usually trace them for more than a
short distance. The longest examples known to me are those which have
been mapped with much patience and skill by Mr. Clough in Eastern
Argyleshire. Some of them he has been able to track over hill and
valley for four or five miles, though the great majority are much
shorter. In Arran and in the Inner Hebrides, it is seldom possible to
follow what we can be sure is the same dyke for more than a few hundred
yards. This difficulty arises partly, no doubt, from the frequent
spread of peat or other superficial accumulation which conceals the
rocks, and partly also from the great number of dykes and the want of
sufficiently distinct lithological characters for the identification of
any particular one. But making every allowance for these obstacles, we
are compelled, I think, to regard the gregarious dykes as essentially
short as well as relatively irregular.
In striking contrast to these, come the great solitary dykes. In
estimating their length, as I have already remarked, we must bear
in mind the fact that they occasionally undergo interruptions of
continuity owing to the local failure of the igneous material to
rise to the level of what is now the surface of the ground. A narrow
wall-like mass of andesite or dolerite, which sinks beneath the surface
for a few hundred yards, or for several miles, and reappears on the
same line with the same petrographical characters, while there may
be no similar rock for miles to right and left, can only be one dyke
prolonged underneath in the same great line of fissure. But even if we
restrict our measurements of length to those dykes or parts of dykes
where no serious interruption of continuity takes place, we cannot
fail to be astonished at the persistence of these strips of igneous
rock through the most diverse kinds of geological structure. A few
illustrative examples of this feature may be selected. It will be
observed that the longest and broadest dykes are found furthest from
the basalt-plateaux, while the shortest and narrowest are most abundant
near these plateaux.
Not far from what I have taken provisionally as the northern boundary
of the dyke region, two dykes occur which have been mapped from the
head of Loch Goil by Arrochar across Lochs Lomond and Katrine by Ben
Ledi to Glen Artney, whence they strike into the Old Red Sandstone of
Strathmore, and run on to the Tay near Perth--a total distance of about
60 miles. If the dyke which continues in the same line on the other
side of the estuary of the Tay beyond Newburgh, is a prolongation of
one of these, then its entire length exceeds 70 miles. A few miles
further south, one of a group of dykes can be followed from the
heart of Dumbartonshire by Callander across the Braes of Doune to
Auchterarder--a distance of 47 miles, with an average breadth of more
than 100 feet. In the district between the Forth and Clyde a number
of long parallel dykes can be traced for many miles across hill and
plain, and through the coal-fields. One of these is continuous for
25 miles from the heart of Linlithgowshire into Lanarkshire. Still
longer is the dyke which runs from the Firth of Forth at Grangemouth
westward to the Clyde, opposite Greenock--a distance of about 36 miles.
Coming southward, we encounter a striking series of single dykes on
the uplands between the counties of Lanark and Ayr, whence they strike
into the Silurian hills of the southern counties. One of these runs
across the crest of the Haughshaw Hills, and can be followed for some
30 miles. But if, as is probable, it is prolonged in one of the dykes
that traverse the moorlands of the north of Ayrshire and south of
Renfrewshire to the Clyde, its actual length must be at least twice
that distance. The great Moffat and Eskdale dyke strikes for more than
50 miles across the South of Scotland and North of England. The Hawick
and Cheviot dyke runs for 26 miles in Scotland and for 32 miles in
Northumberland.
But the most remarkable instance of persistence is furnished by the
Cleveland dyke. From where it is first seen near the coast-cliffs of
Yorkshire the strip of igneous rock can be followed, with frequent
interruptions, during which for sometimes several miles no trace of it
appears at the surface, across the North of England as far as Dalston
Hall south of Carlisle, beyond which the ground onwards to the Solway
Firth is deeply covered with superficial deposits. The total distance
through which this dyke can be recognized is thus about 110 miles. But
it probably goes further still. On the opposite side of the Solway,
a dyke which runs in the same line, rises through the Permian strata
a little to the east of the mouth of the Nith. Some miles further
to the north-west, near Moniaive, Mr. J. Horne, in the progress of
the Geological Survey, traced a dark compact dyke with kernels of
basalt-glass near its margin, running in the same north-westerly
direction. Still further on in the same line, another similar rock is
found high on the flanks of the lofty hill known as Windy Standard.
And lastly, in the Ayrshire coal-field, a dyke still continuing the
same trend, runs for several miles, and strikes out to sea near
Prestwick. It cannot, of course, be proved that these detached Scottish
protrusions belong to one great dyke, or that if such a continuous dyke
exists, it is a prolongation of that from Cleveland. At the same time,
I am on the whole inclined to connect the various outcrops together as
those of one prolonged subterranean wall of igneous rock. The distance
from the last visible portion of the Cleveland dyke near Carlisle to
the dyke that runs out into the Firth of Clyde near Prestwick, is about
80 miles. If we consider this extension as a part of the great North
of England dyke, then the total length of this remarkable geological
feature will be about 190 miles.
8. PERSISTENCE OF MINERAL CHARACTERS
Not less remarkable than their length is the preservation of their
normal petrographical characters by some dykes for long distances.
In this respect the Cleveland dyke may again be cited as a typical
example. The megascopic and microscopic structures of the rock of this
dyke distinguish it among the other eruptive rocks of the North of
England. And these peculiarities it maintains throughout its course.[184]
Similar though less prominent uniformity may be traced among the long
solitary dykes of the South of Scotland, the chief variations in
these arising from the greater or less extent to which the original
glassy magma has been retained. The same dyke will at one part of its
course show abundant glassy matter even to the naked eye, while at a
short distance the vitreous groundmass has been devitrified, and its
former presence can only be detected with the aid of the microscope.
Where a dyke has caught up and absorbed abundant foreign materials its
composition naturally varies considerably from point to point. Mr.
Harker has observed some good examples of this variation in Skye.
[Footnote 184: See the careful examination of this dyke by Mr. Teall,
_Quart. Journ. Geol. Soc._ xl. p. 209.]
CHAPTER XXXV
THE SYSTEM OF DYKES--_continued_
Direction--Termination upward--Known vertical Extension--Evidence
as to the movement of the Molten Rock in the Fissures--Branches and
Veins--Connection of Dykes with Intrusive Sheets--Intersection of
Dykes--Dykes of more than one infilling--Contact Metamorphism of
the Dykes--Relation of the Dykes to the Geological Structure of the
Districts which they traverse--Data for estimating the Geological
Age of the Dykes--Origin and History of the Dykes.
9. DIRECTION
Another characteristic feature of the dykes is their generally
rectilinear course. So true are the solitary dykes to their normal
trend that, in spite of varying inequalities of surface and wide
diversities of geological structure in the districts which they
traverse, they run over hill and dale almost with the straightness
of lines of Roman road. In the districts where the gregarious type
prevails, the dykes depart most widely from the character of the great
solitary series, but still tend to run in straight or approximately
straight lines, or, if wavy in their course, to preserve a general
parallelism of direction.
Yet even among the great persistent dykes instances may be cited where
the rectilinear trend is exchanged for a succession of zig-zags, though
the normal direction is on the whole maintained. In such cases, it is
evident that the fissures were not long straight dislocations, like the
larger lines of fault in the earth's crust, but were rather notched
rents or cracks which, though keeping, on the whole, one dominant
direction, were continually being deflected for short distances to
either side. As a good illustration of this character, reference may be
made to the Cheviot and Hawick dyke. In Teviotdale, this dyke can be
followed continuously among the rocky knolls, so that its deviations
can be seen and mapped. From the median line of average trend the
salient angles sometimes retire fully a quarter of a mile on either
side. Some examples of the same feature may be noticed in the Eskdale
dyke. The large dyke which runs westward from Dunoon has been observed
by Mr. Clough to change sharply in direction three times in four
miles, running occasionally for a short distance at a right angle to
its general direction (see Fig. 257).
Among these solitary dykes also, though the persistence of their trend
is so predominant, there occur instances where the general direction
undergoes great change. Some of the most remarkable cases of this
kind have been mapped by Mr. B. N. Peach and Mr. R. L. Jack, in the
course of the Geological Survey of Perthshire. Several important
dykes strike across the Old Red Sandstone plain for many miles in a
direction slightly south of west. But when they approach the rocks of
the Highland border in Glen Artney, they bend round to south-west, and
continue their course along that new line.
Many years ago I called attention to the dominant trend of the dykes
from north-west to south-east.[185] Subsequent research has shown this
to be on the whole the prevalent direction throughout the whole region
of dykes. But the detailed mapping, carried on by my colleagues and
myself in the Geological Survey, has brought to light some curious and
interesting variations from the normal trend. In the districts where
dykes of the gregarious type abound there is sometimes no one prevalent
direction, but the dykes strike to almost all points of the compass.
Of the Arran dykes, so carefully catalogued by Necker, only about a
third have a general north-westerly course. But in Eastern Argyleshire
the abundant dykes mapped by Mr. Clough trend almost without exception
towards N.N.W. In the North of Ireland, Berger found the direction
of thirty-one dykes to vary from 17° to 71° W. of N., giving a mean
of N. 36° W.[186] In Islay, Jura, Eigg, Mull, and Skye the mean of
several hundred observations has given me similar results. Among the
Inner Hebrides, however, though the general north-westerly trend is
characteristic, many of the later dykes show marked departures from it.
Thus in Strath, Skye, some of the youngest follow a nearly north and
south direction (Fig. 253). In the Blath Bhein hill-range, Mr. Harker
has found that the latest dykes cut the gabbro at right angles to the
prevalent trend and are further distinguished by their low hade.
[Footnote 185: _Trans. Roy. Soc. Edin._ xxii. (1861), p. 650.]
[Footnote 186: _Trans. Geol. Soc._ iii. p. 225.]
It appears, therefore, that though there is sometimes extraordinary
local diversity in the direction of the dykes in those districts where
they present the gregarious type, the general north-westerly trend
can usually still be recognized. But when we turn to the long massive
solitary dykes, we soon perceive a remarkable change in their direction
as we follow them northward into Scotland. I formerly pointed out how
the general north-westerly trend becomes east and west in the Lothians,
with a tendency to veer a little to the south of west and north of
east.[187] This departure from the normal direction is now seen to be
part of a remarkable radial arrangement of the dykes. Beginning at
the southern margin of the dyke-region, we have the notable example
of the Cleveland dyke, which in its course from Cleveland to Carlisle
runs nearly W. 15° N. The Eskdale dyke has an average trend of W.
32° N., and the same general direction is maintained by the group of
dykes which run from the Southern Uplands across the south-west of
Lanarkshire and north-east of Ayrshire. But proceeding northwards
we observe the trend to turn gradually round towards the west. The
dyke that runs from near the mouth of the Coquet across the Cheviot
Hills to beyond Hawick has a general course of W. 8° N. In the great
central coal-field of Scotland the average direction may be taken to
be nearly east and west, the same dyke running sometimes to the north,
and sometimes to the south of that line. But immediately to the north
a decided tendency to veer round southwards makes its appearance. Thus
the long dyke which runs from the Carse of Stirling through the Campsie
Fells to the Clyde west of Leven, has a mean direction of W. 5° S. This
continues to be the prevalent trend of the remarkable series of dykes
which crosses the Old Red Sandstone plains, though some of these revert
in whole or in part to the more usual direction by keeping a little to
the north of west. Even as far as Loch Tay and the head of Strathardle,
the course of the dykes continues to be to the south of west. Tracing
these lines upon a map of the country we perceive that they radiate
from an area lying along the eastern part of Argyleshire and the head
of the Firth of Clyde (see Map I.).
[Footnote 187: _Trans. Roy. Soc. Edin._ xxii. p. 651.]
[Illustration: Fig. 241.--Section along the line of the Cleveland Dyke
at Cliff Ridge, Guisbrough (G. Barrow).
Scale, 12 inches to 1 mile.]
10. TERMINATION UPWARDS
It was pointed out many years ago by Winch that some of the dykes
which traverse the Northumberland coal-field do not cut the overlying
Magnesian Limestone. The Hett dyke, south of Durham, is said to end
off abruptly against the floor of the limestone.[188] Here and there,
among the precipices of the Inner Hebrides, a dyke may be seen to die
out before it reaches the top of the cliff. But in the vast majority
of cases, no evidence remains as to how the dykes terminated upwards.
I have referred to the occasional interruptions of the continuity of
a dyke, where, though the rock does not reach the surface, it must be
present in the fissure underneath. Such interruptions show that, in
some places at least, there was no rise of the rock even up to the
level of what is now the surface of the ground, and that the upward
limit of the dykes must have been exceedingly irregular.
[Footnote 188: This is expressed in the Geological Survey Map, Sheet 93,
N.E.]
Excellent illustrations of this feature are supplied by sections on
the line of the Cleveland dyke. Towards its south-easterly extremity,
this great band of igneous rock ascends from the low Triassic plain of
the Tees into the high uplands of Cleveland. Its course across the
ridges and valleys there has been carefully traced for the Geological
Survey by Mr. G. Barrow, who has shown that over certain parts of its
course it does not reach the surface, but remains concealed under the
Jurassic rocks, which it never succeeded in penetrating. But that in
places it comes within a few feet of the soil is shown by the baked
shale at the surface, for the alteration which it has induced on the
surrounding rocks only extends a few feet from its margin. These
interruptions of continuity show how uneven is the upper limit of the
dyke. The characteristic porphyritic rock may be observed running up
one side of a hill to the crest, but never reaching the surface on the
other side. At Cliff Ridge, for example, about three miles south-west
of Guisbrough, Mr. Barrow has followed it up to the summit on the
west side; but has found that on the east side it does not pierce the
shales, which there form the declivity. This structure is represented
in Fig 241. The vertical distance between the summit to the left, where
the dyke (_b_) disappears, and the point to the right, where the Lias
shale (_a_) of the hillside is concealed by drift (_c_), amounts to 250
feet, the horizontal distance being a little more than 900 feet. But as
the shale when last seen at the foot of the slope is quite unaltered,
the dyke must there be still some little distance beneath the surface,
so that the vertical extension of this upward tongue of the dyke must
be more than 250 feet. Mr. Barrow, to whom I am indebted for these
particulars, has also drawn the accompanying section (Fig. 242) along
the course of the dyke for a distance of nearly 11 miles eastward from
the locality represented in Fig. 241. From this section it will be
observed that in that space there are at least three tongues or upward
projections of the upper limit of the dyke. Several additional examples
of the same structure are to be seen further east towards the last
visible outcrop of the dyke.
[Illustration: Fig. 242.--Section along the course of the Cleveland
Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the _Memoirs of
the Geol. Surrey_, Geology of Cleveland, p. 61).
_a_, Liassic shales, sandstones and ironstones; _b_, the dyke.]
Another feature connected with the upward termination of the dyke is
well seen in some parts of the ground through which the two foregoing
sections are taken. Mr. Barrow informs me that at Ayton a level course
has been driven into the hill for mining operations, at a height of 400
feet above sea-level, and the dyke has there been ascertained to be 80
feet broad. Higher on the hill, close to the 750 feet contour--line,
its breadth is only 20 feet, so that it narrows upward as much as 60
feet in a vertical height of 350 feet. Its contraction in width during
the last twenty feet is still more rapid, and in the last few yards it
diminishes to two or three feet, and has a rounded top over which the
strata are bent upward. The accompanying section (Fig. 243) across the
upper part of the dyke will make these features clear.
[Illustration: Fig. 243.--Section across the extreme upper limit of
Cleveland Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow).
_a_ _a_, Jurassic shales, etc.; _b_, Dyke.]
[Illustration: Fig. 244.--Upper limit of Cleveland Dyke in quarry near
Cockfield (after Mr. Teall).
_a_ _a_, Carboniferous shales; _b_, dyke.]
Further to the west an exposure of the upper limit of the dyke has been
described and figured by Mr. Teall. In 1882, at one of the Cockfield
quarries (Fig. 244), the dyke was "seen to terminate upwards very
abruptly in the form of a low and somewhat irregular dome, over which
the Coal-measure shales passed without any fracture, and only with a
slight upward arching."[189]
[Footnote 189: _Quart. Jour. Geol. Soc._ xl. p. 210.]
Near the other or north-western termination of this great dyke, similar
evidence is found of an uneven upper limit. After an interrupted course
through the Alston moors, the dyke reaches the ground that slopes
eastward from the edge of the Cross Fell escarpment. Its highest
visible outcrop is at a height of 1700 feet. But westwards from that
point the dyke disappears under the Carboniferous rocks, and does not
emerge along the front of the great escarpment that descends upon the
valley of the Eden, where among the naked scarps of rock it would
unquestionably be visible if it reached the surface. Its upper edge
must rapidly descend somewhere behind the face of the escarpment, for
the igneous rock crops out a little to the west of the foot of the
cliff, about 1000 feet below the point where it is last seen on the
hills above. Here the top of the dyke has a vertical drop of not less
than 1000 feet, in a horizontal distance of five miles, as shown in
Fig. 245, which has been drawn for me by Mr. J. G. Goodchild.
It will be observed that in these sections (Figs. 241, 242 and 245)
there is a curiously approximate coincidence between the inequalities
in the upper surface of the dyke and those in the form of the overlying
ground. The coincidence is too marked and too often repeated to be
merely accidental. Whether the ancient topographical features had any
influence in determining, by cooling or otherwise, the limit of the
upward rise of the lava, or whether the dyke, even though concealed,
has affected the progress of the denudation of the ground overlying it,
is a question worthy of fuller investigation.
[Illustration: Fig 245.--Section along the course of the Cleveland Dyke
across the Cross Fell escarpment. The shaded part shows the position of
the dyke, the unshaded part overlying it marks where the dyke does not
reach the surface. Scale of one inch to one mile.]
11. KNOWN VERTICAL EXTENSION
Closely connected with the determination of the upper limit reached by
the dykes, is the total vertical distance to which they can be traced.
Of course, the depth of the original reservoir of molten rock which
supplied them remains unknown, and probably undiscoverable. But it is
possible, in many cases, to determine at least the inferior limit of
the thickness of rock through which the molten material of the dykes
has ascended. Along the great basalt-escarpments of Mull and Skye,
the ascent of dykes from base to summit may often be observed. Thus,
on the cliffs of Dunvegan Head, on the west coast of Skye, which rise
out of the sea to a height of about 1000 feet, several dykes may be
observed rising through the whole series of basalts up to the crest of
the precipice. In the dark gabbro hills of the same island, numerous
dykes may be seen climbing from the glens right up the steep rugged
acclivities and over the crests, through a vertical thickness of more
than 3000 feet of rock (Fig. 333). The dykes which cross Loch Lomond,
and ascend the hills on either side of that deep depression, must rise
through at least as great a thickness. But where a knowledge of the
geological structure of the ground enables us to estimate the bulk of
the successive rock-formations which underlie the surface, it can be
shown that the lava ascended through a much greater depth of rock.
Measurements of this kind can best be made towards the eastern end
of the Cleveland dyke, where the different sedimentary groups have
not been seriously disturbed, and where, from natural sections and
artificial borings, their thicknesses are capable of satisfactory
computation. The highest bed of the Jurassic series anywhere touched
by the dyke is the Cornbrash. It is certain, therefore, that the
igneous rock rises through all the subjacent members of the Jurassic
series up to that horizon. There can be no doubt also that the Trias
and Magnesian Limestone continue in their normal thickness underneath
the Jurassic strata. To what extent the Coal-measures exist under
Cleveland has not been ascertained; possibly they have been entirely
denuded from that area, as from the ground to the west. But the
Millstone Grit and Carboniferous Limestone probably extend over the
district in full development; and below them there must lie a vast
depth of Upper and Lower Silurian strata, probably also of still older
Palæozoic rocks and beneath all the thick Archæan platform. Tabulating
these successive geological formations, and taking only the ascertained
thickness of each in the district, we find that they give the results
shown in the subjoined table.[190]
[Footnote 190: Drawn up for me by Mr. G. Barrow.]
STRATA CUT BY THE CLEVELAND DYKE
Cornbrash--
Feet.
Lower Oolite and Upper Lias, as proved by bore-hole on
Gerrick Moor, 950
Middle and Lower Lias, ascertained from measurement of
cliff-sections and from mining operations to be more than 850
New Red Sandstone and Marl, found by boring close to the Tees
to exceed 1,600
Magnesian Limestone, at least 500
Coal-measures, possibly absent 0
Millstone Grit, not less than 500
Carboniferous Limestone series at least 3,000
Silurian rocks, probably not less than 10,000
-------
17,400
There is thus evidence that this dyke has risen through probably more
than three miles of stratified rocks. How much deeper still lay the
original reservoir of molten material that supplied the dyke, we have
at present no means of computing.
12. EVIDENCE AS TO MOVEMENT OF THE MOLTEN ROCK IN THE FISSURES
It is usual to speak of the molten material of the dykes as having
risen vertically within the fissures. And doubtless, on the whole,
the expression is sufficiently accurate. In the case of such long
dykes as those of Central Scotland and the North of England, where the
petrographical character of the material remains so uniform throughout,
it is obvious that the andesite or dolerite cannot have come from a
mere single pipe like a volcanic orifice. Nor can we easily understand
how it could have been supplied even from a series of such pipes. The
general aspect and structure of the dykes suggest that the fissures
were rent so profoundly in the crust of the earth as to reach down to
a reservoir of molten rock which straightway rose in them. The roof of
such a reservoir, however, may have been irregular and uneven, so that
a fissure need not have traversed it continuously, but may have only
touched its upward projecting vaults. Hence gaps would arise in the
continuity of the dyke-material.
The ascent of lava from a line of such separate openings along a
fissure would necessarily involve lateral as well as vertical movements
in the molten mass which would be forced along the open rent until
the several streams united and filled it up. We might therefore
expect somewhere to find instances of flow-structure in the dykes
pointing to these movements. I have already referred to the lines of
amygdales frequently noticed in dykes, especially towards the centre.
Occasionally these steam-vesicles may be observed to be drawn out in
one general direction indicative of the trend of motion of the molten
rock.
Some of the best examples of this feature which have come under my
observation occur among the trachytic dykes of the south-east coast of
Skye between Kyle Rhea and Loch na Daal, where they have been mapped
and carefully investigated by Mr. Clough, who has conducted me over the
sections. In some of these dykes, as already narrated, the marginal
portions display a finely spherulitic structure, the small pea-like
spherulites being grouped into fine ribs or rods. It is also observable
that the steam-vesicles which may retain their spherical forms in the
centre are elongated in the same direction as the rows of spherulites.
Where this lineation is developed vertically, it no doubt points to the
vertical ascent of the lava between the two walls of the fissure.
But in other examples, the elongation is nearly horizontal, and between
the two positions Mr. Clough has registered many intermediate trends.
It would thus appear that in some places the lava has certainly
flowed laterally between the fissure-walls. Moreover, the trend of
the spherulitic rods and of the amygdales is found to vary in closely
adjoining planes at different distances from the margin, as if after
the outer portions of the dyke had consolidated into position, there
was still movement enough to drag the rows of spherulites and vesicles
up or down along the trend of the fissure.
Mr. Clough has observed that in some dykes, while the amygdaloidal
vesicles are large and undeformed in the centre, they become elongated
and inclined downward in the direction of the margin, as if the central
portions had not only remained fluid longer than the rest, but had a
tendency to rise upwards in the fissure, though there was obviously
less motion after these central vesicles appeared than in the marginal
parts where the vesicles are so much drawn out.
13. BRANCHING DYKES AND VEINS
It might have been anticipated that the uprise of such abundant masses
of molten rock, in so many long and wide fissures, would generally be
attended with the intrusion of the same material into lateral rents and
irregular openings, so that each dyke would have a kind of fringe of
offshoots or processes striking from it into the surrounding ground.
It might have been expected also that dykes would often branch, and
that the arms would come together again and enclose portions of the
rocks through which they rise. But in reality such excrescences and
bifurcations are of comparatively rare occurrence. As a rule, each
dyke is a mere wall of igneous rock, with little more projection or
ramification than may be seen in a stone field-fence. Among the short,
narrow and irregular dykes of the gregarious type branchings are
occasionally seen, and in some districts are extraordinarily abundant.
But among the great single dykes such irregularities are far less
common than might have been looked for. A few characteristic examples
from each type of dyke may here be given.
[Illustration: Fig. 246.--Branching portion of the great Dyke near
Hawick (length about one mile).]
[Illustration: Fig. 247.--Branching Dyke at foot of Glen Artney (length
about four miles).]
The Cleveland dyke, which in so many respects is typical of the great
solitary dykes of the country, has been traced for many miles without
the appearance of a single offshoot of any kind. Yet here and there
along its course, it departs from its usual regularity. As it crosses
the Carboniferous tracts of Durham and Cumberland, there appear near
its course lateral masses of eruptive rock, most of which doubtless
belong to the much older "Whin Sill." But there is at least one
locality, at Bolam near Cockfield, in the county of Durham, where
the dyke, crossing the Millstone Grit, suddenly expands into a boss,
and immediately contracts to its usual dimensions. Around this knot
several short dykes or veins seem to radiate from it. The dyke has been
quarried here, and its relations to the surrounding strata have been
laid bare, as will be again referred to a little further on.[191]
[Footnote 191: This locality was well described by Sedgwick, in his early
paper on Trap-Dykes in Yorkshire and Durham, _Trans. Cambridge Phil.
Soc._ ii. p. 27.]
Among the great persistent dykes of Scotland the absence of bifurcation
and lateral offshoots offers a striking contrast to the behaviour of
the dykes in those districts where they are small in size and many in
number. But exceptions to the general rule may be gathered. Thus the
Eskdale dyke is flanked at Wat Carrick with a large lateral vein, which
is almost certainly connected with the main fissure. The Hawick and
Cheviot dyke splits up on the hill immediately to the east of the town
of Hawick, sends off some branches, and then resumes its normal course
(Fig. 246). Again, one of the two nearly parallel dykes which run from
Lochgoilhead across Ben Ledi into Glen Artney bifurcates at the foot of
that valley, its northern limb (about two miles long) speedily dying
out, and its southern branch throwing off another lateral vein, and
then continuing eastward as the main dyke (Fig. 247).
In the districts of gregarious dykes, however, abundant instances
may be found of dykes that branch, and of others that lose the
parallelism of their walls, become irregular in breadth, direction, and
inclination, so as to pass into those intrusive forms that are more
properly classed as veins. Excellent illustrations of bifurcating dykes
may be observed along the shores of the Firth of Clyde, particularly
on the eastern coast-line of the isle of Arran. The venous character
has become familiar to geologists from the sketches given by Macculloch
from the lower parts of the cliffs of Trotternish in Skye.[192] Still
more striking examples are to be seen in the breaker-beaten cliffs of
Ardnamurchan. The pale Secondary limestones and calcareous sandstones
of that locality are traversed by a series of dark basic veins, and the
contrast of tint between the two kinds of rock is so marked as even to
catch the eye of casual tourists in the passing steamboats. The veins
vary in width from less than an inch to several feet or yards. They run
in all directions and intersect each other, forming such a confused
medley as requires some patience on the part of the geologist who
would follow out each independent ribbon of injected material in its
course up the cliffs, or still more, would sketch their ramifications
in his note-book. A good, though perhaps somewhat exaggerated,
illustration of their general character was given by Macculloch.[193]
The accompanying figure (Fig. 248) is less sensational, but represents
with as much accuracy as I could reach, the network of veins near the
foot of the cliffs. One conspicuous group of veins, which, seen from
a distance, looks like a rude sketch of a lug-sail traced in black
outline upon a pale ground, is known to the boatmen as "M'Niven's
Sail." Another admirable locality for the study of dykes and tortuous
veins is the northern coast of the Sound of Soa, where an extraordinary
number of injections traverse the Torridon Sandstones on which the
plateau-basalts rest (Fig. 323).
[Footnote 192: _Western Islands_, plate xvii.]
[Footnote 193: _Op. cit._ plate xxxiii. Fig. 1.]
As a general rule, the narrower the vein the finer in grain is the
rock of which it consists. This compact dark homogeneous material has
commonly passed by the name of "basalt." Its minuteness of texture
probably in most cases arises from local rapidity of cooling, and
it is doubtless the same substance which, where in larger mass in
the immediate neighbourhood, has solidified as one of the other
pyroxene-plagioclase-magnetite rocks.
With regard to the places where such abundant tortuous veins are more
especially developed, I may remark that they are particularly prominent
under a thick overlying mass of erupted rock, such as a great intrusive
sheet, or the bedded basalts of the plateaux, or where there is good
reason to believe that such a deep cover, though now removed by
denudation, once overspread the area in which they appear. It will be
shown in the sequel that such horizons have been peculiarly liable to
intrusions of igneous material of various kinds, and at many different
intervals, during the volcanic period. A thick cake of crystalline rock
seems to have offered such resistance to the uprise of molten material
through it, that when the subterranean energy was not sufficient to
rend it open by great fissures, and thus give rise to dykes, the lavas
were either forced into such irregular cracks as were made partly in
the softer rocks underneath and partly in the cake itself, or found
escape along pre-existing divisional planes. In Ardnamurchan, round the
Cuillin Hills of Skye, and in Rum, the overlying resisting cover now
consists mainly of gabbro sheets. In the east of Skye, in Eigg, and in
Antrim, it is made up of the thick mass of the plateau-basalts.
[Illustration: Fig. 248.--Basic veins traversing secondary limestone
and sandstone on the coast cliffs, Ardnamurchan.]
14. CONNECTION OF DYKES WITH SILLS
Every field-geologist is aware how seldom he can actually find the vent
or pipe up which rose the igneous rock that supplied the material of
sills and laccolites. He might well be pardoned were he to anticipate
that, in a district much traversed by dykes, there should be many
examples of intrusive sheets and frequent opportunities of tracing the
connection of such sheets with the fissures from which their material
might be supposed to have been supplied. But such an expectation is
singularly disappointed by an actual examination of the Tertiary
volcanic region of Britain. That there are many intrusive sheets
belonging to the great volcanic period with which I am now dealing,
I shall endeavour to show in the sequel. But it is quite certain that
though these sheets have of course each had its subterranean pipe
or fissure of supply, they can only in rare instances be directly
traced to the system of dykes. On the other hand, the districts
where great single dykes are most conspicuous, are for the most part
free from intrusive sheets, except those of much older date, like
the Carboniferous Whin Sill of Durham and those of Linlithgowshire,
Stirlingshire and Fife.
Yet a few interesting examples of the relation of dykes to sheets
have been noticed among British Tertiary volcanic rocks. The earliest
observed instances were those figured and described by Macculloch.
Among them one has been familiar to geologists from having done duty
in text-books of the science for more than half a century. I allude to
the diagram of "Trap and Sandstone near Suishnish."[194] In that drawing
seven dykes are shown as rising vertically through the horizontal
sandstone, and merging into a thick overlying mass of "trap." The
author in his explanation leaves it an open question "whether the
intruding material has ascended from below and overflowed the strata,
or has descended from the mass," though from the language he uses in
his text we may infer that he was inclined to regard the overlying body
as the source of the veins below it.[195]
[Footnote 194: _Western Islands of Scotland_, pl. xiv. Fig. 4.]
[Footnote 195: _Op. cit._ vol. i. pp. 384, 385.]
[Illustration: Fig. 249.--Section showing the connection of a Dyke with
an Intrusive Sheet, Point of Suisnish, Skye.
_g_, Granophyre of Carn Dearg; _f_, similar rock, which appears
eastward under the "sill" (_d_); _e_, intrusive sheet of
fine-grained "basalt"; _d_, intrusive sheet or sill of coarse
dolerite, 200 feet thick at its maximum, and rapidly thinning out;
_c_, dyke or pipe of finer grain than _d_; _b_, yellowish-brown
shaly sandstones, and _a_, dark sandy shales (Lias).
]
The section given by Macculloch, however, does not quite accurately
represent the facts. The narrow dykes there drawn have no connection
with the overlying sheet, but are part of the abundant series of
basaltic dykes found all over Skye. The feeder of the gabbro sill was
presumably the broad dyke which descends the steep bank immediately
on the southern front of Carn Dearg (636 feet high). The accompanying
figure (Fig. 249) shows what seemed to me to be the structure of the
locality, but the actual junction of the dyke and sheet is concealed
under the talus of the slope.[196] I shall have occasion in a later
Chapter to refer again to this section in connection with the history
of intrusive sheets, and also to cite from the neighbouring island of
Raasay another good example of the same relation between dyke and sill.
[Footnote 196: In more recently surveying this ground, Mr. Harker
has been led to regard the coarse sill as independent of the other
intrusions, and as almost certainly later than the basalt-sheets of
the same locality. When it reaches the base of these sills it turns so
as to pass beneath them as a gabbro-sill, which is conspicuous near
the summit of Carn Dearg. It runs westward for some distance, almost
immediately breaking across the bedding so as to leave the basalt, and
rapidly tapering until it dies out.]
Sedgwick, in the paper above quoted, gave an account and figure of the
expansion of the Cleveland dyke at Bolam, to which allusion has already
been made. He showed that from a part of the dyke which is unusually
contracted a great lateral extension of the igneous rock takes place on
either side over beds of shale and coal. While in the dyke the prisms
are as usual directed horizontally inward from the two walls, those
in the connected sheet are vertical, and descend upon the surface of
highly indurated strata on which the sheet rests.
The most important examples known to me are those which occur in
the coal-field of Stirlingshire. In that part of the country, the
remarkable group of dykes already referred to, lying nearly parallel
to each other and from half a mile to about three miles apart, runs in
a general east and west direction. From one of these dykes no fewer
than four sills strike off into the surrounding Coal-measures. The
largest of them stretches southwards for three miles, but the same
rock is probably continued in a succession of detached areas which
spread westwards through the coal-field and circle round to near the
two western sheets that proceeded from the same dyke. Another thick
mass of similar rock extends on the north side of the dyke for two
and a half miles down the valley of the river Avon. These various
processes, attached to or diverging from the dyke, are unquestionably
intrusive sheets, which occupy different horizons in the Carboniferous
series. The one on the north side has inserted itself a little above
the top of the Carboniferous Limestone series. Those on the south side
lie on different levels in the Coal-measures, or, rather, they pass
transgressively from one platform to another in that group of strata.
[Illustration: Fig. 250.--Section to show the connection of a Dyke with
an Intrusive Sheet, Stirlingshire Coal-field.
_a_, Dyke in line of fault; _b_, Sill traversing and altering the coal;
_i_, Slaty-band Ironstone.]
No essential difference can be detected by the naked eye between the
material of the dyke and that of the sheets. If a series of specimens
from the different exposures were mixed up, it would be impossible to
separate those of the dyke from those of the sheets. A microscopical
examination of the specimens likewise shows that they are perfectly
identical in composition and structure, being chiefly referable to
rocks of the dolerite, but partly of the tholeiite type. I have
therefore little doubt that these remarkable appendages to this dyke
are truly offshoots from it, and are not to be classed with the general
mass of the sills of Central Scotland, which are of Carboniferous,
partly of Permian, age. The accompanying diagrammatic section (Fig.
250) explains the geological structure of the ground.
An interesting and important fact remains to be stated in connection
with these sheets. They are traversed by some of the other east
and west dykes. This is particularly observable in the case of the
sheet which extends northwards from the dyke through the parish of
Torphichen. Two well-marked dykes can be seen running westwards among
the ridges of the sheet. It is obvious, therefore that these particular
dykes are younger than the sheet. But, as will be shown in the sequel,
there is abundant evidence that all the dykes of a district are not of
one eruption. The intersection of one eruptive mass by another does
not necessarily imply any long interval of time between them. They
mark successive, but it may be rapidly successive, manifestations of
volcanic action. Hence the cutting of the sheets by other dykes does
not invalidate the identification of these sheets as extravasations
from the great dyke by which they are bounded.
15. INTERSECTION OF DYKES
[Illustration: Fig. 251.--Intersection of Dykes in bedded basalt,
Calliach Point, Mull.]
Innumerable instances may be cited, where one dyke, or one set
of dykes, cuts across another. To some of these I shall refer in
discussing the data for estimating the relative ages of dykes. In
considering the intersection from the point of view of geological
structure, we are struck with the clean sharp way in which it so
generally takes place. The rents into which the younger dykes have
been injected seem, as a rule, not to have been sensibly influenced
in width and direction by the older dykes, but go right across them.
Hence the younger dykes retain their usual breadth and trend (Fig.
251). In trying to ascertain the relative ages of such dykes we obtain
a valuable clue in studying the respective "chilled edges" of the two
intersecting masses, as has already been pointed out.
Not only do dykes cross each other, but still more is this the case
among the narrower tortuous intrusions known as Veins (Fig. 252). Among
the illustrations which the dykes of the Inner Hebrides supply of these
features one further characteristic example may be culled from the
shore of Skye, near Broadford, where the gently-inclined sheets of Lias
limestone are traversed by three systems of dykes (Fig. 253). One of
these systems runs in a N.W. or N.N.W. direction, a second follows a
more nearly easterly trend, while the third and youngest runs nearly
north and south.
[Illustration:
Fig. 252.--Basalt Veins traversing bedded dolerites, Kildonan, Eigg.
]
[Illustration:
Fig. 253.--Ground-plan of intersecting Dykes in Lias limestone,
Shore, Harrabol, East of Broadford, Skye.
]
16. DYKES OF MORE THAN ONE IN-FILLING
The intersections of dykes prove that the process of fissuring in
the earth's crust took place at more than one period, and prepare us
for the reception of evidence that the same line of fissure might be
again re-opened, even after it had been filled with molten material.
Numerous instances have now been accumulated in which dykes are not
single or simple intrusions, but where the original dyke-fissure has
been re-opened and has been invaded by successive uprisings of lava.[197]
Compound dykes have thus been formed, consisting of two or more
parallel bands of similar or dissimilar rock.
[Footnote 197: See an example figured by Macculloch, _Western Isles_,
plate xviii. Fig. 1.]
While it is not difficult to conceive of the re-opening of a vertical
fissure during terrestrial strain, and the injection into it of later
intrusions of a volcanic magma, it is not so easy to understand the
mechanism where the line of weakness has been slightly inclined or
horizontal, and where, consequently, there has been the enormous
superincumbent pressure of the overlying part of the earth's crust to
overcome. Yet gently inclined compound dykes exhibit their parallel
bands with hardly less regularity than do those that are vertical.
The difficulty of explanation is felt most strongly in the attempt to
realize the origin of the compound sills described in Chapter xlviii.
In the re-opening of dyke-fissures the later intrusions have generally
taken place along the walls, or where the dykes were already compound,
between some of the component bands. Less frequently the first dyke has
been split open along the middle, and a second injection has forced its
way along the rent.
Of the first of these two types, numerous instances have now been
observed in the West of Scotland. If the portion of a compound dyke
exposed at the surface be limited in extent, we may be unable to
determine which is the older of two parallel bands of igneous rock,
though the fact that they present to each other the usual fine-grained
edge due to more rapid cooling, shows that they are not one but two
dykes, belonging to distinct eruptions. So far as I have noticed,
where one of the dykes can be continuously traced for a considerable
distance, the other is comparatively short. I infer that the shorter
one is the younger of the two.
In the Strath district of Skye, Mr. Harker has recently observed that
many of the basic dykes, both those older and those younger than the
granophyre protrusions, are double, triple or multiple. Thus in a
conspicuous dyke, more than 100 feet wide, to the south-east of Loch
Kilchrist, belonging to the older series, he has detected at least six
contiguous dykes which as they are traced south-eastward, in spite
of their interruption by the Beinn an Dubhaich granite, can be seen
to separate and take different courses, or successively die out. He
remarks, further, that "many cases of apparent bifurcation of dykes are
really due to the separation of distinct dykes which have run for some
distance in one fissure. Sometimes apparent variations in the width of
a dyke are to be explained by this dying out of one member of a double
dyke. These multiple dykes are less easily detected in the newer than
the older set, owing to greater uniformity of lithological type in the
prevalent kinds and to the frequent absence of chilled selvages."[198] An
example of a compound basic dyke cutting the crest of the gabbro-mass
of the Cuillin Hills is shown in Fig. 333.
[Footnote 198: MS. notes supplied by Mr. Harker.]
Instances of the second type of compound dykes are less common. Here,
instead of being re-opened along one of the walls, the fissure has
been ruptured along the centre of the dyke, and a second injection of
molten material has then taken place. This structure may be observed
where the materials of the compound dyke are on the whole similar,
such as varieties of dolerite, basalt, diabase or andesite. In these
cases the rock of the central dyke is generally rather fine-grained,
sometimes decidedly porphyritic, and often a true basalt. Where broad
enough to show the difference of texture between margin and centre, it
exhibits the usual close grain along its edges, indicative of quicker
cooling. The older dyke presenting no such change at its junction with
the younger, was obviously already cooled and consolidated before its
rupture.
Whilst the centre of a dyke has occasionally proved to be a line of
weakness which has given way under intense strains in the terrestrial
crust, this rupture and the accompanying or subsequent ascent of molten
material in the re-opened fissure may sometimes have been included as
phases of one connected volcanic episode. In those instances, for
example, which have been above described, where a central vitreous band
has risen along the heart of a dyke, the petrographical affinities of
the rocks may be so close as to suggest that although the main dyke had
consolidated and had subsequently been ruptured along its centre by
powerful earth-movements, these changes all belonged to the same period
of dyke-making, and the subsequent uprise of glassy material was merely
a later phase in the movements of the same subterranean magma.
But where, as probably happens in the large majority of compound dykes,
there is a strongly marked difference between the respective bands
of rock, we must either infer that two essentially different magmas
co-existed in the volcanic reservoirs underneath, and were successively
injected into the same fissures, or that a sufficient lapse of time
occurred to permit a total renewal of the nature of the magma, and an
uprise of this changed material into fissures which sometimes coincided
with older dykes. If any interlocking of the crystals of the several
bands of a compound dyke could be detected, we might suppose that the
first-injected material had not become consolidated and cold before
the uprise of the newer rock. But in general it would seem that so
sharp a line of demarcation can be drawn between the two rocks as to
indicate that their protrusion was due to two distinct and perhaps
widely-separated volcanic paroxysms.
Compound dykes of basic material occur not only among the ordinary
straight north-westerly series, but also among the less regular
gregarious dykes and veins, such as abundantly intersect the gabbro
bosses. Moreover they are to be found among the youngest intrusions,
for they traverse the masses of granophyre. Conspicuous examples of
such late compound dykes are displayed along the cliffs of St. Kilda,
as will be more particularly described in a later Chapter. These St.
Kilda dykes often occupy not vertical fissures but parallel rents with
a gentle inclination (see Figs. 367, 368).
The Tertiary volcanic series of Scotland furnishes many examples of
compound dykes of a much more complex character where parallel bands
of some acid (granophyre, felsite, quartz-porphyry) or intermediate
(andesite) rock is associated with others of the more usual basic
material (dolerite, basalt, diabase). As the acid intrusions belong
to a comparatively late part of the volcanic history, their modes of
occurrence will be discussed in Chapters xlvi., xlvii. and xlviii. But
no account of the general system of dykes would be complete without
some reference to these compound examples, which will therefore be
briefly described in the present section of this work.
Early in this century some striking illustrations of the association
of acid and more basic rocks within the same fissure were noticed by
Jameson in the island of Arran. He described and figured instances at
Tormore, on the west side of that island, where a group of pitchstones
and "basalts" or andesites have been successively protruded into
the same fissures in the (probably Permian) red sandstones of that
district.[199]
[Footnote 199: _Mineralogy of the Scottish Isles_, 1800.]
In some instances the more basic rock has been first injected, and has
subsequently been disrupted, by the more acid pitchstone. In other
cases the order has been the reverse. The successive ruptures have
taken place sometimes along the centre, sometimes at the margins, and
sometimes irregularly along the breadth of the dykes. Professor Judd
has recently studied these rocks, and has given descriptions of their
chemical composition and microscopic characters. He regards them as
having been successively injected into the fissures from the same
subterranean reservoir, in which two magmas of very different chemical
constitution were simultaneously present.[200]
[Footnote 200: _Quart. Jour. Geol. Soc._ vol. xlix. (1893), p. 536.
Full details of the compound dykes of Tormore and Cir Mhor in Arran,
and references to previous writers will be found in this paper. The
probable age of the youngest eruptive rocks of this island will be
discussed in Chapter xlvii. p. 418.]
Nowhere in the Tertiary volcanic regions of Britain do compound dykes
appear to be so abundant as in the centre and southern part of the
island of Skye. During the progress of the Geological Survey in that
district, Mr. Clough and Mr. Harker have mapped a large number in the
ground between the Sound of Sleat and the Red Hills. With regard to
these dykes Mr. Harker observes that the several members are generally
petrographically different, some being basic, others intermediate,
and others acid. "There is usually," he remarks, "a symmetrical
disposition, two similar and more basic dykes being divided by a more
acid one; for example, two andesites separated by a pitchstone. Thus at
the mouth of the little stream which runs from Torran into the bay east
from Dùn Beag a dyke, apparently 18 feet wide, is found on examination
to consist of a central dyke (specific gravity 2·86) flanked by two
more basic dykes (specific gravity 3·02)."
In the great majority of examples hitherto observed in Skye the two
lateral dykes consist of some basic rock (diabase or basalt), while
the central and thickest band is of some acid material (granophyre or
quartz-felsite). This triple arrangement occurs both in dykes and sills.
[Illustration: Fig. 254.--Compound dyke, Market Stance, Broadford, Skye.
_a_, Granophyre; _b_ _b_, Basalt; _c_ _c_, Torridon sandstone.]
As an illustration of the association of the two kinds of rock in
dykes I may cite an example which appears on the southern edge of the
Market Stance of Broadford (Fig. 254). Here the characteristic triple
arrangement is typically developed. A central light-coloured band,
about eight to ten feet broad, consists of a spherulitic granophyre
in which the spherulites are crowded together and project from the
weathered surface like peas, though they do not here show the curious
rod-like aggregation so marked in some other dykes. On either side of
this acid centre a narrow basalt dyke intervenes as a wall next to the
Torridon sandstone which here forms the country-rock. Such compound
dykes have sometimes a total width of 100 feet or more.
In this instance, and generally throughout the district, there is
nothing to indicate that the different bands of the dyke have any
relation to each other as connected uprises of material from the
same original magma which was either heterogeneous or was undergoing
a process of differentiation beneath the terrestrial crust. On the
contrary, the several parts of each dyke are as distinctly marked off
from each other as they could have been had they been injected at
widely separated intervals of volcanic activity.
Mr. Harker, in the course of his survey of this Skye ground, has
observed that "where evidence is available, the central acid dyke
is found to be newer than the basic ones. It has not split a single
basic dyke, but has insinuated itself between the two members of a
double dyke. This is more clearly seen when the acid magma has been
forced into a triple or multiple basic dyke; the perfect symmetry
of arrangement may in this case be lost. For instance, on the shore
north-east of Corry, Broadford, a 13 feet dyke of granophyre occurs in
a multiple dyke of basalt, but it has taken its line so as to leave
only a one-foot dyke on one side, and a group with a total width of
12 feet on the other. Also it has not accurately kept its course, but
has cut obliquely across one of the group of dykes alluded to. In some
cases it is certain that the acid magma has to some extent dissolved a
portion of the wall of a basic dyke with which it has come in contact.
This may account for the magma finding its easiest path along, and
especially between, pre-existing more basic dykes." This subject will
be again referred to in Chapter xlviii., when the phenomena of compound
sills are discussed.
Before closing this account of compound dykes, I may remark that no
examples have yet been observed among the ordinary Tertiary dykes
of Britain where, by a process of differentiation between the walls
of a fissure, successive zones have been developed in the dyke,
differing from each other in structure and composition, but becoming
progressively and insensibly more acid towards the centre, such as
have been described from the older rocks of Norway and Canada. Among
the Tertiary gabbro bosses, indeed, there occur sheets or dykes which
present a remarkably banded structure, to which full reference will be
made in later pages. But I have never seen anything at all resembling
such a structure among the dykes of andesite, dolerite, or basalt.
17. CONTACT-METAMORPHISM OF THE DYKES
A geologist might naturally expect that such abundant intrusions of
igneous rock as those of the dykes should be accompanied with plentiful
proofs of contact-metamorphism. But in actual fact, evidence of any
serious amount of alteration is singularly scarce. A slight induration
of the rocks on either side of a dyke is generally all the change that
can be detected.
[Illustration: Fig. 255.--Section of coal rendered columnar by
intrusive basalt, shore, Saltcoats, Ayrshire.
_a_, Fireclay; _b_, Coal rendered prismatic near the basalt; _c_, Dark
shale; _d_, Basalt-rock.]
Some of the larger dykes, however, show more marked metamorphism, the
nature of which appears in many cases to be chiefly determined by
the chemical composition of the rock affected. Thus a considerable
alteration has been superinduced on carbonaceous strata, particularly
on seams of coal. In the Ayrshire coal-field the alteration of the coal
extends sometimes 150 feet from the dyke, the extent of the change
depending not merely on the mass of the igneous rock, but on the nature
of the coal, and possibly on other causes. Close to a dyke, coal passes
into a kind of soot or cinder, sometimes assumes the form of a finely
columnar coke (Fig. 255), and occasionally has become vesicular after
being fused.[201] Shales are converted into a hard flinty substance that
breaks with a conchoidal fracture and rings under the hammer. Fireclay
is baked into a porcelain-like material. Limestone is changed for a few
inches into marble. As an illustration of this alteration, I may cite a
dyke ten feet broad which cuts through the chalk in the Templepatrick
Quarry, Antrim. For about six inches from the igneous rock the chalk
has passed into a finely saccharoid condition, and its organisms are
effaced. But beyond that distance the crystalline structure rapidly
dies away, the micro-organisms begin to make their appearance, and
within a space of one foot from the dyke the chalk assumes its ordinary
character.
[Footnote 201: Explanation of Sheet 22, Geological Survey of Scotland, p.
26.]
Sandstones are indurated by dykes into a kind of quartzite, sometimes
assume a columnar structure (the columns being directed away from
the dyke-walls), and for several feet or yards have their yellow or
red colours bleached out of them. The granite of Ben Cruachan where
quarried on Loch Awe, as I am informed by Mr. J. S. Grant Wilson, is
traversed by a basic dyke, and for a distance of about 20 feet is
rendered darker in colour, becomes granular, and cannot be polished and
made saleable.
Where many dykes have been crowded together, their collective effects
in the alteration of the strata traversed by them have sometimes been
strongly developed. One of the most remarkable illustrations of this
influence is presented by the district of Strathaird, which was cited
by Macculloch for the abundance of its dykes. In recently mapping
this ground for the Geological Survey, Mr. Harker has observed in
some places a score or more dykes in actual juxtaposition, while over
considerable distances he found it difficult to detect any trace of
the Jurassic strata, through which the igneous rocks have ascended.
As might be expected under these circumstances, such portions of
the strata as can be seen display an altogether exceptional amount
of contact-metamorphism. Mr. Harker has noticed some limestones at
Camasunary which have been changed into very remarkable lime-silicate
rocks, with singular bunches of diopside crystals.
These, however, are the extremes of contact-metamorphism by the
Tertiary basic dykes. A geologist visiting the Liassic shores of Strath
in Skye will not fail to be surprised at the very slight degree of
alteration in circumstances where he would have expected to find it
strongly pronounced. The dark shales, though ribbed across with dykes,
are sometimes hardly even hardened, and at the most are only indurated
from an inch or two to about two feet. These baked bands project above
the rest of the more easily denuded shales, and so adhere to the dykes
as almost to seem part of them. Again the limestones, where traversed
by dykes some distance apart, are not rendered in any appreciable
degree more crystalline even up to the very margin of the intrusive
rock. Where the igneous material has been thrust between the strata
in sills, it has produced far more general and serious metamorphism
than when it occurs in the form of single dykes. The famous rock of
Portrush, already referred to as having been once gravely cited as an
example of fossiliferous basalt, is a good illustration of the way in
which Lias shale is porcellanized when the intruded igneous material
has been thrust between the planes of bedding.
In the West of Scotland, where dykes are so abundantly developed,
considerable differences can be observed between the amount of
metamorphism superinduced by adjacent dykes which may be of the same
thickness, and cut through the same kind of strata. Such variations
have not probably arisen from differences in the temperature of the
original molten rock. Perhaps they are rather to be assigned to the
length of time occupied by the ascent of the lava in the fissure.
If, for instance, the fissure opened to the surface and discharged
lava there, the rocks of its walls would be exposed to a continuous
stream of molten rock as long as the outflow lasted. They would thus
have their temperature more highly raised, and maintained at such an
elevation for a longer time than where the magma, at once arrested
within the fissure, immediately proceeded to cool and consolidate
there. It would be an interesting and important conclusion if we could,
from the nature or amount of their contact-metamorphism, distinguish
those dykes which for some time served as channels for the discharge of
lava above ground.
Some dykes which have caught up fragments of older rocks in their
ascent have exercised a considerable solvent action on these
inclusions. Examples of this feature have already been cited from Skye,
where they have been studied by Mr. Harker (pp. 129, 163).
In connection with the metamorphism superinduced by dykes, reference
may again be made to the alteration which they themselves undergo
where they have invaded a carbonaceous shale or coal. The igneous
rock, as we have seen, loses its dark colour and obviously crystalline
structure, and becomes a pale yellow or white, dull, earthy substance,
or "white trap." The chemical changes involved in this alteration
have been described by Sir J. Lowthian Bell.[202] Dr. Stecher has also
discussed the alterations traceable by the aid of the microscope.[203]
Though most of the instances of such transformation in Britain occur
in the Carboniferous system, and have taken place in intrusive rocks
of probably, for the most part, Carboniferous or Permian age, yet they
are not unknown in the Tertiary volcanic series. Some of the "white
trap" of the Coal-measures may indeed belong to the Tertiary period,
but the coals and carbonaceous shales interstratified in the Tertiary
basalt-plateaux have reacted on both the superficial lavas and the
sills, and have given rise to the same kind of alteration as in the
Carboniferous system, as will be shown in a later Chapter.
[Footnote 202: _Proc. Roy. Soc._ xxiii. (1875), p. 543.]
[Footnote 203: Tschermak's _Mineralogische Mittheilungen_, ix. (1887), p.
145, and _Proc. Roy. Soc. Edin._ 1888.]
[Illustration:
Fig. 256.--Dolerite dyke with marginal bands of "white trap," in
black shale, Lower Lias, Pabba.
_a_, Black carbonaceous Lower Lias Shale; _b_ _b_, bands of
indurated shale from 15 inches to 2 feet broad; _c_, dolerite dyke
3 feet 3 inches broad; _d_ _d_, bands of altered dolerite or "white
trap," 3 to 5 inches broad.
]
Some marked examples of this alteration of intrusive igneous material
are to be observed among the basalt dykes which cut the Lower Lias
Shales of Skye. These shales, where black and carbonaceous, as in
the island of Pabba, have exercised an unmistakable influence on the
abundant dykes which intersect them. The chilled selvage of each dyke
has assumed the dull earthy pale-grey or yellowish aspect, which
extends for a few inches from the wall into the interior, where it
rapidly passes into the ordinary black crystalline basalt. These
features will be readily understood from the accompanying diagram (Fig.
256). Where the dykes give off narrow veins a few inches broad, these
consist entirely of the "white trap." The shales are often traversed
with strong joints parallel to the walls of the dykes, and the
transverse joints of the dykes are sometimes prolonged into the bands
of indurated shale.
18. RELATION OF DYKES TO THE GEOLOGICAL STRUCTURE OF THE DISTRICTS
WHICH THEY TRAVERSE.
In no respect do the Tertiary dykes of Britain stand more distinguished
from all the other rocks of the country than in their extraordinary
independence of geological structure. The successive groups of
Palæozoic and Mesozoic strata have been so tilted as to follow each
other in approximately parallel bands, which run obliquely across the
island from south-west to north-east. The most important lines of fault
take the same general line. The contemporaneously included igneous
rocks follow, of course, the trend of the stratified deposits among
which they lie, and even the intrusive sills group themselves along the
general strike of the whole country. But the Tertiary dykes have their
own independent direction, to which they adhere amid the extremest
diversities of geological arrangement.
In the first place, the dykes intersect nearly the whole range of the
geological formations of the British Islands. In the Outer Hebrides and
north-west Highlands, they rise through the most ancient (Lewisian)
gneisses, through the red pre-Cambrian (Torridon) sandstones, and
through the oldest members of the Cambrian system. In the southern
Highlands, they pursue their course across the gnarled and twisted
schists of the younger crystalline (Dalradian) series. In the South of
Scotland and North of England, they traverse the various subdivisions
of the Lower and Upper Silurian rocks. In the basins of the Tay, Forth,
and Clyde they cross the plains and ridges of the Old Red Sandstone,
with its deep pile of intercalated volcanic material. In Central
Scotland, and the northern English counties, they occur abundantly in
the Carboniferous system, and have destroyed the seams of coal. In
Cumberland and Durham, they traverse the Permian and Trias groups.
In Yorkshire, and along the West of Scotland, they are found running
through Jurassic strata. In Antrim, they intersect the Chalk. Both in
the North of Ireland, and all through the chain of the Inner Hebrides,
they abound in the great sheets and bosses of Tertiary volcanic rocks.
These are the youngest formations through which they rise. But it is
deserving of note, that they intersect every great group of these
Tertiary volcanic products, so that they include in their number the
latest known manifestations of eruptive action in the geological
history of Britain.[204]
[Footnote 204: They have not been found cutting the pitchstone-lava of
the Scuir of Eigg.]
In the second place, in ranging across groups of rock belonging to
such widely diverse periods, the dykes must necessarily often pass
abruptly from one kind of material and geological structure to another.
But, as a rule, they do so without any sensible deviation from their
usual trend, or any alteration of their average width. Here and there,
indeed, we may observe a dyke to follow a more wavy or more rapidly
sinuous or zig-zag course in one group of rocks than in another.
Yet, so far as I have myself been able to observe, such sinuosities
may occur in almost any kind of material, and are not satisfactorily
explicable by any difference of texture or arrangement in the rocks
at the surface. No dyke traverses a greater variety of sedimentary
formations than that of Cleveland. In the eastern part of its course,
it rises through all the Mesozoic groups up to the Cornbrash. Further
west it cuts across each of the different subdivisions of the
Carboniferous system; and, of course, it must traverse all the older
formations which underlie these. But the occasional rapid changes
noticeable in its width and direction do not seem to be referable to
any corresponding structure in the surrounding rocks. The Cheviot
dyke crosses from the Carboniferous area of Northumberland into the
Upper Silurian rocks and Lower Old Red Sandstone volcanic tract of
the Cheviot Hills. It then strikes across the Upper Old Red Sandstone
of Roxburghshire, and still maintaining the same persistent trend,
sweeps westward into the intensely plicated Silurian rocks of the
Southern Uplands. Its occasional deviations have no obvious reference
to any visible change of structure in the adjacent formations. Again,
some of the great dykes at the head of Clydesdale furnish striking
illustrations of entire indifference to the nature of the rock through
which they run. Quitting the Silurian uplands, they keep their line
across Old Red Sandstone and Carboniferous rocks, and through large
masses of eruptive material.
In the third place, not only are the dykes not deflected by great
diversities in the lithological character of the rocks which they
traverse, they even cross without deviation some of the most important
geological features in the general framework of the country. Some
of the Scottish examples are singularly impressive in this respect.
Those which strike north-westward from the uplands of Clydesdale
cross without deflection the great boundary-fault which, by a throw
of several thousand feet, brings the Lower Old Red Sandstone against
Silurian rocks. They traverse some large faults in the valley of the
Douglas coal-field, pass completely across the axis of the Haughshaw
Hills, where the Upper Silurian rocks are once more brought up to the
surface, and also the long felsite ridge of Priesthill. The dykes in
the centre of the kingdom maintain their line across some of the large
masses of igneous rock that protrude through the Carboniferous system.
Further north, the dykes of Perthshire cut across the great sheets of
volcanic material that form the Ochil Hills, as well as through the
piles of sandstone and conglomerate of the Lower Old Red Sandstone, and
then go right across the boundary-fault of the Highlands, to pursue
their way in the same independent manner through grit, quartzite, or
mica-schist, and across glen and lake, moor and mountain.
No one can contemplate these repeated examples of an entire want of
connection between the dykes and the nature and arrangement of the
rocks which they traverse without being convinced that the lines of
rent up which the material of the dykes rose were not, as a rule, old
fractures in the earth's crust, but were fresh fissures, opened across
the course of the older dislocations and strike of the country by the
same series of subterranean operations to which the uprise of the
molten material of the dykes was also due.
In the fourth place, the dykes for the most part are not coincident
with visible lines of fault. After the examination of hundreds of dykes
in all parts of the country, and with all the help which bare hillsides
and well-exposed coast-sections can afford, the number of instances
which have been met with where dykes have availed themselves of lines
of fault is surprisingly small. Some of these cases will be immediately
cited. To whatever cause we may ascribe the rupture of the solid crust
of the earth, which admitted the rise of molten rock to form the dykes,
there can be no doubt that it was not generally attended with that
displacement of level on one or both sides of the dislocation, which
we associate with the idea of a fault. Nowhere can this important part
of dyke-structure be more clearly illustrated than along the Cleveland
dyke, where the igneous rock rises through almost horizontal Jurassic
strata and gently inclined Coal-measures (Figs. 241, 242, 243, 244).
Besides the localities already cited, mining operations both for coal
and for the Liassic ironstone have proved over a wide area that the
dyke has not risen along a line of fault. Again, in Skye, Raasay,
Eigg, and other parts of the west coast, where Jurassic strata and
the horizontal basalts of the plateaux are plentifully cut through by
dykes, the same beds may generally be seen at the same level on either
side of them.
In the fifth place, while complete indifference to geological structure
is the general rule among the dykes, instances do occur in which the
molten material has found its way upward along old lines of rupture.
Most of such instances are to be found in districts where previously
existing faults happened to run in the same general direction as that
followed by the dykes. These lines of fracture might naturally be
re-opened by any great earth-movements acting in their direction, and
would afford ready channels for the ascent of the lava, as we have seen
to have not infrequently happened in the case of dyke-fissures, which
are shown by compound dykes to have sometimes been re-opened several
times in succession even after having been filled up with basalt. Yet
it is curious that, even when their trend would have suited the line
of the dykes, faults have not been more largely made use of for the
purpose of relief. Some of the best examples of the coincidence of
dykes with pre-existing faults in the same direction are to be found
in the Stirlingshire coal-field. The dyke that runs from Torphichen
for 23 miles to Cadder occupies a line of fault which at Slamannan has
a down-throw of more than 70 fathoms. The next dyke further south has
also risen along an east and west fault.
But other examples may be observed where pre-existing fissures have
served to deflect dykes from their usual line of trend. Thus the
Cleveland dyke, after crossing several faults in the Coal-measures, at
last encounters one near Cockfield Fell, which lies obliquely across
its path. Instead of crossing this fault it bends sharply round a few
points south of west, and after keeping along the southern flank of
the fault for about a mile, sinks out of reach. Some of the Scottish
examples are more remarkable. One of the best of them occurs in the
Sanquhar coal-field, where a dyke runs for two miles and a half along
the large fault that here brings down the Coal-measures against the
Lower Silurian rocks. At the north-western end of the basin, this fault
makes an abrupt bend of 60° to W.S.W., and the dyke turns round with
it, keeping this altered course for a mile and a half, when it strikes
away from the fault, crosses a narrow belt of Lower Silurian rocks,
and finds its way into the parallel boundary fault which defines the
north-western margin of the Southern Uplands.
[Illustration: Fig. 257.--Map of the chief dykes between Lochs Riddon
and Striven (C. T. Clough, Geological Survey, Sheet 29). The large E.
and W. dyke is a continuation of that which reaches the shore of the
Firth of Clyde at Dunoon.]
Some of the Perthshire dykes, where they reach the great boundary-fault
of the Highlands, present specially interesting features. There can
be no doubt that this dislocation is one of the most important in the
general framework of the British Isles, though no definite estimate has
yet been formed of how much rock has been actually displaced by it. The
fact that in one place the beds of Old Red Sandstone are thrown on end
for some two miles back from it, shows that it must be a very powerful
fracture. Here, therefore, if anywhere, either an entire cessation of
the dykes, or at least a complete deflection of their course might be
anticipated. It would require, we might suppose, a singularly potent
dislocation to open a way for the ascent of the lava through such
crushed and compressed rocks, and still more to prolong the general
line of a fracture across the old fault. Two great dykes, about half a
mile apart, run in a direction a little south of west across the plain
of Strathearn. Passing to the south of the village of Crieff, they hold
on their way until they reach the highly-inclined beds of sandstone
and conglomerate which here lean against the Highland fault in Glen
Artney. They then turn round towards south-west, and run up the glen
along the strike of the beds, keeping approximately parallel to the
fault for about three miles, when they both strike across the fault,
and pursue a W.S.W. line through the contorted crystalline rocks of
the Highlands. About two miles further south, another dyke continues
its normal course across the belt of upturned Old Red Sandstone; but
when it reaches the fault it bends round and follows the line of
dislocation, sometimes coinciding with, sometimes crossing or running
parallel with that line, at a short distance (see Fig. 247).
Some remarkable examples have been mapped by Mr. Clough in Eastern
Argyleshire, where broad bands of basalt or other allied rock run in a
N. and S. direction, and are formed by the confluence of N.W and S.E.
or N.N.W. and S.S.E. dykes, where these are drawn into a line of fault
(Fig. 257). These broad bands, he has found to be not usually traceable
for more than a mile or so, for the dykes of which they are made up
will not be diverted from their regular paths for more than a certain
distance, so that one by one the dykes leave the compound band to
pursue their normal course. He has observed that the occasional great
thickness of these compound bands depends partly on the size and partly
on the number of separate dykes that are diverted into the line of
transverse fissure; for, where the fissure crosses an area with fewer
north-west dykes, the band becomes thinner or ceases altogether.
In some rare cases, the dykes have been shifted by more recent faults.
I shall have occasion to show that faults of more than 1000 feet have
taken place since the Tertiary basalt-plateaux were formed. There is
therefore no reason why here and there a fault with a low hade should
not have shifted the outcrop of a dyke. But the fact remains, that,
as a general rule, the dykes run independently of faults even where
they approach close to them. Mr. Clough has observed some interesting
cases in South-eastern Argyleshire, where the apparent shifting of
a dyke by faults proves to be deceptive, and where the dyke has for
short distances merely availed itself of old lines of fracture. One
of the most remarkable of these is presented by the large dyke which
runs westward from Dunoon. No fewer than three times, in the course of
four miles between Lochs Striven and Riddon, does this dyke make sharp
changes of trend nearly at right angles to its usual direction, where
it encounters north and south faults (Fig. 257). It would be natural to
conclude that these changes are actual dislocations due to the faults.
But the careful observer just cited has been able to trace the dyke in
a very attenuated and uncrushed form along some of these cross faults,
and thus to prove that the faults are of older date, but that they
have modified the line of the long east and west fissure up which the
material of the dyke ascended.
19. DATA FOR ESTIMATING THE GEOLOGICAL AGE OF THE DYKES
I have already assigned reasons for regarding the system of north-west
and south-east or east and west dykes as belonging to the Tertiary
volcanic period in the geographical history of the British Islands.
But I have no evidence that they were restricted to any part of that
period. On the contrary, there is every reason to consider the uprise
of the earliest and latest dykes to have been separated by a protracted
interval. That they do not all belong to one epoch has been already
indicated, and may now be more specially proved.
The intersection of one dyke by another furnishes an obvious criterion
of relative age. Macculloch drew attention to this test, and stated
that it had enabled him to make out two distinct sets of dykes in Skye
and Rum. But he confessed that it failed to afford any information as
to the length of the interval of time between them.[205] It is not always
so easy as might be thought to make sure which of two intersecting
dykes is the older. As was explained in Chapter vi. (vol. i. p. 81),
we have to look for the finer-grained marginal strip at the edge of a
dyke, which, where traceable across another dyke, marks at once their
relative age. The cross joints of the two dykes also run in different
directions. Reference may again be made to the illustration given in
Fig. 253 where three distinct groups of dykes intersect each other as
they traverse the Lias limestones of Skye. The chilled edges and the
different arrangement of joints mark these dykes out from each other,
while the order in which they cross each other furnishes a clue to
their relative age. If from such sections, repeated in different parts
of a district, certain persistent petrographical characters can be
ascertained to distinguish each particular system of dykes, a guide may
thereby be obtained for the chronological grouping of the intrusions
even where evidence of actual intersection is not visible. In the case
just cited from Skye, the later north and south dykes are characterized
by their lines of vesicular cavities and by the large porphyritic
felspars which they contain.
[Footnote 205: _Trans. Geol. Soc._ iii. p. 75.]
It is obvious, however, that although sections of this kind suffice to
prove the dykes to belong to distinct periods of intrusion, no longer
interval need have elapsed between their successive production than was
required for the solidification and assumption of a joint-structure by
an older dyke before a younger broke through it. They may both belong
to one brief period of volcanic activity. But when we pass to a series
of dykes traversing a considerable district of country, and find that
those which run in one direction are invariably cut by those which
run in another, the inference can hardly be resisted that they do not
belong to the same period of eruption, but mark successive epochs of
volcanic energy. An excellent example of this kind of evidence is
furnished by Mr. Clough from Eastern Argyleshire. The east and west
dykes in that district are undoubtedly older than those which run
in a N.N.W. direction (Fig. 257).[206] The latter are by far the most
abundant, and are on the whole much narrower, less persistent, and
finer in grain. On the opposite coast of the Clyde, a similar double
set of dykes may be traced through Renfrewshire, those in an east and
west direction being comparatively few, while the younger N.N.W. series
is well developed. The great sheets or "sills" connected with one of
the Stirlingshire dykes, already described, appear to me to furnish
similar evidence in the younger dykes which run through them. And this
evidence is peculiarly valuable, for it shows a succession even among
adjacent dykes which all run in the same general direction.
[Footnote 206: As already stated, Mr. Clough and also Mr. Gunn are
inclined to separate these older east and west dykes from the Tertiary
series and to regard them as probably of late Palæozoic age.]
But in all these cases it is obvious that we have little indication of
the length of time that intervened between the successive injections
of the dykes. In Skye, however, more definite evidence presents itself
that the interval must have been in some cases a protracted one.
As far back as the year 1857,[207] I showed that the basic dykes of
Strath in Skye are of two ages; that one set was erupted before the
appearance of the "syenite" (granophyre) of that district, and was cut
off by the latter rock; and that the other arose after the "syenite"
which it intersected. Recent re-examination has enabled me to confirm
and extend this observation. The younger series which traverses the
granophyre is much less numerous than the older series in the same
districts. In Chapter xlvi., where the relations of the granophyres
to other members of the volcanic series will be discussed, further
details will be given from that region of Skye to demonstrate that
there is a pre-granophyre and a post-granophyre series of basic dykes.
As a good illustration of the younger series I may refer to the way
in which these rocks make their appearance in the island group of St.
Kilda, where both the gabbros and granophyres of the Tertiary volcanic
series are characteristically developed. Numerous dykes traverse both
these rocks. Those in the gabbro are more abundant than those in the
granophyre--a circumstance which is exactly paralleled among the basic
and acid bosses of Skye. It is not improbable that in these remote
islands a similar difference in age and in petrographical character
may be made out between two series of dykes, one older and the other
younger than the granophyre. There is ample proof, at all events, of a
post-granophyre series.
[Footnote 207: _Quart. Jour. Geol. Soc._ vol. xiv. p. 16.]
[Illustration: Fig. 258.--Basalt-veins traversing granophyre, St.
Kilda.]
The pale colour of the precipices in which the St. Kilda granophyre
plunges into the sea gives special prominence to the dark ribbon-like
streaks which mark the course of basalt-dykes through that rock.
Moreover the greater liability of the material of the dykes to decay
causes them to weather into long lines of notch or recess. Four or five
such dykes follow each other in nearly parallel bands, which slant
upward from the sea-level on the eastern face of the hill Conacher to a
height of several hundred feet.[208] (Fig. 258, see also Fig. 367.)
[Footnote 208: This relation of the later dykes to the granophyre was
observed here by Macculloch (_Western Isles_, vol. ii. p. 55).]
The acid eruptions of the Inner Hebrides are marked by so varied a
series of rocks, and so complex a geological structure, that they may,
with some confidence, be regarded as having occupied a considerable
interval of geological time. Yet we find that this prolonged episode
in the volcanic history was both preceded and followed by the
extravasation of basic dykes.
Reference has already been made to recent observations by Mr. Harker,
who, in mapping the Strath district of Skye for the Geological Survey,
has not only confirmed the generalization as to the existence of a
series of dykes earlier, and another later, than the great granophyre
protrusions of the Inner Hebrides, but has made some progress towards
the detection of a means of distinguishing the two series even where no
direct test of their relative age may be available. He thinks that the
general habit and petrographical characters of the dykes may on further
investigation be found to afford a sufficiently reliable basis for
discrimination. He finds that where the relative ages of the dykes with
reference to the granophyre can be fixed, the earlier or pre-granophyre
series is without exception basic. It consists of fine-textured basalts
or diabases, without any conspicuous porphyritic crystals. Its dykes
are less regular and persistent in their bearing than those of the
later series; have frequently a considerable hade, even as much as 45°,
and often show chilled edges with tachylitic selvages. In Skye many of
these earlier dykes may be connected with the gabbro. They appear to
be more basic and to have a higher specific gravity than those of the
later series which most resemble them.
The later or post-granophyre dykes include several types, the relative
ages of which are not yet definitely fixed. They run in straight
parallel lines, and thus seldom intersect each other. They are
generally vertical or highly inclined, and are much more frequently
characterized by amygdaloiclal structure than the earlier series.
Mr. Harker distinguishes the following varieties among them: (_a_)
Quartz-felsites and other acid rocks; these are not very common.
(_b_) Pitchstones and various spherulitic and variolitic rocks: the
actual pitchstones observed are comparatively few in number, but it is
certain that some of spherulitic varieties are devitrified pitchstones.
(_c_) Basic rocks, not conspicuously porphyritic and less decidedly
basic than the dykes of the pre-granophyre series; most of the later
groups come into this or the next group, (_d_) Porphyritic basic dykes
not infrequently carrying inclusions of gabbro, granophyre or other
rocks. The porphyritic felspars seem to be in great part of foreign
derivation, and the same is certainly true of the augite which
occasionally accompanies them and of the quartz that appears in some
examples.[209]
[Footnote 209: Annual Report of the Director-General of the Geological
Survey in Report of Science and Art Department for 1895.]
In the Carlingford district of the North-east of Ireland, similar
evidence has been obtained that one series of dykes preceded and
another followed the protrusion of the granites and granophyre which
are in all probability geologically coeval with the acid bosses of the
Inner Hebrides. The distinction was observed and mapped by Mr. Traill
for the Geological Survey. Professor Sollas in recently confirming
these observations has not noticed any striking difference between the
pre-granite and post-granite dykes, the whole appearing to consist of
the same coarsely porphyritic material.[210]
[Footnote 210: See Sheets 59, 60, and 71 of the Geological Survey Map of
Ireland; Professor Sollas, _Trans. Roy. Irish Acad._ vol. xxx. (1894),
p. 477; and Annual Report of the Director-General of the Geological
Survey for 1895.]
While the eruption of the granophyre bosses furnishes proof that the
dykes are not all of the same age, other evidence may be gathered
to show how much older some of the dykes are than the youngest
lava-streams in the volcanic history of Tertiary time in Britain.
The Scuir of Eigg, to which fuller reference will be made in Chapter
xxxviii., is formed of a mass of pitchstone, which has filled up an
ancient valley eroded out of the terraced basalts of the plateaux. At
both ends of the ridge, these basalts are seen to be traversed by dykes
that are abruptly cut off by the shingle of the old river-bed which
the pitchstone has occupied (Figs. 279, 282). It is thus evident that,
though these dykes are younger than the plateau-basalts, they are much
older than the excavation of the valley out of these basalts, and still
older than the eruption of pitchstone. The latter rock probably belongs
to the close of the period of lava-eruptions. The enormous denudation
of the basalt-plateaux after the injection of the dykes and before the
outflow of the pitchstone affords a convincing proof of the vastness of
the interval between the eruption of the two kinds of rock.[211]
[Footnote 211: _Quart. Jour. Geol. Soc._ xiv. p. 1.]
It is thus demonstrable that the dykes which in Britain form part of
the great Tertiary volcanic series, were not all produced at one epoch,
but belong to at least two (and probably to many more) episodes in one
long volcanic history. As they rise through every member of that series
of rocks (save the pitchstones), some of them must be among the latest
records of the prolonged volcanic activity. But, on the other hand,
some probably go back to the very beginning of the Tertiary volcanic
period.
20. ORIGIN AND HISTORY OF THE DYKES
Reference has already been made to the doubt expressed by Macculloch
whether the dykes in Skye had been filled in from above or from below.
That the dykes of the country as a whole were supplied from above,
was the view entertained and enforced by Boué. He introduces the
subject with the following remarks:--"Scotland is renowned for the
number of its basaltic veins, which gave Hutton his ideas regarding
the injection of lava from below; but, as the greatest genius is not
infallible, and as volcanic countries present us with examples of such
veins arising evidently from accidental fissures that were filled
up by currents of lava which moved over them, and as the Scottish
instances are of the same kind, we regard it as infinitely probable
that all these veins have been formed in the same way notwithstanding
the enormous denudation which this supposition involves; and that only
rarely do cases occur where they have been filled laterally or in some
other irregular manner."[212] I need not say that this view, which,
except among Wernerians, had never many supporters, has long ago been
abandoned and forgotten. There is no further question that the molten
material came from below.
[Footnote 212: _Essai Géologique sur l'Écosse_, p. 272.]
1. In discussing the history of the dykes, we are first confronted
with the problem of the formation of the fissures up which the molten
material rose. From what has been said above regarding the usual want
of relation between dykes and the nature and arrangements of the rocks
which they traverse, it is, I think, manifest that the fissures could
not have been caused by any superficial action, such as that which
produces cracks of the ground during earthquake-shocks. The fact that
they traverse rocks of the most extreme diversities of elasticity,
structure, and resistance, and yet maintain the same persistent trend
through them all, shows that they originated far below the limits to
which the known rocks of the surface descend. We have seen that in the
case of the Cleveland dyke, the fissure can be proved to be at least
some three miles deep. But the seat of the origin of the rents no doubt
lay much deeper down within the earth's crust.
It is also evident that the cause which gave rise to these abundant
fissures must have been quite distinct from the movements that produced
the prevalent strike and the main faults of this country. From early
geological time, as is well known, the movements of the earth's crust
beneath the area of Britain, have been directed in such a manner as
to give the different stratified formations a general north-east and
south-west strike, and to dislocate them by great faults with the same
average trend. But the fissures of the Tertiary dykes run obliquely
and even at a right angle across this prevalent older series of lines
and are distinct from any other architectonic feature in the geology
of the country. They did not arise therefore by a mere renewal of some
previous order of disturbances, but were brought about by a new set of
movements to which it is difficult to find any parallel in the earlier
records of the region.[213]
[Footnote 213: The only other known example of such a dyke-structure in
Britain is that of the Pre-Cambrian series of dykes in the Lewisian
gneiss of Sutherland, described in Chapter viii.]
We have further to remember that the fissures were not produced merely
by one great disturbance. The evidence of the dykes proves beyond
question that some of them are earlier than others, and hence that
the cause to which the fissures owed their origin came into operation
repeatedly during the protracted Tertiary volcanic period. One of
the most instructive lessons in this respect is furnished by the huge
eruptive masses of gabbro and granitoid rocks in Skye. These materials
have been erupted through the plateau-basalts. The granitoid bosses
are the younger protrusions, for they send veins into the gabbros;
but their appearance was later than that of some of the dykes and
older than that of others. Nevertheless, the youngest dykes generally
maintain the usual north-westerly trend across the thickest masses of
the granophyre. Thus we perceive that, even after the extrusion of
thousands of feet of such solid crystalline igneous rocks, covering
areas of many square miles, the fissuring of the ground was renewed,
and rents were opened through these new piles of material. From the
evidence of the dykes also we learn that some fissures were repeatedly
re-opened and admitted a new ascent of molten magma between their
walls. The general direction of the fissures remained from first to
last tolerably uniform. Here and there indeed, where one set of dykes
traverses another, as in Skye and the basin of the Clyde, we meet with
proofs of a deviation from the normal trend. But it is remarkable that
dykes which pierce the latest eruptive bosses of the Inner Hebrides
rose in fissures that were opened in the normal north-westerly line
through these great protrusions of basic and acid rock.
Such a gigantic system of parallel fissures points to great horizontal
tension of the terrestrial crust over the area in which they are
developed. Hopkins, many years ago, discussed from the mathematical
side the cause of the production of such fissures.[214] He assumed the
existence of some elevatory force acting under considerable areas
of the earth's crust at any assignable depth, either with uniform
intensity at every point or with a somewhat greater intensity at
particular points. He did not assign to this force any definite origin,
but supposed it "to act upon the lower surface of the uplifted mass
through the medium of some fluid, which may be conceived to be an
elastic vapour, or, in other cases, a mass of matter in a state of
fusion from heat."[215] He showed that such an upheaving force would
produce in the affected territory a system of parallel longitudinal
fissures, which, when not far distant from each other, could only have
been formed simultaneously, and not successively; that each fissure
would begin not at the surface but at some depth below it, and would
be propagated with great velocity; that there would be more fissures
at greater than at lesser depths, many of them never reaching the
surface; that they would be of approximately uniform width, the mean
width tending to increase downwards; that continued elevation might
increase these fissures, but that new fissures in the same direction
would not arise in the separated blocks which would now be more or less
independent of each other; that subsequent subsidences would give rise
to transverse fissures, and by allowing the separated blocks to settle
down would cause irregularities in the width of the great parallel
fissures. He considered also the problem presented by those cases where
the ruptures of the terrestrial crust have been filled with igneous
matter, and now appear as dykes. "The results above obtained," he
says, "will manifestly hold equally, whether we suppose the uplifted
mass acted upon immediately through the medium of an elastic vapour
or by matter in a state of fusion in immediate contact with its
lower surface. In the latter case, however, this fused matter will
necessarily ascend into the fissures, and if maintained there till it
cools and solidifies, will present such phenomena as we now recognize
in dykes and veins of trap."
[Footnote 214: _Cambridge Phil. Trans._ vi. (1835), p. 1.]
[Footnote 215: _Ibid._ p. 10.]
The existence of a vast lake or reservoir of molten rock under the
fissure-region of Britain is demonstrated by the dykes. But, if we
inquire further what terrestrial operation led to the uprise of so vast
a body of lava towards the surface in older Tertiary time, we find that
as yet no satisfactory answer can be given.
2. In some districts the dykes can be connected with the gabbros which
occur as intrusive sills and irregular bosses in the basalt-plateaux
and among older rocks. The gabbros, however, are traversed by still
later dykes, which must then be independent of any visible mass of
these rocks. The connection of dykes with the gabbros is what we
might naturally expect to find, if the more coarsely crystalline rock
represents portions of the basic magma which consolidated at some
depth below the surface. If we could penetrate deep enough, it is not
improbable that the dykes might be found in large measure to shade
downward into vast bodies of gabbro. Such a relation has been observed
in the Yellowstone district, where Mr. Iddings has noticed that the
centre toward which the dykes of the Old Crandale volcano converge is a
large mass of granular gabbro, passing into diorite, the dykes becoming
rapidly coarser in grain as they approach the gabbro-core.[216]
[Footnote 216: _Journ. Geol._ i. (1893), p. 608.]
3. The rise of molten rock in thousands of fissures over so wide a
region is to my mind by far the most wonderful feature in the history
of volcanic action in Britain. The great plateaux of basalt, and the
mountainous bosses of rock by which they have been disrupted, are
undoubtedly the most obvious memorials of Tertiary volcanism. But,
after all, they are merely fragments restricted to limited districts.
The dykes, however, reveal to us the extraordinary fact that, at a
period so recent as older Tertiary time, there lay underneath the area
of Britain a reservoir or series of reservoirs of lava, the united
extent of which must have exceeded 40,000 square miles.
That the material of the dykes rose in general directly from below, and
was not, except locally, injected laterally along the open fissures,
may be inferred, although proof of such lateral injection on a small
scale may here and there be detected. The narrowness of the rents, and
their enormous relative length, make it physically impossible that
molten rock could have moved along them for more than short distances.
The usual homogeneous character of the dyke-rocks, the remarkable
scarcity of any broken-up consolidated fragments of them immersed in
a matrix of different grain, the general uniformity of composition
and structure from one end of a long dyke to another, the spherical
form of the amygdales, the usual paucity of fragments from the fissure
walls--all point to a quiet welling of the lava upward. Over the whole
of the region traversed by the dykes, from the hills of Yorkshire and
Lancashire to the remotest Hebrides, molten rock must have lain at a
depth, which, in one case, we know to have exceeded three miles, and
which was probably everywhere considerably greater than that limit.
Forced upwards, partly perhaps by pressure due to terrestrial
contraction and partly by the enormous expansive force of the gases
and vapours absorbed within it, the lava rose in thousands of fissures
that had been opened for it in the solid overlying crust. That in
most cases its ascent terminated short of the surface of the ground
may reasonably be inferred. At least, we know, that many dykes do
not reach the present surface, and that those which do have shared
in the enormous denudation of the surrounding country. That even in
the same dyke the lava rose hundreds of feet higher at one place than
at another is abundantly proved. When, however, we consider the vast
number of dykes that now come to the light of day, and reflect that
the visible portions of some of them differ more than 3000 feet from
each other in altitude, we can hardly escape the conviction that it
would be incredible that nowhere should the lava have flowed out at
the surface. Subsequent denudation has undoubtedly removed a great
thickness of rock from what was the surface of the ground during older
Tertiary time, and hundreds of dykes are now exposed that doubtless
originally lay deeply buried beneath the overlying part of the earth's
crust through which they failed to rise. But some relics, at least, of
the outflow of lava might be expected to have survived. I believe that
such relics remain to us in the great basalt-plateaux of Antrim and
the Inner Hebrides. These deep piles of almost horizontal sheets of
basalt, emanating from no great central volcanoes, but with evidence
of many local vents, appear to me to have proceeded in large measure
from dykes which, communicating with the surface of the ground, allowed
the molten material to flow out in successive streams with occasional
accompaniments of fragmentary ejections.[217] The structure of the
basalt-plateaux, and their mode of origin, will form the subject of the
next division of this volume.
[Footnote 217: It is interesting to note that in the great paper on
Physical Geology already cited, Hopkins considered the question of the
outflow of lava from the fissures which he discussed. "If the quantity
of fluid matter forced into these fissures," he says, "be more than
they can contain, it will, of course, be ejected over the surface; and
if this ejection take place from a considerable number of fissures, and
over a tolerably even surface, it is easy to conceive the formation
of a bed of the ejected matter of moderate and tolerably uniform
thickness, and of any extent" (_op. cit._ p. 71).]
We can hardly suppose that the lava flowed out only in the western
region of the existing plateaux. Probably it was most frequently
emitted and accumulated to the greatest depth in that area. But over
the centre of Scotland and North of England there may well have been
many places where dykes actually communicated with the outer air, and
allowed their molten material to stream over the surrounding country,
either from open fissures or from vents that rose along these. The
disappearance of such outflows need cause no surprise, when we consider
the extent of the denudation which many dykes demonstrate. I have
elsewhere shown that all over Scotland there is abundant proof that
hundreds and even thousands of feet of rock have been removed from
parts of the surface of the land since the time of the uprise of the
dykes.[218] The evidence of this denudation is singularly striking in
such districts as that of Loch Lomond, where the difference of level
between the outcrop of the dykes on the crest of the ridges and in
the bottom of the valleys exceeds 3000 feet. It is quite obvious, for
example, that had the deep hollow of Loch Lomond lain, as it now does,
in the pathway of these dykes, the molten rock, instead of ascending
to the summits of the hills, would have burst out on the floor of
the valley. We are, therefore, forced to admit that a deep glen and
lake-basin have been in great measure hollowed out since the time of
the dykes. If a depth of many hundreds of feet of hard crystalline
schists could have been removed in the interval, there need be no
difficulty in understanding that by the same process of waste, many
sheets of solid basalt may have been gradually stripped off the face of
Central Scotland and Northern England.
[Footnote 218: _Scenery of Scotland_, 2nd edit. (1887), p. 149. But see
the remarks already made (p. 150) on the curious coincidence sometimes
observable between the upper limit of a dyke and the overlying
inequalities of surface.]
The association of fissures and dykes with the accumulation of thick
and extensive volcanic plateaux, over so wide a region of North-western
Europe as from Antrim to the North of Iceland, finds its parallel in
different parts of the world. One of the closest analogies presents
itself among the Ghauts of the Bombay Presidency, where vast basaltic
sheets, probably of Cretaceous age, display topographical and
structural features closely similar to those of the Tertiary volcanic
plateaux of the British Isles. The dykes connected with these Indian
basaltic outflows correspond almost exactly in their general character
and stratigraphical relations to those of this country. They occur in
great numbers, rising through every rock in the district up to the
crests of the Ghauts, 4000 feet above the sea. They vary from 1 or 2
to 10, 20, 40, and even occasionally 100 or 150 feet in width, and are
often many miles in length. They observe a general parallelism in one
average direction, and show no perceptible difference in character even
when traced up to elevations of 3000 and 4000 feet.[219]
[Footnote 219: Mr. G. T. Clark, _Quart. Journ. Geol. Soc._ xxv. (1869) p.
163. For remarks on the connection of dykes with superficial lavas, see
_postea_, p. 268.]
Thousands of square miles in the Western States and Territories of the
American Union have been similarly flooded with basic lavas. Denudation
has not yet advanced far enough to lay bare much of the platform on
which these lavas rest. But the dykes that traverse the rocks outside
of the lava-deserts afford an example of the structure which will
ultimately be revealed when the wide and continuous basalt-plains shall
have been trenched by innumerable valleys and reduced to fragmentary
plateaux with lofty escarpments (p. 267).
It is to the modern eruptions of Iceland, however, that we turn for
the completest illustration of the phenomena connected with dykes
and fissures. An account of these eruptions will therefore be given
in Chapter xl. as an explanation of the history of the Tertiary
basalt-plateaux of Britain.
CHAPTER XXXVI
THE PLATEAUX
Nature and Arrangement of the Rocks: 1. Lavas.--Basalts, Dolerites,
Andesites--Structure of the Lavas in the Field--2. Fragmental
Rocks.--Agglomerates, Conglomerates, and Breccias--Tuffs and their
accompaniments.
We have now to consider the structure and history of those volcanic
masses which, during Tertiary time, were ejected to the surface
within the area of the British Islands, and now remain as extensive
plateaux. Short though the interval has been in a geological sense
since these rocks were erupted, it has been long enough to allow of
very considerable movements of the ground and of enormous denudation,
as will be more fully discussed in Chapters xlviii. and xlix. Hence
the superficial records of Tertiary volcanic action have been reduced
to a series of broken and isolated fragments. I have already stated
that no evidence now remains to show to what extent there were actual
superficial outbursts of volcanic material over much of the dyke-region
of Britain. The subsequent waste of the surface has been so enormous
that various lava-fields may quite possibly have stretched across parts
of England and Scotland, whence they have since been wholly stripped
off, leaving behind them only that wonderful system of dykes from which
their molten materials were supplied.
There can be little doubt, however, that whether or not other Phlegrean
fields extended over portions of the country whence they have since
been worn away, the chief volcanic tract lay in a broad and long hollow
that stretched from the south of Antrim to the Minch. From the southern
to the northern limit of the fragmentary lava-fields that remain in
this depression is a distance of some 250 miles, and the average
breadth of ground within which these lava-fields are preserved may be
taken to range from 20 to 50 miles. If, therefore, the sheets of basalt
and layers of tuff extended over the whole of this strip of country,
they covered a space of some 7000 or 8000 square miles. But they were
not confined to the area of the British Islands. Similar rocks rise
into an extensive plateau in the Faroe Islands, and it may reasonably
be conjectured that the remarkable submarine ridge which extends thence
to the North-west of Scotland, and separates the basin of the Atlantic
from that of the Arctic Ocean, is partly at least of volcanic origin.
Still further north come the extensive Tertiary basaltic plateaux of
Iceland, while others of like aspect and age cover a vast area in
Southern Greenland. Without contending that one continuous belt of
lava-streams stretched from Ireland to Iceland and Greenland, we can
have no doubt that in older Tertiary time the north-west of Europe was
the scene of more widely-extended volcanic activity than had shown
itself at any previous period in the geological history of the whole
continent. The present active vents of Iceland and Jan Mayen are not
improbably the descendants in uninterrupted succession of those that
supplied the materials of the Tertiary basaltic plateaux, the volcanic
fires slowly dying out from south to north. But so continuous and
stupendous has been the work of denudation in these northern regions,
where winds and waves, rain and frost, floe-ice and glaciers reach
their highest level of energy, that the present extensive sheets of
igneous rock can be regarded only as magnificent relics, the grandeur
of which furnishes some measure of the magnitude of the last episode in
the extended volcanic history of Britain.
The long and wide western valley in which the basalt-plateaux of this
country were accumulated seems, from a remote antiquity, to have been
a theatre of considerable geological activity. There are traces of
some such valley or depression even back in the period of the Torridon
Sandstone of the north-west. This formation, as we have seen, was laid
down between the great ridge of the Outer Hebrides and some other land
to the east, of which a few of the higher mountains, once buried under
the sandstone, are now being revealed by denudation between Loch Maree
and Loch Broom, and also in Assynt. The conglomerates and volcanic
rocks of Lorne may represent the site of one of the older water-basins
of this ancient hollow. The Carboniferous rocks, which run through the
North of Ireland, cross into Cantyre, and are found even as far north
as the Sound of Mull, mark how, in later Palæozoic time, the same
strip of country was a region of subsidence and sedimentation. During
the Mesozoic ages, similar operations were continued; the hollow sank
several thousand feet, and Jurassic strata to that depth filled it up.
Before the Cretaceous period, underground movements had disrupted and
irregularly upheaved the Jurassic deposits, and prolonged denudation
had worn them away, so that when the Cretaceous formations came to be
laid down on the once more subsiding depression, they were spread out
with a strong unconformability on everything older than themselves,
resting on many successive horizons of the Jurassic system, and passing
from these over to the submerged hillsides of the crystalline schists.
Yet again, after the accumulation of the Chalk, the sea-floor along
the same line was ridged up into land, and the Chalk, exposed to
denudation, was deeply trenched by valleys, and entirely removed from
wide tracts which it once covered.
It was in this long broad hollow, with its memorials of repeated
subsidences and upheavals, sedimentation and denudation, that the
vigour of subterranean energy at last showed itself in volcanic
outbreaks, and in the gradual piling up of the materials of the
basalt-plateaux. So far as we know, these outbursts were subærial.
At least no trace of any marine deposit has yet been found even at
the base of the pile of volcanic rocks. Sheet after sheet of lava was
poured out, until several thousand feet had accumulated, so as perhaps
to fill up the whole depression, and once more to change entirely the
aspect of the region. But the volcanic period, long and important as
it was in the geological history of the country, came to an end. It,
too, was merely an episode during which denudation still continued
active, and since which subterranean disturbance and superficial
erosion have again transformed the topography. In wandering over
these ancient lava-fields, we see on every hand the most stupendous
evidence of change. They have been dislocated by faults, sometimes with
a displacement of hundreds of feet, and have been hollowed out into
deep and wide valleys and arms of the sea. Their piles of solid rock,
hundreds of feet thick, have been totally stripped off from wide tracts
of ground which were once undoubtedly buried under them. Hence, late
though the volcanic events are in the long history of the land, they
are already separated from us by so vast an interval that there has
been time for cutting down the wide plateaux of basalt into a series of
mere scattered fragments. But the process of land-sculpture has been of
the utmost service to geology, for, by laying bare the inner structure
of these plateaux, it has provided materials of almost unequalled value
and extent for the study of one type of volcanic action.
I. NATURE AND ARRANGEMENT OF THE ROCKS OF THE PLATEAUX
The superficial outbursts of volcanic action during Tertiary time in
Britain are represented by a comparatively small variety of rocks.
These consist almost wholly of basalts, but include a number of less
basic rocks which may be classed as andesites. Many andesitic sheets,
like the andesitic dykes, have been intruded into the basalts, and are
really sills.
Besides the lavas of the basaltic-plateaux there are intercalated
deposits of tuffs and breccias and large masses of agglomerate. A brief
notice of the general petrography of the various constituents of the
plateaux and their mode of occurrence will here be given. The intrusive
bosses which have disrupted the superficial lavas will be discussed in
subsequent chapters.
i. LAVAS
1. _Petrographical Characters_
(_a_) _Basalts and Dolerites._--In external characters these rocks
range from coarsely crystalline varieties, in which the constituent
minerals may be more or less readily detected with the naked eye or a
field-lens, to dense black compounds in which only a few porphyritic
crystals may be megascopically visible. One of their characteristic
features is the presence of the ophitic structure, sometimes only
feebly developed, sometimes showing itself in great perfection.
Many of the rocks are holocrystalline, but usually show more or
less interstitial matter; in others the texture is finer, and the
interstitial matter more developed; in no case, as far as I have
observed, are there any glassy varieties, which are restricted to
the dykes and sills, though in some of the basalts the proportion of
glassy or incompletely devitrified substance is considerable. The
felspars are generally of the characteristic lath-shaped forms, and
are usually quite clear and fresh. The augite resembles that of the
dykes, occurring sometimes in large plates that enclose the felspars,
at other times in a finely granular form. Olivine is frequently
not to be detected, even by green alteration products. Magnetite
is sometimes present in such quantity as to affect the compass of
the field-geologist. Porphyritic varieties occur with large felspar
phenocrysts; but such varieties are, I think, less frequent among the
plateau-rocks than among the dykes. They are well developed in the west
part of the island of Canna, and have been described from the Faroe
islands. Occasionally the plateau lavas are full of enclosed fragments
of other rocks which have been carried up in the ascending magma.
(_b_) _Andesites and Trachytes._--Probably the majority of these rocks
where they occur intercalated between the basalts of the plateaux are,
as already remarked, intrusive sheets rather than true lavas. But
they have also been poured out intermittently among the basalts and
dolerites. The most extensive development of lavas which are readily
distinguishable from the group of plateau-basalts, and must be placed
in the present series, occurs in the island of Mull. These rocks form
part of a group of pale lavas which overlie the main mass of the
plateau-basalts, and cap the mountain Ben More, together with several
of its lofty neighbours. They are interstratified with true ophitic
dolerites, and basalts showing characteristic granular augite. They
are not so heavy as the ordinary plateau-lavas, their specific gravity
ranging from 2·55 to 2·74. Externally they are light grey in colour
and dull in texture, sometimes strongly amygdaloidal, sometimes with a
remarkable platy structure, which, in the process of weathering, causes
them to split up like stratified rocks. In some of their amygdaloidal
varieties the cells are filled with epidote, which also appears in the
fissures, and sometimes even as a constituent of the rock.
Specimens from this "pale group" of Ben More, when examined in thin
slices under the microscope, were found by Dr. Hatch to consist almost
wholly of felspar in minute laths or microlites, but in no instance
sufficiently definite for satisfactory determination. In one of them
he observed that each lath of felspar passed imperceptibly into
those adjacent to it; the double refraction being very weak, and the
twin-striation, if present, not being traceable.[220] More recently my
colleague, Mr. W. W. Watts, has looked at some of the same slides. He
is disposed to class the rocks rather with the trachytes than the
andesites. He remarks that "in the apparent holocrystalline character,
the size and shape of the felspars, the sort of damascened appearance
in polarized light, the finely scattered iron-ores and the presence
of a pale green hornblende, possibly augite, in small, often complex,
grains, these rocks much resemble the Carboniferous trachytes of the
Garlton Hills in Scotland."
[Footnote 220: In the course of my investigations I have had many
hundreds of thin slices cut from the Tertiary volcanic rocks for
microscopic determination. These I have myself studied in so far as
their microscopic structure appeared likely to aid in the investigation
of those larger questions of geological structure in which I was
more especially interested. But for further and more detailed study
I placed them with Dr. Hatch, who submitted to me the results of his
preliminary examination, and where these offered points of geological
import I availed myself of them in the memoir published in 1888 in
the _Transactions of the Royal Society of Edinburgh_. I have retained
most of these citations in their place in the present volume, and have
supplemented them by notes supplied to me from fresh observations by
Mr. Watts and Mr. Harker. Professor Judd, in a series of valuable
papers, has discussed the general petrography of the Tertiary volcanic
rocks (_Quart. Jour. Geog. Soc._ vols. xxxix. xli. xlii. xlvi. xlix.)]
One of the most interesting lavas of the Tertiary volcanic series is
the "pitchstone-porphyry" of the Scuir of Eigg. This rock, the latest
known outflow of lava in any of the volcanic areas of Britain, was
formerly classed with the acid series. Microscopical and chemical
analyses prove it, however, to be of intermediate composition, and
to be referable to the andesites or dacites. It is more particularly
described in Chapter xxxviii.
Professor Judd, collecting the andesitic rocks as a whole (both lavas
and sills), has grouped them into amphibole and mica-andesites, and
pyroxene-andesites.[221] The thick lumpy and non-persistent sheets of
these rocks sometimes found near the centres of protrusion of the
gabbros and granophyres are probably sills.
[Footnote 221: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 356.
Professor Judd has there described under the name of "propylites"
various members of the volcanic series which he believes to have
undergone alteration from solfataric action. I have not been able to
discover any trace of such action, but I have found that the lavas of
the plateaux assume a peculiar condition where they have been affected
by large intrusive masses of granophyre or gabbro. (See _postea_,
Chapter xlvi.)]
(_c_) _Rhyolites._--In the Antrim plateau a group of rhyolite bosses
occurs, some of which have been claimed as superficial lavas. In
some cases it can be demonstrated that they are intrusive, and in no
instance can they be decisively shown to have escaped in streams at
the surface. It is probable, however, that some of these bosses did
actually communicate with the outer air, for between the lower and
upper group of basalts in this plateau, bands of rhyolitic conglomerate
occur which may indicate the degradation of exposed masses of rhyolite.
The description of these Antrim bosses will be given in Chapter xlvii.,
in connection with the acid eruptive rocks of the Tertiary volcanic
series.
2. _Structure in the Field_
Passing now to the consideration of the lavas as they are built up
into the plateaux, we have to note their distinctive characters as
individual sheets of rock, and their influence on the topography of the
regions in which they occur. Every tourist who has sailed along the
cliffs of Antrim, Mull, Skye, or the Faroe Islands is familiar with the
singular terraced structure of the great volcanic escarpments which
stretch as mural precipices along these picturesque shores. Successive
sheets of lava, either horizontal or only gently inclined, rise above
each other from base to summit of the cliffs as parallel bars of brown
rock with intervening strips of bright green grassy slope.
The geologist who for the first time visits these coast-lines is
impressed by the persistence of the same lithological characters
giving rise to the same topographical features. He soon realises
that the plateaux, so imposingly truncated by the great escarpments
that spring from the edge of the sea, are built up essentially of
dark lavas--basalts and dolerites--and that fragmental volcanic
accompaniments, though here and there well developed, play, on the
whole, a quite insignificant part in the structure and composition
of these thick piles of volcanic material. Closer examination in the
field enables him to ascertain that, regarded as rock-masses, the lavas
include four distinct types:--
1st. Thick, massive, prismatic or rudely-jointed sheets, rather
more coarsely crystalline and obviously more durable than the other
types, inasmuch as they project in tabular ledges and tend to retain
perpendicular faces owing to the falling away of slices of the rock
along lines of vertical joints. Many rocks of this type are undoubtedly
intrusive sheets, and as such will be further referred to in a later
chapter. But the type includes also true superficial lavas which
show the characteristic slaggy or vesicular bands at their upper and
lower surfaces. The mere presence of such bands may not be enough,
indeed, absolutely to establish that the rock possessing them flowed
at the surface as a lava, for they are occasionally, though it must be
confessed rarely, exhibited by true sills. But the rough scoriaceous
top of a lava-stream, and the presence of fragments of this surface
in the overlying tuff, or wrapped round by the next succeeding lava,
sufficiently attest the true superficial outflow of the mass.
2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway
and Staffa, have long attracted notice as one of the most striking
topographical elements of the plateaux. Columnar structures are typical
of the more compact heavy basalts. A considerable variety is observable
in the degree of perfection of their development. Where they are least
definite, the rock is traversed by vertical joints, somewhat more
regular and close-set than those in the dolerites, by the intersection
of which it is separated into rude quadrangular or polygonal columns.
The true columnar structure is shown in two chief forms. (_a_) The rock
is divided into close-fitting parallel, usually six-sided columns; the
number of sides varying, however, from three up to nine. The columns
run the whole thickness of the bed, and vary from 8 or 10 to 40 or even
80 feet in length. They are segmented by cross joints which sometimes,
as at Giant's Causeway, take the ball-and-socket form. Occasionally
they are curved, as at the well-known Clam-shell cave of Staffa.
(_b_) The prisms are much smaller, and diverge in wavy groups crowded
confusedly over each other, but with a general tendency upwards. This
starch-like aggregation may be observed superposed directly upon the
more regular columnar form as at the Giant's Causeway and also at
Staffa. Excellent illustrations of both these types may be seen at many
points along the sea-cliffs of the Inner Hebrides; the western coast
of Skye, the south-west side of Mull, and the cliffs of the island of
Canna may be specially cited.
[Illustration: Fig. 259.--Section of scoriaceous and prismatic Basalt,
Camas Tharbernish, north shore of Canna Island.]
Though generally rather compact, becoming indeed dense, almost vitreous
rocks in some sheets, the columnar basalts are often more or less
cellular throughout, and highly slaggy along their upper and under
surfaces. In some cases, as in that of a prismatic sheet which overlies
the rough scoriaceous lava of Camas Tharbernish, in the island of
Canna, the rows of vesicles are disposed in lines parallel to the under
surface of the sheet (Fig. 259.)
As already remarked with regard to the massive, rudely-jointed sheets,
many of the most perfectly columnar rocks of the plateaux are not
superficial lavas, but intrusive sills, bosses or dykes. Conspicuous
examples of such sills are displayed on the coast of Trotternish in
Skye, and of the bosses and dykes at the eastern end of Canna. To these
further reference will be made in the sequel. It is not always possible
to be certain that columnar sheets which appear to be regularly
intercalated among the undoubted lavas of the volcanic series may not
be really intrusive. In some instances, indeed, we can demonstrate that
they are so, when after continuing perfectly parallel with the lavas
above and below them, they eventually break across them. One of the
most remarkable examples of this feature is supplied by the great sill
of the south-west of Stromö, in the Faroe Islands, of which I shall
give some account in Chapter xlii. (Figs. 312, 328, 329).
3rd. Slaggy or amygdaloidal lavas without any regular jointed
structure, but often with roughly scoriform upper and under layers, and
tending to decay into brown earthy debris. Some of the upper surfaces
of such sheets among the Tertiary basalt-plateaux must have resembled
the so-called "Aa" of the Sandwich Islands. A striking example of the
structure may be noticed at Camas Tharbernish, on the north coast of
the Island of Canna. There the hummocks on the upper surface of a
slaggy basalt measure about 15 feet in breadth, and rise about three
feet above the hollows between them, like a succession of waves (see
Fig. 259). The steam-holes are disposed in a general direction parallel
to the strike of the hummocks.
Great variety obtains in the size and shape of the vesicles. Huge
cavities a foot or more in diameter may occasionally be found, and from
such extremes every gradation may be traced down to minute pore-like
vacuoles that can hardly be made out even with a strong lens. In regard
to the deformation of the vesicles, it is a familiar general rule that
they have been drawn out in the direction of the flow of the original
lava. Occasionally they have become straight, narrow, sometimes
bifurcating pipes, several inches long, and only an eighth of an inch
or so in diameter.[222] A number of such pipes, parallel to each other,
resembles a row of worm-burrows (see Fig. 2).
[Footnote 222: Some examples have been deposited by me in the Museum
of Practical Geology, Jermyn Street, in the case illustrating
rock-structures. The elongation of the vesicles into annelide-like
tubes may also be observed among the stones in the volcanic
agglomerates.]
It may often be noticed that, even where the basalt is most perfectly
prismatic, it presents a cellular and even slaggy structure at the
bottom. The rock that forms the Giant's Causeway, for instance, is
distinctly vesicular, the vesicles being drawn out in a general east
and west direction. The beautiful columnar bed of Staffa is likewise
slaggy and amygdaloidal for a foot or so upwards from its base, and
portions of this lower layer have here and there been caught up and
involved in the more compact material above it. Even the bottom of the
confusedly prismatic bed above the columnar one on that island also
presents a cellular texture. A similar rock at Ardtun, in Mull, passes
upward into a rugged slag and confused mass of basalt blocks, over
which the leaf-beds lie.
Amygdaloidal structure is more or less developed throughout the whole
series of basalts. But it is especially marked in certain abundant
sheets, which, for the sake of distinction, are called amygdaloids.
These beds, which form a considerable proportion of the materials of
every one of the plateaux, are distinguished by the abundance and
large size of their vesicles. In some places, the cavities occupy at
least as much of the rock as the solid matrix in which they lie. They
have generally been filled up with some infiltrated mineral--calcite,
chalcedony, zeolites, etc. The amygdales of the west of Skye and of
Antrim have long been noted for their zeolites. As a consequence of
their cellular texture and the action of infiltrating water upon them,
these amygdaloidal sheets are always more or less decomposed. Their
dull, lumpy, amorphous aspect contrasts well with the sharply-defined
columnar sheets above and below them, and as they crumble down they
are apt to be covered over with vegetation. Hence, on a sea-cliff
or escarpment, the green declivities between the prominent columnar
basalts usually mark the place of such less durable bands.
Exceedingly slag-like lavas are to be seen among the amygdaloids,
immediately preceded and followed by beds of compact black basalt
with few or no vesicles. From the manner in which such rocks yield
to the weather, they often assume a singularly deceptive resemblance
to agglomerates. One of the best examples of this resemblance which
have come under my notice is that of the rock on which stands Dunluce
Castle, on the north coast of Antrim. Huge rounded blocks of a harder
consistency than the rest of the rock project from the surface of
the cliffs, like the bombs of a true volcanic agglomerate, while the
matrix in which they are wrapped has decayed from around them. But an
examination of this matrix will soon convince the observer that it is
strongly amygdaloidal, and that the apparent "bombs" are only harder
and less cellular portions of it. The contrast between the weathering
of the two parts of the rocks seems to have arisen from an original
variety in the relative abundance of steam-cavities. The origin of
such nodular or pillow-like blocks has been already referred to at
pp. 26 and 193. Another singular instance occurs at the foot of the
outlier of Fionn Chro (Fig. 360), in the island of Rum. A conspicuous
band underlying the basalts there might readily be taken for a
basalt-conglomerate. But in this case, also, the apparent matrix is
found to be amygdaloidal, and the rounded blocks are really amygdales,
sometimes a foot in length, filled or lined with quartz, chalcedony, &c.
A somewhat different structure, in which, however, the appearance
of volcanic breccia or agglomerate due to explosion from a vent is
simulated, may be alluded to here. The best instance which I have
observed of it occurs at the south end of Loch-na-Mna, in the island of
Eigg, within a basalt which is remarkable for a streaky flow-structure.
On the weathered faces the streaky layers may be observed to have
been broken up, and their disconnected fragments have been involved
in ordinary basalt wherein this flow-structure is not developed,
while large blocks and irregular masses are wrapped round in a more
decomposing matrix. There can be no doubt that in such cases we see the
effects of the disruption of chilled crusts, and the entanglement of
the broken pieces in the still fluid lava.
It is a common belief that the filling in of the steam-cavities has
taken place long subsequent to the volcanic period, by the slow
percolation of meteoric water through the rock. I believe, however,
that at least in some cases, if not in all, the conversion of the
vesicular lavas into amygdaloids was effected during the volcanic
period. Thus it can be shown that the basalts which have been disrupted
by the gabbros and granophyres were already amygdaloids before these
basic intrusions disturbed them, for the kernels of calcite, zeolite,
etc., have shared in the general metamorphism induced in the enclosing
rock. Again, the blocks of amygdaloid contained in the agglomerates of
the volcanic series are in every respect like the amygdaloidal lavas
of the plateaux. It would thus seem that the infilling of the cavities
with mineral secretions was not merely a long secular process of
infiltration from the cool atmosphere, but was more rapidly completed
by the operation of warmer water, either supplied from volcanic sources
or heated by the still high temperature of the cellular lavas into
which it descended from the surface.[223]
[Footnote 223: Professor J. D. Dana, originally an advocate of
infiltration from above, subsequently supported the view that the
kernels of amygdaloids were filled in by the action of moisture within
the rocks during the time of cooling.--_Amer. Journ. Sci._ ser. 3, vol.
xx. (1880), p. 331. Messrs. Harker and Marr have demonstrated that the
Lower Silurian vesicular lavas of the Lake district had already become
amygdaloids before the uprise of the Shap granite.--_Quart. Journ.
Geol. Soc._ vol. xlix. (1893).]
4th. Banded or stratiform lavas, consisting of successive parallel
layers or bands which weather into projecting ribs and flutings. The
deceptive resemblance to sedimentary rocks thus produced has no doubt
frequently led to these lavas being mistaken for tuffs. As I have
recently found them to be much more plentiful than I had supposed, a
more detailed description of them seems to be required.
The banded character arises from marked distinctions in the texture
of different layers of a lava-sheet. In some cases (_a_) these
distinctions arise from differences in the size of the crystals or in
the disposition of the component minerals of the rock; in others (_b_)
from the varying number and size of the vesicles, which may be large or
abundantly crowded together in some layers, and small or only sparsely
developed in others. The structure thus points to original conditions
of the lava at the time of its emission and may be regarded as, to some
extent, a kind of flow-structure on a large scale.
(_a_) Where the banding is due to differences of crystalline texture,
the constituent felspars, augites, and iron-ores may be seen even with
the naked eye as well-defined minerals along the prominent surfaces
of the harder ribs, while the broader intervening flutings of finer
material show the same minerals in minuter forms. The alternating
layers of coarser and finer crystallization lie, on the whole,
parallel with the upper and under surfaces of the sheets in which they
occur. But they likewise undulate like the streaky lines in ordinary
flow-structure.
Banded structure of this type may be seen well developed in the
lower parts of the basalt-plateaux throughout the Inner Hebrides and
the Faroe Islands. A specimen taken from the west end of the island
of Sanday, near Canna, which showed the structure by a conspicuous
parallel fluting on weathered surfaces, was sliced for microscopical
examination. Mr. Harker has been kind enough to supply me with the
following observations regarding this slice:--
"In the slice [6660][224] the banding becomes less conspicuous under the
microscope. The rock is of basaltic composition, and, with reference
to its micro-structure, might be styled a fine-grained olivine-diabase
or olivine-dolerite in some parts of the slice, an olivine-basalt in
others. It consists of abundant grains of olivine, imperfect octahedra
and shapeless granules of magnetite, little simple or twinned prisms
of labradorite, and a pale brown augite. The last-named mineral is
always the latest product of consolidation, but it varies in habit,
being sometimes in ophitic patches moulded upon or enclosing the other
minerals, sometimes in small granules occupying the interstices between
the felspars and other crystals. The ophitic habit predominates in the
slice, while the granulitic comes in especially along certain bands. If
the former be taken as indicative of tranquil conditions, the latter of
a certain amount of movement in the rock during the latest stages of
its consolidation, the banding, though not strictly a flow-structure,
may be ascribed in some degree to a flowing movement of the nearly
solidified rock. There is, however, more than this merely structural
difference between the several bands. They differ to some extent in the
relative proportions of the minerals, especially of olivine and augite;
which points to a considerable flowing movement at an early stage in a
magma which was initially not homogeneous."
[Footnote 224: The figures within square brackets throughout the
following pages refer to the numbers of the microscopic slides in the
Geological Survey collection, where I have deposited all those prepared
from my specimens.]
(_b_) Where the banding arises from the distribution of the vesicles,
somewhat similar weathered surfaces are produced. In some instances,
while the basalt is throughout finely cellular, interposed bands of
harder, rather finer-grained and less thoroughly vesicular character
serve to give the stratified appearance. Instances may be observed
where the vesicles have been crowded together in certain bands, which
consequently weather out differently from the layers above and below
them. An excellent illustration of this arrangement occurs in the
lowest lava but one of the largest of the three picturesque stacks
known as Macleod's Maidens on the west coast of Skye (Figs. 260, 283,
284 and 287). This lava is thoroughly amygdaloidal, but the vesicles
are specially crowded together in certain parallel bands from an inch
to three or four inches thick. Some of these layers lie close to each
other, while elsewhere there may be a band of more close-grained, less
vesicular material between them. But the most singular feature of the
rock is to be seen in the shape and position of the vesicles that are
crowded together in the cellular bands. Instead of being drawn out into
flattened forms in the general direction of banding, they are placed
together at high angles. Each layer remains parallel to the general
bedding, but its vesicles are steeply inclined in one direction, which
was doubtless that of the flow of the still unconsolidated lava.[225]
Weathering along these bands, the lava might easily be mistaken at a
little distance for a tuff or other stratified intercalation.
[Footnote 225: This elongation of vesicles, more or less perpendicular to
the general bedding, may be noticed sometimes even in sills, as will be
shown in a later Chapter.]
[Illustration:
Fig. 260.--Banded amygdaloidal basalt showing layers of elongated
and steeply inclined vesicles, Macleod's Maidens, Skye.
]
Banded lavas possessing the characters now described are of frequent
occurrence among the Inner Hebrides. Many striking examples of them may
be seen along the west coast of Skye. Still more abundant in Faroe,
they form one of the most conspicuous features in the geology of that
group of islands. Along the whole of its western seaboard, on island
after island, they are particularly prominent in the lower parts of
the precipices, while the upper parts consist largely of amorphous or
prismatic sheets. So much do they resemble stratified rocks that it was
not until I had landed at various points that I could satisfy myself
that they are really banded lavas.[226]
[Footnote 226: For recent contributions to the Geology of the Faroe
Islands, see Prof. James Geikie, _Trans. Roy. Soc. Edin._ vol. xxx.
(1880), p. 217, where the banding of the basalts is noticed; Prof. A.
Helland, _Dansk. Geografisk. Tidskr._ (1881); R. Bréon, _Notes pour
servir à l'étude de la Géologie de l'Islande et des Isles Faeroe_
(1884); Mr. J. Lomas, _Proc. Geol. Soc. Liverpool_, vol. vii. (1895),
p. 292. Various writers have treated of the petrography of Faroe,
particularly A. Osann, _Neues Jahrb._ (1884), vol. i. p. 45, and M.
Bréon in the volume here cited.]
5th. Ordinary flow-structure, save in these banded lavas, is rather
rare among the plateaux. It may, however, be occasionally observed,
where there is no distinct banding. On a weathered surface it appears
in fine, widely parallel streaks, which are sometimes wavy, puckered
and broken up, as in rhyolites and felsites, while the porphyritic
felspars are arranged with their long axes in the direction of flow.
A good example of these characters may be seen on the summit of the
Dùn Can--the remarkable truncated cone which forms the highest point
on the Island of Raasay. The rock is a black olivine-basalt, partly
amygdaloidal, with zeolites filling up the cavities, and its flow-lines
are prominent on the weathered faces where they lie parallel to the
general bedding of the lavas. Another illustration may be observed
in the basalt already cited from Loch-na-Mna, in the island of Eigg,
where the rock presents in places a remarkable streaky structure
which, though hardly visible on a fresh fracture, reveals itself on a
weathered face in thin nearly parallel ribs coincident in direction
with the upper and under surfaces of the mass.
Great variety is to be found in the thickness of different sheets of
lava in the plateaux. Some of them are not more than 6 or 8 feet;
others reach to 80 or 100 feet, and sometimes, though rarely, to even
greater dimensions. In Antrim, the average thickness of the flows is
probably from 15 to 20 feet.[227] In the fine coast-sections at the
Giant's Causeway, however, some bands may be seen far in excess of that
measurement. The bed that forms the Causeway, for instance, is about
60 or 70 feet thick, and seems to become even thicker further east.
Along the great escarpment, 700 feet high, which rises from the shores
of Gribon, on the west coast of Mull, there are twenty separate beds,
which give an average of 35 feet for the thickness of each flow. On
the great range of sea-precipices along the west coast of Skye, which
present the most stupendous section of the basalts anywhere to be
seen within the limits of the British Islands, the average thickness
of the beds can be conveniently measured. At the Talisker cliffs some
of the flows are not more than 6 or 8 feet; others are 30 or 40 feet.
The chief precipice, 957 feet high (Fig. 286), contains at least 18 or
20 separate lava-sheets, which thus average of from 47 to 53 feet in
thickness. In the cliffs that form the seaward margin of the tableland
of Macleod's Tables (Fig. 283) fourteen successive beds of basalt can
be counted in a vertical section of 400 feet, which is equal to an
average thickness of about 28 feet. But some of the basalts are only
about 6 feet thick, while others are 50 or 60. The Hoe of Duirinish,
759 feet high, is composed of about sixteen distinct beds, which
thus have a mean thickness of 46 feet. The average thickness of the
successive flows on Dunvegan Head, which is 1000 feet high and contains
at least twenty-five separate sheets, is about 40 feet. Still further
north, the cliffs, 800 feet high, comprise sixteen successive flows,
which have thus an average of 50 feet each. Among the Faroe Islands the
average thickness of the basalt-sheets seems to be nearly the same as
in Britain. Thus in the magnificent ranges of precipices of Kalsö, Kunö
and Borö, forty or more sheets may be counted in the vast walls of rock
some 2000 feet high, giving a mean of about 50 feet.
[Footnote 227: See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11.]
Each bed appears, on a cursory inspection, to retain its average
thickness, and to be continuous for a long distance. But I believe that
this persistence is in great measure deceptive. We can seldom follow
the same bed with absolutely unbroken continuity for more than a mile
or two. Even in the most favourable conditions, such as are afforded by
a bare sea-cliff on which every sheet can be seen, there occur small
faults, gullies where the rocks are for the time concealed, slopes of
debris, and other failures of continuity; while the rocks are generally
so like each other, that on the further side of any such interruption,
it is not always possible to make sure that we are still tracing the
same bed of basalt which we may have been previously following. On
the other hand, a careful examination of one of these great natural
sections will usually supply us with proofs that, while the bedded
character may continue well marked, the individual sheets die out, and
are replaced by others of similar character. Cases may not infrequently
be observed where the basalt of one sheet abruptly wedges out, and is
replaced by that of another. Where both are of the same variety of
rock, it requires close inspection to make out the difference between
them; but where one is a green, dull, earthy, amorphous amygdaloid,
and the other is a compact, black, prismatic basalt, the contrast
between the two beds can be recognized from a distance (Fig. 261). In
the basaltic cliffs of the west coast of Skye, the really lenticular
character of the flows can be well seen. I may especially cite the
great headland south of Talisker Bay, already referred to, where, in
the pile of nearly horizontal sheets, two beds may be seen to die out,
one towards the north, the other towards the south. Further north,
in the cliff of the Hoe of Duirinish, a similar structure presents
itself. Along the coast-cliffs of Mull, Morven and Canna the same
fact is clearly displayed. Thus on the west side of the Sound of Mull
the slopes above Fishnish Bay show a group of basalts, which die out
southward, and are overlapped by a younger group that has been poured
over their ends. Such sections are best seen in the evening, when the
grass-covered lavas show their successive sheets by their respective
shadows, their individuality being lost in the full light of day. A
more striking example occurs beyond the west end of Glen More in Mull,
where one series of basalts has been tilted up, probably during some
volcanic episode, and has had a younger series banked up against its
edges.
[Illustration: Fig. 261.--Termination of Basalt-beds, Carsaig, Mull.]
In Antrim also, remarkable evidence is presented of the rapid
attenuation not of single beds only, but of a whole series of basalts.
Thus, at Ballycastle, the group of lavas known as the Lower Basalts,
which underlie the well-known horizon of iron-ore, are at least 350
feet thick. But, as we trace them westwards, bed after bed thins out
until, a little to the west of Ballintoy, in a distance of only about
6 miles, the whole depth of the group has diminished to somewhere
about 40 feet. A decrease of more than 300 feet in six miles or 50
feet per mile points to considerable inequalities in the accumulation
of the lavas. If the next series of flows came from another vent and
accumulated against such a gentle slope, it would be marked by a slight
unconformability. Structures of this kind are much rarer than we should
expect them to be, considering the great extent to which the plateaux
have been dissected and laid open in cliff-sections.
The basalt-plateau of the Faroe Islands exhibits with remarkable
clearness the lenticular character of the basalt-sheets, and a number
of examples will be cited in the description of that region to be
given in Chapter xxxix. In these northern climes vegetation spreads
less widely over rock and slope than it does in the milder air of the
Inner Hebrides. Hence the escarpments sweep in precipices of almost
bare rock from the level of the sea up to the serrated crests of the
islands, some 2000 feet in height. Each individual bed of basalt can
thus be followed continuously along the fjords, and its variation
or disappearance can be readily observed. Coasting along these vast
natural sections, we readily perceive that, as among the Western Isles,
the successive sheets of basalt have proceeded from no one common
centre of eruption. They die out now towards one quarter, now towards
another, yet everywhere retain the universal regularity and gentle
inclinations of the whole volcanic series.
ii. FRAGMENTAL ROCKS
While the plateaux are built up mainly of successive flows of basaltic
lavas, they include various intercalations of fragmental materials,
which, though of trifling thickness, are of great interest and
importance in regard to the light which they cast on the history of the
different regions during the volcanic period. I shall enumerate the
chief varieties of these rocks here, and afterwards give fuller details
regarding their stratigraphical relations and mode of occurrence in
connection with the succession of beds in each of the plateaux.
(_a_) _Volcanic Agglomerates._--In the tumultuous unstratified masses
of fragmentary materials which fill eruptive vents in and around the
plateaux, the stones, which vary in size up to blocks several feet in
diameter, consist for the most part of basalts, often highly slaggy and
scoriaceous. They include also fragments of different acid eruptive
rocks (generally felsitic or rhyolitic in texture), with pieces of
the non-volcanic rocks through which the volcanic pipes have been
drilled. The paste is granular, dirty-green or brown in colour, and
seems generally to consist chiefly of comminuted basalt. As in the
Carboniferous and Permian necks, the Tertiary agglomerates contain
abundant detritus of a basic minutely cellular pumice.
(_b_) _Volcanic Conglomerates and Breccias in beds intercalated
between the flows of Basalt._--These are of at least three kinds.
(_a_) Basalt-conglomerates, composed mainly of rounded and subangular
blocks of basalt (or allied basic lava), sometimes a yard or more
in diameter, not unfrequently in the form of pieces of rough slag
or even of true bombs, imbedded in a granular matrix of comminuted
basalt-debris. In some cases, the stones form by far the most abundant
constituents of the rock, which then resembles some of the coarse
agglomerates just described. Perhaps the most remarkable accumulations
of this kind are those intercalated among the basalts in the islands
of Canna and Sanday, of which a detailed account will be given in
Chapter xxxviii. These conglomerates, besides their volcanic materials,
contain rounded blocks of Torridon sandstone and other rocks, which
must have been carried from the east by some tolerably powerful river
that flowed across the basalt-plains during the volcanic period.
Again, on the east side of Mull, the slaggy basalts of Beinn Chreagach
Mhor are occasionally separated by volcanic conglomerates. As a rule,
however, such intercalations are seldom more than a few feet or yards
in thickness. Their coarseness and repetition on successive horizons
indicate that they probably accumulated in the near neighbourhood of
one or more small vents, from which discharges of fragmentary materials
took place at the beginning or at the close of an outflow of lava, and
that the stones were sometimes swept away from the cones and rolled
about by streams before being buried under the succeeding lava-sheets.
More commonly the dirty-green or dark-brown granular matrix exceeds in
bulk the stones embedded in it. It has obviously been derived mainly
from the trituration of already cooled basalt--masses, and probably
also from explosions of the still molten rock in the vents. A striking
illustration of this type of rock may be seen on the south side of
Portree Harbour, where a mass of dark-green basalt-conglomerate, with
a coaly layer above it, lies near the base of the bedded basalts, and
attains at one part of its course a thickness of about 200 feet. This
rock will be again referred to in connection with the vent from which
its materials were probably derived. As in the case of the agglomerates
of the vents, pieces of older acid lavas, and still more of the
non-volcanic rocks that underlie the plateaux, are found in the bedded
conglomerates and breccias. In Antrim and Mull, for instance, fragments
of flint and chalk are of common occurrence. A characteristic example
of this kind of rock forms the platform of the columnar bed out of
which Fingal's Cave, Staffa, has been excavated (Fig. 266_a_).
([Greek: beta]) Felsitic Breccia.--This variety, though of rare
occurrence, is to be seen in a number of localities in the island
of Mull. It is composed in great measure of angular fragments of
close-grained flinty felsitic or rhyolitic rocks, sometimes showing
beautiful flow-structure, together with pieces of quartzite and
amygdaloidal basalt, the dull dirty-green matrix appearing to be made
up chiefly of basalt-dust.
([Greek: gamma]) Rhyolitic Conglomerate.--Between the upper and lower
group of basalts in the Antrim plateau there occur bands of a pale
fawn-coloured conglomerate largely made up of more or less rounded
fragments of rhyolite, like some of the varieties of the rock which
occur in place on the plateau. The rhyolitic debris is often mixed with
pebbles of basalt. Sometimes it becomes so fine as to pass into pale
clays.
([Greek: delta]) Breccias of non-volcanic materials.--These, the most
exceptional of all the fragmentary intercalations in the plateaux,
consist almost wholly of angular blocks of rocks which are known
to underlie the basalts, but with a variable admixture of basalt
fragments. They are due to volcanic explosions which shattered the
subjacent older crust of rocks, and discharged fragments of these from
the vents or allowed them to be borne upwards on an ascending column
of lava. Pieces of the non-volcanic platform are of common occurrence
among the fragmentary accumulations, especially in the lower parts of
the plateaux basalts. But I have never seen so remarkable an example
of a breccia of this kind as that which occurs near the summit of
Sgurr Dearg, in the south-east of Mull. The bedded basalt encloses a
lenticular band of exceedingly coarse breccia, consisting mainly of
angular pieces of quartzite, with fragments of amygdaloidal basalt.
In the midst of the breccia lies a huge mass or cake of erupted
mica-schist, at least 100 yards long by 30 yards wide, as measured
across the strike up the slope of the hill. To the west, owing to
the thinning out of the breccia, this piece of schist comes to lie
between two beds of basalt. A little higher up, other smaller but still
large blocks of similar schist are involved in the basalt, as shown
in Fig. 262. As the huge cake of mica-schist plunges into the hill,
its whole dimensions cannot be seen; but there are visible, at least,
15,000 cubic yards, which must weigh more than 30,000 tons. Blocks of
quartzite of less dimensions occur in the basalts on Loch Spelve, in
the same district. There can be no doubt, I think, that these enormous
fragments were torn off from the underlying crystalline schists which
form the framework of the Western Highlands, and were floated upward
in an ascending flow of molten basalt. Had the largest mass occurred
at or near the base of the volcanic series, its size and position
would have been less remarkable. But it lies more than 2000 feet up
in the basalts, and hence must have been borne upward for more than
that height. A similar but less striking breccia occurs on the south
coast of the same island, near Carsaig, made up chiefly of pieces of
quartzite and quartz.[228]
[Footnote 228: This is noticed by Mr. Starkie Gardner, _Quart. Journ.
Geol. Soc._ xliii. (1887), p. 283, note.]
Some remarkable agglomerates, near Forkhill, Armagh, probably belonging
to the Tertiary volcanic series, will be described in the account
of the Irish acid rocks (Chapter xlvii.). They consist entirely of
non-volcanic stones and dust and are traceable for some miles along the
line of a fissure. Where they have been discharged through granite they
consist entirely of the detritus of that rock, but where they have been
erupted in the Silurian area they consist of fragments of grits and
shales. They seem to have been produced by æriform discharges, without
the uprise of any volcanic magma, though eventually andesite and
rhyolite ascended the fissure and became full of granitic and Silurian
fragments.
Some remarkable necks filled almost entirely with fragments of Torridon
Sandstone have been observed in the west of Applecross, Ross-shire, and
some curious plug-like masses of breccia, also made up of fragments of
Torridonian strata, occur in the island of Raasay. These examples will
be more particularly described on later pages (pp. 292, 293).
(_c_) _Tuffs._--The tuffs intercalated in the basalt-plateaux generally
consist essentially of basic materials, derived from the destruction
of different varieties of basalts, though also containing occasional
fragments of older felsitic rocks, as well as pieces of chalk, flint,
quartz, and other non-volcanic materials. They are generally dull,
dirty-green in colour, but become red, lilac, brown, and yellow,
according to the amount and state of combination and oxidation of their
ferruginous constituents. They usually contain abundant fragments
of amygdaloidal and other basalts. As a rule, they are distinctly
stratified, and occur in bands from a few inches to 50 feet or more
in thickness. The matrix being soft and much decomposed, these bands
crumble away under the action of the weather, and contribute to the
abruptness of the basalt-escarpments that overlie them.
[Illustration: Fig. 262.--Breccia and Blocks of mica-schist, quartzite,
etc., lying between bedded Basalts, Isle of Mull.
_a a_, Bedded basalts; _b_, Breccia; _d_, Basic dyke.]
In the group of strata between the two series of basalts in Antrim,
some of the tuffs consist chiefly of rhyolitic detritus, both glassy
and lithoid.
Where the tuffs become fine-grained and free from imbedded stones,
they pass into variously-coloured clays. Among these are the "bauxite"
and "lithomarge" of Antrim, probably derived from pale rhyolitic tuffs
and conglomerates (p. 204). Associated with these deposits in the same
district, is a pisolitic hæmatite, which has been proved to occur over
a considerable area on the same horizon. Many of the clays are highly
ferruginous. The red streaks that intervene between successive sheets
of basalt are of this nature (bole, plinthite, etc.). The source of the
iron-oxide is doubtless to be traced to the decomposition of the basic
lavas during the volcanic period.
(_d_) There occur also grey and black clays and shales, of ordinary
sedimentary materials, containing leaves of terrestrial plants
(leaf-beds), with occasional wing-cases of beetles, sometimes
associated with impure limestones, but more frequently with sandstones
and indurated gravels or conglomerates containing pieces of fossil
wood. These intercalated bands undoubtedly indicate the action of
running water, sometimes even of river-floods, and the accumulation of
sediment in hollows of the exposed flows of basalt at intervals during
the piling up of the successive lava-sheets that form the plateaux.
The alternation of fluviatile gravels with volcanic tuffs, fluviatile
conglomerates, and lava-streams, is admirably displayed in the island
of Canna, as will be narrated in detail in Chapter xxxviii.
The vegetable matter has in some places gathered into lenticular seams
of lignite, and even occasionally of black glossy coal. Amber also has
been found in the lignite. Where the vegetation has been exposed to the
action of intrusive dykes or sheets, it has sometimes passed into the
state of graphite.
The remarkable terrestrial flora found in the leaf-beds, and in
association with the lignites, was first made known by the descriptions
of Edward Forbes already referred to, and has subsequently been studied
and described by Heer, W. H. Baily, and Mr. Starkie Gardner.[229] It
was regarded by Forbes as of Miocene age, and this view has generally
been adopted by geologists. Mr. Starkie Gardner, however, contends
that it indicates a much wider range of geological time. He believes
that a succession of floras may be recognised, the oldest belonging
to an early part of the Eocene period. Terrestrial plants, it must be
admitted, are not always a reliable test of geological age, and I am
not yet satisfied that in this instance they afford evidence of such
a chronological sequence as Mr. Gardner claims, though I am convinced
that the Tertiary volcanic period was long enough to have allowed
of the development of considerable changes in the character of the
vegetation.
[Footnote 229: On this subject consult Duke of Argyll, _Quart. Journ.
Geol. Soc._ vol. vii. (1851), p. 89; E. Forbes, _Ibid._ p. 103; W. H.
Baily, _op. cit._ xxv. (1869), pp. 162, 357; _Brit. Assoc. Rep._ (1879)
p. 162; (1880) p. 107; (1881) p. 151; (1884) p. 209; Mr. J. Starkie
Gardner, _Palæontographical Society_, vols. xxxviii. xxxix. In the last
of Mr. Baily's papers he notices that "the Rev. Dr. Grainger found a
portion of a fish (_Percidæ_, possibly _Lates_)." The discovery of the
remains of a fresh-water fish is an important additional testimony to
the terrestrial conditions under which the lavas were erupted. The
genus _Lates_ now inhabits the Nile and the Ganges.]
For the purpose of the present volume, however, the precise stage
in the geological record, which this flora indicates, is of less
consequence than the broad fact that the plants prove beyond all
question that the basalts among which they lie were erupted on land
during the older part of the long succession of Tertiary periods. Their
value in this respect cannot be overestimated. Stratigraphical evidence
shows that the eruptions must be later than the Upper Chalk; but the
imbedded plants definitely limit them to the earlier half of Tertiary
time.
CHAPTER XXXVII
THE SEVERAL BASALT-PLATEAUX AND THEIR GEOLOGICAL HISTORY,
ANTRIM, MULL, MORVEN AND ARDNAMURCHAN
There are five districts in North-western Europe where the original
widespread Tertiary lava-fields have been less extensively eroded
than elsewhere, or at least where they have survived in larger and
thicker masses. Whether or not each of them was an isolated area of
volcanic activity cannot now be determined. Their several outflows of
lava within the area of the British Isles may have united into one
continuous volcanic tract, and their present isolation there may be due
entirely to subterranean movements and denudation. There is a certain
convenience, however, in treating the districts separately. They
are--1. Antrim; 2. Mull, Morven and Ardnamurchan; 3. Small Isles; 4.
Skye; 5. The Faroe Islands.
i. ANTRIM[230]
[Footnote 230: The basalts of Antrim are the subject of an abundant
literature. I may refer particularly to the papers of Berger and
Conybeare (_Trans. Geol. Soc._ iii.), the Geological Report of
Portlock, and the Explanations of the Sheets of the Geological Survey
of Ireland. Other papers will be afterwards cited. The general features
of the Antrim plateau are shown on Map VII.]
The largest of the basalt-plateaux of Britain is that which forms so
prominent a feature in the scenery and geology of the North of Ireland,
stretching from Lough Foyle to Belfast Lough, and from Rathlin Island
to beyond the southern margin of Lough Neagh. Its area may be roughly
computed at about 2000 square miles. But, as its truncated strata rise
high along its borders, and look far over the surrounding low grounds,
it must be regarded as a mere fragment of the original volcanic plain.
It may be described as an undulating tableland, which almost everywhere
terminates in a range of bold cliffs, but which, towards the centre and
south, sinks gently into the basin of Lough Neagh. The marginal line of
escarpment, however, presents considerable irregularity both in height
and form, besides being liable to frequent local interruptions. It is
highest on the west side, one of its crests reaching at Mullaghmore,
in County Londonderry, a height of 1825 feet. It sinks down into the
valley of the Bann, east of which it gradually ascends, forming the
well-known range of cliffs from the Giant's Causeway and Bengore Head
to Ballycastle. It then strikes inland, and making a wide curve in
which it reaches a height of more than 1300 feet, comes to the sea
again at Garron Point. From that headland the cliffs of basalt form a
belt of picturesque ground southwards beyond Belfast, interrupted only
by valleys that convey the drainage of the interior of the plateau
to the North Channel. Above the valley of the Lagan the crest of the
plateau rises to a height of more than 1500 feet.
Throughout most of its extent the basalt-escarpment rests on the white
limestone or Chalk of Antrim, beneath which lie soft Lias shales and
Triassic marls. Here and there, where the substratum of Chalk is thin,
the action of underground water on the crumbling shales and marls below
it has given rise to landslips. The slopes beneath the base of the
basalt are strewn with slipped masses of that rock, almost all the way
from Cushendall to Larne, some of the detached portions being so large
as to be readily taken for parts of the unmoved rock. On the west side
also, a group of huge landslips cumbers the declivities beneath the
mural front of Benevenagh.
I have found some difficulty in the attempt to ascertain what was the
probable form of surface over which the volcanic rocks of this plateau
began to be poured out. The Chalk sinks below the sea-level on the
north coast, but, in the outlier of Slieve Gallion, three miles beyond
the western base of the escarpment, it rises to a height of 1500 feet
above the sea. On the east side also, it shows remarkable differences
of level. Thus, below the White Head at the mouth of Belfast Lough,
it passes under the sea-level, but only 16 miles to the south, where
it crops out from under the basalt, its surface is about 1000 feet
above that level. If these variations in height existed at the time
of the outpouring of the basalt, the surface of the ground over which
the eruptions took place was so irregular that some hundreds of feet
of lava must have accumulated before the higher chalk hills were
buried under the volcanic discharges. But it seems to me that much
of this inequality in the height of the upper surface of the Chalk
is to be attributed to unequal movements since the volcanic period,
which involved the basalt in their effects, as well as the platform
of Chalk below it. Had the present undulations of that platform been
older than the volcanic discharges, it is obvious that upper portions
of the basalt-series would have overlapped lower, and would have come
to rest directly on the Chalk. But this arrangement, so far as I am
aware, never occurs, except on a trifling scale. Wherever the Chalk
appears, it is covered by sheets of the lower and not of the upper of
the two groups into which the Antrim basalts are divisible. We have
actual proof of considerable terrestrial disturbance, subsequent to the
date of the formation of the volcanic plateau. Thus, near Ballycastle,
a fault lets down the basalt and its Chalk platform against the
crystalline schists of that district. On the east side of the fault,
the Chalk is found far up the slope, circling round the base of the
beautiful cone of Knocklayd--an outlier of the basalt which reaches a
height of 1695 feet (Fig. 263). The amount of vertical displacement of
the volcanic sheets is here 700 feet.[231] Many other displacements, as
shown by the mapping of my colleagues in the Geological Survey, have
shifted the base of the escarpment from a few inches up to several
hundred feet. Besides actual dislocations, the Antrim plateau has
undergone some marked subsidences of which the most notable is that of
Lough Neagh.[232]
[Footnote 231: Explanatory Memoir of Sheets 7 and 8, Geological Survey,
Ireland, by Messrs. Symes, Egan, and M'Henry (1888), p. 37.]
[Footnote 232: These inequalities in the level of the base of the Antrim
plateau will be more particularly discussed in Chapter xlix., in
connection with the subsidences and dislocations which have affected
the region since the close of the volcanic period.]
It is evident, therefore, that the present position of the Chalk
platform is far from agreeing with that which it presented to the
outflow of the sheets of basalt. But, on the other hand, there can be
no doubt that its surface at the beginning of the volcanic outbursts
was not a level plain. It was probably a rolling country of low bare
chalk-downs, like parts of the South-east of England. The Irish Chalk
attains its maximum thickness of perhaps 250 feet at Ballintoy. But it
is liable to rapid diminution. On the shore at Ballycastle about 150
feet of it can be seen, its base being concealed; but only two and a
half miles to the south, on the outlier of Knocklayd, the thickness is
not quite half so much. On the west side of the plateau also, there are
rapid changes in the thickness of the Chalk. Such variations appear
to be mainly attributable to unequal erosion before the overflow of
the basalts. So great indeed had been the denudation of the Cretaceous
and underlying Secondary formations previous to the beginning of the
volcanic eruptions, that in some places the whole of these strata had
been stripped off the country, so that the older platform of Palæozoic
or still more ancient masses was laid bare. Thus, on the west side of
the escarpment, the basalt steals across the Chalk and comes to rest
directly upon Lower Carboniferous rocks.
The authors who have described the junction of the Chalk and basalts in
Antrim have generally referred to the uneven surface of the former rock
as exposed in any given section. The floor on which the basalt lies is
remarkably irregular, rising into ridges and sinking into hollows or
trenches, but almost everywhere presenting a layer of earthy rubbish
made of brown ferruginous clays, mixed with pieces of flint, chalk,
and even basalt.[233] The flints are generally reddened and shattery.
The chalk itself has been described as indurated, and its flints as
partially burned by the influence of the overlying basalt. But I have
not noticed, at any locality, evidence of alteration of the solid
chalk, except where dykes or intrusive sheets have penetrated it.[234]
There can be no doubt that the hardness of the rock is an original
peculiarity, due to the circumstances of its formation. The irregular
earthy rubble, that almost always intervenes between the chalk and the
base of the basalt, like the "clay with flints" so general over the
Chalk of Southern England, no doubt represents long-continued subærial
weathering previous to the outflow of the basalt. Even, therefore, if
there were no other evidence, we might infer with some confidence from
this layer of rubble, that the surface over which the lavas were poured
was a terrestrial one. Here and there, too, we may detect traces of
the subsidence of the basalt into swallow-holes dissolved in the chalk
subsequent to the outflow of the basalt-sheets.
[Footnote 233: Portlock, _Report on Geology of Londonderry_, etc.
(Geological Survey), p. 117.]
[Footnote 234: See Portlock, _op. cit._ p. 116.]
The Antrim plateau is not only the largest in the British Islands, it
is also the most continuous and regular. It may be regarded, indeed,
as one unbroken sheet of volcanic material, not disrupted by any such
mountainous masses of intrusive rock as in the other plateaux interrupt
the continuity of the horizontal or gently inclined sheets of basalt.
Around its margin, indeed, a few outliers tower above the plains, and
serve as impressive memorials of its losses by denudation. Of these,
by much the most picturesque and imposing, though not the loftiest, is
Knocklayd already referred to, which forms so striking a feature in the
north-east of Antrim (Fig. 263).
[Illustration: Fig. 263.--Section of Knocklayd, an outlier of the
Antrim basalt-plateau lying on Chalk.
1. Crystalline schists; 2. Cretaceous strata; 3. Lower basalts; 4.
Group of tuffs, clays and iron-ore; 5. Upper basalts; _f_. Fault.]
The total thickness of volcanic rocks in the Antrim plateau exceeds
1000 feet; but, as the upper part of the series has been removed by
denudation, the whole depth of lava originally poured out cannot now be
told. A well-marked group of tuffs and clays, traceable throughout a
large part of Antrim, forms a good horizon in the midst of the basalts,
which are thus divisible into a lower and upper group (Fig 264).
The Lower Basalts have a thickness of from 400 to 500 feet. But,
as already mentioned (p. 194), they die out in about six miles to
no more than 40 feet at Ballintoy. They are distinguished by their
generally cellular and amygdaloidal character, and less frequently
columnar structure. The successive flows, each averaging perhaps above
15 feet in thickness, are often separated by thin red ferruginous
clayey partings, sometimes by bands of green or brown fine gravelly
tuff. The most extensive of these tuff-bands occurs in the lower part
of the group at Ballintoy, and can be traced along the coast for
about five miles. In the middle of its course, near the picturesque
Carrick-a-raide, it reaches a maximum thickness of about 100 feet and
gradually dies out to east and west. The neck of coarse agglomerate at
Carrick-a-raide, is doubtless the vent from which this mass of tuff was
discharged (see Fig. 301). Owing to the thinning out of the sheets of
basalts, as they approach the vent, the tuff comes to rest directly on
the Chalk, and for some distance westwards forms the actual base of
the volcanic series.[235] Occasional seams of carbonaceous clays, or of
lignite, appear in different horizons among the basalts. Beneath the
whole mass of basalt, indeed, remains of terrestrial vegetation here
and there occur. Thus, near Banbridge, County Down, a patch of lignite,
four feet ten inches thick, underlies the basalt, and rests directly on
Silurian rocks. Such fragmentary records are an interesting memorial
of the wooded land-surface over which the earliest outflows of basalt
spread.
[Footnote 235: See Explanation of Sheets 7 and 8 of the Geological Survey
of Ireland (1888), p. 23.]
[Illustration: Fig. 264.--Diagram-Section of the Antrim Plateau.
1. Triassic series; 2, 3. Rhaetic strata and Lias; 4. Greensand; 5.
Chalk; 6. Gravel and soil; 7. Lower group of basalts; 8. Group of
tuffs, clays and iron-ore; 9. Upper group of basalts.]
In looking at the great basalt-escarpments of Antrim, the Inner
Hebrides or the Faroe Islands, and in following with the eye the
successive sheets of lava in orderly sequence of level bands from
the breaking waves at the base to the beetling crest above, we are
apt to take note only of the proofs of regularity and repetition
in the outflows of molten rock and to miss the evidence that these
outflows did not always rapidly follow each other, but were separated
by intervals of varying, sometimes even of long duration. One of the
most frequent and conspicuous proofs of such intervals is to be found
in the red layers or partings above referred to which, throughout all
the basalt-plateaux, so commonly intervene between successive sheets
of basalt. These red streaks cannot fail to arrest the eye on the
coast-precipices where by their brilliant contrast of colour, they help
to emphasize the bedded character of the whole volcanic series.
Examined more closely, they are found to consist of clay or bole
which shades into the decomposed top of the bed whereon it lies, and
is usually somewhat sharply marked off from that which covers it.
This layer has long, and I think correctly, been regarded as due to
the atmospheric disintegration of the surface of the basalt on which
it rests, before the eruption of the overlying flow. It varies in
thickness from a mere line up to a foot or more, and it passes into the
tuffs and clays which are sometimes interposed between the sheets of
basalts. It may be looked upon as probably furnishing evidence of the
lapse of an interval sufficiently extended to permit a considerable
subserial decay of the surface of a lava-sheet before the outflow
of the next lava. But an attentive study of the plateaux discloses
other and even more remarkable indications that the pauses between
the consecutive basalt-beds were frequently so prolonged as to allow
extensive topographical changes to be made in a district. Nowhere is
the long duration of some of these intervals more impressively taught
than in the central zone of sedimentary strata in Antrim.
This persistent group of tuffs, clays, and iron-ore is generally from
30 to 40 and sometimes as much as 70 feet thick. From the occurrence
of the ore in it, it has been explored more diligently in recent years
than any other group of rocks in the district, and its outcrop is now
known over most of the plateau. The iron-ore bed varies from less
than an inch up to 18 inches in thickness, and consists of pisolitic
concretions of hæmatite, from the size of a pea to that of a hazel
nut, wrapped up in a soft ochreous clayey matrix.[236] Where it is
absent, its place is sometimes taken by an aluminous clay, worked as
"bauxite," which has yielded stumps of trees and numerous leaves and
cones. Beneath the iron-ore or its representative, lies what is called
the "pavement,"--a ferruginous tuff, 8 to 10 feet thick, resting on
"lithomarge,"--a lilac or violet mottled aluminous earth sometimes
full of rounded blocks or bombs of basalt. The well-known horizon for
fossil plants at Ballypallidy is a red tuff in this zone. The section
of strata between the two basalt-groups at this locality may serve as
an illustration of the nature and arrangement of the deposits.[237]
[Footnote 236: Consult a good essay on the Iron-ore and Basalts of
North-east Ireland by Messrs. Tate and Holden, _Quart. Journ. Geol.
Soc._ xxvi. (1870), p. 151. In this paper the nature, composition and
modes of origin of the iron-ore and its associated strata are fully
discussed.]
[Footnote 237: A. M'Henry, _Geol. Mag._ (1895), p. 263.]
Upper Basalt, compact and often columnar sheets.
Brown laminated tuff and volcanic clays.
Laminated brown impure earthy lignite, 2 feet 3 inches.
Brown and red variegated clays, tuffs and sandy layers, with irregular
seams of coarse conglomerate composed of rounded and subangular
fragments of rhyolite and basalt, 3 feet 4 inches.
Brown, red and yellowish laminated tuffs, mudstones, and bole, with
occasional layers of fine conglomerate (rhyolitic and basaltic),
pisolitic iron-ore band and plant-beds, 8 feet 10 inches.
Lower basalt, amygdaloidal.
In some of the Ballypallidy tuffs the most frequent lapilli are pieces
of green and brown glass, which Mr. Watts compares with the pitchstone
of Sandy Braes, though rarely containing phenocrysts as that rock does.
He has found also in these strata a smaller proportion of lithoidal
rhyolites and occasionally fragments of basic rock.
The pale and coloured clays that occur in this marked sedimentary
intercalation have doubtless been produced by the decomposition of
the volcanic rocks and the washing of their fine detritus by water.
Possibly this decay may have been in part the result of solfataric
action. From true bauxite or aluminium-hydrate, the sediments vary in
composition and specific gravity and pass into aluminous silicates
and iron-ores. They seem to indicate a prolonged interval of volcanic
quiescence when the lavas and tuffs already erupted were denuded and
decomposed.[238]
[Footnote 238: See a note on Bauxite by Professor G. A. Cole, _Scientif.
Trans. Royal Dublin Soc._ vol. vi. series ii. (1896), p. 105.]
The area over which this interesting series of stratified deposits now
extends is obviously much less than it was originally. It has indeed
been so reduced by denudation into mere scattered patches that it
probably does not exceed 170 square miles. But the group can be traced
from Divis Hill, near Belfast, to Rathlin Island, a distance of 50
miles, and from the valley of the Bann to the coast above Glenarm, more
than 20 miles. There can be little doubt that it was once continuous
over all that area, and that it probably extended some way further on
each side. If the so-called Pliocene clays of Lough Neagh be regarded
as parts of this group of strata, its extent will be still further
increased. Hence the original area over which the iron-ore and its
accompanying tuffs and clays were laid down can hardly have been less
than 1000 square miles. This extensive tract was evidently the site of
a lake during the volcanic period, formed by a subsidence of the floor
of the lower basalts. The salts of iron contained in solution in the
water, whether derived from the decay of the surrounding lavas or from
the discharges of chalybeate springs, were precipitated as peroxide in
pisolitic form, as similar ores are now being formed on lake-bottoms
in Sweden. For a long interval, quiet sedimentation went on in this
lake, the only sign of volcanic energy during that time being the dust
and stones that were thrown out and fell over the water-basin, or were
washed into it by rains from the cones of the lava-slopes around.
It may here be remarked that the tendency to subsidence in the Antrim
plateau seems to have characterized this region since an early part
of the volcanic period. The lake in which the deposits now described
accumulated was entirely effaced and overspread by the thick group
of upper basalts. But long after the eruptions had ceased, a renewed
sinking of the ground gave rise to the sheet of water which now forms
Lough Neagh.[239]
[Footnote 239: This subject will be discussed in Chapter xlix.]
Nowhere else among the Tertiary basalt-plateaux of Britain has any
trace been found of so marked and prolonged a pause in the volcanic
activity as is indicated by the Antrim zone of tuffs and clays.
Throughout the Inner Hebrides, indeed, numerous intercalations of
sedimentary material occur among the basalts, but these consist
mainly of tuffs and volcanic conglomerates with less frequent shales
and coal-seams, and they never suggest so distinct and lengthened an
interval as is indicated by the Antrim deposit.
It is not improbable that this interval was marked by the outbreak
of rhyolitic eruptions somewhere in the region. The abundance of
rhyolite fragments in some of the tuffs is striking evidence that
acid rocks were in one way or other brought to the surface at this
time. At Glenarm one of the members of the stratified series is a
marked rhyolitic conglomerate, composed of rounded pebbles of a rock
not unlike the well-known rhyolite of Tardree and Carnearny. These
fragments, obviously of local origin, must either have been derived
from a surface of acid rock laid bare by denudation, or from rhyolite
ejected in lapilli or poured out in streams. I formerly believed that
all the Antrim rhyolites had been injected into the basalts after the
close of the plateau-period. But the proved abundance and wide extent
of the rhyolitic detritus among the sediments associated with the
iron-ore point to a possible outflow of acid lavas with accompanying
tuffs during the sedimentary interval between the two groups of
basalt. The characters of the Antrim rhyolites, however, will be more
particularly discussed in Chapter xlvii., in connection with the acid
rocks of the Tertiary volcanic series.
Immediately above the iron-ore of Antrim, or separated from it in
places by only a few inches of tuff, comes the group of Upper Basalts,
which varies up to 600 feet in thickness, though as the upper portion
has been everywhere removed by denudation, no measure remains of what
may have been the original depth of the group. The general character
of these basalts is more frequently columnar, black and compact, and
with fewer examples of a strongly amygdaloidal structure than in the
lower group. But this distinction is less marked in the south than in
the north of Antrim, so that where the intervening zone of tuffs and
iron-ore disappears, no satisfactory line of division can be traced
between the two groups of basalt. The occurrence of that zone, however,
by giving rise to a hollow or slope, from which the upper basalts rise
as a steep bank or cliff, furnishes a convenient topographical feature
for mapping the boundary of these rocks. Among the upper basalts, also,
there is perhaps a less frequent occurrence of those thin red partings
of bole between successive flows, so conspicuous in the lower group.
But the flows are not less distinctly marked off from each other.
Nowhere can their characteristic features be better seen than along the
magnificent range of cliffs from the Giant's Causeway eastwards. The
columnar bed that forms the Causeway is the lowest sheet of the upper
group, and may be seen resting directly on the zone of grey and red
tuffs. It is about 60 or 70 feet thick; and, while perfectly regular in
its columnar structure at the Causeway and the "Organ," assumes further
eastward the confusedly starch-like arrangement of prisms already
referred to. But in the great cliff section of the "Amphitheatre,"
the more regular structure is resumed, the bed swells out to about
80 feet in thickness, and columns of that length run up the face
of the precipice, weathering out at the top into separate pillars,
which, perched on the crest of an outstanding ridge, are known as the
"Chimneys." The basalt-beds that succeed the lowest one are each only
about 10 to 15 feet thick (Fig. 265).
[Illustration: Fig. 265.--View of Basalt escarpment, Giant's Causeway,
with the Amphitheatre and Chimneys. (From a photograph by Mr. R.
Welch.)]
Between the successive sheets of the Upper Basalts thin seams of red
ferruginous clay though, as I have said, less frequent perhaps than in
the lower group, continue to show that the intervals between successive
eruptions were of sufficient duration to admit of some subærial decay
of the surface of a lava before the outflow of the next bed. Occasional
thin layers of tuff also, and even of pisolitic iron-ore, have been
observed among these higher basalts. But the most interesting and
important intercalations are inconstant seams of lignite. One of the
most conspicuous of these lies immediately above the basalt of the
"Causeway," where it was long worked for fuel, and was found to be
more than six feet thick. But it is quite local, as may be seen at the
"Organ" over which it lies, having a thickness of only 12 inches and
rapidly dying out so as to allow the basalts above and below it to come
together. The removal of the upper portion of the basalts by denudation
has destroyed the records of the latest part of the volcanic history of
the Irish plateaux.
It is obvious that nowhere in Antrim does any trace exist of a central
vent or cone from which the volcanic materials were discharged. There
is no perceptible thickening of the individual basalt-sheets, nor of
the whole series in one general direction, in such a manner as to
point to the site of some chief focus of eruption. Nor can we place
reliance on the inclination of the several parts of the plateau. I have
pointed out that the varying dip of the beds must be attributed mainly
to post-volcanic movements, or at least to movements which, if not
later than all the phases of volcanic action, must have succeeded the
outpouring of the plateau-basalts. There has been a general subsidence
towards the central and southern tracts now occupied by the valley of
the Bann and Lough Neagh. But nowhere in the depression is there any
trace of the ruins of a central cone or focus of discharge.
The Antrim plateau, in these respects, resembles the others. But as
has already been remarked, it differs from them in one important
particular. It has nowhere been disrupted by huge bosses of younger
rocks, such as have broken up the continuity of the old lava-fields
further north. Yet it also is not without its memorials of younger
protrusions. It contains not a few excellent examples of true volcanic
vents, and, as above stated, it includes some small acid bosses that
may represent the great protrusions of the Inner Hebrides, and may have
been connected with superficial outflows of rhyolitic lava and showers
of rhyolitic tuff.
ii. MULL, MORVEN AND ARDNAMURCHAN
This plateau covers nearly the whole of the island of Mull, embraces a
portion of Morven on the Argyleshire mainland, and, stretching across
Loch Sunart, includes the western part of the peninsula of Ardnamurchan
(Map VI.). That these now disconnected areas were once united into a
continuous lava-field which extended far beyond its present limits
is impressively indicated by their margin of cliffs and fringe of
scattered islands and outliers. The plateau went west, at least, as far
as the Treshnish Isles, which are composed of basalt. On its eastern
border, a capping of basalt on the top of Beinn Iadain (1873 feet) in
Morven, and others further north, prove that its volcanic sheets once
spread into the interior of Argyleshire (Fig. 266). On the south, its
fine range of lofty cliffs, with their horizontal bars of basalt, bear
witness to the diminution which it has undergone on that side; while,
on the north, similar sea-walls tell the same tale. Not only has it
suffered by waste along its margin, it has also been deeply trenched
by the excavation of glens and arms of the sea. The Sound of Mull
cuts it in two, and the mainland portion is further bisected by Loch
Sunart, and again by Loch Aline. The island of Mull is so penetrated
by sea-lochs and divided by deep valleys that a comparatively slight
depression would turn it into a group of islands. But, besides its
enormous denudation, this plateau has been subjected to disruption,
and perhaps also to subsidence, from subterranean movements. In the
southern portion of the island of Mull it has been broken up by the
intrusion of large bosses and sheets of gabbro, and by masses as
well as innumerable veins of various granitoid and felsitic rocks.
In Ardnamurchan, it has suffered so much disturbance from the same
cause that its original structure has been almost obliterated over a
considerable area. Moreover, it has been dislocated by many faults,
by which different portions have been greatly shifted in level. The
most important of these breaks is one noticed by Professor Judd, and
visible to every tourist who sails up the Sound of Mull. It traverses
the cliffs on the Morven side, opposite Craignure, bringing the basalts
against the crystalline schists, and strikes thence inland, wheeling
round into the long valley in which Lochs Arienas and Teacus lie.
On its western side, the base of the basalt-series is almost at the
sea-level; on its eastern side, that platform rises high into the
outliers of Beinn na h-Uamha (1521 feet) and Beinn Iadain. The amount
of displacement is probably more than 1000 feet. Many other minor
faults in the same district show how much the crust of the earth has
been fractured here since older Tertiary time.
[Illustration: Fig. 266.--Basalt-capping on top of Beinn Iadain, Morven.
The hummocky ground to the right consists of the Highland schists
against which the basalts are brought by lines of dislocation.[240]]
[Footnote 240: There are no fewer than three faults in the basalt-capping
on the summit of Beinn Iadain. By bringing the basalts and schists into
juxtaposition, they have given rise to topographical features that can
be seen even from a distance.]
A little to the west of Mull, and belonging originally to the same
plateau, lies the isle of Staffa, the famous columnar basalts of which
first attracted the attention of travellers, and gave to the Tertiary
volcanic rocks of Scotland their celebrity (Fig. 266_a_).
[Illustration:
Fig. 266_a_.--View of the south side of Staffa, showing the bedded
and columnar structure of the basalt. The rock in which the cave to
the left hand has been eroded is a conglomeratic tuff underlying
the basalt; to the right is Fingal's Cave. These caverns bear
witness to the enormous erosive power of the Atlantic breakers.
]
In spite of the extent to which it has suffered from denudation and
subterranean disturbance, and indeed in consequence thereof, the Mull
plateau presents clear sections of many features in the history of
the basalt-outflows and of the subsequent phases of Tertiary volcanic
action which cannot be seen in the more regular and continuous
tableland of Antrim. Moreover, it still possesses in its highest
mountain, Ben More (3169 feet), a greater thickness, and probably
a higher series, of lavas than can now be seen in any other of the
plateaux.
The difficulties, already referred to in regard to Antrim, of tracing
the probable form of ground on which the volcanic eruptions began, are
even greater in the case of the Mull plateau. We can dimly perceive
that the depression in the crystalline rocks of the Highlands which
had, from at least the older part of the Jurassic period, stretched in
a N.N.W. direction along what is now the western margin of Argyleshire,
lay beneath the sea in Jurassic time, and was then more or less filled
up with sedimentary deposits. The hollow appears thereafter to have
become a land-valley, whence the Jurassic strata were to a large extent
cleared out by denudation before its subsequent submergence under the
sea in which the upper Cretaceous deposits accumulated. Professor
Judd has shown that relics of these Cretaceous strata appear on both
sides of the plateau from under the protecting cover of basalt-sheets.
But, before the volcanic eruptions began, the area had once again
been raised into land, and the youngest Secondary formations had been
extensively eroded.
In their general aspect the basalts of Mull agree with those of
Antrim, and the circumstances under which they were erupted were no
doubt essentially the same. But considerable differences in detail are
observable between the succession of rocks in the two areas. When I
first visited the island in 1866, the only available maps, with any
pretensions to accuracy, were the Admiralty charts; but, as these do
not give the interior except in a generalized way, it was difficult
to plot sections from them, and to arrive at satisfactory conclusions
as to the thickness of different groups of rock. Accordingly, as the
successive nearly flat flows of basalt can be traced from the sea-level
up to the top of Ben More, I contented myself with the fact that the
total depth of lava-beds in Mull was at least equal to the height of
that mountain, or 3169 feet. The publication of the Ordnance Survey
Maps now enables us to make a nearer approximation to the truth. From
the western base of the magnificent headland of Gribon, the basalts in
almost horizontal beds rise in one vast sweep of precipice and terraced
slope to a height of over 1600 feet, and then stretch eastwards to
pass under the higher part of Ben More, at a distance of some eight
miles. They have a slight easterly inclination, so that the basement
sheets seen at the sea-level, at the mouth of Loch Scridain, gradually
sink below that level as they go eastward. It is not easy to get a
measurement of dip among these basalts, except from a distance. If we
take the inclination at only 1°, the beds which are at the base of the
cliff on the west, must be about 700 feet below the sea on the line
of Ben More, which would give a total thickness of nearly 3900 feet
of bedded lava below the top of that mountain. We shall not probably
overestimate the thickness of the Mull plateau if we put it at 3500
feet.
The base of the volcanic series of Mull can best be seen on the
south coast at Carsaig, and at the foot of the precipices of Gribon.
As already stated, it is there found resting above Cretaceous and
Jurassic rocks. The lowest beds are basalt-tuffs, of the usual dull
green colour. They are in places much intermingled with sandy and
gravelly sediment, as if the volcanic debris had fallen into water
where such sediment was in course of deposition. One of the most
interesting features, indeed, in this basement part of the series, is
the occurrence of bands of non-volcanic material which accumulated
after the tuffs and some of the lavas had been erupted, but before the
main mass of basalts. Those at Carsaig include a lenticular bed, 25
feet thick, of rolled flints, which, with some associated sandy bands,
lies between sheets of basalt. On the opposite side of the promontory
is the well-known locality of Ardtun, from which the first land-plants
in the volcanic series were determined. The actual base of the basalts
is not there seen, being covered by the sea. The "leaf-beds," with
their accompanying sandstones, gravels, and limestone, lie upon a
sheet of basalt, which in some parts is exceedingly slaggy on the
top, passing down into a black compact structure, and assuming at
the base of the cliff a columnar arrangement, with the prisms curved
and built up endways towards each other. Some of the gravels exceed
30 feet in thickness, and consist of rolled flints, bits of chalk,
and pieces of basalt and other basic igneous rocks. But some of
their most interesting ingredients are pebbles of sanidine lavas,
which have been recognized in them by Prof. G. Cole.[241] No known
protrusions of such lavas occur anywhere beneath or interstratified
with the plateau-basalts of this district. As will be afterwards
shown, all the visible acid rocks, the geological relations of which
can be ascertained, are here of younger date than these basalts. I am
disposed to regard the fragments found in the Ardtun conglomerates as
probably derived from some of the basalt-conglomerates of the plateau,
in which fragments of siliceous igneous rocks do occur. Though there
is no evidence that any lavas of that nature were here poured out at
the surface before or during the emission of the basalts, the contents
of these fragmental volcanic accumulations suggest that such lavas,
already consolidated, lay at some depth beneath the surface, and that
fragments were torn off from them during the explosions that threw out
the materials of the basalt-conglomerates to the surface.
[Footnote 241: _Quart. Jour. Geol. Soc._ xliii. (1887) p. 277.]
The succession of strata at the Ardtun headland varies considerably in
a short distance, some of the sedimentary deposits rapidly increasing
or diminishing in thickness. The section as measured by Mr. Starkie
Gardner is as follows[242]:--
Columnar basalt, 40 feet.
Position of first leaf-bed, obscured by grass, about 2 feet.
Gravel varying from about 25 feet to a maximum of nearly 40 feet.
Black or second leaf-bed, 2-1/2 feet.
Gravel about 7 feet.
Grey clay, 2 feet.
Laminated sandstone, 6 inches, with 3 inches of fine limestone,
containing leaves at the base.
Clay, with leaves at base, 1 foot.
Clunch, with rootlets, 7 inches.
Amorphous basalt, becoming columnar at base, about 60 feet.
[Footnote 242: _Op. cit._ p. 280.]
Mr. Starkie Gardner has called attention to the extraordinarily fresh
condition of the vegetation in some of the layers of the Ardtun
section. One of the leaf-beds he has found to be made up for an inch or
two of a pressed mass of leaves, lying layer upon layer, and retaining
almost the colours of dead vegetation. Among the plants represented is
a large purple _Ginkgo_ and a fine _Platanites_, one leaf measuring
15-1/2 inches long by 10-1/2 broad. The characteristic dicotyledonous
leaves at this locality possessed relatively large foliage.[243]
[Footnote 243: For fuller local details regarding the Ardtun leaf-beds,
I may refer to the original paper by the Duke of Argyll (_Quart. Jour.
Geol. Soc._ vii. p. 89), and to the memoir by Mr. Starkie Gardner (_op.
cit._ xliii. (1887), p. 270).]
To the early observations of Macculloch we are indebted for the
record of an interesting fact in connection with the vegetation of
the land-surface over which the first lava-flows spread. He figured a
vertical tree trunk, imbedded in prismatic basalt, and rightly referred
it to some species of fir.[244] This relic may still be seen under the
basalt precipices of Gribon. Mr. Gardner found it to be "a large trunk
of a coniferous tree, five feet in diameter, perhaps _Podocarpus_,
which has been enveloped, as it stood, in one of the flows of trap to
the height of 40 feet. Its solidity and girth evidently enabled it to
resist the fire, but it had decayed before the next flow passed over
it, for its trunk is a hollow cylinder filled with debris, and lined
with the charred wood. A limb of another, or perhaps the same tree, is
in a fissure not far off."[245]
[Footnote 244: _Western Islands_, vol. i. p. 568, and plate xxi. Fig. 1.]
[Footnote 245: _Quart. Jour. Geol. Soc._ xliii. p. 283.]
At different levels in the volcanic series of Mull, beds of lignite and
even true coal are observable. These seem to be always mere lenticular
patches, only a few square yards in extent. The best example I have
met with lies among the basalts near Carsaig. It is in part a black
glossy coal, and partly dull and shaly. Some years ago it was between
two and three feet thick, but now, owing to its having been dug away by
the shepherds, only some six or eight inches are to be seen. It lies
between two basalt-flows, and rapidly disappears on either side.
More frequent than these inconstant layers of fossil vegetation are
the thin partings of tuff and layers of red clay, sometimes containing
iron-ore, which occur at intervals throughout the series between
different flows of basalt. But even such intercalations are of trifling
thickness, and only of limited extent. The magnificent precipices of
M'Gorry's Head and Gribon expose a succession of beds of columnar
amorphous and amygdaloidal basalt, which must attain a thickness of at
least 2500 feet, before they are overlain by the higher group of pale
lavas in Ben More. On the east side of the island, thin tuffs and bands
of basalt-conglomerate occur on different horizons among the bedded
basalts, from near the sea-level up to the summit of the ridge which
culminates in Beinn Meadhon (2087 feet), Dùn-da-Ghaoithe (2512 feet),
and Mainnir-nam-Fiadh (2483 feet). Reference has already been made to
the remarkably coarse character of some of the breccias intercalated
among the basalts in this part of Mull, and to the enormous dimensions
of some of the masses of mica-schist and quartzite which have been
carried up from a depth of 2000 feet or more by volcanic agency (see
_ante_, p. 196, and Fig. 262).
Above the ordinary compact and amygdaloidal basalt comes the higher
group of pale lavas already referred to as forming the uppermost
part of Ben More, whence it stretches continuously along the pointed
ridge of A'Chioch, and thence northwards into Beinn Fhada. The same
lavas are likewise found in two outliers, capping Beinn a' Chraig,
a mile further north, and I have found fragments of them on some of
the loftier ridges to the south-east. This highest and youngest group
of lavas in the plateaux has been reduced to mere isolated patches,
and a little further denudation will remove it altogether. Yet it is
not less than about 800 feet thick, and consists of bedded andesitic
or trachytic lavas, which alternate with and follow continuously
and conformably upon the top of the ordinary plateau-basalts. These
dull, finely crystalline or compact, light-grey rocks weather with a
characteristic platy form, which has been mistaken for the bedding
of tuffs. The fissility, however, has none of the regularity or
parallelism of true bedding, and may be observed to run sometimes
parallel with the bedding of the sheets, sometimes obliquely or even
at right angles to it. Even where this structure is best developed,
the truly crystalline nature of the rocks can readily be detected.
Some of them are porphyritic and amygdaloidal, the very topmost bed
of the mountain being a coarse amygdaloid. Intercalated with these
curious rocks there are others in which the ordinary characters of
the dolerites and basalts of the plateaux can be recognised. The
amygdaloids are often full of delicate prisms of epidote.
In Mull, as in the other areas of terraced basalts, we everywhere meet
with gently inclined sheets, which do not thicken out individually or
collectively in any given direction, except as the result of unequal
denudations. So far as I have been able to discover, they afford
no evidence of any great volcanic cone from which they proceeded.
Their present inclinations are unquestionably due, as in Ireland, to
movements subsequent to the formation of the plateau. In Loch-na-keal
they dip gently to the E.N.E.; in Ulva and the north-west coast to
N.N.E.; near Salen to W.S.W. on the one side, and N.W. on the other.
Round the southern and eastern margins of the mountainous tract of the
island, they dip generally inwards to the high grounds.
The Mull plateau presents a striking contrast to that of Antrim, in
the extraordinary extent to which it has been disrupted by later
protrusions of massive basic and acid rocks over a rudely circular
area, extending from the head of Loch Scridain to the Sound of Mull,
and from Loch-na-keal to Loch Buy. The bedded basalts have been invaded
by masses of dolerite, gabbro, and granophyre, with various allied
kinds of rock. They have not only been disturbed in their continuity,
but have undergone considerable metamorphism.
Again, further to the north, in the promontory of Ardnamurchan,
the plateau has been disrupted in a similar way, and only a few
recognisable fragments of it have been left. These changes will be more
appropriately discussed in connection with similar phenomena in the
other plateaux further north.
CHAPTER XXXVIII
THE BASALT-PLATEAU OF THE PARISH OF SMALL ISLES--RIVERS OF
THE VOLCANIC PERIOD
iii. PARISH OF SMALL ISLES PLATEAU
The parish of Small Isles includes the islands of Eigg, Rum, Canna,
Sanday and Muck (Map VI.). The fragmentary basalt-plateau which it
contains, although the smallest of the whole series, is surpassed by
none in the variety and interest of its geology. It contains by far the
most complete records of the rivers which, during the volcanic period,
flowed across the lava plains. And it alone has preserved a relic of
the latest lava which, after the basalt-plateau had been built up and
had been greatly eroded, flowed over the denuded surface in streams of
volcanic-glass that found their way into a river-channel and sealed it
up.
That the fragments of the basaltic plateau preserved in each member
of the group of the Small Isles were once connected as a continuous
volcanic plain can hardly be doubted. Indeed, as already stated, they
were not improbably united with the plateau of Skye on the north, and
with that of Mull, Morven and Ardnamurchan oh the south. Taking the
whole space of land and sea within which the basalt of Small Isles
is now confined, we may compute it at not much less than 200 square
miles. In Eigg, Muck, Canna and Sanday the basalts retain their almost
horizontal position, and from underneath them the Jurassic strata
emerge in the first of these islands. The central part of the plateau
in the island of Rum has suffered greatly from denudation. It now
consists of four small outliers of basalt, which lie at levels of
1200 feet and upwards, on the western slope. The basalt is underlain
by a thick mass of red Torridon Sandstone, which, with some gneisses
and schists, forms the general underlying platform of this island.
These rocks are doubtless a continuation of the red sandstone and
schists of Sleat, in Skye, and like them have been subjected to those
post-Cambrian convolutions and metamorphism whereby the Lewisian
Gneiss and Torridon Sandstone have been brought above younger rocks,
and have been crushed and rolled out so as to assume a new schistose
arrangement. Before the time when volcanic action began, a mass of high
ground, consisting of these ancient rocks, stood where the island of
Rum is now situated. The streams of basalt spread around it, not only
covering the surrounding low tracts of Jurassic rocks, but gradually
accumulating against the hills, and thus reducing them both in area
and in height above the plain.[246] Viewed from Canna the western coast
of Rum presents a striking picture of the general relations of the
volcanic masses of the Inner Hebrides and of the enormous denudation
which they have undergone (Fig. 267). The Torridon Sandstones are
there seen to mount into ranges of hills, capped with outliers of
the basalt-plateau, while behind rise the great eruptive bosses of
gabbro and granophyre. The edges of the sheets that form the outliers
would, if prolonged, cover the northern or lower half of the island,
where pre-Cambrian rocks form the surface. In the southern half,
the continuity of the basalt has been partly obscured and partly
destroyed by the protrusion of the great masses of gabbro that form the
singularly picturesque mountain group to which this island owes its
prominence as a landmark far and wide along the West Coast of Scotland.
[Footnote 246: That the lava-fields did not completely bury this nucleus
of older rocks has been supposed to be shown by the fragments of red
sandstone found in the ancient river-bed of Eigg, which was scooped
out of the basalt-plateau and sealed up under pitchstone. But I am
disposed to think that these fragments, together with those of Jurassic
sandstone, came, not from Rum, but from some district more to the north
and east, as will be explained in a later page. At Canna, a few miles
to the west, fragments of red sandstone not improbably derived from Rum
are abundant in the conglomerates between the basalts.]
[Illustration: Fig. 267.--View of Rum from the harbour of Canna.
The ground indicated by single birds is the area of Torridon Sandstone;
two birds, the plateau basalts; three birds, the gabbro just seen at
one point above the granophyre hills; four birds, the granophyre.]
The most varied and interesting of the fragments of the basaltic
plateau in the area of the Small Isles is that which forms the island
of Canna, with its appendage Sanday. Canna measures five miles in
length by from half a mile to a mile in breadth, and consists entirely
of the rocks of the plateau and their accompaniments. The basalts are
exposed along the north coast in a range of mural precipices rising
to a height of about 600 feet above the sea. From the top of that
escarpment the ground falls by successive rocky terraces and grassy
slopes to the southern shore-line. Sanday, connected with the large
island by a shoal and foot-bridge, is two miles long and 220 to about
1200 yards broad. Its highest cliffs range along its southern shore
to a height of 193 feet, whence they slope gently northward into the
hollow between the two islands. This peculiar topography accounts
for the manner in which the geological sections of most interest are
distributed.
The first, and still the best, account of the geology of these islands
is that of Macculloch. He showed that the rocks all belong to the
series of the plateau-basalts, and he described the presence among them
of a "trap-conglomerate." He noticed the occurrence also of trap-tuff
and the occasional appearance of carbonized wood in these deposits.
Reasoning upon these observations in his characteristically vague and
verbose manner, "bewildered in the regions of conjecture," he concludes
that the basalts instead of belonging to "one general formation" have
been successively deposited on the same spot, "since lapse of time is
evidently implied in the formation of a conglomerate." He inclines to
believe that they have been discharged by ancient volcanoes from which
in the course of time all traces of their original outline have been
more or less completely removed, the existing basalts being merely
fragments of once more extensive masses.[247]
[Footnote 247: _Western Isles_, vol. i. pp. 448-459, and pl. xix. Figs.
2, 3 and 4. See also Jameson's _Mineralogy of the Scottish Isles_.]
Macculloch regarded the intercalated-conglomerates as having been
arranged under water and as marking pauses in the deposition of the
sheets of "trap." He gave two diagrams in illustration of the relations
of these detrital deposits, but he expressed no definite opinion as to
their origin, though from one passage it would seem that he inclined
towards the belief that they were formed in the sea.[248] Since his time,
so far as I am aware, no fresh light has been thrown upon the subject.
[Footnote 248: _Op. cit._ pp. 449, 457, pl. xix. Figs. 2 and 3.]
During a yachting cruise in the summer of 1894 I visited Canna for
the first time and found so much that was new to me in regard to the
history of Tertiary volcanic action, and which demanded a careful
survey, that I returned to the locality the following summer and
remained in the island until I had mapped it and its dependencies upon
the Ordnance Survey sheets on the scale of six inches to a mile. The
following narrative is the result of the observations then made.
As far back as the year 1865 I published an account of an ancient
river-channel which, during the volcanic period, had been eroded on the
surface of the basalt-plateau, and of which a small portion had been
preserved under a stream of pitchstone-lava that had flowed into and
buried it.[249] This water-course, now marked by the picturesque ridge of
the Scuir of Eigg, was shown to have been excavated by a stream which
came from the north-east or east, and to be younger, not only than the
plateau-basalts of the district, but than even the dykes which cut
these basalts. Yet that it belonged to the volcanic period was proved
by the manner in which it had been sealed up and preserved under the
black glassy lava of the Scuir. Its history and the data from which
this history is compiled will be narrated in a later part of this
chapter.
[Footnote 249: _Scenery of Scotland_ (1865); _Quart. Journ. Geo. Soc._
vol. xxvii. (1871), p. 303.]
My examination of the islands of Canna and Sanday, however, brought
to light other and more abundant evidence of river-action in the same
region of the Inner Hebrides, but belonging to an earlier part of the
volcanic period. This evidence reveals that a powerful river, flowing
westwards from the Highland mountains, swept over the volcanic plain,
while the sheets of basalt were still being poured forth, and while
volcanic eruptions were taking place from cones of slag.
The basalt-plateau of Canna resembles in all essential particulars
those of the other Western Isles. Its base is everywhere concealed
under the sea, but from the fragments of Torridon Sandstone in its
agglomerates we may infer that it probably rests on that formation,
like the volcanic outliers in Rum. It is formed of successive sheets of
different basalts including the usual banded, amygdaloidal and columnar
forms. Some of them towards the west are specially marked by the great
abundance and large size of their porphyritic felspars. The magnetic
properties of the basalts at the east end of the island have long been
known, and have given rise to various modern myths regarding their
influence on the compasses of passing vessels.
[Illustration: Fig. 268.--Section of the cliffs below Compass Hill,
Isle of Canna.]
But it is in its conglomerates, tuffs and agglomerates and the light
they cast on some aspects of the volcanic period, elsewhere hardly
recorded, that the geology of Canna possesses a special importance. To
these, therefore, we may at once turn.
The conglomerates are best developed at the eastern end of the island,
where the cliffs present the structure represented in Fig. 268. At the
base, and passing under the level of the sea, lies the agglomerate
(_a_) of a vent which will be described in Chapter xli., together with
other eruptive orifices of the various plateaux (p. 288). This rock
has a somewhat uneven upper surface which rises in places about 150
feet above high tide-mark. Here and there it shades off upward into
the conglomerate that overlies it; water-worn pebbles appear among its
contents, and rude traces of bedding begin to show themselves, until,
within the course of a few feet, we pass upward into an undoubted
conglomerate. Elsewhere, however, and particularly along the precipices
west of Compass Hill, the two deposits are more distinctly marked
off from each other. The agglomerate has there a hummocky, irregular
upper surface, as if it had been thrown down in heaps. The hollows
between these protuberances have been filled up with conglomerate and
sandstone, forming the base of the thick overlying deposit.
It is thus clear that the loose materials of the vent were directly
exposed at the surface when the conglomerate was accumulated, and,
indeed, that these materials served to supply some of the detritus of
which the conglomerate consists. The absence of any trace of a cone and
crater at the vent may perhaps be explicable on the supposition that
their incoherent material was washed down by the currents that swept
along and deposited the conglomerate.
The mass of sedimentary material (_b_) which overlies the agglomerate
of the vent forms a conspicuous feature along the lower half of the
precipices at the eastern end of Canna. It rises to a height of 250
to 300 feet above sea-level, and must reach a maximum thickness of
probably not less than 100 to 150 feet. It gradually descends in a
westward direction, both along the northern cliffs and in the lower
ground round Canna Harbour, insomuch that in about a mile, owing to the
gentle westerly dip of the whole volcanic series, combined with the
effect of a number of small faults, it passes under the level of the
sea.
Great variation in the character of the detritus composing this thick
group of strata may be observed as it is followed westward. On the
cliffs below Compass Hill, as represented in Fig. 268, the coarse
conglomerate with water-worn stones, hardly to be distinguished from
the volcanic agglomerate of the vent, shows more or less distinct
bedding, or at least a succession of coarser and finer bands. Towards
its base it encloses numerous pieces of Torridon Sandstone, sometimes
subangular, but often so well and smoothly rounded as to show that they
must have been long subjected to the action of moving water. It is
further observable that, while in the agglomerate the volcanic stones
have rough surfaces, those in the conglomerate begin to show increasing
evidence of attrition, until, as the deposit is traced upwards, they
become almost as well rounded and water-worn as the non-volcanic stones.
Yet amidst and overlying these proofs of transport from some little
distance lie abundant huge slags and blocks of amygdaloidal lava,
sometimes closely aggregated, sometimes scattered through a volcanic
tuff or ashy sandstone. The composition and structure of these stones,
and the manner of their dispersion through the deposit, leave little
doubt that they were ejected from the vent. We are thus confronted
with the interesting fact that, while the materials of the volcanic
cone were being washed down by running water, eruptions were still
taking place. But by degrees these indications of contemporaneous
volcanic activity diminish. The detrital materials become coarser and
more distinctly water-rolled until they pass into greenish sandstones
and fine conglomerates. Yet the matrix even of these higher sediments
is largely composed of fine volcanic detritus, and probably points to
occasional discharges of dust and ashes.
Various sills or intrusive sheets have been injected into this
sedimentary group along the precipices at the east end of Canna, and
form there lenticular bands. One of these (_c_) is shown in Fig. 268.
Immediately above the massive greenish pebbly sandstone (_d_) which
caps the stratified series lies a group of basalts (_e_), composed of
several distinct beds, having a united thickness of from 80 to 100
feet. The lowest of these has a regular columnar structure, while those
overlying it exhibit the confused starch-like grouping of curved and
rather indistinctly-formed prisms.
The next band in upward succession is one of conglomerate (_f_), which
runs as a continuous and conspicuous feature along the upper part of
the cliff. This rock presents in many respects a strong contrast to the
conglomerates underneath. It is dull-green to yellow in colour, and
is well stratified, being marked by the interstratification of finer
layers, and passing down into a band of pebbly sandstone, which rests
immediately on the basalt (_e_). Its component stones are thoroughly
water-worn, ranging up to six inches or even more in length. But its
most distinctive character lies in the nature of its pebbles. Instead
of consisting mainly of volcanic materials, these stones have almost
all been transported for some distance. They include abundant fragments
of Torridon Sandstone, gneiss, schists, grits, and other rocks like
those in Rum and Western Inverness-shire. No such rocks exist _in situ_
in Canna. The nearest tract of Torridon Sandstone is in Rum, about four
miles to the eastward. But the pieces of schist and epidotic grit like
the rocks of the Western Highlands, have probably travelled at least 30
miles.
It is important to observe that all these transported stones indicate
a derivation from some source lying to the eastward of Canna. The
evidence in this respect agrees with that furnished by the ancient
river-gravel under the pitchstone of the Scuir of Eigg. It is clear
that the waters which found their way across the lava-fields of this
part of the Inner Hebrides took their rise somewhere to the eastward,
probably among the mountains of Inverness-shire.
The conglomerate now described is from 40 to 50 feet thick. It can be
followed along the face of the cliffs for more than a mile on the north
side of Canna. Less persistent on the south side, its outcrop strikes
from the edge of the precipice inland, keeping to the south of the top
of Compass Hill. It is well seen in the ravine above the Coroghon, but
cannot be followed further westward among the basalt-terraces. Yet,
though this stratified intercalation is not traceable far as a band of
conglomerate, the same stratigraphical horizon is probably indicated
elsewhere by other kinds of sedimentary deposits, to which further
reference will be made in the sequel.
The section now described establishes the existence of at least two
successive platforms of conglomerate in the volcanic series. Following
these platforms along their outcrop, we obtain additional light on
their origin, and on the topographical conditions under which they
were deposited, and we learn further that other prolonged intervals,
which were likewise marked by intercalations of sedimentary material,
occurred in the outpouring of the basalts.
Taking first the lower conglomerate of Compass Hill and tracing it
westward, we find it to form the depression in which the sheltered
inlet of Canna Harbour lies. It is exposed along the shores and also
in the islands enclosed within the same bay. But it is not traceable
further west, possibly because it seems to sink beneath the level of
the sea. To the south-east, though it is there likewise for the most
part concealed under the waves, it rises above them in one or two parts
of the coast-line of Sanday, particularly at the Uamh Ruadh or Red
Cave, and likewise on a surf-beaten skerry off Ceann an Eilein, the
highest part of the Sanday cliffs--a distance of about a mile and a
half from Compass Hill. Throughout this space it retains its remarkably
coarse character and is mainly made up of volcanic material.
The numerous sections exposed in Canna Harbour enable us to study the
composition and local variations of this curious deposit. On the north
side of the basin, while the lower part of the sedimentary series
continues to be an exceedingly coarse volcanic conglomerate, it passes
upward into finer conglomerates, tuffs, and shales. In front of Canna
House the imbedded blocks are of large size, occasionally as much as
three or four feet in diameter. They are still more gigantic on the
island of Eilean a' Bhaird, where I found one to contain 150 cubic feet
in the exposed part, the rest being still concealed in the matrix. As
they are generally somewhat rounded, here and there markedly so, most
of these stones have probably undergone a certain amount of attrition
in water. The great majority of them, and certainly all those of
larger size, are pieces of basalt, dolerite, andesite, etc. Among them
huge blocks of amygdaloid and coarsely vesicular lava are specially
abundant. Some of these look like pieces of slag torn from the upper
surface of lava-streams. Others, displaying a highly vesicular centre
and a close-grained outer crust, are suggestive of bombs. It is
interesting to note here again that the amygdaloidal blocks present
their zeolitic infiltrations so precisely like those of the amygdaloids
of the plateau that it seems reasonable to suppose the carbonate of
lime, zeolites, etc. to have been introduced before the blocks were
imbedded in the conglomerate.
The whole aspect of this deposit is eminently volcanic. It looks like
a vast sheet of lava-fragments swept away from one or more cones of
slags and cinders, or from the scoriaceous surface of a lava-stream.
Where the vesicles were still empty, the large boulders could be more
easily swept along by moving water. But a powerful current must have
been needed to transport and wear down into more or less rounded forms
blocks of basic lava, many of which must weigh several tons. The large
block on Eilean a' Bhaird probably exceeds 12 tons in weight.
Besides the obviously volcanic contents of the conglomerate there occur
here also, as in the Compass Hill cliffs, abundant pieces of Torridon
Sandstone. These stones are notably smaller in size and more perfectly
water-worn and even polished than the blocks of lava. Obviously they
have travelled further and have undergone more prolonged attrition.
The matrix of the rock consists essentially of the fine detritus of
basic lavas, probably mingled with true volcanic dust. The coarser
parts display only the feeblest indication of stratification; indeed,
in a limited exposure the rock might be regarded as a tumultuous
agglomerate. But the manner in which the deposit is intercalated
with, and sometimes overlies, green tuffs and shales, together with
the water-worn condition of its stones, shows that it has not been
accumulated in a volcanic chimney, but has been thrown down by some
powerful body of water, with probably the co-operation of volcanic
discharges.
While the composition of the conglomerate suffices to indicate that
this deposit was formed at a time when some volcano was active in the
immediate neighbourhood, singularly convincing proofs of the work of
this vent are to be seen in the form of intercalated sheets of lava.
Thus on Eilean a' Bhaird the boulders of the conglomerate are overlain
and wrapped round by a sheet of rudely prismatic basalt, with lines of
vesicles arranged in the direction of the bedding. A similar relation
can be traced along the beach between Canna House and the wooden pier,
where successive sheets of basalt have flowed over the conglomerate
(Fig. 269).
But, besides coarse volcanic detritus, the sedimentary platform
represented by the lower conglomerate of Compass Hill includes other
deposits of which good sections may be examined all round Canna
Harbour. Beds of fine well-stratified dull-green tuff pass by an
admixture of pebbles into fine ashy conglomerate or pebbly sandstone,
and by an increase in the proportion of their fine detritus into
volcanic mudstone and fine shales. The shales vary from a pale grey
or white tone into blackish grey, brown, and black. They are well
stratified and are frequently interleaved with layers of fine tuff.
The darker bands are carbonaceous, and are not infrequently full
of ill-preserved vegetation. Indeed, leaves and stems in a rather
macerated condition are of common occurrence in all the shaly layers.
Here and there, especially in some ashy shales in front of Canna House,
I observed a recognisable _Sequoia_. The mudstones are dull green,
close-grained shattery rocks composed of fine volcanic detritus,
and pass both laterally and vertically into shales, tuffs, and
conglomerates. They suggest showers of fine dust or streams of volcanic
mud. They, too, contain fragmentary plants.
It is a noteworthy fact that the sedimentary intercalations among the
Canna basalts generally end upward in carbonaceous shales or coaly
layers. The strong currents and overflows of water, which rolled
and spread out the coarse materials of the conglomerates, gave way
to quieter conditions that allowed silt and mud to gather over the
water-bottom, while leaves and other fragments of vegetation, blown
or washed into these quiet reaches, were the last of the suspended
materials to sink to the bottom. Good illustrations of this sequence
in the case of the lower conglomerate zone of Canna may be studied
along the shores of Sanday, from the Catholic Chapel eastwards. The
fine pebbly sandstones, tuffs, and shales, which there overlie the
coarse conglomerate, are surmounted by dark brown or black carbonaceous
shale, with lenticles of matted vegetation that pass into impure coal.
Immediately overlying this coaly layer comes a sheet of prismatic
vesicular basalt, followed by another with an exceedingly slaggy
texture.
Lenticles of shale and mudstone likewise occur in the heart of the
finer parts of the conglomerate, especially towards the top, as may be
seen in the section exposed beneath the basalt behind the first cottage
west from Canna House. One of the most interesting layers in this
section is a seam of tuff, varying up to about two inches in thickness,
which lies at the top of the lenticular band of tuffs and shales, and
immediately beneath the band of basalt-conglomerate, on which a basalt,
carrying a vesicular band near its bottom, rests. Traced laterally, the
dark brown tuff of this seam gradually passes into a series of rounded
bodies and flattened shells composed of a colourless mineral which has
evidently been developed _in situ_ after the deposition of the tuff.
Mr. Harker's notes on thin slices made from this band are as follows:--
"This is a rusty-brown, dull-looking rock, rather soft and seemingly
light, but too absorbent to permit of its specific gravity being
tested. The dark brown mass is in great part studded with little
spheroidal bodies, 1/50 to 1/10 inch in diameter, of paler colour,
but the larger ones having a dark nucleus. In other parts larger flat
bodies have been formed, as if by the coalescence of the spheroids,
extending as inconstant bands in the direction of lamination for
perhaps 1/2 inch, with a thickness of 1/10 inch or less. The appearance
is that of a spherulitic rather than an oolitic structure.
"A slice [6658 A] shows the general mass of the rock to be of an
extremely finely divided but coherent substance of brown colour, which
can scarcely be other than a fine volcanic dust, composed of minute
particles of basic glass or 'palagonite' compacted together. Scattered
through this are fragments of crystals recognizable as triclinic and
perhaps monoclinic felspars, green hornblende, augite, olivine (?), and
magnetite, usually quite fresh.
"The curious spheroidal and elongated growths already mentioned are
better seen in another slide [6658 B], where they occupy the larger
part of the field, leaving only an interstitial framework of the brown
matrix. The substance of the little spheroids is clear, colourless,
and apparently structureless. The centre is often occupied by an
irregularly stellate patch of brown colour, and sometimes cracks tend
to run in radiating fashion, but these are the only indications of
radial structure. The outer boundary is sharply defined, and where the
slice is shattered the spheroids have separated from the matrix. The
matrix is darker than in the normal rock, being obscured by iron-oxide
which we may conceive as having been expelled from the spaces occupied
by the spheroids. The little crystal-fragments are enclosed in the
spheroids as well as in the matrix, but there is no appearance of
their having served as starting-points for radiate growths. The flat
elongated bodies are like the spheroids, with merely the modifications
implied in their different shape.
"The identity of the clear colourless substance seems to be rather
doubtful. It is sensibly isotropic and of refractive power distinctly
lower than that of felspar. These characters would agree with analcime,
which is not unknown as a contact-mineral; but it is difficult to
understand how analcime, even a lime-bearing variety like that of Plas
Newydd,[250] could be formed in abundance from palagonitic material.
An alternative supposition, perhaps more probable, is that the clear
substance is a glass, modified from its former nature, especially by
the expulsion of the iron-oxide into the remaining matrix. A comparison
is at once suggested with certain types of 'Knotenschiefer,' but
respecting the thermal metamorphism of fine volcanic tuffs there seems
to be little or no direct information."
[Footnote 250: Henslow, _Trans. Camb. Phil. Soc._ (1821), vol. i. p.
408; Mr. Harker, _Geol. Mag._ (1887), p. 414. Mr. W. W. Watts suggests
a comparison with the hexagonal bodies figured by Mr. Monckton in an
altered limestone from Stirlingshire: _Quart. Journ. Geol. Soc._, vol.
li. p. 487.]
Lenticular interstratifications of shale and mudstone make their
appearance even in the coarser parts of the conglomerate, as may be
observed on the beach below Canna House where, as shown in Fig. 269,
some shales and tuffs (_a_) full of ill-defined leaves are surmounted
by a conglomerate (_b_). The deposition of this overlying bed of
boulders has given rise to some scooping-out of the finer strata
underneath. Subsequently both the conglomerate and shales have been
overspread by a stream of dolerite (_c_), the slaggy bottom of which
has ploughed its way through them.
[Illustration: Fig. 269.--Lava cutting out conglomerate and shale.
Shore below Canna House.]
Before discussing the probable conditions under which the group of
sedimentary deposits now described was formed, we may conveniently
follow the upper conglomerate band of the Compass Hill, and note the
variations in structure and composition which its outcrop presents.
This yellowish conglomerate can be traced along the cliffs for more
than a mile, when it descends below the sea-level at the solitary stack
of Bod an Stòl. A few hundred yards further west, what is probably
the same band appears again at the base of the precipice overlain
by prismatic basalts. But the conglomerate, here only 12 feet thick,
is made of much finer detritus which, largely composed of volcanic
material, includes small well-rounded and polished pebbles of Torridon
Sandstone. Beneath it lies a bed of dark shale, with remains of plants,
resting immediately on a zeolitic amygdaloid which plunges into the
sea. The chief interest of this locality is to be found in the shale
which, instead of being at the top of the sedimentary group, lies at
the bottom. I was informed by Mr. A. Thom that leaves had been obtained
from this shale; but I was not successful in my search for them. The
locality is only accessible by boat, and, as the coast is fully exposed
to the Atlantic swell, landing at the place is usually difficult and
often impossible.
About a mile and a half still further west, where a foreshore fronts
the precipice of Earnagream at the Camas Tharbernish, a band of
intercalated sedimentary material underlies the great escarpment of
basalts and rests upon the slaggy sheet with the singular surface
already referred to (p. 187). This band not improbably occupies the
same platform as the upper conglomerate of Compass Hill. It is only
about seven feet thick, the lower four feet consisting of a dull green
pebbly tuff or ashy sandstone, with small rounded pieces of Torridon
Sandstone, while the upper three feet are formed of dark shale with
crowded but indistinct remains of plants. Here the more usual order
in the sequence of deposition is restored. The shale is indurated
and shattery, so that no slabs can be extracted without the use of
quarrying tools.
[Illustration:
Fig. 270.--Section of shales and tuffs, with a coniferous stump
lying between two basalt-sheets, Cùl nam Marbh, Canna.
]
Rather less than half a mile towards the south, on the roadside
at the gully of Cùl nam Marbh, the basalts enclose a sedimentary
interstratification which not improbably lies on the same horizon as
those just described along the northern shore. The relations of the
rocks at this locality are shown in Fig. 270. A remarkable slaggy
basalt (_a_) rises into a hummock, against which have been deposited
some fine granular tuffs (_b_) whereof only a few inches are visible,
that pass up into a thin band of dark shale (_c_), including a layer
of pebbly ferruginous tuff, with small rounded pea-like pieces of
basalt, basic pumice, bole, limonite, etc. At the top of this shale an
irregular parting of coaly material (_d_) lies immediately under the
slaggy base of the succeeding basalt (_e_). It will be observed that
this upper lava cuts out the shale and thus comes to rest directly
upon the lower sheet. At the point where it begins to descend it has
caught up and enclosed a small tree-stump (_d´_) which stands upright
on the coaly parting and shale. This stump, at the time of my visit,
measured five inches in height by three inches in breadth; it had
been thoroughly charred and was crumbling away on exposure, but among
the pieces which I took from it sufficient trace of structure can be
detected with the microscope to show the tree to have been a conifer.
We have here another instance of the deposition of volcanic dust and
fine mud in a pool that filled a hollow in the lava-field. Again we see
that the closing act of sedimentation was the subsidence of vegetable
matter in the pool, which was finally buried under another outflow of
basalt.
[Illustration: Fig. 271.--Dùn Mòr, Sanday. (From a photograph by Miss
Thom.)]
It is on the southern coast of the isle of Sanday that the higher
intercalations of sedimentary material among the basalts are most
instructively displayed. At the eastern end of this island, as already
stated, the lowest and coarsest conglomerate is visible on a skerry
immediately to the south of the headland Ceann an Eilein. It doubtless
underlies the Sanday cliffs, but is not there visible, for the
basalts descend below sea-level. These volcanic sheets have a slight
inclination westward; hence in that direction we gradually pass into
higher parts of the series. In the Creag nam Faoileann (Seamews' Crag)
and the gully that cuts its eastern end, likewise in the two singularly
picturesque stacks of Dùn Mòr and Dùn Beag (Big and Little Gull Rocks),
which here rise from the foreshore, two distinct platforms of detrital
material may be noticed among the basalts. Both of these can be well
seen on Dùn Mòr, about 100 feet high, which is represented in Fig.
271. The lower band, four or live feet thick, is here a rather coarse
conglomerate which lies upon a sheet of scoriaceous basalt that extends
up to the base of the Creag nam Faoileann. It is directly overlain by
another basalt, about 30 feet thick, which dips seawards and forms a
broad shelving platform, whereon the tides rise and fall. On this stack
a second coarse conglomerate, about 10 feet thick, forms a conspicuous
band about a third of the height from the bottom; it is composed mainly
of well-rounded blocks of various lavas up to 18 inches or more in
diameter, but it contains also pieces of Torridon Sandstone. It is
covered by about 60 feet of basalt, which towards the base is somewhat
regularly columnar, but passes upward into the wavy, starch-like,
prismatic structure.
If now we trace these two intercalated zones of conglomerate along
the shore, we find them both rapidly to change their characters and
to disappear. The lower, though formed of coarse detritus under the
Dùn Mòr, passes on the opposite cliff in a space of not more than 60
yards, into fine tuff and shale, about six feet thick, which become
carbonaceous at the top where they are overlain by the next basalt.
A hundred yards to the east, the band likewise consists of tuffs and
ashy shales, which underlie the basalts on the Dùn Beag, and again
show the usual coaly layers at the top. On the east side of the gully
in the coast, about 160 yards to the north-east of Dùn Mòr, the same
band is reduced to not more than three feet in thickness, consisting
chiefly of fine conglomerate, wherein well water-worn pebbles of
Torridon Sandstone and epidotic grit appear among the predominant
volcanic detritus. This conglomerate is surmounted by a few inches of
dark carbonaceous mudstone or shale. Rough slaggy basalts lie above and
below the band.
The upper conglomerate dies out, both towards the east and the west,
in the cliff opposite to Dùn Mòr, dwindling down at last to merely a
few pebbles between the basalts. It lies in a kind of channel or hollow
among these lavas. This depression, in an east and west direction,
cannot be more than about 65 yards broad.
Probably still higher in the series of basalts is another intercalation
of sedimentary layers which may be seen in the little bay to the east
of Tallabric, rather more than a mile to the west of the Creag nam
Faoileann. It rests upon a coarsely slaggy amygdaloid, and is from six
to ten feet in thickness. The lower and larger part of the deposit
consists of greenish pebbly sandstone and fine conglomerate, largely
composed of basaltic detritus, but including abundant well-smoothed and
polished pebbles of Torridon Sandstone, green grit, quartzite, etc. The
stones vary from mere pea-like pebbles up to pieces two or three inches
long, the largest being generally fragments of slag and amygdaloid
which are less water-worn than the sandstones and other foreign
ingredients. The uppermost two or three feet of the intercalation
consist of dark carbonaceous mudstone or shale, made up in large
measure of volcanic detritus, which may have been derived partly from
eruptions of fine dust, partly from subærial disintegration of the
basalt-sheets. Some layers of these finer strata are full of remains of
much macerated plants.
Other thin coaly intercalations have been observed among the basalts
of Canna, some of which may possibly mark still higher horizons than
those now described. But, confining our attention to the regular
sequence of intercalations exposed along the Sanday coast, we find
at least four distinct platforms of interstratified sediment among
the plateau-basalts of this district. Each of these marks a longer or
shorter interval in the outflow of lava, and points to the action of
moving-water over the surface of the lava-fields.
We may now consider the probable conditions under which this
intervention of aqueous action took place. The idea that the sea had
anything to do with these conglomerates, sandstones, and shales may
be summarily dismissed from consideration. The evidence that the
basalt-eruptions took place on a terrestrial surface is entirely
convincing, and geologists are now agreed upon this question.
Excluding marine action, we have to choose among forms of fresh
water--between lakes on the one hand and rivers on the other. That the
agency concerned in the transport and deposition of these strata was
that of a river may be confidently concluded on the following grounds:--
1. The large size and rolled shape of the boulders in the
conglomerates. To move blocks several tons in weight, and not only to
move them but to wear them into more or less rounded forms, must have
required the operation of strong currents of water. The coarse detritus
intercalated among the basalts is quite comparable to the shingle of a
modern river, which descends with rapidity and in ample volume from a
range of hills.
2. The evidence that the materials of the conglomerates are not
entirely local, but include a marked proportion of foreign stones.
The proofs of transport are admirably exhibited by pieces of Torridon
Sandstone, epidotic grit, quartzite, and other hard rocks none of which
occur _in situ_ except at some distance from Canna. These stones are
often not merely rounded, but so well smoothed and polished as to show
that they must have been rolled along for some considerable time in
water.
3. The lenticular character and rapid lithological variations of the
strata, both laterally and vertically. The coarse conglomerates die
out as they are followed along their outcrop and pass into finer
sediment. They seem to occur in irregular banks, which may not be more
than 200 feet broad, like the shingle-banks of a river. The coarser
sediment generally lies in the lower part of the sedimentary group.
But cases may be observed, such as that shown in Fig. 269, where fine
sediment, laid down upon the bottom conglomerate, has subsequently been
overspread by another inroad of coarse shingle. Such alternations are
not difficult to understand if they are looked upon as indicating the
successive floods and quieter intervals of a river.
For these reasons I regard the platforms of sedimentary materials
intercalated among the basalts of Canna and Sanday as the successive
flood-plains of a river which, like the rivers that traverse the
lava-deserts of Iceland, flowed perhaps in many separate channels
across the basalt-fields of the Inner Hebrides, and was liable to have
its course shifted from time to time by fresh volcanic eruptions.
That this river came from the east or north-east and had its source
among the Western Highlands of Inverness-shire, may be inferred from
the nature of the stones which it has carried for 30 miles or more
along its bed. And that it crossed in its course the tract of Torridon
Sandstone, of which a portion still remains in Rum, is manifest from
the abundance of the fragments of that formation in the conglomerates.
With the remarkable exception of the section on Dùn Beag, to be
immediately referred to, no trace of any eroded channel of this river
through the lavas of the great volcanic plain has been preserved.
Possibly frequent invasions of its bed by streams of basalt from
different vents hindered it from remaining long enough in one course
to erode anything like a gorge or canon. But, in any case, the main
channel of the river probably lay rather to the east of the present
islands of Canna and Sanday, on ground which is now covered by the
sea. The banks or sheets of boulder-conglomerate undoubtedly show
where its current swept with great force over the lava-plain, but the
manner in which these coarser materials are so often covered with fine
silt suggests that the sedimentary materials now visible were rather
deposited on the low grounds over which the steam rushed in times of
flood. Pools of water would often be left after such inundations, and
in these depressions silt would gradually accumulate, partly carried
in suspension by the river, partly washed in by rain, while drift-wood
that found its way into these eddies, and leaves blown into them from
the trees and shrubs of the surrounding country, would remain for
some time afloat and would be the last of the detritus to sink to the
bottom. Hence, no doubt, the carbonaceous character of the hardened
silt in the upper part of each intercalation of sediment.
If we were to look upon the volcanic materials in the conglomerates
as derived from the subærial disintegration of the fields of basalt,
we should be compelled to admit a very large amount of erosion of the
surface of the volcanic plain during the period when the river flowed
over that tract. It would be necessary to suppose not only that there
was a considerable rainfall, but that the differences of temperature,
either from day to night, or from summer to winter, were so great as
to split up the lavas at the surface, in order to provide the river
with the blocks which it has rolled into rounded boulders. I do not
think, however, that such a deduction would be sound. If we compare
the materials that have filled up the large eruptive vent at the east
end of Canna (to be afterwards described) with the great majority of
the blocks in the coarse conglomerates, we cannot fail to note their
strong resemblance. The abundance of lumps of slaggy lava in the
river-shingle corresponds with their predominance in the agglomerate
of the vent. The boulders of basalt, dolerite, and andesite which
crowd the conglomerates need not have been derived from the action
of atmospheric waste on the lava-fields, but might quite well have
been mainly supplied by the demolition of volcanic cones of fragmental
materials.
[Illustration: Fig. 272.--View of the Dùn Beag, Sanday, seen from the
south.
(From a Photograph by Miss Thom.)]
That such has really been the chief source of the blocks in the
conglomerates I cannot doubt. At the east end of Canna we actually
detect a volcanic cone, partly washed down and overlain by a pile
of river-shingle. There were probably many such mounds of slag and
stones along lines of fissure all over the lava-fields. The river in
its winding course might come upon one cone after another, and during
times of flood, or when its waters burst through any temporary barrier
created by volcanic operations it would attack the slopes of loose
material and sweep their detritus onward. At the same time, the current
would carry forward its own natural burden of far-transported sediment,
and hence on its flood-plains, buried and preserved under sheets of
basalt, we find abundant pebbles of the old Highland rocks which it had
borne across the whole breadth of the basaltic lowland.
But the destruction of volcanic cones was probably not the only source
of the basaltic detritus in the conglomerates of Canna and Sanday. I
have shown that these conglomerates pass laterally into tuffs, and
are sometimes underlain, sometimes overlain, with similar material.
It is quite obvious that their deposition was contemporaneous with
volcanic action in the immediate neighbourhood, and that at least part
of their finer sediment was obtained directly from volcanic explosions.
In wandering over the coast-sections of these coarse deposits, I
have been impressed with the enormous size of many of the stones,
their resemblance to the ejected blocks of the agglomerate, and the
distinction that may sometimes be made with more or less clearness
between their rather angular forms and the more rounded and somewhat
water-worn aspect of the other boulders. It seems to me not improbable
that some of the remarkably coarse masses of unstratified conglomerate
in Canna Harbour consist largely of ejected blocks from the adjacent
vent.
[Illustration: Fig. 273.--View of Dùn Beag, Sanday, from the north. The
island of Rum in the distance.
(From a Photograph by Miss Thom.)]
The only instance which I have observed of erosion of the basalt
contemporaneous with the operations of the river that spread out this
conglomerate is to be found in the striking stack of Dùn Beag already
alluded to.[251] This extraordinary monument of geological history
forms an outlying obelisk which rises from the platform of the shore
to a height of about 70 feet. Seen from the south-west, it appears
to consist entirely of bedded basalt resting on some stratified tuff
and shale which intervene between these lavas and that of the broad
platform of basalt on which the obelisk stands. On that side it
presents no essential difference from the structure of the Dùn Mòr to
the west, save that the lower conglomerate of that outlier is here
represented by fine sediment, and the upper conglomerate is wanting.
The general aspect of this south-western front of the stack is shown in
Fig. 272. If, however, we approach the rock from the coast-gully to the
north, we form a very different impression of its structure. It then
appears to consist chiefly of conglomerate with a capping of basalt on
the top (Fig. 273). Not until a close scrutiny is made of the eastern
and western faces of the column do the true structure and history of
this singular piece of topography become apparent.
[Footnote 251: This pinnacle of rock is referred to by Macculloch in
his account of Canna, and is figured in Plate xix. Fig. 3 of his work
already cited. But neither his description nor his drawing conveys any
idea of the real structure of the rock.]
[Illustration: Fig. 274.--Section of eastern front of Dùn Beag.
_a_, Very shaggy amygdaloidal basalt; _b_, shales and tuff; _c_, slaggy
and jointed basalts; _d_, conglomerate; _e_, prismatic basalt.
The dotted lines indicate the supposed form of the ravine.]
On the eastern front, the section represented in Fig. 274 is exposed.
At the bottom, forming the pediment of the column, lies a sheet of
slaggy and vesicular or amygdaloidal basalt (_a_), which shelves gently
in a south-westerly direction into the sea. The lowest band (_b_)
in the structure of the stack is a thin group of lilac, brown, and
green shale and volcanic mudstone or tuff, which encloses pieces of
coniferous wood, and becomes markedly carbonaceous in its uppermost
layers. Above these strata on the south front comes the pile of bedded
basalts (_c_) with their slaggy lower and upper surfaces. But as we
follow them round the east side, we find them to be abruptly cut off by
a mass of conglomerate (_d_). That the vertical junction-line is not a
fault is speedily ascertained. The lower platform of slaggy basalt runs
on unbroken under both shales and conglomerate. Moreover, the line of
meeting of this conglomerate with the basalts that overlie the shales
is not a clean-cut straight wall, but displays projections and recesses
of the igneous rocks, round and into which the materials of the
conglomerate have been deposited. The pebbles may be seen filling up
little crevices, passing under overhanging ledges of the basalts, and
sharply truncating lines of scoriaceous structure in these rocks. The
same relations may be observed on the west front of the stack. There
the ashy shales and tuffs are sharply cut out by the conglomerate,
which wraps round and underlies a projecting cornice of the slaggy
bottom of the basalt that rests on the stratified band (Fig. 275).
[Illustration:
Fig. 275.--Enlarged Section on the western side of Dùn Beag.
_a_, amygdaloid; _b_, tuff; _c_, ashy shales; _d_, layer of coaly
shale; _e_, amygdaloidal basalts conglomerate.
]
The conglomerate is rudely stratified horizontally, its bedding being
best shown by occasional partings of greenish sandstone. It consists
of well-rounded, polished, and water-worn stones, chiefly of members
of the volcanic series--basalts, and dolerites, both compact and
amygdaloidal or slaggy--but with a conspicuous admixture of Torridon
Sandstone, gneiss, grey granite, grit and different schists. The
coarsest part of the deposit lies toward the bottom where the volcanic
blocks, some of them being six and eight feet in diameter, may have
originally fallen from the basalts against which the conglomerate now
reposes. The far-transported stones are also of considerable size,
pieces of granite and gneiss frequently exceeding a foot in length. The
well-rounded pebbles of foreign materials have been washed into the
interstices between the large volcanic blocks.
It is, I think, tolerably clear that the wall of basalt against which
this conglomerate has been laid down is one of erosion. The beds of
basalt have here been trenched by some agent which has likewise scooped
out the soft underlying shales, and even cut them away from under their
protecting cover of basalt. There can be little hesitation in regarding
this agent as a water-course, which for some considerable interval of
time continued to dig its channel through the hard basalts. There is
not room enough between the basalt-wall of Dùn Beag and the opposite
cliffs of the shore (where no trace of this conglomerate is to be seen)
for any large stream to have found its way. I do not therefore seek to
identify this relic of an ancient waterway with the channel of the main
river which deposited the conglomerate bands of Canna and Sanday. More
probably it was either a mere torrential chasm, or a tributary stream
draining a certain part of the volcanic plateau and allowed to retain
its channel long enough to be able to erode it to a depth of nearly
50 feet. Erosion had reached down through the underlying tuffs to the
slaggy basalt below, but before it had made any progress in that sheet
its operations were brought to an end at this locality by the floods
that swept in the coarse shingle, and by the subsequent stream of
basalt of which a mere outlying fragment now forms the upper third of
the stack (_e_, Fig. 274).
That the ravine or gully of Dùn Beag probably lay within the reach
of the floods of the main river, may be inferred from the number and
size of the far-transported rocks in its conglomerate. It was filled
up gradually, but the conditions of deposition remained little changed
during the process, except that the largest blocks of rock were swept
into the chasm in the earlier part of its history, while much smaller
and more water-worn shingle were introduced towards the close.
Denudation, which has performed such marvels in the topography of the
West of Scotland since older Tertiary time, has here obliterated every
trace of this ancient gully, save the little fragment of one of the
walls which survives in the stack of Dùn Beag. When in the course of
centuries this picturesque obelisk shall have yielded to the action
of the elements, the last leaflet of one of the most interesting
chapters in the geological history of the Inner Hebrides will have been
destroyed.
The question naturally arises--What was the subsequent history of the
river which has left so many records of its floods entombed among the
basalts of Canna and Sanday? In particular, can any connection be
traced or plausibly conjectured between it and the river-bed preserved
under the Scuir of Eigg? To this question I shall return after the
evidence for the existence and date of the latter stream has been laid
before the reader.
In the chain of the Inner Hebrides, broken as it is in outline and
varied in its types of scenery, there is no object more striking than
the island of Eigg. Though only about five miles long and from a mile
and a half to three miles and a half broad, and nowhere reaching
a height of so much as 1300 feet, this little island, from the
singularity of one feature of its surface, forms a conspicuous and
familiar landmark. Viewed in the simplest way, Eigg may be regarded as
consisting of an isolated part of the basaltic plateau which, instead
of forming a rolling tableland or a chain of hills with terraced sides,
as in Antrim, Mull and Skye, has been so tilted that, while it caps
a lofty cliff about 1000 feet above the waves at the north end, it
slopes gently along the length of the island to the south end. In its
southern half, however, the ground rises, owing to the preservation of
an upper mass of lavas, which denudation has removed from the northern
half. On this thicker part of the plateau stands the distinguishing
feature of the island, the strange fantastic ridge of the Scuir, which,
seen from the north or south, looks like a long steep hill-crest,
ending in a sharp precipice on the east. Viewed from the east, this
precipice is seen to be the end of a huge mountain-wall, which rises
vertically above the basalt-plateau to a height of more than 350 feet.
The accompanying map (Fig. 276) shows that the ridge of the Scuir
corresponds with the area occupied by a mass of pitchstone, and that
while the basaltic rocks cover the whole of the rest of the southern
half of the island, they gradually rise towards the north, successive
members of the Jurassic series making their appearance until, at the
cliffs of Dunan Thalasgair, the latter cover the greater part of the
surface, and leave the volcanic rocks as a mere stripe capping the
cliffs. In the section (Fig. 277) the general structure of the island
is represented.
[Illustration: Fig. 276.--Geological Map of the Island of Eigg.
P, Pitchstone-lava of the Scuir; R, old river gravel under
pitchstone; _p p_, small veins of Pitchstone; _b b_, dykes, veins
and sheets of intrusive basalt; the short black lines running
north-west and south-east are basalt dykes; _f f_, granophyre
sills; D, bedded basalts with occasional tuffs; F, andesite; 1, 2,
3, 4, clays, shales, sandstones, limestones, etc. (Jurassic); xx,
Loch Beinn Tighe; x, Loch a Bhealaich. --> General dip of the rocks.
]
[Illustration: Fig. 277.--Section of the geological structure of the
Island of Eigg.
P, Pitchstone-lava of Scuir; _c_, ancient river-gravel; _p p_,
pitchstone veins; _f f_, intrusive granophyre, etc.; _b b_, dolerite
and basalt dykes and veins; B, intrusive dolerite and basalt-sheets; D,
bedded dolerites and basalts; F, andesite bed; 1-4, Jurassic rocks.]
In Eigg the fragment of the basalt-plateau which has been preserved,
rests unconformably on successive platforms of the Jurassic formations.
Its component sheets of lava rise in cliffs around the greater part
of the island. As they dip gently southwards their lower members are
seen along the northern and eastern shores, while on the south-west
side their higher portions are exposed in the lofty precipices which
there plunge vertically into the sea. The total thickness of the
volcanic series may here be about 1100 feet. The rocks consist of the
usual types--black, fine-grained, columnar and amorphous basalts,
more coarsely crystalline dolerites, dull earthy amygdaloids with red
partings, and occasional thin bands of basalt-conglomerate or tuff.
The individual beds range in thickness from 20 to 50 or 60 feet.
Though they seem quite continuous when looked at from the sea, yet, on
closer examination, they are found not unfrequently to die out, the
place of one bed being taken by another, or even by more than one,
in continuation of the same horizon. The only marked petrographical
variety which occurs among them is a light-coloured band which stands
out conspicuously among the darker ordinary sheets of the escarpment on
the east side of the island. The microscopic characters of this rock
show it to belong to the same series of highly felspathic, andesitic,
or trachitic lavas as the "pale group" of Ben More, in Mull. It is
strongly vesicular, and the cells are in some parts so flattened and
elongated as to impart a kind of fissile texture to the rock. There
can be no doubt that this band is a true lava, and that it was poured
out during the accumulation of the basalt-plateau. It supplies an
interesting example of the intercalation of a lighter and less basic
lava among the ordinary heavy basic basalts and dolerites.
That feature of the island of Eigg which renders it so remarkable
and conspicuous an object on the west coast is the long ridge of the
Scuir. Rising gently from the valley which crosses the island from
Laig Bay to the Harbour, the basaltic plateau ascends south-westwards
in a succession of terraces, until along its upper part it forms a
long crest, from 900 to 1000 feet above the sea, to which it descends
on the other or south-west side, first by a sharp slope, and then by
a range of precipices. Along the watershed of this crest runs, in a
graceful double curve, the abrupt ridge of the Scuir, terminating
on the north-west at the edge of the great sea-cliff (975 feet), and
ending off on the south-east in that strange well-known mountain-wall
(1272 feet high) which rises in a sheer cliff nearly 300 feet above
the basalt-plateau on the one side and more than 400 feet on the other
(Fig. 278). The total length of the Scuir ridge is two miles and a
quarter, its greatest breadth 1520, its least breadth 350 feet. Its
surface is very irregular, rising into minor hills and sinking into
rock-basins, of which nine are small tarns, besides still smaller
pools, while six others, also filled with water, lie partly on the
ridge and partly on the basaltic plateau. No one, indeed, who looks
on the Scuir from below, and notes how evenly it rests upon the
basalt-plateau, would be prepared for so rugged a landscape as that
which meets his eye everywhere along the top of the ridge. Two minor
arms project from the east side of the ridge; one of these forms the
rounded hill called Beinn Tighe (968 feet), the other the hill of A
chor Bheinn.
[Illustration: Fig. 278.--View of the Scuir of Eigg from the east.]
Singular as the Scuir of Eigg is, regarded merely as one of the
landmarks of the Hebrides, its geological history is not less peculiar.
The natural impression which arises in the mind when this mountain
comes into view for the first time is, that the huge wall is part of a
great dyke or intrusive mass which has been thrust through the older
rocks.[252] It was not until after some time that the influence of this
first impression passed off my own mind, and the true structure of the
mass became apparent.
[Footnote 252: Hay Cunningham remarks:--"In regard to the relations of
the pitchstone-porphyry of the Scuir and the trap-rocks with which it
is connected, it can, after a most careful examination around the whole
mass, be confidently asserted that it exists as a great vein which
has been erupted through the other Plutonic rocks--thus agreeing in
age with all the other pitchstones of the island." Macculloch leaves
us to infer that he regarded the rock of the Scuir to be regularly
interstratified with the highest beds of the dolerite series (_Western
Isles_, i. p. 522). Hugh Miller speaks of the Scuir of Eigg as "resting
on the remains of a prostrate forest."--_Cruise of the Betsy_, p. 32.]
The ridge of the Scuir, presenting as it does so strong a
topographical contrast to the green terraced slopes of the
plateau-basalts on which it rests, consists of some very distinct
bands of black and grey lava, long known as "pitchstone-porphyry." To
the nature and history of these rocks I shall return after we have
considered a remarkable bed of conglomerate which lies below them. On
the lower or southern side of the ridge the bottom of the pitchstone,
dipping into the hill, is exposed on the roof of a small cave where the
ends of its columns form a polygonal reticulation. It is there seen
to repose upon a bed of breccia or conglomerate, having a pale-yellow
or grey felspathic matrix like the more decomposing parts of the grey
devitrified parts of the pitchstone. Through this deposit are dispersed
great numbers of angular and subangular pieces of pitchstone, some
of which have a striped texture. Fragments of basalt, red (Torridon)
sandstone, and other rocks are rare; and the bed suggests the idea that
it is a kind of brecciated base or floor of the main pitchstone mass. A
similar rock is found along the bottom of the pitchstone on both sides
of the ridge (_c_, in Fig. 279). Here and there where this breccia is
only a yard or two in thickness, it consists of subangular fragments
of the various dolerites and basalts of the neighbourhood, together
with pieces of red sandstone, quartzite, clay-slate, etc. The matrix
is in some places a mass of hard basalt debris; in others it becomes
more calcareous, passing into a sandstone or grit in which chips and
angular or irregular-shaped pieces of coniferous wood are abundant.[253]
A little further east, beyond the base of the Scuir, a patch of similar
breccia is seen, but with the stones much more rounded and smoothed.
This outlier rests against the denuded ends of the basalt-beds forming
the side of the hill. Its interest arises from the evidence it affords
of the prolongation of the deposit eastward, and consequently of the
former extension of the precipice of the Scuir considerably beyond its
present front.
[Footnote 253: The microscopic structure of this wood was briefly
described by Witham (_Fossil Vegetables_, p. 37), and two magnified
representations were given to show its coniferous character. Lindley
and Hutton further described it in their "Fossil Flora," naming it
_Pinites eiggensis_, and regarding it as belonging to the Oolitic
series of the Hebrides--an inference founded perhaps on the erroneous
statement of Witham to that effect. William Nicol corrected that
statement by showing that the wood-fragments occurred, not among the
"lias rocks," but "among the debris of the pitchstone" (_Edin. New
Phil. Journal_, xviii. p. 154). Hay Cunningham, in the paper already
cited, states that the fossil wood really lies in the pitchstone
itself! The actual position of the wood, however, in the breccia and
conglomerates underlying the pitchstone is beyond all dispute. I have
myself dug it out of the bed. The geological horizon assigned to this
conifer, on account of its supposed occurrence among Oolitic rocks,
being founded on error, no greater weight can be attached to the
identification of the plant with an Oolitic species. Our knowledge
of the specific varieties of the microscopic structure of ancient
vegetation is hardly precise enough to warrant us in definitely fixing
the horizon of a plant merely from the examination of the minute
texture of a fragment of its wood. From the internal organization of
the Eigg pine, there is no evidence that the fossil is of Jurassic age.
From the position of the wood above the dolerites and underneath the
pitchstone of the Scuir it is absolutely certain that the plant is not
of Jurassic but of Tertiary date.]
It is at the extreme north-western extremity of the pitchstone ridge,
however, that the most remarkable exposure of this intercalated
detrital band is now to be seen. Sweeping along the crest of the
plateau the ridge reaches the edge of the great precipice of Bideann
Boidheach, by which its end is truncated, so as to lay open a section
of the gravelly deposit along which the pitchstone flowed.
The accompanying diagram (Fig. 279) represents the natural section
there exposed. Rising over each other in successive beds, with a hardly
perceptible southerly dip of 2°, the sheets of basalt form a mural
cliff about 700 feet high. The bedded character of these rocks and
their alternations of compact, columnar, amorphous and amygdaloidal
beds are here strikingly seen. They are traversed by veins and dykes
of an exceedingly close-grained, sometimes almost flinty, basalt. But
the conspicuous feature of the cliff is the hollow which has been
worn out of these rocks, and which, after being partially filled with
coarse conglomerate, has been buried under the huge pitchstone mass of
the Scuir. The conglomerate consists of water-worn fragments, chiefly
of dolerite and basalt, but with some also of the white Jurassic
sandstones, imbedded in a compacted sand derived from the waste of the
older volcanic rocks. The grey devitrified bands in the pitchstone, so
conspicuous at the east end of the Scuir, here disappear and leave the
conglomerate covered by one huge overlying mass of glassy pitchstone.
[Illustration: Fig. 279.--Natural Section at the Cliff of Bideann
Boidheach, north-west end of the Scuir of Eigg.
_a a_, Bedded dolerites and basalts; _b_, basalt dykes and veins; _c_,
ancient river-bed filled with conglomerate; _p_, pitchstone of the
Scuir.]
If any doubt could arise as to the origin of the mass of detritus
exposed under the pitchstone at the east end of the Scuir it would be
dispelled by the section at the west end, which shows with unmistakable
clearness that the conglomerate is a fluviatile deposit and lies in the
actual channel of the ancient river which was eroded out of the basalt
plateau, and was subsequently sealed up by streams of pitchstone-lava.
An examination of the fragments of rock found in the conglomerate
affords here, as in Canna and Sanday, some indication of the direction
in which the river flowed. The occurrence of pieces of red sandstone,
which no one who knows West-Highland geology can fail to recognize
as of Torridonian derivation, at once makes it clear that the higher
grounds from which they were borne probably lay to the north or
north-east. The fragments of white sandstone may also have been
derived from the same quarter, for the thick Jurassic series of Eigg
once extended further in that direction. The pieces of quartzite and
clay-slate bear similar testimony to an eastern or north-eastern
source. In short, there seems every probability that this old Tertiary
river flowed through a forest-clad region, of which the red Torridon
mountains of Ross-shire, the white sandstone cliffs of Raasay and Skye,
and the quartzite and schist uplands of Western Inverness-shire are but
fragments, that it passed over a wide and long tract of the volcanic
plateau, and continued to flow long enough to be able to carve out for
itself a channel on the surface of the basalt. Its course across what
is now the island of Eigg took a somewhat north-westerly direction,
probably guided by inequalities on the surface of the lava-plain. It is
there marked by the winding ridge of the Scuir, the pitchstone of which
flowed into the river-bed and sealed it up. Several minor spurs, which
project from the eastern side of the main ridge, show the positions
of small tributary rivulets that entered the principal channel from
the slopes of the basaltic tableland. One of these, on the south-east
side of the hill called Corven, must have been a gully in the basalt
with a rapid or waterfall. The pitchstone has flowed into it, and some
of the rounded pebbles that lay in the channel of this vanished brook
may still be gathered where the degradation of the pitchstone has once
more exposed them to the light. That the Eigg river here flowed in a
westerly direction may be inferred from the angle at which the beds of
the small tributaries meet the main stream, and also from the fact that
the old river-bed at the east end of the Scuir is considerably higher
than at the west end.
Several features in the geological structure of this locality serve to
impress on the mind the great lapse of time represented by the erosion
of the river-channel of Eigg. Thus at the narrowest point of the
pitchstone ridge, near the little Loch a' Bhealaich, the bottom of the
glassy lava is about 200 feet above its base on the south side, so that
the valley cut out of the plateau-basalts must have been more than 200
feet deep. Even the little tributaries had cut ravines or cañons in the
basalts before the ground was buried under the floods of pitchstone. In
the most northerly spur of the ridge, for example, the hill of Beinn
Tighe, which represents one of these tributaries, shows a considerable
difference between the level of the bottom of the pitchstone on the
east and west sides.
Again, all along the ridge of the Scuir, the basalt-dykes are abruptly
cut off at the denuded surface on which the pitchstone rests. This
feature is conspicuously displayed on the great sea-wall at the
west end (Fig. 279). The truncation of the dykes demonstrates that
a considerable mass of material must have been eroded before these
lava-filled fissures could be laid bare at the surface. And the removal
of this material shows that the denudation must have been continued for
a long period of time.
The river-channel of Eigg, since it was eroded long after the cessation
of the outflows of basalt in the plateau of Small Isles, must be much
later in origin than those of Canna and Sanday which, as we have seen,
were contemporaneous with the basalt-eruptions. But the river that
excavated the channels and deposited the gravels may have been the same
in both areas.
In dealing with this subject, though the evidence is admittedly scanty,
we are not left wholly to conjecture. A consideration of the general
topographical features of the wide region of the Inner Hebrides, from
the beginning of the volcanic period onward, will convince us that, in
spite of the effects of prolonged basalt-eruptions, the persistent flow
of the drainage of the Western Highlands must have taken a westerly
direction. It was towards the west that the low grounds lay. Though
the long and broad valley which stretched northwards from Antrim,
between the line of the Outer Hebrides and the West of Scotland, was
gradually buried under a depth of two or three thousand feet of lava,
the volcanic plain that overspread it probably remained even to the
end lower than the mountainous Western Highlands. Hence the rivers,
no matter how constantly they may have had their beds filled up and
may have been driven into new channels, would nevertheless always seek
their way westwards into the Atlantic.
On Canna and Sanday the traces of a river are preserved which poured
its flood-waters across the lava-fields in that part of the volcanic
region, while streams of basalt were still from time to time issuing
from vents and fissures. Not more than fourteen miles to south-east
stands the Scuir of Eigg, with its buried river-channel and its
striking evidence that there, also, a river flowed westwards, but at a
far later time, when the basalt-eruptions had ceased and the volcanic
plain had been already deeply trenched by erosion, yet before the
subterranean fires were finally quenched, as the pitchstone of the
Scuir abundantly proves.
When one reflects upon the enormous denudation of this region, to which
more special reference will be made in the sequel, one is not surprised
that many connecting links should have been effaced. The astonishment
rather arises that so continuous a story can still be deciphered.
Even, however, had the original record been left complete, it would
have been exceedingly difficult to trace the successive mutations of a
river-channel during long ages of volcanic eruptions. Such a channel
would have been concealed from view by each lava-stream that poured
into it, and would not have been again exposed save by the very process
of erosion that destroys while it reveals.
While, therefore, there is not and can never be any positive proof that
in the fluviatile records of Canna, Sanday and Eigg successive phases
are registered in the history of one single stream, I believe that this
identity is highly probable. It was a river which seems to have risen
among the mountains of Western Inverness-shire, and it had doubtless
already taken its course to the sea before any volcanic eruptions
began. It continued to flow westwards across the lava-floor that
gradually spread over the plains. Its channel was constantly being
filled up by fresh streams of basalt or deflected by the uprise of new
cinder-cones. But, fed by the Atlantic rains, it maintained its seaward
flow until the general subsidence which carried so much of the volcanic
plain below the sea. Yet the higher part of this ancient water-course
is no doubt unsubmerged, still traversing the schists of the Western
Highlands as it has done since older Tertiary time. It may, perhaps, be
recognized in one of the glens which carry seaward the drainage of the
districts of Morar, Arisaig, or Moidart.
[Illustration: Fig. 280.--View of the Scuir of Eigg from the South.]
Let us now turn to the remarkable lava which has sealed up the
river-channel of Eigg, and of which the remaining fragment stands up
as the great ridge of the Scuir. This rock presents characters that
strongly distinguish it from the surrounding basalts. It is not one
single uniform mass, but consists of a number of distinct varieties,
some of which are a volcanic glass, while others are a grey "porphyry,"
or devitrified pitchstone. These are arranged in somewhat irregular,
but well-marked, and, in a general sense, horizontal sheets. On the
great eastern terminal gable of the Scuir this bedded structure is not
clearly displayed, for the cliff seems there to be built up of one
homogeneous mass, save a markedly columnar band that runs obliquely up
the base of the precipice (Fig. 278). If, however, the ridge is looked
at from the south, the truly bedded character of its materials becomes
a conspicuous feature. Along the cliffs on that side the two varieties
of rock are strongly distinguished by their contrasting colour and
mode of weathering, the sombre-hued pitchstone standing up in a huge
precipice striped with columns, and barred horizontally with bands of
the pale-grey "porphyry," which, from its greater proneness to decay,
seems sunk into the face of the cliff. At the south-east end of the
ridge the bedding is especially distinct. West of the precipices, to
the south of the Loch a' Bhealaich, the dark pitchstone which forms the
main mass is divided by two long parallel intercalations of grey rock,
and two other short lenticular seams of the same material (see Figs.
280, 281). It is clear from these features, which are not seen by most
travellers who pass Eigg in the tourist-steamer that the Scuir is in
no sense of the word a dyke.
But although the Scuir is thus a bedded mass, the bedding is far
different from the regularity and parallelism of that which obtains
among the bedded basalt-rocks below. Even where no intervening
"porphyry" occurs, the pitchstone can be recognized as made up of many
beds, each marked by the different angle at which its columns lie. And
when the "porphyry" does occur and forms so striking a division in the
pitchstone, its beds die out rapidly, appearing now on one horizon, now
on another, along the face of the cliffs, and thickening and thinning
abruptly in short distances along the line of the same bed. Perhaps the
best place for examining these features is at the Bhealaich, the only
gully practicable for ascent or descent, at the south-eastern face of
the ridge.
[Illustration: Fig. 281.--View of the Scuir of Eigg from the South-west
of the Loch a' Bhealaich, showing the bedded character of the mass.]
By much the larger part of the mass of the Scuir consists of vitreous
material. As a rule this rock is columnar, the columns being much
slimmer and shorter than those of the basalt-rocks. They rise sometimes
vertically, and often obliquely, or project even horizontally from the
face of the cliff. They are seldom quite straight, but have a wavy
outline; and when grouped in knolls here and there along the top of
the ridge they remind one of gigantic bunches of some of the Palæozoic
corals, such as _Lithostrotion_. In other cases they slope out from
a common centre, and show an arrangement not very unlike that of a
Highland peat-stack.
The pitchstone of the Scuir differs considerably in petrographical
character from other pitchstones of the island which occur in dykes
and veins. Its base is of a velvet-black colour, and is so much less
vitreous in aspect than ordinary pitchstone as to have been described
by Jameson and later writers as intermediate between pitchstone and
basalt.[254] A chemical analysis of the rock by Mr. Barker North,[255] gave
the following composition:--
Silica 65·81
Alumina 14·01
Ferric oxide 4·43
Lime 2·01
Magnesia 0·89
Soda 4·15
Potash 6·08
Loss in ignition 2·70
------
100·08
[Footnote 254: _Mineralogy of the Scottish Isles_, vol. ii. p. 47. See
also Macculloch, _Western Isles_, vol. i. p. 521, and Hay Cunningham,
_Mem. Wern. Soc._ vol. viii. p. 155.]
[Footnote 255: _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 379.]
The grey devitrified bands, which occur as a subordinate part of
the mass of the Scuir ridge, are usually somewhat decomposed. Where
a fresh fracture is obtained, the material shows a fine-grained,
sometimes almost flinty, grey felsitic base, containing clear granules
of quartz, and facets of glassy felspar. In some places the rock is
strongly porphyritic. Examined under the microscope it presents a more
thoroughly devitrified groundmass, with the minutest depolarizing
microlites, large porphyritic crystals of plagioclase and sanidine,
grains of augite, and sometimes exceedingly abundant particles of
magnetite.[256]
[Footnote 256: The microscopic structure of the identical pitchstone of
Hysgeir is given on p. 247.]
[Illustration: Fig. 282.--Section at the base of the Scuir of Eigg
(east end).]
Although the line of separation between the grey dull felsitic sheets
and the more ordinary glassy pitchstone is usually well defined,
the two rocks may be observed to shade into each other in such a
manner as to show that the lithoid material is only a devitrified and
somewhat decomposed condition of the glassy rock. This connection is
particularly to be observed under the precipice at the east end of the
Scuir. At that locality the pitchstone is underlain by a very hard
flinty band, varying in colour from white through various shades of
flesh-colour and brown into black, containing a little free quartz and
crystals of glassy felspar. Where it becomes black it passes into a
rock like that of the main mass of the Scuir. Such vitreous parts of
the bed lie as kernels in the midst of the more lithoid and decomposed
rock. The lower six feet of the "porphyry" are white and still more
decomposed. The relations of this mass are represented in Fig. 282,
where the basalt-rocks of the plateau (_a_) are shown to be cut through
by basalt dykes (_b b_), and overlain by the "porphyry" (_c_) and the
pitchstone (_d_). In the porphyry are shown several pitchstone kernels
(_p_, _p_). It is deserving of remark also that in different parts
of the Scuir, particularly along the north side, the bottom of the
pitchstone beds passes into a dull grey earthy lithoid substance, like
that now under description.
The bedded character of the rock of the Scuir and the well-marked
lithological distinction between its several component sheets show the
lava to have been the product of a number of separate outflows that
found their way one after another into the river-valley, which was the
lowest ground in the vicinity of the active vent. There can be little
doubt, I think, that the lava flowed down the valley. Its successive
streams are still inclined from east to west. The vent of eruption,
therefore, ought to be looked for towards the east. Nowhere within the
Tertiary volcanic region is there any boss of pitchstone or any mass
the shape or size of which is suggestive of this vent. In the island of
Eigg no boss of any kind exists, save those of granophyric porphyry to
be afterwards referred to. But none of these affords any satisfactory
links of connection with the rock of the Scuir. More probably the
vent lay somewhere to the east on ground now overflowed by the sea.
The pitchstone veins of Eigg may represent some of the subterranean
extrusions from the same volcanic pipe, and if so, its site could not
be far off.
The rock of the Scuir of Eigg has a special importance in the history
of the volcanic plateaux. It is, so far as we know, the latest of all
the superficial lavas of Britain.[257] From the basalts on which it rests
it was separated by an enormous interval of time, during which these
older lavas were traversed by dykes and were worn down into valleys.
Its presence shows that long after the basalts of Small Isles had
ceased to be erupted, a new outbreak of volcanic activity took place in
this district, when lavas of a more acid composition flowed out at the
surface. Whether this outburst was synchronous with the appearance of
the great granophyric protrusions of the Inner Hebrides, or with the
still later extravasation of pitchstone dykes, can only be surmised.
[Footnote 257: The rocks of Beinn Hiant in Ardnamurchan have been
claimed by Professor Judd as superficial lavas. For reasons to be
afterwards given (p. 318) I regard them as intrusive sheets. Professor
Cole believes the rhyolites and pitchstones of Tardree to be probably
evidence of a volcano later than the basalts of Antrim. As I have not
been able to detect any actual proofs of superficial outflow there, I
relegate the description of the rocks to a future chapter, in which the
acid protrusions will be discussed (p. 426).]
When one scans the great precipice on the west side of Eigg, with
its transverse section of the pitchstone-lava, buried river-bed and
basalt-plateau underneath, there seems no chance of any further
westward trace of the pitchstone being ever found. The truncated end
of the Scuir looks from the top of the cliff out to sea, and the
progress of denudation might have been supposed to have effectually
destroyed all evidence of the continuation of the rock in a westerly
direction. Some years ago, however, my friend Prof. Heddle, while
cruising among the Inner Hebrides, landed upon the little uninhabited
islet of Hysgeir, which, some eighteen miles to the westward of Eigg,
rises out of the open sea. He at once recognized the identity of the
rock composing this islet with that of the Scuir, and in the year 1892
published a brief account of this interesting discovery.[258]
[Footnote 258: Appendix C to _A Vertebrate Fauna of Argyle and the Inner
Hebrides_, by Messrs. J. A. Harvie-Brown and Thomas E. Buckley, p. 248.]
I have myself been able to land on Hysgeir in two successive summers,
and can entirely confirm Prof. Heddle's identification. The islet
stands on the eastern edge of the submarine ridge which, running in a
north-easterly direction, culminates in the island of Canna. Hysgeir
is a mere reef or skerry, of which the top rises only 38 feet above
the Ordnance datum-level. Its surface is one of bare rock, save where
a short but luxuriant growth of grasses has found root on the higher
parts of two or three of its ridges, and on the old storm-beach of
shingle which remains on the summit. The rock undulates in long low
swells, that run in a general direction 20° to 45° west of north, and
are separated by narrow channels or hollows. The place is a favourite
haunt of gulls, terns, eider-ducks and grey seals, and is used by the
proprietor of Canna for the occasional pasturage of sheep or cattle.
So numerous are the sea-fowl during the breeding-season that the
geologist, intent upon his own pursuits, may often tread on their nests
unawares, while he is the centre of a restless circle of white wings
and anxious cries.
The pitchstone of Hysgeir, like that of Eigg, is columnar, the columns
being irregularly polygonal and varying from three to ten inches in
diameter. They are packed so close together that the domes of rock on
which their ends appear look like rounded masses of honeycomb. They may
here and there be observed to be arranged radially with their ends at
right angles to the curved exterior of the ridges, as if this external
surface represented the original form of the cooled pitchstone, and
were not due to mere denudation. There can be no doubt, however, that
the island has been well ice-worn.
At the north-west promontory a beautiful example of fan-shaped grouping
of columns may be observed on a face of rock which descends vertically
into the sea. Here, too, is almost the only section on which the sides
of the columns may be examined, for, as a rule, it is merely their ends
on the rounded domes which are to be observed, and which everywhere
slip under the waves. The columns in a cliff from 15 to 20 feet high
show the slightly wavy, starch-like arrangement so often to be met with
among the plateau-basalts.
The rock presents a tolerably uniform texture throughout, though
in some parts it is blacker, more resinous, and less charged with
porphyritic enclosures than in the general body of the rock. Large
fresh felspars are generally scattered through it. To the naked eye it
reproduces every feature of the pitchstone of the Scuir of Eigg.
A microscopic examination completes our recognition of the identity of
these two rocks. Mr. Harker has examined a thin slice prepared from the
Hysgeir pitchstone, and remarks regarding it that "the large felspars
are not the only porphyritic element. The microscope shows the presence
also of smaller imperfect crystals of augite, very faint green in the
slice, and small grains of magnetite. The felspars have been deeply
corroded by the enveloping magma, and irregular included patches of the
groundmass occupy nearly half the bulk of some of the crystals. This
latter feature is seen especially in some of the larger crystals, which
seem to be sanidine. They are, for the most part, apparently simple
crystals, but in places there is a scarcely defined lamellar twinning,
or, again, small patches not extinguishing with the rest; so that
we are probably dealing with some perthitic intergrowth on a minute
scale.[259]
[Footnote 259: Comp. Prof. Judd's remarks on the Scuir of Eigg rock,
_Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. 380.]
"Rather smaller felspar-crystals are rounded by corrosion, but
lack the inclusions of groundmass; these have albite-and sometimes
pericline-lamellation, and may be referred to oligoclase-andesine.
The groundmass of the rock is a brown glass with perlitic cracks,
enclosing very numerous microlites of felspar about ·001 inch in length
[6619]. The rock is probably to be regarded as a dacite rather than a
rhyolite, and thus agrees with Mr. Barker North's analysis of the Eigg
pitchstone."[260]
[Footnote 260: _Op. cit._ p. 379.]
There is no trace of any conglomerate _in situ_ like that under the
Scuir of Eigg, nor of any other rock, aqueous or igneous. As the
pitchstone everywhere slips under the sea, its geological relations are
entirely concealed.
The great variety of materials met with in the form of boulders on
the island is a testimony to the transport of erratics from the
neighbouring islands and the mainland during the Glacial Period. The
most abundant rock in these boulders is Torridon Sandstone, derived
no doubt from the hills of Rum, but there occur also various kinds of
schist, gneisses, quartzites, granites, porphyries, probably from the
west of Inverness-shire, as well as pieces of white sandstone, probably
Jurassic, which may have come from Eigg.
That the pitchstone of Hysgeir is a continuation of that of the Scuir
may be regarded as highly probable. If not a continuation, it must be
another stream of the same kind, and doubtless of the same date. If
it be regarded as probably a westward prolongation of the Eigg rock,
and if it be about as thick as that mass at the west end of the Scuir,
then its bottom lies 200 or 300 feet under the waves. The river-channel
occupied by the Eigg pitchstone undoubtedly sloped from east to west.
The position of Hysgeir, 18 miles further west, may indicate a further
fall in the same direction at the rate of perhaps as much as 35 feet in
the mile.[261] Unfortunately, however, as no trace of the river-bed can
now be seen on this island, any statement in regard to it must rest on
mere conjecture.
[Footnote 261: _Rep. Brit. Assoc._ 1894, p. 653.]
Although the question of the denudation of the basalt-plateaux since
the close of the volcanic period will be the subject of a special
chapter in a later part of this volume, I cannot here refrain from
calling attention to the pitchstone of Eigg and Hysgeir as one of the
most impressive monuments of denudation to be found within the British
Isles. Though now so prominent an object in the West Highlands, this
rock once occupied the bottom of a valley worn out of the basaltic
tableland. Prolonged and stupendous denudation has destroyed the
connection with its source, has cut down its ends into beetling
precipices, has reduced the former surrounding hills into gentle slopes
and undulating lowland, and has turned the bottom of the ancient valley
into a long, narrow and high crest. Moreover, we see that the erosion
has not been uniform. The great wall of the Scuir does not stand fairly
on the crest of the basalt-plateau but on the south side of it, so that
the southern half of the old valley, with all its surrounding hills,
has been entirely cut away. That subsidence has also come into play in
the destruction of even the youngest parts of the volcanic plateaux
will be more fully discussed in a later chapter. I need only remark
here that the submergence of Hysgeir probably points to extensive
depression of the land-surface on which the lavas were poured out.
CHAPTER XXXIX
THE BASALT-PLATEAUX OF SKYE AND OF THE FAROE ISLES
iv. THE SKYE PLATEAU
This largest and geologically most important of all the Scottish
plateaux comprises the island of Skye, at least as far south as Loch
Eishort, and the southern half of Raasay, but is shown by its sills
to stretch as far as the Shiant Isles on the north, and the Point of
Sleat on the south (see Map VI.). It may be reckoned to embrace an area
of not less than 800 square miles. The evidence that its limits, like
those of the other plateaux, are now greatly less than they originally
were, is abundant and impressive. The truncated edges of its basalts,
rising here and there for a thousand feet as a great sea-wall above the
breakers at their base, and presenting everywhere their succession of
level or gently inclined bars, are among the most impressive monuments
of denudation in this country. But still more striking to the geologist
is the proof, furnished beyond the margins of the plateau, that the
Jurassic and other older rocks there visible were originally buried
deep under the basalt-sheets, which have thus been entirely stripped
off that part of the country.
Throughout most of the district, wherever the base of the basalts can
be seen, it is found to rest upon some member of the Jurassic series,
but with a complete unconformability. The underlying sedimentary strata
had been dislocated and extensively denuded before the volcanic period
began. On the southern margin, however, the red (Torridon) sandstones
emerge from under the basalts of Loch Scavaig, and extending into
the island of Soay are prolonged under the sea into Rum. This ridge
probably represents the range of the ancient high ground of the latter
island already referred to.
Nowhere are the distinctive topographical features and geological
structure of the basalt-plateaux better displayed than in the northern
half of the island of Skye. The green terraced slopes, with their
parallel bands of brown rock formed by the outcrop of the nearly flat
basalt-beds, rise from the bottoms of the valleys into flat-topped
ridges and truncated cones (Fig. 283). The hills everywhere present
a curiously tabular form that bears witness to the horizontal sheets
of rock of which they are composed.[262] And along the sea-precipices,
each excessive sheet of basalt can be counted from base to summit,
and followed from promontory to promontory (Figs. 284, 286). In the
district of Trotternish, the basalt hills reach a height of 2360 feet.
Further west, the singular flat-topped eminences, called "Macleod's
Tables" (Fig. 283) ascend to 1600 feet.
[Footnote 262: These features are more fully described in my _Scenery of
Scotland_, 2nd edit (1887), pp. 74, 145, 216.]
[Illustration: Fig. 283.--Terraced Hills of Basalt Plateau (Macleod's
Tables), Skye.]
Along the western side of Skye, the basalts descend beneath the level
of the Atlantic, save at Eist in Duirinish, where the Secondary strata,
with their belt of intrusive sills, rise from underneath them, and at
the Sound of Soa, where they rest on the Torridon Sandstone. Along
the eastern side, their base runs on the top of the great Jurassic
escarpment, whose white and yellow sandstones rise there, and on the
east side of Raasay, into long lines of pale cliffs. To the south-east,
the regularity of the volcanic plateau is effaced, as in Mull and
Ardnamurchan, by the protrusion of extensive masses of eruptive rocks
constituting the Cuillin and Red Hills, east of which the basalts have
been almost entirely removed by denudation, so as to expose the older
rocks which they once covered, and through which the younger eruptive
bosses made their way. This is undoubtedly the most instructive
district for the study of that late phase in the volcanic history of
Britain comprised in the eruptive bosses of basic and acid rocks.
The magnificent plateau of this island has been so profoundly cut down
into glens and arms of the sea, and its component layers are exposed
along so many leagues of precipice, that its structure is perhaps more
completely laid open than that of any of the other Tertiary volcanic
areas in Britain. It is built up of a succession of basalts and
dolerites of the usual types, which still reach a thickness of more
than 2000 feet, though in this instance, also, denudation has left
only a portion of them, without any evidence by which to reckon what
their total original depth may have been. In rambling over Skye, the
geologist is more than ever struck with the remarkable scarcity and
insignificance of the interstratifications of tuff or of any other
kind of sedimentary deposit between the successive lava-sheets. One
of the thickest accumulations of volcanic tuff and conglomerate has
already been referred to as occurring on the south side of Portree
Harbour, where it attains a depth of about 200 feet. As it is in
immediate connection with its parent vent, it will be more fully
alluded to in Chapter xli. Here, as is so generally observable among
the basalt-plateaux, traces of vegetation are plentiful among the
stratified intercalations, even forming thin seams of lignite and coal,
one of which was formerly worked. That volcanic eruptions, though
possibly of a feebler kind, continued during the interval between the
basalt-outflows at this locality, is shown by the thick accumulation
of tuff and by the occurrence of abundant lapilli of fine basic pumice
among the shales, even to a distance of several miles from the vent.
[Illustration: Fig. 284.--"Macleod's Maidens" and part of Basalt Cliffs
of Skye.]
Another conspicuous intercalation of sedimentary materials in the Skye
plateau occurs on the Talisker cliffs at the mouth of Loch Bracadale,
where, on the face of the great precipice of Rudha nan Clach, some
conspicuous bands of lilac and red are interspersed among the basalts.
These bands were noticed by Macculloch, who described them as varieties
of "iron-clay."[263] I have not had an opportunity of examining them
except from the sea at a little distance. But they suggest a similarity
to some of the variegated clays between the upper and lower basalt
series of Antrim.
[Footnote 263: _Western Islands_, vol. i. p. 376.]
Though good coal is not well developed in the Tertiary volcanic
plateaux of the British Isles, it has already been pointed out that
coaly layers are abundant, and that as the vegetable matter may
confidently be assumed always to indicate terrestrial vegetation, the
presence of the carbonaceous bands may be regarded as good evidence
of some lapse of time between the eruption of the basalts which they
separate. I have also called attention to the fact that the vegetable
material is more especially observable in the highest parts of a
group of intercalated sediments between two sheets of basalt. This
relation, so strikingly exhibited in the isle of Canna, as already
observed, is also to be remarked in the Skye plateau. I may here cite
an interesting example which occurs at the base of the lofty sea-cliff
of An Ceannaich, to the south of Dunvegan Head, on the west coast of
Skye (Fig. 285). At the base of the precipice, ledges of a highly
cellular basalt (_a_) show a singularly scoriaceous and amygdaloidal
structure, with abundant and beautiful zeolites, the hollows of the
upper surface of the sheet being filled in with dark brown carbonaceous
shale, forming a layer from one to fourteen inches thick, marked by
coaly streaks and lenticles (_b_). A band of green and yellow sandstone
(_c_) next supervenes, which, from its pale colour, attracts attention
from a distance, and led me, while yachting along the coast, to land
at the locality in the hope that it might prove to be a plant-bearing
limestone. This sandy stratum is only some three or four inches thick
at the north end of the section, but increases rapidly southward to a
thickness of as many feet or more, when, owing to the cessation of the
underlying shale, it comes to lie directly on the amygdaloid and to
enclose slaggy portions of that rock. Immediately above the sandstone
two or three feet of fissile shale, black with plant-remains (_d_),
include brown layers that yield to the knife like some oil-shales. The
next stratum is a seam of coal (_e_) about a foot thick, of remarkable
purity. It is glossy, hard, and cubical, including layers that break
like jet. It has been succeeded by a deposit of green sand (_f_), but
while this material was in course of deposition another outpouring of
lava (_g_) took place, whereby the terrestrial pool or hollow of the
lava-field, in which the group of sedimentary materials accumulated,
was filled up and buried. This lava is about 20 feet thick, and
consists of a coarsely-crystalline, jointed dolerite with highly
amygdaloidal upper and under surface. Its slaggy bottom has caught
up or pushed aside the layer of green sand, so as to lie directly on
the coal, and has there been converted into the earthy modification
so familiar under the name of "white trap" among our coal-fields. It
is interesting to find that this kind of alteration, where molten
rock comes in contact with carbonaceous materials, is not confined to
subterranean sills, but may show itself in lavas that have flowed over
a terrestrial surface.
[Illustration:
Fig. 285.--Intercalated group of strata between Basalts, An
Ceannaich, western side of Skye.
]
From the frequent intercalation of such local deposits of sedimentary
material between the basalts, we may reasonably infer that during older
Tertiary time the rainfall in North-Western Europe was copious enough
to supply many little lakes and streams of water. As the surface of
the lava-fields decayed into soil, vegetation spread over it, so that,
perhaps for long intervals, some tracts remained green and forest-clad.
But volcanic action still continued to show itself, now from one vent,
now from another. These wooded tracts were buried under overflows of
lava, and, the water-courses being filled up, their streams were driven
into new channels, and other pools and lakes were formed.
[Illustration: Fig. 286.--Escarpment of Plateau-basalts, Cliffs of
Talisker, Skye.]
In no part of the Tertiary volcanic area of Britain can the characters
of the lavas and the structure of the plateaux be better seen than
along the west side of Skye, north of Loch Bracadale. The precipices
rise sheer out of the sea, to heights of sometimes 1000 feet, and from
base to summit every individual bed may be counted. Some particulars
have already been given (p. 192) regarding the average thickness of the
basalt-sheets on this coast-line. The general aspect of these cliffs
and the arrangement of their component lavas is shown in Fig. 286.
As a further detailed illustration of the general succession of the
basalts in the Skye plateau, I give a diagrammatic view of the largest
of Macleod's Maidens--the three weird sea-stalks that rise so grandly
in front of the storm-swept precipice at the mouth of Loch Bracadale.
The height of the stack must be at least 150 feet (Figs. 284 and 287).
About ten distinct sheets of igneous rock can be counted in it, which
gives an average thickness of 15 feet for the individual beds. It will
be observed that there is a kind of alternation between the compact,
prismatic basalts and the more earthy amygdaloids, but that the former
are generally thickest.[264]
[Footnote 264: A striking and illustrative contrast between the relative
thickness of the beds of the two kinds of rock is supplied by the fine
sections of this district. The amygdaloids range from perhaps 6 or 8
to 25 or 30 feet; but the prismatic basalts, while never so thin as
the others, sometimes enormously exceed them in bulk. In the island of
Wiay, for example, a bed of compact black basalt, with the confused
starch-like grouping of columns, reaches a thickness of no less than
170 feet. Its bottom rests upon a red parting on the top of a dull
greenish earthy amygdaloid. It is possible, however, that some of these
columnar sheets of basalt are really sills.]
[Illustration: Fig. 287.--Section of the largest of Macleod's Maidens.]
These features, which are repeated on cliff after cliff, may be
considered typical for all the plateaux. Another characteristic point,
well displayed here, is the intervening red parting between the
successive beds. If the occurrence and thickness of this layer could
be assumed as an indication of the relative lapse of time between
the different flows of lava, it would furnish us with a rude kind of
chronometer for estimating the proportionate duration of the intervals
between the eruptions. It is to be noticed on the top both of the
compact prismatic and of the earthy amygdaloidal sheets; but it is more
frequent and generally thicker on the latter than on the former, which
may only mean that the surfaces of the cellular lavas were more prone
to subærial decay than those of the compact varieties. Nevertheless,
I am disposed to attach some value to it, as an index of time. In the
present instance, for example, it seems to me probable that the lavas
in the lower half of Macleod's Maiden, where the red layers are very
prominent, were poured out at longer intervals than those that form the
upper half. The remarkable banded arrangement of the vesicles in one of
the cellular lavas of this sea-stack has been already referred to (p.
191).
Another characteristic plateau-feature is admirably displayed in
Skye--the flatness of the basalts and the continuity of their level
terraces (though not of individual sheets) from cliff to cliff and
hillside to hillside. This feature may be followed with almost tiresome
monotony over the whole of the island, north of a line drawn from Loch
Brittle to Loch Sligachan. Throughout that wide region, the regularity
of the basalt-plateau is unbroken, except by minor protrusions of
eruptive rock, which, as far as I have noticed, do not seriously affect
the topography. But south of the line just indicated, the plateau
undergoes the same remarkable change as in Rum, Ardnamurchan and Mull.
Portions of it which have survived indicate with sufficient clearness
that it once spread southwards and eastwards over the mountainous
district, and even farther south into the low parts of the island. Its
removal from that tract has been of the utmost value to geological
research, for some of the subterranean aspects of volcanism have
thereby been revealed, which would otherwise have remained buried under
the thick cover of basalt. Denudation has likewise cut deeply into the
eruptive bosses, and has carved out of them the groups of the Red Hills
and the Cuillins, to whose picturesque forms Skye owes so much of its
charm.
In this, as in each of the other plateaux, there is no trace of
any thickening of the basalts towards a supposed central vent of
eruption. The nearly level sheets may be followed up to the very
edge of the great mountainous tract of eruptive rocks, retaining all
the way their usual characters; they do not become thicker there
either collectively or individually, nor are they more abundantly
interstratified with tuffs or volcanic conglomerates. On the contrary,
their very base is exposed around the mountain ground, and the thickest
interstratifications of fragmentary materials are found at a distance
from that area. So far as regards the structure of the remaining part
of the plateau, the eruption of the gabbros and granitoid rocks might
apparently have taken place as well anywhere further north.
v. THE FAROE ISLANDS[265]
[Footnote 265: For references to the recent geological literature
connected with these islands see the footnote _ante_, p. 191.]
Though these islands lie beyond the limits of the region embraced by
the present work, I wish to cite them for the singular confirmation
and extension they afford to observations made among British Tertiary
volcanic rocks. Over a united extent of coast-cliffs which may be
roughly estimated at about 500 English miles, the nearly level sheets
of basalts, with their occasional tuffs, conglomerates, leaf-beds and
coals, can be followed with singular clearness. Although the Faroe
Islands have been so frequently visited and so often described that
their general structure is sufficiently well known, they present in
their details such a mass of new material for the illustration of
volcanic action that they deserve a far more minute and patient survey
than they have yet received. They cannot be adequately mapped and
understood by the traveller who merely sails round them. They must be
laboriously explored, island by island and cliff by cliff.
While I cannot pretend to more than a mere general acquaintance
with their structure, I have learnt by experience that one may sail
near their precipices and yet miss some essential features of their
volcanic structure. In the summer of the year 1894 I passed close to
the noble range of precipices on the west side of Stromö, at the mouth
of the Vaagöfjord, and sketched the sill which forms so striking a
part of the geology of that district (Figs. 312, 328 and 329). But I
failed to observe a much more remarkable and interesting feature at
the base of the same sea-cliffs. The following summer, probably under
better conditions of light, I was fortunate enough to detect with my
field-glass, from the deck of the yacht, what looked like a mass of
agglomerate, and found on closer examination the interesting group of
volcanic vents described in Chapter xli. The magnificent precipices
of Faroe, which in Myling Head reach a height of 2260 feet, present
a series of natural sections altogether without a rival in the rest
of Europe. They are less concealed with verdure than those of Mull
and Skye, and therefore display their geological details with even
greater clearness than can be found either in Scotland or in Ireland.
I would especially refer to the bare precipitous sides of the long
narrow islands of Kalsö and Kunö, as admirable sections wherein the
characters of the plateau-basalts are revealed as in a series of
gigantic diagrams. The scarcity of vegetation, and the steepness of
the declivities which prevents the abundant accumulation of screes
of detritus, enable the observer to trace individual beds of basalt
with the eye for several miles. Thus on the west side of Kunö, one
conspicuous dark sheet in the lower part of the section can be followed
from opposite Mygledahl in Kalsö to the southern end of the island.
There is one concealed space at the mouth of the corrie behind Kunö
village, but the same, or at least a similar band of rock at the same
level, emerges from the detritus on the further side, and may possibly
run into the opposite promontory of Bodö. It extends in Kunö for at
least six geographical miles.
[Illustration: Fig. 288.--Dying out of Lava-beds, east side of Sandö,
Faroe Isles.]
These vast escarpments of naked rock show, with even greater clearness
than the precipices of the Inner Hebrides, how frequently the basalts
die out, now in one direction now in another. The two sides of the
Kalsöfjord exhibit many examples of this structure, and some striking
instances of it are to be seen on the west side of Haraldsfjord. In
these cliffs, which must be about 2000 feet high, upwards of forty
distinct flows can sometimes be traced from the sea-level to the crest.
The average thickness of each bed is thus somewhat less than 50 feet.
Such vast escarpments, with wide semicircular corries scooped out of
their sides, such serrated crests and dark rifts in the precipices,
such deep fjords winding through nearly horizontal basalts, of which
the parallel sheets can be followed by the eye from island to island,
fill the mind with a vivid conception at once of the enormous scale
of the volcanic eruptions and of the stupendous denudation which this
portion of North-Western Europe has undergone since Tertiary time.
As the lenticular character of the basalts, and the evidence they
supply of having been discharged from many small local vents are of
great importance in the comprehension of the volcanic history of
the plateaux, some further illustrations of these features may with
advantage be given here. Thus the traveller who skirts the western
precipices of Suderö will notice some good examples to the north of
the highest part of the cliffs. On Stromö he will detect other cases
of the same structure. Similar features will arrest his attention on
the precipices of Sandö, where, though at first sight the basalts
seem to be regular and continuous, a nearer view of them reveals such
sections as that shown in Fig. 288, where a group of sheets rapidly
dies out towards the north against a thicker band that thins away in
the opposite direction. Further north he will come upon other examples
in the range of low cliffs between Kirkebonaes and Thorshaven, and more
impressive still in the rugged precipices that front the Atlantic on
the western front of Hestö (Fig. 289), where the disappearance is in a
northerly direction.
[Illustration: Fig. 289.--Lenticular lavas, western front of Hestö,
Faroe Isles.]
But it is in the northern part of the Faroes, where the basalt-plateau
has been so deeply trenched by parallel fjords as to be broken up into
a group of long, narrow, lofty, and precipitous insular ridges, that
the really local and non-persistent character of the lavas can best be
seen. The eastern cliffs of Svinö present admirable examples, where
in the same vertical wall of rock some of the basalts die out to the
south, others to the north, while occasionally a shorter sheet may be
seen to disappear in both directions as if it were the end of a stream
that flowed at right angles to the others (Fig. 290).
[Illustration: Fig. 290.--Lenticular lavas east side of Svinö, Faroe
Isles.]
The more the basalt-plateaux of Britain and the Faroe Islands are
studied, the more certain does the conclusion become that these
widespread sheets of lava never flowed from a few large central
volcanoes of the type of Etna or Vesuvius, but were emitted from
innumerable minor vents or from open fissures. In a later chapter an
account will be given of the vents, which may still be seen under the
overlying sheets of basalt, and, in particular, a remarkable group in
the Faroe Islands will be described.
[Illustration: Fig. 291.--Section at Frodbonyp, Suderö, Faroe.]
The occurrence of tuffs, leaf-beds and thin coals between the
plateau-basalts of the Faroe Islands has long been known. These
stratified deposits are well seen in the island of Suderö, where they
serve to divide two distinct series of basalts, like the iron-ore and
its accompaniments in Antrim. As a characteristic illustration of the
same diversity of deposits observable between the lava-sheets of the
basalt-plateaux of the British Isles I give here a section exposed on
the east side of this island--a locality often visited and described
in connexion with its coal-seams (Fig. 291). At the base lies a sheet
of basalt (_a_) with an irregularly lumpy upper surface. It may be
remarked that the lower group of basalts is marked by the occurrence of
numerous columnar sheets, some of them possibly sills, and also more
massive, solid, and durable basalts than the sheets above. The lowest
of the intercalated sediments are light-coloured clays, passing down
into dark nodular mudstone and dark shale, the whole having a thickness
of at least 20 feet (_b_). These strata are succeeded by (_c_) pale
clays with black plant-remains, about three feet thick. Immediately
above this band comes the coal or coaly layer (_d_), here about six
inches thick, which improves in thickness and quality further inland,
where it has been occasionally worked for economic purposes. A deposit
of green and brown volcanic mudstone (_e_), twelve feet in thickness,
overlies the coal and passes under a well-bedded granular green tuff
and mudstone three feet thick (_f_). The uppermost band is another
volcanic mudstone (_g_) four feet in thickness, dark green in colour,
and more or less distinctly stratified, with irregular concretions, and
also pieces of wood. Above this layer comes another thick overlying
group of basalts (_h_) distinguished by their abundantly amygdaloidal
character, and by their weathering into globular forms which at a
little distance give them a resemblance to agglomerates.
We have here an intercalated group of strata upwards of 40 feet thick,
consisting partly of tuffs and partly of fine clays, which may either
have been derived from volcanic explosions or from the atmospheric
disintegration of basaltic lavas. Through some of these strata abundant
carbonaceous streaks and other traces of plants are distributed,
while among them lies a band almost wholly composed of compressed
vegetation. Unfortunately none of the strata at this locality seem
to have preserved the plant-remains with sufficient definiteness
for identification. There can be no doubt, however, that they were
terrestrial forms like those of Mull and Antrim.
This coal, with its accompanying sedimentary deposits, has been traced
through Suderö, and another outcrop, possibly of the same horizon,
occurs on Myggenaes, the extreme western member of the group of
islands, at a distance of some 40 miles.[266]
[Footnote 266: See in particular Prof. J. Geikie, _Trans. Roy. Soc.
Edin._ vol. xxx. (1880), p. 229.]
CHAPTER XL
THE MODERN VOLCANOES OF ICELAND AS ILLUSTRATIVE OF THE
TERTIARY VOLCANIC HISTORY OF NORTH-WESTERN EUROPE
From the facts stated in the foregoing chapters concerning the
structure of the basalt-plateaux of North-Western Europe, it is evident
that in none of these areas have the eruptions come from one great
central volcano like Etna or Vesuvius. On the contrary, in every
instance there is abundant evidence that the basalt has flowed from
many scattered points of eruption. The uniformity of the lava-sheets in
petrographical characters, their continuity when viewed in mass, their
general horizontality, and their constant thinning away in different
directions, show that the eruptive vents must have been distributed
over the whole plateau-areas.
The conditions under which such eruptions took place can be most
readily understood by a comparison of the phenomena with those
observable in modern volcanic tracts where extensive outflows of lava
have taken place without the existence of any great central cones. Of
these regions the most instructive is undoubtedly to be found among the
recent lava-deserts of Iceland. There the parallels to the structures
described from the British and Faroe plateaux are so numerous and so
close that an account of the Icelandic region may appropriately be
inserted here.
The evidence furnished by Iceland is of special value in our present
enquiry, inasmuch as that island, besides its modern eruptions,
includes vast basaltic plateaux of Tertiary age. These areas of nearly
level sheets of basalt belong to the same geological period as those of
the British and Faroe Islands, and display the same internal structure
and external features. But they have this distinguishing peculiarity
that the volcanic fires beneath them are not yet extinguished. They
have been broken through again and again in recent times by volcanic
eruptions which have repeated many of the characteristics of their
Tertiary predecessors. The old and the new development of the same
volcanic type are thus visible side by side.
The Tertiary volcanic series of Iceland reaches a thickness of
upwards of 3000 metres, or nearly 10,000 English feet, but as its
base is nowhere seen, it may be still thicker. Its successive sheets,
piled over each other in parallel layers, form terraced hills and
bold escarpments along the coast, whence they slope gently inland.
The plateau, as in the Faroe Islands and in Scotland, has been
extensively eroded, and has been trenched by many long valleys and
fjords The composition of the basalts remains remarkably uniform over
the island. The lava sheets are often decomposing, amygdaloidal, and
filled with zeolites; while higher in the series compact basalts
abound, the uppermost fine-grained sheets being especially constant
in structure and composition. Numerous dykes traverse the plateau,
and some of them cut even its highest members. The parallel with the
geological structure of the Inner Hebrides is continued in Iceland by
the appearance of intrusive masses of gabbro and granophyre, which
represent the deeper parts of the Tertiary volcanic series, while the
basalts were poured out at the surface. Thus, at Papafjord, the gabbro
rises into mountainous peaks and, like the similar rock in Mull and
Skye, is intersected by dykes of a coarse-grained granitoid liparite or
granophyre. Large dykes and ramifying veins of the same acid material,
often with a thoroughly granitic aspect, extend into the basalts.[267]
[Footnote 267: Mr. Thoroddsen, _Dansk. Geografisk Tidsskrift_, vol. xiii.]
A long series of eruptions has taken place in Iceland since the Glacial
Period. There were likewise pre-glacial eruptions. The glaciated
lava-streams are found underneath the modern lavas. So far indeed as is
known, no evidence exists of any important cessation of subterranean
activity there since Tertiary time.[268] The existing volcanic phenomena
may with probability be regarded as the survival of those which were
so widely manifested over the Icelandic area and the north-west
of Europe in the older Tertiary ages. A careful study of them may
therefore be expected to throw light on the history of the Tertiary
basaltic plateaux; while, on the other hand, the thorough dissection of
these plateaux by the denuding agencies will not improbably be found
to explain some parts of the subterranean mechanism of the modern
Icelandic volcanoes.
[Footnote 268: See Dr. Johnston-Lavis, _Scottish Geographical Magazine_,
1895, p. 442.]
In calling attention to some of the more obvious analogies which
may be traced between the modern and the ancient volcanoes, I am
more particularly indebted to the excellent memoirs of the resident
Icelandic geologist, Mr. Th. Thoroddsen, who has examined so large a
part of the island.[269] The account given by Mr. A. Holland of the Laki
craters has likewise been of much service to me.[270] Among other recent
observers I may cite Dr. Tempest Anderson,[271] who has made himself
familiar with extensive tracts of Iceland. He was accompanied one year
by Dr. Johnston-Lavis, who has published a narrative of the journey.[272]
[Footnote 269: See In particular his paper on the volcanoes of north-east
Iceland (_Bihang till. k. Svensk. Vet. Akad. Handl._ xiv. ii. No. 5,
1888) and that on Snaefell and Faxebugt in the south-west of the island
(_op. cit._ xvii. ii. No. 2, 1891); also papers in _Dansk. Geografisk
Tidsskrift_, vols. xii. xiii. (1893-95); _Verhand. Gesellsch. Erdkunde
zu Berlin_, 1894-95.]
[Footnote 270: "Lakis Kratere og Lavaströmme, Universitætsprogram,"
Christiania, 1885. See Mr. Thoroddsen's remarks on this paper,
_Verhand. Gesell. Erdkunde_, 1894, p. 289.]
[Footnote 271: _Brit. Assoc. Rep._ 1894, p. 650.]
[Footnote 272: Dr. Johnston-Lavis, _Scottish Geographical Magazine_,
September 1895.]
It is a mistake to suppose that the Icelandic volcanoes are generally
built on the plan of such mountains as Vesuvius or Etna. Mr. Thoroddsen
can evidently hardly repress his impatience to find these two Italian
cones cited in almost every handbook of geology as types of modern
volcanoes and their operations. The regular volcanic cone, composed
of alternations of lavas and tuffs, plays a very subordinate part in
Iceland.
[Illustration: Fig. 292.--Fissure (gjá) in a lava-field, Iceland. (From
a photograph by Dr. Tempest Anderson.)]
The fundamental feature in the Icelandic eruptions is the production
of fissures which reach the surface and discharge streams of lava
from many points. Two systems of such fissures appear to be specially
marked, one in southern Iceland running from south-west to north-east,
the other, in the north part of the island, stretching from south
to north.[273] Hekla and Laki belong to the former. The dislocations
have often followed the boundaries of the "horsts," or solid blocks
of country which have withstood terrestrial displacement. The vast
outbreaks of Odádahraun and Myvatn have almost all issued from fissures
of that nature.
[Footnote 273: In the Snaefell promontory they run nearly east and west.
Mr. Thoroddsen, _Bihang. Svensk. Akad._ xvii. (ii.) No. 2, p. 91.]
The violent eruption of 1875 in Askja found its exit at the
intersection of two lines of fissures. Many large fissures were opened
on the surface in a nearly north and south direction, which could be
followed for 80 kilometres or nearly 50 English miles. Some of them
became the theatre of intense volcanic activity.[274]
[Footnote 274: Mr. Thoroddsen, _op. cit._ xiv. ii. No. 5, p. 63.]
Many lines of fissure are traceable at the surface as clefts or "gjás,"
that run nearly straight for long distances, with a width of one to
three yards, and sometimes of unknown depth.[275] The most stupendous
example of the structure yet discovered is probably the Eldgjá found by
Dr. Thoroddsen in the year 1893, below the Mýrdalsjökull. This gigantic
chasm has a length of 30 kilometres (more than 18 English miles), and
a depth of 130 to 200 metres (426 to 656 feet). Over its vertical walls
lofty waterfalls plunge from the crest to the bottom.
[Footnote 275: On the various modes of origin of these chasms, see Dr.
Tempest Anderson, _Brit. Assoc. Rep._ p. 650. The gjá shown in Fig. 292
is not an eruptive fissure. For this and the following illustration
I am indebted to the kindness of Dr. Tempest Anderson, who himself
photographed the scenes.]
Occasionally a fissure has not been continuously opened to the surface.
An interesting example of such intermittent chasms is supplied by the
great rent which gave forth the enormous volume of lava in 1783. The
mountain of Laki, composed of palagonite tuff, stands on the line of
this dislocation, but has not been entirely ruptured. The fissure has
closed up beneath the mountain, a short distance above the bottom of
the slope, as is shown by the position of a couple of small craters.[276]
[Footnote 276: Mr. A. Helland, _op. cit._ p. 25.]
Some fissures have remained mere open chasms without any discharge of
volcanic material; others have served as passages for the escape of
lava and the ejection of loose slags and cinders.[277]
[Footnote 277: Mr. Thoroddsen has observed that in the Reykjanes
peninsula in the south-west of Iceland, by the subsidence of one side
of a fissure, a row of four craters has been cut through, leaving their
segments perched upon the upper side. _Globus._ vol. lxix. No. 5.]
[Illustration: Fig. 293.--Cones on the great Laki fissure, Iceland.
(From a photograph by Dr. Tempest Anderson.)]
In some instances, according to Mr. Thoroddsen, lava wells out from
the whole length of a fissure without giving rise to the formation of
cones, the molten material issuing either from one or from both sides
and flowing out tranquilly. Thus from three points on the great Eldgjá
chasm lava spread out quietly without giving rise to any craters,
though at the southern prolongation of the fissure, where it becomes
narrower, a row of low slag-cones was formed. The three lava-streams
flooded the low ground over an area of 693 square kilometres, or 270
English square miles. In the great majority of cases, however, the
lava as it ascends in the fissure gives rise to long ramparts of slags
and blocks of lava piled up on either side, or to a row of cones along
the line of the open chasm. Thus, on the Laki fissure, which runs for
about 20 miles in a north-east direction, the cones amount to some
hundreds in number.
[Illustration: Fig. 293_a_.--Plan of small craters along the line of
great Laki fissure, Iceland. (After Mr. Helland, reduced.)]
The cones consist generally of slags, cinders, and blocks of lava.
They are on the whole not quite circular but oblong, their major axis
coinciding with the line of the chasm on which they have been piled up,
as along the marvellous line of the Laki fissure. In many places they
are exceedingly irregular in form, changes in the direction of outflow
of lava or of escape of steam having caused the cones partially to
efface each other.
As regards their size, the cones present a wide range. Some of them
are only a few yards in diameter, others several hundred yards.
Generally they are comparatively low mounds. On a fissure hardly 30
feet long, Mr. Thoroddsen found a row of twelve small cones built
exactly like those of largest size, but with craters less than three
feet in diameter. On the Laki fissure some are only a couple of yards
high; the majority are much less than 50 yards in height, and hardly
one is as much as 100 yards.[278] And yet these little monticules, as
Mr. Helland remarks, represent the pipes from which milliards of
cubic metres of lava have issued. While other European volcanoes form
conspicuous features in the landscape, the Icelandic volcanoes of the
Laki district, from which the vastest floods of lava have issued in
modern times, are so low that they might escape notice unless they were
actually sought for.[279]
[Footnote 278: Mr. Thoroddsen, however, states that there are about 100
ranging between 20 and 100 metres in height.]
[Footnote 279: _Op. cit._ p. 27.]
As they have generally arisen along lines of fissure, the cones are,
for the most part, grouped in rows. The hundreds of cones that mark the
line of the Laki fissure present an extraordinary picture of volcanic
energy of this type. In other instances the cones occur in groups,
though this distribution may have arisen from the irregular uprise
of scattered vents along a series of parallel fissures. Thus to the
north-east of Laki a series of old cones entirely surrounded by the
lavas of 1783 lie in groups, the most northerly of which consists of
about 100 exceedingly small craters that have sent out streams of lava
towards the N.N.E.[280]
[Footnote 280: _Op. cit._ p. 25. The great lava-fields of Iceland are
likewise dotted over with secondary craters or "hornitos" which have
no direct connection with the magma below, but arise from local causes
affecting the outflowing lava. They are grouped in hundreds over a
small space.]
It would appear from Mr. Helland's observations that the same fissure
has sometimes been made use of at more than one period of eruption. He
describes some old craters on the line of the Laki fissure, which had
been active long before the outbreak of 1783.[281]
[Footnote 281: _Op. cit._ p. 26.]
When the lava issues from fissures it is in such a condition of
plasticity that it can be drawn out into threads and spun into
ropes. When the slope over which it flows is steep it often splits
up into blocks on the surface. Where the ground is flat the lava
spreads out uniformly on all sides, forming wide plains as level as
a floor. Thus the vast lava-desert of Odádahraun covers a plain 3640
square kilometres in area, or, if the small-lava-streams north from
Vatnajökull be included, 4390 square kilometres. This vast flood of
lava (about 1700 English square miles in extent) would, according to
Mr. Thoroddsen, cover Denmark to a depth of 16 feet. The whole of this
enormous discharge has been given forth from more than twenty vents
situated for the most part on parallel fissures.
Not less striking is the picture of fissure-eruption to be met with at
Laki--the scene of the great lava-floods of 1783. "Conceive now," says
Mr. Helland, "these hundreds of craters, or, as they are called by the
Icelanders, 'borge,' lying one behind another in a long row; every one
of them having sent out two or more streams of lava, now to the one
side, now to the other. Understand further that these streams merge
into each other, so as to flow wholly round the cones and form fields
of lava miles in width, which, like vast frozen floods, flow down to
the country districts, and you may form some idea of this remarkable
region."[282]
[Footnote 282: _Op. cit._ p. 24. Mr. Helland allows an average thickness
of 30 metres for the mass of lava which issued in two streams, one
80 kilometres (nearly 50 miles), the other 45 kilometres (about 28
miles) long. He estimates the total volume of lava discharged in
the 1783 eruption at 27 milliards of cubic metres, equal to a block
10 kilometres (6 miles 376 yards) long, 5 kilometres (3 miles 188
yards) broad, and 540 metres (1771 feet) high; _op. cit._ p. 31. Mr.
Thoroddsen remarks that the older estimates of the volume of lava
discharged by this eruption have been greatly exaggerated. He puts
the area covered by lava at 565 square kilometres and the contents at
12-1/3 cubic kilometres. Verhand. _Gesell. Erdkunde Berlin_, 1894, p.
296.]
The basaltic lavas have issued in a comparatively liquid state, form
thin sheets and reach to great distances. The western stream from the
Laki eruption of 1783 flowed for upwards of 40 miles; a prehistoric
lava from Trölladyngjá in Odádahraun flowed for more than 60 miles.
In the course of time the successive streams of lava poured out upon
one of these wide volcanic plains gradually increase the height of
the ground, while preserving its generally level aspect. The loose
slag-cones of earlier eruptions are effaced or swallowed up, as one
lava-stream follows another. Eventually, when, by the operation of
running water or by fissure and subsidence, transverse sections are
cut through these lava-sheets, the observer can generally notice only
horizontal beds of lava piled one above another, including the dykes
connected with them and intercalated masses of loose slag, that remain
as relics of the old craters.
In some places the lava has gradually built up enormous domes, like
those of Hawaii, having a gentle inclination in every direction, as may
be seen especially in the district between Floderne Skjalfanafljot and
Jökulsà Most of the large volcanic piles of North Iceland are of this
nature. The highest of them are 1209 and 1491 metres high by from 6
to 15 kilometres in diameter. The elliptical crater of the highest of
these eminences measures 1100 by 380 metres.[283]
[Footnote 283: Mr. Thoroddsen, _op. cit._ xiv. ii. No. 5, pp. 10, 23.]
Large conical volcanoes of the Vesuvian type built up of alternating
lavas and tuffs are not common in Iceland, but some occur and rise
into lofty glacier-covered mountains, such as Öræfajökull (6241 feet),
Eyjafjallajökull (5432), and Snaefellsjökull (4577). Hekla (4961) also
is similarly composed of sheets of lava and tuffs, but has not been
built as a cone. It forms an oblong ridge which has been fissured
in the direction of its length and bears a row of craters along the
fissure.[284]
[Footnote 284: Mr. Thoroddsen, _Dansk. Geograf. Tidsskrift_, vol. xiii.]
Explosion-craters likewise occur among the modern volcanic phenomena
of Iceland. One of these was formed by a violent explosion at Askja
on 29th March 1875. It has a diameter of only about 280 feet, yet so
great was the vigour of the outburst that pumiceous stones were spread
over an area of more than 100 Danish (468 English) square miles, and
the dust was carried as far as Norway and Sweden. Nine years later Mr.
Thoroddsen found the bottom of this crater filled with bluish-green
boiling mud, which will probably in the end become a sheet of still
water. The borders of these Icelandic explosion-craters seem to be very
little higher than the ground around them. Most of the ejected material
is expelled with such force and to such a distance that only a small
fraction of it falls down around the orifice of eruption.[285]
[Footnote 285: Mr. Thoroddsen, _op. cit._]
There is still another feature of the Icelandic volcanic regions which
may be cited as an interesting parallel to the sequence of eruptive
discharges among the Inner Hebrides. While the lavas are as a rule
more or less basic--many of them being true basalts--they have been
at different times pierced by much more acid liparites and obsidians.
Examples of these rocks of post-Glacial age have recently been traced
on the ground by Mr. Thoroddsen,[286] and their petrographical characters
have been studied by Mr. Bäckström.[287] The wide distribution of such
rocks all over the island, their occurrence in isolated bosses among
the more basic lavas, and their remarkable internal structures have
been noted by several observers.[288] The liparites and obsidians are
contrasted with the basalt by the colours and forms of their streams.
Some of them are so black as to look like heaps of coal, though their
surfaces pass into grey pumice. They have flowed out in a much less
liquid condition than the basalts, and have consequently formed short,
thick and irregular sheets. The liparites and basalts appear to have
been nearly contemporaneous. They certainly belong to the same volcanic
cycle and their vents lie close to each other. Though none of the
acid eruptions are known to have occurred in modern times, some of the
liparites are crusted with sulphur and from the connected fissures
steam still rises.
[Footnote 286: _Geol. Fören. Stockholm Förhandl._ xiii. (1891), p. 609;
_Bihang. Svensk. Vet. Akad. Handl._ xvii. ii. p. 21 (1891); _Dansk.
Geograf. Tidsskrift_, xiii. (1895).]
[Footnote 287: _Geol. Fören. Stockholm Förhandl._ xiii. (1891), p. 637.]
[Footnote 288: See in particular C. W. Schmidt, _Zeitsch. Deutsch. Geol.
Gesellsch._ xxxvii. (1885), p. 737.]
It will thus be seen how entirely the modern volcanic eruptions
of Iceland agree with the phenomena presented by our Tertiary
basalt-plateaux. It is, therefore, to the Icelandic type of
fissure-eruptions, and not to great central composite cones like
Vesuvius or Etna that we must look for the modern analogies that will
best serve as commentary and explanation for the latest chapter in the
long volcanic history of the British Isles.[289]
[Footnote 289: In his memoir of 1874, Professor Judd announced his
conclusion that there were formerly five great volcanoes amongst
the Western Isles, and that the lavas of the plateaux had issued
from these. He subsequently reiterated this view (_Quart. Journ.
Geol. Soc._ xlv., 1890, p. 187), and ridiculed the explanation of
fissure-eruptions. The evidence adduced by me in a paper published in
1896 (same journal, vol. lii. p. 331) and reprinted with additions
in this chapter, will, I trust, be regarded by geologists as having
finally settled this question.]
As a further but more ancient illustration of the type of volcanic
action which appears to have been prevalent during the formation of the
Tertiary volcanic plateaux of Britain, I may again refer to the vast
basalt-fields of Western America. The basalt of Idaho stretches out as
an apparently limitless plain. Along its northern boundary, this sea
of black lava runs up the valleys and round the promontories of the
older trachytic hills with almost the flatness of a sheet of water.
It has been deeply trenched, however, by the streams that wind across
it, and especially by the Snake River, which has cut out a gorge some
700 feet deep, on the walls of which the successive beds of basalt lie
horizontally one upon another, winding along the curving face of the
precipice exactly as those of Antrim and the Inner Hebrides do along
their sea-worn escarpments. Here and there, a low cinder-cone on the
surface of the plain marks the site of a late outflow. One is struck,
however, with the singular absence of tuffs and volcanic conglomerates.
The basalts appear to have flowed out stream after stream with few
fragmentary discharges.
These characteristic features of one distinctive type of volcanic
action have been repeated over a vast region, or rather a whole series
of regions, in Western America, the united area of which must equal
that of a considerable part of Europe. From Idaho, the basalt-fields
may be followed southwards interruptedly into Utah and Nevada, and
across the great plateau-country of the cañons into Arizona and New
Mexico, northwards into Montana, and westwards into Oregon. The tract
which has as yet been most carefully traversed and described is
probably that of the high plateaux of Utah and Arizona. Thus on the
Uinkaret plateau, which measures some 45 to 50 miles in length by 8 to
12 in breadth, a thick covering of basalt has been spread composed of
many successive flows. Between 160 and 170 separate cones have been
counted on this area, most of them quite small, mere low mounds of
scoriæ, though a few reach a height of 700 or 800 feet, with a diameter
of a mile. From three to seven or eight may be found in a row, as if
springing from a single line of fissure. But generally the grouping
is quite irregular.[290] My friend Captain C. E. Dutton, from whose
admirable memoir these details are quoted, remarks further that among
the Utah plateaux no trace of a cone is to be found at or near some of
the most recent basalt-fields, and that the most extensive outpours are
most frequently without cones. "The lavas," he adds, "appear to have
reached the surface and overflowed like water from a spring, spreading
out immediately and deluging a broad surface around the orifice."[291]
The deep gorges cut by the rivers through these thick accumulations of
horizontal or nearly horizontal basalts, have here and there revealed
parallel dykes that traverse the rocks, and in at least one case
have shown the dyke running for half a mile up a cliff and actually
communicating with a crater of scoriæ at the top.[292] Again, in New
Mexico, Captain Dutton noticed vast tracts of younger basalt, about
which "a striking fact is the entire absence of all distinguishable
traces of the vents from which they came. Some of them, however,
indicate unmistakably their sources in small depressed cones of very
flat profiles. No fragmental ejecta (scoriæ, lapilli, etc.) have been
found in connection with these young eruptions."[293] Such I believe
to have been the general conditions under which the basalts of the
Tertiary plateaux of the British Isles were also erupted.[294]
[Footnote 290: Captain C. E. Dutton, "Tertiary History of the Grand Cañon
District," _U.S. Geol. Survey_ (1882), p. 104.]
[Footnote 291: Captain C. E. Dutton, "Geology of the High Plateaux
of Utah," _U.S. Geol. Survey of the Rocky Mountain Region_ (1880),
pp. 198, 200. See also pp. 232, 234, 276 of the same Monograph for
additional examples.]
[Footnote 292: _Tertiary History of the Grand Cañon_, etc., p. 95.]
[Footnote 293: _Nature_, xxxi. (1884), p. 49.]
[Footnote 294: I may again refer to Hopkins's _Researches in Physical
Geology_, where the conditions of the problem here discussed have
been distinctly realized. Speaking of the ejection of lava from a
number of fissures, he remarks that the imperfect fluidity of the
melted material "would seem to require a number of points or lines
of ejection as a necessary condition." "If there were only a single
centre of eruption, a bed of such matter approximating to uniformity
of thickness, could only be produced on a surface of a conical form."
"Where no such tendency to this conical structure can be traced, it
would probably be in vain to look for any single centre of eruption.
On the supposition, too, of ejection through continued fissures, or
from a number of points, that minor unevenness of surface which must
probably have existed under all circumstances during the formation of
the earth's crust, would not necessarily destroy the continuity of a
comparatively thin extensive bed of the ejected matter, in the same
degree in which it would inevitably produce that effect in the case of
central ejection" (_Cambridge Phil. Trans._ vi. 1835, p. 71).]
Although we may be convinced, from their general structure and
relations, that the stratified lavas of these plateaux have been poured
out from fissures and not from great central cones, it must obviously
be difficult to obtain demonstrative evidence of this origin from any
single section. Of the thousands of dykes which traverse the British
plateaux and the ground around them, I am not aware of a single one
which can be actually seen to have ever communicated with the surface.
The very process of denudation which has revealed these dykes has at
the same time removed all trace of any former connection they may have
had with the surface. The only places where we may hopefully search
for the missing evidence are the fronts of the escarpments. On these
precipices dykes may sometimes be seen to end off at some particular
platform among the basalt-sheets, but I have never found a case which
could be confidently cited as an example of lava rising in a fissure
and spreading out as a superficial sheet. That this connection may
eventually be found when a more detailed survey is made of these great
sea-walls I fully anticipate.
In recently mapping the basalt-plateau of Strathaird in Skye, Mr.
Harker has made some interesting observations regarding the probable
connection of the dykes with the plateau basalts. He has noticed that
the flanks of Slat Bheinn, a portion of the plateau, are abundantly
traversed by dykes containing numerous enclosed pieces of gabbro,
while the basalt on the summit of the plateau is full of similar
fragments--an occurrence not observed elsewhere. It is conceivable that
the gabbro-bearing basalt-sheets are sills, but Mr. Harker has found no
proof that they are so, the evidence so far as it has been collected
being rather in favour of the view that these sheets are superficial
lavas, and that they have been supplied from the dyke-fissures.
Various considerations suffice to assure us that actual instances of
the outflow of the basalt from its parent fissures should be expected
to be exceptional. The absence or scarcity of beds of scoriæ among the
basalt-plateaux may be taken as an indication that the lava as a rule
flowed out without the formation of cinder-cones, and therefore that
these conspicuous monuments of the eruptive vents were probably always
rare in Britain. If the lava was poured out tranquilly from one or two
points along a fissure which were subsequently buried under floods of
similar lava issuing from other fissures, the chances that such points
of emission should be laid open along the front of any escarpment are
small. And, even when so exposed, it might be difficult to feel sure
that the dyke below was really the feeder of the basalt above, unless
the cliff were accessible and the rocks could be scrutinized foot by
foot. These elements of uncertainty are happily removed where the
volcanic energy has drilled well-marked funnels of discharge and left
them filled with the erupted materials, as will be narrated in the next
chapter.
CHAPTER XLI
THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX
Vents filled with Basalt or other Lava-form Rock--Vents filled
with Agglomerate
It is one of the most interesting points in the Tertiary volcanic
history that, in spite of the enormous geological revolutions that
have passed since they became extinct, the sites of many scattered
vents can still be recognized. A far greater number must lie buried
under the basalts, and of others the positions are concealed by
the sea, which now covers so large an area of the old lava-fields.
Nevertheless, partly within the area of the plateaux, but still more
on the surrounding tracts from which the basalts have been removed
by denudation, the traces of unmistakable vents of discharge may be
recognized amid the general wreck.
In Britain and the Faroe Isles, it is chiefly along the coast-line
that the process of denudation has revealed the volcanic vents of
Tertiary time. The interior of the country is often loaded with peat,
covered with herbage, or strewn with glacial detritus: and even where
indications of the vents are to be detected, it is not always possible
to ascertain their true limits and connections. But where the structure
of the plateaux has been laid bare along ranges of rocky precipice, the
vents have sometimes been so admirably dissected by the sea that every
feature of their arrangements can be satisfactorily determined.
As the actual physical connexion of these volcanic orifices with the
plateaux has been in most cases removed by denudation, we can usually
only by inference place them in what was probably their true relation
to the plateau-eruptions. Those which project from the surface of the
plateaux must, of course, be younger than the basalts through which
they rise; how much younger we cannot tell. They may possibly be later
than any of the plateau-sheets; they may even belong to a subsequent
and waning condition of volcanic action. On the other hand, the vents
which can now be traced outside of the present limits of the edges
of the plateaux may, like those just mentioned, be younger than the
basalt-sheets, or, on the contrary, they may be records of a period
of eruptivity anterior to the emission of any of the rocks of the
plateaux, and may have been deeply buried under a mass of basalt-beds
subsequently removed. Positive demonstration is, from the nature of
the case, impossible in these instances. But examples will be cited
from the Western Isles and from Faroe, where the vents can be proved
to belong to the time of the plateau-eruptions, for they are seen to
have broken through some of the basalt-sheets and to have been buried
under others. With this clear evidence of relationship in some cases,
there need be little hesitation in believing that in other instances
where no such positive connexion can be found, but where the vents are
obviously such as the general structure of the plateaux would have led
us to expect, they may be confidently regarded as part of the phenomena
of the plateau-eruptions.
Sometimes the vents can be linked with lines of fissures or dykes. This
is especially the case where they are small in size. More usually,
however, no such relation can be demonstrated. It will be remembered
that among the modern Icelandic eruptions, some eruptive vents, like
the later cinder-cones of Laki, are ranged in a linear direction
along the great fissure, while others, of an older series in the same
district, almost engulphed amidst the more recent lavas, are clustered
irregularly in groups. A similar diversity of arrangement has been
observed among the volcanic cones of the Velay in Central France.
Considering as a whole the volcanic necks or eruptive vents which rise
from the older rocks around the Tertiary basalt-plateaux, and sometimes
even from the surface of these plateaux themselves, we may conveniently
follow the same classification as was adopted in dealing with those
of Palæozoic age, and, according to the nature of the material that
now fills them, arrange them in two series: (1) Those occupied by some
form of crystalline eruptive rock, and (2) those filled with volcanic
agglomerate.
i. VENTS FILLED WITH DOLERITE, BASALT, ETC.
These, as the composition of the plateaux would lead us to anticipate,
are numerous. They perhaps attain their most conspicuous development
in Antrim, either on the tableland or among the underlying rocks
round its edges. The finest example in that district is undoubtedly
furnished by the lofty eminence called Slemish, which rises above the
surrounding basalt-terrace, to a height of 1437 feet above the sea
(Fig. 294). It is elliptical in ground-plan, measuring some 4000 feet
in length by 1000 in breadth. Seen from the north, it appears as a
nearly perfect cone. The material of which it consists is a coarsely
crystalline olivine-dolerite, presenting under the microscope a nearly
holocrystalline aggregate, in which the lath-shaped felspars penetrate
the augite, with abundant fresh olivine, and wedge-shaped patches
of interstitial matter. The rock is massive and amorphous, except
that it is divided by parallel joints into large quadrangular blocks
like a granitic rock, and wholly different from the character of the
surrounding basalts. The latter, which possess the ordinary characters
of the rocks of the plateaux, can be followed to within 80 yards of
this neck, which rises steeply from them, but their actual junction
with it is concealed under the depth of talus.
[Illustration: Fig. 294.--Slemish, a Volcanic Neck or Vent on the
Antrim Plateau, seen from the north.]
[Illustration: Fig. 295.--Section of Volcanic Vent at Carnmony Hill (E.
Hull).
T, Lower basalt; C, Cretaceous strata; L, Lower Lias; M, Triassic
marls; V, Vent.]
At the nearest point to which the two rocks are traceable, the basalts
appear somewhat indurated, break with a peculiar splintery fracture,
and weather with a white crust. These characters are still better shown
on abundant fragments which may be picked up among the debris further
up the slope. There can be no doubt, I think, that a ring of flinty
basalt, differing considerably in texture from the usual aspect of that
rock in the district, surrounds the neck. The meaning of this ring
will be more clearly seen from the description of another example in
Mull. About four miles to the north-east of Slemish, a smaller and less
conspicuous neck rises out of the plateau-basalts. The rock of which
it consists is less coarsely crystalline than that of Slemish, but its
relations to the surrounding volcanic rocks are obviously the same. On
the west side of Belfast Lough a boss of similar rock, about 1200 feet
in diameter, rises at the very edge of the basalt escarpment into the
eminence known as Carnmony Hill (Fig. 295). On its northern side it
presents along its wall a mass of interposed volcanic agglomerate.[295]
On visiting with Mr. M'Henry the quarry opened on the eastern face of
this vent, I was much struck with the remarkable cellular structure
of some parts of the dolerite. Many of the vesicles are lined with a
thin pellicle of black glass, and the same substance occurs in minute
patches in the body of the rock. A thin slice exhibiting this structure
was found by Mr. Watts to possess the following characters:--"The
rock is an ophitic dolerite consisting of plagioclase, augite, and
iron ores, without olivine, enclosing one or two patches of finer
basalt. The vesicles in the latter, and certain angular spaces between
the crystals of the former, have been wholly or partially filled with
brown glass, the outer part of which has been converted into radiating
crystals of a brown mineral." The occurrence of patches of glass which
seem to have been squeezed into vesicles or cracks in the body of a
dolerite or andesite has been noticed in some of the Tertiary dykes.
But in the present case the glass occurs as a mere coating on the walls
of the larger spheroidal vesicles, the interior of which generally
remains empty.
[Footnote 295: This neck was recognised by Du Noyer in 1868 as "one of
the great pipes or feeders of the basaltic flows." See Prof. Hull,
Explanation of Sheets 21, 28 and 29, _Geol. Survey of Ireland_ (1876),
p. 30.]
Of the other doleritic necks scattered over the surface of the Antrim
plateau, I will refer to only one which occurs on the hillslopes
between Glenarm and Larne. It forms a prominence known as the Scawt
Hill, and consists of a boss of basalt, which, in rising through a vent
in the plateau-sheets, has carried up with it and converted into marble
a large mass of chalk which is now exposed along its eastern wall (Fig.
296).
[Illustration: Fig. 296.--Section of the east side of Scawt Hill, near
Glenarm.
_a_, bedded basalt; _b_, mass of chalk; _c_, basalt neck.]
[Illustration: Fig. 297.--Section of Neck of Basalt, Bendoo, Ballintoy.
_a_ _a_, Chalk; _b_, neck.]
As examples of similar necks which have been exposed by denudation
outside the present limits of the same plateau, I may allude to those
which rise through the Cretaceous and other Secondary strata on the
northern coast near Ballintoy. One of the most striking of these may
be seen at Bendoo, where a plug of basalt, measuring about 1400 feet
in one diameter and 800 feet in another, rises through the Chalk, and
alters it around the line of contact (Fig. 297). Another remarkably
picturesque example is to be seen near Cushendall, where a prominent
doleritic cone rises out of the platform of Old Red Sandstone, some
distance to the north of the present edge of the volcanic escarpment
(Fig. 298).
[Illustration: Fig. 298.--Volcanic Neck of Dolerite near Cushendall.]
The greater coarseness of grain of the material filling these pipes,
compared with that of the sheets in the terraces, is only what the very
different conditions of cooling and consolidation would lead us to
expect. There is no essential difference of composition between the two
rocks. Where the erupted material has been poured out at the surface,
it has assumed a finely crystalline texture, while, where it has slowly
solidified within a volcanic pipe at some depth beneath the surface,
and where consequently its component crystals have had more time for
development, the resulting structure is much more largely crystalline,
with a more or less complete development of the ophitic structure.
[Illustration: Fig. 299.--Section of Volcanic Neck at 'S Airde Beinne,
near Tobermory, Mull.
_a_ _a_, bedded basalts; _b_ _b_, bedded basalts altered along the side
of vent; _c_ _c_, dolerite.]
In the island of Mull, another instance of the same kind of vent has
been observed and described by Professor Judd.[296] It rises in the
conspicuous hill, 'S Airde Beinne (Sarta Beinn), about two miles
south-west from Tobermory, and consists of a coarsely crystalline
dolerite, which becomes finer in grain towards the outer margin
(Fig. 299). No bedding, or structure of any kind beyond jointing, is
perceptible in it. Examined in thin sections under the microscope,
this rock is found to be another typical ophitic dolerite, consisting
of lath-shaped felspars embedded in augite, with here and there
wedge-shaped portions of interstitial matter and grains of olivine. Dr.
Hatch found the felspars to contain spherical inclusions of devitrified
glass, filled with black granules and trichites, and he observed that,
under a high power, the interstitial matter is seen to consist mainly
of a greenish-brown isotropic substance, in which are inclosed small
crystals of augite, skeleton-forms and microlites of felspar, sometimes
in stellate aggregates, as well as club-shaped, cruciform, arrow-headed
and often crested microlites of magnetite.
[Footnote 296: _Quart. Jour. Geol. Soc._, xxx. (1874), p. 264.]
[Illustration: Fig. 300.--Interior of the Volcanic Neck of 'S Airde
Beinne, near Tobermory, Mull.]
Towering prominently above the flat basalt sheets, this neck has an
oval form, measuring about half a mile in length by a quarter of a
mile in breadth. Its central portion, however, instead of rising into
a rugged hill-top, as is usually the case, sinks into a deep hollow,
which is filled with water, and reminds one of a true crater-lake
(Figs. 299, 300). The middle of the neck is thus concealed from view,
and we can only examine the hard prominent ring of dolerite that
surrounds the tarn. The material occupying the hollow may be softer
than that of the ring, and may have been scooped out by denudation.
What we now see may not be the original surface, but may have been
exposed after the removal of possibly hundreds of feet of overlying
material. On the other hand, it is conceivable that the hollow is
really a crater-lake which was filled up with detritus and may have
been overspread with basalt, since removed. It may be suggestively
compared with the crater-hollows revealed by denudation on the cliffs
of Stromö and Portree Harbour, which will be described in a later part
of this chapter. Possibly some more easily removable agglomerate,
representing an eruption later than that of the dolerite, may occupy
the centre of the volcanic pipe.
One of the most interesting features of this vent is to be found in
its relation to the surrounding basalts. The marginal parts of the
rock along the line of contact are much finer in grain than the rest,
and have obviously cooled more rapidly. The contrast between them and
the ordinary dolerite nearer the centre, however, cannot be properly
understood, except in thin sections under the microscope. Dr. Hatch, to
whom I submitted my specimens, observed that, in place of the structure
above described, the marginal parts show an absence of the ophitic
grouping except in small isolated patches. Instead of occurring in
large grains or plates enveloping the felspars, the augite is found in
numerous small roundish grains, together with grains of magnetite, in
equal abundance and of similar size. The felspars are speckled over
with opaque particles; olivine has not been detected.
For miles around the vent, the plateau-rocks are of the usual
type--black, compact, sometimes amygdaloidal, alternating with more
coarsely crystalline decomposing bands, the separation between
different sheets being often marked by the ordinary red ferruginous
partings. But around the margin of the neck, they have undergone a
remarkable metamorphism. The portions of them which adhere to the outer
wall of the neck have lost their distinct bedding, and have been, as
it were, welded together into an indurated compact, black to dull-grey
rock, so shattery and jointed that fresh hand-specimens, three or four
inches in length, are not easily obtainable. Especially marked is one
set of joints which, running approximately parallel, cause the rock to
split into plates or slabs. These joints are sometimes curved. Yet, in
spite of the alteration from its normal character, the basalt retains
in places some of its more usual external features, such, for instance,
as its amygdaloidal structure, the amygdales consisting of calcite,
finely acicular mesotype, and other minerals.
Examined under the microscope, this altered basalt presents "a confused
aggregate of colourless microlites (felspar?) and innumerable minute
granules of magnetite, these two constituents being very unequally
distributed. Sometimes the colourless portions preponderate, in other
places the opaque granules are heaped together in black patches, which
may possibly mark the position of fused augites."[297]
[Footnote 297: Notes by Dr. Hatch.]
In the zone of contact-metamorphism around some of the volcanic pipes
in the plateaux, we see changes analogous to, but less developed than,
those which have been superinduced on so large a scale round the great
eruptive bosses of gabbro, granophyre, etc., that have broken up the
terraced basalts along the west coast of Scotland. I shall accordingly
return to this subject in connection with phenomena presented by these
younger rocks (p. 386).
ii. VENTS FILLED WITH AGGLOMERATE
While the necks of dolerite or basalt cannot always be satisfactorily
discriminated from bosses which may never have established a connection
with the surface, there is no room for any doubt in this respect in
the case of those filled with fragmentary materials. As has been
already pointed out, the occurrence of true volcanic agglomerate
may be accepted as evidence of the existence of an eruptive vent
communicating with the surface of the earth. The agglomerate in the
vents associated with the basalt-plateaux, like that of the Palæozoic
vents, is generally exceedingly coarse, and without any trace of
structure. Blocks of all sizes up to masses some yards in length, and
of the most diversified materials, both volcanic and non-volcanic, are
dispersed confusedly through a granular paste of similar miscellaneous
composition.
[Illustration: Fig. 301.--Diagram to show the probable relation of the
Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs.
_a_ _a_, Chalk; _b_ _b_, lower group of bedded basalts; _c_, vent
of Carrick-a-raide, filled with coarse volcanic agglomerate; _d_
_d_, bedded tuffs; _e_ _e_, large veins of basalt traversing the
agglomerate; _f_ _f_, zone of tuffs and pisolitic iron ore; _g_ _g_,
upper group of bedded basalts.]
An instructive example of the general characteristics of
agglomerate-vents, and of the relation of these vents to the
surrounding tuffs and basalts, is to be found at the island of
Carrick-a-raide, on the north coast of Antrim, and on the opposite
mainland. The visible mass of this neck is about 1000 feet in diameter,
but the boundaries, except on the land side, are concealed by the sea.
The material filling up the vent is a coarse agglomerate, in which
blocks and bombs of basalt, with pieces of chalk and flint, are stuck
at all angles in a dull dirty-green granular tuff. Some large and small
intrusions of basalt rise through it. Owing partly to these intrusions,
and partly to the grass-covered slope that separates it from the line
of cliff, the actual contact of this neck with the volcanic beds of the
escarpment cannot be seen. I have no doubt, however, that the tuff,
which has already been referred to as so conspicuous a member of the
series here, was discharged from this vent.[298] The materials are as
usual coarser in the pipe than beyond it, but the finer portion or
matrix of the agglomerate is similar to many bands of the tuff. The
structure of the locality may be diagrammatically represented as in
Fig. 301. The bedded tuff is thickest in the neighbourhood of the vent,
and gradually dies away on either side of it.
[Footnote 298: See Explanation of Sheets 7 and 8, _Geol. Survey of
Ireland_ (1888), p. 31.]
But another important inference may be drawn from this locality. I have
already pointed out that the lower basalts here reach their minimum
thickness. Their basement beds thin away towards the vent as markedly
as the tuff thickens. Obviously they cannot have proceeded from that
point of eruption. Yet, that they had begun to be poured out before
the discharge of the tuff is shown by their underlying as well as
overlying that rock, though westward, owing to the thinning away of
the undermost basalts, the tuff comes to lie directly on the Chalk.
Hence, we may legitimately infer that in this neighbourhood one or more
other vents supplied the sheets of the lower basalts.
In the island of Mull a number of detached bosses or patches of
agglomerate much obscured by invasions of granophyre probably mark
the sites of volcanic vents. They will be more particularly noticed
in Chapter xlvii. One of their most interesting features is the large
number of fragments of felsitic or rhyolitic rocks which they contain.
In the promontory of Ardnamurchan, where the basalt-plateau has been
invaded and displaced by later intrusions of crystalline rocks, and has
likewise been reduced to such a fragmentary condition by denudation,
some interesting examples of agglomerate necks have been laid bare.
One of the largest of these occurs on the north shore at Faskadale.
Cut open by the sea for more than a quarter of a mile, this neck
is seen to be filled with a coarse agglomerate, composed mainly of
basalt-blocks and debris, but crowded also with angular and subangular
pieces of different close-grained andesitic, felsitic and porphyritic
rocks belonging to the acid series to be afterwards described.[299] Some
of these stones exhibit a very perfect flow-structure, and closely
resemble certain fine-grained, flinty, intrusive rocks in Mull, to
which allusion will subsequently be made. The matrix of the agglomerate
is of the usual dull dirty-green colour, but is so intensely indurated
that on a fresh fracture it can hardly be distinguished from some of
the crystalline rocks of the locality. The neck is pierced in all
directions with dykes and veins of basalt, dolerite, andesite, gabbro,
and felsitic rocks. Similar intrusions continue and increase in numbers
farther west until the cliffs become a labyrinth of dykes and veins
running through a mass of rocks which appears to consist mainly of dull
dolerites and fine gabbros. Though the relations of this vent to the
plateau-basalts are not quite plain, the agglomerate seemed to me to
rise out of these rocks. At least the basalts extend from Achateny to
Faskadale, but, as they are followed westwards, they are more and more
invaded by eruptive sheets, and assume the indurated character to which
I have already referred.
[Footnote 299: One of these felsites when viewed under a high magnifying
power is seen to present an abundant development of exceedingly minute
micropegmatite arranged in patches and streaks parallel with the
lines of flow-structure in the general cryptocrystalline groundmass.
The close relationship between the felsites, quartz-porphyries, and
granophyres will be afterwards pointed out in the description of the
acid rocks. It is remarkable that, though these rocks occur abundantly
in fragments in the volcanic necks and agglomerates of the plateaux,
not a single instance has been observed of their intercalation as
contemporaneous sheets among the basic lavas. The analogous case of the
interstratification of felsitic tuffs among basic lavas in the volcanic
series of the Old Red Sandstone of Central Scotland has been described
(vol. i. p. 279). It is interesting to note that liparitic pumice and
dykes have been erupted by some of the basaltic craters of Iceland, for
example at Askja, Öræfajökull and Snaefellsjökull. (Mr. Thoroddsen,
_Dansk. Geograf. Tidsskrift_, vol. xiii. 7th and 8th parts.)]
On the south side of the peninsula of Ardnamurchan, another
agglomerate, noticed by Professor Judd,[300] rises into the bold headland
of Maclean's Nose, at the mouth of Loch Sunart, and affords better
evidence of its relation to the bedded basalts. It measures about
1000 yards in length by 300 in breadth, and its summit rises more
that 900 feet above the sea, which washes the base of its southern
front. It is filled with an agglomerate even coarser than that on
the northern coast. The blocks are of all sizes, up to eight or ten
feet in diameter. By far the largest proportion of them consists of
varieties of basalt and andesite, slaggy and vesicular structures
being especially conspicuous. There are also large blocks of different
andesitic porphyries and felsitic rocks like those just referred to,
a porphyry with felspar crystals two inches long being particularly
abundant. All the stones are more or less rounded, and are wrapped
up in a dull-green compact matrix of basalt-debris. There is no
stratification or structure of any kind in the mass. Numerous dykes
or veins of basalt, of andesite, and of a porphyry, resembling that
of Craignure, in Mull, traverse the agglomerate. Some of the narrow
basalt-dykes cut through the others.
[Footnote 300: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 261. Professor
Judd has subsequently (_op. cit._ xlvi. 1890, pp. 374 _et seq._) given
a map, section and description of what he believes to be the structure
of this ground, with numerous details as to the petrography of the
rocks. The geological structure of this area is more fully referred to
on pp. 318 _et seq._]
[Illustration: Fig. 302.--Section of agglomerate Neck at Maclean's
Nose, Ardnamurchan.
_a_ _a_, quartzites and schists; _b_, bedded basalts lying partly
on the schists and partly on patches of Jurassic sandstones that
occupy hollows of the older crystalline rocks; _c_, agglomerate;
_d_ _d_, dykes and veins traversing the agglomerate; _e_, dolerite
sheets of Ben Hiant.
]
The position of the vent, with reference to the surrounding rocks, will
be understood from the accompanying section (Fig. 302). On the eastern
side, the agglomerate can be seen to abut against the truncated ends
of the flat beds of the plateau-basalts, which are of the usual bedded
compact and amygdaloidal character. There can be no doubt, therefore,
that the vent has been opened through these basalts. But it will be
observed that the latter belong to the lower part of the volcanic
series. These lowest sheets are exposed on the slope, resting upon
yellowish and spotted grey sandstone, with seams of jet and a reddish
breccia, which, lying in hollows of the quartzites, quartz-schists, and
mica-schists, form no doubt the local base of the Jurassic rocks of
the district. Hence, the vent, though younger than the older sheets of
the plateau, may quite well be contemporaneous with some of the later
sheets.[301]
[Footnote 301: It may here be remarked that there is evidence of great
differences in the level of the base of the Jurassic series and
the bottom of the volcanic plateau in this district. On the south
and west sides of Ben Hiant the Jurassic conglomerates may be seen
lying on the edges of the crystalline schists only a little above
high-water mark, while on the north side, the schists, with their
overlying unconformable cake of limestones, rise several hundred feet
above sea-level. The surface on which the basalts were poured out was
probably very uneven, but there may also have been some considerable
displacements of these basalts either before or during the injection of
the dolerite sills of Ben Hiant.]
An interesting feature at this locality is the peculiar grouping of
some of the large dykes in the area around the agglomerate. They run
in the direction of the vent, and one or other of them may represent
the fissure or fissures on which the volcanic orifice was blown open
to the surface. Another notable element in the geological structure
of the ground is the vast amount of intrusive material, both in dykes
and sheets, which has been erupted. The intrusive sheets of Ben
Hiant form the most prominent eminence in this part of Ardnamurchan.
Reserving them for description in the following Chapter (p. 318), I
will only remark here that they partly overlie the agglomerate, and
are therefore, to some extent at least, younger than the vent. They
belong to that late stage in the history of the basalt-plateaux when
the molten material, no longer getting ready egress to the surface,
forced its way among the rocks about the base of the bedded basalts,
and more especially on the sites of older vents, which were doubtless
weak places, where it could more easily find relief.
The large neck now described is only one of a group scattered around
it in the ground to the north. Two of these may be seen rising through
a detached area of Jurassic limestones and shales at the northern base
of Ben Hiant. A third, almost obliterated by the intrusive sheets, may
be traced at the western end of that mountain above Coiremhuilinn.
Two others rising through the schists on either side of Beinn na
h-Urchrach, have been much invaded by the sills of that eminence
(Fig. 326). It is doubtless owing to the extensive denudation of the
basalt-plateau, and the consequent uncovering of the rocks underneath
it, that this series of vents has been laid bare.[302]
[Footnote 302: Professor Judd has united these scattered vents into a
continuous platform of volcanic agglomerates, which he represents as
underlying the supposed lavas of Ben Hiant. Since the publication
of his map and description, I have re-examined the ground without
being able to discover any trace of this platform. All the visible
agglomerates are separate necks, their actual walls being sometimes
exposed, as in the neck immediately north of the base of Ben Hiant,
where the limestone in contact is marmorised, though twelve yards of it
is an ordinary dull blue rock.]
By far the largest mass of agglomerate in any of the Tertiary volcanic
areas of Britain is that which occurs on the north side of the main
valley of Strath, in Skye.[303] Unfortunately, it has been so seriously
invaded by the eruptive rocks of the Red Hills, that its original
dimensions and its relations to the surrounding rocks, especially
to the bedded basalts, are much obscured (see Fig. 348). It can be
followed continuously from the lower end of Loch Kilchrist along the
southern slopes of Beinn Dearg Bheag round to the western roots of
Beinn Dearg Mhor--a distance of more than two miles in a straight
line, and from Kilbride to the flank of Beinn na Caillich above
Coire-chat-achan--a direct distance of two miles and a quarter. A
similar rock, possibly a portion of the same mass, appears in Creagan
Dubha, on the north side of the Red Hills. If the whole of this
agglomerate forms part of one originally continuous mass, it must have
been upwards of two miles in diameter. There may, however, have been
two or three closely adjacent vents. The Beinn na Caillich patch, for
example, appears to belong to a different area, and that of Creagan
Dubha is also probably distinct. But there seems no reason to doubt
that the mass which forms Cnoc nam Fitheach, and all the long declivity
on the southern flank of Beinn Dearg Bheag, occupies part of the site
of a single volcano. Owing to the absence of sufficient sections, it
is hardly possible to determine how much of this fragmentary material
should be assigned to the actual chimney. The diameter of the whole
mass is almost two miles. But possibly a considerable proportion of
this accumulation belongs to the external cone which gathered round
the vent, so that the eruptive pipe might thus be of much smaller
dimensions than the superficial area of the agglomerate. The subsequent
invasion of so much granophyre, not only that of the Red Hills, but
that of numerous smaller intrusions, has indurated the agglomerate and
made the investigation of its structure somewhat unsatisfactory.
[Footnote 303: This extensive mass was not separated from the "syenite"
of the Red Hills by Macculloch. Von Oeynhausen and Von Dechen noticed
it as a conglomerate with quartz pebbles, but did not realise its
volcanic nature (_Karsten's Archiv_, i. p. 90). In my map of Strath
(_Quart. Jour. Geol. Soc._ xiv. plate i.) I distinguished it from the
rock of the Red Hills, but no name for it appears in the legend of
the map, nor is it referred to in the text. Its character as a true
volcanic agglomerate was recognised by Professor Judd, _op. cit._ p.
255. See _postea_, pp. 384 _et seq._]
It might be supposed that the mere existence of intrusive bosses and
veins rather furnishes an argument in favour of considering the visible
agglomerate to belong to a deeper-seated part of the erupted material
than the external cone. But, as will be afterwards shown, there is
some reason to regard the present conical or dome-shaped outlines of
the granophyre hills as not far from their original forms, and to
believe that, like the trachytic Puys of Auvergne, they were much more
superficial than plutonic eruptions. A study of the cinder cones of
Central France shows that even these superficial accumulations have
been invaded not only by bosses but by dykes.[304]
[Footnote 304: The existence of a small dyke of andesite on the northern
rim of the well-known crater of the Puy Parion has already been
noticed.]
The agglomerate of the great Strath vent is a coarse tumultuous
assemblage of blocks and bombs, imbedded in the usual dull, dirty-green
matrix. Among the stones, grit and sandstone, together with
scoriaceous, vesicular and amygdaloidal basalts are specially abundant;
also pieces of various quartz-porphyries and granophyres, among which
a black felsite like that of Mull may often be recognised. In some
places, large masses of altered limestone and quartzite (Cambrian)
are included; in others, pieces of yellow sandstone and dark shale
(Jurassic), or of the bedded lavas. Some of these masses may be 100
yards or more in length. Occasionally a breccia, mainly made up of acid
materials--granophyre or granite,--has been noticed by Mr. Harker along
the north side of the Red Hills, which he thinks may rather be of the
nature of a crush-breccia than a part of the true agglomerate.
The agglomerate of this district is wholly without stratification
or structure of any kind. On the north-west side of Loch Kilchrist,
indeed, it weathers into large tabular forms, the parallel surfaces
of which dip to south-west; but this is probably due only to
jointing. Here and there, dykes of basalt cut the rock in a general
north-westerly direction, but their number is remarkably small when
compared with the prodigious quantity of them in the limestone at the
bottom and opposite side of the valley, some of which may possibly mark
the fissure on which the vent was placed. More abundant and extensive
are the masses of granophyre that rise particularly along the outer
margin of the agglomerate near Loch Kilchrist. These may be connected
with the great boss that forms the Red Hills, of which further details
will be given in Chapter xlvi.[305]
[Footnote 305: The granophyre intrusions in this agglomerate have been
found by Mr. Harker to have taken up and dissolved a considerable
proportion of fragments of gabbro, Chapter xlvi. p. 392.]
The important question of the relation of this agglomerate to the
plateau-basalts does not admit of satisfactory treatment, owing to
destruction of the evidence by the intrusion of the granophyre, and
likewise to enormous denudation. Nevertheless, some traces still remain
to indicate that the basalts once stretched over the site of the vent,
which probably rose through them. Looking westward from the Hanks of
Beinn Dearg Bheag to the other side of Loch Slapin, the geologist sees
the bold basalt-escarpment of Strathaird presenting its truncated beds
to him at a distance of only two miles. That these lavas were once
prolonged eastwards beyond their present limits is obvious, and that
they stretched at least over these two intervening miles can hardly
be doubted. But we can still detect relics of them on the flanks of
Beinn Dearg. As we follow the agglomerate round the margin of the
granophyre that mounts steeply from it, we lose it here and there under
beds of amygdaloidal basalt. The rocks next the great eruptive mass of
the mountain are so indurated and shattered that it is difficult to
separate them from each other and determine their relative positions.
But, so far as I could ascertain, these basalts are fragments of beds
that overlie the agglomerate (Fig. 303). This is not the only place
along the flanks of the Red Hills where portions of the bedded basalts
have survived. Other localities will be subsequently alluded to.
[Illustration:
Fig. 303.--Diagram to show the probable relations of the rocks on
the southern flank of Beinn Dearg Bheag.
_a_, agglomerate; _b_, amygdaloidal and compact basalt-rocks; _c_,
granophyre.
]
The Strath vent has been drilled through the Cambrian limestone, and
as the result of protracted denudation it now towers steeply 500 or
600 feet above that formation on the floor of the valley. Of the
material discharged from it over the surrounding country no certain
trace now remains. We may infer from the nature of the rock which fills
it that towards the end, if not from the beginning of its activity,
its discharges consisted mainly of dust and stones. A cone, of which
the remains are two miles in diameter, must surely have sent its
fragmentary materials far and wide over the surrounding region. But on
the bare platform of older rocks to the south, beyond the bottom of
the agglomerate declivities, not a vestige of these erupted materials
can now be found. Westward the escarpment of Strathaird remains to
assure us that no thick showers of ashes fell at even so short a
distance as two miles, either before or during the outpouring of the
successive basalt sheets still remaining there. We may therefore
conclude with some confidence that here, as at Ardnamurchan, the
vent is younger than at least the older parts of the basalt-plateau.
Unfortunately the uprise of the large bosses of granophyre that stretch
from the Red Hills to Loch Sligachan has entirely destroyed the vent
and its connections in that direction. There is no certain proof that
any molten rock ever issued from this orifice, unless we suppose the
fragmentary patches of amygdaloid on the southern flank of Beinn Dearg
Bheag to be portions of flows that proceeded from this centre of
eruption. The basalt-plateau which still remains in Strathaird no doubt
formerly extended eastwards over Strath and northwards across the site
of the Red Hills and Cuillins, joining on to the continuous tableland
north of Lochs Brittle and Sligachan. How much of the plateau had been
built up here before the outburst of the vent cannot be ascertained.
The agglomerate may possibly, of course, belong to the very latest
period of the plateau-eruptions, or even to a still younger phase of
Tertiary volcanic history. The impression, however, made on my mind by
a study of the evidence from the Western and Faroe Isles is that the
necks of agglomerate, like those of dolerite and basalt, really belong
to different epochs of the plateau period itself; and mark some of
the vents from which the materials of the plateaux were successively
emitted.
The example of Carrick-a-raide (p. 277) is peculiarly suggestive when
we regard it in connexion with the great Strath vent. Already the
progress of denudation has removed at least half of the layer of dust
and stones which, thrown out from that little orifice, fell over the
bare chalk-wolds and black basalt-fields of Antrim. The neck that
marks the position of the volcanic funnel has been largely cut away
by the waves, and is almost entirely isolated among them. The vents
at Canna, Portree and the Faroe Isles, to be afterwards described,
unquestionably belong to the eruptions of the plateau-period, for their
connection with the basalts can be clearly established. At the Strath
vent, however, the march of destruction has been greater. The connexion
between this vent and the materials ejected from it has been entirely
removed, and we can only guess from the size of the remaining neck what
may have been the area covered by the discharges from this largest of
all the volcanic cones of the Inner Hebrides.
Other masses of similar agglomerate are observable in the same region
of Skye, where they not improbably mark the sites of other vents.
Unfortunately their original limits and relations to the rocks through
which the eruptive orifices were drilled have been much obscured by
the uprise of the great masses of gabbro and granophyre of the Cuillin
Hills. Several of these isolated intrusions occur in the midst of the
gabbro, as in Harta Corry and on the west side of the Blaven ridge.
Another mass is interposed between the gabbro and granophyre on Druim
an Eidhne and at the base of the lavas between Druim an Eidhne and
the Camasunary valley. Mr. Harker has found a huge mass of agglomerate
underlying the bedded basalts to the north and west of Belig, one of
the hills on the west side of the large valley that runs from the head
of Loch Slapin to Loch Aynort. This mass has its bottom concealed by
the granophyre which underlies it; but it reaches a maximum thickness
of perhaps 1000 feet, rapidly thinning out and disappearing. It
generally resembles the Strath agglomerate, but is distinguished by
including a large proportion of fragments of gabbro. Mr. Harker remarks
that "a study of these agglomerates points to the existence of both
gabbros and granophyres older than the volcanic series, and therefore
distinct from the gabbros and granophyres now exposed at the surface."
It is a suggestive fact that so many detached masses of agglomerate
should occur around and within the areas of the great eruptive bosses
of gabbro and granophyre. They seem to indicate the former existence
of groups of volcanic vents in these tracts, and may thus account for
the uprise of such large bodies of intrusive material through what must
have been a weakened part of the terrestrial crust.
Further north in Skye a much smaller but more perfectly preserved
vent has been laid open by denudation on the south side of Portree
Bay--a deep inlet which has been cut out of the plateau-basalts and
their underlying platform of Jurassic sandstones and shales. The great
escarpment of the basalts has, at the recess of Camas Garbh, been
trenched by a small rivulet, aided by the presence of two dykes. The
gully thus formed exposes a section of a neck of agglomerate that
underlies the basalts of the upper half of the cliff. This neck is
connected with a thick deposit of volcanic conglomerate and tuff which,
lying between the basalts, extends from the neck to a considerable
distance on either hand. The general relations of the rocks at this
locality are represented in Fig. 304.
[Illustration: Fig. 304.--Section of Volcanic Vent and connected lavas
and tuffs, Scorr, Camas Garbh, Portree Bay, Skye.
_a_, Rudely-bedded dull green tuff; _b_, coarse agglomerate;
_c_, prismatic basalt; _d_, massive jointed basalt; _e_, red
banded decomposing rock, probably of detrital origin; _f_,
plateau-basalts, prismatic and rudely columnar; _g_, dyke of
dolerite, somewhat vesicular, five to six feet broad; _h_, basalt
dyke two to three feet broad; _i_, dyke or sill of similar basalt
to _h_, and possibly connected with it.
]
The agglomerate (_b_) is quite tumultuous, and here and there
strikingly coarse. Some of its included blocks measure five feet in
length. These fragments represent most of the varieties of the lavas
of the district. Large slaggy masses are abundant among them, and
sometimes exhibit the annelide-like elongation of the vesicles which
I have referred to as occasionally displayed by the plateau-basalts.
More than 60 feet of agglomerate are visible in vertical height from
where its base is concealed by debris and vegetation to where its
upper surface passes under a banded rock to be afterwards described.
That this unstratified mass of volcanic detritus marks the site of a
vent can hardly be doubted, although denudation has not revealed the
actual walls of the chimney. The steep grassy slopes do not permit
the relations of the rocks to be everywhere seen, but the agglomerate
appears to pass laterally into finer, rudely-stratified material of a
similar kind, which extends towards east and west as a thick deposit
between the bedded basalts. Possibly denudation has only advanced far
enough to lay bare the crater and its surrounding sheets of fragmentary
material, while the chimney lies still buried underneath.
To the east or left of the agglomerate the detritus becomes less
coarse, and shows increasing indications of a bedded arrangement. Close
to the agglomerate the dip of the coarse tuff is towards that rock at
about 10°. A few yards further east a sheet of very slaggy basalt is
seen to lie against the tuff, which it does not pierce. The vesicles
in this adhering cake of lava have been pulled out in the direction of
the slope till they have become narrow tubes four or five inches long
and parallel to each other. Some parts of this rock have a curved ropy
surface, like that of well-known Vesuvian lavas, suggestive of the
molten rock having flowed in successive thin viscous sheets down the
slope, which has a declivity of about 30°. This part of the section may
possibly preserve a fragment of the actual inner slope of the crater
formed of rudely-bedded tuffs.
Continuing still eastward, we find the feebly stratified tuff (_a_) to
be perhaps 200 feet thick. It forms a grassy declivity that descends
from the basalt-escarpment above to the grass-covered platform which
overlies a lower group of basalts. The visible portion of this tuff
presents a thoroughly volcanic character, being made up of the usual
dull dirty-green granular paste, through which are dispersed angular
and rough lumps of slag and pieces of more solid basalt, varying up
to a foot or two feet in length. These stones are generally disposed
parallel to the indistinct bedding, but are sometimes placed on end,
as if they had assumed that position on falling from an explosive
shower. Among the smaller stones, pieces of a finely vesicular basic
pumice are frequent and are among the most strikingly volcanic products
of the deposit. From a characteristic sample of these stones, a thin
slice was prepared and placed in Mr. Harker's hands. The following are
his observations on it:--"A very compact dark grey rock, amygdaloidal
on a minute scale. The lighter grey crust is probably due merely to
weathering, and the specimen seems to be a distinct fragment, not a
true bomb. The slice shows it to be essentially a brown glass with only
occasional microscopic crystals of a basic plagioclase. It has been
highly vesicular, and the vesicles are now filled by various secondary
products, including a chloritic mineral, nearly colourless and singly
refracting in thin section, and a zeolite."
Tracing now the tuff from the west or right side of the vent, we can
follow it to a greater distance. No abrupt line can be detected here,
any more than on the other side, between the agglomerate and the tuff.
The latter rock extends under the overlying plateau of basalt, at least
as far west as Portree Loch, a distance of fully a mile, but rapidly
diminishes in thickness in that direction. Traces of what is probably
the same tuff can be detected between the basalts at Ach na Hannait,
more than three miles to the south (Fig. 305). It is thus probable that
from the Portree vent fragmentary discharges took place over an area of
several square miles.
Above the agglomerate of this vent two lavas may be seen to start
towards opposite directions. One of these (_c_), already referred to,
is a dull prismatic basalt with a slaggy bottom, its vesicles being
pulled out in the direction of the general bedding of the section. It
descends by a twist or step, and then lies on the inclined surface of
the tuff which dips towards the agglomerate and seems to pass into
that rock. Further east this basalt increases in thickness and forms
the lowest of the basalt-sheets of the cliff. The lava that commences
on the west side of the agglomerate (_d_) is a massive jointed basalt,
which, though not seen at the vent, appears immediately to the west of
it and rapidly swells out so as to become one of the thickest sheets of
the locality. It lies upon the rudely-bedded tuff, and is covered by
the other basalts of the cliff.
That these two basalts came out of this vent cannot be affirmed. If
they did so at different times, their emission must have been followed
by the explosion which cleared the funnel and left the central mass
of agglomerate there. But that some kind of saucer-shaped depression
was still left above the site of the vent is indicated by the curious
elliptical mass of rock (_e_) that lies immediately above the
agglomerate, from which it is sharply marked off. This is one of the
most puzzling rocks in the district, probably in large measure owing to
its advanced state of decay. It is dull-red in colour, and decomposes
into roughly parallel layers, so that at a short distance it looks like
a bedded tuff, or like some of the crumbling varieties of banded lavas.
I could not obtain specimens fresh enough to put its nature and origin
beyond dispute. Whatever may have been its history, this ferruginous
rock rests in a flat basin-shaped hollow directly above the agglomerate
of the vent. The form of this depression corresponds fairly well with
what we may suppose to have been the final position and shape of the
crater of the little volcano. The rock that occupies the bowl dies out
towards the east on the face of the cliff, and the prismatic basalt
(_c_) is then immediately covered by the rest of the basalt-sheets
of the plateau (_f_). On the west side its precise termination is
concealed by grass. But it must rapidly dwindle in that direction
also, for not many yards away it is found to have disappeared, and the
basalts (_d_ and _f_) come together.
Though the decayed state of this rock does not warrant any very
confident opinion regarding its history, I am inclined to look upon
it as a deposit of much disintegrated volcanic detritus washed into
the hollow of the old crater when it had become filled with water, and
had passed into the condition of a _maar_. The peculiarly oxidized
condition of its materials points probably to long atmospheric
exposure, and an examination of the surrounding parts of the district
furnishes more or less distinct evidence that a considerable lapse of
time did actually intervene between the cessation of the eruptions of
the Portree volcano and the next great basalt-floods of this part of
Skye.
That volcanic eruptions from other vents continued after the Portree
vent had become extinct is proved by the great sheets of basalt (_f_)
that overspread it, and still bury a large tract of the fragmentary
material which it discharged. At a later time a fissure that was
opened across the vent, allowed the uprise of a basalt dyke (_g_), and
subsequently another injection of similar material took place along the
same line of weakness (_h_).
Before leaving this interesting locality we may briefly take note
of the distribution of the ashes and stones ejected by the volcano,
and the evidence for the relative length of the interval between
the outflow of the lavas below and that of those above the tuff
and volcanic conglomerate. These deposits may be traced in clear
sections along the base of the cliffs for a mile to the west of the
vent. They thin away so rapidly in that direction that at a distance
of three-quarters of a mile they do not much exceed fifty feet in
thickness. At Camas Bàn they consist mainly of a fine, dull-green,
granular, rudely-stratified basalt-tuff, through which occasional
angular pieces of different lavas and rough slags are irregularly
dispersed. These stones occur here and there in rows, suggestive of
more vigorous discharges, the layers between the platforms of coarser
detritus being occupied by fine tuff. Some of the ejected blocks are
imbedded on end--an indication of the force with which they were
projected so as to fall nearly a mile from the crater.
The upper parts of the tuff pass upward into fine yellow, brown, and
black clays a few feet in thickness, the darker layers being full of
carbonaceous streaks. On this horizon the coal of Portree was formerly
mined. The workings, however, have long been abandoned, and, owing to
the fall of large blocks from the basalt-cliff overhead, the entrance
to the mine is almost completely blocked up. One wooden prop may still
be seen keeping up the roof of the adit, which is here a slaggy basalt.
To the east and south-east of the Portree vent, extensive landslips
of the volcanic series and of the underlying Jurassic formations make
it hardly possible to trace the continuation of the tuff-zone in that
direction. To the south, however, at a distance of rather more than
three miles, what is probably the same stratigraphical horizon may be
conveniently examined from Ach na Hannait for some way to the north
of Tianavaig Bay. At the former locality the calcareous sandstones
of the Inferior Oolite are unconformably covered by the group of
rocks represented in Fig. 305. At the bottom of the volcanic series
lies a sheet of nodular dolerite with a slaggy upper surface (_a_).
Wrapping round the projections and filling up the depressions of this
lava comes a thin group of sedimentary strata from an inch or two to
eighteen inches or more in thickness (_b_). These deposits consist of
hardened shale charged with macerated fragments of linear leaves and
other plant-remains, including and passing into streaks of coal, which
may be looked upon as probably occupying the same horizon with the
coal of Portree. But here, instead of reposing on a mass of stratified
tuff, the carbonaceous layers lie on one of the bedded lavas. The tuff
has died out in the intervening three miles, yet that some of the
discharges of volcanic detritus reached even to this distance, and
that they took place during the accumulation of these layers of mud
and vegetation, is shown by the occurrence in the shales of pieces of
finely amygdaloidal basalt, from less than an inch to six inches in
length, likewise lapilli of a fine minutely cellular basic pumice, like
some varieties of palagonite. The overlying dolerite (_c_) becomes
finely prismatic at its junction with the sedimentary layers and has
probably indurated them.
[Illustration:
Fig. 305.--Section of the Volcanic Series at Ach na Hannait, south
of Portree, Skye.
]
This intercalation of a shaly and coaly band among the lavas can be
followed northward along the coast. In some places it has been invaded
by dykes, sills, and threads of basalt on the most remarkably minute
scale, of which I shall give some account in Chapter xlii. (see Fig.
321). North of Tianavaig Bay--that is, about three-quarters of a mile
nearer to the Portree vent--a perceptible increase in the amount of
volcanic material is observable among the shales and leaf-beds. Not
only are lapilli of basic pumice abundant, but the volcanic detritus
has accumulated here and there in sufficient amount to form a band of
dull greenish-brown tuff.
These coast-sections in the neighbourhood of Portree afford additional
illustrations of the characteristic fact, on which I have already
insisted, that the interstratifications of sedimentary material in the
basalt-plateaux frequently terminate upward in leaf-beds, thin coals,
or layers of shale, full of indistinctly preserved remains of plants.
As I have endeavoured to show, this vegetation, which was undoubtedly
terrestrial, probably grew not far from the sites where its remains
have been preserved. Leaves and seeds would naturally be blown or
washed into pools on the lava-fields, and would gather there among
the mud and sand carried by rain from the surrounding ground. Such a
topography and such a sequence of events point to intervals of longer
or shorter duration between the successive outpourings of basalt. It
was probably during one of these intervals of quietude that the crater
of the Portree volcano became a _maar_ and was finally silted up.
Reference has already been made to a conspicuous mass of agglomerate
which occurs at the east end of the island of Canna, and marks the site
of an important volcanic vent belonging to the Small Isles plateau. A
portion of it projects from the grassy slopes, and rises vertically
above the beach as a picturesque crag, in front of the precipice of
Compass Hill (Fig. 306). But the same rock may be traced southward to
the Coroghon Mòr, and north-westward in the lower part of the cliffs to
a little beyond the sea-stack of An Stòll. It has thus a diameter of at
least 3000 feet. Westward it passes under the conglomerate described in
Chapter xxxviii. Its eastern extension has been concealed by the sea.
[Illustration: Fig. 306.--View of part of a Volcanic Neck at the
eastern end of the island of Canna. (From a photograph by Miss Thom.)]
The materials that fill this vent consist of a typical agglomerate
composed entirely, or almost entirely, of volcanic detritus. The
embedded blocks vary up to eight feet in diameter or even more. They
are chiefly fragments of various basalts and andesites, generally
vesicular or amygdaloidal. Some of these, which have evidently been
broken off from already consolidated lavas, are angular or subangular
in shape, and their steam-holes are cut across by the outer surfaces
of the stones. Where they consist of calcite, zeolite, etc., the
amygdales so exactly resemble those of the bedded basalts of the
plateaux that, as already remarked, we must believe them to have been
already filled by infiltration before the disruption of the rocks by
volcanic explosions. Other blocks are true bombs, with a fine-grained
crust outside and a more cellular texture inside, the vesicles of the
outer crust being sometimes dragged round the surface of the stone.
The variety of materials included among the ejected blocks and the
abundance of pieces of the red bole which so generally separates the
plateau-basalts indicate that a considerable thickness of bedded lavas
has probably been broken through by the vent.
Beside the volcanic materials, occasional angular pieces of red
(Torridon) sandstone may be observed in the agglomerate. The paste is a
comminuted mass of the same material as the blocks, tolerably compact,
and entirely without any trace of stratification.
The actual margin of this vent has nowhere been detected by me. We
never reach here the base of the volcanic series, for it is sunk under
the sea-level. On the other hand, the upper limits of the agglomerate
have been partially effaced or obscured by the conglomerates which
overlie it. From the breadth of ground across which the agglomerate
can be followed along the shore, the vent might be regarded as having
been perhaps not less than three-quarters of a mile in diameter. But
there is the same difficulty here as at the Strath vent in Skye in
determining the actual limits of the volcanic funnel. Possibly there
may have been more than one vent in close proximity. Even if there was
only one, the existing agglomerate may include not only what filled the
chimney, but also a portion of what had accumulated round the orifice
and formed the external cone. That the volcano continued for some time
in vigorous eruption may be judged from the amount of material ejected
from it, the large size of its blocks, and the distance to which they
were sometimes thrown.
The pieces of Torridon Sandstone were no doubt derived from the
extension of that formation underneath Canna. On the opposite island of
Rum, where these pre-Cambrian red sandstones are copiously developed,
they form the platform through which the Tertiary volcanic series has
been erupted. The several remaining outliers of the bedded basalts,
referred to in a previous chapter (p. 215 and Fig. 267) as visible on
the west side of this island, show that the basalt-plateau of Small
Isles, which once covered that area, rested immediately on the inclined
edges of the Torridon Sandstones. Probably the same structure stretches
westward under Canna and Sanday. No traces of any Jurassic strata
have been detected beneath the volcanic rocks of Rum, though they
are so well developed a few miles to the east in the island of Eigg.
Either they were not deposited over the pre-Cambrian rocks of Rum, or
they had been removed from that ancient ridge before the beginning
of the Tertiary volcanic period. Certainly I have not detected a
single recognizable fragment of any Jurassic sedimentary rock in the
agglomerate of Canna.
This Canna vent exhibits, better than is usually shown, the occurrence
of dykes and irregular injections of lava through the agglomerate.
A large mass of a finely columnar basalt runs up from the beach at
Garbh Asgarnish. A similar rock forms several detached crags a little
further south, particularly in the headland of Coroghon Mòr and the
island of Alman. Here the basalt is beautifully columnar, its slender
prisms curving from a central line until their ends abut against the
agglomerate. The truly intrusive character of this basalt is well shown
on the southern front of Coroghon Mòr, and on the northern face of
Alman, as represented in the accompanying diagrams (Figs. 307 and 308).
[Illustration:
Fig. 307.--Columnar Basalt invading Agglomerate of Volcanic Vent,
Coroghon Mòr, Isle of Canna. (Height above 20 feet.)
]
[Illustration:
Fig. 308.--Columnar Basalt invading Volcanic Conglomerate, north
side of Alman Islet, Canna.
]
Although there is no conclusive evidence that these intrusions belong
to the time of the activity of the vent, yet they differ so much from
the ordinary dykes (one of which also cuts the agglomerate and ascends
through the conglomerates and basalts above), are confined so markedly
to the vent and its immediate proximity, and resemble so closely the
basalt-injections of other vents, such as those of the Carboniferous
and Permian necks of Scotland, that they may with probability be
regarded as part of the mechanism of the Canna volcano.
Though the form and size of the vent of this volcano cannot be
precisely defined, the upper part of its agglomerate, as we have seen
(_ante_, p. 219), is dovetailed in the most interesting way with the
series of coarse conglomerates, which indicate strong river-action in
this part of the volcanic area during the time of the eruption of the
plateau-basalts.
The agglomerate vents described in the foregoing pages as occurring
in Antrim and among the Inner Hebrides all appear either in the midst
of the plateau-basalts or in close proximity to them. Before quitting
the Scottish examples, I may refer to some that rise through much more
ancient formations at a distance from any portion of the volcanic
plateaux, and yet may with probability be assigned to the Tertiary
volcanic period.
During the progress of the Geological Survey through the district of
Applecross, in the western part of the mainland of Ross-shire, and far
away from the basalt-plateau of Skye, Mr. John Horne[306] has found two
small necks rising on each side of a line of fracture, through gently
inclined Torridon Sandstones. They are conspicuous from a distance by
the verdure of their slopes, in contrast with the brown tints of the
surrounding moorland. The larger of the two necks measures about 180 by
150 feet, and abruptly truncates the beds of Torridon Sandstone, which
as they approach it assume a bleached aspect and become indurated.
The material filling this vent is an agglomerate made up mainly of
pieces of Torridon Sandstone and grit which, though generally small,
occasionally measure a foot across, and in one case were found to reach
a length of four feet. They are not as a rule markedly altered, but
some of them have acquired a glazed or vitreous texture. Besides these
fragments of the general rock of the district, there occur abundant
lapilli of a basic volcanic rock, found by Mr. Teall to consist of
porphyritic felspar, extremely minute acicular microlites of felspar,
somewhat irregular transparent spaces now occupied by a yellowish-green
substance, and interstitial matter. At the south end of the vent a
small mass of decayed basalt appears to pierce the agglomerate.
[Footnote 306: _Trans. Geol. Soc. Edin._ vii. (1894), p. 35.]
[Illustration: Fig. 309.--View of neck-like mass of breccia, Brochel,
Raasay.]
Though there is no indication of the age of these necks, they agree
so closely in general character with known vents of the Tertiary
volcanic plateaux that there cannot be much hesitation in regarding
them as dating from the same great period of basalt-eruption. But no
relic now exists anywhere around of lavas or tuffs ejected from them.
They rise on the bare Applecross hills, 1000 feet above sea-level, two
miles from the shore, and about ten miles from the nearest outlier of
the basalt-plateau in the Dùn Can of Raasay. If they once discharged
streams of lava that united with the rest of the plateau, the total
destruction of this lava affords another impressive picture of the
waste which the volcanic rocks of the Inner Hebrides have undergone.
The large proportion of Torridon Sandstone blocks in these two
Applecross necks suggests, however, that the orifices never became
active volcanic vents. They may have been mere spiracles, or
blow-holes, where the funnels drilled by explosive vapours were filled
up with the debris of the rocks that were blown out. But that lava did
rise within them is shown by the basic lapilli in the agglomerate, and
by the basalt which in both vents has found its way up the chimney.
In the island of Raasay Mr. Teall, during the summer of 1894,
observed a group of curious neck-like masses of breccia which pierce
the Torridon Sandstone near Brochel (Fig. 309). The blocks in them
are large angular unaltered pieces of the surrounding sandstones
and shales, sometimes ten feet or more in length, and the matrix is
sometimes pure crystalline calcite like Iceland spar. The breccia is
generally coarsest towards the outer margin. But though the Lewisian
gneiss exists immediately below the thin cake of Torridonian strata,
not a fragment of it could either Mr. Teall or I, when I visited the
locality with him, find among the components of the breccia. Nor did
we detect any trace of volcanic material. The general ground-plan of
these masses is elliptical, the most northerly measuring 30 yards in
diameter. Where the junction of the breccia with the Torridon strata
can be seen it is a nearly vertical one, the sandstones and shales
being much jumbled and broken, but not sensibly indurated. This little
cluster of patches of breccia can hardly be due to local crushing of
the rocks. Their definite outlines and composition seem rather to
indicate spiracles of Tertiary time, which never became vents erupting
lava or ashes. The absence of fragments of the underlying gneiss may be
accounted for if we suppose that the orifices were completely cleared
out by the violence of the explosions and were afterwards filled up
by the falling in of the walls of the higher parts now removed by
denudation, which consisted of Torridon Sandstone and shale.[307]
[Footnote 307: It is on one of these neck-like patches of breccia that
Brochel Castle stands, of which Macculloch gave so sensational a
picture in one of the plates of his _Western Isles_.]
Further research may detect at still greater distances from the
basalt-plateaux ancient volcanic necks that might, with more or less
probability, be referred to the Tertiary period. As an instance of this
kind, I refer to the neck at Bunowen, County Galway, recently described
by Mr. M'Henry and Professor Sollas. Though so remote from the Tertiary
basalt-plateaux, the rock of this boss is an olivine-basalt presenting
a close resemblance to some of the rocks of Antrim.[308]
[Footnote 308: _Trans. Roy. Irish Acad._, 1896].
As a final illustration of Tertiary volcanic vents I will now describe
the Faroe group already alluded to (vol. i. p. 63, vol. ii. p. 256).
It was almost by a kind of happy accident that these vents were
discovered. Noticing at a distance of a mile or more from the deck of
a steam-yacht that the base of the great basalt cliffs on the west
side of Stromö were varied by what looked like agglomerate, I steamed
inshore, and was delighted to find, as the vessel drew near to the
cliff, that the agglomerate assumed definite boundaries and occurred in
several distinct patches, until at last it presented the unmistakable
outlines of a group of vents underlying and overspread by the bedded
basalts of the plateau. Favoured by an unusually calm sea, I was
enabled to boat into every nook and round every buttress and islet of
this part of the coast-line.
[Illustration: Fig. 310.--View of Volcanic Neck piercing and overlain
by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands.
(From a photograph by Colonel Evans.)]
The basalt-plateau here presents to the western ocean a nearly vertical
escarpment which must reach a height of at least 1000 feet (see Fig.
328), and displays a magnificent section of the bedded lavas. The
lower part of this section shows chiefly the banded structure already
described, the layers of different consistency being etched out by the
weather in such a way as to give them the look of stratified rocks.
In the upper part of the precipice columnar and jointed or prismatic
sheets are more common, but the most prominent band is the great sill,
to which further reference will be made in the next Chapter.
In the course of the gradual retreat of the cliff, as the waves tunnel
its base, and slice after slice is detached from its vertical front,
a group of at least five small vents has been uncovered lying along a
nearly north and south line. Of two of these a segment remains still
on the cliff-wall and passes under the basalts; the others have been
dissected and half cut away from the cliff, while groups of stacks and
rocky islets of agglomerate may mark the position of others almost
effaced. The horizontal distance within which the vents are crowded
is probably less than half a mile, but the lofty proportions of the
precipice tend to lead the eye to underestimate both heights and
distances.
[Illustration: Fig. 311.--Section of the same Neck as that shown in
Fig. 310.]
The agglomerate is a thoroughly volcanic rock, consisting of large and
small blocks of various basalts, among which large slags are specially
conspicuous, the whole being wrapped in a granular matrix of comminuted
volcanic detritus. The arrangement of this material is best seen in the
fourth vent (Figs. 310 and 311). In this characteristic volcanic neck
(_b_ in Fig. 311) the boundary walls, as laid bare on the face of the
precipice, are vertical, and are formed of the truncated ends of the
banded lavas (_a_ _a_) which have been blown out at the time of the
formation of the orifice. The visible diameter of the vent was roughly
estimated by me to be about 100 yards. No appreciable alteration was
observed in the ends of the lavas next the vent.
The agglomerate is coarsest in the centre, where huge blocks of
slaggy lava lie imbedded in the amorphous mass of compacted debris.
On either side of this structureless central portion the agglomerate
is distinctly stratified from the walls towards the middle, at angles
of 30° to 35°. Even from a distance it can be observed that the upper
limit of the agglomerate is saucer-shaped, the sloping sides of the
depression dipping towards the centre of the neck at about the same
angle as the rudely-stratified agglomerate underneath. From the bottom
of this basin to the sea-level may be a vertical distance of some 30
yards. The basin itself has been filled up by three successive flows
of basalt, of which the first (_c_) has merely overflowed the bottom,
the second (_d_), entering from the northern rim of the basin, extends
across to the southern slope, while the third (_e_), also flowing from
the north, has filled up the remainder of the hollow and extended
completely across it. The next succeeding lava (_f_) stretched over the
site in such a way as to bury it entirely, and to provide a level floor
for the piling up of the succeeding sheets of basalt.
[Illustration: Fig. 312.--Volcanic Neck close to that shown in Figs.
310 and 311.]
[Illustration:
Fig. 313.--Section of wall of another Neck of agglomerate in the
same group with those represented in Figs. 310, 311, and 312.
]
The second vent, which is represented in Fig 312, exhibits the same
features, but with some additional points of interest. It measures
roughly about 20 yards in diameter at the sea-level, rises through the
same group of banded basalt (_a_ _a_), and is filled with a similar
agglomerate (_b_). Its more northerly wall is now coincident with a
line of fault (_h_) which ascends the cliff, and probably marks some
subsidence after the eruptions had ceased. The southern wall shows
that a dyke of basalt (_g_) has risen between the agglomerate and the
banded basalts, and that a second dyke (_g´_) traverses the latter at
a distance of a few feet. In this instance, also, the upper surface of
the agglomerate forms a cup-shaped depression which has been filled
in by two successive streams of lava (_c_, _d_). Among the succeeding
lavas (_e_) the prominent sill (_f_) has been intruded, to which
further allusion is made on p. 323.
These necks are obviously volcanic vents belonging to the time of the
basaltic eruptions. They have been drilled through the basalts of
the lower part of the cliff, but have been buried under those of the
central and higher parts. The arrangement of their component materials
in rude beds dipping towards the middle of each vent shows that the
ejected dust and stones must have fallen back into the orifice so as
to be rudely stratified towards the centre of the chimney, which was
finally closed by its own last discharges of coarse detritus. The
saucer-shaped upper limit of the agglomerate seems to indicate, as has
been suggested above in the case of the Portree volcano, that after
the eruptions ceased each vent remained as a hollow or _maar_ on the
surface of the lava-fields. And the manner in which they are filled
with successive sheets of basalt shows that in course of time other
eruptions from neighbouring orifices gave forth streams of lava which,
in flowing over the volcanic fields, eventually buried and obliterated
each of the vents.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map VI MAP OF THE TERTIARY VOLCANIC REGION OF THE INNER HEBRIDES
The Edinburgh Geographical Institute Copyright J. G. Bartholomew]
In the destruction of the precipice some of the vents have been so much
cut away that only a small part of the wall is left, with a portion of
the agglomerate adhering to it. The third neck, for instance, affords
the section represented in Fig. 313, where the horizontal sheets of
basalt (_a_) have still a kind of thick pellicle of the volcanic
detritus (_b_) adhering to what must have been part of the side of the
orifice of eruption. The waves have cut out a cave at the base, so that
we can, by boat, get behind the agglomerate and see the margin of the
volcanic funnel in the roof overhead.
The fragment of geological history so picturesquely laid bare on the
Stromö cliffs presents a significant illustration of what seems to
have been a frequent, if not the normal type of volcanic vent in the
Tertiary basalt-plateaux. By the fortunate accident that denudation has
not proceeded too far, we are able to observe the original tops of at
least two of the vents, and to see how such volcanic orifices, which
were doubtless abundant all over these plateaux, came to be entombed
under the ever-increasing pile of accumulating basalt.
There is still one feature of interest in these cliff-sections which
deserves notice here. Every geologist who has studied the composition
of the basalt-plateaux has remarked the comparatively insignificant
part played by tuffs in these volcanic accumulations. Hundreds of
feet of successive basalt-sheets may often be examined without the
discovery of any intercalation of fragmental materials, and even where
such intercalations do occur they are for the most part quite thin and
extremely local. I found it impossible to scale the precipice for the
purpose of ascertaining whether around the Stromö vents, and connected
with them, there might not be some beds of tuff interstratified between
the basalts. If such beds exist, they can only be of trifling thickness
and extent. Here, then, are examples of once active vents, the funnels
of which are still choked up with coarse fragmentary ejections, yet
from which little or no discharge of ashes and stones took place over
the surrounding ground. They seem to have been left as crater-like
hollows on the bare surface of the lava-fields.
CHAPTER XLII
THE BASIC SILLS OF THE BASALT-PLATEAUX
We have now followed the distribution of the basalt-plateaux, the
arrangement of their component materials which were erupted at the
surface, and the character of the dyke-fissures and vents from which
these materials were ejected. But there remains to be considered
an extensive series of rocks which display some of the underground
phenomena of the Tertiary volcanoes. The injection of many basaltic
sheets had been clearly enforced by Macculloch. In 1871 I pointed out
that at different horizons in the plateau-basalts, but especially at
their base and among the stratified rocks underneath them, sheets
of basalt and dolerite occur which, though lying parallel with the
stratification of the volcanic series, are not truly bedded, but
intrusive, and therefore younger than the rocks between which they
lie.[309] The non-recognition of their true nature had led to their being
regarded as proofs of volcanic intercalations in the Jurassic series of
Scotland. There is, however, no trace of the true interstratification
of a volcanic band in that series, every apparent example being due
to the way in which intrusive sheets simulate the characters of
contemporaneous flows.
[Footnote 309: _Quart. Jour. Geol. Soc._ xxvii. (1871), p. 296.]
If such sheets had been met with only at one or two localities, we
might regard them as due to some mere local accident of structure in
the overlying crust through which the erupted material had to make its
way. But when we find them everywhere from the cliffs of Antrim to
the far headlands of Skye and the Shiant Isles, and see them reappear
among the Faroe Islands, it is obvious that, like those of Palæozoic
time, they must be due to some general cause, and that they contain the
record of a special period or phase in the building up of the Tertiary
volcanic tablelands. I will first describe some typical examples
of them from different districts, and then discuss their probable
relations with the other portions of the plateaux.
i. ANTRIM
First to be examined, and now most familiar to geologists, are the
remarkable sheets that underlie the plateau of Antrim, and project at
various parts of the picturesque line of coast between Portrush and
Fair Head. From the shore at Portrush, as I have already remarked,
came the evidence that was supposed to prove basalt to be a rock of
aqueous origin, inasmuch as shells were obtained there from what was
believed to be basalt. The long controversy to which this supposed
discovery gave rise is one of the most curious in the history of
geology.[310] It continued even after the illustrious Playfair had shown
that the pretended basalt was in reality highly indurated shale, and
hence that, instead of furnishing proof of the aqueous formation
of basalt, the Portrush sections only contributed another strong
confirmation of the Huttonian theory, which claimed basalt to be a rock
of igneous origin.
[Footnote 310: For an excellent summary of this controversy and an
epitome of the descriptions of the Portrush section, see the _Report
on the Geology of Londonderry_, etc. (_Mem. Geol. Survey_), by J. E.
Portlock (1843), p. 37.]
It is now well known that the rock which yielded the fossils is a
Liassic shale, that it is traversed by several sheets of eruptive rock,
and that by contact-metamorphism it has been changed into a highly
indurated substance, breaking with a splintery, conchoidal fracture,
but still retaining its ammonites and other fossils. The eruptive
material is a coarse, distinctly crystalline dolerite, in some parts of
which the augite, penetrated by lath-shaped crystals of plagioclase, is
remarkably fresh, while the olivine has begun to show the serpentinous
change along its cracks.[311] This rock has been thrust between the
bedding planes of the shales, but also breaks across them, and occurs
in several sheets, though these may all be portions of one subterranean
mass. Some of the sheets are only a few inches thick, and might at
first be mistaken for sedimentary alternations in the shale. But their
mode of weathering soon enables the observer readily to distinguish
them. It is to be noticed that these thin layers of eruptive material
assume a fine grain, and resemble the ordinary dykes of the district.
This closeness of texture, as Griffith long ago pointed out,[312] is
also to be noticed along the marginal portions of the thicker sheets
where they lie upon or are covered by the shales. But away from the
surfaces of contact, the rock assumes a coarser grain, insomuch that
in its thickest mass it presents crystals measuring sometimes an inch
in length, and then externally resembles a gabbro. A more curious
structure is shown in one of these coarsely crystalline portions by the
occurrence of a band a few inches broad which is strongly amygdaloidal,
the cells, sometimes three inches or more in diameter, being filled
with zeolites.[313] The general dip of the shales and of the intrusive
sheets which have been injected between them is towards the east. From
underneath them a thick mass of dolerite rises up to form the long
promontory that here projects northwards from the coast-line, and is
prolonged seawards in the chain of the Skerries.
[Footnote 311: Dr. F. Hatch, Explanation of Sheets 7 and 8, _Geol. Survey
of Ireland_, p. 40.]
[Footnote 312: "Address to Geological Society of Dublin, 1835," p. 13,
_Jour. Geol. Soc. Dublin_, vol. i. The varieties of the Portrush rock
were described by the late Dr. Oldham, in Portlock's _Report on the
Geology of Londonderry_, p. 150; see also the same work for Portlock's
own remarks, p. 97.]
[Footnote 313: For a list of the minerals in this rock, see Oldham, _op.
cit._ p. 151.]
An interesting feature of the Portrush sections is the clear way
in which they exhibit the phenomena of "segregation-veins"--so
characteristic of the thicker and more coarsely crystalline sills.
These veins or seams here differ from the rest of the rock mainly in
the much larger size and more definitely crystalline form of their
component minerals. Though sharply defined, when looked at from a
little distance, they are found on closer inspection to shade into the
surrounding rock by a complete interlacing of crystals. On the shore,
they can be seen to lie, on the whole, parallel with the bedding of
the sheets in which they occur, but without rigidly following it,
since they undulate and even ramify. A good section across their dip
has been exposed in a quarry near the end of the promontory, and shows
that they are considerably less regular than the plan of their outcrop
on the shore would have led us to anticipate. The accompanying drawing
(Fig. 314) represents the veins laid bare on a face of rock nine feet
in length by five feet in height. It will be seen that while there is a
general tendency to conform to the dip-slope, which is here from right
to left, the seams or layers unite into a large rudely-bedded mass,
which sends out processes at different angles. The peculiar aggregation
of minerals which distinguishes such veins is perhaps best seen at Fair
Head, and I reserve for the description of that locality what I have
to say on the subject, only remarking with regard to the Portrush rock
that the felspar shows a disposition to collect in the centre of the
veins with the augite and the other dark minerals at the outer margins.
[Illustration: Fig. 314.--View of "Segregation-Veins" in a dolerite
sill, Portrush, Antrim.]
The contact-metamorphism at this locality is of more historical
interest in connection with the progress of geological theory than of
scientific importance. It consists mainly in an intense induration of
the argillaceous strata. These pass here from their usual condition of
fissile, laminar, dull, dark shales into an exceedingly compact, black,
flinty substance, which in its fracture, colour and hardness reminds
one of Lydian stone. Yet the ammonites and other organic remains have
not been destroyed. They are preserved in pyrites.
[Illustration: Fig. 315.--View of Fair Head, from the east, showing the
main upper sill and a thinner sheet cropping out along the talus slope.]
Of all the examples of Tertiary sills in Britain few are more imposing
than that of the noble range of precipices which form the promontory
of Fair Head. Leaving out of account the minor masses of eruptive rock
which occur underneath it, we find the main sheet to extend along the
coast for nearly four miles, to rise to a height of 636 feet above the
sea, and to attain a maximum thickness of 250 feet. This enormous bed
dies out rapidly both to the east and west, and seems also to thin away
inland. Seen from the north, it stands upon a talus of blocks as a
sheer vertical wall, 250 feet high, and the rude prisms into which it
is divided are continuous from top to bottom (Fig. 315). So regular is
this prismatic structure, and so much does it recall the more minute
columnar grouping of the bedded basalts, that at a little distance we
can hardly realize the true scale of the structure. It is only when
we stand at the base of the cliff or scramble down its one accessible
gully, the "Grey Man's Path," that we appreciate how long and thick
each of the prisms actually is (Fig. 316). It may here be remarked
that this regular prismatic jointing is one of the distinguishing
features of the large sills, and serves to mark them off from the
bedded basalts, even when these have assumed a columnar structure. The
prisms are much larger than the basalt-columns, and never display the
irregular starch-like arrangement so common among the plateau-basalts.
[Illustration: Fig. 316.--View of Fair Head from the shore. (From a
Photograph by Mr. R. Welch.)]
The rock composing this magnificent sheet is a coarsely crystalline,
ophitic, olivine-dolerite.[314] The same diminution of the component
crystals, which is so marked along the margins of the eruptive masses
at Portrush, is strikingly exhibited at the borders of the Fair Head
sill. For about 18 or 20 inches upward from the bottom, where the
bed rests on the black, Carboniferous shales, the dolerite is dark
and finely crystalline, weathering spheroidally in the usual manner.
But immediately above that bottom layer of closer grain, the normal
coarsely crystalline texture rapidly supervenes. A similar closeness of
grain is observable at the surfaces of contact where the sheet splits
up on its western border.
[Footnote 314: Professor Judd has described what he calls a
"glomero-porphyritic structure" in this rock (_Quart. Journ. Geol.
Soc._ xlii. (1886), p. 71).]
Nowhere, so far as I know, can the phenomena of "segregation-veins"
be so instructively studied as along the abundant exposures of this
great sheet. The veins are most conspicuous where the rock occurs
in thickest mass. They vary up to three or four feet in thickness,
and, as at Portrush and elsewhere, lie on the whole parallel to the
upper and under surfaces of the sheet. An erroneous impression may be
conveyed by the term "veins" applied to them. They are quite as much
layers, parallel on the whole with the bedding of the sheet, yet not
adhering rigidly to one plane, but passing across here and there from
one horizon to another. That they are not due to any long subsequent
protrusion of younger material through the main sheet is made manifest
by the thorough interlocking of their component crystals with those
of the body of the rock in which they lie. The material that fills
these veins has obviously been introduced into them while there was
still some freedom of movement among the crystals of the surrounding
rock, which must thus have been still not quite consolidated and
therefore intensely hot. Both crystallized slowly, and in so doing
their component minerals dovetailed with each other. The constituents
of the veins consist of an exceedingly coarse aggregate of crystals, or
rather of crystalline lumps of the same minerals that constitute the
general mass of the rock, the felspar and augite showing the ophitic
intergrowth of the main rock, but on a far larger scale. Some of the
pieces of augite measure two inches or more in diameter. The conditions
under which these veins were produced must have differed in some
essential respects from those that prevailed during the formation of
the fine-grained, highly siliceous veins already described as occurring
in some dykes and sills.
This great Fair Head sill lies upon Carboniferous strata, but that
it is to be classed with the Tertiary volcanic series is, I think,
demonstrated by its relations to the Chalk at its eastern end. It has
there broken through that rock, and converted it for a short distance
into a white, granular marble. But it is at the western side that the
most interesting sections occur to show the truly intrusive nature of
the mass. The rock there splits up into about a dozen sheets, which,
keeping generally parallel with each other, have forced their way
between and partly across the bedding planes of the Carboniferous
shales (Fig. 317). In this way the huge, unbroken mass, 250 feet thick,
subdivides itself and disappears in a few hundred yards, though it
continues a little further inland, and approaches the shore again half
a mile to the south-west. Further evidence of the intrusive nature of
this rock may be observed along the base of the precipice, where at
least one sheet 70 feet thick diverges from the main mass and runs
eastwards between the Carboniferous shales (Fig. 315). At the contact
with the eruptive rock the shales are everywhere much indurated.
[Illustration: Fig. 317.--Section at Farragandoo Cliff, west end of
Fair Head, showing the rapid splitting up and dying out of an Intrusive
Sheet.
_a_, Carboniferous sandstone; _b_, Carboniferous shale; _c_, intrusive
sheet.]
ii. SKYE
All through the Inner Hebrides the base of the basalt-plateaux presents
abundant examples of sills. The general parallelism of these intrusive
sheets to the bedding of the Jurassic strata among which they lie has
been above referred to as having given rise to the erroneous conclusion
that in Skye and elsewhere the basalts are interstratified with
Jurassic rocks, and are consequently of Jurassic age. It was Macculloch
who first described and figured in detail the proofs of their intrusive
nature. His well-known sections in plate xvii. of the illustrations to
his work on the _Western Islands_ have been repeatedly copied, and have
served as typical figures of intrusive igneous rocks.
Nowhere in North-Western Europe can the phenomena of sills be studied
so fully and with such exuberance and variety of detail as in the
island of Skye and its surrounding islets. On the western coast the
greater subsidence of the basaltic plateau has for the most part
submerged the platform of intrusive sheets, though wherever the base
of the bedded lavas is brought up to the surface the accompanying
sills are exposed to view. The east coast of the island has been
classic ground for this part of volcanic geology since it supplied the
materials for Macculloch's descriptions and diagrams. From the mouth of
Loch Sligachan to Rudha Hunish, at the north end of Skye, a series of
sills may be traced, sometimes crowning the cliffs as a columnar mural
escarpment, sometimes burrowing in endless veins and threads through
the Jurassic rocks. The horizontal distance to which this continuous
band of sills extends in Skye is not far short of 30 miles. But it
stretches beyond the limits of the island. It forms the group of islets
which prolongs the geological structure and topographical features of
Trotternish for 4 miles further to the north-west. It reappears 10
miles still further on in the Shiant Isles. Thus its total visible
length is fully 40 miles, or if we include some outlying sills near the
Point of Sleat, to be afterwards described, it extends over a distance
of not less than 60 miles. From the last outlier in Skye to the sills
of the Isle of Eigg is a distance of only 8 miles, thence to those
of Ardnamurchan 17 miles, and to those of the south coast of Mull 25
miles. Thus this platform of intrusive sheets of the Inner Hebrides can
be interruptedly followed for a space of not less than 110 miles.
[Illustration: Fig. 318.--View of the Trotternish Coast, showing the
position of the band of Sills.
The dark band crowning the first slope above sea-level marks a
conspicuous band of sills which towards the right descends to the
beach and is prolonged seaward in the group of islands. The Storr Rock
appears as a slanting obelisk of rock on the hill to the left.]
Though none of the sills in Skye itself attain the dimensions of
the Fair Head sheet, they present a greater variety of rock and of
geological structure than is to be found in Antrim. They are specially
developed at the base of the thick, overlying, basalt-plateau--a
platform on which such a prodigious quantity of eruptive material
has been injected. Part of this material consists of basic rocks in
the form of dykes, veins, or sills; part of it is included in the
intermediate and acid groups, and comprises veins, sheets, and bosses
of granitoid, felsitic, rhyolitic, trachytic, and pitchstone rocks.
One of the peculiarities of the Skye sills is the occurrence among
them of compound examples, where sheets of basic and acid material
have been injected along the same general platform. These will be more
specially referred to in Chapter xlviii. With regard to the basic
sills (dolerites, basalts, etc.), I would remark that while in Western
Scotland the Antrim type of short, thick intrusions, or laccolites,
is also found, the vast majority of the sheets are much thinner, more
persistent, and less easily distinguishable from the bedded basalts.
[Illustration: Fig. 319.--Columnar Sill intrusive in Jurassic Strata
east of Kilmartin, Trotternish, Skye.
[The high ground to the left is a portion of the basalt-plateau to the
north of the well-known Quiraing.]]
In describing the sills of Skye I shall take first those of the eastern
and then those of the western side of the island. Along the east coast,
from Loch Sligachan to the most northerly headlands and islets the
sills play a notable part in the scenery, inasmuch as they cap the
great sea-cliff of Trotternish and run as a line of ridges parallel
to the trend of the coast, while the plateau-basalts rise above them
further inland as a lofty escarpment, which includes the picturesque
landslips of the Storr Rock and Quiraing (Figs. 318, 319). Beneath
the thick sills, the Jurassic sandstones form a range of pale yellow
precipices, along which many thinner sheets of eruptive material have
been intruded. As Macculloch well showed, many of these sheets, if seen
only at one point, might readily be taken for regularly interstratified
beds, but perhaps only a few yards distant they may be found to break
across the strata and to resume their course on a different level.
The sills of this Trotternish coast may be distinguished even at some
distance from the bedded basalts by the regular prismatic jointing,
already referred to, and by their frequently greater thickness, while
on closer inspection they are characterized by their much coarser
texture. They are generally somewhat largely crystalline ophitic
dolerites, gabbros or diabases, and exhibit the persistent uniformity
of composition and structure so characteristic of intrusive sheets and
dykes. These characters are well exhibited in the Kilt Rock, a columnar
sill capping the cliffs to the south of Loch Staffin (Fig. 319).
These massive sills are prolonged in a series of picturesque flat
tabular islets beyond the most northerly headlands of Skye. They
probably continue northwards under the sea at least 12 miles further,
for sills of the same type rise there in the singularly striking group
of the Shiant Isles (Fig. 320). These lonely islets, extending in
an east and west direction for about three miles, display in great
perfection most of the chief characters of the Skye sills. They are
especially noteworthy for including the thickest intrusive sheet and
the noblest columnar cliff in the whole of the Tertiary volcanic series
of Britain. The larger of the two chief islands consists of two masses
of rock connected by a strip of shingle-beach, and having a united
length from north to south of about two miles. The northern half, or
Garbh Eilean, presents towards the north a sheer precipice 500 feet
high. This magnificent face of rock consists of one single sill, but as
its original upper limit has been removed by denudation and its base,
where it is thickest, is concealed under the sea, the sill may exceed
500 feet in thickness. The rock has the usual prismatic structure,
which imparts to it an impressive appearance of regularity. The columns
retain their individuality to a great height, and though none of them
perhaps can be followed from base to crest of the cliff, many of them
are evidently at least 300 or 400 feet long.
Macculloch, who gave the first geological description of the Shiant
Isles, showed the intrusive nature of the igneous rocks, and described
the remarkable globular or botryoidal structure of the Jurassic
shales between which they have been injected.[315] Professor Heddle has
published a brief account of the geology of the islands.[316] Professor
Judd visited the group and brought away a series of specimens of
their eruptive rocks, which he found to include basic and ultra-basic
varieties.[317]
[Footnote 315: _Western Islands_, vol. i. p. 441.]
[Footnote 316: _Trans. Norfolk Nat. Hist. Soc._ vol. iii. (1880) p. 61.]
[Footnote 317: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878) p. 677, and
xli. (1885) p. 393. My description in the text is the result of three
successive visits to the islands.]
[Illustration: Fig. 320.--View of the northern precipice (500 feet
high) of the largest of the Shiant Isles.
(From a Photograph by Colonel Evans.)]
In Garbh Eilean, where the thickest mass of erupted material presents
itself, at least three sills may be observed. Some low reefs that
run parallel with the northern coast of the island consist of coarse
ophitic gabbro in two or more sheets which have been intruded between
the Jurassic shales. Above these strata comes the great columnar sill,
its base gradually sinking towards the west until it passes under the
sea, and the vertical columns then plunge abruptly into the water.
The rock of which this massive sill consists is another large-grained
gabbro or dolerite, with an ophitic structure. Owing to the form of
the ground it cannot be so satisfactorily examined as the neighbouring
island of Eilean Mhuire, which, though less lofty and rather smaller
than Garbh Eilean, affords a succession of admirable and easily
examined sections along its precipitous shores.
Professor Judd found that while the rocks are mainly ophitic gabbros
and dolerites, they include such highly basic compounds as dunite.
An examination of the Eilean Mhuire cliffs enables the observer to
ascertain that the sills display considerable variety in texture and
in the character and arrangement of their component minerals. They
are marked by a persistent, more or less distinct disposition in rude
beds, and these again often display a banding of their constituents
in lines parallel with the general bedding. Some of these bands are
largely felspathic, and are thus paler in colour. Others, where the
ferro-magnesian minerals and ores are more specially aggregated, are
dark in colour. In some layers the long black prisms of augite are
ranged in a general parallelism with the banding.
A specimen selected as typical of the ordinary coarse-grained amorphous
rock was sliced and placed in Mr. Harker's for microscopic examination,
and he has supplied the following observations regarding it: "The
gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the
eye lustrous black augites, half an inch long, and (predominating)
felspar. The microscope reveals, in addition, irregular grains of black
iron-ore and little hexagonal prisms of apatite. No olivine is to be
detected. As regards structure, the augite has tended to crystallise
out in advance of the felspar, but this relation is not constant.
"The augite is of a light-brown tint in slices, and has an unusual
kind of pleochroism. The colour for vibrations parallel to the [Greek:
b]-axis is of the purplish-brown tone seen in some soda-bearing
augites; parallel to [Greek: g] and [Greek: a] it has a yellow or
citron tint. The colour and pleochroism are more marked in the
interior of a crystal than towards the margin, but some crystals
pass at the margin into a slightly pleochroic, pale-green, recalling
ægerine-augite. The felspar tends to build elongated crystals. It is a
rather finely lamellated labradorite, sometimes showing pericline- as
well as albite-lamellae."
Another specimen from one of the black bands in the same island, with a
linear arrangement of its component minerals, is thus described by the
same petrographer: "This rock [7111] is of darker appearance than the
preceding, and contains abundant black iron-ore, besides some pyrites.
It also differs in having a marked parallel disposition of its crystals.
"Except for the greater prominence of large irregular grains of
iron-ore, this rock under the microscope closely resembles the last
described, the parallel structure not being conspicuous in the slice.
The augite has the peculiar colour and pleochroism already noted, and
the felspar is of the same kind as before."
I did not succeed in finding in place any bands of dunite, but this
basic material probably occurs at the base of some of the sills where
it has segregated from the rest of the mass, like the picrite at the
bottom of the Bathgate diabase.
The amount of contact-metamorphism effected even by such thick sills as
those of Trotternish and Shiant is much less than might be expected.
It seldom goes beyond a mere induration of the strata for a few yards,
often only for a few inches from the surface of junction. In the Shiant
Isles, however, the shales between the sills have undergone a more
remarkable alteration. They have not only been greatly indurated, but
have acquired the globular or botryoidal structure so fully described
by Macculloch. The spheroidal aggregates vary from not more than a
line to more than half an inch in diameter, and appear on the surface
as dark, irregularly grouped, pea-like aggregates. This structure is
perhaps best developed immediately under the thick sill on the west
side of Eilean Mhuire.
The massive sills are not the only evidence of the injection of igneous
material on the Shiant Isles. The sill, or more probably group of
sills, forming Eilean Mhuire is traversed by a number of sheets of
basalt varying from only two or three inches to 20 feet in thickness.
These black fine-grained rocks invariably present chilled selvages next
the coarse gabbro, and though they have been on the whole injected
parallel to the general bedding or banding, they here and there break
across it as veins. The most important of these later intrusions forms
a columnar sill on the eastern side of the island, and can be followed
for several hundred yards. It consists of a dark finely crystalline
olivine-basalt, which towards the margin assumes a dense black texture.
Under the microscope Mr. Harker found a thin slice of this rock to
be "an olivine-basalt of semi-ophitic, semi-granulitic structure
[7112]. The olivine is mostly fresh, but part of it is converted
into a yellowish-brown pseudomorph like iddingsite. Magnetite occurs
chiefly in imperfect octohedra. The felspar is in little lath-shaped
sections, many of which are finely striated, and give extinction-angles
indicating a labradorite. The augite, light brown in the slice, never
has crystal-boundaries, and often enwraps the felspars."
The narrow veins are composed of a much closer-grained basalt in which
a few scattered felspars are visible. Mr. Harker remarks, with regard
to a thin slice of one of these rocks [7113], that "the microscope
shows this, too, to be an olivine-basalt. The porphyritic felspars
are twinned on the Carlsbad and albite laws. Olivine and pseudomorphs
after it are well represented. Magnetite is only sparingly present.
The general mass of the rock consists of very small striated prisms of
labradorite, granules of augite, and interstitial matter which must be
partly glassy."
This is perhaps the most striking of all the examples known to me where
an older sill has been split open to receive a subsequent injection of
molten material. The Eilean Mhuire gabbro must be at least 200 feet
thick, and it not impossibly passes under the still thicker pile of
Garbh Eilean. Yet it has been horizontally ruptured near its base, and
into the rent thus produced another mass of molten matter has been
thrust. This subject will be again referred to in connection with
another remarkable example on the west coast of Skye.
[Illustration: Fig. 321.--Section of thin Intrusive Sheets and Veins in
carbonaceous shales lying among the Plateau-basalts, cliffs north of
Ach na Hannait, between Portree Bay and Lock Sligachan.]
In contrast to such enormous thicknesses of intrusive material as those
of Trotternish and the Shiant Isles, instances may be culled from the
same belt of sills where the molten rock has been injected in thin
leaves and mere threads into the Jurassic sandstones and shales, or
into the shales and coals intercalated among the plateau-basalts. Thus,
on the cliff immediately to the north of Ach na Hannait, between Loch
Sligachan and Portree Bay, the section may be seen which is represented
in Fig. 321. At the base lies a vesicular dolerite with a slaggy upper
surface (_a_). Next comes a zone of sedimentary material about five
or six feet thick, the lower portion consisting of an impure coal,
which passes towards the right hand into brown and grey carbonaceous
shale with plant-remains (_b_). This coaly layer has been already
alluded to as probably lying on the same horizon with the coal of
Portree (p. 288). Traced northward, it is found to have a bed of fine
tuff beneath it, and sometimes a volcanic breccia or conglomerate.
It fills up rents in the underlying slaggy lava, and was undoubtedly
deposited upon the cooled surface of that rock. Immediately above this
lower band the black carbonaceous shale which follows has been invaded
by an extraordinary number of thin cakes or sills and also by veins
or threads of basalt. For a thickness of two or three feet the band
(_d_) consists mainly of these intrusions, which, in the form of a
fine grey basalt, vary from less than an inch to three or four inches
in thickness. They are separated by thin partings of coaly shale,
and as they tend to break up into detached nodule-like portions,
especially towards the right hand of the section, they might, on casual
inspection, be easily mistaken for nodules in the dark shales. Somewhat
later in the time of intrusion are veins of basalt which, as at _c_,
break across the nodular sills, and sometimes expand into thicker beds
(_c´_).
I have never seen such a congeries of minute sills among the Tertiary
basalt-plateaux as that here exhibited. In a space of about three feet
of vertical height there must be more than a dozen of roughly parallel
leaves of intrusive rock. Veins (_e_) run up from the chief band of
eruptive material into the overlying finely vesicular basalt (_f_). The
dyke (_g_) is probably the youngest rock in the section.
The more general and extensive submergence of the base of the
basalt-plateau on the west side of Skye has for the most part carried
the platform of sills below sea-level, so that it is only exceptionally
where, owing to local irregularities, that base has been brought up to
the air, that the intrusive sheets show themselves. Yet the persistence
of the platform on that side is indicated by its extension even as far
as the southern promontory of the island.
The Trotternish type of sill extends down the west coast under the
headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the
underlying Jurassic platform has been ridged up above the surface of
the sea, it has carried with it the marked sill which forms the islets
of Mingay and Clett that lie as a protecting breakwater across the
entrance of the inlet. The intrusive rock rests on shell-limestones
full of oysters (_Ostrea hebridica_), and referable to the Loch
Staffin group of the Great Oolite Series. This sill, when observed
from a little distance, presents the usual regularly prismatic or
columnar structure so well developed among the Trotternish examples,
but on a closer view shows this structure less distinctly. It is an
olivine-dolerite of medium and fine texture, which in thin slices
displays under the microscope a distinctly ophitic structure, the
abundant light-brown augite enclosing the striated felspars. Its
lowest portion, from three to seven or eight inches upward from the
bottom, is much closer-textured than the rest of the rock and is finely
amygdaloidal. Its vesicles are in many cases drawn out to a length of
three or four inches, and the zeolites which now fill them look like
parallel annelid tubes or stems of _Lithostrotion_. It is noteworthy
also that the elongation of the vesicles has sometimes taken place at
a right angle to the surface of contact with the underlying strata.
But the most remarkable feature in this sill is the surface which it
presents to the oyster-beds on which it rests. The fine-grained dark
dolerite has there assumed the aspect of a sheet of iron-slag, with a
smooth or wrinkled, twisted, ropy surface, which displays fine curving
flow-lines. No one looking at a detached specimen of this surface would
be ready to admit that it could possibly have come from anything but
a true lava-stream that flowed out at the surface. The contours of a
viscous lava are here precisely reproduced on the under surface of a
massive sill.
A little further south, the promontory of Eist, forming the western
breakwater of Moonen Bay, consists of an important sill or group of
sills which has insinuated itself among shales, shell-limestones, and
shaly sandstones, full of _Ostrea hebridica_, _Cyrena aurata_, etc.,
and belonging to the Loch Staffin group of the Great Oolite Series.
The shore-cliff below the waterfall affords the section given in Fig.
322, illustrating the manner in which a thick intrusive sheet may
sometimes give off thin veins from its mass. The rock attains on the
Eist promontory a thickness of probably at least 100 feet, where it
is thickest and undivided. But the two main sheets, or branches of
one great sheet, on this peninsula have probably a united depth of
more than 300 feet. Landwards the rock splits up and encloses cakes
of the Jurassic strata. It possesses the usual prismatic structure
and doleritic composition. In Moonen Bay, as shown in Fig. 322, it
presents a banded structure, marked especially by an alternation of
lines of amygdales and layers of more compact and solid dolerite,
with occasional enclosed cakes of baked shale or sandstone. Its upper
surface is somewhat uneven, and from it are given off narrow, wavy,
ribbon-like veins (_d_), from less than an inch to three inches or
more in width, which keep in a general sense parallel to the top of
the sill, but at a distance of a few inches or feet from it. The sill
becomes as usual fine-grained towards the contact, the shales and
sandstones being indurated and the limestone marmorized.
[Illustration: Fig. 322.--Upper part of Sill, Moonen Bay, Waternish,
Skye, showing the divergence of veins.
_a_, false-bedded shaly sandstone; _b_, shell-limestone; _c_, dolerite
sill; _d_, veins proceeding from the sill.
Length of section about five yards.]
The next uprise of the base of the basalt-plateau on the west side of
Skye lies about 25 miles to the south-east, where it emerges from the
sea in the Sound of Soa (Fig. 323). A vast volcanic pile has there
been heaped up on the Torridon sandstone, the whole of the thick
Jurassic series, which is found in force only three miles distant in
Strathaird, having been removed by denudation from this area before the
beginning of the Tertiary volcanic period. The plateau-basalts rests on
the upturned edges of the Torridonian sandstones and shales, and are
accompanied as usual by their underlying network of intrusive rocks. It
is hardly possible to exaggerate the wild confusion of sills, dykes and
veins which have been injected among the rocks, at and on both sides of
the unconformability. Endless sheets of basalt and dolerite have forced
their way between the bedded basalts and the sandstones, while across
the whole rise vast numbers of dykes and veins. Narrow, black, wavy
ribbons of basic material cross many of these veins, while the later
north-west dykes cut sharply through everything older than themselves.
As a natural section for the study of the phenomena of intrusion in
many of their most characteristic phases, I know no locality equal
to the northern coast-line of the Sound of Soa, unless it be the
cliffs of Ardnamurchan. But the Skye cliffs, though less imposing than
those of the great Argyllshire headland, have this advantage, that
instead of being exposed to the full roll of the open Atlantic, they
form the margin of a comparatively sheltered strait, and can thus be
conveniently examined.
[Illustration: Fig. 323.--Section of the base of the Basalt-plateau
with sill and dykes, Sound of Soa, Skye.
_a_ _a_, Torridon Sandstone; _b_, Bedded basalts; _c_, Sill; _d_ _d_,
Dykes.]
Following still the western seaboard of Skye, we meet with other
striking examples of sills at a distance of some eight miles in a
straight line eastward where, between Lochs Slapin and Eishort, the
prominent headland of Suisnish juts out into the sea. This promontory
has long been known to geologists from the section of it given by
Macculloch as an instance of the connection between overlying rocks
and dykes. I have already alluded to it in that relation, and refer
to it again as an example of one of the thicker intrusive sheets of
the Inner Hebrides. Denudation has here also proceeded so far that
the whole of the volcanic plateau has been stripped off, only some of
the underlying sills being left, together with the platform of older
rocks between which and the vanished basalts they were injected.
Most of these sills consist of granophyres belonging to the acid
group of rocks to be afterwards described. But basic sheets occur not
infrequently interposed between the granophyres and the subjacent Lias,
and sometimes even intercalated in the former rock. Though at first
sight it might be thought that these sills had insinuated themselves
after the eruption of the granophyre, and there are instances where
this cannot be shown not to be the case, I have obtained so many proofs
of the invasion of the basic by the acid rock that I have no doubt the
former is, as a general rule, the older of the two.
The Suisnish headland exhibits the structure represented in Fig.
249. For about 300 feet above the sea-level the steep grassy slope
shows outcrops of the dark, sandy shales and yellowish brown, shaly
sandstones of the Lias which form the range of cliffs to the eastward.
These gently inclined strata are cut through by many vertical
basalt-dykes, some of which intersect each other, but among which
by far the largest is the mass shown in the figure. This broad dyke
consists of a dolerite or gabbro the largely crystalline texture of
which marks it off at once from the others, which are of the usual
dark, heavy, fine-grained type, with an occasional less basic and
porphyritic variety. Traced up from the sea-margin, the dyke loses
itself in a talus of blocks from the cliff above, so that its actual
junction with the mural front of the sill cannot be seen. But that it
joins that mass, with which it agrees in petrographical characters,
hardly admits of question. The cliff consists of a thick sheet of
coarsely crystalline dolerite or gabbro (_d_ in Fig. 249), which in
its general aspect at once recalls the rock of Fair Head. It varies
considerably in texture, some parts of the mass are exceedingly coarse,
like the Skye gabbros, and present a fibrous structure in their augite
resembling that of the diallage in these rocks; other portions assume
the compactness of basalt. A specimen of medium grain under the
microscope shows the typical ophitic structure so generally found among
the dolerites both of the plateaux and of the intrusive sheets. This
sill must be about 200 feet thick, and like the rock at Fair Head is
traversed from top to bottom by joints that divide it into prisms. It
appears to bifurcate eastward, one portion running with a tolerably
uniform thickness of a few feet as a prominent band at the top of
the shales and sandstones, the other slanting upwards and gradually
thinning away in the granophyre.
Towards its base, near the contact with the underlying shales, the rock
as usual becomes finer grained, and the thin band just referred to
resembles in texture one of the wider basalt-dykes. Westwards the rock
can be followed round the top of the grassy slopes formed by the decay
of the shales. Though concealed by intervals of moorland and peat, it
is visible in the stream sections, and I think must be continuous, as
a band only a few yards thick, round the northern side of the hills as
far as Beinn Bhuidhe, where a similar sill makes a prominent crag. Its
total area measures a mile and a quarter in length by half a mile in
breadth. The granophyre which overlies it forms part of an interesting
series of sheets which I have traced all the way from Suisnish to the
braes above Skulamus.
Whether or not the whole sheet of basic rock is continuous, and whether
it all proceeded from the great Suisnish dyke, cannot be confidently
decided until the ground is mapped in detail, though from the great
thickness of the sill at the dyke, its attenuation outwards from that
centre and its uniformity of petrographical character, I am disposed
to answer affirmatively. There is no other probable vent to be seen
in the neighbourhood, unless a massive dyke that runs from Loch Fada
north-westwards into Glen Boreraig can be so regarded.
Not far from the extreme southern point of Skye a singularly
interesting example of a sill remains as a detached survival of the
basaltic plateau and its accompaniments. In his map of Skye, Macculloch
showed the position of this outlier, which he classed with the
general "trap" formation of the island. The locality was visited by
Professor Judd, who regarded the intrusive rock as a "phonolite"[318]
In 1894, during an excursion with my colleague Mr. C. T. Clough, I had
an opportunity of examining the rocks and collecting notes for the
following account of them.
[Footnote 318: _Quart. Jour. Geol. Soc._ vol. xxxiv. (1878) p. 692.]
At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small
outlier of conglomerate lies on the edges of the Torridon Sandstone.
This deposit has been correctly identified by Professor Judd with
the similar strata which, in Skye and elsewhere on the west coast of
Scotland, underlie the Liassic series. It is here about 10 or 12 feet
thick, reddish and yellowish in colour, and distinctly calcareous. Its
component pebbles consist largely of Cambrian (Durness) limestone,
quartzite, and Torridon Sandstone--rocks which all occur _in situ_ in
Sleat. It may be compared with the limestone conglomerates of Strath
and those which underlie the Lias at Heast on Loch Eishort.[319] That
here, as elsewhere in this region, the basement conglomerate was
followed by the rest of the Lias and Oolites may be inferred with some
confidence from the copious development of the Jurassic series a few
miles off, both to north and south. But the whole of this overlying
succession of formations has here been swept away, and, but for the
protection afforded by the eruptive rocks of Rudh' an Iasgaich, the
conglomerate would likewise have disappeared.
[Footnote 319: _Op. cit._ vol. xiv. (1857), p. 9; vol. xliv. (1888), p.
71.]
[Illustration: Fig. 324.--Section of Dolerite Sill cut by another sill,
both being traversed by dykes, Rudh' an Iasgaich, western side of
Sleat, Skye.]
Above the conglomeratic band lies a sheet of intrusive rock, which in
one place has apparently cut it out, so as to rest directly upon the
Torridon Sandstone (_a_, Fig. 324). The decay of the softer detrital
rock underneath has caused the sill to break off in slices, which have
left behind them a bold mural escarpment (_b_ _b_).
The rock of this sill is a rather coarsely crystalline porphyritic
olivine-dolerite, which towards the north attains a thickness of about
70 feet. It exhibits the usual prismatic jointing, but less perfectly
than some of the Trotternish sills already referred to. Besides these
vertical joints, it is also traversed by a system of horizontal
divisional planes which, though somewhat irregular in their course,
run, in a general sense, parallel to the upper and under surfaces of
the sill.
It seems to have been along this transverse series of joints that a
second sill (_c_), five or six feet thick, has been injected. The
material of this younger intrusion is a black, finely crystalline
dolerite or basalt, with rudely prismatic jointing. Its most striking
feature, besides its regularity of position and persistency for several
hundred yards as a platform along the shore, is the basalt-glass which
marks both its under and upper surfaces of contact, and which is here
developed upon a scale to which I have not met with an equal among the
Tertiary sills of this country.
The selvage of glass appears as a black tar-like layer, varying from
a mere film to two or three inches in thickness. It is found not only
on the upper and under surfaces, but descends along abrupt step-like
interruptions of the upper surface, a foot or more in height, as if the
sill had been broken by a series of subsidences. The apparent fracture,
however, is probably due to the irregularities of the passage forced
for itself by the molten rock as it passed from one line of horizontal
joint to another through the heart of the older sheet.
The exposed surface of black glass on the top of the younger sill
exhibits long parallel lines, probably marking flow-structure, which
are made conspicuous by a pale yellow ferruginous weathered crust.
Portions of the larger intrusive sheet have been broken off and
involved in the later rock. The younger sill disappears to the north,
and is not found in the cliff of Rudha Chàrn nan Cearc, where the
thick sill, lying once more on the band of conglomerate, forms a fine
escarpment above the shore. Dykes of fine-grained basalt (_d_ _d_) with
compact chilled margins rise through both sills, together with veins
which pursue a wavy upward path like strips of black ribbon.
This example, and that of the Shiant Isles already described, cannot
but impress the observer with the prodigious force with which the
material of the sills was injected. In these instances solid sheets
of intrusive rock have subsequently been rent open, doubtless under a
superincumbent pressure of many hundreds of feet of the terrestrial
crust, and a new injection of molten magma has made its way into
the rents thus caused. In each case, the position of the rents was
obviously determined by structural lines in the older sills, but we are
lost in astonishment at the energy required to split open, even along
these lines, such solid crystalline masses as the thick sills, and to
overcome the superincumbent pressure of so deep a pile of rock.
The isolation of a relic of the Tertiary sills on the west side of
the promontory of Sleat presents some interesting problems to the
mind of the geologist. The locality lies about midway between the
basalt-plateau of Strathaird and that of Eigg, and some eight or nine
miles in a direct line from either. The basalts cannot be proved to
have once stretched continuously between Eigg and Strathaird, and to
have covered this part of Sleat; but the position of the Sleat sills
makes it probable that this continuation did formerly exist. The
denudation of the West of Scotland since early Tertiary time has been
so stupendous that I am prepared for almost any seemingly incredible
evidence of its effects. There can hardly be any doubt, however, that
the sills here described belong to the great platform of intrusive
sheets, and that they were injected under a pile of Secondary strata,
if not also of Tertiary basalts, which has here been entirely removed.
Reference may be made, in conclusion, to a not infrequent feature of
the Skye sills. Like the dykes, they are often double or multiple,
molten material having been successively injected along the same plane.
The example just cited from the west side of Sleat illustrates one
type of such compound sills. More frequently, however, the subsequent
injections have been made along the floor or roof of the first sheet.
Mr. Harker has found numerous cases of this structure in the Strath
district. They are recognizable even from a distance by their terraced
contours when seen in profile. They often vary considerably in
thickness owing to the dying out or coming-in of their separate bands;
while, on the other hand, single sills tend to maintain a uniform
thickness for long distances, or taper away gradually. The compound
arrangement of the basic sills is well brought out where acid material
has been injected between the sheets, as will be more fully described
in Chapter xlviii.
iii. EIGG, ARDNAMURCHAN
The phenomena of the coasts of Skye are repeated on the east side of
Raasay, in Eigg, and still more magnificently along the south coast
of Mull. A single example is here given (Fig. 325) from the east side
of Eigg. Over the Jurassic sandstones (_a_ _a_) a sill of basalt (1)
four to six feet thick has been injected between the stratification,
and another (2) two to four feet thick has forced its way across the
middle of one of the bedded basalts (_b_ _b_) in which it bifurcates,
and above which comes the thick series of lavas of the plateau (_c_,
_d_). In one of the streamlets, which exposes a section of the Jurassic
strata below the volcanic escarpment, more than twenty intrusive sheets
may be counted among the shales and limestones. They are sometimes not
six inches thick, and seldom exceed six or eight feet.
[Illustration: Fig. 325.--Section to show Bedded and Intrusive Sheets,
Eigg.]
I will conclude this account of the Tertiary basic sills of Britain
by referring to one further district in the West of Scotland,
where they are well displayed on bare hillslopes and also along a
picturesque sea-coast. In the promontory of Ardnamurchan in the west of
Argyleshire, one of the most conspicuous eminences, known as Ben Hiant,
affords a striking mass of intrusive material, which, extending along
a rugged shore for three-quarters of a mile, mounts thence inland in a
series of rocky knolls, and in rather less than a mile culminates in a
summit, 1729 feet above sea-level.[320] The rocks which cover this large
space are disposed in numerous rude beds, which have a seaward dip of
perhaps 15° to 20°, and are sometimes distinctly prismatic, the prisms
being not infrequently grouped in fan-shape. They are evidently due
to successive intrusions. Although generally coarsely crystalline in
texture, they include also intermediate and fine-grained sheets. They
are never, so far as I have been able to discover, amygdaloidal,[321]
nor do they present the ordinary external characters of the beds of
the plateaux, though here and there they appear to have caught up
portions of the plateau-series. They distinctly overlie the bedded
basalts on their eastern and southern margins; but westwards they
appear to lie transgressively across the edges of these rocks, while
to the north-west and north they rest on quartzites and schists and on
Jurassic limestones. An outlier from the main mass forms the prominent
hill of Sròn Mhor, and can be seen distinctly overlying the bedded
basalts as well as the neck of agglomerate already described (Fig. 302).
[Footnote 320: This locality has been described by Professor Judd
(_Quart. Jour. Geol. Soc._ xxx. (1874), p. 261; and xlvi. (1890), p.
373).]
[Footnote 321: As amygdaloidal structure is occasionally to be found
among both dykes and sills its presence in the Ben Hiant rocks would
not be inconsistent with their intrusive origin.]
The prevalent rocks of Ben Hiant are well crystallized, ophitic
olivine-dolerites and gabbros. A specimen taken from the shore on the
west side of the mass was found by Dr. Hatch to present under the
microscope its augite in large plates, which enclose narrow laths and
needles of plagioclase felspar as well as grains of olivine. All the
felspars are in lath-shapes, sometimes extremely long and narrow. The
iron-ore likewise assumes an ophitic character, enclosing rectangular
portions of felspar. Dr. Hatch observed in another specimen, taken
from the south-east side of the hill, "a curious intermixture of two
different structures. Scattered portions which show the usual ophitic
structure, their felspar and augite occurring in large crystals,
are, so to speak, imbedded in a groundmass which presents rather
a basaltic type, its felspar, augite, and magnetite, in long thin
needles, microlites, and other skeleton forms, being enclosed in a
dark devitrified base." A third specimen, selected from one of the
columnar sheets near the top of Ben Hiant, is "a fine-grained dolerite
(or gabbro) showing little ophitic structure, the augite occurring in
roundish grains, and only slightly intergrown with the felspars, which
are more or less lath-shaped. The rock contains a considerable quantity
of black iron-ore in irregular grains and some dirty-green viridite."
Still another variety of structure occurs in a specimen which I broke
from one of the shore crags on the south-west side of the hill.
Under the microscope, Dr. Hatch found in it a beautiful aggregate of
"skeleton crystals and microlites of plagioclase, with here and there
a rectangular crystal, long slender microlites of augite, and short
serrated microlites of magnetite, the whole being confusedly imbedded
in a dark glassy base powdered over with a fine magnetite dust."[322] A
sill of pitchstone lies among the bedded basalts on the east side of
the hill.
[Footnote 322: Professor Judd has called the rocks of Beinn Hiant
augite-andesites, and has given descriptions and figures of their
structure, and analyses of their chemical composition (_op. cit._).]
From a number of specimens collected by me during a second visit to
this district in the summer of 1896, I selected some for microscopic
examination and submitted them to Mr. Harker, who has furnished me with
the following descriptions of them: "The sill at the north end of Camas
na Cloiche, Ben Hiant [7114] is an olivine-gabbro of medium grain and
fresh appearance. Olivine, fresh or partly serpentinized, is plentiful.
The felspar is a labradorite with Carlsbad- and albite- (rarely
pericline-) twinning, and some of it has zonary banding. It is for the
most part in crystals giving rectangular sections, but there are some
of allotriomorphic form. Magnetite occurs chiefly in shapeless grains
of later crystallization than the felspar, but sometimes presenting
crystal-faces to the augite. The augite is light-brown in the slice,
without any true diallage-structure, and tends to enwrap the earlier
minerals in ophitic patches.
"The sill south of Uamh na Creadha, on the west side of Ben
Hiant [7115], is a rock of different type, having porphyritic
crystals of felspar, up to an inch or more in length, in a rather
finely-crystalline groundmass. The microscope shows it to be a dolerite
of granulitic structure, the main mass of the rock consisting of little
striated labradorite-crystals, grains of pyroxene, and rather abundant
crystal-grains of magnetite. The pyroxene seems to be chiefly augite,
but hypersthene is also present, and builds rather larger and more
idiomorphic crystals with characteristic pleochroism."
In rambling over this Ardnamurchan district I have often been reminded
of the great intrusive sheets of Fair Head. One of the features in
which the rocks of the two localities resemble each other is their
tendency to assume a coarsely crystalline texture. In some parts of
Ben Hiant the individual crystals reach an inch or more in length.
These more largely crystalline portions, however, do not form distinct
bands so much as patches in the midst of the general mass; at least I
have not noticed any examples of such veins of segregation as are so
prominent in Antrim.
No one familiar with the well-marked distinctions between the lavas of
the plateaux and the sills which traverse them can hesitate in which
series to place the rocks of Ben Hiant. Since, however, these rocks
have been claimed by Professor Judd as the superficial lava-currents of
a volcano which broke out after the time of the plateau-basalts, like
the Scuir of Eigg, some further details in regard to the geological
structure of the district, which would otherwise be superfluous, may
here be given.
The number of sills and dykes in Ardnamurchan is astonishingly great.
There must be hundreds of them visible, and perhaps as many more
concealed under superficial coverings. They are well exposed on the
shore traversing the Jurassic strata and the schists. The sills become
especially large and abundant in the direction of Ben Hiant, which has
evidently been the principal centre from which their materials were
injected. The rocks composing these sills are quite similar to those
of Ben Hiant, save that, as they occur in thinner sheets than in that
mountain, they do not attain the same coarseness of texture which the
more massive beds there display. They generally possess fine-grained
chilled selvages along their upper and under surfaces.
[Illustration: Fig. 326.--Ground-plan of Sills at Ben Hiant,
Ardnamurchan.
_a_ _a_, crystalline schists; _b_ _b_, necks of volcanic agglomerate;
_c_ _c_, numerous thin sills; D, massive sill of Beinn na h-Urchrach;
E, north side of Ben Hiant; F, sill proceeding from the series forming
Ben Hiant and joining that of Beinn na h-Urchrach. The arrows mark the
dip.]
These abundant sills may be traced up into the mass of Ben Hiant from
which they have issued, and of the individual sheets of which they are
a continuation. One of the most striking and easily-followed examples
of this connection is to be seen on the north side of the mountain.
A thick sheet in the middle of Ben Hiant descends from among its
contiguous sheets and, as a prominent rib, runs down the scree-slope
into the valley below, where it forms a prominent feature. Crossing
the streamlet in the middle of the valley, where a section has been
cut through its upper surface, it gradually bends round towards the
north-east, mounts the side of Beinn na h-Urchrach until it reaches the
crest of the ridge and joins the other sills of which this eminence is
built up. The route of this band of rock will be understood from the
annexed ground-plan (Fig. 326).
That this prolongation of one of the thick beds of Ben Hiant is in no
respect a superficial lava-stream but a true sill, is proved not only
by its escarpment and dip-slope, but by its actually passing under and
indurating the schistose grits, as may be seen in the stream-section.
Again Beinn na h-Urchrach, which is mapped by Professor Judd as a
northern expansion of Ben Hiant, is likewise not a lava but a true
sill. Not only does it dip northwards at an angle of about 20°, having
the schists immediately below its crest on the one side and descending
with a long dip-slope on the other, but dwindling down rapidly from
a thickness of 100 or 200 feet in the centre to no more than a few
feet in a south-westerly direction, it there passes under schistose
grits like those on which it lies. The strata that adhere to its upper
surface are as usual indurated.
A section drawn across this attenuated development of the Beinn na
h-Urchrach sill and that from Ben Hiant shows the structure represented
in the accompanying diagram (Fig. 327), which simply gives the facts
as exposed on the ground. The lower sill is that which issues from the
main body of Ben Hiant, massive at first but diminishing in thickness
as it recedes from its source.
Again, among the sheets which descend from the northern face of the
summit of Ben Hiant and strike into the Jurassic outlier below,
intensely indurated shale may be seen lying between two of the
dolerites, which are unquestionably sills that have been injected into
the Jurassic series.
The ridge of Ben Hiant is thus found to consist of a thick and complex
series of sills, some of which are not traceable beyond the side of the
mountain, while others can be followed outwards among the surrounding
rocks. The specially marked dyke-like sills diverge from the main mass
and run for some distance north-eastward, one of them, fully a mile
long, descending among the schists into the valley and ascending into
the basalt-plateau on the opposite side.[323]
[Footnote 323: The sills of Ben Hiant descend on the south-west side into
the sea, and can be examined along the slopes and the beach, where
Professor Judd has mapped a continuous platform of agglomerate. The
broad hollow between that mountain and Beinn na h-Urchrach, over which
he has spread his "augite-andesite lavas," appears to be underlain
mainly by the crystalline schists through which sills from Ben Hiant
have been injected. The northern eminence, which he has united with Ben
Hiant, is entirely separate and, as above shown, is an obvious sill.]
[Illustration: Fig. 327.--Section of two Sills in schistose grits, west
end of Beinn na h-Urchrach, Ardnamurchan.
_a_ _a_, crystalline schists; _b_, neck of volcanic agglomerate; _c_,
small sill; D, massive sill of Beinn na h-Urchrach; F, sill proceeding
from the series forming Ben Hiant and joining that of Beinn na
h-Urchrach.]
On the south-east side of the mountain where the bedded basalts can
be traced close up to the intrusive dolerites, they are found to
present the usual dull indurated aspect so characteristic of contact
alteration among these rocks. There cannot therefore be any doubt that
Ben Hiant never was itself a volcano. Its rocks are characteristically
those of subterranean intrusions. They seem to have been injected from
a line of fissure or from several such lines, running in a general
north-easterly direction, at some late part of the volcanic period. The
group of agglomerate necks of older date shows that already the ground
underneath had been drilled by a number of distinct volcanic funnels,
and discloses a weak part in the terrestrial crust.
iv. FAROE ISLES
In the Faroe Islands the actual base of the volcanic series is
nowhere visible. Hence, the great lower platform of intrusive sheets
being there concealed, this feature of the basalt-plateaux is less
conspicuous than it is in the Inner Hebrides. A number of sills,
however, have been noticed by previous observers,[324] and I have
observed others on the sides of Stromö, Kalsö, Kunö and other islands.
In the lofty precipices of the Haraldsfjord, many of the massive
light-coloured prismatic sheets are intrusive, for though they preserve
their parallelism with the bedded sheets for considerable distances,
they may be seen sometimes to break across these, as is strikingly
shown in one of the great corries on the east side of Kunö.
[Footnote 324: See in particular Prof. James Geikie and Mr. Lomas, in the
papers already cited on p. 191.]
[Illustration: Fig. 328.--Sill traversing bedded Basalts, cliffs of
Stromö, at entrance of Vaagöfjord.
The caves and notches shown at the bottom of the precipice mark the
position of the vents represented in Figs. 311, 312, 313, 314.]
One of the most remarkable sills in the Faroe Islands is probably that
which forms so prominent an object on the western cliffs of Stromö, at
the entrance into the Vaagöfjord (Figs. 328, 329). It is prismatic in
structure, and where it runs along the face of the cliffs, parallel
to the bedded basalts among which it has been intruded, presents the
familiar characters of such sheets. The precipice of which it forms
a part is that which rises above the row of volcanic vents already
described. But it there begins to ascend the cliffs obliquely across
the basalts until it reaches the crest of the great wall of volcanic
rock at a height of probably about 1000 feet above the waves. From the
crest of the precipice the upward course of the sill is continued into
the interior of the island. It pursues its way as a line of bold crag
along the ridges of the plateau, gradually ascending till it forms the
summit of one of the most prominent hills in the district (Fig. 329).
Some further idea of the enormous energy with which the sills were
injected may be formed from this example, where the eruptive materials
followed neither the line of bedding nor a vertical fissure, but took
an oblique course through the plateau-basalts for a vertical distance
of probably more than 1500 feet.
V. GENERAL DEDUCTIONS REGARDING THE TERTIARY BASIC SILLS
If we consider the facts which have now been adduced regarding
the position and structure of the sills, we are led, I think, to
regard these masses as certainly belonging to the history of the
basalt-plateaux, but, on the whole, to a comparatively late part of
it. They consist of essentially the same materials as the lavas that
form these plateaux, though with the differences of structure which the
conditions of their production would lead us naturally to expect. Where
they occur in thick masses, which must obviously have cooled much more
slowly at some depth beneath the surface than the comparatively thin
sheets could do that were poured out above ground, they have assumed
a far more largely crystalline texture than that of the superficial
lavas. As a rule, we may say that the thicker the sill the coarser is
its texture, while the thinnest sheets are the most close-grained.
Sills are especially abundant about the base of the basaltic-plateaux.
We may examine miles of the central and higher parts of the escarpments
without detecting a single example of them, but if the escarpment is
cut down to the base we seldom need to search far to find them.
[Illustration: Fig. 329.--View of the same Sill seen from the channel
opposite the island of Kolter.]
That the efforts of the internal magma to establish an outlet
towards the surface were accompanied by powerful disturbances of the
terrestrial crust is shown by the abundant dykes which traverse all the
volcanic districts from Antrim to Iceland, and some of which ascend
even to the very highest remaining lavas of the basalt-plateaux. The
parallel fissures filled by these dykes prove that even after the
accumulation of more than 3000 feet of basalt-sheets, the movements
continued to be so powerful as to disrupt these vast piles of volcanic
material. But undoubtedly the highest parts of the plateau-basalts are
less cut by dykes than the lower parts. There would no doubt come a
time when the dislocations would more seldom reach the surface, when
dykes would not be formed so abundantly or up to such a high level, and
when the volcanic energies would more and more sparingly result in the
opening of new vents or in the discharge of fresh eruptions from old
ones.
It appears to me most probable that the injection of the sills was
connected with the same terrestrial disturbances that produced the
dykes which traverse the plateaux. Besides being dislocated by parallel
fissures, the earth's crust in North-Western Europe seems to have been
ruptured internally along lines more or less at right angles to the
vertical fissures. The deep accumulation of bedded basalts presented
an increasing obstacle to the ascent of the magma to the surface.
Unable to gain ample enough egress through such vertical fissures as
might be formed in the volcanic pile, the molten rock would find its
lines of least resistance along the planes of the strata and the lower
basalt-beds, either by the aid of terrestrial ruptures there, or in
virtue of its own energy. On these horizons, accordingly, the sills
occur in extraordinary profusion throughout the volcanic regions. They
are no doubt of all ages in the progress of the building up of the
volcanic plateaux, but I am disposed to believe that a large number
of them may belong to the very latest period of the uprise of basalt
within the area of Britain.
One of the most suggestive features of the abundant Tertiary sills
lies in the evidence they furnish of the enormous energy concerned
in the ascent and intrusion of volcanic material. The infilling of
dykes or the outpouring of successive streams of lava at the surface
hardly appeals to our imagination so strikingly as the proof that the
sills have been impelled into their places with a vigour which, even
when guided and aided by gigantic terrestrial ruptures, was capable of
overcoming the vertical pressure of hundreds, or even thousands of feet
of overlying rock. Had these intrusive sheets been mere thin layers,
their horizontal extent and persistence would still have excited our
astonishment, but when we find them sometimes several hundred feet
thick, and to extend in a continuous series for horizontal distances
of 50 miles or more, we are lost in wonder at the prodigious expansive
strength of the volcanic forces. Again, the intrusions have not always
taken place between the bedding-planes of the stratified or igneous
rocks, but, as we have seen, solid sheets of already deeply buried
lavas have sometimes been split open and the intrusive material has
forced itself between the disrupted portions. Such subterranean proofs
of the vigour of volcanic energy teach some of the most impressive
lessons in the chronicles of volcanic action in the British Isles.
* * * * *
In closing this history of the accumulation of the great Tertiary
volcanic plateaux of North-Western Europe, I would remark that as the
result of prolonged eruptions from innumerable vents, the depression
that extended from the south of Antrim to the Minch was gradually
filled up with successive sheets of basalt to a depth of more than 3000
feet. A succession of lava-fields stretched from the North of Ireland
across the West of Scotland, and perhaps even to the Faroe Islands,
Iceland and Greenland. That the lava spread round the base of the
Highland mountains and ran up the Highland glens, much as the sea now
does, is made clear from the position of the outliers of it which have
been left perched on the ridges of Morven and Ardnamurchan. So far as
can now be surmised, these wide Phlegræan fields were only varied by
occasional volcanic cones scattered over their surface, marking some
of the last vents from which streams of basalt had flowed. But the
volcanic energy was still far from exhaustion. After the accumulation
of such a deep and far-extended sheet of lava, those underground
movements which produced the fissures that served as channels for the
uprise of the earliest dykes continued to show their vigour. The pile
of bedded lavas was rent open by innumerable long parallel fissures
in the prevalent north-westerly direction, up which basic lavas rose
to form dykes, while vast numbers of sills were injected underneath.
Whether the outflow of basalt at the surface had wholly ceased when
the last of these dykes were injected into the plateaux cannot be
told. Nor is there any evidence whether it had ended before the next
great episode of the volcanic history--the extravasation of the gabbro
bosses. All that we can affirm with certainty is, that the formation
of north-west fissures and the uprise of basalt in them were again
repeated, for we find north-west dykes traversing even the crests of
the later eruptive masses of basic and acid rocks. It is difficult to
suppose that none of these latest dykes communicated with the surface,
and gave rise to cones with the outpouring of basalt and the ejection
of dust and stones. But of such later outflows of basic material over
the surface of the plateaux no undoubted trace has yet been recognised.
CHAPTER XLIII
THE BOSSES AND SHEETS OF GABBRO
Petrography of the Rocks--Relations of the Gabbros to the other
members of the Volcanic series--Description of the Gabbro
districts--Skye
In singular contrast to the nearly flat basalts of the plateaux,
another series of rocks rises high and abruptly above these tablelands
into groups of dome-shaped, conical, spiry, and rugged hills. It is
these heights which, more than any other feature, relieve the monotony
of the wide areas of almost horizontal stratification so characteristic
of the volcanic region of the north-west. Their geological structure
and history are much less obvious than those of the bedded basalts.
Their mountainous forms at once suggest a wholly different origin. Some
portions of them have even been compared with the oldest or Archæan
rocks.[325] That they are really portions of the Tertiary volcanic
series, and that they reveal a wholly distinct phase in the history of
volcanic action, is now frankly admitted. Whether we regard them from
the petrographical or structural point of view, they naturally arrange
themselves into two well-defined groups. Of these one consists of
highly basic compounds, of which olivine-gabbro is the most prominent.
The other comprises numerous varieties--granite, granophyre, felsite,
quartz-porphyry, pitchstone and others--all of them being more or less
decidedly acid, and some of them markedly so. For reasons which will
appear in the sequel, the former group must be considered as the older
of the two, and it will therefore be described first.
[Footnote 325: This was my own first impression, when I began, as a
boy, to ramble among them. The remarkable resemblance of some parts
of them to ancient gneisses will be afterwards dwelt upon. Macculloch
had correctly grouped them with the other overlying rocks, and this
conclusion was afterwards confirmed by Prof. Zirkel.]
i. PETROGRAPHY OF THE GABBRO AREAS
Since the publications of Macculloch, the occurrence of beautiful
varieties of highly basic rocks among the igneous masses of the
Western Isles has been familiar to geologists. They were named by him
"hypersthene rock" and "augite rock,"[326] names which continued in use
until 1871, when my friend Professor Zirkel published the results
of his tour through the West of Scotland, and showed that the rocks
in question were mostly true gabbros.[327] Since his observations were
published some of these rocks have formed the subject of important
papers by Professor Judd.[328]
[Footnote 326: _Western Islands_, vol. i. pp. 385, 484.]
[Footnote 327: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871), p.
1.]
[Footnote 328: _Quart. Jour. Geol. Soc._ xli. (1885), p. 354; xlii.
(1886), p. 49.]
The general petrographical characters of the gabbro areas of Western
Scotland may be summarized as follows:--A very considerable variety of
petrological structure and chemical composition is observable among the
rocks. At the one end of the series are compounds of plagioclase and
augite, which, though wanting in olivine, have the general structure
and habit of dolerites. At the other end are mixtures wherein felspar
is scarce or absent, and where olivine becomes the chief constituent.
Between these two extremes are many intermediate grades, of which the
most important are those containing the variety of augite known as
diallage and also olivine. These are the olivine-gabbros, which form so
marked a feature in the central parts of the great basic bosses. That
some of these varieties of rock pass into each other cannot be doubted.
Their distinctive composition and structure appear to have been
largely determined by their position in the eruptive mass. The outer
and thinner sheets are in great measure dolerites, with little or no
olivine. Coarse gabbros are abundant in the inner portions. Rocks rich
in olivine, however, occur at the outer and especially the lower part
of the gabbro masses of Rum and in some parts of Skye. The following
leading varieties may be enumerated:--
Dolerite.--This rock varies from an exceedingly close grain (when
it approaches and graduates into basalt) up to a coarse granular
crystalline texture, in which the component minerals are distinctly
visible to the naked eye. An average sample is found to consist of
plagioclase, usually lath-shaped, and crystals or grains of augite
with or without olivine. Under the microscope, the different varieties
are distinguished by the presence of more or less distinct ophitic
structure, the felspar being enveloped in the augite. For the most
part they are holocrystalline, but occasionally show traces of a
glassy base. Ilmenite is not infrequent, with its characteristic
turbid decomposition product (leucoxene). In other cases, the iron-ore
is probably magnetite. Between the dolerites and gabbros no line of
demarcation can be drawn in the field, nor can a much more satisfactory
limitation be made even with the aid of the microscope. As a rule, the
thickest and largest intrusive masses or bosses are gabbro, those of
less size are dolerite, while the smallest (and sometimes the edges of
the others) assume externally the aspect of basalts.
Gabbro.--Under this term I arrange, as proposed by Professor Judd,
all the coarse-grained granitoid basic rocks of the region without
reference to the variety of augite present in them. Under the
microscope, they are found to be holocrystalline, but with a granitic
or granulitic rather than an ophitic structure, though traces of the
latter are by no means rare. To the naked eye their component minerals
are usually recognizable. Professor Zirkel, from his examination
of the Mull gabbros, believed them to consist of three parts of
plagioclase, two parts of olivine, and one part of diallage.[329]
Olivine, however, is not invariably present.[330] The pyroxene also does
not always show the peculiar fibrous structure of diallage. Professor
Judd, indeed, maintains that the diallagic form is due to a deep-seated
process of alteration (schillerization), and that the same crystal may
consist partly of ordinary augite and partly of diallage.[331] Ilmenite
(with leucoxene), magnetite, apatite, biotite, and epidote are not
infrequent constituents.
[Footnote 329: _Zeitschr. Deutsch. Geol. Gesellsch._ xxiii. (1871), p.
59.]
[Footnote 330: Professor Judd (_Quart. Jour. Geol. Soc._ xlii. p. 62)
believes that originally all the gabbros contained olivine, and
that where it is now absent, it has been altered into magnetite or
serpentine. But in some coarse massive gabbros this mineral does not
appear to have been an essential constituent. See _op. cit._ vol. l. p.
654.]
[Footnote 331: _Op. cit._ xli. In a later paper he insists on
the gradation of the coarse granitoid varieties (gabbros) into
holocrystalline compounds, where the felspar appears in lath-shapes
with crystals or rounded grains of augite and olivine (dolerites), and
thence into true basalts, magma-basalts, and tachylytes (_op. cit._
xlii. p. 62).]
In a recent study of the gabbros of the Cuillin Hills of Skye by
Mr. J. J. H. Teall and myself, four characteristic types have been
recognized.[332]
[Footnote 332: _Quart. Journ. Geol. Soc._ vol. 1. (1894), pp. 645-659,
and Plates xiii. xxvi.-xxviii. See also Prof. Judd's paper, _op. cit._
(1886), p. 49.]
(1) _Granulitic Gabbros._--These are dark, fine-grained rocks which
externally resemble some of the altered basalts of the plateau-series.
They occur in bands or sheets which, so far as can be made out, are the
oldest portions of the whole gabbro mass. Under the microscope they
are found to possess a finely granulitic structure, and to consist
of grains of pyroxene (augite, but more usually with the inclusions
characteristic of diallage and pseudo-hypersthene), and of felspar
allied to labradorite, with green pseudomorphs agreeing in form and
size with the pyroxene-grains, but made of minute prisms and fibres of
green hornblende and a little chlorite.
(2) _Banded Gabbros._--These are characterized by a remarkable
arrangement in parallel bands of different mineral composition
like the banding of ancient gneisses. This structure will be more
particularly described in later pages. They are coarse-grained rocks
composed of pyroxene, plagioclase, olivine and magnetite. But these
minerals are not distributed equally through the mass. The pale bands
contain much felspar; the dark bands are largely composed of the
ferro-magnesian minerals and magnetite. The pyroxene, occurring as
ordinary augite, not uncommonly shows a tendency to ophitic structure.
The felspar, a variety closely allied to labradorite, occurs as
grains, as irregular ophitic patches, and also in forms that give
broad rectangular sections. Olivine in an unaltered condition has
been detected by Mr. Teall in only one specimen, and he thinks that
this mineral probably never played an important part in the original
constitution of these rocks. Its rounded grains may be observed to
have the other minerals moulded round them, whence it may be inferred
to be of older consolidation. Magnetite is generally present, either
in rounded grains or in large irregular masses. Though it occurs also
in strings traversing the other minerals as a secondary product, it
must undoubtedly have entered largely into the original composition
of these rocks. It is found enclosing the augite grains and behaving
like a groundmass between the felspars. Among the dark bands there
occur narrow lenticular black layers ('schlieren') composed entirely of
augite and iron-ore.
The extraordinary differences between the composition of the pale
felspathic and the dark ultra-basic bands are well brought out in
the following analyses by Mr. J. Hort Player, No. 1 being from a
light-coloured band consisting mainly of labradorite with some augite,
uralitic hornblende and magnetite; No. 2 from a dark band composed of
augite, magnetite and labradorite; and No. 3 from a thin ultra-basic
layer mainly formed of augite and magnetite. All these specimens were
taken from the ridge of Druim an Eidhne, on the eastern side of the
Cuillin Hills, Skye.[333]
I. II. III.
Silica 52·8 40·2 29·5
Titanic acid ·5 4·7 9·2
Alumina 17·8 9·5 3·8
Ferric oxide 1·2 9·7 17·8
Ferrous oxide 4·8 12·2 18·2
Ferric sulphide ··· ·4 ·4
Oxide of manganese ··· ·4 ·3
Lime 12·9 13·1 10·0
Magnesia 4·8 8·0 8·7
Soda 3·0 ·8 ·2
Potash ·5 ·2 ·1
Loss by ignition 1·2 ·5 1·0
---- ---- ----
99·5 99·7 99·2
---- ---- ----
Spec. grav. 2·91 3·36 3·87
==== ==== ====
[Footnote 333: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 653. Banded
structures have been recognized in many gabbros of different ages. See
the references in this paper; also Mr. W. S. Bayley, _Journ. Geol._
Chicago, ii. (1895), p. 814, and vol. iii. p. 1.]
(3) _Coarse-grained massive Gabbros._--These rocks, so abundant among
the great basic bosses of the Inner Hebrides, are characterized by
their coarse granitic structure, their component crystals being
sometimes more than an inch long. They occur as sheets, veins and
irregular masses traversing the varieties of gabbro already mentioned.
They consist of the same minerals as the banded forms, and indeed are
themselves sometimes banded. They are more uniform in composition than
the typical banded gabbros, though showing also some variation in the
relative proportions of their constituents. The specific gravity of
three specimens was found to be 2·82, 2·97, and 3·06.
[Illustration: Fig. 330.--Granulitic and coarsely foliated gabbro
traversed by later veins of felspathic gabbro, Druim an Eidhne, Cuillin
Hills, Skye.]
(4) _Pale Gabbros of the Veins._--These occur abundantly as irregular
branching veins, from less than an inch to several yards in width,
and cross all the other varieties (Fig. 330[334]). Their whiteness on
weathered surfaces makes them conspicuous by contrast with the dark
brown or black hue of the rocks which they traverse, and shows at
once that they must be poorer in bases than these. They are found on
microscopic examination to consist of the same minerals as the more
coarsely crystalline gabbros, but with a much greater abundance of
the felspar. They contain also apatite, and hornblende appears to
predominate in them over augite. They are to be distinguished from the
pale veins that form apophyses from the intrusive granophyres.
[Footnote 334: Figs. 330, 336 and 337 are from photographs taken for the
Geological Survey by Mr. R. Lunn.]
Troctolite (Forellenstein).--This beautiful variety of
plagioclase-olivine rock occurs as a conspicuous feature on the east
side of the gabbro-area of the island of Rum. It forms a sill on the
side of the mountain Allival, in which the component minerals are drawn
out parallel with the upper and under surfaces of the bed (Fig. 341).
So marked is this flow-structure that hand-specimens might readily
be taken at the first glance for ancient schistose limestone. "The
felspathic ingredient (probably labradorite or anorthite) is white,
and its lath-shaped crystals have ranged themselves with their long
axes parallel to the line of flow. The olivine occurs in perfectly
fresh grains, which in hand-specimens have a delicate green tint.
Under the microscope they appear colourless, and are penetrated by
the felspar prisms in ophitic intergrowth. There is a small quantity
of a pale brownish augite, which not only occurs in wedge-shaped
portions between the felspars, but also as a narrow zone round the
olivines."[335] Considerable differences are visible in the development
of the flow-structure, and with these there appear to be accompanying
variations in the microscopic structure. Dr. Hatch, to whom I submitted
my specimens, informed me that in one of them, where the flow-structure
is so marked as to give a finely schistose aspect to the rock, "there
is a larger proportion of augite, some of which exhibits a distinct
diallagic striping; the olivine grains show no ophitic structure, but
are sometimes completely embedded in the augite." To this remarkable
flow-structure I shall again refer in connection with the light it
throws on the bedded character of much of the gabbro bosses.
[Footnote 335: MS. of Dr. Hatch.]
Between the different basic intrusive igneous rocks of the Inner
Hebrides, as Professor Judd has shown, there are many gradations
according to the varying proportions of the chief component minerals.
Thus from the olivine-gabbros, by the diminution or disappearance of
the augite we get such rocks as troctolite; where the plagioclase
diminishes or vanishes, we have different forms of picrite; where the
olivine is left out, we come to compounds, like eucrite; while by the
lessening or disappearance of the felspar and augite, we are led to
ultra basic compounds, consisting in greatest part of olivine, like
lherzolite, dunite and serpentine. To some of the features and probable
origin of these chemical and mineralogical diversities in the same
great eruptive mass further reference will be made in later pages.
ii. RELATIONS OF THE GABBROS TO THE OTHER MEMBERS OF THE VOLCANIC SERIES
Various opinions have been expressed regarding the connection between
the amorphous eruptive rocks of the hill-groups and the level
basalt-sheets of the plateaux. Jameson, though he landed at Rudh' an
Dunain, in Skye, where this connection can readily be found, does not
seem to have made any attempt to ascertain it. He noticed that the
lower grounds were formed of basalt, and that the mountains "appeared
to be wholly composed of syenite and hornblende rock, traversed
by basalt veins."[336] Macculloch, in many passages of his _Western
Islands_, alludes to the subject as one which he knew would interest
geologists, but about which he felt that he could give no satisfactory
information, and with characteristic verbiage he refers to the
impossibility of determining boundaries, to the transition from one
rock into another, to the inaccessible nature of the ground, to the
almost insuperable obstacles that impede examination, to the distance
from human habitation, and to the stormy climate,--a formidable list of
barriers, in presence of which he leaves the relative position and age
of the rocks unsettled.[337]
[Footnote 336: _Mineralogical Travels_ (1813), vol. ii. p. 72.]
[Footnote 337: See his _Western Islands_, vol. i. pp. 368, 374, 385,
386. With much admiration for the insight and zeal, amounting almost
to genius, which Macculloch displayed in his work among the Western
Islands, at a time when, with poor maps and inadequate means of
locomotion, geological surveying was a more difficult task than it
is now, I have found it impossible to follow in his footsteps with
his descriptions in hand, and not to wish that for his own fame he
had been content to claim credit only for what he had seen. His
actual achievements were enough to make the reputation of half a
dozen good geologists. It was unfortunate that he did not realize how
inexhaustible nature is, how impossible it is for one man to see and
understand every fact even in the little corner of nature which he may
claim to have explored. He seems to have had a morbid fear lest any one
should afterwards discover something he had missed; he writes as if
with the object of dissuading men from travelling over his ground, and
he indeed tacitly lays claim to anything they may ascertain by averring
that those who may follow him "will find a great deal that is not
here described, although little that has not been examined" (p. 373).
Principal Forbes long ago exposed this weak side of Macculloch and his
work (_Edin. New Phil. Journ._ xl. 1846, p. 82).]
Von Oyenhausen and Von Dechen, who wrote so excellent an account of
their visit to Skye, and who traced much of the boundary-line between
the gabbros and the other mountainous eruptive masses ("syenite"), seem
to have made no attempt to work out the connection between the former
and the rest of the volcanic rocks.[338]
[Footnote 338: Karsten's _Archiv_, i. p. 99. They frankly admit that
"the relation of the hypersthene rock to the other trap rocks was not
ascertained."]
J. D. Forbes, in his able sketch of the _Topography and Geology of
the Cuchullin Hills_, was the first to recognize the superposition
of the "hypersthene rock" upon the "common trap rocks"--that is, the
plateau-basalts. He was disposed to consider the "hypersthene mass as
a vast bed, thinning out both ways, and inclined at a moderate angle
towards the S.E."[339]
[Footnote 339: _Edin. New Phil. Journ._ xl. (1846), pp. 85, 86.]
Professor Judd regarded the bosses of basic and acid rocks that rise
out of the bedded basalts as the basal cores of enormously denuded
volcanic cones. He believed the granitoid rocks to have been first
erupted, and that after a long interval the basic masses were forced
through them, partly consolidating underneath and partly appearing at
the surface as the plateau-basalts.[340] That the order of appearance
of the several rocks has been exactly the reverse of this supposed
sequence was fully established by me in the year 1888, and has since
been amply confirmed.[341] Professor Zirkel recognized that the gabbros
are a dependence of the basalts, that they overlie them, and that
on the naked flanks of the mountains they are regularly bedded with
them.[342]
[Footnote 340: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 249.]
[Footnote 341: _Trans. Roy. Soc. Edin._ xxxv. (1888), pp. 122 _et seq._;
_Quart. Journ. Geol. Soc._ vol. 1. (1894), pp. 216, 645; vol. lii.
(1896), p. 384, and Mr. Harker, _ibid._ p. 320.]
[Footnote 342: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871),
pp. 58, 92.]
Up to the time of the publication of my memoir in 1888 no one had
traced out in more detail the actual boundaries of the several rocks on
the ground, so as to obtain evidence of their true relations to each
other as regards structure and age. Some of the numerous impediments
recorded by Macculloch no doubt retarded the investigation. But, as
Forbes so well pointed out, there is really no serious difficulty in
determining the true structural connection of the amorphous rocks
with each other and with the bedded basalts of the plateaux. I have
ascertained them in each of the districts,[343] and have found that there
cannot be the least doubt that the amorphous bosses, both basic and
acid, are younger than the surrounding bedded basalts, and that the
acid protrusions are on the whole younger than the basic, I shall now
proceed to show how these conclusions are established by the evidence
of each of the areas where the several kinds of rock occur.
[Footnote 343: In two of my excursions in Mull, and once in Skye, I was
accompanied by my former colleague Mr. H. M. Cadell, who rendered me
great assistance in mapping those regions.]
iii. DESCRIPTIONS OF THE SEVERAL GABBRO-DISTRICTS
1. _The Gabbro of Skye_
The largest, most picturesque, and to the geologist most important area
of Tertiary gabbro in Britain, is that of Skye (Map. VI.). Though, like
every other portion of the Tertiary volcanic districts, it has suffered
enormous denudation, and has thereby been trenched to the very core,
it reveals, more conspicuously and clearly than can be seen anywhere
else, the relation of the gabbro to the bedded basalts on the one hand,
and to the acid protrusions on the other. Its chief portion is that
which rises into the group of the Cuillin Hills, which for blackness of
hue, ruggedness of surface, jaggedness of crest, and general grimness
of aspect, have certainly no rivals within the limits of the British
Isles (Fig. 331). It has long been known to extend eastwards into
Blath Bheinn (Blaven) and its immediate northern neighbours. There is,
indeed, no break whatever between the rock of the Cuillins and that of
the hills on the east side of Strath na Creitheach. In Strath More the
gabbro is interrupted by the granitoid mass of the Red Hills. Patches
of it, however, occur further to the east, even as far as the Sound of
Scalpa.
[Illustration: Fig. 331.--Scuir na Gillean, Cuillin Hills, shewing the
characteristic craggy forms of the Gabbro. (From a photograph by Mr.
Abraham, Keswick.)]
If we throw out of account the invading granitoid rocks, and look
upon the whole tract within which the gabbro occurs as originally
one connected area, we find that it covered an elliptical space
measuring about nine miles from south-west to north-east and six
miles from north-west to south-east, and embracing at least 40 square
miles.[344] But that its original size was greater is strikingly shown
more particularly on the western margin, which like that of the
basalt-escarpments, has obviously been determined by denudation, for
its separate beds present their truncated ends to the horizon all along
the flanks of the Cuillins, from the head of Glen Brittle round to Loch
Scavaig (Fig. 332), and from Strath na Creitheach round the southern
flanks of Blath Bheinn to Loch Slapin and Strath More.
[Footnote 344: Though this and the other bosses are here spoken of
as consisting of gabbro, it will be understood that this rock only
constitutes the larger portion of their mass, which includes also
dolerites and other more basic compounds, together with involved
portions of the plateau-basalts and masses of agglomerate which
probably mark the position of older vents.]
[Illustration: Fig. 332.--Section across Glen Brittle, to show the
general relations of the Bedded Basalts (_a_) and the Gabbros (_b_).]
The first point to be ascertained in regard to the gabbro and
associated basic rocks of the mountainous tract is their connection in
geological structure and age with the bedded basalts of the plateau.
This initial and fundamental relation, as Forbes long ago said, can
be examined along the whole western and southern flank of the Cuillin
Hills, from the foot of Glen Sligachan round to the mouth of Loch
Scavaig. Even from a distance, the observer, who is favoured with clear
weather, can readily trace the almost level sheets of basalt till they
dip gently under the darker, more massive rock of the hills. Tourists,
who approach Skye by way of Loch Coruisk, have an opportunity, as
the steamer nears the island of Soay, of following with the eye the
basalt-terraces of the promontory of Rudh' an Dunain until they
disappear under the gabbro of the last spur of the Cuillins that guards
the western entrance to Loch Scavaig.
What is so evident at a distance becomes still more striking when
viewed from nearer ground. Nowhere can it be more impressively seen
than at the head of Glen Brittle. Looking westwards, the traveller
sees in front of him only the familiar level terraces and green slopes
of the basalt-plateau, rising platform above platform to a height of
nearly 1500 feet above the sea. But turning to the east, he beholds
the dark, gloomy, cauldron-like Corry na Creiche, from which rise some
of the ruggedest and loftiest crests of the Cuillins. On the hills
that project from either side of this recess and half enclose it,
the bedded basalts mount from the bottom of the valley, with their
lines of parallel terrace dipping gently inward below the black rugged
gabbro that crowns them and sweeps round to form the back or head of
the corry. Down the whole length of Glen Brittle the same structure
conspicuously governs the topographical features. On the right hand,
the ordinary terraced basalts form the slopes; and they rise for some
500 or 600 feet up the eastern side, until they pass under the darker,
more rugged, and less distinctly bedded rocks of the mountains (Fig.
332). The dip of the whole series is here at a gentle angle towards
south-east, that is, into or under the main mass of the Cuillin group.
When, however, we proceed to examine the junction between the two
rocks we find it to be less simple than it appears. It is not an
instance of mere superposition. The gabbro unquestionably overlies
the basalts, and is therefore of younger date. But it overlies them,
not as they rest on each other, in regular conformable sequence of
eruption, but intrusively, as a sill does upon the rocks on which it
appears to follow in the unbroken order of accumulation. This important
structure may be ascertained in almost any of the many sections cut
by the torrents which have so deeply trenched with gullies the flanks
of the hills. Starting from the ordinary bedded basalts, we observe,
in mounting the slopes and approaching the gabbro, that the rocks
insensibly assume that indurated shattery character, which has been
referred to as characteristic of them round the margins of vents,
and which will be shown to be not less so in contact with large
eruptive masses of basic or acid rock.[345] Beds of dolerite make their
appearance among the basalts, so distinctly crystalline, and so similar
in character to the rocks of the sills, that there can be little
hesitation in regarding them as intrusive. These sills increase in size
and number as we ascend, though hardened amygdaloidal basalts may still
be observed. True gabbros then supervene in massive beds, and at last
we find ourselves entirely within the gabbro area, where, however, thin
bands of highly altered basalt may still for some distance appear. One
further fact will generally be noticed, viz. that before reaching the
main mass of gabbro, veins and sills of basalt, as well as of various
felsitic and porphyritic members of the acid group, come in abundantly,
crossing and recrossing each other in the most intricate network. The
base of the thick gabbro-sheets is thus another horizon on which, as on
that below the plateau-basalts, intrusive masses have been especially
developed. Through all these rocks numerous parallel basalt-dykes,
running in a general persistent N.N.W. direction, with a later N.E.
series, rise from below the sea-level up even to the very crests of the
Cuillins (Fig. 333).
[Footnote 345: This indurated, altered character of the bedded basalts
near the intrusive bosses and sills will be more particularly described
in a later chapter in connection with the granophyre intrusions (see p.
386). The metamorphism induced by the basic rocks has generally been
less pronounced than that effected by the acid masses.]
The sections on the western side of the gabbro area of Skye thus prove
that this rock inosculates with the bedded basalts by sending into
them, between their bedding planes, sheets which vary in texture from
fine dolerites at the outside into coarse gabbros further towards the
central mass, and that this intrusion has been accompanied by a certain
amount of induration of the older rocks.
[Illustration: Fig. 333.--View of the crest of the Cuillin Hills,
showing the weathering of the gabbro along its joints, and of a
compound basic dyke which rises through it. (From a photograph by Mr.
Abraham, Keswick.)]
On the eastern side, the same structure can be even more distinctly
seen, for it is not only exposed in gullies and steep declivities, but
can be traced outward into the basalt-plateau. In the promontory of
Strathaird, Jurassic sandstones and shales, which form the coast-line
and lower grounds, are surmounted by the bedded basalts. Denudation
has cut the plateau into two parts. The smaller of these makes the
outlier that rises into Ben Meabost (1128 feet). The larger stretches
continuously from Glen Scaladal and Strathaird House northward into
Blath Bheinn. Hence from the ordinary terraced basalts, with their
amygdaloids, thin tuffs, red partings, and seams of lignite, every
step can be followed into the huge gabbro mountain. Starting from the
black Jurassic shales on which the lowest basalt lies, we walk over
the successive terraces up into the projecting ridge of An da Bheinn.
But as we ascend, sheets of dolerite and gabbro make their appearance
between the basalts, which gradually assume the altered aspect already
noticed. The dip of the whole series is at a low angle northwards, and
the beds can be followed round the head of the Glen nan Leac into the
southern slopes of Blath Bheinn. Seen from the eastern side of this
valley, the bedded character of that mountain is remarkably distinct,
but it becomes less marked towards the upper part of the ridge where
the gabbros preponderate. One of the most striking features of the
locality is the number and persistence of the dykes, which strike
across from the ordinary unaltered basalts of the plateau up into the
highest gabbros of the range. Where less durable than the intractable
gabbro, they have weathered out on the face of the precipices, thereby
causing the vertical rifts and gashes and the deep notches on the crest
that form so marked a feature in the scenery. On the other hand, they
are often less destructible than the plateau-basalts, and hence in the
Glen nan Leac they may be seen projecting as low dams across the stream
which throws itself over them in picturesque waterfalls. The youngest
dykes in the Blath Bheinn group of hills, have been found by Mr. Harker
to have a north-easterly trend, and a north-westerly hade of about 40°,
and to give a stratified appearance to the gabbro when viewed from a
distance.
The deep dark hollow of the Coire Uaigneich has been cut out of the
very core of Blath Bheinn, and lays bare the structure of the east part
of the mountain in the most impressive as well as instructive way (Fig.
334). By ascending into this recess from Loch Slapin, we pass over the
whole series of rocks, and can examine them in an almost continuous
section in the bed of the stream and on the bare rocky slopes on
either side. Sandstones and shales of the Jurassic series extend up
the Allt na Dunaiche for nearly a mile, much veined with basalt and
quartz-porphyry, by which the sandstones are locally indurated into
quartzite. At last these strata are overlapped by the basalts of the
Strathaird plateau, which with a marked inclination to N.N.W., here dip
towards the mountains. But by the time these rocks have reached the
valley, they have already lost their usual brown colour and crumbling
surfaces, and have assumed the indurated splintery character, though
still showing their amygdaloidal structure. They are much traversed
by felsitic veins and strings which proceed from a broad band of
fine-grained hornblende-granite that runs up the bottom of the Coire
Uaigneich and, ascending the col, crosses it south-westwards into the
Glen nan Leac. On the left or south-eastern side of this intrusive
mass, a portion of Lias shales and limestone (here and there altered
into white marble) is traceable for several hundred yards up the
stream.[346]
[Footnote 346: This limestone was formerly identified by me with the
Cambrian strata of the district. It was noticed by Von Oyenhausen and
Von Dechen, who, as Mr. Harker has recently ascertained, correctly
believed it to be a portion of the Lias torn off and carried upward by
the eruptive rocks (Karsten's _Archiv_, i. p. 79).]
The bedded basalts of Strathaird, after dipping down towards the
N.N.W., bend up where they are interbanded with dolerites and gabbros,
and form the prominence called An Stac, which rises as the eastern
boundary of the Coire Uaigneich. Their steep dip away from the mountain
is well seen from the east side, and their outward inclination is
continued into the ridge to the southward. Similar rocks appear on the
other flank of the band of granite, and form the base of Blath Bheinn.
They are likewise continued in the mountains further north called
Sgurr nan Each and Belig, where they dip in a northerly direction
away from Blath Bheinn, which seems to be the centre of uprise, with
the gabbro-sheets dipping away from it. The bedded basalts have been
traced by Mr. Harker up to a height of well over 2000 feet on the Blath
Bheinn range. They are of the usual altered, indurated, and splintery
character. The intrusive sheets interposed between them become thicker
and more abundant higher up, until they constitute the main mass of
the mountain. But that they are in separate sheets, and not in one
amorphous mass, can be recognized by the parallel lines that mark their
boundaries. The junction of the gabbro sills and the lavas is a very
irregular one, portions of the latter rocks being enveloped in the
intrusive sills.
The granite which sends out veins into the surrounding rocks is
obviously the youngest protrusion of the locality, except of course
the basalt-dykes which cross it, and which are nowhere seen in a more
imposing display than round the flanks of Blath Bheinn. A section
across the corry shows the structure represented in Fig. 334.
It is thus demonstrable that when its line of junction with the
surrounding plateau-basalts is traced in some detail, the gabbro is
found to overlie them as a whole, but also to be intercalated with
them in innumerable beds, bands, or veins which rapidly die out as
they recede outwards from the main central mass; that these interposed
beds are intrusive sheets or sills from that mass which have cut off
and enveloped portions of the basalts, and that the contiguous bedded
basalts show more or less marked metamorphism.
We have now to consider the structure of the interior of the gabbro
area of the Cuillin Hills. The first impression of the geologist
who visits that wild district is that the main mass of rock is as
thoroughly amorphous as a core of granite. Yet a little further
examination will reveal to him many varieties of texture, sometimes
graduating into, sometimes sharply marked off from, each other, and
suggesting that the rock is not the product of one single protrusion.
He will notice further indications of successive discharges or
extravasations of crystalline material during probably a protracted
period of time, and in the intricate network of veins crossing each
other and the general body of the rock in every direction, as well as
in the system of basalt-dykes that traverse all the other rocks, he
will recognize the completion of the evidence of repeated renewals of
subterranean energy.
[Illustration: Fig. 334.--Section across the Coire Uaigneich, Skye.
_a_, _b_, Jurassic sandstones and shales; _c_ _c_, bedded basalts
and dolerites; _d_ _d_, gabbros and dolerites with indurated
basalts; _e_, fine-grained hornblende-granite sending veins into
surrounding rocks; _f_ _f_, basalt-dykes running through all the
other rocks.
]
But the observer will be struck with the absence of the more usual
proofs of volcanic activity in such forms as vesicular lavas and
abundant masses of slag, bombs and tuffs, which are commonly associated
with the idea of the centre of a volcanic orifice, though he will meet
with isolated masses of coarse volcanic agglomerate within the gabbro
area and along some parts of its junction with the granophyre. The
general characters of the rocks around him suggest that he stands, as
it were, far beneath that upper part of the earth's crust which is
familiar to us in the phenomena of modern volcanoes; that he has been
admitted into the heart of one of the deeper layers, where he can study
the operations that go on at the very roots of an active vent.
When the geologist begins a more leisurely and systematic examination
of the interior of the gabbro area of Skye he soon sees reason to
modify the impression he may at first have received that this rugged
region presents the characters of one single eruptive mass. The more
he climbs among the hills the more will he meet with evidence of
long-continued and oft-repeated extravasation, one portion having
solidified before another broke through it, and both having been
subsequently disrupted by still later protrusions.
But if by chance he should begin his examination of the ground upon
some of the more typically banded varieties of rock, he may for a time
almost refuse to admit that these can be either of volcanic origin or
of Tertiary age.[347] He will find among them such startling counterparts
of the structure of the ancient Lewisian gneiss of the North-West
of Scotland that he may well be pardoned if for a time he seeks for
evidence that they really do belong to that primeval formation, and
have only been accidentally involved among the Tertiary volcanic rocks.
If, for instance, he should land in Loch Scavaig, and first set foot
upon the gabbros as they appear around Loch Coruisk, he would find
himself upon masses of grey coarsely crystalline, rudely banded rock,
like much of the old gneiss of Sutherland and Ross. Ascending over
the ice-worn domes, he would notice that the banding becomes here and
there more definitely marked by strong differences in texture and
colour, while elsewhere it disappears and is replaced by a granitoid
arrangement of the crystals, which are often as large as walnuts.
[Footnote 347: See _Quart. Journ. Geol. Soc._ vol. l. pp. 217, 657, and a
paper by the author, "Sur la Structure rubannée des plus anciens Gneiss
et des Gabbros Tertiaires," _Compt. rend. Cong. Géol. Internat._ 1894,
p. 139.]
[Illustration: Fig. 335.--Banded and puckered gabbro, Druim an Eidhne,
Glen Sligachan, Skye.]
Nowhere is the gneissoid banding more beautifully developed than on
the east side of the Cuillin group near the head of Glen Sligachan
along the ridge of Druim an Eidhne. It was at this locality that the
four typical structures were observed which have already been referred
to (p. 329). The varieties of colour and composition depend upon the
exceedingly irregular distribution of the component minerals. The
paler bands, rich in felspar, lie parallel with dark brown bands full
of pyroxene, olivine and magnetite, in which, moreover, thin ribs of
glistening black consist in large part of the iron ore. These layers
vary in thickness from mere pasteboard-like laminæ to beds a yard
or more in thickness. Within a space of a few square yards their
parallelism reminds one of stratified deposits (Fig. 336), but traced
over a wider space they are found to be more or less irregular in
thickness and lenticular in form.
[Illustration: Fig. 336.--Banded structure in the Gabbro, from the
ridge of Druim an Eidhne between Loch Coruisk and Glen Sligachan.]
The resemblance to gneisses, and sometimes to the flow-structure of
coarse rhyolites, is still further sustained by occasional undulations
or minute puckerings (Fig. 335). Still more extraordinary are the
examples of the actual plication of a group of successive bands, as
shown in Fig. 337, wherein such a group about ten feet thick is shown
to have been doubly folded between parallel bands above and below. This
structure is not due to any deformation of the gabbro long subsequent
to the consolidation of the mass. It belongs to the phenomena of
protrusion and solidification. An examination of thin slices of
these rocks under the microscope reveals no evidence of crushing. On
the contrary, the minerals of one band interlock with those of the
band adjoining, in such a manner as to prove that the differences of
composition cannot be due to crushing and shearing or to successive
intrusion, but must have been present before the final consolidation of
the whole rock.[348]
[Footnote 348: Mr. J. J. H. Teall and A. G., _Quart. Journ. Geol. Soc._
vol. 1. (1894), p. 652.]
The conclusion which seems most consonant with the facts is that the
magma which supplied the visible masses of gabbro in Skye existed
below in a heterogeneous condition, that portions of it, differing
considerably from each other in composition, were simultaneously
intruded, and that by the deformation of these portions during
their intrusion their present plicated structures were produced. A
careful study of these banded gabbros offers many suggestive points
of comparison with the gneisses and anorthosite (Norian) rocks of
pre-Cambrian age. It seems in the highest degree probable that the
banded structures and peculiar mineral aggregation in these ancient
rocks arose under conditions closely analogous to, if not identical
with, those in which the Tertiary gabbros of Skye originated.[349]
[Footnote 349: Consult the Memoirs cited in the footnote on p. 342.]
Similar structures are found to be widely developed through the gabbros
of the Cuillin Hills. Not only are these rocks disposed in distinct
beds, but many of the beds display the most perfect banding. Thus
the mountains that surround the head of Loch Scavaig and sweep round
Loch Coruisk up to the great splintered crests of Sgurr na Banachdich
display on their bare black crags a distinct bedded structure. On the
east side of Loch Scavaig the rock presents a rudely-banded character,
the bands or beds being piled over each other from the sea-level up
to the summits of the rugged precipices, and dipping into the hill
at angles of 25° to 35°. Abundant dykes and veins of various basic,
intermediate and acid rocks cut this structure. The individual layers
here show sometimes the wavy and puckered condition already referred to.
Even from a distance the alternating lighter and darker bands can
readily be seen, so that this structure, with the variations in
its inclination, can be followed from hill to hill (Fig. 338). The
regularity of the arrangement, however, is often less pronounced on
closer inspection. While the gabbro is rudely disposed in thick beds,
indicative of different intrusive sheets or sills, with which the
banding is generally parallel, considerable irregularities may be
observed in the arrangement of the structure of individual sheets.
These sheets may be parallel to each other, and yet, while in some the
banding is tolerably regular in the direction of the planes of the
sheets, in others it is much twisted or inclined at various angles.
[Illustration: Fig. 337.--Banded and doubly-folded Gabbro, Druim an
Eidhne, 10 feet broad.]
On the west side of the Coruisk river the banding is vertical;
southward from that stream it inclines slightly towards the south, but
soon again becomes vertical, and continues conspicuously so at the
junction of the gabbro with the Torridon sandstones and plateau-basalts
on the west side of Loch Scavaig.
Thus, instead of being one great eruptive boss, the gabbro of this
district is in reality an exceedingly complicated network of sills,
veins and dykes. While the general inclination of the bedding sometimes
continues uniform in direction and amount from one ridge to another,
it is apt to change rapidly, as if the complex assemblage of intruded
masses had been disrupted and had subsided in different directions. For
example, after overlying the bedded basalts of the plateau all the way
from Glen Brittle to the west side of Loch Scavaig, the gabbro descends
abruptly across these basalts and also across the Torridon sandstones,
on which they unconformably rest. These two groups of rocks are not
only truncated by the gabbro, but are traversed by the intricate system
of sills, dykes and veins already referred to. Where it abuts against
the sandstones and basalts in Loch Scavaig, the gabbro is arranged in
vertical bands of different mineral composition and texture. Much of it
is remarkably coarse, some bands displaying pyroxene crystals more than
an inch in length. There is no fine-grained selvage here, indicative
of more rapid cooling. So coarse, indeed, is the rock close up against
the sandstone, that the junction-line can hardly be supposed to be the
normal contact of the intrusive rock. This inference is confirmed by
the existence of a singular kind of breccia between the gabbro and the
sandstones. It is a tumultuous mass of fragments of coarse and fine
gabbro, Torridon sandstone and shale, and plateau-basalts, embedded
in a pale crystalline matrix of fine granular granophyre; veins from
this acid intrusion run off into the gabbro on the one side as well
as into the Torridon sandstones on the other. It would seem that this
junction-line has been one of great movement, that the gabbro-sheets
have subsided against a fault-wall of plateau-basalt and Torridon
sandstone, and that subsequently an intrusion of finely granular
granophyre has come up the fissure, involving in its ascent fragments
of all the materials around.
The rocks for a considerable distance to the south of the gabbro are
intensely altered. The Torridon sandstone has been so indurated as to
pass into a bleached white quartzite, while the shales interstratified
with it have been converted into a kind of porcellanite. But the
most interesting alterations are those to be observed in the
plateau-basalts, which at a height of about 300 feet above the sea,
are to be seen in nearly horizontal sheets that lie immediately on the
upturned edges of the Torridon sandstones. These lavas have suffered
great metamorphism, to which more particular reference will be made in
Chapter xlvi. in connection with the action of the granophyre. Whether
this alteration has been produced by the intrusion of the gabbro or of
some concealed mass of granophyre underneath, of which only projecting
dykes and veins reach the surface, must remain a matter of doubt. On
the whole, as the gabbro is here undoubtedly thrown against the basalts
and Torridon sandstone by a fault, it seems most probable that the
change has been mainly due to the influence of the acid rock.
In the Blath Bheinn group of hills the relations of the gabbro to the
bedded basalts have recently been mapped in detail by Mr. Harker during
the progress of the Geological Survey of Skye. He has observed that,
allowing for irregularities of form, the mass of gabbro obliquely
overlies the basalts as a great sheet, not necessarily due to a
single intrusion, which dips towards the west. He has found the rock
to vary from a coarse gabbro to a diabasic type, and to vary also in
mineralogical constitution, becoming in places very rich in olivine,
though the banded structure is here only exceptionally developed. North
of Garbh Bheinn the gabbro is much crushed and the outlying patch to
the north of Belig is in part a crush-breccia. Mr. Harker remarks that
similar brecciated structures are common among the granophyres of the
Red Hills, and that it is sometimes difficult to distinguish their
structure from that of the true volcanic agglomerates.
[Illustration: Fig. 338.--Sketch of Banded Structure in the Gabbros of
the hills at the head of Loch Scavaig.]
Besides the main area of gabbro in Skye, a great many small detached
bosses, sills and dykes lie further east on the flanks of the Red
Hills. One of the best marked of these detached areas forms a
conspicuous crag on the east side of Strath More, immediately to the
north of Beinn na Cro. It consists of beds of coarse gabbro, with
others of dolerite intercalated in an outlier of the plateau-basalts,
and is traversed by veins from the granophyre of the glen, as well as
by the usual north-west basalt dykes (Fig. 349). It appears to be a
marginal portion of the main gabbro area separated by the intrusion of
the great granitoid boss of the Red Hills. On the north-eastern side
of Beinn na Caillich numerous intrusive sheets of gabbro and dolerite
traverse the quartzite and limestone, and extend down to the sea-margin
in the Sound of Scalpa.
There is an important feature in the main gabbro area of Skye not yet
clearly understood, and which only a minute and patient survey can
elucidate. Though I have found among the Cuillin Hills no distinct
proof that the mass of gabbro ever gave rise to discharges of material,
either lava-form or fragmentary, which reached the surface, the
gabbro area, as already remarked, contains unquestionable evidence
of explosions and the production of pyroclastic masses. Among the
moraine-mounds of Harta Corry, blocks of basalt-agglomerate are strewn
about, full of angular fragments of altered basalt, sometimes highly
amygdaloidal, and also boulders in which lumps of coarse gabbro are
enveloped in a matrix of finer material. I did not find the parent
rocks from which these glacier-borne masses had been derived, but there
can be no doubt that they exist among the gabbro crags that surround
that deep glen. Reference has already been made to the similar rock
found _in situ_ on the opposite side of the Cuillin ridge at the head
of the great cauldron of Corry na Creich; likewise to the mass of
coarse agglomerate which forms a group of knolls and crags on the east
side of Druim an Eidhne above the head of Glen Sligachan. This rock
contains abundant blocks of various slaggy lavas like those of the
basalt-plateau, and runs for some distance along the eastern limit of
the gabbro, between that rock and the granophyre. It is intersected by
numerous basalt-veins. Mr. Harker, as above mentioned, has recently
found some considerable strips of agglomerate which, like that which I
traced round the west side of Beinn Dearg, are interposed between the
gabbro and the bosses of granophyre, or lie at the base of the volcanic
series (p. 284).
There does not, however, appear to be any evidence to connect these
isolated masses of agglomerate with the phenomena attending the uprise
of the gabbro. They seem to be more probably related to the plateau
eruptions, and may be compared with those of Strath, Ardnamurchan and
Mull (pp. 278, 280, 384). That the huge gabbro mass of Skye, besides
invading and altering the bedded basalts, may have communicated
eventually with the surface, and have given rise to superficial
discharges, is not at all improbable, but of any such outflows not
a vestige appears now to remain. We must remember, however, that
the gabbro no doubt in many places found its readiest upward ascent
in vents belonging to the plateau-period, and that portions of the
agglomerates of these earlier vents may be expected to be found
involved in it, as the agglomerate of the great vent of Strath has been
invaded by the granophyre.
CHAPTER XLIV
THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM,
ARDNAMURCHAN, MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF
THE GABBRO INTRUSIONS
2. _The Island of Rum_
The mountains of the island of Rum, rising as they do from a wide
expanse of open sea, present one of the most prominent and picturesque
outlines in the West Highlands (Map VI.). More inaccessible than
most of the other parts of the volcanic region, they have been less
visited by geologists. They were described by Macculloch as composed
of varieties of "augite rock." He noticed in this rock "a tendency
to the same obscurely bedded disposition as is observed in other
rocks of the trap family," and found at one place that it assumed "a
regularly bedded form, being disposed in thin horizontal strata, among
which are interposed equally thin beds of a rock resembling basalt
in its general characters."[350] Professor Judd repeats Macculloch's
observation, that "the great masses of gabbro in Rum often exhibit that
pseudo-stratification so often observed in igneous rocks." He regards
these masses, like those of Skye and Mull, as representing the core
of a volcano from which the superficial discharges have been entirely
removed, and he gives a section of the island in which the gabbro is
represented as an amorphous boss sending veins into a surrounding mass
of granite.[351] In a subsequent paper he gave an excellent detailed
account of the mineralogical composition of some of the remarkably
varied and beautiful basic rocks constituting the hills of Rum, but
added no further information regarding the geological structure of the
island.[352]
[Footnote 350: _Western Islands_, i. p. 486.]
[Footnote 351: _Quart. Journ. Geol. Soc._ xxx. p. 253.]
[Footnote 352: _Op. cit._ xli. (1885) p. 354. See also his paper in vol.
xlii. of the same Journal.]
Even from a distance of eight or ten miles, the hills of Rum are seen
to be obviously built up of successive nearly horizontal tiers of rock.
As the summer tourist is carried past the island, in that wonderful
moving panorama revealed to him by the "swift steamer" of modern days,
these great dark cones remind him of colossal pyramids, and as the
ever-varying lights and shadows reveal more prominently the alternate
nearly level bars of crag and stripes of slope, the resemblance to
architectural forms stamps these hills with an individuality which
strikes his imagination and fixes itself in his memory. If choice or
chance should give him a nearer view of the scene, he would not fail
to notice that it is among the northern hills of the island that the
bedded character is so conspicuous, and that it ceases to be prominent
in the southern heights, though here and there, as in the upper part
of Scuir na Gillean, it may in certain lights be detected even from
a distance. Crossing over from Eigg, he would recognize each of the
features represented in the sketch reproduced in Fig. 339. Along the
shore, red sandstones rise in naked cliffs, from the top of which the
ground slopes upward in brown moors to the bare rocky declivities. A
deep valley (Glen Dibidil) is seen to run into the heart of the hills,
between the bedded group to the north and the structureless group to
the south. If the weather is favourable, some eight or more prominent
parallel bars of rock may be counted on the two higher cones to the
right. These bars are not quite level, but slope gently from right to
left. They remind one of the terraced basalts of the plateaux, but
present a massiveness and a breadth of intervening bare talus-slope
such as are not usual among those rocks.
[Illustration: Fig. 339.--Outline of the Hills of the Island of Rum,
sketched from near the Isle of Eigg.]
Nor is this impression of regularity and bedded arrangement lessened
when we actually climb the slopes of the hills. I had for years been
familiar with the outlines of Rum as seen from a distance, and had
sketched them from every side, but I shall never forget the surprise
and pleasure when my first ascent of the cones revealed to me the
meaning of these parallel tiers of rock. I found it to be the structure
of the Cuillin Hills repeated, but with some minor differences which
are of interest, inasmuch as they enlarge our conceptions of the
process by which the gabbro-bosses were formed.
The northern half of the island of Rum consists almost entirely of red
sandstone, which, as already stated, is a continuation of the same
formation (Torridonian) so well developed in the south-east of Skye,
Applecross and Loch Torridon, and traceable between the Archæan gneiss
and the Cambrian strata up as far as Cape Wrath. The sandstones, though
full of false bedding, show quite distinctly their true stratification,
which is inclined with singular persistence towards W.N.W., at angles
averaging from 15° to 20°. If they are not repeated by folds or faults,
they must reach in this island a thickness of some 10,000 feet. Their
red or rather pinkish tint seems mainly to arise from the pink felspar
so abundant in them, for in many places they really consist of a kind
of arkose. Pebbly bands with rounded pieces of quartz are of common
occurrence throughout the whole formation. Dykes and veins of basalt
are profusely abundant. Sometimes these run with the bedding, and might
at a distance be taken for dark layers among the pink sandstones. They
often also strike obliquely up the face of the cliffs like ribbons.
But, notwithstanding their apparent continuity, there can be no doubt
that these sandstones have suffered from those powerful terrestrial
disturbances which have affected all the older rocks of the North-West
Highlands. On the west side, where they plunge steeply into the sea,
they have undergone a change into fine laminated rocks, which might at
first be mistaken for shales, but which owe their fissility to shearing
movements. Along their southern border, from a point on the east coast
near Bagh-na-h-Uamha, south of Loch Scresort, to the head of Kilmory
Glen, they are abruptly truncated against a group of dark, flaggy and
fissile schists and fine quartzites or grits, which in some places are
black and massive like basalt, and in others are associated with coarse
grey gneiss. That some of these rocks are portions of the Lewisian
series can hardly be doubted, and their structure and relations are
probably repetitions of those between the Lewisian gneiss and Torridon
sandstone of Sleat in Skye. I found also on the northern slopes of Glen
Dibidil a patch of much altered grey and white limestone or marble,
which reminded me of the Cambrian limestone of Skye. The red sandstones
in a more or less altered condition are prolonged to the south-east
promontory of the island.
In passing over the zone of these more ancient rocks, we find them
to present increasing signs of alteration as they are traced up the
slopes towards the great central mass of erupted material. The pink
sandstones gradually lose their characteristic tint, and grow much
harder and more compact, while the veins and dykes of basalt and sheets
of dolerite intersecting them increase in number. The zone of black
compact quartzite, which lies to the south of the sandstones, and which
at one point reminds us of basalt, at another of the flinty slate of
the schistose series, likewise displays increasing induration. Its
bedding, not always to be detected, is often vertical and crumpled.
But the most remarkable point in its structure is the intercalation in
it of bands of breccia. These vary from less than an inch to several
yards in diameter; they run mostly with the bedding, but occasionally
across it. The stones in them are fragments of the surrounding rock
embedded in a matrix of the same material, but also with pieces of a
somewhat coarser grit or quartzite. A band of coarse breccia forms
the southern limit of this zone along the northern base of Barkeval
and Allival. In general character it resembles the thinner seams of
the same material just referred to. The matrix so closely agrees with
the black flinty quartzite, that but for the included stones it could
hardly be distinguished; so greatly has the mass been indurated that
the stones seem to shade off into the rest of the rock. But here and
there its true brecciated nature is conspicuously revealed by prominent
blocks of hardened sandstone. This band of breccia must in some places
be 150 or 200 feet broad. It has no distinct bedding, but seems to
lie as a highly inclined bed dipping into the hill. It may possibly
be a crush-breccia belonging to a period earlier than the volcanic
eruptions. It is at once succeeded by a black flinty felsite like
that of Mull. The groundmass of this rock, so thickly powdered with
magnetite grains as to be almost opaque under the microscope, displays
good flow-structure round the turbid crystals of orthoclase and the
clear granules of quartz. Further up the hill, the rock becomes lighter
in colour and less flinty in texture--a change which is found to arise
from more complete devitrification, the groundmass having become a
crystalline granular aggregate of quartz and felspar with scattered
porphyritic crystals of these minerals (microgranite). In some places,
the felsite incloses fragments of other rocks. A specimen of this
kind, taken from the head of Coire Dubh, shows under the microscope a
brown micro-felsitic groundmass, with crystals of felspar and augite,
inclosing a piece of basalt, composed of fine laths of plagioclase,
abundant magnetite and a smaller proportion of granules of augite.
[Illustration: Fig. 340.--View of Allival, Rum, sketched from the base
of the north-east side of the cone.]
This band of felsite and microgranite may be traced continuously from
Loch Gainmich along the base of Barkeval and Allival, and similar
rocks appear at intervals on the same line round the eastern base of
the hills. Immediately above this belt of felsitic protrusions comes
the great body of gabbro. It will be observed that here, as in Skye,
the base of the gabbro mass presents a horizon on which injections of
acid rocks have been particularly abundant. Whether the breccias be
regarded as the result of earlier rock-crushings, or as due to volcanic
explosions during the Tertiary period, they are evidently older than
the eruption of the gabbros. In that respect they may be compared
with the agglomerates through which the youngest eruptive bosses of
Skye have made their way; but their component materials have been
derived from the surrounding platform of ancient rocks, and not from
subterranean lavas.
[Illustration: Fig. 341.--Section of foliated gabbros in the Tertiary
volcanic series of Allival, Rum.
_a_, massive gabbro with rude lamination parallel to bedding, only seen
in some weathered surfaces; _b_, laminated troctolite; _c_, massive
coarsely crystalline gabbro rudely laminated.]
For my present purpose, however, the chief point of importance is the
structure of the gabbro mass that springs from that platform into
the great conical hills of Rum. The accompanying sketch (Fig. 340)
will convey a better idea of this structure than a mere description.
At the base, immediately above the felsite just referred to, bedded
dolerites make their appearance, much intersected with veins from
the siliceous rock. Veins and dykes of basalt also cut all the rocks
here, the newest being those which run in a north-west direction. The
lowest sheets of dolerite are succeeded by overlying sills of coarser
dolerites, gabbros, troctolites, etc., which are as regular in their
thickness and continuity as the ordinary basalts of the plateaux. The
band of light-coloured troctolite, in particular (Fig. 341), about 20
to 30 feet thick, which has been already referred to for its remarkable
laminar structure, can be followed for some distance along the base
of the hill as a marked projecting escarpment. This rock at once
arrests attention by its platy or fissile structure, parallel to the
bedding-surfaces of the sheet. Indeed hand-specimens of it, as I have
said, might readily pass for pieces of schistose limestone, especially
if taken from the upper part. It consists of successive layers, which
on the weathered surface divide it into beds almost as regular as those
of a flagstone, each bed being further separated into laminæ marked
off by the darker and lighter tints of their mineral constituents. The
darker layers consist of olivine, and the lighter of plagioclase. This
segregation here and there takes the form of rounded masses, where the
minerals are more indefinitely gathered together. The affinity of the
rock with intrusive sheets is further displayed by the occurrence of
abundant nut-like aggregates of pale green olivine. Examined under the
microscope, flow-structure is admirably seen, the lath-shaped felspars
being drawn out parallel to the planes of movement, and giving thereby
the peculiarly schistose structure which is so deceptive.
The massive and coarsely crystalline gabbros below and above this
troctolite are all more or less affected by the same laminar structure.
Some of those in higher parts of the mountain are quite massive in
part, but also include bands of lamination. Banding like that of the
Skye gabbros is generally developed among them, the individual bands
varying from less than an inch to a foot or more in thickness. This
structure, like the lamination, is parallel to the general bedding of
the sheets. As in the Cuillin Hills, the bands differ from each other
in the relative proportions of the constituent minerals, especially
the predominant pyroxene and olivine. The crystals or crystalline
aggregates are often from a quarter of an inch to an inch in diameter,
and in these large forms are crowded together in certain bands.
Magnetite, on the whole, is rather less conspicuous than in the Cuillin
gabbro: at least, it is not so prominently aggregated in special
layers. In one or two instances I have observed curvature of the
banding, but no example so striking as that cited from the Cuillin area
(Fig. 337).
On weathered surfaces, where the felspars decay into a creamy white
and the ferro-magnesian minerals assume tints of green, brown and red,
the resemblance of the rocks to schists is striking. This external
likeness is combined with a tendency to split into thin plates parallel
to the lamination, which still further increases their schistose
appearance. Though less developed than in Skye, the banding appears
to be of the same kind and origin; but in Rum it is combined with the
remarkable lamination above mentioned, produced by the arrangement of
the component minerals with their longer axes parallel to the planes
of bedding, as in flow-structure--a combination which I have not yet
observed in Skye.
The bedded arrangement of the gabbros of Rum, so conspicuous in
the great eastern cones (Figs. 339 and 340), is emphasized by the
fact that some sheets, of a more durable kind, stand out boldly as
prominent ribs, while the softer crumble into a kind of sand, which
forms talus-slopes between the others. Alternations of this nature are
continued up to the very top of the mountains. The beds are nearly
flat, but dip slightly into the interior or towards the south-west.
On the west side of the island also, beyond Loch Sgathaig, a distinct
bedding may be traced, the inclination being here once more inwards
or to the east. But from Glen Harris and the base of Askival this
structure becomes less marked, and gradually disappears. There is thus
a central or southern more amorphous region, while round the margin
towards the north and east the rock appears in frequent alternating
beds.
It is clear that in the broad features of their architecture the hills
of Rum follow closely the plan shown in the Cuillin Hills of Skye. But,
unfortunately, in the former island denudation has gone so far that
no connection can be traced on the ground between the gabbros and the
plateau-basalts. As already stated, the latter rocks have been almost
entirely stripped off from the platform of sandstones and schists
which they undoubtedly at one time covered, and the few outliers of
them that remain lie at some little distance from the margin of the
gabbro area (_ante_, p. 216). Nevertheless, we are not without some
indications of them underneath the gabbros. I have alluded to the
basalts that lie at the base of the eastern cones. As we follow the
bottom of the gabbro southward round the flanks of the hills, dull
compact black shattery basalts, with a white crust, appear from under
the more crystalline sheets. These at once remind one of the altered
basalts of Skye and Mull. On the west side also, beds of basalt emerge
from under the gabbro, but they have been so veined and indurated by
the granophyre of that district, that their relations to the gabbro
are somewhat obscured. If we could restore the lost portions of the
plateau, I believe we should find the gabbros of Rum resting on part of
the volcanic plateau, and some of the gabbro-beds prolonged as sills
between the sheets of basalt.
3. _The Gabbro of Ardnamurchan_
The promontory of Ardnamurchan reveals as clearly as the flanks of
the Cuillin Hills, though in a less imposing way, the relations of
the gabbros to the plateau-basalts (Map VI.). From the southern shore
at Kilchoan to the northern shore at Kilmory, bedded basalts, of the
usual type, amygdaloidal and compact, weathering into brown soil, may
be followed along the eastern slopes of the hills, resting upon the
schists and Jurassic series of western Argyleshire. These rocks are a
continuation of those that cap the ridges further to the south-east and
cross Loch Sunart into Morven. They dip westwards, and followed upwards
in that direction, they soon present the usual marks of alteration.
They weather with a white crust and become indurated and splintery.
Sheets of dolerite with many veins and dykes of basalt run between and
across them. Bands of gabbro make their appearance, and these, as we
advance westwards, increase in number and in coarseness of grain until
this rock, in its rudely bedded form, constitutes practically the whole
of the promontory from Meall nan Con to the light-house. Many admirable
sections may be seen on the coast-cliffs and in the rugged interior,
showing the irregular bedding of the gabbro, and how prone this rock is
to develop its component minerals in bands or ribbons, sometimes made
up of large crystals, as in Skye, Rum and Mull.
4. _The Gabbro of Mull_
In the island of Mull, the conclusions to which the geology of the
other volcanic districts leads us as to the position of the gabbros
in the series of volcanic phenomena, are further confirmed. The first
geologist who appears to have observed the relation of these rocks
in that island was Jameson, who classed them under the old name of
"greenstone," including in the same designation rocks now termed
dolerites and gabbros. He ascended one of the hills above Loch Don,
probably Mainnir nam Fiadh (2483 feet), which he found to consist of
"strata of basalt and greenstone," with some basalt-breccia or tuff and
a capping of basalt. He speaks of the "singular scorified-like aspect"
of the weathered greenstone--a description which applies to some of
the coarser gabbro bands of that locality. But he appears to have
recognized the general bedded arrangement of the rocks up even to the
summit of the hill.[353]
[Footnote 353: _Mineralogy of the Scottish Isles_ i. p. 205.]
It was not, however, until the visit of Professor Zirkel in 1868,
that the true petrographical characters of the gabbro of Mull were
recognized. This observer remarked that the rock is regularly
interstratified with the basalt.[354] Professor Judd, as already stated,
has supposed the gabbros to be the deep-seated portion of the masses
which when poured out at the surface became the plateau-basalts, and he
represents them in his map and sections of Mull as ramifying through
the granitic rocks.[355]
[Footnote 354: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) p. 58.]
[Footnote 355: _Quart. Jour. Geol. Soc._ xxx. (1874).]
In Mull the disposition of the gabbro in beds, sheets or sills is well
displayed, for there is here no great central complicated mass of
interlacing banded and amorphous sheets. We have seen that a higher
group of plateau-basalts has survived in this island better than in the
other plateaux, and it would seem that denudation has not yet succeeded
here in cutting down so deeply into the gabbro core as in Skye, Rum
and Ardnamurchan. Only the upper or outer fringe of intrusive sheets
among the bedded basalts has been laid bare. The district within which
this fringe may be observed is tolerably well-defined by the difference
of contour between the long terraced uplands of the ordinary basalts
and the more conical forms of the southern group of gabbro hills
between Loch na Keal and Loch Spelve. The number and thickness of the
gabbro-sheets increase as we proceed inwards from the basalt-plateau.
These sheets are specially prominent along the higher parts of the
ridge that runs northwards from the northern end of Loch Spelve, and
along the west side of Glen Forsa. But they swell out into the thickest
mass in the south-western part of the hilly ground, where, from above
Craig, in Glenmore, they cross that valley, and form the rugged ridge
that rises into Ben Buy (2354 feet), and stretches eastward to near
Ardara (Map VI.). It is in this southern mass that the Mull gabbro
approaches nearest in general characters to that of Skye. But even here
its true intercalation above a great mass of bedded basalt may readily
be ascertained in any of the numerous ravines and rocky declivities.
One of the best lines of section for exhibiting the relations of
the rocks is the declivity to the west of Ben Buy and Loch Fhuaran.
Ascending from the west side, we walk over successive low escarpments
and terraces of the plateau-basalts with a gentle inclination towards
north-east or east. These rocks weather in the usual way, some into
a brown loam, others into spheroidal exfoliating masses. But as we
advance uphill they gradually assume the peculiar indurated shattery
character already referred to. The soft earthy amygdaloids become dull
splintery rocks, in which the amygdales are no longer sharply defined
from the matrix, but rather seem to shade off into it, sometimes
with a border of interlacing fibres of epidote. The compact basalts
have undergone less change, but they too have become indurated, and
generally assume a white or grey crust, and none of them weather out
into columnar forms. Strings and threads full of epidote run through
much of these altered rocks. Abundant granophyric and felsitic veins
traverse them. Sheets of dolerite likewise make their appearance
between the basalts, followed further up the slope by sheets of gabbro
until the latter form the main body of the hill.
On the north side of the same ridge similar evidence is obtainable,
though somewhat complicated by the injections of granophyric and
felsitic veins and bosses, to which more detailed reference will
afterwards be made. But the altered basalts with their amygdaloidal
bands and their intercalated basalt-tuffs and breccias, can be followed
from the bottom of the glen up to a height of some 1700 feet, above
which the main gabbro mass of Ben Buy sets in. Many minor sheets of
dolerite and gabbro make their appearance along the side of the hill
before the chief overlying body of the rock is reached. Some of these
can be distinctly seen breaking across or ending off between the bedded
basalts which here dip gently into the hill (Fig. 342). A conspicuous
band of coarse basalt-agglomerate, containing blocks of compact and
amygdaloidal basalt a yard or more in diameter, shows by the excessive
induration of its dull-green matrix the general alteration which the
rocks of the basalt-plateau have here undergone. An almost incredible
number of veins of fine basalt, porphyry and felsite has been injected
into these rocks--a structure which is precisely a counterpart of what
occurs under the main body of gabbro in Skye, Ardnamurchan and Rum.
[Illustration:
Fig. 342.--Altered Plateau-Basalts invaded by Gabbro, and with a
Dyke of prismatic Basalt cutting both rocks, north slope of Ben
Buy, Mull.
_a_ _a_, amygdaloidal basalt, much altered; _b_, gabbro; _c_, finely
prismatic basalt.]
The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge
overlying sheet, which probably reaches a thickness of at least 800
feet. It seems to descend rather across the bedding into the hollow of
Glen More, and possibly its main pipe of supply lay in that direction.
Being enormously thicker than any other sheet in the island, it
exhibits the crystalline peculiarities which are so well developed in
the central portions of the larger bosses of gabbro. It presents more
coarsely crystalline varieties than appear in the thinner sheets, some
portions showing crystals of diallage and felspar upwards of an inch in
length. It likewise contains admirable examples of banded structure,
which, as in Skye and elsewhere, is best developed where the texture
becomes especially coarse. Veins or bands, in which the constituent
minerals have crystallized out in more definite and conspicuous forms,
here and there succeed each other so quickly as to impart a bedded or
foliated look to the body of rock, recalling, as in Skye, the aspect
of some coarsely crystalline granitoid gneiss. In these respects the
Mull gabbro closely resembles that of the Cuillin Hills. Occasionally,
on the exposed faces of crags, portions of such bands or veins are
seen to be detached and enveloped in a finer surrounding matrix. The
thick belts or bands of coarser and finer texture alternate, and give
an appearance of bedding to the mass. Nevertheless they are really
intrusive sills, which run generally parallel with beds of finer
gabbro or with sheets of highly indurated basalt, that may be detached
portions of the ordinary rocks of the plateau. The thick sheet of Ben
Buy, like the mass of the Cuillin Hills, is thus the result not of one
but of many uprises of gabbro.
Of the thinner sheets of dolerite and gabbro in Mull little need here
be said. I have referred to their great abundance in the range of
eastern hills that rise from the Sound of Mull between Loch Spelve
and Fishnish Bay. Though obviously intrusive, they lie on the whole
parallel to the bedding of the basalts. The latter rocks exhibit the
usual dull indurated shattery character which they assume where large
bosses of gabbro have invaded them, and which gradually disappears as
we follow them down hill away from the intrusive sheets to the shores
of the Sound. They dip towards the centre of the hill group, that is,
to south-west in the ridge of Mainnir nam Fiadh, Dùn da Ghaoithe, and
Beinn Meadhon, the angle increasing southwards to 15°-20°, and at the
south end reaching as much as 35°-40°. Some fine crags of gabbro and
dolerite form a prominent spur on the east side of the ridge of Ben
Talaidh, in the upper part of Glen Forsa. These consist of successive
sheets bedded with the basalts, and dipping south-west. A large sheet
stands out conspicuously on the north front of Ben More, lying at the
base of the "pale lavas," and immediately above the ordinary basalts.
It circles round the fine corry between Ben More and A'Chioch, some
of its domes being there beautifully ice-worn. This is the highest
platform to which I have satisfactorily traced any of the intrusive
sheets of Mull. Another dyke-like mass emerges from beneath the talus
slopes of A'Chioch, on the southern side, and runs eastward across the
col between the Clachaig Glen and Loch Scridain.
5. _The Gabbros of St. Kilda and North-east Ireland_
Sixty miles to the westward of the Outer Hebrides lies the lonely
group of islets of which St. Kilda is the chief. As the main feature
of geological interest in this group is the relation of the acid
protrusions to the other rocks, the account of the geology will be
more appropriately given as a whole in Chapter xlvii. I need only
remark here that the predominant rocks of these islands are dark basic
masses, chiefly varieties of gabbro, but including also dolerites and
basalts. Reasons will be afterwards brought forward for regarding
these rocks as parts of the Tertiary volcanic series. They present a
close parallel to the gabbros and associated rocks of Skye. But in one
important respect they stand alone. No certain trace remains of any
basalt-plateau at St. Kilda such as those through which the gabbros of
Skye, Mull and Ardnamurchan have been injected. In regard to their mode
of production they have doubtless been intruded at some considerable
depth beneath the surface. But no relic appears to have survived of the
overlying cover of rock under which they consolidated, and into which
they were injected.
In the remarkable volcanic district of the north-east of Ireland a
series of basic rocks appears, which in its mode of occurrence and its
relation to the other members of the series presents many points of
resemblance to the gabbros of the Inner Hebrides. The Irish gabbros are
well developed in the Carlingford district, where they form intrusive
bosses and sheets which have been erupted through the Palæozoic rocks
(Map VII.). They are themselves pierced by later masses of granophyre
and other acid rocks. Further reference will be made to these gabbros
in later pages, where an account will be given of the granite masses of
Mourne, Barnavave and Slieve Gullion.
* * * * *
It is interesting to observe that, while in St. Kilda no relic of any
basaltic plateau has been preserved, in the Faroe Islands, on the
other hand, no sign has been revealed by denudation that the volcanic
plateau of that region is pierced by any eruptive core of gabbro or of
granophyre. During my cruises round these islands and through their
channels, I was ever on the outlook for any difference in topography
that might indicate the presence of some eruptive boss like the gabbro
and granophyre masses of the Inner Hebrides. But nothing of that nature
could be discerned. Everywhere the long level lines of the bedded
basalts were seen mounting up to the crests of the ridges and the tops
of the highest peaks. Though I cannot assert that no intrusions of
gabbro or of granophyre exist among the Faroe Islands, I feel confident
that any such masses which may appear at the surface must be of quite
insignificant dimensions, and do not make the important feature in
geology and topography which they do among the Inner Hebrides. It is,
of course, possible that, vast as the denudation of these islands has
undoubtedly been, it has not yet trenched the plateau deeply enough
to expose any great intrusive bosses and sills which may underlie and
invade the basalts.
iv. HISTORY OF THE GABBRO INTRUSIONS
We are now in a position to draw, from the observations which have been
given in this and the preceding chapter regarding the different areas
of gabbro in the Tertiary volcanic region of Britain, some general
conclusions with respect to the type of geological structure and the
phases of volcanic energy which they illustrate.
1. No evidence exists to show that the masses of gabbro ever
communicated directly with the surface. They never exhibit the
cellular, slaggy and other structures so characteristic of
surface-flows. They are, on the whole, free from included pyroclastic
material, though masses of agglomerate are enclosed in, and have
probably been invaded by, the gabbro of the Cuillin Hills. If the
gabbro-bosses ever were continuous with sheets of rock emitted above
ground, all such upward continuations have been entirely removed. In
any case, we may be quite certain that in an outburst at the surface,
the rock would not have appeared in the form of a coarsely crystalline
or granitoid gabbro.
2. The crystalline structures of the gabbros point unmistakably to
slow cooling and consolidation at some depth beneath the surface. The
most coarsely-crystalline varieties, and those with the best developed
banded structure, occur in the largest bodies of rock, where the
cooling and consolidation would be most prolonged.[356]
[Footnote 356: On this subject, see the papers by Professor Judd already
cited.]
3. The remarkable differences in composition between the dark and pale
layers in the banded gabbros cannot be accounted for by segregation
or successive intrusion, but seem to point to the existence of a
heterogeneous magma from which these distinct varieties of material
were simultaneously intruded.
4. From the prevalence of a bedded structure and the occurrence of
bands and more irregular portions of considerably different texture and
even mineralogical composition which intersect each other, it may be
confidently inferred that even what appears now as one continuous mass
was produced by more than one intrusion.
5. In every case there would necessarily be one or more pipes up which
the igneous material rose. These channels might sometimes be wider
parts of fissures, such as those filled by the dykes. In other places,
they may have been determined by older vents, which had served for the
emission of the plateau-basalts and their pyroclastic accompaniments.
There can be no doubt that some of these vents afforded egress for the
subsequent eruption of granitoid rocks, as will be pointed out in the
following chapters. In the case of the gabbros, however, the position
of the vents seems to have been generally concealed by the tendency
of these rocks to spread out laterally. Denudation has cut deeply
into the gabbro-masses, but apparently not deep enough to isolate any
of the pipes from the larger bodies of material which issued from
them, and thus to leave solitary necks like those in and around the
basalt-plateaux. In Skye, where the central core of gabbro is largest
and most completely encircled, we cannot tell how much of it lies above
the true pipe or pipes, and has spread out on all sides from the centre
of eruption. The prevalence of rude bedding and a banded structure
indicate that most of the visible rock occurs in the form of sills,
successively injected not only into the plateau-basalts, but between
and across each other. Round the margin of the gabbro we undoubtedly
reach horizons below that rock, and see that it lies as a cake or
series of cakes upon the plateau-basalts. The actual pipe or fissure of
supply must in each case lie further inward, away from the margin, and
may be of comparatively small diameter.
6. From the central pipe or group of pipes or fissures which rose
from the platform of older rocks into the thick mass of the
basalt-plateaux, successive sheets of dolerite and gabbro were forced
outward between the layers of basalt. This took place all round the
orifices of supply, on many different horizons, and doubtless at many
different times. In some cases, the intrusive sheets were injected
into the very bottom of the basalts, and even between these rocks
and the older surface on which they rested. This is particularly
the case in Rum, where the gabbro-cones spring almost directly from
the ancient grits, schists and sandstones on which they rest. The
intrusive sheets have likewise found egress at every higher platform
in the basalt-series, up at least to the base of the "pale group" in
Mull--that is, through a continuous pile of more than 2000 feet of
bedded basalt. But the intrusion did not proceed equally all round an
orifice. At all events, the progress of denudation has revealed that on
one side of a gabbro area the injected portions may occur on a lower
stratigraphical level than they do on the opposite side. At the Cuillin
Hills, for example, the visible sheets of dolerite and gabbro to the
north of Coire na Creiche begin about 1600 feet above the sea, which
must be much more than that distance above the bottom of the basalts.
On the south-east side, however, they come down to near the base of the
basalts at Loch Scavaig; that is to say, their lowest members lie at
least 1600 feet below those on the opposite margin.
7. The uprise of so much igneous material in one or more funnels, and
its injection between the beds of plateau-basalt, would necessarily
elevate the surface of the ground immediately above, even if we
believe that surface to have been eventually disrupted and superficial
discharges to have been established. If no disruption took place, then
the ground would probably be upraised into a smooth dome, the older
lavas being bent up over the cone of injected gabbro until the portion
of the plateau so pushed upward had risen some hundreds of feet above
the surrounding country. The amount of elevation, which would of course
be greatest at the centre of the dome, might be far from equable all
round, one side being pushed up further or with a steeper slope than
another side. But even in the case of the Cuillin Hill area, it is
conceivable that the total uplift produced at the surface a gentle
inclination of no more than 8° or 10°.
It is along the periphery of a gabbro area that we may most hopefully
search for traces of this uplift. But unfortunately it is just there
that the work of denudation has been most destructive. There appears
also to have been a general tendency to sagging subsequent to the
gabbro protrusions, and the inward dip thereby produced has probably
been instrumental in effacing at least the more gentle outward
inclinations caused by the uprise of the eruptive rock. In one striking
locality, however, to which I have already referred, the effects
of both movements are, I think, preserved. The basalt-plateau of
Strathaird, which in its southern portion exhibits the ordinary nearly
level bedding, dips in its northern part at an unusually steep angle
to the north-west, towards the gabbro mass of Blath Bheinn. But before
reaching that mountain the basalts, much interbanded with sheets of
dolerite and gabbro, suddenly bend up to form the prominent eminence
of An Stac, where they dip rapidly towards south-east and south (Fig.
334). This steep dip away from the central mass of gabbro, is repeated
in the hills to the north, where the beds are inclined to north-east,
the angle gradually lessening northwards till they are truncated by the
granophyre of Strathmore. The mass of Blath Bheinn thus occupies the
centre of the dome or anticline. The theoretical structure of one of
the gabbro bosses is represented in Fig. 343. It will be understood,
however, that what for the sake of clearness is here represented in one
uniform tint of black in reality consists of an exceedingly complex
network of sheets and dykes differing from each other in texture and
structure, as well as in the relative dates of their intrusion.
8. The injection of so much igneous material among the bedded basalts
has induced in these rocks a certain amount of contact metamorphism.
I have referred to it as showing itself in the field as a marked
induration, the rocks becoming closer grained, dull, splintery, and
weathering, with a grey or white crust, while their amygdales lose
their definite outlines, and epidote and calcite run in strings, veins
and patches through many parts of the rocks. As already remarked,
it is difficult to determine how much of this change should be
referred to the influence of the gabbro, and how much to that of the
numerous intrusions of granophyre which may be apophyses of much
larger bodies of that rock lying not far underneath. On account of
this difficulty, the more detailed description of the metamorphism
of the plateau-basalts is reserved for Chapter xlvi., where it will
find a place in connection with the effects produced by the intruded
granophyres, which have undoubtedly been more extensive than those
effected by the gabbros.
[Illustration: Fig. 343.--Theoretical representation of the structure
of one of the Gabbro Bosses of the Inner Hebrides.
_a_ _a_, platform of older rock on which the bedded basalts (_b_ _b_)
have been poured out; _c_, gabbro.]
The structure and history of the gabbro bosses of the Inner Hebrides
find a close parallel in those of the Henry Mountains of Southern Utah,
so well described by Mr. G. K. Gilbert of the United States Geological
Survey.[357] In that fine group of mountains, rising to an extreme height
of 5000 feet above the surrounding plateau, and 11,000 feet above
the level of the sea, masses of trachyte have been injected between
sedimentary strata belonging to the Jura-Triassic and Cretaceous
systems. These masses, thirty-six in number, have consolidated in
dome-shaped bodies, termed by Mr. Gilbert "laccolites," which have
arched up the overlying strata, sending sheets, veins and dykes into
them, and producing in them the phenomena of contact metamorphism.
There is no proof that any of these protrusions communicated with the
surface, and there is positive evidence that most if not all of them
did not. The progress of denudation has laid bare the inner structure
of this remarkable type of hill, and yet has left records of every
stage in its sculpture. In one place are seen only arching strata,
the process of erosion not having yet cut down through the dome of
stratified rocks into the trachyte that was the cause of their uprise.
In another place, a few dykes pierce the arch; in a third, where a
greater depth has been bared away, a network of dykes and sheets is
revealed; in a fourth, the surface of the underlying "laccolite" is
exposed; in a fifth, the laccolite, long uncovered, has been carved
into picturesque contours by the weather, and its original form is more
or less destroyed.[358]
[Footnote 357: See the remarks and diagram, _ante_, p. 86.]
[Footnote 358: "Geology of the Henry Mountains," by Mr. G. K. Gilbert,
_U.S. Geographical and Geological Survey of the Rocky Mountain Region_,
1877.]
The gabbro "laccolites" of the West of Scotland belong to an older
geological period than those of Utah, and have, therefore, been longer
subject to the processes of denudation. They have been enormously
eroded. The overlying cover of basalt has been stripped off from
them, though from the escarpments beyond them it is not difficult in
imagination to restore it. In Rum it has been so completely removed,
that only a few fragments remain at some distance from the core of
gabbro, which now stands isolated. In Ardnamurchan, and still more
in Skye, the surrounding plateau of basalt remains in contact with
the gabbro bosses. But in Mull, where the plateau-basalts reach now,
and perhaps attained originally a greater thickness than anywhere
else, they have protected the intrusive sheets, which are therefore
less deeply cut away than in any of the other districts, and no great
central core of gabbro has yet been uncovered.
CHAPTER XLV
THE ACID ROCKS
Their Petrography--Their Stratigraphical Position and its
Analogies in Central France
We now come to the examination of another distinct phase of volcanic
action during Tertiary time in Britain. The igneous rocks that have
been under consideration in the foregoing chapters, whether poured
out at the surface or injected below ground, have been chiefly of
basic, partly indeed, like the peridotites, of ultra-basic character.
Some, however, have shown an andesitic or intermediate composition.
Reference has also been made to the probable eruption of acid rhyolites
in the long interval between the outflow of the lower and the upper
basalts in Antrim. But we now encounter a great series, decidedly acid
in composition, in the more largely crystalline members of which the
excess of silica is visible to the eye in the form of free quartz.
While there is a strong contrast in chemical composition between this
series and the rocks hitherto under discussion, there are also marked
differences in structure and mode of occurrence. Like the gabbros,
all the masses of acid rock now visible appear to be intrusive. They
have been injected beneath the surface, and therefore record for us
subterranean rather than superficial manifestations of volcanic action.
The existence of rocks of this class in the midst of the basic
masses has long been recognized. They were noticed by Jameson, who
described the hills between Loch Sligachan and Broadford as composed
of "a compound of felspar and quartz, or what may be called a
granitel, with occasional veins of pitchstone."[359] Macculloch gave a
fuller account of the same region, and classed the rocks as chiefly
"syenite" and "porphyry."[360] In Antrim, also, even in the midst of the
basalt-tableland, masses of "pitchstone-porphyry "pearlstone-porphyry,"
"clay-porphyry," and "greystone" were observed and described.[361] In
more recent years Professor Zirkel has given a brief account of the
so-called "syenite and porphyry" of Mull and Skye,[362] and the late
Professor Von Lasaulx fully described the "trachyte" or rhyolite of
Antrim.[363]
[Footnote 359: _Mineralogical Travels_, ii. 90.]
[Footnote 360: _Western Isles_, see the descriptions of Skye, Mull and
Rum.]
[Footnote 361: Berger, _Trans. Geol Soc._ iii. (1816), p. 190; Portlock,
_Journ. Geol. Soc. Ireland_, vol. i. (1834), p. 9.]
[Footnote 362: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871), pp.
54, 77, 84, 88.]
[Footnote 363: Tschermak's _Min. und Petrog. Mittheilungen_, 1878, p.
412. The chemical composition of this rock and its place among the
rhyolites had already been determined by E. T. Hardman from analysis,
_Journ. Geol. Soc. Ireland_, vol. iii. (1871), p. 32.]
This interesting series of rocks embraces a greater variety of
petrographical characters than any other portion of the British
Tertiary volcanic rocks. On the one hand, it presents thoroughly
vitreous masses, some of which in their colour, lustre and microscopic
structure remind us of recent obsidians. On the other hand, it affords
coarsely crystalline compounds, to which no other name than granite
can be assigned, and which, did we not know their geological position,
might almost be classed with some of the most ancient eruptive rocks.
Between these two extremes abundant gradations may be found, including
beautiful spherulitic rocks, felsites and rhyolites.
In dealing with such a series of intrusive rocks, we again encounter
the difficulty of reaching certainty as to their relative dates of
eruption, since in each case all that can usually be affirmed is that
the intrusive mass is younger than that into which it is injected. It
is quite possible that protrusions of acid rocks occurred at intervals
during the accumulation of the basic masses, as may perhaps be inferred
from the rhyolite-tuffs and conglomerates of Antrim and from the
occurrence of fragments of siliceous lavas in the gravels near the base
of the basalt-plateau of Mull, and in the agglomerates of that island
as well as of other districts.[364] It is probable, therefore, that at
the time when the basalts of the plateaux were emitted, there existed,
within reach of volcanic explosions, masses of granophyric, felsitic
or rhyolitic rocks, fragments from which were shot up the funnels of
discharge. That portions of these rocks were actually intruded into
the basalt-sheets before the building up of the plateaux was completed
appears to be proved in Antrim. Elsewhere, however, no evidence has
yet been obtained of any such intrusion until after the close of the
plateau-period. On the contrary, in every case where the relative
ages of the rocks can be fixed, the acid are younger than the basic
protrusions.
[Footnote 364: Reference may also again be made to the agglomerates of
Strath, Skye, which contain in some parts abundant fragments of acid
rocks that closely resemble some of the masses of granophyre which
disrupt these agglomerates.]
The only known exceptions to this rule are the latest basalt-dykes.
Hence, while amid the large and varied series of acid rocks, which
no doubt represents a wide interval of time, some may belong to
comparatively early epochs in the protracted volcanic period, the
actual available evidence places the emission of these rocks, as a
whole, towards the end of the volcanic history. This evidence I shall
bring, forward in full detail, since it necessitates an abandonment of
what has been the general belief in regard to the relative ages of the
rocks.
i. PETROGRAPHY OF THE ACID ROCKS
The classification of the rocks which best harmonizes the
field-evidence and the detailed study of their mineralogical
composition, is one that arranges these volcanic protrusions into two
series. In the one, the orthoclase is sanidine, and the rocks range
from the most vitreous pitchstone through perlitic and spherulitic
varieties to rhyolite ("quartz-trachyte"). In the other series, which
embraces by far the largest proportion of the whole, the orthoclase is
always turbid, and in this respect as well as in many others the rocks
remind us rather of ancient eruptive masses than of those which have
appeared in Tertiary time. They range from flinty felsitic varieties,
which are obviously devitrified glasses, through different textures of
quartz-porphyry into granophyre, and finally into granite. As I have
been unable to recognize any essential difference of structure and
composition between these acid Tertiary rocks and those of far earlier
geological time, I give them the names which no petrographer would
hesitate to apply to them if they were of Palæozoic age. It has long
appeared to me that these rocks furnish conclusive evidence of the
misleading artificiality of any petrographical nomenclature in which
relative antiquity is made an essential element of discrimination.
_Granite._--That true granites form part of the Tertiary volcanic
series of the British Isles has now been completely established. They
occur as bosses and sills which have been intruded into the gabbros
and all older rocks. They are thus proved not only to belong to the
Tertiary period, but to one of the latest phases of its volcanic
history. But besides these granites, the relative age of which can be
definitely fixed, there occur others which, standing alone and at some
distance from the basaltic plateaux, can only be inferentially classed
in the Tertiary series. To this group belong the granite masses of the
Isle of Arran and the Mourne Mountains in north-eastern Ireland.
Taking first the unquestionably Tertiary granites which occur as
bosses and intrusive sheets, we have to note that the more coarsely
crystalline granophyres are hardly to be distinguished externally from
granite. As the dark ferro-magnesian constituent of these rocks was
generally believed to be hornblende, they were called by the older
petrographers "syenite"; that is, granite with hornblende instead of
mica. The peculiar micropegmatitic groundmass, which constitutes the
distinguishing feature of the granophyres, may occasionally be observed
so reduced in amount as only to appear here and there between the
other minerals, which are grouped in a granitic structure. From this
condition, one step further carries us into a true granite, from which
all trace of the granophyric character has disappeared. Such gradations
may be traced even within short distances in the same boss of rock.
Thus, in the hornblende-biotite-granite boss of Beinn-an-Dubhaich,
Skye, a thoroughly granitic arrangement of the component minerals is
observable in the centre, while a specimen taken from near the edge
on the shore of Camas Malag shows the development of a granophyric
groundmass. But, though the large bosses are usually somewhat coarsely
crystalline in the centre, and tend to assume finer felsitic textures
around their borders, as was observed long ago by Oeynhausen and Von
Dechen,[365] the granitic structure is sometimes exhibited even at the
very edge, and not only so, but in the dykes that protrude from the
bosses into the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at
its margin in Camas Malag, sends a vein into the surrounding limestone,
but though more close-grained than the main body of the rock, this vein
is neither felsitic nor granophyric, but truly granitic in structure.
[Footnote 365: Karsten's _Archiv_, i. p. 89.]
So far as I have observed, the true granites contain a brown mica and
also a little hornblende, both visible to the naked eye, but generally
somewhat decomposed. These rocks are thus hornblende-biotite-granites
(amphibole-granitites of Rosenbusch). They may be defined as
medium-grained aggregates of quartz, orthoclase (also plagioclase),
biotite and hornblende, with sometimes magnetite, apatite, epidote and
zircon. Dr. Hatch found that in some instances (Beinn-an-Dubhaich) the
quartz contains minute inclusions (glass?), bearing immovable bubbles
with strongly-marked contours; while in others (Beinn-na-Chro, Skye)
this mineral is full of liquid inclusions with bubbles, sometimes
vibratile, sometimes fixed. He remarked that the quartz and felspar
have consolidated almost simultaneously, but that in some instances
(Marsco, Glen Sligachan) there are isolated roughly idiomorphic
crystals, of a white, less turbid orthoclase, which belong to a
slightly earlier consolidation than that of the more kaolinized felspar
of the rest of the rock.
The granite of the island of Arran, in the Birth of Clyde, which is
here included in the Tertiary volcanic series, has long been recognized
as consisting of two distinct portions, an eastern or coarse-grained,
and a western or fine-grained variety. The latter sends veins into the
former. These granites contain orthoclase, plagioclase, quartz and dark
mica, the quartz being often idiomorphic with respect to the felspar,
and a tendency towards a micropegmatitic structure being sometimes
observable. A distinguishing characteristic of the Arran granite is
the cavernous or drusy structure which it presents, the cavities being
often lined with well-crystallized orthoclase and smoky quartz.[366] The
granite of the Mourne Mountains in Ireland closely resembles that of
Arran. Its druses, with their beautifully terminated minerals, have
long been well known.
[Footnote 366: See Mr. Teall's _British Petrography_, p. 328.]
_Microgranite._--This term is applied to certain intrusive masses,
which megascopically may be classed with the quartz-porphyries
and felsites, but which microscopically are found to possess a
holocrystalline granitic groundmass of quartz and orthoclase, through
which are scattered porphyritic crystals of the same two minerals,
sometimes also with plagioclase, augite, magnetite or apatite. Rocks of
this type do not appear to be abundant. They occur as dykes and bosses,
but occasionally also as sheets. I have collected them from Skye, Rum
and Ardnamurchan.
_Granophyre._--Under this name may be grouped the large majority of
the acid rocks which play an important part in the geology of the West
of Scotland. They are typically developed in the islands of Mull and
Skye. Generally pale grey or buff in colour, they range in texture
from the true granites, into which, as above stated, they graduate,
to exceedingly close-grained varieties like the felsites of Palæozoic
formations. In the great majority of them the micrographic intergrowth
of quartz and felspar, known as micropegmatite, is their conspicuous
structure, and even constitutes most of their substance. They may
thus be classed generally as granophyres, in the sense in which this
term is employed by Rosenbusch, but without his limitation of it to
pre-Tertiary rocks.
The specific gravity of these rocks has been determined from a series
of specimens by Mr. A. Harker to range from about 2·3 among the
felsites to 2·7 among the granites. No chemical analyses of these rocks
have yet been made, but they have been subjected to microscopical
examination, and their general structure and composition are now known.
The typical granophyre of the Inner Hebrides outwardly closely
resembles an ordinary granite of medium grain, in which the component
dull felspar and clear quartz can be readily distinguished by the naked
eye. Throughout all the varieties of texture there is a strong tendency
to the development of minute irregularly-shaped drusy (miarolitic)
cavities, which here and there give a carious aspect to the rock.
That these cavities, however, are part of the original structure
of the rock, and are not due to mere weathering, is shown by the
well-terminated crystals of quartz and felspar which project into them.
On a small scale, it is the same structure so characteristic of the
granite of the Mourne Mountains and of parts of that of Arran.
Examined under the microscope, a normal specimen of the granophyre of
the Western Isles presents a holocrystalline groundmass, which fills
all the interspaces between the crystals of earlier consolidation.
This groundmass consists of an aggregate of clear quartz and turbid
orthoclase, arranged as micropegmatite, but also in more or less
idiomorphic crystals. In some parts, the two dominant minerals are
grouped in alternate parallel fibres, diverging from the surface of the
enclosed crystals, which are thus more or less completely surrounded by
a radially fibrous mass. The felspathic portion of the micropegmatite
which usually surrounds the orthoclase crystals, when viewed between
crossed Nicols, is found to extinguish simultaneously with the central
crystal.[367] In other parts, the felspar forms a kind of network, the
meshes of which are filled up with quartz. Through the groundmass,
besides the clear quartz and dull orthoclase, some ferro-magnesian
or other additional constituent is generally distributed, but
usually somewhat decomposed. In certain varieties Dr. Hatch found an
abundant brown mica, as in the rock at Camas Malag, Skye. In others,
a pyroxene occurs, which he observed in minute greenish grains,
sometimes completely enclosed in the quartz. In a third variety, the
dark constituent is hornblende, the most remarkable example of which
is one to be seen at Ishriff, in the Glen More of Mull, where the
ferro-magnesian mineral takes the form of long dirty-green needles,
conspicuous on a weathered surface of the rock. A fourth variety is
distinguished by containing plagioclase in addition to or instead of
orthoclase. In the rock of the sheet forming Cnoc Carnach, near Heast,
in Skye, Dr. Hatch observed both orthoclase and plagioclase scattered
through a fine micropegmatitic groundmass, and in a part of the boss at
Ishriff he found the rock to be composed mainly of plagioclase, in a
micropegmatitic groundmass of quartz and felspar, with a few scattered
grains of a pale brown augite and grains of magnetite. A fifth variety
is marked by the prominence of the crystals of quartz and felspar of
earlier consolidation, and by the fineness of grain in the surrounding
micropegmatitic groundmass, whereby a distinct porphyritic structure
is developed. Rocks of this kind are megascopically like ordinary
quartz-porphyries. Still another variety has been detected by Mr. Teall
in the rock of Meall Dearg, at the head of Glen Sligachan, Skye, in
which, besides irregular patches which may represent decayed biotite,
and others which are possibly ilmenite, the rare mineral riebeckite is
present.[368]
[Footnote 367: Mr. Teall, _Quart. Journ. Geol. Soc._ vol. 1. (1894) p.
219. See also his _British Petrography_, p. 327.]
[Footnote 368: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 219.]
_Felsite._--The close-grained rocks into which the ordinary granophyres
frequently graduate may be conveniently grouped under the general name
of Felsite. They differ in no essential feature from the felsites of
the Palæozoic formations. They are more particularly developed, as
might be expected, in those places where the conditions have been most
favourable for rapid cooling, while the more coarsely crystalline
granophyres occur where the material may be supposed to have
consolidated most slowly. Where the acid magma has been injected into
chinks and fissures so as to take the form of veins or dykes, it is
sometimes felsitic, sometimes granophyric, in texture. Along the margin
of large bosses, like those of Mull and Skye, it frequently though
not invariably has assumed a fine texture, with even spherulitic and
flow-structures. But in the centre of large bosses it usually appears
as coarse granophyre or as granite.
The felsites vary in texture from flinty or horny to dull
finely-granular, and in colour from white through shades of grey,
buff and lilac, to black, generally with porphyritic felspars and
blebs of quartz. Where these porphyritic enclosures increase in size
and number, the rocks cannot be distinguished externally from ancient
quartz-porphyries. In general the groundmass of these rocks has been
completely devitrified. But in some dykes enough of the glassy base
remains to show their original vitreous condition. A gradation can thus
be traced from thoroughly glassy pitchstone into completely lithoid
felsites and crystalline granophyres.
A characteristic feature of the felsitic varieties of acid rock is
their flow-structure, which they often display in great perfection.
Sometimes, indeed, this structure has been so strongly developed as to
cause the rock to weather along the planes of flow and to break up into
thin slabs.
Many of these rocks also present admirably developed spherulitic
structures, varying from microscopic minuteness up to large round or
egg-shaped balls nearly two inches in diameter, and often distributed
in lines along those of flow-structure. They likewise exhibit a
frequent development of micropegmatite. No line indeed can be drawn
between these felsites and the granitoid varieties, for the same
characteristic granophyric intergrowth of felspar and quartz runs
through them all.
_Pitchstone._--This name is applied to the glassy varieties apart from
their chemical composition, and specially denotes the possession of
a vitreous structure. Some of the rocks to which it has been applied
are probably glassy varieties of andesite, others are dacites, while
some may be as acid as the most acid felsites and granophyres. The
pitchstones are found in veins or dykes which traverse different
geological formations up to and including the great granophyre bosses
of the Inner Hebrides. They vary in colour from a deep jet-black or
raven-black to a pale bottle-green, and in lustre from an almost
glassy obsidian-like to a dull resinous aspect. Occasionally they
assume a felsitic texture, owing to devitrification, and also a finely
spherulitic structure. Some varieties appear to the naked eye to be
perfectly homogeneous, others become porphyritic by the appearance of
abundant sanidine crystals.
The microscopic structure of the British pitchstones has not yet been
fully worked out. The beautiful feathery microlites of the Arran dykes,
first made known by David Forbes, and subsequently described by Zirkel,
Allport and others, are well known objects to geological collectors.
Dr. Hatch, in whose hands I placed my tolerably large collection of
specimens and their thin slides, furnished me with some preliminary
notes on the slides, from which the following generalized summary is
compiled.
At the one end of the pitchstone group we have a nearly pure glass,
with no microlites, and only a few scattered crystals of sanidine,
quartz, augite or magetite. The glass in thin slices is almost
colourless, but generally inclines to yellow, sometimes to dark-grey.
Some varieties of the rock are crowded with microlites, in others these
bodies are gathered into groups, the glass between which is nearly
free from them. Among the minerals that have been observed in this
microlitic form are sanidine, augite, hornblende (forming the beautiful
green feathery or fern-like aggregates in the Arran pitchstones, Fig.
3) and magnetite. Sometimes the rudimentary forms appear as globulites,
or as belonites, but more commonly as dark trichites. Among the more
definite mineral forms are grains of sanidine, quartz and augite. The
porphyritic crystals are chiefly sanidine, augite and magnetite, but
plagioclase occasionally occurs. The development of spherulites is well
seen in a few of the slides, and occasionally perlitic structure makes
its appearance.
The interesting rhyolitic areas of Antrim include several varieties of
pitchstone. One of these is described by Professor Cole as "a glassy
pyroxene-rhyolite, on the verge of the rhyolitic andesites." Another is
a blue-black porphyritic obsidian.[369]
[Footnote 369: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77.]
_Rhyolite (Quartz-Trachyte)._--This rock has been abundantly erupted
in north-east Ireland, where it rises in occasional bosses among the
plateau-basalts.[370] It is best exposed at the Tardree and Carnearny
Hills, where it has long been quarried. Its petrographical characters
at that locality were described by Von Lasaulx as those of a typical
quartz-trachyte rich in tridymite, and containing large crystals
of glassy sanidine, isolated narrow laths of plagioclase (probably
andesine), grains of smoky-grey quartz, partly bounded by dihexahedral
faces, and a few scattered flakes of a dark-coloured mica. The
groundmass is microgranitic, and under a high power is resolvable
into a confused aggregate of minute microlites of felspar, with
interstitial quartz-granules.[371] More recently a detailed investigation
of the petrography of the Antrim rhyolites has been conducted by
Professor Cole, who has called attention to their remarkable varieties
of structure, ranging from perfect volcanic glass to a thoroughly
lithoidal texture, and exhibiting flow, perlitic and spherulitic
structures.[372]
[Footnote 370: Fragments of acid rock were detected by Prof. Cole in the
gravel among the Ardtun basalt of Mull, as already noticed on p. 212.]
[Footnote 371: Tschermak's _Min. und Pet. Mittheil._ 1878, p. 412.]
[Footnote 372: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77. This paper gives an excellent account of the microscopical
character and mineralogical and chemical compositions of these rocks.]
Intrusive masses of rhyolite are also found in the Carlingford region.
One of these, seen at Forkhill, is a velvet-black almost resinous rock
with abundant quartz and felspar, and sometimes displaying beautiful
flow-structure. It will be more particularly described in Chapter
xlvii. Some of the acid dykes and sills of the Inner Hebrides are
varieties of rhyolite. No undoubted example has yet been observed of a
superficial rhyolite-lava, though such not improbably appeared in the
interval between the lower and upper basalts of Antrim.
ii. STRATIGRAPHICAL POSITION.--ANALOGIES FROM CENTRAL FRANCE
In the history of opinion regarding the relative position of the
Tertiary eruptive rocks, no feature is so remarkable as the universal
acceptance of the misconception regarding the place of the acid
protrusions. In tracing this mistake to its source, we find that
it probably arose from the fact that along their line of junction
the granitoid masses generally underlie the basic. This order of
superposition, which would usually suffice to fix the age of two groups
of stratified rocks, is obviously not of itself enough to settle the
relative epochs of two groups of intrusive rocks. Yet it has been
assumed as adequate for this purpose, and hence what can be proved
to be really the youngest has been placed as the oldest part of the
Tertiary volcanic series.
Macculloch, who showed that his "syenites" and "porphyries" had invaded
the Secondary strata of the Inner Hebrides, and must therefore be of
younger date than these, left their relations to the other igneous
rocks of the region in a curiously indefinite position. He was disposed
to regard them all as merely parts of one great series; and seems to
have thought that they graduate into each other, and that any attempt
to discriminate between them as to relative age is superfluous. Yet
he evidently felt that the contrasts of topography which he described
could hardly fail to raise the question of whether rocks so distinct
in outward form did not differ also in relative antiquity. But he
dismissed the question without answering it, remarking that if there
is any difference of age between the two kinds of rock, "there appears
no great prospect of discovering it."[373] He records an instance of
a vein of "syenite" traversing the "hypersthene rock" in the valley
of Coruisk. "If this vein," he says, "could be traced to the mass of
syenite, it might be held a sufficient ground of judgment, but under
the present circumstances it is incapable of affording any assistance
in solving the difficulty."[374] Instead, however, of being a solitary
instance, it is only one of hundreds of similar intrusions which can be
connected with the general body of granitic and granophyric masses, and
which put the relative ages of the several groups of rock beyond any
further doubt.
[Footnote 373: _Western Islands_, i. p. 368; see also pp. 488, 575, 578.]
[Footnote 374: _Op. cit._ p. 370.]
Boué, who knew the geology of some of the extinct volcanic regions of
Europe, recognized the similarity of the Scottish masses to those of
the Continent, and classed the acid rocks as "trachytes." He saw in
each of the volcanic areas of the West of Scotland a trachytic centre,
and supposed that the more granitoid parts might represent the centres
in the European trachytic masses. He traced in imagination the flow of
the lava-streams from these foci of volcanic activity, distinguishing
them as products of different epochs of eruption, among the last
of which he thought that the trachytic porphyries might have been
discharged. He admitted, however, that his restoration could not be
based on the few available data without recourse to theoretical notions
drawn from the analogy of other regions.[375]
[Footnote 375: _Essai Géologique sur l'Écosse_, pp. 291, 322, 327.]
In the careful exploration of the central region of Skye made by Von
Oeynhausen and Von Dechen, these able observers traced the boundary
between the "syenite" and the "hypersthene rock"; and as they found
the former lying underneath the latter, they seem naturally to have
considered it to be the older protrusion of the two.[376] Principal
Forbes came to a similar conclusion from the fact that he found the
dark gabbro always overlying the light-coloured felspathic masses.[377]
Professor Zirkel also observed the same relative position, and adopted
the same inference as to the relative age of the rocks.[378] Professor
Judd followed these writers in placing the acid rocks before the basic.
He supposed the granitoid masses to form the cores of volcanic piles
probably of Eocene age, through and over which the protrusions of
gabbro and the eruptions of the plateau-basalts took place.[379]
[Footnote 376: Karsten's _Archiv_, i. p. 82. It will be shown in later
pages that the apparent infraposition of the granophyre is often
deceptive, the real junction being vertical.]
[Footnote 377: _Edin. New Phil. Jour._ xl. (1846) p. 84.]
[Footnote 378: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) pp. 90,
95. He says that the gabbro seems to be the younger rock, so far as
their relations to each other can be seen.]
[Footnote 379: _Quart. Jour. Geol. Soc._ xxx. (1874) p. 255.]
The evidence for the posteriority of the acid rocks will be fully
detailed in later pages. Before entering upon its consideration,
however, I would remark that the uprise of the British granophyres
presents so many points of resemblance to that of the trachytes and
phonolites among the basalt-plateaux of Auvergne and the Velay in
Central France, that a brief account of the acid protrusions of these
regions may be suitably given here as an introduction to the account of
those of the Inner Hebrides. A succession of stages in the progress of
denudation allows us to follow the gradual isolation and dissection of
the French volcanic groups. The youngest examples occur in the chain of
cones and craters, in the region of the Puy de Dôme. These may be of
Pleistocene, or even of more recent date. Older and more deeply eroded
than these are the numerous domes and cones in the territory of Haute
Loire. Yet more ancient and still more stupendously denuded come the
bosses, sills and dykes of Britain. Nevertheless, the geologist, by
the methods so admirably devised by Desmarest, may follow the chain of
relationship through these different regions and trace a remarkable
continuity of structure. The younger rocks serve to illustrate the
original condition of the more ancient, while the latter, by their
extensive denudation, permit points of structure to be seen which in
the former are still concealed.
No feature in the interesting volcanic district of Auvergne has
attracted more attention than the trachytic protrusions.[380] Rising
conspicuously along the chain of puys, they claim notice even from
a distance owing to the topographical contrast which their pale
rounded domes offer to the truncated, crater-bearing cones of dark
cinders around them. They consist of masses of a pale variety of
trachyte (domite), which in ground-plan present a circular or somewhat
elliptical outline. They vary in size from the nearly circular dome of
the Grand Sarcoui, which measures about 400 yards in diameter, to the
largest mass of all--that of the Puy de Dôme, which extends for some
1500 yards from north to south with a breadth varying from 500 to 800
yards. They are likewise prominent from their height; in the Puy de
Dôme they form the highest elevation of the whole region (1465 metres),
and even in the less conspicuous hills they rise from 500 to 600 feet
above the surrounding plateau.
[Footnote 380: The admirable Map and Memoirs of Desmarest on Auvergne
are classics in geology. Scrope's work, vol. i. p. 45, gives still
the best published account of this district. See also the work of
Lecoq (_ibid._). The results of more detailed petrographical research
regarding the rocks will be found in the essays of M. Michel Lévy
(_Bull. Soc. Géol. France_, 1890, p. 688) and in the Clermont sheet of
the Geological Survey Map of France (Feuille, 166). A bibliography of
the district up to the year 1890 is given in the volume of the _Bull.
Soc. Géol. France_ just cited, p. 674.]
Five such dome-shaped protrusions of trachyte have made their
appearance among the cinder-cones in a space of about five English
miles in length by about two miles in extreme breadth. Though opinions
have varied as to the mode of formation of these domes, there has been
a general agreement that their present topographic contours cannot be
far from the original outlines assumed by the masses at the time of
their production. The position of the trachyte bosses among the puys
serves to show that they were not deep-seated masses which have been
entirely uncovered by denudation, but were essentially superficial,
and were protruded to the surface at various points along the plateau
in the midst of already existing cinder-cones. In some cases, they have
risen on or near the position of the vents of these cones. Thus the Puy
de Chopine is half encircled by the crater of the Puy de la Goutte,
and the Grand Sarcoui stands in a similar relation to the fragmentary
crater-wall of the Petit Sarcoui.
M. Michel Lévy, in pointing out the superficial character of the
domitic protrusions, has forcibly dwelt on the evidence that these
rocks have undergone a comparatively trifling denudation, and that
they could never have extended much beyond their present limits.[381] As
Scrope pointed out, they were obviously protruded in a pasty condition,
not flowing out in streams like the other lavas of the district, but
consolidating within their chimneys and rising from these in rounded
domes.
[Footnote 381: _Op. cit._ p. 711.]
[Illustration: Fig. 344.--Section through the Puy de la Goutte and Puy
de Chopine.
1, Mica-schist; 2 2, Granite; 3 3, Tuffs; 4, Trachyte; 5, Basalt dyke.]
Undoubtedly denudation, cannot have left them altogether unaffected,
but must have removed some amount of material from their surface.
There is reason to believe that the material so removed may have been
in large part of a fragmental character, and that it was under a
covering of loose pyroclastic debris that the upward termination of the
trachyte column assumed its typical dome-form. Thus in the crater-wall
of the Puy de la Goutte, layers of buff-coloured trachytic tuff dip
gently away from the central domite mass of the Puy de Chopine. That
this material was thrown out from the vent previous to the uprise of
the domite may be inferred from the way in which the latter rock has
obliterated the northern half of the crater. The relations of the rocks
are somewhat obscured by talus and herbage, but when I last visited
the locality in the spring of 1895 the structure seemed to me to be as
expressed in the accompanying diagram (Fig. 344).[382]
[Footnote 382: Compare M. Michel Lévy, _ibid._]
The relative date of the protrusion of the trachytic domes cannot
be very precisely defined. There can, indeed, be no doubt that it
belongs to a late phase of the volcanic history. It came long after the
outpouring of the older basaltic plateaux, of which large fragments
emerge from beyond the limits of the younger lavas on both sides of the
great ridge of the puys, and not only long after that outpouring, but
even after the widespread sheets of basalt had been deeply trenched by
valleys and isolated into outliers capping the hill-tops. Yet there is
good evidence also that the uprise of the comparatively acid trachytes
was not the last volcanic episode of the district. The abundance of
dark slags and fragments of basalt lying on the domite hills shows that
discharges of more basic detritus occurred after these hills had taken
their place in the landscape.
Since the latest eruptions, a gradual alteration of the topographical
features by denudation has been slowly but continuously going on. The
Grand Sarcoui, possibly from having originally had a considerable
covering of fragmentary material, shows least the effects of this
waste. Its remarkably regular form, like that of an inverted cauldron
(the "Chaudron," as it is called in the district), presents, in a
distant view, a smooth grassy surface which slopes steeply down into
the great volcanic plain. But on a nearer examination these declivities
are found to be seamed with trenches which the rain-storms of centuries
have dug out. The covering of loose debris has been largely washed
away, though many fragments of dark slag are still strewn over the
slopes, and the scars are now being cut into the domite below. A more
advanced stage of decay may be seen on the Puy de Dôme, where, from
greater elevation and exposure, the domite is already deeply gashed
by gullies and ravines, while the slopes below are strewn with its
detritus.
The region of the Velay displays on a far more extensive scale the
protrusion of trachytic and phonolitic bosses, but as its volcanic
history goes back beyond the time of the Puys of Auvergne, its
volcanic monuments have consequently been more extensively affected
by denudation.[383] A series of basaltic eruptions forming extensive
sheets can there be traced, the oldest dating from Miocene time, the
youngest coming down to the age of the mammoth, cave-bear and early
man. During this prolonged outpouring of basic lavas there were several
intervals during which materials of a more acid nature--trachytes and
phonolites--were erupted. These rocks occur partly as extensive tracts,
covering five or six square miles, like those of the Mezenc, the Megal,
the Pic de Lizieux, and the Rand, and partly in isolated conical or
dome-shaped prominences, sometimes only a few hundred feet in diameter.
Upwards of one hundred distinct eruptions of phonolite have been
observed in the Velay. Even in the tracts where they cover the largest
space, several prominent eminences may usually be observed, not unlike
in general shape the isolated cones and domes of Auvergne. In these
wider areas there appears to be evidence of the outcome of the lava
from one or more vents, either as superficial streams or as underground
intrusive sheets. M. Boule has expressed his opinion that most of the
masses of trachyte and phonolite have been the result of local and
limited eruptions, the pasty rock having risen in and accumulated
around its pipe, without flowing far in any direction. A section across
one of these masses would present a somewhat mushroom-shaped form.[384]
[Footnote 383: In addition to the work of Scrope, the student of this
important volcanic district will find an invaluable guide in the Le
Puy Sheet (No. 186) of the Geological Survey Map of France, and in the
_Bulletins_ of the Survey, particularly those by MM. Termier and Boule,
No. 13 (1890) and No. 28 (1892).]
[Footnote 384: _Bull. Carte. Géol. France_, No. 28 (tome iv.) p. 125.]
That fragmentary ejections accompanied the protrusion of these rocks,
though probably on a very limited scale, is shown by the occasional
survival of portions of trachyte tuff around them. One of the most
notable of these deposits occurs in the hollow between the Suc du
Pertuis and the next dome to the south. It consists of fine and coarse,
trachytic detritus, which in one place is rudely bedded and appears to
dip away from the phonolite dome behind it at an angle of 30°. This
material and its inclination are what might be expected to occur round
an eruptive vent, and may be compared with those of the crater-wall
of the Puy de la Goutte in relation to the domite boss of the Puy de
Chopine.
The denudation of Velay has undoubtedly advanced considerably further
than that of the Puys of Auvergne. The pyroclastic material which may
have originally covered the domes of trachyte and phonolite has been
in great part swept away. The surrounding rocks, too, both aqueous and
igneous, have been extensively removed from around the necks of more
enduring material. Hence the trachyte and phonolite bosses stand out
with so striking a prominence as to arrest the eye even for a distance
of many miles.
[Illustration: Fig. 345.--View of the Huche Pointue and Huche Platte
west of Le Pertuis.
The cone is one of the trachytic domes, while the flat plateau to the
left is a denuded outlier of the basalt sheets.]
There cannot be any doubt that these necks have pierced the older
basalts, and therefore belong to a later epoch in the volcanic history.
The approximately horizontal sheets of basalt have been deeply eroded
and reduced to mere fragments, and in some instances their existing
portions owe their survival to the protection afforded to them by the
immense protrusions of more acid material. But there is here, as well
as in Auvergne, evidence of the uprise of a later more basic magma, for
sheets of basalt are found overlying some parts of the trachytes and
phonolites.
While the external forms of these Velay necks recall with singular
vividness the features of many more ancient necks in Britain, an
examination of the internal structure of some of them affords some
further interesting points of resemblance. The slabs into which, by
means of weathering along the joints, the rock is apt to split up are
sometimes arranged with a general dip outwards from the centre of the
hill, so that their flat surfaces roughly coincide with the hillslopes.
In other cases the peculiar platy structure, so characteristic of
phonolite, is disposed vertically or dips at a steep angle into the
hill, so that the edges of the slabs are presented to the declivities,
which consequently become more abrupt and rugged.
Though none of the volcanic series in Auvergne or the Velay is so acid
in composition as the more acid members of the Tertiary volcanic series
of Britain, the manner in which the trachytes and phonolites of the
French region make their appearance presents some suggestive analogies
to that of the corresponding rocks in this country. We see that they
were erupted long after the outpouring of extensive basaltic plateaux,
that they belonged to successive epochs of volcanic activity, that they
were protruded in a pasty condition to the surface, where, more or less
covered with fragmentary ejections, they terminated in dome-shaped
hills or spread out to a limited distance around the vents, and
lastly, that they were succeeded by a still later series of more basic
eruptions, which completed the long volcanic history. We shall see in
the following pages how closely the various stages in this complex
record of volcanic activity may be paralleled in the geological records
of Tertiary time in Britain.[385]
[Footnote 385: The phonolite necks of Bohemia, which form so prominent
a feature in the Tertiary geology of that country, might likewise be
cited here in illustration of the acid domes and bosses of the British
Isles.]
CHAPTER XLVI
TYPES OF STRUCTURE IN THE ACID ROCKS--BOSSES
Returning now to the consideration of the acid rocks as these manifest
themselves in the volcanic areas of Britain, I would remark that
three distinct types of structure may be noted among them, viz. (1)
bosses, (2) sills or intrusive sheets, (3) veins and dykes. These
types, as above remarked, belong entirely to the underground operations
of volcanism, for though the rhyolitic fragments in the tuffs and
agglomerates of the plateaux prove that acid lavas existed near the
surface, no undoubted case of superficial lava belonging to the acid
series has yet been observed.[386]
[Footnote 386: The rhyolites of Tardree in Antrim have recently been
claimed by Professor Cole as true lavas grouped round an eruptive vent.
For reasons to be given in the next chapter I regard them as intrusive
masses, though they may not improbably have been connected with streams
of lava now entirely removed.]
The bosses of acid material in the British Tertiary volcanic series
are irregular protrusions, varying in size from knobs only a few
square yards in area up to huge masses many square miles in extent,
and comprising groups of lofty hills. As a rule, their outlines are
markedly irregular. Beneath the surface they plunge down almost
vertically through the rocks which they traverse, but in not a few
instances their boundaries are inclined to the horizon, so that the
contiguous rocks seem to rest against them, and sometimes lie in
outliers on their sides and summits. From the margins of these bosses
apophyses are given off into the surrounding rocks, sometimes only
rarely and at wide intervals, in other places in prodigious numbers.
Sometimes the acid material has been injected in thousands of veins and
minute threads, which completely enclose fragments of the surrounding
rock.
The rock of which the bosses consist is generally granophyric in
texture, passing on the one hand, particularly in the central parts,
into granite, and on the other, and especially towards the margin, into
various more compact felsitic varieties, and sometimes exhibiting along
the outer edge more or less developed spherulitic and flow-structures.
Decided contact metamorphism is traceable round the bosses, but is
by no means uniform even in the same rock, some parts being highly
altered, while others, exposed apparently to the same influences, have
undergone little change. The most marked examples of this metamorphism
are those in which the Cambrian limestone of Skye has been converted
into a pure white saccharoid marble. But the most interesting to the
student of volcanic action are those where the altered rocks are older
parts of the volcanic series. As the bosses of each volcanic area offer
distinctive peculiarities they will here be described geographically.
i. THE ACID BOSSES OF SKYE
It is in the island of Skye that the granophyre and granite bosses
attain their largest dimensions and afford, on the whole, the most
complete evidence of their structures and their relations to the other
parts of the volcanic series (Map VI.). They cover there a total area
of about 25 square miles, and form characteristic groups of hills from
2000 to 2500 feet in height. On the south-east side, three conspicuous
cones (the Red Hills) rise from the valley of Strath (Beinn Dearg
Mhor, Beinn Dearg Bheag and Beinn na Caillich). A solitary graceful
pointed cone (Beinn na Cro) stands between Strathmore and Strathbeg,
while to the north-west a continuous chain of connected cones runs from
Loch Sligachan up into the heart of the Cuillin Hills. Their conical
outlines, their smooth declivities, marked with long diverging lines
of screes, and their pale reddish or reddish-yellow hue, that deepens
after a shower into glowing orange, mark off these hills from all the
surrounding eminences, and form in especial a singular contrast to the
black, spiry, and rugged contours of the gabbro heights to the west of
them.
Besides this large continuous mass, a number of minor bosses are
scattered over the district. Of these the largest forms the ridge of
Beinn an Dubhaich, south of Loch Kilchrist. Several minor protrusions
lie between that ridge and the flank of Beinn Dearg. Others protrude
through the moory ground above Corry; several occur on the side of
the Sound of Scalpa, about Strollamus; and one, already referred to,
lies at the eastern base of Blath Bheinn. In the neighbouring island
of Raasay, a large area of granophyre likewise occurs, which will be
described with the Sills in later pages.
In so extensive a district there is room for considerable diversity
of composition and texture among the rocks. As already stated, in
some places, more particularly in the central parts of the hills,
the acid material assumes the character of a granite, being made up
of a holocrystalline aggregate of quartz, orthoclase, plagioclase,
hornblende and biotite, without granophyric structure, and thus becomes
a hornblende-biotite-granite (quartz-syenite, granite-syenite of
Zirkel, or amphibole-granitite of Rosenbusch). By the development of
the micropegmatitic structure and radiated spherical concretions, it
passes into granophyre. By the appearance of a felsitic groundmass, it
shades off into different varieties of quartz-porphyry or rhyolite,
sometimes with distinct bi-pyramidal crystals of quartz.[387] This
change, which here and there is observable along the edge of a boss,
is sometimes accompanied with an ample development of spherulitic and
flow-structures. As it is convenient to adopt some general term to
express the whole series of varieties, I have used the word granophyre
for this purpose.
[Footnote 387: The best account yet published of these varieties in Skye
is that by Prof. Zirkel, _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii.
(1871) p. 88.]
[Illustration: Fig. 346.--View of Glamich, 2537 feet, Glen Sligachan.
(From a photograph by R. J. A. Berry, M.D., lent by the Scottish
Mountaineering Club).]
That the large area of these rocks in Skye was the result of many
separate protrusions from distinct centres of emission may be inferred,
I think, not only from the varieties of petrographical character in
the material, but also from the peculiar topography of the ground,
and perhaps from the curious relation which seems, in some instances
at least, to be traceable between the external features and apparent
internal structure of the hills. It will be seen from the Map (No. VI.)
that in the area lying to the east of Strath More the granophyre is
broken up into nearly detached portions by intervening patches of older
rocks. There can be little doubt that the mass of Beinn na Caillich
and the two Beinn Deargs is the product of a distinct orifice, if not
of more than one. Beinn na Cro, lying between its two deep bounding
glens, is another protrusion. The western cones stand so closely
together that their screes meet at the bottoms of the intervening
valleys. Yet each group is not improbably the result of emission from
an independent funnel, like the separate domite puys of Auvergne.
But, though I believe this large area of granitoid rock to have
proceeded not from one but from many orifices, I have only here and
there obtained, from the individual hills themselves, indications of an
internal structure suggestive of distinct and successive protrusions
of material from the same vent of discharge. On the outer declivities
of some of the cones we may detect a rudely bedded structure, which
will be subsequently referred to as well displayed in Rum (p. 403).
This structure is specially observable along the east side of Glen
Sligachan. Down the northern slopes of Marsco the granophyre (here in
part a hornblende-biotite-granite) is disposed in massive sheets or
beds that plunge outwards from the centre of the hill at angles of 30°
to 40°. On the southern front of the same graceful cone, as well as on
the flanks of its neighbour, Ruadh Stac, still plainer indications of
a definite arrangement of the mass of the rock in irregular lenticular
beds may be noticed. These beds, folding over the axis of the hill, dip
steeply down as concentric coats of rock. The external resemblance of
the red conical mountains of Skye to the trachyte puys of Auvergne was
long ago remarked by J. D. Forbes,[388] and in this internal arrangement
of their materials, indefinite though it may be, there is a further
resemblance to the onion-like coatings which Von Buch and Scrope
remarked in the structure of the interior of the Grand Sarcoui.[389]
[Footnote 388: _Edin. New Phil. Jour._ xl. p. 78.]
[Footnote 389: Von Buch, _Geognostische Beobachtungen auf Reisen durch
Deutschland und Italien_, vol. ii. (1809) p. 245; Scrope, _Geology
and Extinct Volcanoes of Central France_, 2nd edit. p. 68. Von Buch
regarded the external form of this Puy as having been determined by its
internal structure.]
Where the contour of the cones is regular, and the declivities are not
marked by prominent scars and ribs of rock, this monotony of feature
betokens a corresponding uniformity of petrographical character. But
where, on the other hand, the slopes are diversified by projecting
crags and other varieties of outline, a greater range of texture and
composition in the material of the hills is indicated. This relation
is well brought out on the western front of Marsco, where numerous
alternations of granitoid and felsitic textures occur. On many
declivities also, which at a distance look quite smooth, but which
are really rough with angular blocks detached from the parent mass
underneath, an occasional basalt-dyke will be observed to rise as a
prominent dark rib. A good example of this structure is to be seen on
the south front of Beinn na Caillich. Where a group of dark parallel
dykes runs along the sides of one of these pale cones, it sometimes
produces a curiously deceptive appearance of bedding. A conspicuous
illustration may be noticed on the southern front of Beinn Dearg
Meadhonach, north from Marsco. When I first saw that hillside I could
not realize that the parallel bars were actually dykes until I had
crossed the valley and climbed the slopes of the hill.[390]
[Footnote 390: The difference of contour and colour between the ordinary
reddish smooth-sloped "syenite" and the black craggy "hypersthene rock"
and "greenstone" in the Glamaig group of hills caught the eyes of Von
Oeynhausen and Von Dechen (Karsten's _Archiv_, i. p. 83).]
Good evidence of successive protrusions of the acid rock within the
great area of the Red Hills may be found on the south side of Meall
Dearg at the head of Glen Sligachan, where the granophyre is traversed
by a younger band or dyke of fine-grained spherulitic material
about ten feet broad. The rock exhibits there the same beautiful
flow-structure with rows of spherulites as is to be seen along the
contact of the main granophyre mass with the gabbro on the same hill,
which will be afterwards described. This dyke, vein or band, though
possibly belonging to the same epoch of protrusion as the surrounding
granophyre, must obviously be later than the consolidation of the rock
which it traverses.
Occasionally round the margin of the granophyre a singular brecciated
structure is to be seen. I have found it well marked on weathered
faces, along the flanks of Glamaig and of Marsco, and Mr. Harker has
observed many examples of it on the north side of the granophyre
mass of the Red Hills. When the rock is broken open, it is less easy
to detect the angular and subangular fragments from the surrounding
matrix, which is finely crystalline or felsitic.
The actual junction of the eruptive mass with the surrounding rocks
through which it has ascended is generally a nearly vertical boundary,
but the granophyre sometimes plunges at a greater or less angle under
the rocks that lie against or upon it. On the north side of Glamaig,
for instance, the prophyritic and felsitic margin of the great body
of eruptive rock descends as a steeply inclined wall, against which
the red sandstones and marls at the base of the Secondary formations
are sharply tilted. On the south side of the area a similar steep
face of fine-grained rock forms the edge of the granophyre of the
great southern cones, and plunges down behind Lias limestone and
shale, Cambrian limestone and quartzite, or portions of the Tertiary
volcanic series. Where the granophyre cuts vertically through the
gabbro, the latter rock being more durable is apt to rise above the
more decomposable granophyre as a crag or wall, and thus the deceptive
appearance arises of the basic overlying the acid rock. As above
mentioned, there seems every reason to believe that this peculiarity of
weathering has given rise to or confirmed the mistaken impression that
the granophyre is older than the gabbro.
There can be no doubt, however, that along many parts of the
boundary-line the acid eruptive mass extends underneath the surface
far beyond the actual base of the cones, for projecting knobs as
well as veins and dykes of it rise up among the surrounding rocks.
This is well seen along the northern foot of Beinn na Caillich. But
of all the Skye bosses none exhibits its line of junction with the
surrounding rocks so well and continuously as Beinn an Dubhaich. This
isolated tract of eruptive material lies entirely within the area
of the Cambrian limestone, and its actual contact with that rock,
and with the basalt-dykes that traverse it, can be examined almost
everywhere. The junction is usually vertical or nearly so, sometimes
inclining outwards, sometimes inwards. It is notched and wavy, the
granite sending out projecting spurs or veins, and retiring into little
bays, which are occupied by the limestone. The subdivisions of the
latter rock have recently been traced by Mr. Harker up to one side of
the granite and recognized again on the other side, with no apparent
displacement, as if so much limestone had been punched out to make way
for the uprise of the acid boss. The older dykes, too, are continuous
on either side of the ridge. The granite is massive and jointed,
splitting up into great quadrangular blocks like an ancient granite,
and weathering into rounded boulders. Its granitic composition and
texture are best seen where the mass is broadest, south of Kilbride.
Towards its margin, on the shore of Camas Malag, the granophyric
structure appears, especially in narrow ribbons or veins that run
through the more granitic parts of the rock. These may be compared with
the much larger dyke of spherulitic rock above noticed as traversing
the granophyre of Meall Dearg.
[Illustration: Fig. 347.--Section across the north slope of Beinn an
Dubhaich, Skye.
_a_ _a_, Cambrian limestone; _b_ _b_, basalt dykes; _c_, granite.]
Immediately to the south of Camas Malag the junction with the limestone
is well displayed, and the eruptive rock, which is there granitic
in character, sends out into the limestone a vein or dyke about two
feet broad, of closer grain than the main body of the boss, but still
distinctly granitic in structure. The junction on the north side
is equally well seen below the crofts of Torran. Here the rock of
the boss, for a few yards from its margin, assumes a fine-grained
felsitic aspect, and under the microscope presents a curious brecciated
appearance, suggestive of its having broken up at the margin before
final consolidation. Portions of the already crystallized granite seem
to be involved in a microgranitic base. The rock has here truncated a
number of basalt-dykes which intersect the Cambrian limestone. To one
of these further reference will be made in the sequel.
On the surface of the mass of Beinn an Dubhaich, a few little patches
of limestone occur to the south of Kilchrist Loch. Considering the
nearly vertical wall which the granophyre presents to the adjacent
rock all round its margin, we may perhaps reasonably infer that these
outliers of limestone are remnants of a once continuous limestone sheet
that overlay the eruptive rock, and hence that, with due allowance for
considerable denudation, the present surface of the boss represents
approximately the upper limit to which the granophyre ascended through
the limestone. The actual facts are shown in Fig. 347.
All round the margin of this boss, the limestone has been converted
for a variable distance of a few feet or many yards into a granular
crystalline marble. The lighter portions of the limestone have become
snowy white; but some of the darker carbonaceous beds retain their
dark tint. The nodules of chert, abundant in many of the limestones,
project from the weathered faces of the marble. The dolomitic portions
of the series have likewise undergone alteration into a thoroughly
crystalline-granular or saccharoid rock. The most thorough metamorphism
is exhibited by portions of the limestone which are completely
surrounded by and rest upon the granite. The largest of these overlying
patches was many years ago quarried for white marble above the old
Manse of Kilchrist. I have shown by lithological, stratigraphical and
palæontological evidence that this limestone, instead of belonging
to the Lias, as was formerly believed, forms a part of the Cambrian
or possibly the very lowest Silurian series, being a continuation of
the fossiliferous limestone of western Sutherland and Ross-shire.[391]
Mr. Clough and Mr. Harker, in the progress of the Geological Survey
in Skye, have ascertained that the distinctive characters of the
three groups of strata into which the limestone can be divided may be
recognized even through the midst of the metamorphism.[392]
[Footnote 391: _Quart. Journ. Geol. Soc._ vol. xliv. (1888) p. 62.]
[Footnote 392: _Annual Report of Director-General of the Geological
Survey for 1895._]
The generally vertical line of separation between the rock of Beinn
an Dubhaich and the contiguous limestone has been taken advantage of
for the segregation of mineral veins. On the southern boundary at
Camas Malag, a greenish flinty layer, from less than an inch to two
or three inches in width, consisting of a finely-granular aggregate
of some nearly colourless mineral, which polarizes brilliantly, coats
the wall of the granophyre, and also both sides of the vein which
proceeds from that rock into the limestone. But the most abundant
and interesting deposits are metalliferous. Fragments of a kind of
"gossan" may be noticed all along the boundary-line of the boss, and
among these are pieces of magnetic iron-ore and sulphides of iron and
copper. The magnetite may be seen in place immediately to the south of
Kilbride. A mass of this ore several feet in diameter sends strings
and disseminated particles through the surrounding granophyre, and is
partially coated along its joints with green carbonate of copper.
From the Skye area important evidence is obtainable in regard to the
relation of the acid eruptions to (1) earlier eruptive vents filled
with agglomerate; (2) the bedded basalts of the plateaux; (3) the
bosses, sills and dykes of gabbro and dolerite; and (4) the great
system of basic dykes.
(1) _Relation of the Granophyre to older Eruptive Vents._--The
granophyre of Beinn na Caillich and the two Beinn Deargs has invaded
on its north-eastern side the Cambrian limestone and quartzite, and has
truncated the sheets of intrusive dolerite and gabbro that have there
been injected into them. But to the south-west it rises through the
great Strath agglomerate already described, and continues in that rock
round to the entrance into Strath Beg. The eruptive mass is in great
part surrounded with a ring of agglomerate, as if it had risen up a
huge volcanic chimney and solidified there, though probably there were
more than one vent in this agglomerate area. Again the thick mass of
agglomerate north of Belig is interposed between the bedded lavas and
the great granophyre mass which extends northwards to Loch Sligachan.
On the west side of the Blaven ridge, a number of masses of agglomerate
are found on both sides of Glen Sligachan, along the border of the same
great tract of acid rock.
[Illustration: Fig. 348.--Section from Beinn Dearg to Beinn an
Dubhaich, Skye.
_a_ _a_, Cambrian limestone; _b_ _b_, volcanic agglomerate; _c_ _c_
_c_, basalt-dykes older than granophyre; _d^1_, granophyre of Beinn
Dearg; _d^2_, granophyre in the agglomerate neck; _d^3_, granite of
Beinn an Dubhaich; _e_, basalt-dyke younger than granite.
]
With regard to the relation of the granophyre of the Red Hills to the
great agglomerate of Strath, we may infer that the granophyre has
not risen exactly in the centre of the old funnel, but rather to the
north of it, unless we suppose, as already suggested, that some of
the agglomerate belongs to the cone that gathered round the eruptive
orifice. It is interesting to observe, however, that granophyre, from
the same or from another centre of protrusion, has likewise risen along
the outer or southern margin of the agglomerate, generally between that
rock and the limestone, but sometimes entirely within the agglomerate.
The distance between the nearest part of this ring of eruptive rock
and the edge of the boss of Beinn an Dubhaich is under 400 yards,
the intervening space being occupied by limestone (or marble), much
traversed by north-west basalt-dykes. Most of these dykes do not enter
the rocks of the vent, and are abruptly truncated by the mass of Beinn
an Dubhaich. The probable structure of this locality is shown in Fig
348.
The masses of agglomerate which further westward so curiously follow
the margin of the great granophyre bosses, and those which are
entangled in that rock and in the gabbro, probably indicate, as already
suggested, the position of a group of older volcanic funnels which
provided facilities for the uprise of the basic and acid magmas.
The group of vents which, as we have seen, probably rose out of the
plateau-basalts, and first served for the rise of the masses of gabbro,
has by the subsequent protrusion of the granophyres been still further
destroyed and concealed.
The granophyre intrusions in the great Strath agglomerate have lately
been mapped and described by Mr. Harker. As regards their internal
structure and composition, this observer remarks that compared with the
normal granophyres of the Red Hills and other bosses of the district,
these smaller intrusive masses are darker and manifestly richer in the
iron-bearing minerals, and have a slightly higher specific gravity. But
in their general characters they agree with the other granophyres. The
most interesting feature in them is the evidence they afford that they
have enclosed and partially dissolved fragments of basic rocks. To this
evidence further reference will be made on a later page (see p. 392).
(2) _Relation of the Granophyre to the Bedded Basalts of the Plateaux.
Metamorphism of the Basalts._--On the north-west side, the granophyre
of Glamaig and Glen Sligachan mounts directly out of the bedded
basalts. These latter rocks, which rise into characteristic terraced
slopes on the north side of Loch Sligachan, appear on the south side
immediately to the west of Sconser, and stretch westwards round the
roots of Glamaig into the Coire na Sgairde. As they approach that hill
they assume the usual dull, indurated, splintery, veined character of
their contact metamorphism, and weather with a pale crust. Some of them
are highly amygdaloidal, and between their successive beds thin bands
of basalt-breccia, also much hardened, occasionally appear. Veins of
granophyre become more numerous nearer the main mass of that rock.
The actual line of junction runs into the Coire na Sgairde and slants
up the Druim na Ruaige, ascending to within a few feet of the top of
that ridge. A dark basic rock lies on the granophyre, the latter being
here finer grained and greenish in colour, and projecting up into the
former.[393] There is so much detritus along the sides and floor of
Glen Sligachan that the relations of the two groups of rock cannot be
well examined there. But the basalts, which present their ordinary
characters to the north of the Inn, are observed to become more and
more indurated, close-grained, dull and splintery, as they draw nearer
to the granophyre of Marsco. This part of the district furnishes the
clearest evidence of the posteriority of the great cones of Glamaig and
its neighbours to the plateau-basalts which come up to the very base of
these hills.[394]
[Footnote 393: I think it probable that some of the greenish portions of
the granophyre along this part of the junction-line will be found to
have had their structure and composition altered by having incorporated
into their substance a proportion of the bedded basalts through which
they have been disrupted.]
[Footnote 394: The dykes of granophyre in these basalts are referred to
at p. 444.]
Round the eastern group of cones some interesting fragments of the
once continuous sheet of plateau-basalts remain, and show the same
relation of the acid protrusions on that side. One of these lies on
the granophyre of the flanks of Beinn na Caillich, a little to the
west of the loch at the northern base of that hill. Another of larger
size forms a prominent knob about three-quarters of a mile further
west, and is prolonged into the huge dark excrescence of Creagan Dubha,
which rises in such striking contrast to the smooth red declivities
of the granophyre cones around it. This prominence at its eastern
and northern parts consists of highly indurated splintery basalt in
distinct beds, some of which are strongly amygdaloidal. The bedding is
nearly vertical, but with an inclination inwards to the hill. Towards
the south-west end a thin band of basalt-breccia makes its appearance
between two beds of basalt. Its thickness rapidly increases southward
until it is the only rock adhering to the granophyre. Beyond the foot
of the hill, limestone and quartzite occupy for some distance the
bottom of Strath Beg, much invaded by masses of quartz-porphyry. At the
summit of Creagan Dubha abundant veins run into the basic rocks from
the granophyre, which is here finer grained towards the margin; and
there are likewise veins of quartz-porphyry which, though their actual
connection with the main mass of granophyre cannot be seen, are no
doubt apophyses from it.
This outlier of altered basalt and breccia appears to me to be a
fragment of the plateau-basalts which once overlay the Cambrian and
Jurassic rocks of Strath Beg, and were disrupted by the uprise of the
granophyre. It continues to adhere to the wall of the eruptive mass
that broke up and baked its rocks. Its breccia, passing southward into
a coarse agglomerate, may be a product of the same vent or group of
vents that discharged the great agglomerate mass above Kilbride and
Kilchrist. I have already (p. 282) referred to what appears to be
another outlier of the basalts on the south side of Beinn Dearg.
On the northern and southern flanks of Beinn na Cro, similar evidence
may be observed of the posteriority of the granophyre to the basic
rocks. Round the northern base of the hill a continuous tract of
plateau-basalts, dolerites and gabbros forms the ridge between
Strathmore and Strathbeg. There is an admirable section of the relation
of the two groups of rock on the eastern side of the western glen.
Along the lower part of the declivity, coarsely-crystalline gabbros,
like some of those in the Cuillin Hills, are succeeded by sheets of
dolerite and basalt, the whole forming an ascending succession of beds
to the summit of the ridge. The edges of these beds are obliquely
truncated by the body of granophyre, which slants up the hill across
them and sends veins into them. They are further traversed by basalt
dykes, which here, as almost everywhere, abound (Fig. 349). On the
south side of Beinn na Cro, highly indurated black and grey Lias
shales and sandstones have been tilted up steeply and indurated by the
eruptive rock of the hill; and at one place some 800 feet above the
sea, a little patch of altered basalt, lying on the shale, but close
up against the steep declivity of granophyre, forms a conspicuous
prominence on the otherwise featureless slope.
Reference has already been made to the mass of fine-grained
hornblende-granite which runs for several miles at the base of the
volcanic series on the eastern side of the Blaven group of hills. Mr.
Harker has traced a great development of granophyre on the west side of
these hills, where the acid rock sends apophyses both into the bedded
basalts and into the gabbros.
[Illustration: Fig. 349.--Section at north end of Beinn na Cro, Skye.
_a_, basalt, dolerite and gabbro; _b_, granophyre of Beinn na Cro;
_b′_, dyke of granophyre; _c_ _c_, basalt dykes.]
Combining the results of observations made not only in Skye but in
Mull, Rum and Ardnamurchan, I shall here give a fuller account of the
metamorphism of the basalts, to which frequent allusion has been made
as one of the evidences of the posteriority of the eruptive bosses of
rock round which it occurs.[395] The field-geologist observes that the
basalts, as they are traced towards these bosses, lose their usual
external characters. They no longer weather into spheroidal blocks with
a rich brown loam, but project in much jointed crags, and their hard
rugged surface shows when broken a thin white crust, beneath which the
rock appears black or dark bluish-grey, dull and splintery. They are
generally veined with minute threads or strings of calcite, epidote and
quartz, which form a yellowish-brown network that projects above the
rest of the weathered surface. Where they are amygdaloidal, the kernels
no longer decay away or drop out, leaving the empty smooth-surfaced
cells, but remain as if they graduated into the surrounding rock by
an interlacing of their crystalline constituents. They then look at a
distance more like spots of decoloration, and even when seen close at
hand would hardly at first betray their real nature.
[Footnote 395: Many years ago I was much struck with the evidence of
alteration in the igneous rocks of Mull, and referred to it in several
papers, _Proc. Roy. Soc. Edin._ (1866-67) vol. vi. p. 73; _Quart.
Journ. Geol. Soc._ xxvii. (1871) p. 282, note. The subject was more
fully discussed in my memoir in the _Trans. Roy. Soc. Edin._ vol. xxxv.
(1888) p. 167, from which the account in the text is taken. Prof.
Judd has more recently referred the alteration to solfataric action
(_Quart. Journ. Geol. Soc._ xlvi. 1890, p. 341). As already mentioned,
I have been unable to detect evidence of such action. The alteration
is always intimately connected with the presence of intrusive masses,
and it affects indifferently any part of the basalt-plateaux which may
chance to lie next to these masses. The bedded lavas can be traced step
by step from their usual unaltered condition in the plateaux to their
metamorphosed state next to the eruptive rocks. The nature or degree
of the metamorphism has doubtless somewhat varied with the composition
and structure of the rocks affected, and with the character and mass
of the eruptive material; but it is certainly not confined to the
older parts of the plateaux, nor to any supposed pre-basaltic group of
andesites. I have found no evidence that such a group anywhere preceded
the plateau-basalts. The andesites, so far at least as my observations
go, were erupted at intervals during the plateau period, and alternate
with the true basalts. The greatest accumulation of them lies not below
but above the general body of the basalts, in the "pale group" of Mull.
Nor even if the term "propylite" be adopted for these altered rocks,
can it be applied to any special horizon in the volcanic series. The
alteration of the basic rocks by the granophyre of St. Kilda will be
described in the account of that island in Chapter xlvii.]
From the specimens collected by me among the Inner Hebrides up to the
year 1888, I selected two dozen which seemed to be fairly typical of
these altered rocks, and placed thin slices of them for microscopic
examination in Dr. Hatch's hands. His notes may be condensed into the
following summary. One of the most frequent features in the slides is
the tendency in the component minerals to assume granular forms. In
one specimen from Loch Spelve, Mull, the rock, probably originally
a dolerite, shows only a few isolated recognizable crystals of
plagioclase and augite, the whole of the rest of the rock consisting of
roundish granules embedded in a felspathic matrix. The felspar crystals
are sometimes broken up into a mosaic, though retaining their external
contours. Besides the granules, which are no doubt augite, a few grains
of magnetite are scattered through the rock, aggregated here and there
into little groups. In another specimen, taken from the junction with
the granophyre in Glenmore in the same island, parts of the augite
crystals are converted into granular aggregates associated with large
grains and patches of magnetite. The latter mineral also assumes in
some of the rocks granular and even globular shapes suggestive of
fusion.
The felspars, which in most of the basic rocks are usually remarkably
clear and fresh, show marked kaolinization in some of these altered
masses. Minute dusky scales of kaolin are developed, sometimes also
with the separation of minute grains of quartz. The augite shows
frequent alteration to hornblende, proceeding as usual from the
exterior inward. In some cases only an envelope of uralite appears
round the augite, while in others only a kernel of the original mineral
is left, or the whole crystal has been changed. In many cases the
altered substance appears as minute needles, blades and fibres of
actinolite. Occasionally, besides the green hornblende, shred-like
pieces of a strongly pleochroic brown hornblende make their appearance.
Serpentinous and chloritic substances are not infrequent. Epidote is
sometimes abundant. The titaniferous iron has commonly passed more
or less completely into leucoxene. Here and there a dark mica may be
detected.
Since the year 1888 I have continued the investigation of this subject,
and have especially studied the metamorphism of the bedded basalts
on the western shores of Loch Scavaig, where, as already described,
they are truncated by vertical beds of gabbro, and are traversed by
basalt-dykes and by abundant veins of fine-grained granophyre. The
alteration here effected affords excellent materials for study, as the
very same sheets of basalt can be followed from the normal conditions
outside to the altered state within the influence of the metamorphic
agent. The alternations of amygdaloidal and more compact sheets
can still be recognized, although their enclosed amygdales have in
places been almost effaced. They show the dull, indurated, splintery
character, with the white weathered crust, so distinctive of this type
of contact-metamorphism. They are traversed by numerous sills and veins
of gabbro. As has been already suggested, although no large mass of
granophyre appears here at the surface, the alteration of the basalts
is probably to be attributed not so much to the influence of the
gabbro, as to the abundant acid sills, dykes and veins, for there may
be a considerable body of granophyre underneath the locality, the dykes
and veins being indications of its vicinity.
In the summer of 1895 I examined the locality with much care, and
collected some typical specimens illustrative of the conditions of
metamorphism presented by different varieties of the bedded basalts.
Thin slices cut from these specimens were placed in Mr. Harker's hands
for microscopical examination, and he furnished me with the following
notes regarding them.
"In hand-specimens the bedded basalts from the neighbourhood of the
gabbro of Loch Scavaig [6613-6618] do not appear very different from
the normal basalts of this region. The most conspicuous secondary
mineral is yellowish-green epidote in patches, and especially in the
amygdales.
"The texture of the rocks varies, and the slices show that the
micro-structure also varies, the augite occurring sometimes in small
ophitic plates, sometimes in small rounded granules. The chief
secondary change in the body of the rock is shown by the augite, which
is seen in various stages of conversion to greenish fibrous hornblende.
Some round patches seem also to consist mainly of the latter mineral,
and are probably pseudomorphs after olivine. Here the little fibres are
confusedly matted together, without the parallelism proper to uralite
derived from augite. No fresh olivine has been observed. The felspar
and magnetite of the basalts show little or no sign of metamorphic
processes, unless a rather unusual degree of clearness in the felspar
crystals is to be regarded in that light.
"The contents of the metamorphosed amygdales are not always the same.
Epidote is usually present in some abundance, and in well-shaped
crystals. It has a pale citron tint in the slices, with marked
pleochroism; but a given crystal is not always uniform in its optical
characters. Frequently the interior is pale, and has a quite low
birefringence. This is probably to be regarded as an intergrowth of
zoisite in the epidote, and there are a few distinct crystals of
zoisite seen in some places.
"In the slide which best exhibits these features [6613] the crystals
of epidote are in part enwrapped and enclosed by what are doubtless
zeolitic minerals. At least two of these are to be distinguished. One,
very nearly isotropic, and with a pale-brownish tint, is probably
analcime. Associated with this is a colourless mineral with partial
radiate arrangement and with twin lamellation; the birefringence is
somewhat higher than that of quartz, and the γ-axis of optic elasticity
makes a small angle with the twin-line. These characters agree with
those of epistilbite. In other parts of the same large amygdale, the
epidote crystals are embedded in what seems to be a felspar. This
latter mineral is rather obscure, and twin-lamellation is rarely to
be detected; but it seems highly probable that felspar has here been
developed by metamorphic agency at the expense of zeolites which once
occupied the amygdale. I have observed undoubted examples of this in
metamorphosed basalts from other parts of Skye, _e.g._ from Creagan
Dubha, near the granophyre mass of Beinn Dearg.[396] The felspar occurs
there in the same fashion, and in the same relation to epidote [2700,
2701]. In the specimens now described the chief minerals in the
metamorphosed amygdales are those already named: others occur more
sparingly, associated with them. In some cases there is a grass-green,
strongly pleochroic, actinolitic hornblende, accompanied by a little
iron pyrites [6615].
[Footnote 396: Compare _Trans. Roy. Soc. Edin._ vol. xxxv. p. 166.]
"Epidote and various hornblendic and augitic minerals are
characteristic products in the metamorphism of amygdaloidal basalts
in other regions: felspar with this mode of occurrence I have not
seen except in Skye, where it seems to connect itself naturally with
the abundance of zeolites in the amygdales of the non-metamorphosed
lavas. It is to be observed that in these basalts from Loch Scavaig the
alteration is shown especially in the amygdales, the body of the rock
not being greatly affected: this indicates a not very advanced stage
of metamorphism. The production of uralitic hornblende, rather than
brown mica, from the augite and its decomposition-products, seems to be
characteristic of the metamorphism of basaltic as distinguished from
andesitic rocks, and is well illustrated by a comparison of the two
sets of lavas near the Shap granite."[397]
[Footnote 397: _Quart. Journ. Geol. Soc._ vol. xlix. (1893) p. 361.]
Mr. Harker, who is at present engaged in mapping the central region of
Skye, has had occasion to go over a number of the localities (Creagan
Dubha, etc.) originally cited by me, and, while corroborating my
general conclusions regarding them, has been able to obtain much fresh
evidence regarding the nature and extent of the metamorphism which the
bedded basalts have undergone. The results of his investigations will
be published when the Geological Survey of Skye is further advanced.
(3) _Relation of the Granophyre to the Gabbros._--That the granophyres
invade the gabbros has been incidentally illustrated in the foregoing
pages. But as the mutual relations of the two rocks in the island of
Skye have been the subject of frequent reference in previous writings
of geologists, it is desirable to adduce some detailed evidence from a
region which has been regarded as the typical one for this feature in
the geological structure of the Inner Hebrides. No geological boundary
is more easily traced than that between the pale reddish granophyre and
the dark gabbro. It can be followed with the eye up a whole mountain
side, and can be examined so closely that again and again the observer
can walk or climb for some distance with one foot on each rock. That
there should ever have been any doubt about the relations of the two
eruptive masses is possibly explicable by the very facility with which
their junction can be observed. Their contrasts of form and colour make
their boundary over crag and ridge so clear that geologists do not
seem to have taken the trouble to follow it out in detail. And as the
pale rock undoubtedly often underlies the dark, they have assumed this
infraposition to mark its earlier appearance.
I will only cite one part of the junction line, which is easily
accessible, for it lies in Glen Sligachan immediately to the south of
the mouth of Harta Corry. The rounded eminence of Meall Dearg, which
rises to the south of the two Black Lochs, belongs to the granophyre,
while the rugged ground to the west of it lies in the gabbro. The
actual contact between the two rocks can be followed from the side of
Harta Corry over the ridge and down into Strath na Creitheach, whence
it sweeps northward between the red cone of Ruadh Stac and the black
rugged declivities of Garbh Beinn. There is no more singular scene
in Skye than the lonely tract on the south side of Meall Dearg. The
ground for some way is nearly level, and strewn with red shingle from
the decomposing granophyre underneath. It reminds one of some parts of
the desert "Bad lands" of Western America. Grim dark crags of gabbro,
with veins from the granophyre, rise along its western border, beyond
which tower the black precipices of the Cuillins, while the flaming
reddish-yellow cones of Glen Sligachan stand out against the northern
sky.
Having recently described in some detail the relations of the boss
of granophyre at this interesting locality, I will only here offer a
brief summary of the chief features.[398] The granophyre of Meall Dearg
forms a marginal portion of the great mass of the Red Hills. It has
broken across the banded gabbros, and also cuts an isolated boss of
agglomerate in the ridge of Druim an Eidhne. Its line of junction is
nearly vertical, but along part of its course the wall of gabbro rises
higher than that of the more decomposable granophyre. Hence the origin
of the black crags that crown the red slopes of granophyre debris. Seen
from a distance the basic rock seems to rest as a great bed upon the
acid mass.
[Footnote 398: See _Quart. Journ. Geol. Soc._ vol. 1. (1894) p. 212.]
The younger date and intrusive nature of the granophyre are well shown
by the change in the texture of the mass as it approaches the rocks
against which it has cooled. The ordinary granophyric characters
rapidly pass into a fine-grained felsitic texture, and this change
is accompanied with the development of a remarkably well-defined
flow-structure and of rows of spherulites which run parallel to the
boundary wall. In a ravine on the west side of Meall Dearg, the lines
of flow-structure and rows of large spherulites are seen to be arranged
vertically against the face of gabbro.
Further proof of the later date of the protrusion of the granophyre
is supplied by abundant felsitic dykes and veins which traverse the
gabbro, and some of which can be seen to proceed from the main body of
granophyre. These intrusions will be described in the next chapter, in
connection with the dykes and veins of the acid rocks.
Additional evidence as to the posteriority of the granophyre to the
gabbro has recently been obtained by Mr. Harker from a study of the
internal structure and composition of the masses of these rocks which
have been intruded into the agglomerate above Loch Kilchrist in
Strath. He has found that the granophyre has there caught up from some
subterranean depth portions of gabbro, and has partially dissolved
them, thereby undergoing a modification of its own composition. "The
gabbro-debris," he remarks, "has been for the most part completely
disintegrated by the caustic or solvent action of the acid magma on
some of its minerals. Those constituents which resisted such action
have been set free and now figure as xenocrysts [foreign crystals],
either intact or more or less perfectly transformed into other
substances. At the same time the material absorbed has modified the
composition of the magma, in the general sense of rendering it less
acid." Mr. Harker has traced the fate of each of the minerals of the
gabbro in the process of solution and isolation in the acid magma,
which, where this process has been most developed, is believed by him
to have taken up foreign material amounting to fully one-fourth of its
own bulk, derived not from the rocks immediately around, but from a
gabbro probably at a considerable depth beneath.[399]
[Footnote 399: _Quart. Journ. Geol. Soc._ vol. lii. (1896) p. 320.]
[Illustration:
Fig. 350.--Ground-plan of basic dyke in Cambrian Limestones
truncated by granophyre which encloses large blocks of the dyke,
Torrin, Skye.
]
(4) _Relation of the Granophyre to the Basic Dykes and
Veins._--Reference has already been made to the fact that the "syenite"
bosses of Skye cut off most of the basalt-dykes, but are themselves
traversed by a few others.[400] The locality that furnished me with
the evidence on which this statement was originally made nearly
forty years ago affords in small compass a clearer presentation of
the facts than I have elsewhere met with. The sections described by
me are visible at the eastern end of the boss of Beinn an Dubhaich,
Strath; but similar and even better examples may be cited from the
whole northern and southern margins of that eruptive mass. On the
north side an extraordinary number of dykes may be traced in the
Cambrian limestone from the shores of Loch Slapin eastwards. They
have a general north-westerly trend, but one after another, as I have
already remarked, is abruptly cut off by the granophyre. As an example
of the way in which this truncation takes place, I may site a single
illustration from the northern margin of the eruptive mass, near
Torrin. It might perhaps be contended that the numerous dykes which
traverse the limestone and stop short at the edge of the acid rock,
are not necessarily older than the granophyre, but may actually be
younger, their sudden termination at the edge of the acid boss being
due to their inability to traverse that rock. That this explanation
is untenable is readily proved by such sections as that given in Fig.
350, where a basic dyke (_b_) 9 or 10 feet broad running through the
Cambrian Limestone (_a_ _a_) is abruptly cut off by the edge of the
great granophyre boss. Not only is the dyke sharply truncated, but
numerous pieces of it, from an inch to more than a foot in length,
are enclosed in the granophyre. The latter is well exposed along the
shore of Loch Slapin in an almost continuous section of nearly a mile
in length. The contrast therefore between the development of dykes
within and beyond its area cannot but arrest the attention of the
observer. Though I was on the outlook for dykes in the granophyre, I
found only one. Yet immediately beyond the eruptive boss they at once
appear on either side up to its very edge, where they suddenly cease.
The conclusion cannot be resisted that the protrusion of the acid rock
took place after most of the dykes of the district had been formed, but
before the emission of the very latest dykes, which pursue a north-west
course across the boss (Fig. 348).
[Footnote 400: _Ante_, p. 173, and _Quart. Journ. Geol. Soc._ vol. xiv.
(1857) p. 16.]
Some sections on the southern margin of Beinn an Dubhaich complete
the demonstration that such has been the order of appearance of the
rocks. Near the head of the Allt Lèth Slighe (or Half-way Burn), where
the granite has pushed a long tongue into the limestone, a north-west
basalt-dyke is abruptly cut off by the main body of the boss and by the
protruded vein (Fig. 351). Besides this truncation, the acid rock sends
out strings and threads of its own substance into and across the dyke,
these injected portions being as usual of an exceedingly fine felsitic
texture.
[Illustration: Fig. 351.--Section on south side of Beinn an Dubhaich,
Skye, showing the truncation of a basalt-dyke (_b_), in Cambrian
Limestone (_a_), by the granite (_c_) of Beinn an Dubhaich, Skye.]
Similar evidence may be gathered from the area of the great granophyre
cones further north. The profusion of basalt-dykes in the surrounding
rocks stops short at the margin of that area. The comparatively few
dykes which cross the boundary pursue a general north-west course
through the granophyre, and, as already remarked, from their dark
colour, greater durability and straightness of direction, stand out as
prominent ribs on the flanks of the pale cones which they traverse.
CHAPTER XLVII
THE ACID BOSSES OF MULL, SMALL ISLES, ST. KILDA, ARRAN AND
THE NORTH-EAST OF IRELAND
ii. THE ACID BOSSES OF MULL
Though of comparatively small extent, the granophyre bosses of the
island of Mull afford to the geologist a large amount of instruction in
regard to the relations of the different members of the volcanic series
to each other. Especially important is the evidence which they contain
of the connection between the acid and basic groups of rocks. They have
been laid bare in many natural sections, some of which, forming entire
hillsides, are among the most astonishing in the whole wonderful series
which, dissected by denudation, reveal to us the structure of these
volcanic regions. They lie in two chief areas. One of these extends
along the northern flanks of the mountainous tract from the western
side of Beinn Fhada across Loch Ba' to the west side of Glen Forsa. The
other occupies for over three miles the bottom of Glen More, the deep
valley which, skirting the southern side of the chief group of hills,
connects the east side of the island by road with the head of the great
western inlet of Loch Scridain. There are other minor areas. One of
these extends for about a mile along the declivities to the south of
Salen, across the valley of the Allt na Searmoin; another occurs at
Salen; a third runs along the shore at Craignure. In the interior also,
many isolated areas of similar rocks, besides thousands of veins, occur
in the central group of hills and valleys which form the basins of the
Glencannel and Forsa rivers (Map VI.).
The chief northern boss, which for the sake of convenience of reference
may be called that of Loch Ba', has a length of nearly six miles,
with a breadth varying from a quarter of a mile to about a mile and a
quarter. It descends to within 50 feet of the sea-level, and is exposed
along the crest of Beinn Fhada at a height of more than 1800 feet.
It chiefly consists of a grey crystalline rock which might readily
be identified as a granite, but which when examined microscopically
is found to possess the granophyric structure. With this distinctly
granular-crystalline rock are associated various porphyritic and
felsitic masses, which pass into it, and are more specially observable
along its border. An exceedingly compact black quartz-felsite or
rhyolite forms its southern boundary, runs as a broad dyke-like ridge
from the head of the Scarrisdale Water north-eastward across Loch
Ba' (Fig. 352), and spreads out eastward into a mass more than a
mile broad on the heights above Kilbeg in Glen Forsa. The sharp line
of demarcation of this felsite, and its mass and extent, point to a
different period of extravasation.
[Illustration: Fig. 352.--View of the hills on the south side of the
head of Loch na Keal, showing the junction of the granophyre and the
bedded basalts.
One bird, the bedded basalts of the Gribon plateau; two birds,
the bedded dolerites and basalts of Beinn a' Chraig adhering to
the northern slope and capping the hill; three birds, summit of
Ben More, with A'Chioch to the left and the top of Beinn Fhada
appearing in the middle distance between them; four birds, the
granophyre slopes of Beinn a' Chraig with the great dyke-like mass
of felsite on the left.
]
The geologist, who approaches this district from the north-east, has
his attention arrested, even at a distance of several miles, by the
contrast between the outer and inner parts of the hills that lie to the
south-west of Loch Ba'. He can readily trace from afar the dark bedded
basic rocks rising terrace above terrace, from the shores of Loch na
Keal, to form the seaward faces of the hills along the southern side
of that fjord. But he observes that immediately behind these terraces
the mass of the rising ground obviously consists of some amorphous
rock, which weathers into white debris. Nothing can be sharper than
the contrast of colour and form between the two parts of the hills.
The bedded plateau-rocks lie as a kind of wall or veneer against a
steep face of the structureless interior (Fig. 352). Seen from the
other or hilly side, the contrast is perhaps even more striking.
But the astonishment with which it is beheld at a distance becomes
intensified when one climbs the slopes, and finds that the sheets of
dolerite and basalt (which from some points of view look quite level,
yet dip towards the north-east at a gentle angle) are immediately
behind the declivity abruptly truncated by a mass of granophyre. Of
all the junction-lines between the acid bosses and the lavas of the
plateaux, those exposed on these Mull hillsides are certainly the
most extraordinary. So little disturbed are the lavas, that one's
first impulse is to search for pebbles of the granophyre between the
basalts, for it seems incredible that the inner rock should be anything
but a central core of older eruptive material, against and round which
the younger basic rocks have flowed. But, though the granophyre is so
decomposing and covers its slopes with such "screes" of debris, that
had the basalts been poured round it, they must infallibly have had
some of its fragments washed down between their successive flows, not a
single pebble of it is there to be found. This might not be considered
decisive evidence, but it is extended and confirmed by the fact that
the acid rock gives off veins which ramify through the basalts.
Before examining the actual contact of the two rocks, however, the
geologist will not fail to observe here an admirable example of the
gradual change which was described in the foregoing chapter as coming
over the bedded basalts near the acid bosses. As he approaches the
nucleus of white rock, the basalts assume the usual hard indurated
character, not decaying into brown sand as on the plateaux, but often
standing out as massive crags with vertical clean-cut joint-faces.
This metamorphosed condition extends in some cases to a considerable
distance from the main body of acid rock, especially where knobs of
that material, protruding through the more basic lavas, show that it
must extend in some mass underneath. Thus along the shore at Saline
the bedded basalts succeed each other in well-defined sheets, some
being solid, massive and non-amygdaloidal, others quite vesicular, and
recalling the black scoriform surfaces of recent Vesuvian lavas; yet
they are all more indurated than in the normal plateau-country, and
they break with a hard splintery fracture. Immense numbers of dykes
cut these rocks, and they are likewise pierced by occasional felsitic
intrusions.
If we cross to the other side of the island and trace the bedded
basalts away from the central masses of acid rock we meet with so
gradual a diminution of the induration that no definite boundary-line
for the metamorphism can be drawn. As we recede from the centre of
alteration, the rocks insensibly begin to show brown weathered crusts,
with spheroidal exfoliation, the reticulations of epidote and calcite
become much less abundant, the amygdaloids gradually assume their
normal earthy character, and eventually we find ourselves on the
familiar types of the plateau. This transition is well seen along the
shores of Loch na Keal.[401]
[Footnote 401: Some of the thick massive sheets of basic rock along the
south side of this inlet may possibly be altered sills.]
These proofs of the alteration of the plateau-basalts are accompanied
in Mull as in Skye by further abundant evidence that the acid rocks are
of younger date than the basic. In particular, dykes and veins may be
traced proceeding from the former and intersecting the latter. Thus, in
the bed of the south fork of the Scarrisdale stream, a separate mass
of granophyre (which under the microscope exhibits in perfection the
characteristic structure of this rock) protrudes through the basalts
in advance of the main mass, and a little higher up on the outskirts
of that mass narrow ribbons of the granophyre run through the basic
rocks. The contrast of colour between the pale veins of the intrusive
rock and the dark tint of the basalts is well shown in the channel of
the water. Similar sections may be seen on the flanks of Beinn Fhada,
especially in the great corry north of Ben More, where the granophyre
sends a tongue of finer grain between the beds of basalt. On the east
side of Loch Ba' numerous proofs of similar intrusion may be observed.
Thus at the east end of Loch na Dàiridh, where the granophyre has been
intruded into the basalts, hand-specimens may be obtained showing the
two rocks welded together. On the slopes of Cruach Tòrr an Lochain,
where the granophyre has a felsitic selvage, the bedded basalts are
traversed by veins of the latter material (Fig. 353). A little further
east, at the head of the Allt na Searmoin, the bedded basalts, some of
which are separated by slaggy scoriaceous surfaces, are intersected by
another protrusion from the compact felsitic porphyry (Fig. 354).[402] A
mile lower down the same valley a separate mass of granophyre sends out
veins into the basalt, which as usual is dark bluish-grey in colour,
indurated and splintery.
[Footnote 402: This rock appears to the eye as a black finely
crystalline-granular felsite. Under the microscope, it was found by
Dr. Hatch to "present a markedly granulitic structure, consisting
mainly of small rounded grains of dirty brown turbid felspar, with
isolated granules of colourless quartz. Scattered through the rock, or
accumulated in patches, are small spherical or drop-like granules of a
bright green augite (coccolite)."]
[Illustration: Fig. 353.--Section on south side of Cruach Tòrr an
Lochain, Mull.
_a_, bedded basalts and dolerites; _b_, granophyre; _c_, marginal
finer-grained band; _d_ _d_, veins from the granophyre traversing the
basic rocks.]
[Illustration: Fig. 354.--Section at head of Allt na Searmoin, Mull.
_a_, basalts and dolerites, with slaggy upper surfaces; _b_, felsite.]
As the posteriority of the Mull granophyre and felsites to the basalts
is thus proved, the further question remains as to their manner of
intrusion. Here and there, especially on the south-eastern side,
between the head of the Scarrisdale river and Loch Ba', the line of
junction between the two rocks is nearly vertical, but a body of black
felsite intervenes as a huge wall between the ordinary granophyre and
the basalt. On Beinn Fhada and Beinn a' Chraig the line of separation,
as I have above remarked, is inclined outwards, and plunges under the
basalts at an angle of 30° to 40°. The terraced basalts and dolerites
are not sensibly disturbed, but end off abruptly against the steep
face of intrusive rock. We might suppose that in this case the younger
rock had merely carried upward the continuation of the beds that are
truncated by it, as if an orifice had been punched out for its ascent.
But on the top of the ridge of Beinn a' Chraig we find that the
outliers which there remain are not portions of the lower basalts, but
of the upper "pale group" of Ben More. The same rocks are prolonged
on the other side of the Scarrisdale Glen, sweep over the summit of
Beinn Fhada, and run on continuously into the crest of A'Chioch and the
upper part of Ben More. The granophyre has usurped the place of the
lower dolerites and basalts, but has left the more felspathic lavas of
the "pale group" in their proper position. And to make this remarkable
structure still more clear, sections may be seen on the southern flanks
of Beinn Fhada, where the upper surface of the granophyre comes down
obliquely across the edges of the lavas, and allows the junction of the
basalts and the "pale group" to be seen above it (Fig. 355). As in the
case of Beinn an Dubhaich, it is as if the granophyre had eaten its way
upward and dissolved the rocks which it has replaced.
[Illustration: Fig. 355.--Section on south side of Beinn Fhada, Mull.
_a_, bedded basalts and dolerites; _b_, "pale group" of Ben More; _c_,
granophyre.]
The usual kind of contact-metamorphism has been produced around this
intrusive boss. It is most marked in the outliers that cap Beinn a'
Chraig and on the two ridges to the south-west, where it is seen to
consist in a high degree of induration, the production of a shattery,
irregularly-jointed structure, and the effacement of the obvious
bedding which characterizes the unaltered rocks.
The position of this eruptive mass, quite a mile broad, breaking
through, without violently tilting, more than 1800 feet of the bedded
basalts, and then stopping short about the base of the "pale group,"
presents a curious problem to the student of geological physics. It at
once reminds him of many sections among Palæozoic granites where an
eruptive boss has ascended and taken the place of an equivalent volume
of the surrounding rocks, which, though more or less metamorphosed,
are not made to dip away from it as from a solid wedge driven upwards
through them. In this Mull case, however, there are some peculiar
features that deserve consideration, for they seem to show that here,
as elsewhere, passages for the uprise of the intrusive rock were
already provided by the presence of volcanic pipes, which, even if
filled up with fragmentary materials, would no doubt continue to be
points of weakness. Round the flanks of the Loch Ba' boss, and here
and there on its surface, patches of intensely indurated volcanic
agglomerate may be detected. A little to the south of the tarn called
Loch na Dàiridh, the granophyre is succeeded by the black, flinty
felsite or rhyolite already referred to. This rock in some places
exhibits a beautiful flow-structure, with large porphyritic felspars,
and encloses a great many fragments of dolerite and gabbro, varying
from the size of a pea up to blocks several inches in diameter. Lying
on its surface are detached knolls of much altered dolerite, basalt,
and coarse breccia or agglomerate. On its southern margin one of these
patches of agglomerate contains abundant fragments of various felsitic
rocks, among which are pieces of a compact rock with flow-structure
like that found in place immediately to the north; also rounded pieces
of quartzite, and of compact and amygdaloidal basalt wrapped up in a
very hard matrix which seems to consist largely of basalt-dust. No
bedding can be made out in this rock, and the mass looks like part of a
true neck. Further down the slope the bedded basalts appear. The actual
junctions of the different rocks cannot be satisfactorily traced, but
the structure of the ground appears to me to be as shown in Fig. 356.
A patch of similar agglomerate appears a little to the south-west of
the last section in front of a cliff of the felsite, and seems to
be enclosed in the latter rock, and other exposures of agglomerate,
underlain and intensely indurated by the felsite, may be noticed on the
ground that slopes towards Loch Ba'.
[Illustration: Fig. 356.--Section to south of Loch na Dàiridh, Mull.
_a_, basalts; _b_, dolerite; _c_, volcanic agglomerate; _d_, black
felsite; _e_, granophyre.]
That these agglomerates do not belong to the period of the eruption of
the granophyre and felsite, but to that of the bedded basalts, may be
inferred from their intense induration next the acid rocks, and also
from the fact that similar breccias are actually found here interposed
between the bedded basalts. This is well shown on the hill above the
Coille na Sròine, where the accompanying section can be seen (Fig.
357). The broad dyke-like mass of black flinty felsite already referred
to runs as a prominent rib over the southern end of Beinn a' Chraig
into the head of the Scarrisdale glen (see Fig. 352). It cuts across
the bedded basalts, and immediately to the south of where these appear,
a thin intercalated bed of breccia crops out, of the usual dull-green
colour, with abundant fragments of basalt and many of yellow and grey
felsite.
From these various facts we may, I think, conclude that along the
strip of ground now occupied by the Loch Ba' boss of granophyre and
felsite, there once stood a line or group of vents, from which, besides
the usual basalt-debris, there were ejected many pieces of different
felsitic or rhyolitic rocks, and that these eruptions of fragmentary
material took place during the accumulation of the plateau-basalts.
These volcanic funnels occasioned a series of points or a line of
weakness of which, in a long subsequent episode of the protracted
volcanic period, the acid rocks took advantage, forcing themselves
upwards therein, and leaving only slight traces of the vents which
assisted their ascent. The mingling of acid and basic fragments in the
material ejected from these vents is another proof of the existence
of acid rocks in the volcanic reservoirs before the advent of the
great granophyre intrusions. The evidence thus entirely confirms the
conclusions deduced from the Skye area.
[Illustration: Fig. 357.--Section of junction of south side of Loch Ba'
granophyre boss, with the bedded basalts, Mull.
_a_, bedded basalts; _b_ _b_, basalt-tuff and breccia; _c_, granophyre;
_d_, black felsite; _e_, coarse dolerite dyke, 30 or 40 feet wide.]
The second or Glen More boss, instead of rising into hilly ground, is
confined to the bottom of the main and tributary valleys, and has only
been revealed by the extensive denudation to which these hollows owe
their origin. It begins nearly a mile below Torness and extends up to
Loch Airdeglais--a distance of almost four miles. Though singularly
devoid of topographical feature, it exhibits with admirable clearness
the relation of the granophyres to the gabbros, and thus deserves an
important place among the tracts of acid rocks in the Western Islands.
Its petrographical characters change considerably from one part of its
body to another. For the most part, it is a true granophyre, sometimes
with orthoclase, sometimes with plagioclase as its predominant
felspar. At Ishriff, as already stated, it is sprinkled with long
acicular decayed crystals of hornblende; but at the watershed the
ferro-magnesian mineral is augite. The surrounding rocks are mainly the
plateau-basalts, with their sills of dolerite and gabbro.
[Illustration: Fig. 358.--Mass of dark gabbro about two feet in
diameter traversed by pale veins of granophyre, lying on north slope of
Creag na h-Iolaire, Mull.]
[Illustration: Fig. 359.--Section at Creag na h-Iolaire, Glen More,
Mull, showing basalts and gabbros resting on and pierced by granophyre.
_a_, much indurated and altered basalts and dolerites; _b_ _b_, gabbro;
_c_, granophyre; _d_ _d_, basalt dykes.]
This strip of granophyre sends abundant apophyses from its mass into
the dark basic rocks around it. Some of the best sections to show the
nature of these offshoots are to be found on the steep hillslope which
mounts from the watershed in Glen More southward into the Creag na
h-Iolaire (Eagle's Crag), and thence up into the great gabbro ridge of
Ben Buy. From the main body of granophyre a multitude of veins ascends
through the basalts and gabbros from two feet or more in breadth down
to mere filaments (Fig. 358). Even at a height of 300 feet up the hill
some of these veins are still three inches broad, and present the
usual granophyric structure, though rather finer in grain than the
general mass of the boss, and sometimes assuming a compact felsitic
and spherulitic texture at the immediate contact with the surrounding
rock. One of the most striking proofs of the posteriority of these
veins is furnished by the perfect flow-structure they not infrequently
exhibit along their margins, their long felspar crystals being arranged
parallel to the walls in lines that follow the sinuosities of the
boundary between the two rocks. Patches of gabbro and of the indurated
basalts may be seen lying on the granophyre, from which veins and
strings ramify through them (Fig. 359). Similar veins can be traced
upward into the main body of coarse gabbro, forming the ridge of Ben
Buy. Some of them are of the usual granular granophyric texture, others
are dull and fine-grained (claystones of the older authors).
Hence it is evident that the granophyres of Mull have been protruded
not only after the accumulation of the plateau-basalts, but after
these were traversed by the sheets and veins of gabbro. The amount
of acid rock injected into these older rocks over the mountainous
part of the island is enormous; but I reserve further reference to it
for the section on acid Dykes and Veins, for these are the forms in
which it chiefly occurs in that region. It should be added, that in
the localities here referred to basalt-veins and dykes are generally
abundant, cutting through all the other rocks (Fig. 359). So numerous
are they that the geologist ceases to take note of them when his
thoughts are engaged upon the problems presented by the masses through
which they rise.
iii. THE ACID BOSSES OF SMALL ISLES
In the island of Eigg three small bosses or sheets of acid rock occur.
That at the northern end rises through the Jurassic sedimentary rocks,
and forms a bold cliff from 150 to 200 feet high. It is a light grey
granophyric porphyry, with rounded blebs of quartz in a micropegmatic
base of quartz and felspar. The other two masses, of smaller size, cut
through the bedded basalts[403] (Map VI.).
[Footnote 403: _Quart. Journ. Geol. Soc._ xxvii. (1871) p. 294.]
In the opposite island of Rum, the acid protrusions play a much more
important part. On the east side of the hills, they occur in sheets at
the base of the gabbros; on the west side, they form a large tract of
hilly ground, which, stretching along the coast-line for about three
and a half miles from the headland of A' Bhrideanach to Harris, forms
there a range of shattered sea-cliffs, that tower for 1000 feet above
the Atlantic breakers that beat about their base. The area extends
inland to the slopes on the west side of Loch Sgathaig, a distance of
about three and a half miles, descending in a range of precipices along
its northern front, and reaching in its culminating summit, Orval, a
height of 1868 feet above the sea. The rocks of which this triangular
area consists resemble those of the Mull bosses. They are chiefly
quartz-porphyries, becoming felsitic in texture towards their contact
with adjacent rocks. In some places, as was noticed by Macculloch on
the sea-cliffs,[404] they have a rudely bedded structure. Thus on the
north-west front of Orval, this structure is shown by parallel planes
that dip outwards or north-west at 30° to 40°, and which are made
still more distinct by an occasional intrusive dyke or sheet of basalt
between their surfaces. I have already alluded to indications of an
internal arrangement in the granitoid bosses of Skye (p. 381).
[Footnote 404: _Western Islands_, vol. i. p. 487.]
[Illustration: Fig. 360.--Section on north side of Orval, Rum.
_a_, Torridon sandstones; _b_, bedded basalts of Fionn Chro; _c_,
dolerite; _d_, quartz-porphyry.]
[Illustration: Fig. 361.--Junction of Quartz-Porphyry (Microgranite)
and Basic Rocks, south-east side of Orval, Rum.
_a_, basalts and dolerites; _b_, dolerite and gabbro veins; _c_,
quartz-porphyry cutting _a_ and _b_.]
As in the other islands, the granophyres, porphyries and felsites
of Rum have been intruded at the base of the volcanic series. Over
much, if not all, of their area they lie directly on the red Torridon
sandstone. That the bedded basalts once covered them is indicated
by the position of the three outliers of the basalt-plateau already
noticed. But a fourth outlier still lies upon the porphyry of Orval
as a cake that dips gently northward. It consists of a bedded, dark,
finely-crystalline, ophitic dolerite, porphyritic in places, with a
rudely prismatic or columnar structure (Fig. 360). It has undergone
contact-metamorphism, and tongues from the underlying rock project
up into it. On the south-eastern side of the same hill, still more
striking evidence is presented of the posteriority of the acid to
the basic rocks. The porphyry shows here the same tendency to assume
a bedded structure, the parallel "beds" again dipping outward or
south-east at 40°. They plunge under the body of gabbro, dolerite and
other intrusive masses which from this point stretch eastward into the
great cones of Allival and its neighbours. The rock at the junction
is a fine microgranite with traces of micropegmatite. It is composed
of a holocrystalline base of quartz and orthoclase, with porphyritic
crystals of microcline, blebs of quartz and scattered granules of
augite. The rocks that rest immediately next it are basalt and
dolerite, into which it has sent an intricate network of veins (Fig.
361).[405] It has also pushed long tongues down the slope into them,
which may be seen traversing the dolerite and gabbro veins that cut
the basalts. The basic rocks next the porphyry have been intensely
altered. They seem in places as if they have been shattered by some
explosive force, and had then been invaded by the mass that rushed into
all the rents thus caused. This remarkable structure is still better
displayed on St. Kilda, and is more fully described in the following
account of the geology of that island.
[Footnote 405: In a thin slice cut from a specimen showing the junction,
there is a minute vein of the porphyry penetrating the basalt which is
much altered, while the porphyry becomes much finer in grain than at a
distance from the contact.]
iv. THE ROCKS OF ST. KILDA
Brief allusions to St. Kilda and its rocks have already been made (pp.
173, 358). We may now enter more fully upon the consideration of its
geological structure and history.
When the weather is clear there may be seen from the western headlands
of the Outer Hebrides a small blue cone rising above the Atlantic
horizon at a distance of about 60 miles. As the voyager approaches
this distant land it gradually shapes itself into a group of islets of
which St. Kilda, the largest and only inhabited, has an extreme length
of about four miles, a breadth of less than two miles, and a height of
1262 feet above the sea. Four miles to the north-east Borrera, about
one square mile in extent, rises with precipitous sides to a height
of 1000 feet. Off the north-western promontory of St. Kilda the huge
rock of Soay, half a square mile in area, towers from 600 to 800 feet
above the waves. Borrera has two attendant rocks--Stack Li and Stack
an Armin--huge pyramidal masses several hundred feet high, and the
home of thousands of gannets. St. Kilda possesses two less imposing
islets between its north-western headland and Soay, and a third to the
south-east known as Levenish.
The scenery of this picturesque group affords a good indication of its
geological structure. It displays two distinct types of topographical
form. In Borrera the marvellous combination of spiry ridges, deep
gullies and clefts, notched crests and splintered pinnacles, at once
reminds the visitor of the outlines of the Cuillin Hills of Skye. The
same features are repeated on a less magnificent scale in Soay and
along the whole of the south-western precipitous coast-line of St.
Kilda.
In marked contrast to these varied outlines, the eastern half of St.
Kilda rises with a smooth green surface, varied with sheets of grey
screes, up to the rounded summit of Conagher, the highest point in the
island. If the dark crags of the rest of the island group remind one of
the Cuillins, this eastern tract recalls at once the form and colour
of the Red Hills of Skye. A closer examination shows that in each case
the topography arises from the influence of the very same rocks and
geological structure as in that island.
There is, however, one aspect in which St. Kilda has no rival
throughout the Western Isles. Its russet-coloured cone, though rising
on the west side with gentle green slopes from the central valley,
plunges on the eastern side in one vast precipice from a height of
1000 feet or more into the surge at its base. Nowhere among the Inner
Hebrides, not even on the south-western side of Rum, is there any such
display of the capacity of the youngest granite to assume the most
rugged and picturesque forms. It is hardly possible to exaggerate the
variety of outline assumed by the rock as it yields along its system
of joints to the influence of a tempestuous climate. It has been carved
into huge projecting buttresses and deep alcoves, the naked stone
glowing with tints of orange and fawn colour, veiled here and there
with patches of bright green slope, or edged with fringes of sea-pink
and camomile. Every outstanding bastion is rent with chasms and split
into blocks, which accumulate on the ledges like piles of ruined
walls. To one who boats underneath these cliffs the scene of ceaseless
destruction which they present is vividly impressive.
The geology of St. Kilda was sketched by Macculloch, who recognized
the close resemblance of its two groups of rock to the "augite-rock"
(gabbro) and "syenite" (granophyre) of Skye and other islands of
the Inner Hebrides. But he left the relations of the two groups to
each other undetermined.[406] Professor Heddle has published a brief
reference to the rocks of St. Kilda, without, however, offering any
definite opinion as to the geological structure of the islands.[407]
The best account of the geology has been given by Mr. Alexander Ross,
who obtained evidence that the acid sends veins into the basic rock.
He brought away specimens clearly showing this relation, but in his
description left the question open for further inquiry.[408] To some of
the observations in these papers reference will be made in the sequel.
The following account is based on the results of two visits paid by
me to St. Kilda in the summers of 1895 and 1896, during which I was
enabled to examine the rocks on land, and to sail several times round
the islands, boating along those parts of the cliffs which presented
features of special geological importance.
[Footnote 406: _Description of the Western Isles_, vol. ii. p. 54.]
[Footnote 407: In an article on the general geological features of
the Outer Hebrides contributed to _A Vertebrate Fauna of the Outer
Hebrides_, by J. A. Harvie-Brown and T. E. Buckley, 1888.]
[Footnote 408: _British Association Report_, 1885, p. 1040, and a much
fuller paper in the _Proceedings of the Inverness Field Club_, vol.
iii. (1884), p. 72.]
In the St. Kilda islets three groups of rock differing from each other
in age may be recognized. 1st, A series of gabbros, dolerites and
basalts which have been intruded through and between each other as
sills; 2nd, a mass of granophyre which invades these sills; and 3rd,
abundant dykes and veins of basalt which occur both in the basic and
acid masses.
From the extension of the basalt-dykes across the Outer Hebrides it is
clear that the Tertiary volcanic region reached at least to within 60
miles of St. Kilda. Whether or not it stretched over the intervening
space now overflowed by the Atlantic must be matter for conjecture.
There can be no doubt that the intrusive rocks of St. Kilda are in
age and origin the equivalents of those of the Inner Hebrides. The
remnants left of them were assuredly not superficial extrusions, but
are characteristic examples of the more deep-seated intrusions of the
Tertiary volcanic period. Down to the most minute details of structure
they reproduce the features so well displayed by the gabbros and
granophyres of Skye, Rum and Mull. If it is demonstrable in the case
of these islands that the intrusions have taken place under a deep
cover of basalt-sheets, now in large part removed, the inference may
legitimately be drawn that at St. Kilda a basalt-plateau once existed
which has been more completely destroyed than in the other regions.
Not a fragment of such a plateau has survived, unless we may perhaps
be allowed to recognize it in some of the basalts enclosed among the
gabbro-sills. Placed far amid the melancholy main and exposed to the
full fury of the Atlantic gales, these islets must be regarded as the
mere fragmentary cores of a once much more extensive volcanic area.
The geologist who visits them is deeply impressed at every turn by the
evidence of the active and unceasing destruction which their cliffs
are undergoing. Nothing now remains save the deep-seated nucleus of
intrusive sills, bosses and dykes.
1. _The Gabbro Sills._--The rudely-bedded arrangement of these rocks is
conspicuous along the west side of St. Kilda, in Soay and in Borrera.
They consist of coarse and fine varieties disposed in successive sheets
which dip at angles varying from as little as 15° up to as much as 60°
or even more. In St. Kilda they form the picturesque promontory of
the Dune, and extend thence along the western side of the island to
its extreme northern end. Their escarpments face the ocean, and their
dip-slopes descend towards the north-east in grassy declivities to the
south bay and the long verdant glen which runs thence across to the
north bay. The same strike is prolonged into Soay, but further east in
Borrera the direction curves so as to present vast escarpments towards
the west and shelving sheets of rock towards the east.
None of the gabbros seen by me are as coarse as the large-grained
varieties of Skye, nor does there appear ever to be such a marked
banded structure among them as that displayed by the Cuillin rocks.
Faint banding, however, may be noticed. A series of specimens which
I collected from the west side of the island has been sliced for
microscopic examination, and Mr. Harker has furnished me with the
following notes regarding them.
"An olivine-gabbro from the west side of St. Kilda [7107] is a
dark, heavy, medium-grained rock, in which augite and felspar are
conspicuous. The microscope shows, in addition, plentiful grains of
olivine, with but little original iron-ore, and some apatite-needles.
The structure is ophitic, the plates of pale-brown augite enveloping
both olivine and felspar. A little brown hornblende and red-brown mica
are probably original, the rock showing little sign of alteration. The
felspar is labradorite, with albite- and Carlsbad-twinning, and forms
elongated rectangular crystals.
"Another specimen [7108] is a rock of similar appearance but somewhat
coarser texture, and structurally is a more typical gabbro than the
preceding, the felspar having little of the 'lath' shape, while the
augite, though still moulded on the felspar, scarcely assumes an
ophitic habit. A striking feature in this rock is the way in which
the augite is crowded with 'schiller'-inclusions, in places so
closely as to be almost opaque. A high magnification shows that these
inclusions are dark, linear in form, and disposed along two directions
intersecting at a high angle. The labradorite has unusually close
twin-lamellation on both albite and pericline laws, and it is possible
that this is a strain-effect.
"A third specimen [7109] is from a rock in every respect identical
with the preceding, except that the olivine is rather more plentiful,
and in some grains is partially serpentinized."
While the gabbros of St. Kilda are not a mere uniform boss, but a
series of sills and irregular masses which have been successively
injected into each other, they have subsequently been cut through by
many basalt-dykes and veins. These, which are sometimes as abundant
as in the gabbro of the Cuillin Hills, traverse the rocks both in
the line of bedding and also at many different angles across it. As
they generally weather faster than the gabbros, they give rise to
deep narrow clefts which may be traced up the whole height of the
precipices, occasioning sea-caves below and sharp notches on the crests
above.
These scenic features, so indicative of the geological structure that
causes them, are specially well seen on the western face of the Dune or
south-western promontory of the island, and likewise in the strangely
rifted precipices further north and in Soay. They are, however, most
impressively displayed around the naked walls of Borrera, which in
their marvellous combination of spiry ridges, deep straight gullies,
and splintered crests, remind one at every turn of the scenery of
Blaven and the Cuillin Hills.
2. _The Granophyre Boss and its Apophyses._--The eastern half of the
island of St. Kilda consists of a pale rock which Macculloch long ago
identified with the granophyre of Skye, and which, as he pointed out,
has much resemblance to parts of the granite of Arran.[409] Not only does
it give rise to topographical forms like those of the Red Hills, but it
weathers, like the Skye granophyre and the Arran granite, into thick
bed-like sheets divided by transverse joints into large quadrangular
blocks. On closer inspection it is found to resemble still more
precisely the acid rocks of the Inner Hebrides. It possesses the same
drusy micropegmatitic structure as the granophyres of Skye, Rum and
Mull. The ferro-magnesian constituents are present in small quantity,
hence the pale hue of the stone. The quartz and felspar project in
well-terminated crystals into the drusy cavities, which are sometimes
further adorned with delicate tufts of clear crystallized epidote. In
these and other respects the rock displays the familiar external forms
of the younger or Tertiary granites of Britain.
[Footnote 409: _Description_, vol. ii. p. 54.]
Mr. Harker's notes on the microscopic structure of this granophyre are
as follows:--"The prevailing felspar is orthoclase, often very turbid
from secondary products. Even what appear to be distinct crystals are
sometimes seen in the slices to be invaded on the margin by quartz in
rough micrographic intergrowths, and much of the finer intergrowth
occurs as a fringe to the crystals. In this case the felspar of the
micropegmatite can often be verified to be in crystalline continuity
with the crystal which has served as a nucleus [6624]. Quartz occurs
in distinct crystals and grains as well as in the micropegmatite.
There is a more granitoid variety of the rock, in which only a very
rude approach to micrographic intergrowths is seen [6623]. In both
varieties there is but little trace of any ferro-magnesian mineral; the
more typical granophyre has what seems to be destroyed augite, while
the granitoid rock contains a little deep-brown biotite. Scattered
crystal-grains of magnetite occur in both."
Narrow ribbon-like veins of a finer material, sometimes only an inch in
breadth, traverse the ordinary granophyre. Similar veins run through
the rock of the Red Hills in Skye; they are sharply defined from
the enclosing rock, as if the latter had already solidified before
their intrusion. With regard to the microscopic structure of some
thin slices prepared from these veins, Mr. Harker remarks that "the
material of the veins is of a type intermediate between granophyre
and microgranite [6622, 6623]. The chief bulk is a finely-granular
aggregate of quartz and felspar, the latter very turbid; but in this
aggregate are imbedded numerous patches of micropegmatite, often of
perfect and delicate structure. These areas of micropegmatite show
some approach to a radiate or rudely spherulitic structure, and, in
some cases, are clustered round a crystal of felspar or quartz. Some
granules of magnetite and rare flakes of brown biotite are the only
other constituents of the rock. Although they must be of somewhat later
date, there is evidently nothing in the petrographical characters of
these fine-textured veins to separate them widely from the ordinary
granophyres of the region."
These veins may be compared with the spherulitic dyke that traverses
the granophyre of Meall Dearg at the head of Glen Sligachan (described
at p. 381), which, though undoubtedly somewhat younger than the rock
that contains it, yet presents the very same structures as are visible
at the margin of that rock.[410] The material of this dyke and of the
finer veins of St. Kilda and the Red Hills probably belongs to a later
period of protrusion from a deeper unconsolidated portion of the same
acid magma as at first supplied the general body of granophyre.
[Footnote 410: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 220.]
Undoubtedly the most interesting feature in the granophyre of St.
Kilda is its junction with the mass of basic rock to the west of it.
This junction-line runs from about the middle of the chief or south
bay (where, however, its precise position is concealed under detritus)
across the island to the north shore, where it descends the face of the
precipice and plunges under the sea. Important as the actual contact
of the two rocks obviously is in regard to their relative date, it has
not hitherto been observed or described. Macculloch noticed "numerous
fragments of trap penetrated by veins of syenite," but he did not see
these rocks in place, and, in spite of their apparent testimony to the
posteriority of the acid intrusions, he was inclined to believe that
the veins were not real veins, but that the "trap" and "syenite" had a
common origin and would be found to pass into each other, as he thought
also occurred in Mull and Rum. In recent years Mr. Alexander Ross,
during his visit to St. Kilda, collected specimens illustrating the
varieties of gabbro, dolerite and basalt, and showing the intrusion of
the acid into the basic rocks. As already stated, he was disposed to
regard the "granite" as of younger date than the gabbros, but left the
question undecided.[411]
[Footnote 411: In his paper, _Proceed. Inverness Field Club_, vol.
iii. (1884), p. 78, Mr. Ross quotes a letter from Prof. Judd, who
there states that the rock supposed to be granite "is seen under the
microscope to be a quite different rock--a quartz-diorite." Some of
the specimens from St. Kilda collected by Mr. Ross were exhibited at
a meeting of the Geological Society on 25th January 1893. With regard
to these Prof. Judd, in the course of the discussion on his paper on
"Inclusions of Tertiary Granite in the Gabbro of the Cuillin Hills,"
remarked:--"They show a dark rock traversed by veins of a light one,
but the dark rock is not a gabbro and the light one is not a granite"
(_Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 198).]
The best locality for the examination of the junction of the main
granophyre mass with the gabbros is inaccessible save by boat, and only
in the calmest weather. It occurs in the great cliff on the northern
side of the island between the north bay and the sea-stack known as
the Bragstack. The line of contact emerges from below the sea-level,
and ascends the cliff with a westward inclination of from 60° to 80°.
Here, as in Skye, the acid rock underlies the basic masses, which are
rudely bedded and much jointed. About 150 feet above the sea-level,
the nearly vertical cliff breaks up into an exceedingly rocky and
rugged acclivity, across which the junction seems to slope at a lower
angle. But the place is hardly reachable, save perhaps by the intrepid,
barefooted cragsmen of St. Kilda.
[Illustration: Fig. 362.--Junction of granophyre and gabbro, north side
of St. Kilda.]
Along the sharply defined line of contact the granophyre is
close-grained, and sends a network of veins into the dark sheets of
gabbro. The general features of the junction are represented in Fig.
362. The veins are narrow, those nearest the main body of granophyre
diverging from it at a still more acute angle than those from the
mass of Meall Dearg (Fig. 376), and then branching so as to enclose
masses of the gabbro and to run across them in long parallel veins.
A characteristic feature of many of these veins, besides their
narrowness, is their tendency to split up at the ends into mere fingers
and threads as represented in Fig. 363.
Owing to the depth of soil on the cultivated land, and of boulders and
sand on the beach, the actual junction of the main body of granophyre
with the gabbro is not seen on the southern shore. But a few yards to
the westward of where it must lie, the beach is cumbered with large
blocks of rock broken up from the mass, which can be seen _in situ_
a little further south forming a line of low cliff with a rugged
foreshore. These rocks consist of various gabbros and basalts of rather
fine grain, profusely traversed with veins of white granophyre. Some
of these veins are two feet or more in breadth, and, when of that
size, show the distinctive granular texture and drusy structure of
the main part of the acid rock. But from these dimensions they can
be traced through every stage of diminution until they become mere
threads. When they are only an inch or two broad, they assume a finely
granular texture like that of the veins that run through the body of
the granophyre.
[Illustration: Fig. 363.--Veins of granophyre traversing gabbro and
splitting up into thin threads, north side of St. Kilda.]
The amount of injected material in the dark basic rocks is here and
there so great as to form a kind of breccia (Fig. 364), which, from
the contrast of tone between its two constituents, makes a conspicuous
object on the shore. Here, as in the example already cited from Rum,
the basic rocks seem to have been shattered into fragments, and the
acid material to have been injected into the minutest interstices
between them. The enclosed fragments are of all sizes from mere grains
up to blocks a foot or more in length. They are generally angular, like
rock-chips from a quarry. Moreover, they are not all of the same kind
of material. While at this locality most of them consist of basalt,
they include also a few large and small pieces of rather coarse gabbro.
There has evidently been a certain amount of transport of material, as
well as an extensive disruption of the rocks _in situ_. The granophyre
here and there assumes a darker or greener tint, as if it had dissolved
and absorbed some portion of the older rock.
Still more astonishing are the sections to be seen on the western
cliffs and rocky declivities of the ridge to the north of the Dune,
at a distance of perhaps 500 or 600 yards westwards from those of
the South Bay. Here the gabbro-sheets are traversed by a number of
conspicuous white bands, which on examination prove to be veins or
dykes of granophyre. As viewed from the sea, the general disposition
of the two groups of rocks is represented in Fig. 366. The broadest
mass of granophyre breaks out towards the bottom of the precipice, and
slants upward as a sheet intercalated between the gabbro sills, with
a breadth of about 40 or 50 feet, but rapidly thinning away in its
ascent. One of the bands below it has a breadth of about 15 feet. The
material of these intrusions is a pale fine-grained granophyre like
that of the South Bay, I did not detect, either here or anywhere else
in St. Kilda, a definite spherulitic structure such as is so common in
the granophyre dykes of Skye.
[Illustration: Fig. 364.--Pale granophyre injected into dark basalt,
South Bay, St. Kilda.
The crags on the further side of the bay are the gabbro sheets of the
Dune. (From a photograph by Colonel Evans.)]
Though the acid intrusions are somewhat irregular both in thickness
and direction, they lie generally parallel to each other in the line
of strike of the bedding of the gabbros. They are no doubt apophyses
from the main body of granophyre, which emerges to the surface about a
third of a mile to the eastward, but may of course be at no great depth
underneath.
[Illustration:
Fig. 365.--Veins of granophyre traversing a line-grained gabbro and
scarcely entering a coarse-grained sheet, west side of Rueval, St.
Kilda.
]
Besides the broader bands of acid rock, and diverging from them, a
complicated network of veins ramifies in all directions through the
gabbros, as at the South Bay. The extraordinary degree to which the
basic rocks have been shattered into fragments is strikingly displayed
here, likewise the extreme liquidity of the acid magma, whereby it
was able to insinuate itself into every chink and cranny. But the
observer notices that this condition of excessive disruption is not
shared by all the basic sills, and is not attendant upon all the acid
dykes. As an example of this irregular distribution of the structure,
I give the accompanying sketch (Fig. 365), where a fine-grained gabbro
has been completely broken up and intersected with granophyre veins,
while the coarser sheet overlying it has almost entirely escaped.
The dark basalt-like sheets appear generally to have been much more
disrupted than the more largely-crystalline varieties. It is noticeable
here, also, that the fragments entangled in the network of granophyre
veinings do not entirely belong to the rock that has been shattered,
but sometimes include large and small lumps of different gabbros,
showing some transference of material with the inrush of the acid magma.
Though closer in grain where it comes in contact with the gabbro,
the granophyre never assumes any vitreous texture along its margin.
A series of thin slices, prepared from specimens collected by me in
the South Bay in the summer of 1895, was examined by Mr. Harker, who
furnished the following notes regarding them:--"The basalt traversed
by the granophyre is a fine-textured variety with small porphyritic
felspars. These latter seem to be usually unaltered, retaining the
glass cavities which in some of the crystals are abundant. The
groundmass, however, shows minerals of metamorphic origin which must
be derived mainly from the original augite. A brown mica is the most
conspicuous; but with it are associated some brownish-green hornblende
and certain chloritic and perhaps serpentinous substances. It is
chiefly near the margin of a fragment of basalt that the mica gives
place to these minerals. The basalt still retains plenty of unaltered
granules of augite in the central parts of a fragment. It is not
certain that the secondary minerals named come exclusively from the
augite of the basalt; from their form and mode of occurrence they may
in part have replaced olivine or even rhombic pyroxene.
[Illustration: Fig. 366.--View of sills and veins of pale granophyre
traversing dark sheets of gabbro, west side of St. Kilda.
(From a photograph by Colonel Evans.)]
"The acid rock, though styled granophyre above, belongs to a granitoid
variety of that group of rocks, and has but little indication of
micrographic structures. Compared with the other granophyres from
St. Kilda, sliced and examined, these examples show a less acid
composition. This is expressed mineralogically in the presence of
a somewhat larger proportion of ferro-magnesian minerals and of
soda-lime felspar. These features might indeed be matched in many
normal granophyres among the Western Isles, but in the present case it
can hardly be doubted that they are to be explained, at least in some
degree, by the acid magma having taken up a certain amount of material
from the basalt. Many of these Tertiary granophyres have undoubtedly
been modified by the incorporation of pieces of basalt and gabbro,
and a collection made in the Strath district of Skye will furnish
examples for future study. Professor Sollas's description of similar
phenomena in the Carlingford district has already proved the importance
of this kind of action.[412] In the present instance, both brown mica
and hornblende occur plentifully in the granophyre, and especially
round the basalt fragments. This latter point is conclusive as to the
derivation of the basic material, and further proves a certain degree
of viscosity in the acid magma at the time of its intrusion."
[Footnote 412: _Trans. Roy. Irish Acad._ vol. xxx. (1894), pp. 477-572.]
Another series of specimens which I collected in the following year
was submitted to Mr. Harker for petrographical determination, and his
observations on two of the microscopic slices are as follow: "A breccia
from the South Bay, St. Kilda [7105], consists of angular fragments up
to two inches in diameter set in a matrix of grey granophyre of medium
texture. The fragments belong to two types--one of very close texture
(basalt), the other more evidently crystalline (diabase). Both are cut
by the slice.
"The basalt shows very evident metamorphism, its augite being wholly
transformed into greenish-brown hornblende. The little felspar-laths
and granules of iron-ore seem to be unaltered, though the latter may
perhaps have contributed to the formation of the hornblende. Another
fragment of basalt has some larger crystal-grains of augite, and these
are not converted into hornblende.
"The diabase shows a less marked boundary under the microscope, but
otherwise has similar characters to the preceding. The striated
felspar-crystals and grains of iron-ore have not been re-crystallized.
A considerable amount of pale augite remains, but there is also plenty
of deeply-coloured hornblende, both fibrous and compact. This diabase
is certainly an intrusive rock, but the basalt, from its petrographic
character, might be from a lava-flow or from a dyke.
"The granophyre is of somewhat coarse texture, the micrographic
structure being only of a rude type. It is notably richer in the darker
constituents than is usual in such rocks. Further, the hornblende and
magnetite tend to cluster in little patches which suggest destroyed
fragments of basic rocks. A grain or two of sphene occur, a mineral
foreign to the normal granophyres.
"Another similar specimen [7106] from the same locality shows a basic
rock of coarser texture, approaching some of the gabbros in appearance
and with boundaries in places not very sharply defined. The grey matrix
is again relatively rich in the dark elements, and the manner in which
they occur in little patches, like nearly obliterated 'xenoliths,'
points unmistakably to a certain amount of absorption of basic material
by the acid magma, with consequent enrichment in the ferro-magnesian
minerals.
"The slice cuts only the acid rock, which is seen to be of granitoid
rather than granophyric structure, though the tendency of the felspar
to enclose quartz-grains is unlike a typical granite. Oligoclase, with
combined albite- and Carlsbad-twinning, is well represented in addition
to orthoclase, and some zoned crystals seem to be of albite with a
border of oligoclase. Brown hornblende and a little brown mica are the
coloured constituents. Magnetite and apatite are also observed."
The testimony of the rocks of St. Kilda to the posteriority of the
granophyre to the gabbros and basalts is thus clear and emphatic.
It entirely confirms my previous observations regarding the order
of sequence of these rocks in Mull, Rum and Skye. But the St. Kilda
sections display, even more strikingly than can be usually seen in
these islands, the intricate network of veins which proceed from the
granophyre, the shattered condition of the basic rocks which these
veins penetrate, the remarkable liquidity of the acid magma at the time
of its intrusion, and the solvent action of this magma on the basic
fragments which it enveloped.
3. _The Basic Dykes._--Reference has already been made to the numerous
dykes by which the gabbros of the St. Kilda group of islets is
traversed. Similar dykes occur also, though less plentifully, in the
granophyre. It remains for future observation to determine whether
there is one series older and another later than the intrusion of the
acid rock. In any case, it is quite certain that the dykes in the
gabbro do not all belong to one period of injection, for frequent
examples of intersection may be noticed, especially on the cliffs of
Borrera, and also cases of double and even treble dykes which have been
formed by successive infillings within the same fissure. The remarkably
varied precipices of that island are marked by the long narrow rifts
left by the weathering of vertical dykes, which, as above remarked, may
be followed with the eye from the sea-level to the sky-line, ascending
obliquely across the bedding of the gabbro sheets. Another group of
dykes may be traced sloping upward at low angles along the face of
the cliffs and affording admirable ledges with overarching roofs
for innumerable gannets, kittywakes and guillemots. Other dykes and
ribbon-like veins may be seen traversing the gabbro in many different
directions, precisely as among the Cuillin Hills. As no similar network
of dykes and veins is to be observed in the granophyre, I am disposed
to regard a large number of these intrusions as older than that rock.
But I did not observe any actual example of a basic dyke truncated by
the granophyre.
There can be no doubt, however, that an injection of similar dykes and
veins took place after the invasion of the granophyre. These later
intrusions are conspicuously displayed along the cliffs that extend
from the gabbro junction on the north side of St. Kilda round the
eastern coast into the South Bay. They maintain a general parallelism
and ascend from the sea-level at varying angles of inclination, running
up the pale sea-wall as dark bands. They consist of basalt-rocks, and
may often be seen to branch and to die out. Like those in the gabbro,
they are not infrequently compound, being made up of two or three or
even more distinct dykes. This is well seen on the great precipice
below Conacher, where the section given in Fig. 367 is displayed. Here
in a vertical height of about 800 or 900 feet, there must be at least
seven dykes, simple and compound. A little further south a triple dyke
may be seen to be composed of a thick central zone and two thinner
marginal bands, of which the lower strikes off from the others and
maintains an independent course through the granophyre (Fig. 368).
[Illustration: Fig. 367.--Section of the sea-cliff below Conacher, St.
Kilda, showing basic dykes in granophyre.]
[Illustration: Fig. 368.--Triple basic dyke, sea-cliff, east side of
St. Kilda.]
V. THE GRANITE OF ARRAN
The northern half of the island of Arran is mainly occupied by one
of the most compact and picturesque groups of granite mountains in
Scotland.[413] These heights, rising out of the Firth of Clyde to a
height of 2866 feet, present, in their spiry and serrated crests, a
contrast to the smoother contours of the older granitic elevations of
this country. The granite is surrounded by a ring of schistose rocks,
belonging to the metamorphic series of the Southern Highlands, save for
a short distance on the eastern margin, where it comes in contact with
and indurates the Lower Old Red Sandstone. Macculloch long ago pointed
out that no pebbles of the granite are to be found in the surrounding
conglomerates and red sandstones of Carboniferous and younger age.[414]
Geologists accordingly came to the conclusion that the protrusion of
the granite took place after Carboniferous time, and hence that it had
no connection with the appearance of the far older granites of the
Highlands. In the year 1873 I gave reasons for believing the granite
to be not only younger than the Carboniferous formations, but to be
referable with most probability to the Tertiary volcanic series.[415]
The progress of inquiry has tended to confirm this inference, though
no direct proof of its correctness has been obtained. Two lines of
investigation may be pursued, and each leads to the conclusion of the
probability of the Tertiary age of the granite. One of these proceeds
on a comparison of the petrographical characters of the Arran rocks
with those of undoubted members of the Tertiary series among the
Western Isles. The other inquiry deals with the relation of the rocks
to each other in the general geological structure of Arran itself.
[Footnote 413: The rocks of Arran have often been described. Besides the
work of Macculloch above quoted, reference may be made to the paper by
Sedgwick and Murchison, _Trans. Geol. Soc._ 2nd Ser. vol. iii. p. 21;
A. C. Ramsay's _Geology of the Island of Arran_, 1841, the paper of
Necker de Saussure quoted on p. 412; J. Bryce's _Geology of Clydesdale
and Arran_, 3rd edit. 1865. The island is at present being surveyed for
the Geological Survey by Mr. W. Gunn.]
[Footnote 414: _Description of the Western Islands of Scotland_, vol. ii.
p. 388.]
[Footnote 415: _Trans. Edin. Geol. Soc._ vol. ii. part iii.]
Macculloch first remarked the strong lithological resemblance of
the Arran granite to the "syenite," or granophyre, of Skye and St.
Kilda.[416] More recent petrographical investigation, as already stated,
has furnished additional proofs of the connection between the acid
rocks of these islands. So closely indeed are these rocks linked by
megascopic and microscopic characters, that the petrologist has no
hesitation in placing them together as probably products of the same
period of igneous activity.
[Footnote 416: _Description_, vol. ii. p. 352.]
From the general geological structure of Arran, a further strong
argument may be deduced in favour of the late date of the eruptions
of granite. Good reasons have been given for classing as Permian the
bright red sandstones which occupy much of the central and southern
parts of this island, and include the little volcanic group already
referred to. These sandstones have been invaded by a complex series of
eruptive rocks which would thus be later than the Permian period. No
igneous masses posterior to this period are certainly known in Britain
save those of Tertiary age. The larger body of granite in the northern
half of the island nowhere comes into direct contact with the newer red
sandstones, but these strata are pierced by smaller bodies of granite.
Hence, both by the evidence of their internal structure and by the
stratigraphy of the ground, the later igneous rocks of Arran may be
reasonably grouped together as one important and consecutive series,
comparable in age and general characters with those of Tertiary date in
the Inner Hebrides.
[Illustration: Fig. 369.--Jointed structure of the granite near the top
of Goatfell Arran.
(From a photograph by Mr. W. Douglas, lent by the Scottish
Mountaineering Club.)]
The igneous rocks of Arran, later than the probably Permian sandstones,
range from acid to basic in composition. Besides the northern granite,
there are in the southern part of the island acid rocks that include
granite, coarse-grained quartz-porphyry and fine-grained felsite. Where
the relations of these rocks to each other can be seen, the felsite is
found by Mr. Gunn to be newer than the porphyry, into which it sends
sills and dykes.
A feature observed by the same geologist in Arran offers a further
point of resemblance to the acid sills and dykes of Skye. He has
noticed that accompanying the quartz-porphyry of Drumadoon and Bennan,
a mass of basic rock forms a kind of fringe or selvage round it,
sometimes with what appears to be a rock of intermediate character
between them. Basic sills are abundant south of Glen Ashdale, though
to the west of Whiting Bay most of the intrusive sheets are of acid
material.
Some of the quartz-porphyry sheets are markedly columnar. One of
them, near Corriegills, displays a divergent grouping of the prisms,
not unlike parts of the pitchstone sheets of Eigg and Hysgeir, and
suggestive of the rock having flowed along a hollow like that of a
valley. No certain trace, however, has been found of any Tertiary
lava-stream in Arran, nor has evidence of tuffs been detected in
any part of the younger igneous series. All the rocks appear to be
intrusive, though so abundant and varied are they as to indicate that
they belong to a vigorous eruptive centre, which may have poured out at
the surface lavas and ashes, since entirely removed by denudation.
The numerous basic dykes for which the south end of Arran has long
been celebrated have a general northerly trend, and appear to be all
of the same or nearly the same age. They undoubtedly cut through the
quartz-porphyries and the coarse-grained basic sills, but are less
numerously visible in the finer-grained basic sills, while in the
felsitic sheets they are seldom to be seen. In several places dykes
running in an E.N.E. direction cut the others, and are therefore of
later date.[417] The compound dykes of Tormore on the west side of the
island have been already noticed (p. 161).
[Footnote 417: _Ann. Rep. of Geol. Surv._ for 1894, p. 286.]
VI. THE NORTH-EAST OF IRELAND
In the north-eastern counties of Ireland there are two regions which
afford ample material for discussion in connection with the protrusion
of acid rocks during the Tertiary volcanic period. One of these, which
for distinction may be called the Carlingford region, embraces the
tract of country which includes the Mourne Mountains on the north-east
side of Carlingford Lough and the ranges of Slieve Foye and Slieve
Gullion on the south-west side. The other lies mainly within the
basaltic plateau, the largest of its scattered portions forming parts
of the hills of Carnearny and Tardree in the county of Antrim (Map
VII.).
1. The Carlingford Region
a. _The Mourne Mountains._--This compact and picturesque group of
hills, about twelve miles long and six miles broad, and reaching a
height of 2798 feet in Slieve Donard, presents a comparatively simple
geological structure, since it consists almost entirely of granitic
rocks which pierce, overlie and underlie Upper Silurian grits
and shales. So far as regards the contact of these rocks with the
disrupted sedimentary formations, all that can be asserted is that
the granite must be later than at least the older part of the Upper
Silurian period. But for at least two reasons, the eruptive rocks
may be regarded with some confidence as part of the Tertiary series.
In the first place, there is a strong petrographical resemblance
between the Mourne Mountain granite and that of the Island of Arran
and the granitic parts of the granophyre of the Western Isles. And
this resemblance is so close as to furnish a cogent argument in favour
of grouping all these rocks together as parts of one geologically
contemporaneous series. In the second place, the Mourne Mountain
granite abruptly cuts off a large number of basic dykes which, running
in a general N.N.W. direction, may be looked upon as almost certainly
members of the Tertiary system of protrusions.
The manner in which the granite of the district behaves towards certain
detached areas of Silurian strata with their accompanying dykes is one
of the most astonishing features in the whole assemblage of intrusive
rocks in Britain. As has been excellently shown in the Geological
Survey Map and sections by Mr. W. A. Traill, the granite has carried up
on its surface broad cakes of vertical Silurian strata, together with
all their network of dykes.[418] A cake of this kind, from 50 to about
200 feet thick and nearly two miles broad, has been bodily uplifted
from the rest of the mass and carried upward by the granite, so that
the truncated ends of the beds of grit and shale with their system of
dykes stand upon a platform of granite, from which also numerous veins
penetrate them. There can be little doubt that the basic dykes thus
broken through are parts of the great Tertiary system, and if so, the
granite which disrupts them cannot be older than Tertiary time.
[Footnote 418: See Sheets 60, 61 and 71 of the one-inch map of the
Geological Survey of Ireland, and Sheets 22, 23 and 24 of the
Horizontal Sections. The Explanation to these Sheets of the map was
written by Professor Hull, Mr. Traill having previously retired from
the service. The Mourne Mountain area is now undergoing critical
revision by Prof. Sollas for the Geological Survey, and important
additional material for the elucidation of this district may be
expected from him.]
Besides the older basic dykes disrupted by the granite, a younger but
much less abundant series traverses that rock, and also follows a
general north-westerly direction. These later dykes in some cases cross
more acid dykes which have risen through the granite. There is no trace
of any superficial discharge from the Mourne Mountain area. But from
the analogy of other districts we may easily conceive that the granite
represents the underground parts of volcanic material which has now
been entirely removed.
b. _Slieve Foye and Barnavave District._--This area embraces the
mountainous ground lying between Carlingford Lough and Dundalk Bay, and
culminating in Slieve Foye (1935 feet). It measures roughly about six
miles in extreme length and four miles in breadth.
The remarkable assemblage of basic and acid materials in this area
has received considerable attention from geologists. The relative
order of the two groups of rocks was first clearly recognized by
Griffith, who showed that the granite (granophyre) is intruded into the
gabbro.[419] Professor Haughton subsequently confirmed this observation,
and proved the post-Carboniferous date of the intrusive materials,
which he compared with those of Skye.[420] The general distribution of
the rocks was traced out in some detail by the Geological Survey, and
described in the official _Memoirs_.[421] More recently the district
has been examined by Professor Sollas, who, bringing the photographic
camera and the microscope to the aid of field-geology, has elucidated
the structure and relations of the rocks, and has obtained abundant
evidence that the acid and basic rocks maintain there the same relative
order as among the Inner Hebrides.[422]
[Footnote 419: _Journ. Geol. Soc. Ireland_ (1843), p. 113.]
[Footnote 420: _Quart. Journ. Geol. Soc._ vol. xii. (1856), p. 171; xiv.
p. 300; and _Journ. Geol. Soc. Ireland_ (1876), p. 91.]
[Footnote 421: Sheet 71 of the Geol. Surv. Ireland, and accompanying
Explanation. These were the work of Mr. W. A. Traill.]
[Footnote 422: _Trans. Roy. Irish Acad._ vol. xxx. (1894), p. 477. This
is part i. of what is intended to be a series of papers.]
One of the first features in this tract of country to arrest the eye of
the geologist is the situation of this centre of protrusion and that
of Slieve Gullion along a north-west line, coincident with the general
direction of the numerous basic dykes of the region. Whether or not the
successive intrusions took place contemporaneously in the two areas,
they have followed each other in the same order. In the Barnavave
district the igneous rocks occupy an area of about 20 square miles.
They consist of a central and chief mass composed of acid materials,
which have risen through the basic rocks now found as an interrupted
ring round them.
In his more recent examination, Prof. Sollas has devoted special
attention to the influence of the solvent action of the acid magma
upon the basic rocks and upon its own composition and structure.
Besides confirming the work of previous observers as to the order of
appearance of the two kinds of material, he has obtained evidence that
the gabbro had not only completely solidified, but was traversed by
contraction-joints, possibly even fractured by earth-movements, before
the injection of the granophyric material. He found that this material,
like that of the Inner Hebrides and St. Kilda, must have been in a
state of great fluidity at the time of its intrusion, and made its way
into the minutest cracks and crevices. In observing the solvent action
of the granophyre, he ascertained that this action took place even in
comparatively narrow dykes, which probably consolidated at no great
depth beneath the surface.[423]
[Footnote 423: _Op. cit._]
c. _The Slieve Gullion District._--This area is separated from that
just described by a narrow strip of Silurian strata, so that its
isolation as a separate igneous district is complete. It will be
observed from the map to continue the same north-westerly line as the
Slieve Foye tract, the two together running in that direction for a
distance of some 16 miles. It is interesting to note the adoption of
this predominant north-westerly trend even by eruptive masses which
were mainly of acid material.
This district measures about ten miles in length by from one to five
miles in breadth. The rocks are, on the whole, similar to those in the
area south of Carlingford Lough, and bear the same relation to each
other, the acid being intrusive in the basic series. It is worthy of
remark that the Tertiary eruptive rocks have made their appearance in
the midst of the older granite of Newry. This granite has been already
alluded to as disrupting Upper Silurian strata, and being probably
of the age of the Lower Old Red Sandstone (vol. i. p. 290). In long
subsequent ages, after protracted denudation, during which its cover
of Silurian and Carboniferous formations was stripped off and it was
laid bare, it was broken through by the whole series of basic and acid
protrusions of Slieve Gullion.
This district is portrayed on Sheets 59, 60, 70 and 71 of the
Geological Survey of Ireland, which show a central core of basic and
acid material piercing the Newry granite.[424] Round this core and
touching it at its north-western and south-eastern end, but elsewhere
separated from it by a space of several miles, runs a curiously
continuous band of igneous material which is marked as "quartziferous
porphyry" and "felstone-porphyry" on the Survey maps.
[Footnote 424: The ground was chiefly mapped and described by Mr. Joseph
Nolan and Mr. F. W. Egan.]
The south-western portion of this elliptical ring possesses a peculiar
interest from its including certain remarkable masses of breccia
or agglomerate. These rocks have been mapped by Mr. Nolan, and are
described by him in the official _Explanation_, but in more detail in
two separate papers.[425] Having had an opportunity of paying a brief
visit to the ground, I can confirm the general accuracy of his mapping
and description, and am able to add a few further particulars to the
facts enumerated by him.
[Footnote 425: Sheet 70 of the Geol. Surv. Map of Ireland and Explanation
thereto; also _Journ. Roy. Geol. Soc. Ireland_, vol. iv. (1877), p.
233; _Geol. Mag._ 1878.]
The tract of ground where these agglomerates appear forms a prominent
ridge which rises several hundred feet above the lower country on
either side, and extends in a W.N.W. direction for about seven miles,
nearly along the line of junction between the Newry granite and the
Silurian strata. The ridge has a breadth varying from a few hundred
yards to upwards of a mile. It is separated from the main igneous mass
of the Slieve Gullion area by an intervening strip of lower ground from
three-quarters of a mile to about a mile and a half in width, which is
occupied by the Newry granite. At the north-west end of the ridge the
newer eruptive rocks lie within the area of that granite, while at the
south-east end they rise entirely amongst the Silurian strata.
Beginning at the south-eastern extremity, we find the agglomerate
occupying several detached eminences and surrounded by altered
Silurian grits and shales. Further west the rock occurs in larger and
more continuous masses, appearing at intervals, especially along the
southern borders of the quartz-porphyry which forms by much the greater
part of the ridge. Actual junctions of the agglomerate with the older
rocks around seem to be seldom visible. I found one, however, above
the gamekeeper's house on the southern flanks of the hill called
Tievecrom. The Upper Silurian grits and shales, in a much indurated and
shattered condition, are there traceable for several hundred feet up
the slope, until they are abruptly cut off by the agglomerate. The line
of separation appears to be nearly vertical, the truncated ends of the
strata being wrapped round by the mass of fragmental material.
The most remarkable features of this agglomerate, which has been well
described by Mr. Nolan, are the notable absence of truly volcanic
stones in it, and the derivation of its materials from the rocks
around it. I found only one piece of amygdaloid, but not a single lump
of slag, no bombs, no broken fragments of lava-crusts, and no fine
volcanic dust or enclosed lapilli. The rock may be said to consist
entirely of fragments of Silurian grits and shales where it lies
among these strata, and of granite where it comes through that rock.
Blocks of these materials, of all sizes up to two feet in breadth, are
confusedly piled together in a matrix made of comminuted debris of the
same ingredients.
The agglomerate on the ridge of Carrickbroad has no definite boundary,
but seems to graduate into an andesitic rock, and then into a
quartz-felsite or rhyolite. This apparent gradation is one of the
most singular features of the ridge. The andesite resembles some of
the "porphyrites" of the Old Red Sandstone. It is close-grained,
with abundant minute felspar-laths, and numerous large porphyritic
felspars, which latter are sometimes aggregated in patches, as in the
old porphyries of Portraine, Lambay Island and the Chair of Kildare.
This rock has undoubtedly been erupted at the time of the formation of
the agglomerate, or at least before the loose materials were compacted
together; for it is full of separate stones of the same materials, and
becomes so charged with them as to become itself a kind of agglomerate,
with a small proportion of andesitic matrix cementing the blocks.
A thin slice prepared from one of the specimens obtained by me
from this hill has been studied by Mr. Watts, who reports that the
fine-grained andesitic matrix in which the stones are imbedded has
often been injected into their minute fissures, and that the minute
fragments enclosed in this matrix consist here of a trachyte-like
porphyry, felsite, andesites, basalts of various degrees of fineness
and olivine-basalt, together with isolated grains of felspar, such as
might have been derived from the breaking up of some of these fragments.
Westward from Carrickbroad, the chief eruptive rock is a dark,
sometimes nearly velvet-black, flinty, occasionally almost resinous,
quartz-porphyry or rhyolite, with abundant quartz and large felspars
and occasional well-marked flow-structure. This material, near the
much smaller protrusion of andesite, is curiously mixed up with that
rock, as if the two had come up together. Sometimes they seem to
pass into each other, at least the separation between them cannot
be sharply drawn. There can be little doubt, however, that the acid
magma continued to ascend after the other, for it sends veins and
strings into the more basic material, and encloses blocks of it. This
thoroughly acid porphyry plays the same part as the andesite in regard
to the stones of the agglomerate. Throughout its whole extent, it is
found to enclose these stones, which here and there become so numerous
as to form the main bulk of the mass, leaving only a limited amount of
quartz-porphyry (rhyolite) matrix to bind the whole into an exceedingly
compact variety of breccia. Occasionally the acid rock cuts through the
ordinary clastic agglomerate, as may be well seen on the southern face
of Tievecrom.
A specimen of this porphyry with its enclosed fragments, which was
collected by me from above the old tower at Glendovey, Carrickbroad,
has been sliced and examined by Mr. Watts under the microscope, and is
thus described by him: "The large fragment in this slide consists of
ophitic olivine-dolerite full of large phenocrysts of olivine. It is
broken up and penetrated by veins of quartz-porphyry, rich in quartz,
which exhibits a beautiful flow-structure. The felspars and augite of
the dolerite do not appear to have suffered much alteration at the
margin of the fragment, but the olivines are much serpentinized, the
serpentine passing into a border of actinolite which runs in veins into
the neighbouring rock and even passes out into the quartz-porphyry at
the junction, impregnating it with actinolite and chlorite for some
distance. A few particles of basalt also occur and a portion of a
granite-fragment comes into the slide, from the edge of which a piece
of biotite has floated off into the quartz-porphyry."
The essentially non-volcanic material of the agglomerate shows, as Mr.
Nolan pointed out, that it was produced by æriform explosions, which
blew out the Silurian strata and granite in fragments and dust. These
discharges probably took place either from a series of vents placed
along a line of fissure running in a north-westerly line, or directly
from the open fissure itself. Possibly both of these channels of escape
were in use; detached vents appearing at the east end and a more
continuous discharge from the fissure further west.
After the earliest explosions had thrown out a large amount of
granitic and Silurian detritus, andesitic lava rose in the fissure,
and solidifying there enclosed a great deal of the loose fragmentary
material that fell back into the chasm. Subsequently, and on a more
extensive scale, a much more acid magma ascended from below, likewise
involving and carrying up a vast quantity of loose stones, among which
are pieces of basalt and dolerite.
No evidence remains as to the extent of the material discharged
over the surface from this fissure. Denudation has removed all the
surrounding fragmental sheets as well as any lava that may have flowed
out upon or become intercalated among them. There remains now only
the cores of the little necks at the east end, and the indurated
agglomerate and lava that consolidated along the mouth of the fissure
or vents.
This is the only example of such a line of fissure-eruption which
has yet been met with in the British Isles. Its connection with the
eruptive masses of Slieve Gullion and Carlingford links it with
the Tertiary volcanic series. But no evidence appears to remain
regarding the epoch in the long volcanic period when the eruptions
from it took place. They may possibly date back to the time of the
plateau-basalts; but the abundant acid magma, which constitutes one of
their distinguishing characteristics, suggests that they more probably
belong to the later time when the main protrusions of acid material
took place. They suggest that coeval with the uprise of the great domes
of Slieve Gullion, Carlingford and the Mourne Mountains there may have
been many superficial eruptions of which, after prolonged denudation,
all trace has now been effaced.
2. The Antrim Region
Reference was made in Chapter xxxvii. to the occurrence of rhyolitic
conglomerate and tuff between the lower and upper series of basalts in
the Antrim plateau, and to the evidence furnished by these detrital
deposits either that masses of rhyolite appeared at the surface, or
that rhyolitic ashes were discharged from volcanic vents in the long
interval that elapsed between the two groups of basalt. The further
consideration of this question, and an account of the rhyolite bosses,
were reserved for the present chapter, that they might be taken in
connection with the other acid eruptions of Tertiary time in Britain.[426]
[Footnote 426: For an early account of the Antrim trachytic rocks, see
Berger, _Trans. Geol. Soc._ iii. (1816), p. 190. Professor Hull has
described the Tardree rock in the Explanation to Sheets 21, 28 and 29,
_Geol. Survey of Ireland_ (1876), p. 17, and has supposed it to be
older than the basalts, referring it to the Eocene period (_Physical
Geology and Geography of Ireland_, 2nd edit. (1891), pp. 87, 95).
Duffin (quoted by Mr. Kinahan) believed that "the trachytes occur at
the centre of eruption, and were probably poured out at the end of the
outburst." Du Noyer also (quoted by the same writer) thought them to be
newer than the plateau-basalts, and to have lifted up masses of these
rocks. Mr. Kinahan himself (_Geology of Ireland_, p. 172) has pointed
to the absence of any rhyolitic fragments between the basalts as an
argument against the supposed antiquity of the acid protrusions. A
petrographical account of the Tardree rock is given by Von Lasaulx in
the paper already cited, Tschermak's _Min. Pet. Mittheil._ (1878), p.
412. A more elaborate discussion of the petrography by Prof. Cole will
be found in the Memoir above referred to (_Scientif. Trans. Roy. Dublin
Soc._ vol. vi. 1896), and the geological relations of the rocks are
discussed by him in another shorter paper, _Geol. Mag._ (1895), p. 303.
See also Mr. M'Henry on the trachytic rocks of Antrim, _Geol. Mag._
(1895), p. 260, and _Proc. Geol. Assoc._ vol. xiv. (1895), p. 140.]
With one exception, all the known protrusions of acid material in the
Antrim area lie within the limits of the basalt-plateau (see Map. No.
VII.). They occur along a line at intervals for a distance of about 17
miles, from Templepatrick to a point four miles north of Ballymena.
It is worthy of remark that here again the line of protrusion has a
north-west trend. It not improbably indicates the position of a fissure
up which the acid material rose at various points.
The petrography of the rocks has been frequently discussed. They
include several varieties of rhyolite, generally rather coarsely
crystalline, but sometimes becoming compact, and even passing into
dark obsidian. No undoubted tuff occurs associated with them in any of
the exposures, nor do the rhyolites anywhere display structures that
point to their having flowed out at the surface.[427] That the masses now
visible may have communicated with the surface is quite conceivable,
but what we now see appears in every case to be a subterranean and not
a superficial part of the protrusion.
[Footnote 427: At Sandy Braes an exposure is visible of what at first
might be thought to be a volcanic conglomerate, but closer examination
shows the rock to consist of obsidian, which decomposes into a
clay, leaving round sharply-defined glassy cores enclosed in the
decayed material. The "banded rhyolites" do not exhibit any kind of
flow-structure that may not be met with in dykes and bosses. Nor have
any satisfactory traces been found of vesicular or pumiceous bands such
as might mark the upper surfaces of true lava-streams. Professor Cole
has described what he calls "The Volcanoe of Tardree" (_Geol. Mag._
July 1895). If the Tardree mass ever was a volcano, which is far from
improbable, its superficial ejections have long ago disappeared. At
least, after the most diligent search, I have been unable to discover
any trace of them, all that now remains appearing to me to be the neck
or core of protruded material.]
[Illustration: Fig. 370.--Intrusive rhyolite in the Lower Basalt group
of Antrim, Templepatrick.
1 1, Chalk; 2 2, Gravel; 3 3, Bedded basalt; 4 4, Rhyolite, intrusive.]
Most of the rhyolitic exposures are extremely limited in area--mere
little knobs, sometimes rising in the middle of a bog, and never
forming conspicuous features in the landscape. The relation of these
rocks to the basalts are generally concealed, but the isolation of
the small rhyolitic patches leaves no doubt that they are intrusive
as regards the surrounding basalts. This relation is well seen at
Templepatrick, where it was first observed by Mr. M'Henry of the
Geological Survey (Fig. 370). The rhyolite there forms a sill which has
been thrust between the basalts and the gravel that underlies them, the
basalts being bent back and underlain by the acid rock.[428]
[Footnote 428: The progress of quarrying operations during the last eight
years has somewhat destroyed the section as exposed in 1888. But we now
see that the basalt has not only been bent back but is underlain by the
acid rock.]
The largest and most interesting of the Antrim rhyolite tracts covers a
space of about ten square miles in the heart of the basalt-plateau to
the north-east of the town of Antrim. It rises to about 1000 feet above
the sea, and forms a few featureless hills, some of which are capped
with basalt. The best known localities in this tract are Tardree and
Carnearny. The rock is chiefly a somewhat coarse-textured lithoidal
rhyolite, but includes also vitreous portions.
[Illustration: Fig. 371.--Section across the southern slope of
Carnearny Hill, Antrim.
_a_ _a_ _a_, bedded basalts; _b_, rhyolite.]
Owing to the cover of soil and turf, the junction of this mass with
the surrounding basalts cannot be so clearly seen as in the sections
of the Inner Hebrides, and hence the stratigraphical relations of
the two groups are apt to be misunderstood. What is actually seen is
represented in Fig. 371. The lithoidal rhyolite emerges from underneath
the basalts which abut against its sloping surface, forming on the
north side of Carnearny Hill a steep bank about 150 feet above the more
gently inclined slope below. The basalts consist of successive nearly
level sheets of compact and amygdaloidal rock.
It is obvious that only two explanations of this section are possible.
Either the rhyolite was in existence before the basalts which flowed
round it and gradually covered it, or it has been erupted through these
rocks, and is therefore of later date.
The former supposition has been the more usually received. The rhyolite
has been supposed to form the summit of an ancient volcanic dome,
perhaps of Eocene age, which had been worn down before the outflow
of the plateau-basalts under which it was eventually entombed. Had
this been the true history of the locality, it is inconceivable that
of a rock which decays so rapidly as this rhyolite, and strews its
slopes with such abundance of detritus, not a single fragment should
occur between the successive beds of basalt which are supposed to have
surrounded and buried it. Though the several beds of basalt are well
exposed all round, I could not, on my first visit, find a trace of any
rhyolitic fragments between them, nor had Mr. Symes, who mapped the
ground in detail for the Geological Survey, been more successful. I
have since made a second search with Mr. M'Henry, but without detecting
a single pebble of the acid rock among the basalts. Yet it is clear
from the upper surfaces of some of these lavas that a considerable
interval of time separated their successive outflows, so that there was
opportunity enough for the scattering of rhyolite-debris had any hill
of that rock existed in the vicinity.
Again, little more than a mile to the east of Carnearny Hill, an
outlier of the basalts forming the prominent height of the Brown Dod
lies upon and is completely surrounded by the rhyolite, which along the
east side of the hill can be traced as it passes under the level sheets
of basalt. The line of junction ascends and descends on that flank of
the outlier, so that successive flows of basalt are truncated by the
acid rock. But I could find no rhyolitic debris between them.
It appears to me, therefore, that the relations between the two groups
of rock in this area are similar to those between the granophyres and
bedded basalts on the south side of Loch na Keal in Mull (p. 396). In
other words, the rhyolites have risen through the basalts, and are
therefore younger than these lavas. This conclusion is corroborated
by the actual proofs of the intrusion of rhyolite into the basalts at
Templepatrick.
All the known rhyolitic masses in Antrim are confined to the Lower
group of basalts.[429] And as they traverse some of its highest
members, they may be regarded as certainly younger than that group.
Mr. M'Henry, who first indicated this relation, suggested that the
rhyolites were erupted in the interval between the two basaltic
series, and he connected with their eruption the rhyolitic detritus
found in association with the iron-ore at so many places in Antrim. It
appears to me that this suggestion carries with it much probability.
The rhyolitic conglomerate of Glenarm proves that, in the long period
represented by the iron-ore and its associated group of sedimentary
deposits, there were masses of rhyolite at the surface, the waste of
which could supply such detritus. The resemblance between the material
of that conglomerate and the rhyolites now visible at Tardree and
elsewhere is so close that we cannot doubt that, if not derived from
some of the known rhyolitic protrusions, this material certainly came
from exposed masses that had the same general petrographic characters.
[Footnote 429: The only exception to this rule was believed to be that
of the mass at Eslerstown, four miles east of Ballymena, which, as
originally mapped, was shown as crossing from the Lower into the Upper
basalts. Mr. M'Henry, however, has recently ascertained that the acid
rock is entirely restricted to the area of the older group.]
While the rhyolite pebbles in the Glenarm conglomerate are distinctly
rounded and water-worn, showing that some prominences of acid rock were
undergoing active denudation at the time when this conglomerate was
laid down, the finer rhyolitic detritus in the tuffs of Ballypallidy
rather suggests the actual discharge of rhyolitic ashes during the same
period. But it would appear that the superficial outbursts of rhyolitic
material, whether in the form of lava or of tuff, were only of trifling
extent, or else that the interval between the eruption of the two
basalt-groups was so prolonged that any such superficial material was
then removed by denudation. The varieties of lithological character to
be met with among the acid protrusions of Antrim suggests a succession
of uprises of rhyolites differing from each other more or less in
composition and structure. Unfortunately the ground is generally so
covered with superficial accumulations, and the exposures of rock are
so poor and limited, that no sequence has yet been determined among the
several kinds of acid rock. The only locality where I have observed
clear evidence of such a sequence is on the old quarries half a mile
west of Shankerburn Bridge, and three miles north-west of Dromore,
County Down. A small boss of rhyolite there rises through the Silurian
strata. It consists partly of a coarse-grained lithoidal rhyolite, with
large smoky quartzes and felspars, and partly of a much finer textured
variety. The latter, on the south side of the small brook which
separates the quarries, can be seen to ascend vertically through the
coarse-grained rock into which it sends a projecting vein. Its margin
shows a streaky flow-structure parallel with its vertical wall and is
in places spherulitic. Here the closer-grained rock is certainly later
than the rest of the mass.
CHAPTER XLVIII
THE ACID SILLS, DYKES AND VEINS
i. THE SILLS
Not only have the acid rocks been protruded in small and large bosses,
they have also been injected as sills between the bedding-planes of
stratified rocks, between the surfaces of the basalt-beds, and between
the bottom of the plateau-basalts or of the gabbros and the platform
of older rock on which the volcanic series has been piled up. Every
gradation of size may be observed, from mere partings not more than
an inch or two in thickness, up to massive sheets, which now, owing
to the removal of their original covering of rock by denudation, form
minor groups and ranges of hills. Where the sheets are numerous, they
are usually small in size; where, on the other hand, they are few in
number, they reach their greatest dimensions.
It is not always possible to discriminate between bosses and large
irregular sills. A good illustration of the connection between these
two forms of intrusion will be cited from the island of Raasay, where a
widespread intrusive sheet is in part connected with a true boss.
In Mull, sills of acid eruptive rocks are profusely abundant throughout
the central mountainous tract between Loch na Keal and Loch Spelve. If
we ascend the slopes from the Sound of Mull, for instance, we have not
gone far before some of these sheets make their appearance. They are
usually dull granular quartz-porphyries, or granophyres, often only two
or three feet in thickness, and interposed between the beds of basalt
that form the mass of the hills. Along the crest of the ridge that
stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take
a prominent place among the ledges of basalt, basalt-conglomerate and
dolerite. The largest sheet in Mull is probably that which has thrust
itself between the base of the basalts and the underlying Jurassic
strata and crystalline-schists on the shore of the Sound of Mull at
Craignure. The porphyry of this sheet is referred to by Professor
Zirkel as only a finer-grained variety of the same quartziferous rock,
with hornblende and orthoclase crystals, which in Skye breaks through
the Lias.[430] On the south coast also, at the base of the thick basalt
series, similar porphyries have been injected into the underlying
strata; and under the great gabbro mass of Ben Buy similar protrusions
occur. But as we retire from the mountainous tract into the undisturbed
basalts of the plateau, these acid intercalations gradually disappear.
[Footnote 430: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. p. 54.]
In the islands of Eigg and Rum, excellent examples occur of the
tendency which the sheets of porphyry or granophyre manifest to appear
at or about the base of the bedded basalts. I have already alluded to
the boss or sheet at the north end of the former island. A still more
striking illustration occurs in Rum. All along the base of the great
mass of gabbro, protrusions of various kinds of acid rock have taken
place. The great mass of Orval, already described, is one of these.
Below Barkeval and round the foot of the hills to the south-east of
that eminence an interrupted band of quartz-porphyry may be traced,
from which veins proceed into the gabbros and dolerites.
But it is in Skye and Raasay that the intrusive sheets of the acid
group of rocks reach their chief development. They have been most
abundantly injected underneath the bedded basalts, particularly among
the Jurassic strata. A band or belt of them, though not continuous,
can be traced round the east side of the main body of granophyre, at
a distance of from a mile and a half to about three miles. Beginning
near the point of Suisnish, this belt curves through the hilly ground
for some five miles, until it dies out on the slopes above Skulamus.
It may be found again on the west side of the ridge of Beinn Suardal,
and on the moors above Corry, till it reaches the shore at the Rudh'
an Eireannich (Irishman's Point). It skirts the west side of Scalpa
Island, and runs for some miles through Raasay. Another series of sills
occurs below the basalts and gabbros in the Blaven group of hills.
Over a large part of their course, the rocks of the eastern belt rest
in great overlying sheets upon the Jurassic strata, which may almost
everywhere be seen dipping under them. From the analogy of other
districts, we may, I think, infer that the position of these sills here
points to their having been intruded at the base of the plateau-basalts
which have since been removed from almost the whole tract. Fortunately,
a portion of the basalts remains in Raasay, and enables us to connect
that island with the great plateau of Skye of which it once formed a
part. There can be no doubt that the basalts of the Dùn Caan ridge once
extended westwards across the tract of granophyre which now forms most
of the surface between that ridge and the Sound of Raasay. A thin sheet
of quartz-porphyry, interposed among the Oolitic strata, may be seen
a little inland from the top of the great eastern cliff and below the
position of the bedded basalts.
The great sheet, or rather series of sheets, which stretches
north-eastwards from Suisnish at the mouth of Loch Eishort in
Skye, consists of a rock which for the most part may readily be
distinguished in the field from the granitoid material of the
bosses. It appears to the naked eye to be a rather close-grained or
finely crystalline-granular quartz-porphyry, with scattered blebs
or bi-pyramidal crystals of quartz and crystals of orthoclase. At
the contact with adjacent rocks, the texture becomes more felsitic,
sometimes distinctly spherulitic (west side of Carn Nathragh, next
Lias shale). Under the microscope the rock is seen to be a fine-grained
granophyric porphyry or porphyritic granophyre. It caps Carn Dearg (636
feet) above Suisnish, where it covers a space of nearly a square mile,
and reaches at its eastern extremity (Beinn Bhuidhe), a height of 908
feet above the sea (Fig. 249). This rock rests upon a sill of dolerite,
and is apparently split up by it. But, as I have already stated, the
basic rock is probably the older of the two, and the granophyre seems
to have wedged itself between two earlier doleritic sheets. To the
north-west of Carn Dearg, above the northern end of the crofts of
Suisnish, the same sill, or one occupying a similar position, crops out
between masses of granophyre, and is intersected by narrow veins from
that rock.
Though severed by denudation, the large sheets of granophyre to the
east of Beinn Bhuidhe are no doubt continuations of the Carn Dearg
mass, or at least occupy a similar position. That they are completely
unconformable to the Jurassic strata is shown by the fact, that while
at Suisnish they lie on sandstones which must be fully 1000 feet above
the bottom of the Lias, only two miles to the east they are found
resting on the very basement limestones, within a few yards from the
underlying quartzite and Torridon sandstone. I do not think that this
transgression can be accounted for by intrusion obliquely across the
stratification. I regard it as arising from the eruptive rock having
forced its way between the bottom of the now vanished basalt-plateau
and the denuded surface of Jurassic rocks, over which the basalts were
poured. The platform underneath these granophyre sills thus represents,
in my opinion, the terrestrial surface before the beginning of the
volcanic period.
But there is abundant proof that though the intruded granophyre sills
followed generally this plane of separation, they did not rigidly
adhere to it, but burrowed, as it were, along lower horizons. Thus
on the south-east front of Beinn a' Chàirn, which forms so fine an
escarpment above the valley of Heast, the base of the granophyre,
after creeping upward across successive beds of limestone, sends out
a narrow tongue into these strata, and continues its course a little
higher up in the Lias. The same rock, after spreading out into the
broad flat tableland of Beinn a' Chàirn (983 feet), rapidly contracts
north-eastwards into a narrow strip which forms the crest of the ridge,
and at once suggests a much-weathered lava-stream. The resemblance
to a _coulée_ is heightened by the curious thinning off of the rocks
where the two streams emerge from the Heast lochs; it looks as if the
igneous mass were a mere superficial ridge which had been cut down by
erosion, so as to expose the shales beneath it. But that the granophyre
is really a sill becomes abundantly clear at its eastern end, where we
find that it consists of two separate sheets with intervening Liassic
shales. The structure of this interesting locality is shown in Fig.
372. In this instance also, there is evidence that the acid sills are
younger than the basic, for the upper sheet of granophyre sends up
into the overlying dark basaltic rock narrow vertical felsitic veins,
a quarter of an inch to an inch in width, which being more durable,
stand out above the decomposable surface of the containing rock, and
show their quartz-blebs and felspar crystals on the weathered surface.
Perhaps the most striking feature of the granophyre sills of Skye is
their general association with thinner basic intrusive sheets between
which they have insinuated themselves. This characteristic structure,
pointed out by me in 1888, has recently been more minutely mapped
in the progress of the Geological Survey. Mr. Harker has found the
typical arrangement to be the occurrence of a thick sill of granophyre
interposed between two sills of basalt, each of which is usually not
more than six or eight feet thick. Where the granophyre has been
intruded independently among the Lias formations, it does not assume
the regularity and persistence which mark it where it has followed the
course of basic sills.
[Illustration: Fig. 372.--Section across the Granophyre Sills at Loch
a' Mhullaich, above Skulamus, Skye.
_a_, Jurassic sandstones and shales; _b_, Jurassic dark brown
sandy shales; _c_, sills of basalt, some bands highly cellular;
_c´_, basalt-sill with veins of felsite rising into it from the
granophyre below; _d_ _d_, intrusive sheets or sills of granophyre.
]
"The acid rock," Mr. Harker observes, "is invariably the later
intrusion, for it sends narrow veins into the basalts, metamorphosing
them to some extent and frequently enclosing fragments of them. These
fragments are always rounded by corrosion, and show various stages
of dissolution down to mere darker patches as seen by the naked
eye. Such inclusions and patches are found in the marginal part of
a granophyre, where no continuous basalt occurs, but where the acid
magma has evidently in places completely destroyed the earlier basic
sheets between which it was forced. It seems probable that in all cases
a certain amount of solution of the basalt by the granophyre magma
took place at their contact, facilitating the injection of the later
intrusion and accounting for its persistent choice of the contact-plane
of two basalt-sills as the surface offering least resistance to its
injection."
These observations throw fresh light on the remarkable original
regularity and persistence of the basic sills. Where one of these
sills disappears above or below a granophyre sheet its probable former
presence is often indicated by corroded fragments of the basic in the
acid rock. Mr. Harker remarks that the acid magma seems to have been
"in itself less adapted than the basic to follow accurately a definite
horizon and to maintain a uniform thickness in its intruded sheets, but
could do both when guided by a pre-existing basalt-sill, or especially
when insinuated between contiguous basalt-sills." The corrosive action
of the acid magma on the surface of the basalt, which enabled it to
force its way more readily between the basic sills, might proceed so
far as partially or wholly to destroy these sills.
This solvent action may serve to explain some of the irregularities
of the granophyre intrusions. According to the same observer, such
irregularities are found "where the granophyre sheet and its encasing
basalt-sills are not co-extensive, or again where the two basalt-sills
separate, owing to one of them cutting obliquely across the bedding.
In the latter case, which is not common, the granophyre follows one of
the basalt-sills, necessarily parting from the other. When one of the
two guiding basalt-sills dies out, the granophyre may still continue,
following the sill which persists. If the latter also dies out, while
the granophyre is still in some force, the acid magma seems to have
been reluctant to travel beyond the limit of the basalt, but has drawn
towards it, and the granophyre presents a blunt laccolitic form, which
contrasts with the acutely tapering edge of a granophyre which dies out
before reaching the limit of its basalt-sills. If, on the other hand,
on reaching the limit of the basalt, the acid magma has been in such
force as to be driven further, it is usually found to lose something
of its regularity and to depart from the exact horizon which it has
hitherto followed. This seems to happen, for instance, in the Beinn
a' Chàirn sheet, which, when traced westward, is found to behave as a
'boss' and is obviously transgressive, having cut across the bedding of
the strata so as to enter the limestones, where it no longer behaves
in any degree as a sill. The district affords many examples of the
tendency of intrusive masses in general to cut sharply across the beds
when they enter a group of limestones."
More complex examples of acid sills are to be found where there have
been three or more basic sheets together. The great granophyre sheet
already referred to at Suisnish affords the best illustration of this
structure. Mr. Harker has noticed that "round most of its circumference
there is seen merely a single basalt-sill passing under the granophyre.
Probably there has been another similar sheet over the acid rock, but
if so, it has been removed by erosion, the granophyre itself forming
everywhere the surface of the plateau. On the southern side, however,
we see that the original basalt must have been at least triple, or
counting the uppermost member, now removed, quadruple. The granophyre
has forced its way in between the several members of the multiple
basalt-sill, the intermediate ones being thus completely enveloped.
They are evidently metamorphosed as well as veined by the granophyre,
and when traced onward they give place to detached portions which,
floating as it were in the acid rock, are soon lost."
It is seldom easy to determine where lay the vent or vents from which
the granophyre sills proceeded. Those of the Skye platform just
described may be chiefly concealed under some of the larger areas of
the rock, such as the sheets of Carn Dearg or Beinn a' Chàirn. But
in several places, in close association with the compound sills of
granophyre and basalt, Mr. Harker has found large dyke-like bodies of
the acid rock, which may with considerable probability be regarded
as marking the position of the channels by which the material of the
sills ascended. "These bodies," he remarks, "either occur isolated by
erosion, the sills or the parts of the sills presumed to have been
in connection with the dykes having been removed, or are only very
partially exhibited in direct connection with sills still remaining.
Where they can be examined in detail they are seen to be dykes varying
up to about 100 feet in width, but of no great longitudinal extent.
Between Suisnish and Cnoc Carnach they bear E.N.E., that is, at right
angles to the ordinary basic dykes of the district and parallel to the
general direction of the axes of folding, though further north they
change this trend, but still remain parallel to the strike of the Lias.
"These dykes are composed essentially of granophyre, identical with
that of the sills. In some cases, they are flanked with basalt-dykes
on one or both sides, or the former existence of such lateral dykes
is indicated by partly-destroyed inclusions of the basic rock in the
granophyre. The basalt found in these cases is identical with that
of the basic sills, and shows the same relation to the granophyre.
Discontinuity and failure of the basalt are commoner, however, in the
dykes than in the sills--a difference presumably attributable to more
energetic destructive action of the acid magma when it was hotter and
fresher. These supposed feeders of the granophyre sills are certainly
in some cases, and have possibly been in all, double or triple dykes.
The acid magma thus appears not only to have spread laterally along the
same platforms as the earlier basalts, but to have reached these levels
by rising through the same fissures which had already given passage to
the basic magma."[431]
[Footnote 431: MS. notes supplied by Mr. Harker.]
The granophyre sills which, as already stated, can be followed as an
interrupted band from Suisnish Point to the Sound of Scalpa, emerge
again beyond Loch Sligachan and also in the island of Raasay, where
a great sheet of the acid rock covers an area of about five square
miles. This tract has recently been mapped for the Geological Survey by
Mr. H. B. Woodward, who has found it to have been intruded across the
Jurassic series, a large part of its mass coming in irregularly about
the top of the thick white sandstones of the Inferior Oolite. But it
descends beneath the Secondary rocks altogether, and in some places
intervenes between the base of the Infra-liassic conglomerates and the
Torridon sandstone. Its irregular course transgressively across the
Mesozoic formations is probably to be regarded as another example of
the intrusion of the acid material preferentially along the line of
unconformability between the older rocks and the Tertiary basalts, now
nearly all removed from Raasay by denudation, though the intrusion does
not rigidly follow that line of division, but sometimes descends below
it.
The central portions of this Raasay granophyre possess the ordinary
structures of the corresponding rocks in Skye. They show a finely
crystalline-granular, micropegmatitic base, through which large
felspars and quartzes are dispersed. But at the upper and under
junction with the sedimentary rocks, beautiful spherulitic structures
are developed. This is well seen on the shore near the Point of
Suisnish (Raasay), where, below the Lias Limestones, the top of the
granophyre appears, and where its bottom is seen to lie on the Torridon
sandstone.
This granophyre sheet presents a further point of interest inasmuch as
it appears to have preserved one of the dyke-like masses which may mark
channels of escape from the general body of the acid magma below. Near
the Manse the section represented in Fig. 373 may be observed. Owing
to great denudation, the massive sheet of granophyre has been cut into
isolated outliers which cap the low hills, and the rock may be seen
descending through the Jurassic sandstones, which in places are much
indurated. It is observable that the amount of contact-metamorphism
induced by the granophyre sills upon the rocks between which they have
been injected is, in general, comparatively trifling. It is for the
most part a mere induration, sometimes accompanied with distortion and
fracture.
[Illustration: Fig. 373.--Section to show the connection of a sill of
Granophyre with its probable funnel of supply, Raasay.
_a_ _a_, Jurassic sandstones; _b_, granophyre.]
[Illustration: Fig. 374.--Granophyre sill resting on Lower Lias shales
with a dyke of basalt passing laterally into a sill, Suisnish Point,
Isle of Raasay.]
Although the intrusion of the granophyre sills has been subsequent to
that of the basalt-sheets with which they are so generally associated,
we may expect that as there is a series of post-granophyre basic dykes,
so there may be some basic sills later than the injections of the acid
sheets. The Raasay granophyre appears to furnish an example of such a
later basic intrusion. At the Point of Suisnish on that island I have
observed the relations shown in Fig. 374. There the dark shales of the
Lower Lias (_a_ _a_) are immediately overlain by the granophyre sill
(_b_), and are cut by a basalt-dyke which, when it rises to the base
of the granophyre, turns abruptly to one side, and then pursues its
course as a sill (_c_) between the granophyre and the shales. There can
be little doubt that this intrusion is later than the granophyre. Here
a basic sill is interposed at the bottom of the acid sheet; and is
visibly connected with the actual fissure up which its molten material
was impelled.
ii. THE ACID DYKES AND VEINS
Besides bosses and sills, the acid rocks of the Inner Hebrides take the
form of Dykes and Veins which have invaded the other members of the
volcanic series. Some of these have already been referred to; but a
more particular description of the venous development of the acid rocks
as a whole is now required.
As regards their occurrence and distribution, they present two phases,
which, however, cannot always be distinguished from each other. On the
one hand, they are found abundantly either directly proceeding from the
bosses (more rarely from the sills), or in such immediate proximity
and close relationship to these as to indicate that they must be
regarded as apophyses from the larger bodies of eruptive material. On
the other hand, they present themselves as solitary individuals, or in
groups at a distance of sometimes several miles from any visible boss
of granophyre. In such cases, it is of course obvious that though not
exposed at the surface, there may be a large mass of the acid magma at
no great distance beneath, and that these isolated dykes and veins do
not essentially differ in origin from those of which the relations to
eruptive bosses can be satisfactorily observed or inferred.
Considered as a petrographical group, these Dykes and Veins are marked
by the following characters. At the one extreme, we have thoroughly
vitreous rocks in the pitchstones. From these, through various degrees
of devitrification, we are led to completely lithoid felsites,
quartz-porphyries or rhyolites. Micropegmatitic structure is commonly
present, and as it increases in development, the rocks assume the
ordinary characters of granophyre. Occasionally the structure becomes
microgranitic in the immediate periphery of a boss wherein a granitic
character has been assumed. Viewed as a whole, however, it may be said
that the dull lithoid rocks of the dykes and veins can generally be
resolved under the microscope into some variety of granophyric porphyry
or granophyre.
A characteristic feature in the granophyric, felsitic or rhyolitic
dykes and veins is the presence of spherulitic structure (Figs. 375,
377). In some cases this structure is hardly traceable save with
the aid of the microscope, but from these minute proportions it may
be followed up to such a strong development that the individual
spherulites may be an inch or two in diameter, and lie crowded
together, like the round pebbles of a conglomerate. The structure is
a contact phenomenon, being specially marked along the margin of the
dykes, as it is on the edge of sills and bosses. In the Strath district
of Skye, Mr. Clough and Mr. Harker have observed that the spherulites
are apt to be grouped in parallel lines so as to form rod-like
aggregates along the walls, and that where the rock is fairly fresh the
centre of the dyke sometimes consists of glassy pitchstone, so that the
spherulitic felsite or granophyre is probably devitrified pitchstone.
Frequently flow-structure is admirably developed in these dykes, the
streaky layers of devitrification flowing round the spherulites and any
enclosed fragments as perfectly as in any rhyolitic lava (Fig. 378).
[Illustration: Fig. 375.--Weathered surface of spherulitic granophyre
from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size.]
In regard to their modes of occurrence, the dykes of acid material
differ in some important respects from those of basic composition. More
especially they are apt to assume the irregular venous form, rather
than the vertical wall-like character of ordinary dykes. They take
the form of dykes, particularly where their material has been guided
in its uprise by one or more already existent basic or intermediate
dykes, as in the compound dykes, already described. The conditions for
their production must thus have been essentially different from those
of the great body of the basic dykes. Their intrusion was not marked
by any general and widespread fissuring of the earth's crust, such as
prepared rents for the reception of the basalt and andesite dykes.
They were rather accompaniments of the protrusion of large masses of
acid magma into the terrestrial crust. This magma, as we have seen,
was often markedly liquid, and was impelled, sometimes with what
might be called explosive violence, into the irregular cracks of the
shattered surrounding rocks or into pre-existing dyke-fissures. Hence
long straight dykes of the acid rocks are much less common than short
irregular tortuous veins and strings.
[Illustration:
Fig. 376.--Plan of portion of the ridge north of Druim an Eidhne,
Glen Sligachan, Skye, showing three dykes issuing from a mass of
granophyre.
_a_, gabbros; _b_, granophyre; I. II. III., three dykes proceeding
from the granophyre. The arrows show the direction of dip of the
bands of gabbro.
]
Much difference may be noticed among the granophyre bosses in regard to
their giving off a fringe of apophyses. Thus, along the well-exposed
boundary of Beinn-an-Dubhaich in Skye, though the edge of the boss is
remarkably notched, hardly any veins deserving the name diverge from
it. On the other hand, the ridge of Meall Dearg at the head of Glen
Sligachan, already referred to, is distinguished by the number and
variety of the dykes and veins which proceed from the granophyre and
traverse the banded gabbros. As this locality has been elsewhere fully
described, I will give here only the leading structural features which
it presents.[432]
[Footnote 432: Professor Judd (_Quart. Journ. Geol. Soc._ vol. xlix.
(1893), p. 175) described the granophyre dykes of this locality as
inclusions of Tertiary granite in the gabbro, and cited them in proof
of his contention that the acid eruptions of the Western Isles are
older than the basic. Their true character was shown by me in a paper
published in the _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 212.]
[Illustration: Fig. 377.--Weathered surface of spherulitic granophyre
from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size.]
Within a horizontal distance of less than 100 yards three well-marked
dykes issue from the spherulitic edge of the Meall Dearg granophyre,
and run in a south-easterly direction in the handed gabbros (Fig. 376).
The most northerly of these is traceable in a nearly straight line for
800 feet. The central dyke, which can be followed for 200 feet or more,
rises as a band six to ten feet broad between the dark walls of gabbro
as represented in Fig. 379.
These dykes are marked by the most perfectly developed spherulitic
and flow-structures (Figs. 375, 377). Numerous detached portions of
other dykes and also irregular veins are to be observed cutting the
banded gabbros all over the ridge of Druim an Eidhne for a distance of
a mile or more. Many of these exhibit the same exquisitely beautiful
spherulitic and flow-structure displayed by the dykes which can
actually be traced into the main body of granophyre. The lines of
flow conform to every sinuosity in the boundary-walls of gabbro, and
sometimes sweep round and enclose blocks of that rock. The example
of this structure, given in Fig. 378, shows how these lines, curving
round projections and bending into eddy-like swirls, exhibit the
motion of a viscous lava flowing in a cleft between two walls of solid
rock. Sometimes the laminæ of flow have been disrupted, and broken
portions of them have been carried onward and enveloped in the yet
unconsolidated material. Certain portions of this dyke are richly
spherulitic, the spherulites varying from the size of small peas up to
that of tennis-balls. Occasionally two large spherulites have coalesced
into an 8-shaped concretion, and it may be observed in some cases that
the spherulites are hollow shells.
[Illustration: Fig. 378.--Plan of pale granophyric dyke, with
spherulitic and flow-structure, cutting and enclosing dark gabbro,
Druim an Eidhne.]
[Illustration: Fig. 379.--Dyke (six to ten feet broad) proceeding
from a large body of granophyre and traversing gabbro, from the same
locality as Figs. 375 and 377.]
A remarkable feature has been recently observed by Mr. Harker among the
abundant granophyre dykes and veins which intersect the gabbros and
older rocks, along the eastern flanks of the Red Hills of Skye between
Broadford and the Sound of Scalpa. Broad dykes of granophyre which
traverse the Cambrian limestone of that district might be supposed
at first sight to be cut off by the intrusions of gabbro. But closer
examination proves that their apparent truncation arises from their
suddenly breaking up into a network of small veins where they abut
against the basic rock. This structure evidently belongs to the same
type as that of the St. Kilda granophyre.
[Illustration: Fig. 380.--Section of intruded veins of various acid
rocks above River Clachaig, Mull.
_a_ _a_, basalt, dolerite, etc.; _b_ _b_, granophyre.]
Compound dykes and sills, where one or more of the injections has
consisted of acid material, have been already noticed as intimately
associated together in Skye (p. 162). Dykes of this nature are more
particularly abundant in Strath, especially along its eastern side. In
addition to the examples cited already from that district, I may refer
to other two which intersect the Middle Lias shales and limestones
in the island of Scalpa. They are both compound dykes, but the more
basic marginal bands are not always continuous, having possibly been
here and there dissolved by the acid invasion. Though they do not show
any distinct spherulitic forms, the presence of flow-structure is
indicated by the thin slabs into which the rocks weather parallel to
the dyke-walls. The rock in each case is a fine-grained felsitic mass,
with bi-pyramidal crystals of quartz. It is observable that where these
dykes come directly against the Liassic strata, the latter are more
seriously indurated than where they are traversed by the ordinary basic
dykes.
In the central mountainous tract of the island of Mull veins of
acid material are extraordinarily abundant. They probably proceed
from a much larger subterranean body of granophyre than any of the
comparatively small bosses of this rock which appear at the present
surface of the ground. They show themselves partly at the margins of
the visible bosses, but much more profusely in that tract of altered
basalt, with intrusive sheets and dykes of basalt, dolerite and gabbro,
which lies within the great ring of heights between Loch na Keal and
Loch Spelve. In some areas, the amount of injected material appears to
equal the mass of more basic rock into which it has been thrust. Pale
grey and yellowish porphyries and granophyres, varying from thick dykes
down to the merest threads, ramify in an intricate network through the
dark rocks of the hills, as shown in the accompanying illustration
(Fig. 380), which represents a portion of the hillside between Beinn
Fhada and the Clachaig River. Such a profusion of veins probably
indicates the existence here of some large mass of granophyre or
granite, at no great depth beneath the surface.
In Mull, as in the other islands of the Inner Hebrides, two horizons
on which protrusions of acid materials have been specially abundant,
are the base of the bedded basalts of the plateau and the bottom of the
thick sheets of gabbro. Dykes and veins of granophyre, quartz-porphyry,
felsite and other allied rocks are sometimes crowded together along
these two horizons, though they may be infrequent above or below them.
Illustrations of solitary veins in the midst of unaltered
plateau-basalts or in older rocks may be gathered from many parts of
the Western Isles. Some remarkable instances are to be seen among
the basalts that form the terraced slopes on the north side of Loch
Sligachan. Several thick dykes of granophyre run up the declivity,
cutting across hundreds of feet of the nearly level basalt-beds. Some
of them can be seen on the shore passing under the sea. They trend in a
S.S.E. direction towards Glamaig, and they are not improbably apophyses
from that huge boss, the nearest edge of which is three-quarters of
a mile distant. Another example may be cited from the basalt-outlier
of Strathaird, where two veins of felsite, one of them a pale flinty
rock showing flow-structure parallel to the walls, may be seen on the
west front of Ben Meabost. In this case, the veins are three miles and
a half from the granophyre mass of Strath na Creitheach to the north,
four miles from that of Beinn an Dubhaich to the north-east, and nearly
three miles from that of Coire Uaigneich at the foot of Blath Bheinn.
A special place must be reserved for the pitchstone-veins. Ever
since the early explorations of Jameson and Macculloch, the West of
Scotland has been noted as one of the chief European districts for
these vitreous rocks. From Skye to Arran, and thence to Antrim, many
localities have furnished examples of them, but always within the
limits of the Tertiary volcanic region. That all of the pitchstones
are of Tertiary age cannot, of course, be proved, for some of them are
found traversing only Palæozoic rocks, and of these all that can be
absolutely affirmed is that they must be younger than the Carboniferous
or even the Permian system. But, as most of them are unquestionably
parts of the Tertiary volcanic series, they are probably all referable
to that series. Not only so, but there is, I think, good reason to
place them among its very youngest members. It is a significant fact
that they almost always occur either in or close to granophyre or
granite bosses, the comparatively late origin of which has now been
proved.
[Illustration:
Fig. 381.--Pitchstone vein traversing the bedded basalts, Rudh an
Tangairt, Eigg.
]
The first pitchstone observed in Skye was found by Jameson on the
flanks of the great granophyre cone of Glamaig. Another rises on the
side of the porphyry mass of Glas Bheinn Bheag, in Strath Beg. Several
occur at the foot of Beinn na Callich. In Rum, I found a pitchstone
vein traversing the western slopes of the wide granophyre boss of
Orval. In Eigg, the well-known veins of this rock intersect the
plateau-basalts (Fig. 381), but they are accompanied, even within the
same fissure, with granophyre, and in their near neighbourhood lie the
masses of this rock already alluded to.[433] In Antrim, pitchstone and
obsidian occur in the midst of the rhyolite. The only marked exceptions
to the general rule, with which I am acquainted, are those of the
island of Arran. Most of the pitchstone-veins in that district traverse
the red sandstones which may be Permian. But none of them are far
removed from the great granite boss of the northern half of the island,
while large masses of quartz-porphyry, which strikingly resemble some
of those of Skye and Mull, lie still nearer to them. It is also worthy
of notice that pitchstone-veins rise through the Arran granite boss
itself, the probably Tertiary date of which has been already discussed.
[Footnote 433: For an account of the pitchstone veins of Eigg, see
_Quart. Journ. Geol. Soc._ xxvii. p. 299.]
This common association of pitchstone-veins with the Tertiary eruptive
bosses of acid rocks can hardly be a mere accidental coincidence.
It seems to prove a renewed extravasation of acid material, now in
vitreous form, from the same vents that had supplied the granitoid,
granophyric, porphyritic and felsitic varieties of earlier protrusions.
We must remember that the pitchstone-veins are not mere local glassy
parts of the larger bodies of granophyre or granite in which they
lie. Their margins are sharply defined; they are indeed in all
respects as manifestly intruded, and therefore later masses, as are
the basalt-dykes. Their occurrence, therefore, within the acid bosses
proves them to be younger than these members of the Tertiary volcanic
series. Whether they are also later than the latest basalt-dykes cannot
yet be decided, for I have never succeeded in finding an example of the
intersection of these two groups of veins and dykes. But, with this
possible exception, the pitchstones are the most recent of all the
eruptive rocks of Britain.
As a rule, the intrusive pitchstones occur as veins which cannot be
traced far, and which vary from a few yards to less than an inch in
width. They generally show considerable irregularity in breadth and
direction, sometimes sending out strings into the surrounding rock
(Fig. 381). The outer portions are not infrequently more glassy and
obsidian-like than the interior. Occasionally the vitreous character
disappears by devitrification, and the rock assumes the texture of a
compact felsite or of a spherulitic rock.
Among the later movements of the acid magma account must be taken here
of the pale fine-grained veins which have already been referred to as
traversing the granophyre bosses. These intrusions, so well seen in
the bosses of Skye and St. Kilda, are often so close in texture that
they may be called quartz-felsites. Their sharply-defined edges and
felsitic character suffice to separate them from what are termed "veins
of segregation." In at least one instance, that of Meall Dearg, already
cited, a mass of typical granophyre which has developed spherulitic
and flow-structures along its margin, and which sends out dykes having
the very same structures for a distance of several hundred feet across
the banded gabbros, is itself traversed by a dyke of precisely similar
character. Here we see that after the intrusion of its apophyses,
and after its own consolidation in the upper parts, the granophyric
magma that rose into rents in the solidified portion retained the same
tendency to produce large spherulites as it had shown at first.
The fine felsitic veins that traverse the granophyre of the Red
Hills are now being mapped by Mr. Harker during the progress of the
Geological Survey. He has not yet obtained evidence of the age of these
veins in relation to the latest basic dykes. He has observed that they
appear to be on the whole rather less acid than the material of the
surrounding bosses, though they were probably all connected with the
same underlying acid magma from which the bosses were protruded. A
somewhat similar relation has been noticed between older granites and
their surrounding dykes, as in Cornwall and Galloway.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map VII MAP OF THE TERTIARY VOLCANIC DISTRICT OF NORTH EAST IRELAND
The Edinburgh Geographical Institute Copyright J. G. Bartholomew]
CHAPTER XLIX
THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX
There can be no doubt that considerable alterations of level have
taken place over the volcanic areas of North-Western Europe since
the eruptions that produced the basalt-plateaux, These alterations
embrace general and local subsidences, and also dislocations by which
considerable displacements of the crust either in a downward or upward
direction have been effected.
i. SUBSIDENCES
The mere fact that in many places the lower members of the series of
terrestrial lavas have been submerged under the sea may be taken to
prove a subsidence since older Tertiary time. Along the west coast of
Skye this depression is well shown by the almost entire concealment
of the bottom of the plateau under the Atlantic. In the Faroe Isles
the subsidence has advanced still further, for not a trace of the
underlying platform on which the basalts rest remains above water. In
Iceland, too, the complete submergence of the base of the Tertiary
volcanic sheets points to a widespread subsidence of that region.
Another strong argument in favour of considerable depression may be
derived from a comparison of the submarine topography with that of the
tracts above sea-level. It is obvious that the same forms of contour
which are conspicuous on the land are prolonged under the Atlantic.
If we are correct in regarding the valleys as great lines of subærial
erosion, their prolongations as fjords and submarine troughs must be
considered as having had a similar origin. We can thus carry down the
surface of erosion several hundred feet lower than the line along which
it disappears under the waves.
I know no locality where this kind of reasoning is so impressively
enforced upon the mind as the west end of the Scuir of Eigg. The
old river-bed and its pitchstone terminate abruptly at the top of
a great precipice. Assuredly they must once have continued much
further westward, as well as the sheets of basalt that form the main
part of the cliff. Yet the sea in front of this truncated face of
rock rapidly deepens to fully 500 feet in some places. Had any such
hollow existed in the volcanic period it would have been filled up
by the long-continued outflowings of basalt. Making every allowance
for concealed faults and local subsidences, we can only account for
this submarine topography by regarding it as having been carved out,
together with the topography of the land, at a time when the level of
the latter was at least 500 feet higher than it is now.
The subsidence which is thus indicated along the whole of the
North-West of Europe probably varied in amount from one region to
another. We seem to have traces of such inequalities in the varying
inclinations of different segments of the basalt-plateaux. The angles
of inclination are almost always gentle, but they differ so much in
direction from island to island, and even among the several districts
of the same island, as to indicate that certain portions of the
volcanic plain have sunk rather more than other portions.
Thus in the Faroe Islands, where the bare cliffs allow the varying
angles of inclination to be easily determined, a general gentle dip
of the basalts in a south-easterly direction has been noted among the
central and northern islands by previous observers. This inclination,
however, is replaced among the southern islands by an equally gentle
dip towards the north-east. The centre of depression would thus seem to
lie somewhere about Sandö and Skuö. The highest angle of inclination
which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E.
at about 15°.
Among the Western Isles, also, where similar variations in the
inclination of the basalt-sheets are observable, it might be possible
by careful survey to ascertain the probable position of the areas of
maximum depression, and to show to what extent differential movements
have affected the originally nearly level volcanic floor. It would
doubtless be found that everywhere the dominant movement has been one
of subsidence. The vast outpourings of lava would tend to leave the
overlying crust unsupported, and to cause it to sink into the cavities
thus produced.
Perhaps the most extensive subsidence of this kind, at least that which
admits of most satisfactory investigation, because it still remains
above sea-level, is displayed by the vast hollow in the Antrim plateau,
which embraces the basin of Lough Neagh and the valley of the Lower
Bann. This depression measures about 60 miles in length by about 20
in breadth. Its axis follows the N.N.W. trend so characteristic of
the volcanic features of Tertiary time. The depression may be said to
involve the entire basaltic plateau of Antrim, for with the exception
of a few insignificant areas along the borders, especially on the east
side between Larne and Cushendall, the whole region slopes inward from
its marginal line of escarpments, which reach heights of 1800 feet and
upwards, towards the great hollow in its centre (see Map VII.).
Lough Neagh, which occupies the deepest part of this hollow, and covers
about one-eighth of the whole area of subsidence, is the largest sheet
of fresh water in the British Isles, for it exceeds 150 square miles in
extent of surface. Yet, for its size, it is one of the shallowest of
our lakes, its average depth being less than 40 feet. Its shallowness,
compared with its wide area, marks it out in strong contrast to most
of the larger British lakes. Its surface is only 48 feet above the
level of the sea.
The origin of Lough Neagh, the theme of various legends, has been
seriously discussed by different writers, but most exhaustively by the
late E. T. Hardman of the Geological Survey.[434] This author connected
the formation of the lake-basin with a series of large faults which
are found intersecting the rocks around the basin, and passing under
the water in a general north-easterly direction. He showed that these
faults have produced serious displacements of the strata, amounting
sometimes to as much as 2000 feet, and he believed that it was by the
concurrent effect of such dislocations that the depression of Lough
Neagh had been caused.
[Footnote 434: "On the Age and Formation of Lough Neagh," _Journ. Roy.
Geol. Soc. Ireland_, vol. iv. (1875-76), p. 170; also Explanation of
Sheet 35 of the _Geol. Surv. Ireland_ (1877), p. 72.]
It is possible that these displacements may have contributed to at
least the earlier stages in the history of the Antrim subsidence. They
have undoubtedly taken place after the outpouring of the basalts, for
these rocks are involved in their effects. But in the hollow of the
Bann valley north of Lough Neagh the faults which have been detected
in the basaltic plateau are few and trifling. The bold and bare
escarpments, that so clearly display the relations of the rocks, reveal
few traces of any important transverse dislocations. Nor has any proof
of large longitudinal faults parallel with the axis of depression been
obtained within the area of the Bann valley.
The earliest evidence for the existence of a lake on the site of the
present Lough Neagh has been supposed to be furnished by certain fine
clays, sands, seams of lignite and clay-ironstone, which have been
referred to the Pliocene period. These deposits have been regarded as
indicating the accumulation of fine sediment with drift vegetation
brought down into a quiet lake by streams entering from the south.
Their fresh-water origin was believed to be further corroborated by the
occurrence of shells belonging to the lacustrine or fluviatile genus,
_Unio_.[435]
[Footnote 435: These shells were regarded as forms of _Unio_ by the late
W. H. Baily; but Dr. Henry Woodward assigned them to _Mytilus_. See
Prof. Hull's _Physical Geology and Geography of Ireland_, 2nd edit. p.
101. The shells have been more recently dug out by Mr. Clement Reid,
who has found them to be the common _Mytilus edulis_.]
The thickness of this series of strata, their position above sea-level,
and their distribution are important parts of the evidence for the
geological history of the locality. At one place the deposits are said
to have been bored through to a depth of 294 feet, and Mr. Hardman
believed them to be not less than 500 feet deep. The same observer
found that they certainly reach a height of 120 feet above the sea,
and he was of opinion that in some places their height was not less
than 140 feet. The deposition of strata to the depth of 300 feet below
a level of 120 feet above the sea would, of course, entirely fill up
Lough Neagh, and spread over a large tract of low ground around it.
The pottery-clays and lignites, however, appear to be confined to the
southern half of the lake, from which they rise gently into the low
country around.
The distribution of these deposits and their extraordinary variations
in altitude, as described by Mr. Hardman, present great difficulties
in the attempt to regard them as the sediments of a Pliocene lake.
A more recent examination of the ground by Mr. Clement Reid of the
Geological Survey has led that able observer to believe that two
totally different groups of strata at Lough Neagh have been confounded.
He noticed the _Mytilus_-clay to be a dark blue mass full of derived
boulder-clay stones, and yielding _Mytilus edulis_ and seeds of a
sedge. This deposit cannot be Pliocene, but must be of Glacial or
post-Glacial age, possibly contemporary with the Clyde beds. The
junction of this clay with the pipe-clays is not at present seen,
but the lithological contrast between the two groups of strata is so
strong as to indicate their independence of each other. Mr. Reid found
the white, red and mottled pipe-clays with their masses of lignite
to present a strong resemblance to the Bagshot group in the Tertiary
series. It is possible, as already suggested, that the pipe-clays and
lignites may belong to the sedimentary zone that separates the lower
and upper basalts of Antrim. At all events they furnish no proof of
any Pliocene lake, and may not indicate more than a deeper part of the
depression in which the tuffs, lignites and iron-ore were laid down.
The existence of the _Mytilus_-clay shows that in Glacial or
post-Glacial times the valley of the Bann was a strait or fjord into
which the sea entered. Thick masses of drift have been laid down all
round and over the depression now occupied by Lough Neagh, insomuch
that had any older lake existed here in Glacial times, it could hardly
have escaped being filled up.
The observer, who from one of the basalt-heights looks down upon the
expanse of Lough Neagh and the broad peat-covered plain that continues
the level platform of the lake-surface down the valley of the Bann,
cannot but be impressed with the size of this wide hollow in the heart
of the Antrim plateau, and with the evident continuity of the whole
depression from the lake to the sea. If he be a geologist, he will
be further struck by the fact that while the Chalk and other older
rocks appear from under the basalt-escarpments all round the plateau,
at heights of many hundred feet above the sea, the floor of this
wide hollow is entirely covered with basalt. Had the depression been
merely due to denudation, the rocks that underlie the volcanic series
would have been exposed to view. The base of the basalts which, on
either side of the depression, is often more than 1000 feet above the
sea-level, sinks below that level in the hollow of the Bann and Lough
Neagh.
This inequality of position may have been partially brought about by
faults like those around Lough Neagh, and may thus have been begun long
before the Glacial period. But it appears to me to be mainly due to a
wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction
from the present coast up the valley of the Bann and the basin of Lough
Neagh to beyond Portadown.
We may conceive that after the cessation of the outflows of basalt,
the territory overlying the lava-reservoir that had been emptied would
tend to subside, partly by ruptures of the crust producing faults
and partly by a downward movement of a more general kind. In course
of time, these disturbances turned the drainage into the hollow now
traversed by the Bann. Denudation would necessarily accompany them, and
the surface of the country would be continually eroded and lowered.
Lough Neagh has been carefully sounded by the Admiralty, and its
chart affords much suggestive material for the consideration of the
geologist.[436] From the soundings there given it has long been known
that the lake deepens towards its northern end, and attains a maximum
depth of 102 feet. But it is not until we trace on the chart a series
of contour-lines for successive depths, as shown by the soundings, that
we realize the remarkable form of the lake bottom. We then discover
that below a depth of 50 feet a well-defined channel extends for
rather more than half the length of the lake. This channel begins to
be distinctly perceptible between Kiltagh Point and Langford Lodge. It
first runs in a northerly course on the west side of the centre of the
Lough, but when it comes into a line with Saltera Castle on the western
shore, it wheels round so as to conform to the curve of the Antrim
coast-line, which it follows northward until, about two miles from the
exit of the lake, its outline ceases to be traceable on the gently
shelving bottom. Its total length is thus about 12 miles.
[Footnote 436: Lough Neagh surveyed and sounded by Lieut. Thomas Graves,
R.N.]
There can hardly be any doubt that this channel is a former bed of the
River Bann. It occupies exactly the position which that stream would
take if the lake were drained, and its depth and breadth correspond to
those of the valley-bottom of the present river. If this conclusion be
accepted, some important conclusions may be further deduced from it.
1. The presence of a former course of the Bann on the bottom of Lough
Neagh proves the lake to be much younger than the Ice Age. The thick
boulder-clays and Glacial gravels which so encumber the country
around and descend under the lake, would assuredly have filled up the
river-channel had it existed at the time of their deposition. The
channel has obviously been cut out of these drifts since the Glacial
period. When the erosion took place, the present Lough Neagh could
not have existed, but the Bann followed a continuous course across
the plain which the lake now covers. The river probably maintained
its place for a long period, so as to be able to excavate so wide and
deep a bed in the drifts, if, indeed, it did not to some extent slowly
carve its bed out of the underlying basalts. It must be remembered that
sediment is being continually poured into Lough Neagh, and that some
of the silt must have accumulated in the submerged river-course, thus
lessening its depth and width. That the channel should still be so
marked may be used as an argument for the comparatively late date of
the subsidence.
2. The submerged river-course is a clear proof of subsidence. The
present Lough Neagh cannot be looked upon as a glacial lake formed by
rock-erosion or by irregular deposition of drift. Its floor must have
been a land surface when the Bann cut out its bed upon it. The whole
area has sunk down, the drainage has been arrested, and some 20 miles
of the course of the Bann are now under a sheet of shallow water.
This subsidence was not brought about by faults. It seems rather to
have resulted from a general sinking of the ground. The movement was
probably comparatively rapid, otherwise the river-course would hardly
have survived so well.
3. These inferences, based upon purely geological considerations, have
an interesting bearing upon the allusions to the origin of Lough Neagh
contained in some ancient historical documents. Various legends have
from an early period been handed down as to the first appearance of
this sheet of water. These myths, though differing in details, agree in
describing such a sudden or rapid accumulation of water as destroyed
human life, in a district which had previously been inhabited by man.
The earliest records indicate that the alleged catastrophe took place
in the first century of the Christian era.[437] It appears to me not
improbable that the tradition,thus preserved in these legends, may
have had its basis in the actual disturbance which, on geological
grounds, can be shown to have determined the existence of Lough Neagh.
Though the event may go back far beyond the first century, there can
be no doubt that, in a geological sense, it was one of the most recent
topographical changes which the British Isles have undergone.
[Footnote 437: For versions of the legends, see Dr. Todd's "Irish Version
of the Historia Britonum of Nennius," _Roy. Hist, and Archæol. Assoc.
Ireland_; Dr. Reeves' "Ecclesiastical Antiquities of Down," etc., p.
370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1
in _Journ. Roy. Hist. and Archæol. Assoc. Ireland_, vol. i. (1870-71),
p. 94; _Giraldus Cambrensis_, vol. v. cap. ix. p. 91--"de lacu magno
miram originem habente." Moore's well-known lines embody the popular
belief that round towers and other buildings were submerged by the
inundation.]
Thus the Antrim basalt-plateau, in addition to the high interest of its
volcanic history, has the additional claim to our attention that it
has preserved, more fully and clearly than any other of the plateaux,
the evidence for the latest subterranean movements that followed the
long series of volcanic eruptions during Tertiary time. It contains the
record of a post-Glacial subsidence that gave birth to the largest lake
in Britain.
ii. DISLOCATIONS
Though I have not observed any features among the Tertiary
basalt-plateaux of the British Isles that can be compared to the
remarkable rifts and subsidences of Iceland, it can be shown that these
piles of volcanic material have undoubtedly been fractured, and that
portions of them have subsided along the lines of dislocation.
Careful examination of the basalt-escarpments of the Inner Hebrides
discloses the existence of numerous faults which, though generally
of small displacement, nevertheless completely break the continuity
of all the rocks in a precipice of 700 or 1000 feet in height. Not
infrequently such dislocations give rise to clefts in the cliffs. Some
good illustrations of this feature may be noticed on the north side
of the island of Canna, where the precipice has been fissured by a
series of dislocations, having a hade towards the west and a throw
which may in some cases amount to about 20 or 25 feet. The cumulative
effect of this system of faulting, combined with a gentle westerly
dip, is to bring down to the sea-level the upper band of conglomerate
which further to the east lies at the top of the cliff. Again, the
basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch
Eynort, is traversed by a series of small faults. On the east side of
Skye and in Raasay, a number of faults, some of them having perhaps a
throw of several hundred feet, has been mapped by Mr. H. B. Woodward.
The largest dislocation observed by me among the basalt-plateaux of
the Inner Hebrides is that already referred to (p. 209), which runs at
the back of the Morven outlier, in the west of Argyllshire, from the
Sound of Mull by the head of Loch Aline to the mouth of Loch Sunart,
along the line of valley that contains the salt-water fjord Loch Teacus
and the fresh-water lakes Loch Durinemast and Loch Arienas. While the
Cretaceous deposits and the bottom of their overlying basalts rise but
little above the sea-level on the south-west side of this line, they
are perched as outliers on hill-tops on the north-east side, where they
rise to 1300 feet above the sea. The amount of vertical displacement
here probably exceeds 1000 feet. The fault runs in a north-westerly
direction, and has obviously been the guiding influence in the erosion
of the broad and deep valley which marks its course at the surface.
This dislocation is only the largest of a number by which the
basalt-plateau has been broken in the district of Morven. Their effects
are well shown in the outlier of basalt which caps Ben Iadain, where
two parallel faults bring down the lavas against the platform of
schists on which they lie (see Fig. 266).
Many faults have been traced in the Antrim plateau, and are represented
on the Geological Survey Maps. In general they are of comparatively
trifling displacement. Occasionally, however, they amount to several
hundred feet, as in those already referred to as occurring near
Ballycastle and around the southern part of the basin of Lough Neagh.
To what extent the dislocations that traverse the British Tertiary
basalts are to be regarded as comparable to those which in Iceland
have been referred to subsidence caused by the tapping and outflow of
the lower still liquid parts of lava-sheets must be matter for further
inquiry. So far as my own observations have yet gone, the faults do not
seem explicable by any mere superficial action of the kind supposed.
Where they descend through many hundreds of feet of successive sheets
of basalt, and dislocate the Secondary formations underneath, they
must obviously have been produced by much more general and deep-seated
causes.
It is conceivable that, if these dislocations took place during the
volcanic period, they broke up the lava-plains into sections, some
of which sank down so as to leave a vertical wall at the surface on
one side of the rent, or even to form open "gjás," like those of
Iceland. But it is noteworthy that the fissures, which have been filled
with basalt and now appear as dykes, comparatively seldom show any
displacement in the relative levels of their two sides. In Iceland,
also, the great lava-emitting fissures seem to be in general free from
marked displacements of that kind.
[Illustration: Fig. 382.--Reversed fault on the eastern side of Svinö,
Faroe Isles.]
The faults in the Inner Hebrides, so far as I have observed, are all
normal, and indicate nothing more than gentle subsidence. But among the
Faroe Islands I have come upon several instances of reversed faults,
which, in spite of the usually gentle inclinations of the basalts,
probably point to more vigorous displacement within the terrestrial
crust.
On the east side of Svinö a fault with a low hade runs from sea-level
up to the top of the cliff, a height of several hundred feet. It has
a down-throw of a few yards, but is a reversed fault, as will be
seen from Fig. 382. Another similar instance may be noticed on the
north-east headland of Sandö, where, however, on the upcast side, the
basalts appear as if they had been driven upward, a portion of them
having been pushed up into a low arch (Fig. 383).
[Illustration: Fig. 383.--Reversed fault on the north-east headland of
Sandö, Faroe Isle.]
When the Tertiary basalt-plateaux of the Hebrides and the Faroe
Isles come to be worked out in detail, many examples of dislocation
will doubtless be discovered. We shall then learn more of the amount
and effects of the terrestrial disturbances which have affected
North-Western Europe since older Tertiary time. In the meantime
evidence enough has been adduced to prepare us for proofs of very
considerable recent displacements even among regions of crystalline
schists, like that which has been disrupted by the Morven faults above
alluded to. While the study of the Tertiary volcanic rocks demonstrates
the vast general denudation of the country since older Tertiary time,
the proofs that these rocks have been faulted acquire a special
interest in relation to the origin and evolution of the topography of
the region.
CHAPTER L
EFFECTS OF DENUDATION
Among the more impressive lessons which the basalt-plateaux of
North-Western Europe teach the geologist, the enormous erosion of the
surface of this part of the continental area since older Tertiary time
takes a foremost place. He may be ready almost without question to
accept the evidence adduced in favour of a vast amount of denudation
among such soft and incoherent strata as those of the older Tertiary
formations of the south-east of England or the north-west of France.
But he is hardly prepared for the proofs which meet him among the
north-western isles that such thick masses of solid volcanic rocks have
been removed during the same geological interval.
To gain some idea of the amount of this waste we must, in the first
place, picture to our minds the extent of ground over which the lavas
were poured, and the depth to which they were piled upon it. Though we
may never be able to ascertain whether the now isolated basalt-plateaux
of Britain were once united into a continuous plain of lava, we can
be quite certain that every one of these plateaux was formerly more
extensive than it is now, for each of them presents, as its terminal
edge, a line of wall formed by the truncated ends of horizontal
basalt-sheets. And there seems no improbability in the assumption that
the whole of the great hollow from the centre of Antrim up to the Minch
was flooded with lavas which flowed from many vents between the hills
of ancient crystalline rocks forming the line of the Outer Hebrides on
the west, and those of the mainland of Scotland on the east.
It is certain that the depth to which some parts of this long hollow
were overflowed with lava exceeded 3000 feet, for more than that depth
of rock can be shown to have been in some places removed. The original
inequalities of surface were buried under the volcanic materials which
were spread out in a vast plain or series of plains, like those that
have been deluged by modern eruptions in Iceland. Owing, however, to a
general but unequal movement of subsidence, the lava-fields sank down
here and there to, perhaps, an extent of several hundred feet, so that
the old land-surface on which they began to be poured out now lies in
those places below the level of the sea.
I have shown that even during the volcanic period, while the lavas
were still flowing from time to time, erosion was in active progress
over the surface of the volcanic plain. The records of river-action in
Canna and Sanday, and the buried river-channel of the Scuir of Eigg,
prove that, while eruptions still continued, rivers descending from
the mountains of the Western Highlands carried the detritus of these
uplands for many miles across the lava-fields, swept away the loose
material of volcanic cones, and cut channels for themselves out of the
black rugged floor of basalt.
The erosion thus early begun has probably been carried on continuously
ever since. The present streams may be looked upon as practically the
same as those which were flowing in the Tertiary period. There may have
been slight changes of level, oscillations both upward and downward
in the relative positions of land and sea, and shiftings of the
water-courses to one side or other; but there seems no reason to doubt
that the existing basalt-plateaux, which were built up as terrestrial
areas, have remained land-surfaces with little intermission ever since,
although their lower portions may have been in large measure submerged.
In the existing valleys, fjords and sea-straits by which these plateaux
have been so deeply and abundantly trenched, we may recognize some of
the drainage-lines traced out by the rivers which flowed across the
volcanic plains. The results achieved by this prolonged denudation are
of the most stupendous kind. The original lava-floor has been cut down
into a fragmentary tableland. Hundreds of feet of solid rock have been
removed from its general surface. Outliers of it may be seen scattered
over the mountains of Morven, whence they look into the heart of the
Highlands. Others cap the hills of Rum, where they face the open
Atlantic. Several miles from the main body of the plateau in Skye,
a solitary remnant, perched on the highest summit of Raasay, bears
eloquent witness that the basaltic tableland once stretched far to the
east of its present limits.
Two lines of observation and of argument may be followed in the effort
to demonstrate how great the denudation has been since older Tertiary
time. In the first place, there is the evidence of the level or nearly
level sheets of basalt that form the plateaux, and, in the second
place, there is the testimony of the dykes, sills and bosses by which
these lavas have been disrupted.
1. The study of the denudation of the Tertiary volcanic rocks of
North-Western Europe is most satisfactorily begun by an attempt to
measure the minimum amount of waste which in certain places the
basalt-plateaux can be proved to have undergone. For the purposes
of this study, the stratification of the lavas and their nearly
horizontal, or at least very slightly disturbed, position afford
exceptional facilities. Amorphous rocks, such as granites and gabbros,
or even foliated masses like the old gneisses and schists, may have
been enormously denuded. Their mere presence at the existing surface
may be taken as proof of such waste, yet they furnish in themselves no
criterion by which the amount of removed material may be estimated.
But in the case of the basalt-plateaux, as in that of horizontal
sedimentary formations, the successive lines of superposition of the
component beds of the whole stratigraphical series supply admirable
datum-lines which, on the one hand, vividly impress the imagination
by the demonstration which they afford of the reality and magnitude of
the denudation, and, on the other hand, furnish a measure by which the
minimum amount of this denudation may be actually computed.
Availing ourselves of this kind of evidence it is easy to show that
valleys many miles long, several miles broad, and from crest to
bottom several thousand feet deep, have been excavated out of the
basalt-plateaux since the close of the volcanic period. And if this
conclusion can be demonstrated for these plateaux, it must obviously
apply equally to the rest of the country. We thus obtain a most
important contribution to the investigation of the origin and relative
age of the present topographical features of the surface of the land.
Let me give a few illustrations of the nature of the investigation
and of the results to which it leads. Throughout the Western and
Faroe Islands the level bars of basalt present their truncated ends
in the great escarpment-cliffs which wind mile after mile along their
picturesque coasts. Where they front the open sea, it is obviously
impossible to say how much further seaward they once extended. But
where they retire in fjords or sea-lochs, and sweep inland into glens,
it is easy to measure the distance from the bottom of the eroded hollow
to its bounding watersheds, and to estimate the amount of material
that has been worn out of it. The only uncertainty in this computation
arises from our inability to determine to what extent movements of
subsidence may have come into play to aid in the disappearance of the
basalts. Where the bottom of the lavas can be seen at the same level
on either side of an inlet, with no evidence of faulting, or where a
definite horizon in the volcanic series can be traced round the head
of a glen or sea-loch, the influence of underground movements may be
eliminated. The evidence of vast denudation is always visible, the
proofs of subsidence are much less frequently observable.
The island of Mull supplies many striking examples of the enormous
waste of the basalt-plateau. The Sound of Mull, for instance, has been
eroded out of the volcanic series for a distance of 20 miles, with a
mean breadth of about two miles. From the deepest part of this fjord
to the summit of the Mull plateau is a vertical height of 3600 feet.
The whole of this vast excavation has taken place since older Tertiary
time. On the opposite side of Mull the hollow of Loch Scridain has been
eroded to a mean depth of at least 1200 feet below the average level of
the surrounding plateau, with a breadth of rather more than a mile.
The scattered islands which lie to the west of Mull tell the same tale.
They are all outliers of the same basalt-plateau, and have not only
been greatly lowered by the removal of their upper lavas, but have been
separated by the erosion of long and deep hollows between them. Thus
from the summit of the Gribon cliffs in Mull to the deepest part of the
sea-floor between that precipice and the Treshnish Isles a vertical
depth of at least 2000 feet of rock has been removed since the basalts
ceased to be erupted.
I have referred to the impressive evidence of denudation displayed
on the west side of the island of Eigg. The vertical distance from
the summit of the Eigg plateau to the bottom of the submarine valley
between this island and Rum is about 1500 feet, but as that summit lies
below the original surface of the lava-field, the depth of rock which
has been removed must exceed 1500 feet. We thus learn that since the
close of the volcanic period the hollow between the islands of Eigg and
Rum has been eroded to this great depth.
Still more striking is the evidence of enormous waste presented by the
Faroe Islands. The cliffs there are loftier and barer, and the fjords
have been cut more deeply and precipitously out of the basalt-plateau.
I shall never forget the first impression made on my mind when the
dense curtain of mist within which I had approached the southern end
of the archipelago rapidly cleared away, and the sunlit slopes and
precipices of Suderö, the two Dimons, Skuö and Sandö, rose out of a
deep blue sea. Each island showed its prolongation of the same long
level lines of rock-terrace. The eye at once seized on these features
as the dominant element in the geology and the topography, for they
revealed at a glance the true structure of the islands, and gave a
measure of the amount and irregularity of the erosion of the original
basalt-plateau. And this first impression of stupendous degradation
only deepened as one advanced further north into the more mountainous
group of islands. Probably nowhere else in Europe is the potency of
denudation as a factor in the evolution of topographical features so
marvellously and instructively displayed as among the north-eastern
members of the Faroe group.
Availing ourselves of the datum-lines supplied by the nearly level
bars of basalt, we easily perceive that in many parts of the Faroe
Isles the amount of volcanic material left behind, stupendous though
it be, is less than the amount which has been removed. Thus the island
of Kalsö is merely a long narrow ridge separating two broad valleys
which are now occupied by fjords. The material carved out of these
valleys would make several islands as large as Kalsö. Again, the lofty
precipice of Myling Head, 2260 feet high, built up of bedded basalts
from the summit to below sea-level, faces the north-western Atlantic,
and the sea rapidly deepens in front of it to the surface of the
submarine ridge 200 to 300 feet below. The truncated ends of the vast
pile of basalt-sheets which form that loftiest sea-wall of Europe bears
testimony to the colossal denudation which has swept away all of the
volcanic plateau that once extended further towards the west.
Nevertheless, enormous as has been the waste of this plateau of the
Faroe Islands, we may still trace some of its terrestrial features
that date back probably to the volcanic period. Even more distinctly,
perhaps, than among the Western Isles of Scotland, we may recognize the
position of the original valleys, and trace some of the main drainage
lines of the area when it formed a wide and continuous tract of land.
A line of watershed can be followed in a south-westerly direction
from the east side of Viderö, across Borö to the centre of Osterö,
and thence by the Sund across Stromö and Vaagö. From this line the
fjords and valleys diverge towards the north-west and south-east.
There can hardly be any doubt that on the whole this line corresponds
with the general trend of the water-parting at the time when the
Tertiary streams were flowing over the still continuous volcanic plain.
Considerable depression of the whole region has since then sent the
sea up the lower and wider valleys, converting them into fjords, and
isolating their intervening ridges into islands.
The topography of the Faroe Islands seems to me eminently deserving of
careful study in the light of its geological origin. There is assuredly
no other region in Europe where the interesting problems presented
by this subject could be studied so easily, where the geological
structure is throughout so simple, where the combined influences of
the atmosphere and of the sea could be so admirably worked out and
distinguished, and where the imagination, kindled to enthusiasm by the
contemplation of noble scenery, could be so constantly and imperiously
controlled by the accurate observation of ascertainable fact.
2. Impressive and easily comprehended as are the proofs of denudation
supplied by the basalts of the plateaux, they are perhaps to a
geological eye less overwhelming than those furnished by the eruptive
rocks which have been injected into these plateaux. In the case of at
least the basic intrusions, we may reasonably infer that they assumed
their present position under a greater or less depth of overlying rock
which has since been removed. When, therefore, they are found at or
above the summits of the plateaux, they demonstrate that a vast amount
of material has been removed from these summits.
The argument from the position of the dykes has already been enforced.
It is absolutely certain that valleys several thousand feet deep
must have been excavated since these dykes were erupted, for had
such valleys existed at the time when the dykes were injected across
their site, the molten rock, instead of ascending to the tops of the
surrounding mountains, would obviously have rushed forth over the
valley-bottoms. I have shown that this reasoning applies not merely to
the volcanic districts, but to the whole surface of the country within
the region of dykes. Thus the uplands of Southern Scotland, and wide
areas in the Southern and Western Highlands, can be proved to have had
glens cut out of their mass to a depth of hundreds of feet since the
Tertiary volcanic period.
Not less convincing is the evidence afforded by the great eruptive
masses of gabbro. We have seen that these complex accumulations of
sills, dykes, and bosses include rocks so coarse in grain as to show
that they must have consolidated at some considerable depth, but that
they now appear in hill-groups 2000 to 3000 feet in height, the whole
of the original basaltic cover having been stripped off from them. But
these gabbro hills have been in turn traversed up to the very crests by
later basalt-dykes, which thus supply additional proof that the erosion
here has been stupendous.
The granophyre bosses tell the same tale. Though, like the domite Puys
of Auvergne, they may still retain, in their conical forms, indications
of the original shapes which their component material assumed at
the time of its protrusion, we may be confident that their existing
surfaces have been reached after the removal of much rock which once
lay above them. This inference is confirmed by the fact that these
eruptive bosses have been invaded by a younger system of dykes. The
black ribs of basalt which may be traced along their pale declivities,
which cross the glens that have been eroded in them and which mount
up to their very crests, prove that since the latest manifestations
of volcanic energy in the West of Scotland, extensive changes in the
topography of the land have been effected by the operation of the
subærial agents of degradation.
So much for what can be demonstrated. But how much more may, with the
highest probability, be inferred! The original limits of the plateaux
are unknown. The waves of the wide Atlantic now roll over many a square
league of the old lava-plains, and wide tracts of the islands and the
mainland from which the basalt has been entirely stripped, or where it
remains only in scattered outliers, were once deeply buried under piles
of lava-sheets. It would probably be no exaggeration to affirm that
over the British area, as well as over the Faroe Isles, the amount of
Tertiary volcanic rock that now remains, large as it is, falls short
in amount of what has been removed. The geologist who has made himself
familiar with the effects of denudation in other Tertiary volcanic
districts, such as Central France, Saxony and Bohemia, will be prepared
for almost any conceivable amount of erosion among the far older
volcanic series of the north-west of Europe.
To the student of the origin of the existing topography of the land
there is a profound interest in the demonstration which these volcanic
rocks supply of the vast changes which the terrestrial surface has
undergone within a period geologically so recent as older Tertiary
time. When, on the one hand, he finds himself more and more restricted
in his demands for time by the confident assertions of the physicist
that all the phenomena of geological history must have been comprised
within a few millions of years, and when, on the other hand, he watches
the seemingly feeble and tardy operations of the forces of denudation
and sedimentation which have played the chief parts in that history,
he may well be excused if sometimes he is apt to despair of ever
reconciling the facts which he observes with the physical deductions
that are somewhat dogmatically brought forward in opposition to his
interpretation of them. He may feel sure that his facts cannot be
gainsaid, and he may be unable to find any other way of comprehending
them save by the admission that they necessitate a liberal allowance
of time. Yet he may not feel himself to be in a position to offer any
valid objections to the arguments from physical considerations that
would so seriously abridge the length of time which geology requires.
In these circumstances it is some satisfaction to be provided with
definite measurements of the amount of geological change which has
been effected within a limited and relatively recent period of time.
This change has resulted from the operation of the same agents by
which it is still being carried on. No break in the history can be
detected. There is not the least reason to suppose that the agents
of denudation and sedimentation have, during the period in question,
differed in their rate of working. Their activity at the present time
is probably neither greater nor less than it was then. If, therefore,
during so recent an interval such a stupendous amount of material
has been worn away from the surface of the land and deposited on the
sea-floor as the Tertiary volcanic rocks demonstrate, the geologist
may surely contemplate without misgiving the lapse of time required
for the completion of older geological revolutions. He may oppose
to the arguments of the physicist the measurements and computations
which he himself makes from data which are at least as reliable as
the postulates whereon these arguments are based. The rate at which
denudation and sedimentation are now taking place has been measured
with tolerable accuracy, and a fair average for it has been obtained.
Whatever may be maintained as to this rate in early geological ages,
there can be no serious opposition to its being taken as fairly
constant since older Tertiary time. We are thus provided with data for
estimating the minimum amount of time that can have elapsed since the
volcanic plateaux began to be denuded. But as no relic remains of the
original upper surface of those plateaux, and as we are consequently
ignorant of how much rock has been removed from their highest surviving
outliers, it is obvious that such estimates are more likely to err in
understating than overstating the amount of time required.
It would be beyond the scope of the present volume to enter fully into
the measurements and calculations required for the adequate treatment
of this subject. I will merely illustrate my argument by again taking a
few data from the plateau of Mull. The original height of this plateau
is shown by the outlier of Ben More to have been at least 3200 feet.
If to this figure we add the portion of the basalt-group submerged
under the sea the height will probably be increased by several hundred
feet. But let us take 3000 feet as a moderate computation for the
average thickness of the volcanic series here at the close of the
plateau-period. Until a number of sections have been carefully plotted
from the Ordnance Maps, in order to ascertain with approximate accuracy
the average height of the present surface of the Mull basaltic plateau,
making due allowance for the vast erosion of the Sound of Mull and the
numerous glens and sea-lochs that traverse the island, any estimate
which may be offered as to this average must be merely provisional.
If, in the meantime, we suppose the present mean level of the plateau
to be 1000 feet above the sea, the difference between this amount and
the assumed original height will be 2000 feet. If, further, we take the
present average rate of degradation of the Mull plateau to be 1/6000 of
a foot in a year, which has been shown to be probably a fair estimate,
then the time required for the lowering of the Mull plateau from its
original to its present average level amounts to twelve millions of
years. Yet this period, vast though it be, does not carry us back even
as far as the beginning of Tertiary time.
* * * * *
In concluding this lengthened discussion of the Tertiary volcanic
history of Britain, I may, perhaps, usefully add a brief summary of the
leading features of the long record.
The region within which volcanic activity displayed itself during older
Tertiary time in the British Isles, if our estimate of its area is
restricted to those parts of the country where igneous rocks, probably
of that age, now appear at the surface, embraces the North of England
and of Ireland, the southern half and the west coast of Scotland--a
total area of more than 40,000 square miles. Over that extensive region
volcanic phenomena were displayed during an enormously protracted
interval of geological time. The earliest beginnings of disturbance may
possibly have started in the Eocene, and the final manifestations may
not have ceased until the Miocene period. So prolonged was the duration
of the eruptions, that enormous topographical changes from denudation,
and probably also considerable variation in the fauna and flora, alike
of land and sea, may have been effected.
Owing to some cause which has not yet in this relation been
investigated, but which is probably referable to secular terrestrial
contraction, the volcanic region underwent elevation, while, at the
same time, a vast subterranean lake or sea of molten rock existed
underneath it. Enormous horizontal tension thus arose, and at last
the stretched terrestrial crust gave way. A system of approximately
parallel fissures opened in it, having a general direction towards
north-west. The rapid and simultaneous production of such a gigantic
series of rents must have given rise to earthquakes of enormous
magnitude and destructive force. The great majority of the fractures,
doubtless, did not reach to the surface of the ground, though
probably not a few did so. Such was the potency of this development
of terrestrial energy, that the fissures ran through the most varied
kinds of rocks and the most complicated geological structures, crossing
even earlier lines of powerful dislocation, and yet retaining their
direction and parallelism for sometimes 50 or 100 miles.
Into the fissures thus formed the molten magma from underneath was
forced for many hundreds or even thousands of feet above the surface
of the subterranean lava-reservoir. Solidifying between the fissure
walls, it formed the crowd of basic dykes that stand out as the most
widespread and distinctive feature of the volcanic region.
Where the fissures reached the surface or near to it, the molten
rock would seek relief by egress in streams of lava. This probably
occurred in many places from which subsequent denudation has removed
all vestige of superficial volcanic manifestations. But, in the great
range of basalt-plateaux, from Antrim northwards through the chain
of the Inner Hebrides, there are still left abundant remains of the
surface-outflows. Like the modern lavas of Iceland, the molten material
probably flowed out sometimes from the open fissures, sometimes
from vents formed along the chasms. After the convulsions ceased
which produced the earliest dykes, the communication that had been
established between the magma-reservoir underneath and the air above
would be maintained, and repeated eruptions might take place, either
from the original fissures and vents or from others afterwards opened
by the volcanic energy.
As in the modern eruptions of Iceland, new fissures are successively
opened through the older lava-sheets, so in the Tertiary volcanic
areas, renewed ruptures of the earth's crust allowed later dykes to be
formed. The basalt-plateaux are traversed by such dykes, even up to
their highest sheets. It is impossible to say how often the process of
dyke-making may have been repeated. Not improbably it recurred again
and again during the building of the basalt-plateaux, and we know that
it was renewed even after the protrusion of the granophyre bosses which
mark one of the latest phases of volcanism in the region.
For a protracted geological period, with long intervals of quiescence,
various basic lavas (basalts, dolerites, etc.), with occasionally some
of intermediate composition (andesites, trachytes), and perhaps in
Antrim acid rhyolites, flowed out from fissures and vents until they
had filled up the hollows of the great valley, which then stretched
from the south of Antrim northwards between the west coast of Scotland
and the chain of the Outer Hebrides. In some places the accumulated
pile of these ejections even now exceeds 3000 feet in thickness, but
we cannot tell how much material has been bared away from its top
by denudation. The volcanic discharges consisted mostly of lava,
fragmentary materials being comparatively insignificant in amount and
local in origin, though layers of fine tuff and basalt-breccias occur
in all the plateaux. None of the erupted materials thicken towards any
centres that might be taken to mark volcanoes of the type of Vesuvius
or Etna. On the contrary, the persistent flatness and uniformity of
the volcanic series, and the thinning out of the separate beds in
different directions, show that the lavas issued from many points all
over the region. The positions of some of the actual vents can still
be ascertained. They are now filled sometimes with dolerite, sometimes
with coarse agglomerate.
The surface over which the lava flowed seems to have been mainly
terrestrial. Here and there, between the successive sheets of basalt,
the leaves, stems, and fruit of land-plants, sometimes in most perfect
preservation, may be observed, together with the remains of insects
and fresh-water fish. Distinct relics of old river-channels can be
recognized which have been buried under streams of lava. Among the
deposits left by these streams the uppermost layers are commonly dark
with decayed vegetation, while layers of coal are found here and there
between the basalts.
As the pile of erupted materials gradually thickened, and the
subterranean energy possibly grew feebler, the ascending magma was
forced between the layers of sedimentary strata underneath the basalts,
or between these strata and the overlying volcanic series, or along
any other plane of weakness in the terrestrial crust. In this way arose
the multitudinous sills or intrusive sheets.
When the great volcanic plateaux had been built up to a thickness
of several thousand feet, another remarkable episode in the history
occurred. At certain points large bodies of coarsely crystalline basic
rocks were pushed into and through the plateaux-basalts, upraising them
in dome-shaped elevations, and ultimately solidifying as dolerites,
gabbros, troctolites, picrites, etc. There is reason to believe that
the points of extravasation of these materials were mainly determined
by the positions of the larger or more closely clustered vents of the
plateau-period, where points of weakness consequently existed in the
terrestrial crust. Rising as huge bosses through such weak places,
the gabbros and associated rocks raised up the overlying bedded
basalts, and forced themselves between them, forming thus a fringe of
finer-grained intrusive sills and veins around the central banded and
amorphous masses of more coarsely crystalline material. Whether, in any
of these vast domes of upheaval, the summit was disrupted, so as to
allow the basic intrusion to flow out as lava at the surface, cannot
now be told, owing to enormous subsequent denudation.
The next chapter in the chronicle shows us that probably long after
the eruption of the gabbros, when possibly all outward symptom of
volcanic action had ceased, a renewed outbreak of subterranean activity
gave rise to the protrusion of another and wholly different class of
materials. This time the rocks were of a markedly acid type. They
included varieties that range from obsidians, pitchstones, flinty
felsites and rhyolites, through porphyries and granophyres, into
compounds which cannot be classed under any other name than granite.
These masses likewise availed themselves of older vents in the
plateaux, and broke through them. They now form huge conical hills,
which, in their outer aspect, and even to some extent in their inner
structure, recall the trachytic puys of Auvergne. But the granophyres
not only ascended through the basalt-plateaux and the gabbro-bosses;
they sent into these rocks a network of veins, pushed their way in huge
sheets or sills between the strata below, and actually incorporated
a considerable proportion of the basic materials into their own
substance. Around the bosses of gabbro and granophyre, the bedded
basalts have undergone considerable contact-metamorphism.
The gabbro and granophyre bosses of the Inner Hebrides demonstrate with
singular force how unreliable petrographical characters are as a test
of the relative age of rocks. No one, looking at hand-specimens of
these rocks, or even studying them in the field, would at first suspect
them to be of Tertiary date. They closely resemble rocks of similar
kinds in Palæozoic and even Archæan formations. Yet, of their late
appearance in geological time, there cannot be any possibility of doubt.
After the uprise of the granophyre, and the injection of the network
of felsitic veins, there came once more a period of terrestrial
convulsion, like that of the earliest basic dykes, but of less
intensity. Again, the crust of the earth over the volcanic region was
pushed upward and rent open by another system of parallel fissures.
Again, from a reservoir or basin of basic lava underneath, molten rock
was forced upwards into the rents, and thus another system of basic
dykes was formed. These dykes are found crossing those of earlier
date, and rising through the other volcanic rocks. They traverse the
plateau-basalts from bottom to top; they climb to the summits of the
gabbro mountains, and they even pursue their undeviating course over
the huge domes of granophyre. No proof has yet been found that from any
of these dykes there was a superficial outflow of lava. But so great
has been the subsequent denudation of the areas, that such outflows
might quite well have taken place, and have subsequently been destroyed.
Whether these basic dykes were the last manifestation of volcanic
energy in our region cannot yet be decidedly affirmed. So far as the
evidence at present goes, they are possibly older than another series
of acid veins and dykes (pitchstone, felsite, and granophyre), which
are found at many points from Antrim to the far end of the Inner
Hebrides. These protrusions traverse every other member of the volcanic
series, except some of the youngest basic dykes, and do not appear to
be themselves cut by any.
Since the close of the volcanic period considerable disturbance of
the basalt-plateaux has taken place. The whole volcanic region has
subsided, some districts having sunk more than others. In Britain the
most striking evidence of such depression is supplied by the basin
of Lough Neagh. But throughout the Inner Hebrides much of the lower
portion of the terrestrial lava-plateaux is now below sea-level. In the
Faroe Islands and in Iceland the subsidence has been still more marked.
Dislocations, also, sometimes amounting to more than a thousand feet
of displacement, have occurred among the volcanic masses. The bedded
basalts, originally on the whole nearly flat, have thus been broken up
into large blocks of country wherein the sheets are now inclined in
various directions.
One of the most important lessons taught by the Tertiary volcanic
series of the north-west of Europe is the extent of the denudation of
the land since the close of the volcanic period. The horizontal or
gently inclined layers of bedding among the basalts afford datum-lines
from which the minimum amount of material removed may be measured. As
a reasonable estimate it may be inferred that in the case of the Mull
plateau, for example, the average amount by which its surface has been
lowered since the close of the volcanic period cannot be less than 2000
feet. If the rate of lowering of the land-surface in western Europe
by subærial denudation be taken as 1/6000 of a foot in a year, then
the lapse of time required for the degradation of the Mull plateau
must amount to about twelve millions of years. Some such interval has
therefore elapsed since the last Tertiary volcanoes became extinct.
CHAPTER LI
SUMMARY AND GENERAL DEDUCTIONS
The foregoing chapters comprise a connected narrative of the history
of volcanic action in the area of the British Isles during the vast
succession of ages from the early Archæan dawn down to the latest
eruptions of Tertiary time. In this final chapter I propose to present
a brief summary of the facts of largest import and widest interest
which this protracted history has placed before us, together with a
statement of deductions which may be drawn from them regarding the
nature and progress of volcanism in the evolution of the globe.
1. Among the broad features which soonest arrest attention in such a
survey is the geographical position of the theatre of this volcanic
activity. In the distribution of volcanoes at the present time we are
familiar with their tendency to range themselves along continental
borders or in oceanic islands. The volcanic energy so conspicuous in
the geological history of Britain has shown itself along the western
or Atlantic margin of the European continent. When the eruptions have
not been actually on the land itself, they have taken place within
the shallow tracts near the land, where the lavas and tuffs have been
interstratified with sediments derived from the adjacent coasts.
Moreover the volcanic rocks in Britain are ranged along the greatest
length of the group of islands, in a general north and south line, from
the south of Devonshire to the far Shetlands. It is on the western
side of the country that they occur. East of a line drawn from Berwick
by Leicester to Exeter, although the geological formations, ranging
from the Carboniferous Limestone to the latest Pleistocene deposits,
are there abundantly exposed to view, they include no contemporaneous
volcanic rocks.
2. A second and still more remarkable feature in the geological history
of Western Europe is the persistence of volcanic activity along the
site of the British Isles. Evidence has been brought forward in these
volumes that from the primeval time vaguely termed Archæan, onward
to that of the older Tertiary clays and sands of the south-east of
England--that is to say, through by far the largest part of geological
history, as chronicled in the stratified crust of the globe--this
long strip of territory continued to be intermittently a theatre of
volcanic action. Every great division of Palæozoic time was marked by
volcanic eruptions, sometimes over tracts hundreds of square miles in
area and on a colossal scale. After a long period of quiescence during
the Mesozoic ages, the renewed outbreak of volcanic energy in older
Tertiary time, so marked over the western half of Europe, reached its
maximum of development along the Atlantic border, from the north of
England and Ireland through the chain of the Inner Hebrides to the
Faroe Islands, Iceland and Greenland.
3. Not only has there been a remarkable persistence of volcanic
activity over the comparatively limited area of the British Isles,
viewed as a whole, but if we examine the different parts of this area
we perceive that many of them, of relatively restricted extent, have
been the sites of a recrudescence of volcanic action, again and again,
through a vast succession of geological periods. While the whole region
has been in different quarters and at different times affected, there
have been districts where the volcanic fires have been rekindled after
long intervals of quiescence, the new vents being opened among or near
to the sites of earlier volcanoes. In the south-west of England, for
example, the Middle Devonian tuffs and diabases were succeeded in the
Carboniferous period by the eruptions of the Culm-measures, and in the
very same tracts came last of all the lavas and tuffs of the Permian
conglomerates. Still more astonishing is the record of volcanic energy
in the south of Scotland, where, within a space of not many hundred
square miles, there are the chronicles of the Arenig, Llandeilo and
Bala eruptions of the Southern Uplands, the huge piles of lavas and
tuffs of the Lower Old Red Sandstone, the long succession of the
plateaux and then of the puys of the Carboniferous period, the groups
of tuff-cones of the Permian period, and, lastly, the numerous dykes
connected with the Tertiary volcanoes.
While some portions of the region have been specially liable to
exhibitions of volcanic action, others have continuously escaped. Some
of these "horsts," or stationary and unaffected blocks of country, have
been surrounded by or have risen close to the borders of this volcanic
district, yet have maintained their immunity through a long series of
ages. Thus the Central Highlands of Scotland, though they were flanked
on the south and south-west by the active volcanoes of the Old Red
Sandstone, and again on the south by those of Carboniferous time,
had no vents opened on their surface after the metamorphism of their
schists. Still more striking perhaps is the immunity of the Southern
Uplands. Though they were in large measure surrounded by the volcanoes
of the Lower Old Red Sandstone, then by those of the Calciferous
Sandstones and Carboniferous Limestone, and though they looked down on
the Permian eruptions of Ayrshire and Nithsdale, which spread streams
of lava and showers of ash along their flanks, these hills formed a
solid block that seems to have resisted perforation by the volcanic
funnels. Again, the tracts covered with Carboniferous Limestone in
England and Ireland almost entirely escaped from invasion by volcanic
eruptions.
We thus learn that even within comparatively restricted regions some
portions of the terrestrial crust have been areas of weakness, liable
to serve again and again as lines of escape for volcanic energy, while
close to them other portions of greater solidity have been persistently
left intact.
4. The sites of volcanic vents in all the geological systems wherein
they occur in Britain have not usually been determined by any obvious
structure in the rocks now visible. They comparatively seldom depend
on ascertainable lines of fault, even when faults, probably already
existent, occur in their near neighbourhood. This independence, to
which, however, there are occasional marked exceptions, comes out more
particularly in the coal-fields pierced by vents, for mining operations
have there revealed the positions of many more faults than can be
traced at the surface. If the sites of the vents have been fixed by
dislocations or lines of weakness in the terrestrial crust, these must
generally lie below the formations now visible at the surface.
There is one striking connection between the sites of the vents and
ancient topographical features to which frequent reference has been
made in the foregoing chapters. All through the long volcanic history,
as far back as such features can be traced, we see that orifices of
discharge for the erupted materials have been opened along low grounds
and valleys rather than on ridges and hills. The great central hollow
of the Scottish midlands was a depression even as long ago as the time
of the Lower Old Red Sandstone, and though it has probably been several
times since then filled up, and more or less completely effaced, its
ancient features have been partially revealed by extensive denudation.
This vast depression, 40 miles broad, between the Highland mountains
on the one side and the Southern Uplands on the other, was the chief
centre of volcanic activity in western Europe during the latter half of
Palæozoic time. The vents of the Old Red Sandstone, Carboniferous and
Permian series are scattered all over it, but few or none of them are
to be found on the high grounds that bound it. Again, in Tertiary time,
the great outpouring of lava took place in the hollow that lay between
the ridge of the Outer Hebrides and the mainland of Scotland. This wide
and long tract of low ground was buried under upwards of 3000 feet of
lava and tuff, but these materials were erupted from fissures and vents
within its own border and not from the mountains on either side.
But perhaps the most conspicuous example of any in which the vents keep
to the valleys is that supplied by the Permian necks of Nithsdale and
the neighbouring glens. These depressions are as old as Permian, and
even as Carboniferous time, but they appear to be entirely hollows of
erosion; at least they have yielded no evidence that their direction
has been determined by lines of fault. The chain of vents can be
followed from the lowlands of Ayrshire up to the base of the Southern
Uplands, down the wide valley cut by the Nith in these hills and up
some of the tributary valleys, and though the volcanoes continued for
some time in vigorous eruption, not a trace of any contemporary vent
has yet been met with on the surrounding hills.
While the position of volcanic vents in lines of valley may be
generally due to guiding lines of fissure in the crust underneath,
either within or below the rocks visible at the surface, there may
sometimes be conditions in which other dominant causes come into play.
The curious coincidence between variations in the upper limit of
dykes and inequalities in the configuration of the overlying ground,
suggest that where the subterranean magma has ascended to within a
comparatively short distance from the surface, a difference of a few
hundreds or thousands of feet in the depth of overlying rock, such as
the difference of height between the bottom of a valley and the tops
of the adjacent hills, may determine the path of escape for the magma
through the least thickness of overarching roof.
5. Volcanic phenomena cannot be regarded as a mere isolated and
incidental feature in the physics of the globe. During the short
time within which man has been observing the operations of existing
volcanoes, he has hardly yet had sufficient opportunity of watching
how far they can be correlated with other terrestrial movements. Nor,
when he endeavours to trace some such connection among the records of
the geological past, has he yet collected materials enough to furnish
a sufficiently broad and firm basis of comparison. One formidable
obstacle is presented by the difficulty in determining chronological
equivalents in separated groups of rock. Geologists have tried to
discover whether the volcanoes of some particular period or region
were in any way connected with such geological changes as extensive
plication, dislocations of the crust, or elevation of mountain-chains.
In regard to the volcanic history of Britain, various possible
relations of this kind obviously suggest themselves. Thus the division
of geological time comprised within the Lower Silurian period was
undoubtedly an interval of considerable terrestrial disturbance in
western Europe. The unconformabilities and overlaps in the series
of formations belonging to that period, the frequent conglomerates,
the great and often rapid changes in the thickness and lithological
characters of the strata, all point to instability of land-surface and
sea-floor. During these oscillations a prolonged and widespread series
of volcanic eruptions took place. The volcanic manifestations began
in Cambrian time and continued in intermittent activity till towards
the close of the deposition of the Lower Silurian formations. It is
certainly a significant fact that the Upper Silurian deposits, in
their lithological characters, present a strong contrast to those that
preceded them. They point, on the whole, to quiet sedimentation, during
an interval of comparative calm in the terrestrial crust. With this
evidence of tranquillity there is, over almost the whole of the British
Isles, an entire absence of any trace of renewed volcanic activity.
With the exception of the Dingle lavas and tuffs, in the extreme west
of Ireland, not a single undoubted instance is yet known of an Upper
Silurian volcano.
After the deposition of the Upper Silurian rocks an interval of great
terrestrial disturbance ensued, and these rocks over a large part of
Britain were intensely plicated and crushed. The movements, continued
into the period of the Lower Old Red Sandstone, were, in their later
stages, accompanied or, at least, followed by the vast outpourings of
lava which now cover so much of the tracts of Old Red Sandstone in
Scotland and Ireland.[438]
[Footnote 438: _Trans. Geol. Soc. Edin._ vol. ii. part iii. (1874).]
In proportion as the volcanic energy was vigorous, widespread and
long-continued, we may expect it to have been connected with important
terrestrial movements affecting extensive regions of the earth. The
Tertiary volcanic history seems to afford a remarkable instance of this
connection. A wide area of the European continent is dotted over with
old centres of volcanic activity which were in eruption at successive
epochs throughout the Tertiary period. Of all these centres the most
important was that of the north-western basalt-plateaux, where floods
of lava were discharged over many thousand square miles from Ireland
to Greenland. The geological date of these outpourings probably
coincides with the last great orographic movements that gave to the
mountain-chains of Europe their latest elevation and dimensions.
But without entering into what must be for the present a field of
speculation, we can be assured of one important fact in the connection
of ancient volcanoes with movements of the terrestrial crust. A study
of the records of volcanic action in Britain proves beyond dispute that
the volcanoes of past time have been active on areas of the earth's
surface that were sinking and not rising. We usually associate volcanic
action with elevation rather than subsidence, and there are certainly
abundant proofs of such elevation around active or recently extinct
volcanoes. Many of the active vents of the present time, like Vesuvius
and Etna, began with submarine eruptions and have been gradually
upraised into land. It may be, however, that such uprise is merely a
temporary incident, and that if we could survey the whole geological
period of which human history chronicles so small a part, we might find
that subsidence, and not upheaval, is ultimately the rule over volcanic
areas.
Be this as it may, there can be no question that with the one solitary
exception of the Tertiary volcanoes, which were terrestrial and not
submarine, all the British vents were carried down and eventually
buried under aqueous sediments. Even the Tertiary lava-fields have in
many places sunk down below sea-level since their eruptions ceased.
That there are any Palæozoic volcanic rocks now visible at the surface
is obviously due to subsequent movements not immediately connected with
their original conditions of eruption, and to gigantic denudation. The
amount of subsidence which followed on a volcanic episode was sometimes
enormous, even within the same geological period, as one may see by
observing the prodigious piles of sedimentary material heaped over
the lavas and tuffs of Arenig time, or over those of the Lower Old
Red Sandstone. I do not wish to maintain that the downward movement
was necessarily a consequence of volcanic ejections, for we know that
it took place over tracts remote from centres of eruption. But I have
sometimes asked myself whether it was not possibly increased as a
sequel to vigorous volcanic action; whether, for instance, the great
depth of the Palæozoic sedimentary rocks in some regions, as compared
with their feeble development in others, may not have been due to an
acceleration of subsidence consequent upon volcanic action.
6. A review of the geological history of Britain cannot but impress the
geologist with a conviction of the essential uniformity of volcanism
in its manifestations since the early beginnings of geological time.
The composition and structure of the materials erupted from the
interior have remained with but little change. The manner in which
these materials have been discharged has likewise persisted from the
remotest periods. The three modern types of Vesuvian cones, puys and
fissure-eruptions can be seen to have played their parts in the past as
they do to-day.
Among the earliest igneous masses of which the relative geological
date can be fixed are the dykes which form so striking a system among
the Archæan rocks of the north-west, and show how far back the modern
type of volcanic fissures and dykes can be traced. No relic, indeed,
has survived of any lavas that may have flowed out from these ancient
fissures, but so far as regards underground structure, the type is
essentially the same as that of the Tertiary and modern Icelandic
lava-fields.
The early Palæozoic volcanoes formed cones of lava and tuff comparable
to those of such vents as Vesuvius and Etna. In the Lake District the
pile of material ejected during Lower Silurian time was at least 8000
or 9000 feet thick. In the Old Red Sandstone basins of Central Scotland
there were more than one mass of lavas and tuffs thicker than those of
Vesuvius.
The puys of the later half of Palæozoic time closely resembled their
Tertiary successors in Central France, the Eifel, and the Phlegræan
Fields.
Nor, as regards extent and vigour, did the eruptions of the geological
past differ in any important respect from those of the present time.
There is assuredly no evidence that volcanic energy has gradually
waned since the dawn of geological history. The latest eruptions
of North-Western Europe, forming the Tertiary basalt-plateaux, far
exceeded in area, and possibly also in bulk of material discharged, all
the eruptions that had preceded them in the geological record.
7. Nevertheless, while the Tertiary eruptions showed no diminution
of vigour, it is undoubtedly true that the volcanic energy has not
manifested itself in a uniform way since the beginning of geological
time. There have been periods of maximum activity followed by others of
lessened force. Thus if we take a broad view of the general features
of volcanic action during the Palæozoic ages in Britain, we see
clear evidence of a gradual diminution in its vigour. The widespread
outpourings of lava and tuff in the Silurian period in England, Wales,
Scotland and Ireland were succeeded by the somewhat diminished,
though still important, eruptions of the Lower Old Red Sandstone
basins. The latter were followed by the still lessened outflows of
the Carboniferous plateaux, which in turn were succeeded by the yet
feebler and more localized eruptions of the Carboniferous puys, the
whole prolonged volcanic succession ending in the small scattered vents
of the Permian period. There were of course oscillations of relative
energy during this history, some of the maxima and minima being of
considerable moment. But though progress towards extinction was not
regular and uniform, it was a dominant feature of the phenomena.
8. The Permian volcanoes were the last of the long Palæozoic series,
and, so far as we yet know, the whole of the Mesozoic periods within
the area of Britain were absolutely unbroken by a single volcanic
eruption. The chronological value of this enormous interval of
quiescence may, perhaps, never be ascertainable, but the interval
must assuredly cover a large part of geological time. It was an
era of geological calm, during which the Triassic, Jurassic and
Cretaceous formations were slowly accumulated over the larger part
of Europe. The stratigraphical quietude was not indeed unbroken. The
widespread subsidence of the sea-bottom was interrupted here and
there by important upheavals, and considerable geographical changes
were in process of time accomplished. But, save in one or two widely
separated areas of Europe, there were no active volcanoes over the
whole continent.[439] Here again the scarcity or absence of intercalated
volcanic rocks is in harmony with the general stratigraphy of the
formations.
[Footnote 439: The Triassic eruptions of Predazzo and Monzoni were
important, and traces of others are said to occur in the Cretaceous
system in Portugal and Silesia.]
9. After the prodigious interval represented by the whole of the
Mesozoic and the earlier part of the Tertiary formations, a time
of disturbance arose once more, and the great basalt-floods of the
north-west were poured forth. Evidence has been adduced in the
foregoing chapters that this latest volcanic period was one of vast
duration; that it was marked by long intervals of quiescence, and by
repeated renewals of volcanic energy. Yet over the area of Britain the
whole of its manifestations were probably comprised within the earlier
(Oligocene and perhaps early Miocene) part of older Tertiary time.
Since its eruptions ceased, another interval of profound quiescence has
succeeded, which still continues. But this interval is almost certainly
of less duration than that which elapsed between the Palæozoic and
Tertiary outbursts. In other words, remote as the date of these
Tertiary volcanoes appears to be from our own day, it comes much nearer
to us than did the era of the last Permian eruptions to the earliest of
the Tertiary series.
10. By the dissection which prolonged denudation has effected among the
old volcanic centres of Britain, materials are supplied for studying
the sequence of events from the beginning to the end of a volcanic
period. These events have generally followed the same tolerably
well-defined order.
In the case of fissure-eruptions, rents formed in the crust of the
earth and communicating with the surface have allowed lava to rise and
flow out above ground, either from the lips of the fissures or from
vents opened along the lines of chasm. The thousands of parallel dykes
in Britain remain as evidence of this mode of the ascent of the molten
magma. Lines of large cones of the Vesuvian type may be presumed to
have risen along guiding fissures in the terrestrial crust.
But it is evident from a study of the British examples that the
existence of a fissure in the visible part of the crust is not always
necessary for the production of a volcanic vent. In hundreds of
instances, communication from the internal magma to the surface was
effected by successive explosions, which finally blew out an orifice
at the surface with no visible relation to any fissures or dykes. Of
course, beneath the formations that now form the surface, and through
which the necks rise, there may be lines of fault or weakness in older
rocks which we cannot see. But, in what can be actually examined, vents
have commonly been drilled through rocks independently of faults.
The discharge of explosive vapours was sometimes the first and only
effort of volcanic energy. Generally, however, fragmentary volcanic
materials were ejected, or, if the eruption was more vigorous, lava was
poured out. In a vast number of cases, especially in the later ages
of Palæozoic time, only ashes were projected, and cones of tuff were
formed. In the earlier ages, on the other hand, there was a much larger
proportion of lava expelled. Towards the close of a volcanic period,
the vents were gradually choked up with the fragmentary materials
that were ejected from and fell back into them. Occasionally, during
the process of extinction, an explosion might still occur and clear
the chimney, so as to allow of the uprise of a column of molten rock
which solidified there; or the sides of the crater, as well as of the
cavernous funnel underneath, fell in and filled up the passage. Heated
vapours sometimes continued to ascend through the debris in the vent,
and to produce on it a marked metamorphism.
There seems to have been commonly a contraction and subsidence of the
materials in the vents, with a consequent dragging down or sagging of
the rocks immediately outside, which are thus made to plunge steeply
towards the necks.
When the vents were plugged up by the consolidation of fragmentary
matter or the uprise of lava in them, the final efforts of the
volcanoes led to the intrusion of sills and dykes, not only into the
rocks beneath the volcanic sheets, but also, in many instances, into
at least the older parts of the sheets themselves. These subterranean
manifestations of volcanic action may be recognized in almost every
district. They vary greatly in the degree to which they are developed.
Sometimes, as in the Cader Idris, Arenig and Snowdon regions, they
attain considerable importance, alike as regards the number and
thickness of the sheets. In other cases, they are exhibited on so
small a scale that they might be overlooked, as in the tract of
Carboniferous puy-eruptions in the north of Ayrshire. But they are so
generally present as to form a remarkably characteristic feature of
the volcanic activity of each geological period from the earliest time
to the latest. The basic sheets in the Dalradian series of Scotland
display early and colossal examples. All through the successive
eruptive periods of Palæozoic time, sills are found as accompaniments
of superficial ejections.
The Tertiary basalt-plateaux supply numerous and gigantic examples
of intruded sheets. Tertiary cones of Vesuvian type are not found in
Britain, but where on the continent they have been sufficiently laid
open by denudation, they present sometimes an astonishing series of
sills. As a striking illustration of this structure reference may be
made to the sheets of trachyte that have been injected between and
have marmorized the Cretaceous strata on which Monte Venda stands,
among the Euganean Hills.[440]
[Footnote 440: G. vom Rath, _Zeitsch. Deutsch. Geol. Gesellsch._, xvi.
(1864), p. 461. E. Suess, _Sitzungsber. k. Akad. Wien._, lxxi. (1875),
p. 7; _Antlitz der Erde_, vol. i. p. 193. E. Reyer, _Die Euganeen_,
1877. This volcano is further referred to, _postea_, p. 477.]
It is obvious that the time of intrusion of the sills cannot be
precisely determined. They were not likely to be injected at an epoch
when the volcanic magma could find ready egress to the surface. That
they did not arise before such egress was obtained may be inferred
from their petrographical characters, which are usually those of the
later and not of the earlier outflows of the magma; and from the fact
that they not only lie among the rocks below the volcanic series, but
intersect the lower parts of that series, sometimes even the higher
parts. We may therefore, with every probability, regard the sills as
among the closing phases of a volcanic period.
As the lavas and tuffs of each volcanic period are intercalated among
the successive geological formations, a definite beginning and end to
the period are stratigraphically fixed. We see exactly where in the
sedimentary series the first showers of ashes fell, and where the last
mingled with the ordinary sand and mud of the sea-door. The same record
shows that the volcanic accumulations were finally washed down, that
they subsided with the rest of the ground around them, and that usually
they were buried under overlying conformable sedimentary deposits. Thus
cones of ashes and lava which may have been several thousand feet high
completely disappeared.
10. A consideration of the distribution of the volcanic rocks in time
shows not only how singularly uniform the course of volcanic activity
has been, but that there is no evidence of the cessation of any of the
broader petrographical types during geological history. Quite as much
variety may be observed among the erupted materials of Tertiary time in
Britain as among those of the early ages, when the earth was younger
and its volcanic vigour might be supposed to have been greater and more
varied than it is now. The table on the following page will make these
features at once apparent. From this table it will be seen that while
some of the acid rocks have not always been extruded, the basic masses
have played their part in every volcanic period.
11. A study of the volcanic products of a long series of eruptions
within the same geographical region may be expected to throw light
on the changes that take place during the course of ages in the
character of the internal molten magma. In a former chapter (vol. i.
p. 27) reference was made to the subject of volcanic cycles and to the
sequence, observed in various widely separated parts of the world,
among the materials erupted from below. Allusion was likewise made
in a later chapter (vol. i. p. 90) to the remarkable differences in
texture and composition noticeable within some large bodies of eruptive
material, and to the evidence which these differences furnish of a
segregation or differentiation among the constituents of an eruptive
mass after it has been injected into its position within the crust of
the earth.
Table of the Periods of Volcanic Action in the British Isles and of the
Chronological Distribution of the Volcanic Products.
Key to Columns
====================================
Gr = Granites, Granophyres, etc.
Fe = Felsites, Rhyolites, etc.
Da = Dacite, "Pitchstone" of Eigg.
Tr = Trachytes.
An = Andesites (Porphyrites).
Ga = Gabbros.
Do = Dolerites, Basalts (Diabases).
Pi = Picrites and highly basic lavas.
Tu = Tuffs, acid or basic.
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| | Gr | Fe | Da | Tr | An | Ga | Do | Pi | Tu |
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Older Tertiary | | | | | | | | | |
| (Plateaux, dykes, | | | | | | | | | |
| necks, bosses, | | | | | | | | | |
| sills) | * | * | * | * | * | * | * | * | * |
| | | | | | | | | | |
| Mesozoic | | | | | | | | | |
| No volcanic rocks.| | | | | | | | | |
| | | | | | | | | | |
| Permian | ··· | * | ··· | ··· | * | ··· | * | * | * |
| | | | | | | | | | |
| Carboniferous | ? | | | | | | | | |
| Puy type | ··· | * | ··· | ··· | * | ··· | * | * | * |
| Plateau type | ··· | * | ··· | * | * | ··· | * | * | * |
| | | | | | | | | | |
|{Devonian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * |
|{ | | | | | | | | | |
|{Old Red Sandstone | | | | | | | | | |
|{ Upper | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * |
|{ Lower | * | * | ··· | * | * | ··· | * | ···| * |
| | | | | | | | | | |
| Silurian | | | | | | | | | |
| Upper | ··· | * | ··· | ··· | ··· | ··· | ··· | ···| * |
| Lower, Bala | * | * | ··· | * | * | * | * | * | * |
| " Arenig | * | * | ··· | * | * | * | * | ···| * |
| | | | | | | | | | |
| Cambrian | ··· | * | ··· | ··· | * | ··· | * | ···| * |
| | | | | | | | | | |
| Uriconian | ··· | * | ··· | ··· | ··· | ··· | * | ···| * |
| | | | | | | | | | |
| Dalradian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| ? |
| | | | | | | | | | |
| Torridonian | | | | | | | | | |
| | | | | | | | | | |
| Lewisian | * | ··· | ··· | ··· | ··· | ··· | * | * | ··· |
| | | | | | | | | | |
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
From the history of volcanic action in the British Isles it is clear
that differentiation is effected under three distinct conditions.
In the first place, a notable difference may be occasionally observed
between two adjacent parts of the same mass of lava which has flowed
out at the surface. Thus, in the Carboniferous picrite of Blackburn,
there has been a separation of the heavy basic constituents, which have
in great part settled down into the lower part of the sheet, while the
lighter felspar has mainly come to the top. In this case the gradual
transition from top to bottom suggests that the separation occurred
after the lava had reached the surface and taken the form of a stream
or sheet.
In the second place, segregation has taken place in the magma within
the terrestrial crust after intrusion, for it is frequently observable
in large bosses and sometimes in sills, the basic elements having
tended to mass themselves towards the margins of the rock, leaving
more acid material in the centre. The cases of Garabol Hill among the
Dalradian schists of Scotland, of Carrock Fell among the Silurian
strata of the Lake District, and of the Cramond picrite among the
Carboniferous formations of Midlothian, with others that might be cited
from various other regions and geological formations in Britain, prove
to what a considerable extent a separation of ingredients may take
place in a boss, and even sometimes in a comparatively thin sill before
the molten mass consolidates.
In the third place, there is good evidence that already before the
magma is either intruded or extruded, and while it still lies within
the internal reservoir, it may not possess a general uniformity of
composition, but may have become more or less heterogeneous. In regard
to intrusive rocks, the extraordinarily banded gabbros of the Tertiary
series of Skye obviously proceeded from a magma in which the molten
material consisted in some parts mainly of felspar, and in others
mainly of the ferro-magnesian minerals and iron-ores. Streams from
these differently constituted parts of the magma were simultaneously or
successively injected as sills into the older portions of the volcanic
series, while, as the process of differentiation within the magma
proceeded, still more felspathic liquid was left behind, to be thrust
into cracks in the sills previously consolidated.
Moreover, the banded basalts of the Tertiary plateaux show that this
heterogeneity was not confined to internal intrusions, but maintained
its place even when the molten material was ejected to the surface.
The differentiation indeed is not so striking there as among the sills
of gabbro; but its presence, even in a less degree, proves that the
separation of constituent minerals was not due to any general cooling
of an erupted body of igneous rock, but was already developed in the
reservoir from which the molten material was propelled to the surface.
Attention has been called to the remarkable similarity of structure
between these banded intrusive rocks and some of the ancient gneisses.
The resemblance is so close that we may with every probability infer
that the gneisses acquired their characteristic banding as intrusive
masses of igneous rocks, discharged from heterogeneous magmas, like
that which supplied the gabbros of the Cuillin Hills. And as these
gneisses belong to pre-Cambrian formations, we are thus led to
the interesting result that the tendency to develop heterogeneity
was already as characteristic of the magma-basins of the earliest
geological time as it has been of those of later periods.
The evidence of differentiation presented by superficial lavas, and by
intrusive sills and bosses, acquires great interest when considered
in connection with the changes which are seen to have occurred in the
character of the materials erupted during the course of a definite
volcanic period. An attentive examination of the volcanic products of
the various ages, so fully recorded in the geological structure of the
British Isles, shows that a recognizable sequence in the nature of the
materials erupted during a single volcanic period can be traced from
the earliest to the latest times, and that, in spite of occasional
departures, the normal order remains broadly uniform.
With the important exception of the Snowdonian region and possibly
others, we find that the earlier eruptions of each period were
generally most basic, and that the later intrusions were most acid.
Thus the diabase-lavas and tuffs at the base of the Cambrian series of
St. David's are pierced by quartz-porphyry veins. The andesites of the
Lower Old Red Sandstone were succeeded by bosses, sills, and dykes of
granite, felsite, and lamprophyre. The eruptions of the Carboniferous
plateaux began with extremely basic lavas, and ended with trachytes,
felsites, and quartz-porphyries. The basalts of the great lava-fields
of the Tertiary period are pierced by masses of granophyre and even
granite.
There has evidently been, on the whole, a progressive diminution in
the quantity of bases and a corresponding increase in the proportion
of acid in the lavas erupted during the lapse of one volcanic period.
This sequence is so well marked and so common that it cannot be merely
accidental. The acid and basic rocks, occurring as they do at each
volcanic centre in the same relation to each other, are obviously parts
of one connected series of eruptions. We seem to see in this sequence
an indication of what was taking place within the subterranean magma.
There was first an extensive separation of the more basic constituents,
such as the ferro-magnesian minerals and ores, and the lavas which
came off at that time were heavy and basic basalts, and even picrites.
The removal of these elements left the magma more acid, and such rocks
as andesites were poured out, until at last the deeper intrusive
sills, dykes and bosses became thoroughly acid rocks, such as felsite,
quartz-porphyry and granite, while if any superficial outflow took
place it was such a rock as dacite.
In the case of the Tertiary volcanic series there is evidence that
after the acid protrusions a final uprise of basic material occurred.
No satisfactory proof of any similar return to basic eruptions has been
detected among the Palæozoic formations. But it is possible that some
of the basic sills and dykes, the precise age of which cannot be fixed,
may really mark such a reversion, even in the earlier volcanic periods.
Some illustrative examples of volcanic cycles from other countries
were cited in Chapter iii. To these I may add another instance which
presents a close analogy to some of the phenomena characteristic of
the British examples of Palæozoic as well as of Tertiary age. Monte
Venda in the Euganean Hills, already alluded to (p. 474), may be cited
as an interesting specimen of an older Tertiary volcano, which has
been so dissected by denudation as to show not only the succession
of its superficial discharges, but the position and order of its
subterranean intrusions. The volcanic eruptions of this neighbourhood,
judging from the area which they still cover and the height they
reach, may have piled up a mountain rivalling or surpassing Etna in
dimensions. In Monte Venda the lowest visible igneous rocks are sills
of oligoclase-trachyte that have been thrust between and have highly
altered Cretaceous (Tithonian) limestones. Other intrusive sheets
of trachyte follow in the overlying Cretaceous strata (Neocomian
and _Scaglia_). It is not until the older Tertiary formations are
reached that undoubted tuffs and lavas occur, indicative of truly
interstratified volcanic materials. These formations, consisting of
nummulitic limestones and other strata together with fossiliferous
tuffs, show that the volcano began as a submarine vent. It discharged
dark basic dolerites and tuffs. The highest lava, however, crowning
the summit of the mountain is a trachyte. There appears to have been
a rapid decrease of the bases in the magma, for the later lavas were
rhyolites, accompanied with rhyolitic tuffs of Oligocene age, and
followed in the end by the black vitreous trachyte of Monte Sieva.
12. From the evidence detailed in these volumes, it appears that the
sequence from basic to acid discharges was on the whole characteristic
of each eruptive period. It is obvious, however, that as the
protrusions of successive periods took place within the same limited
geographical area, the internal magma during the interval between
two such periods must in some way have been renewed as regards its
constitution, for when, after long quiescence, eruptions began once
more, basic lavas appeared first and were eventually followed by
acid kinds. This cycle of transformation is admirably exhibited in
Central Scotland, where the andesites of the Old Red Sandstone with
their felsite sills are followed by the limburgites, picrites and
other highly basic lavas at the bottom of the Carboniferous plateaux,
succeeded in turn by the andesites, trachytes and acid sills of that
series. When the puy eruptions ensued, the magma had once more become
decidedly basic.
That the true explanation of these alterations is of a complex order
may be inferred from the exceptions which occur to the general rule.
I have alluded to the Snowdon region, where the acid rhyolites are
followed by more basic andesites, and where the sills are also more
basic than the superficial lavas. In the Arenig and Cader Idris country
the sills are likewise more basic than the bedded lavas. Among the
Carboniferous puys of the basin of the Firth of Forth, the sills are
not sensibly more acid than many of the superficial basalts, and they
even include such rocks as picrite. Possibly in this last-named region
we see an arrested sequence, the volcanic protrusions having from some
cause ceased before the general uprise of the more acid magma.
INDEX
Aa form of lava in the Sandwich Islands, ii. 187
Abereiddy Bay, i. 206
Abich, H., i. 32
Acid igneous rocks, silica percentage of, i. 14;
devitrification of, 19;
flow-structure of, 21;
occur in thicker sheets than basic, 24;
alternations of, with basic, 28, 61, 152, 157, 165, 207, 213, 233,
284, 318; ii. 236, 266, 278;
metamorphic action of, i. 95, 96;
connection with mountains, ii. 98;
scenery of, 102.
Acids, mineral, at volcanoes, i. 72
Acland, Mr. H. D., i. 133
Aegean Sea, volcanoes of, i. 1
Agglomerates, i. 31, 57, 58;
in dykes, 70;
Archæan, 120, 130, 135;
Cambrian, 148, 149, 167;
Silurian, 178, 180, 181, 184, 185, 194, 199, 206, 214, 237, 241,
244, 247, 253, 255;
Old Red Sandstone, 279, 285, 289, 300, 313, 325, 338, 349, 352;
Carboniferous, 381, 399, 402, 404, 427, 429, 439, 440; ii. 13, 24,
28, 29;
Permian, 62, 64, 99;
Tertiary, 194, 277, 278, 281, 289, 292, 293, 384, 400, 423
Allan, T., i. 363
Allotriomorphic minerals, i. 21
Allport, Mr., i. 95, 130, 131, 260, 451; ii. 11, 42, 102, 103, 104,
106, 370
Amber in Tertiary volcanic series, ii. 198
America, Western North, volcanic rocks of, i. 10, 100; ii, 267
Amygdales, origin of, i. 15; ii. 189, 221, 285, 290
Amygdaloidal structure, i. 15, 16, 17, 59, 274, 385; ii. 3, 31, 57,
129, 188
Analyses of Cambrian tuffs, i. 148, 149;
of Cambrian diabases, 153;
of Old Red Sandstone diabases, 274;
of Old Red Sandstone andesites, 275;
of Old Red Sandstone trachytes, 276;
of Old Red Sandstone felsites, 278;
of Carboniferous limburgite, 377;
of Carboniferous basalts, 379;
of Carboniferous trachytes, 380;
of Carboniferous phonolite, 381;
of Tertiary trachyte, ii. 139;
of Tertiary dacite, 244
Anderson, Dr. Tempest, ii 261, 262, 263
Andesite, i. 24, 131, 136, 164, 165, 167, 178, 180, 184, 189, 190, 204,
212, 213, 214, 215, 229, 230, 245, 246, 247, 252, 274, 275,
(analyses), 277, 292, 300, 306, 309, 315, 318, 325, 330, 333, 345,
377, 379, 386, 403, 421; ii. 45, 57, 96, 125, 137, 184, 236, 424
Anglesey, gneisses and schists of, i. 126;
volcanic rocks of, 189, 219
Anhydrite deposits, ii. 54
Annandale, Permian volcanic rocks of, ii. 56, 58, 60, 61, 66
Antrim, Old Red Sandstone volcanic rocks of, i. 314;
Tertiary volcanic rocks of, 47, 52; ii. 109, 110, 113, 139, 140, 199;
basalts of, 192, 193, 199, 202, 206;
clays and iron-ore of, 204;
rhyolites of, 185, 364, 370, 371, 426, 445;
deceptive agglomerate of, 188;
rhyolitic conglomerate of, 195, 206;
plateau of, 199;
tuffs of, 202, 204;
vents of, 271, 277;
sills of, 298;
central subsidence of basalt-plateau of, 448
Apatite, ii. 135
Apjohn, J., ii. 42
Applecross, volcanic vents in, ii. 292
Arans, the, i. 175, 176, 179, 184, 186, 207
Archæan period, i. 110, 111;
volcanic rocks of, 120
Ardnamurchan, dykes and veins of, ii. 154, 320;
basalt-plateau of, 208;
vents of, 287;
sills of, 318;
gabbro of, 355
Arenig group, i. 175;
lower limit of, 177, 185;
top of, 178, 228, 246
---- volcano of, i. 42, 175, 176, 179, 186, 207
---- rocks in Scottish Highlands, i. 123, 126;
in Merionethshire, 176, 179;
of Shropshire, 189;
of Ayrshire, 196;
of Scottish Highlands, 201;
of Anglesey, 221;
of Lake district, 229;
of Ireland, 239
Argyll, Duke of, ii. 113, 114, 198
Argyllshire, dykes of, ii. 127, 128, 138, 142, 146, 171, 172;
vents of, 278
Arizona, explosion crater in, i. 58;
laccolites in, 86
Arran, Old Red Sandstone volcanic rocks of, i. 298, 311;
Carboniferous volcanic rocks of, 386, 392;
possible Permian volcanic rocks of, ii. 58;
granite of, i. 93; ii. 366, 367, 418;
pitchstone of, i. 19; ii. 445;
dykes of, 123, 139, 140, 142, 146, 154, 161
Arthur Seat, i. 364, 373, 378, 385, 386; ii. 67
"Arvonian," i. 145, 156
Asbestos in volcanic breccia, ii. 51
Ascension Island, cellular lava of, i. 15
Ashes, volcanic (_see_ Tuffs)
Ashprington volcanic series, i. 262
Asphalt, ii. 79
Atherstone, i. 170
Augite, loose crystals of, in volcanic vents, i. 62, 178, 181; ii. 58, 79;
lumps of, in volcanic vents, i. 352
Augite-aphanites, i. 178
Auvergne, old volcanoes of, i. 29, 32, 66, 70, 100; ii. 373
Aveline, Mr. W. T., i. 227, 230; ii. 32
Ayrshire, example of volcanic neck in, i. 56;
Silurian volcanic rocks of, 192;
Old Red Sandstone volcanic rocks of, 275, 282, 283, 285, 291, 331;
Carboniferous volcanic plateau of, 102, 368, 388, 393, 398, 410;
Carboniferous Puys of, 415, 416, 434, 440, 474;
Permian volcanic rocks of, ii. 55, 58, 62
Azoic period, i. 109
Bäckström, Mr., ii. 266
Baily, W. H., i. 251, 252; ii. 198, 449
Bala group, i. 175, 190, 196, 201, 206, 207, 223, 242;
limestone of, 47, 175, 229, 245, 251;
volcanic rocks of, 186, 190, 207, 213, 221, 241, 248
Balbriggan, igneous rocks of, i. 244
Ballagan beds (Lower Carboniferous), i. 384, 387, 392, 393, 412, 447
Ballantrae, volcanic rocks at, i. 192, 199
Ballypallidy, tuffs and leaf-beds of, ii. 204, 429
Bamborough, Whin Sill at, ii. 2, 3, 5
Banding of igneous rocks, i. 84, 207; ii. 189, 294, 329, 354, 357, 476
---- of gneiss, i. 116
Bangor group, i. 166
Banks, Sir Joseph, ii. 109
Barnavave, eruptive rocks of, ii. 421
Barrow, Mr. G., i. 201, 226, 272, 279, 380; ii. 147, 148
Basalt, columnar structure of, i. 24, 25;
relation to gabbro, 78;
altered by carbonaceous strata, 95;
shells supposed to occur in, ii. 110;
banded, 189;
thickness of sheets of, 192;
meaning of red layer between sheets of, 197, 203, 206, 254;
metamorphism of, 272, 276, 337, 339, 340, 347, 355, 356, 357, 358,
362, 378, 383, 386, 397, 399, 400, 404, 413
---- pre-Cambrian, i. 119, 131;
Silurian, 206, 207, 230, 245;
Carboniferous, 378, 403, 407, 417; ii. 11, 45, 46;
Permian, 57, 96;
Tertiary, 125, 136, 183, 199, 208, 291
Basalt-conglomerate, ii. 195
Basic volcanic rocks, silica-percentage of, i. 14;
devitrification of, 20;
flow-structure of, 21;
occur in thinner sheets than the acid, 24;
metamorphic action of, 94;
erupted at low levels, 98;
scenery of, 102;
converted into schists by deformation, 75, 114, 118, 119, 124, 129;
alternation with acid, 28, 61, 131, 157, 165, 207, 213, 233, 284, 318;
ii. 236, 266, 278
Bass Rock, i. 372, 373, 403
Bassenthwaite Lake, i. 335
Bathgate, puy eruptions of, i. 440, 442, 445, 456, 461
Bauer, Dr. M., i. 62
Bauxite, ii. 197, 204
Bayley, Mr. W. S., ii. 330
Bedding in lavas, i. 24
Bell, Sir I. Lowthian, ii. 1, 113, 137, 165
Bemrose, Mr. H. A., ii. 10, 11, 13, 16, 17, 18, 20, 21
Ben Cruachan, alteration of granite at, i. 343
---- Hiant, basic sills of, ii. 318
Benaun More, felsite of, i. 347
Berger, J. F., ii. 22, 95, 110, 113, 139, 140, 141, 145, 199, 364, 426
Bertrand, Prof. M., i. 28
Berwickshire, i. 272, 290, 338, 375, 385, 401, 413
Berwyn Hills, i. 176, 186, 208, 218
Biggar, volcanic area, i. 287, 325
Binney, E., ii. 56
Binny Craig type of basalt, i. 419, 421 (444)
Biotite (_see_ Mica)
Bitumen in intrusive rocks, i. 421
Blackstone (Derbyshire), ii. 18, 21
Blair-Atholl Limestone, i. 122
Blake, Rev. J. F., i. 126, 130, 144, 160, 161, 162, 163, 165, 166, 168,
220, 221, 222
Blocks, ejected, i. 36, 423, 438; ii. 197, 221
Bole between lavas, i. 442; ii. 197, 203, 206, 254
Bombay, volcanic plateau of, ii. 180
Bombs, volcanic, i. 60; ii. 39
Bonney, Prof., i. 95, 126, 130, 136, 144, 160, 162, 163, 164, 165, 166,
167, 168, 192, 210, 227
Borrowdale Volcanic Series, i. 227
Bosses, volcanic, i. 56, 78, 88;
petrography of, 89;
differentiation in, 90; ii. 476;
granitic, i. 93;
metamorphism around, 94, 95;
conditions of their intrusion, 97, 98;
weathering of, 102
---- Silurian, i. 215, 235;
Old Red Sandstone, 277, 288;
Carboniferous, 403, 458
---- Tertiary, ii. 271, 284, 327, 366, 378, 395, 403;
boundaries of, 382;
relation to older eruptive vents, 280, 384, 399;
relation to plateau basalts, 386, 396, 402, 404;
relation to gabbro intrusions, 391, 402, 404;
relation to the basic dykes, 395
Bostonite, ii. 47
Boué, Ami, i. 268, 363; ii. 112, 372
Boule, M., i. 27, 29, 44, 45, 46, 61; ii. 375
Boutan, M., i. 62
Bowden Hill, type of doleritic basalt, i. 418, 421
Braid Hills, great vent of, i. 289, 293, 311, 318, 323
Branco, Prof. W., i. 46, 417
Breccias, volcanic, i. 31, 32, 120, 131, 135, 147, 165, 189, 190, 197,
213, 224, 225, 233, 234, 246, 252, 255, 289, 347; ii. 39, 41, 49, 195
---- of non-volcanic materials, ii. 196, 423
Brecciated structure, i. 162, 211
Breidden Hills, i. 176, 190, 208
Brent Tor, ii. 33, 35, 36
Bréon, M. R., ii. 191
Britain, advantageous position of, for the study of ancient volcanic
action, i. 6;
completeness of the Geological Record in, 6;
direction of folds and fractures in, 11;
chief lavas found in, 31;
Vesuvian cones of, 42;
volcanic plateaux of, 43;
puys of, 46;
lacustrine volcanoes of, 49;
fissure eruptions of, 52;
scenery of volcanic rocks of, 100, 101;
pre-Cambrian rocks of, 111;
in Cambrian time, 141;
in Silurian time, 173;
in Devonian time, 258;
in Old Red Sandstone time, 263;
in Carboniferous time, 355;
in Permian time, ii. 53;
in older Tertiary time, 108
Brögger, Prof., i. 28, 88, 90, 91, 92
Bryce, J., i. 314, 369
Buch, L. von, i. 27; ii. 381
Buckland, W., ii. 95, 110, 113
Buddle, J., ii. 113
Builth, i. 176, 203
Burdiehouse Limestone, i. 361, 374, 388, 415, 463
Burnt Country of Asia Minor, i. 2
Burntisland, Binn of, i. 428, 429, 433, 435, 457, 459
Burntisland Sill type of dolerite, i. 418, 421
Busz, Mr. K., i. 261
Bute, Isle of, i. 369, 378, 407
Cadell, Mr. H. M., i. 114, 423; ii. 334
Cader Idris, volcanic rocks of, i. 42, 175, 176, 177, 178, 179, 180,
181, 182, 188, 207
Caer Caradoc, i. 131, 132, 170
Caerfai group (Cambrian), i. 155
Caernarvonshire, volcanic rocks of, i. 159, 207
Caithness Flags, i. 343, 352
---- volcanic vents in, i. 352
Calciferous Sandstones, i. 361, 366, 415
Calcite as a matrix of tuffs, ii. 27, 39, 41
Caldecote volcanic rocks, i. 170
Callaway, Dr. C., i. 126, 130, 132, 134, 220, 221
Calton Hill, lavas and tuffs of, i. 373, 378, 385, 386, 389
Cambrian system, i. 112, 123, 133, 139, 143, 144;
volcanoes of, 145, 159
Campbeltown, volcanic rocks of, i. 312, 386
Campsie Fells, i. 102, 368, 369, 384, 386, 389, 393, 397, 398, 400, 403,
410, 412, 447
Canary Islands, i. 27
Canna, basalts of, ii. 184, 187, 190, 215, 216;
vent in, 288
Cantyre, volcanic rocks of, i. 311, 369, 370, 386
Caradoc group, i. 175, 196
Carbonaceous rocks, influence of, on igneous masses, i. 95, 426, 449, 456;
ii. 65, 87, 104, 165
Carboniferous Limestone, origin of, i. 357
---- system, subdivisions of, in Britain, i. 358, 360, 366;
ancient geography of, 355, 361, 362, 432, 462;
flora and fauna of, 356
---- volcanic plateaux, distribution of, i. 364, 367;
nature of materials constituting, 377;
structure of, 383;
bedded lavas and tuffs of, 383;
vents of, 54, 394, 399
---- Puys, i. 46, 47, 308, 364;
of Scotland, 414;
nature of the materials erupted by, 416;
necks of, 424;
bedded lavas and tuffs of, 417, 436, 440;
sills of, 446, 472;
bosses of, 458, 465;
dykes of, 460;
of Derbyshire, ii. 8;
Isle of Man, 22;
of Somerset, 32;
of Devonshire, 32;
of King's County, 37;
of Limerick, 41
Carlingford, igneous rocks of, i. 96; ii. 175, 371, 420
Carnedd Dafydd, i. 209
Carnmony Hill, ii. 272
Carrock Fell, differentiation in rocks of, i. 91;
metamorphism at, 94, 96;
as a volcanic boss, 235, 236
Cement-stone group, i. 362, 366, 387, 418, 462
Cellular structure of volcanic rocks, i. 15, 33
Chalk, metamorphism of, by a dyke, ii. 164
Champernowne, A., i. 260, 262
Charnwood Forest, i. 134; ii. 53
Cherts associated with volcanic rocks, i. 123, 167, 169, 173, 174, 184,
196, 197, 240, 254; ii. 25, 36
Cheviot Hills, i. 102, 271, 272, 274, 275, 277, 278, 290, 293, 336
Chilled margin in intrusive rocks, i. 81, 83; ii. 126, 158, 160, 172,
299, 303, 310, 317, 321, 392, 402
Christiania, eruptive rocks of, i. 28
Chronology, volcanic, how determined, i. 46
Clark, Mr. G. T., ii. 180
Claystone, i. 277, 279, 318, 324, 327; ii. 403
Cleavage, effects of, on igneous rocks, i. 162, 165, 224, 231, 232, 234,
237, 260, 261; ii. 36
Clee Hills, ii. 101, 102
Cleveland Dyke, ii. 1, 122, 139, 140, 142, 144, 146, 147, 150, 153, 167,
168, 169
Clough, Mr. C. T., i. 114, 201, 236, 274, 290, 337; ii. 123, 124, 127,
128, 132, 137, 138, 142, 145, 146, 152, 162, 171, 172, 316, 384, 437
Clyde, Carboniferous volcanic plateau of, i. 368, 384, 385, 393, 400,
407, 411
Coal interbedded among volcanic rocks, i. 392, 423; ii. 198, 213, 251, 287
---- alteration of, at volcanic vents, i. 72; ii. 64
---- alteration of intrusive rocks by, i. 95, 451
---- alteration of by sills, dykes, etc., ii. 67, 164, 166
Coal-measures, i. 358, 360, 366
Coalbrookdale Coal-field, ii. 103
Cole, Prof. G. A., i. 176, 177, 178, 179, 180, 181, 184, 187, 188, 210,
211; ii. 134, 205, 212, 245, 370, 371, 378, 426
Colorado, Grand Cañon of, i. 30;
laccolites of, 86
Columnar structure, i. 25, 27, 343, 378, 385, 459; ii. 164, 186, 206, 301
Comley Sandstone, i. 144
Cones, volcanic, connection of, with necks, i. 70;
contemporaneous denudation of, 73; ii. 202, 218, 230;
entombment of, i. 433, 463; ii. 66
Conglomerates, volcanic, i. 31, 37, 183, 190, 286, 300, 307, 309, 310,
314, 315, 330, 341; ii. 195, 198, 218, 284
Coniston Limestone, i. 229, 231
Contemporaneity in Geology, 201
Continents, origin of, i. 11
Contraction, effects of terrestrial, i. 12, 97, 98
Conybeare, J. J., ii. 95
---- W., i. 171; ii. 9, 95, 110, 113, 199
Cooling, effects of, in inducing varieties of texture in igneous rocks,
i. 78, 79, 81; ii. 274, 275, 299, 303, 310, 317, 392, 402
Coon Butte, Arizona, i. 58
Cork, County, volcanic breccias of, ii. 49
Corndon, sill of, i. 176, 189, 190
Corston Hill, i. 373, 386, 387
Craiglockhart type of dolerite and basalt, i. 418
Crater, consolidation of tuff within a, i. 429
Crater-lakes, i. 58; ii. 266, 275
Cretaceous period, geography of the, ii. 108, 182
Cross Fell, i. 228, 229, 238
Cross, Mr. Whitman, i. 86
Crush-conglomerates or breccias, i. 32, 220, 223, 225, 244; ii. 281,
347, 352
Crushing, mechanical effects of, i. 315 (_see_ Schist)
Crust, contraction of the terrestrial, i. 12, 97, 98;
oldest rocks of, 110;
deformation of, 117, 121, 264, 295, 297
Cryptocrystalline type of basalt, i. 419
Crystallites of volcanic rocks, i. 18
Crystals, different periods of formation of, in volcanic rocks, i. 19, 20,
21, 421; ii. 128, 131, 134, 135;
ejected by volcanic vents, i. 62, 178, 180, 181, 195, 213, 234, 245;
ii. 27, 49, 58, 79
Cuillin Hills, scenery of, i. 106;
gabbro of, ii. 329, 361;
acid rocks of, 391
Culots, i. 78, 88
Culm-measures, ii. 33
Cumbrae Islands, i. 368, 369, 378, 407
Cumming, J. G., ii. 22
Cycles, volcanic, i. 27, 92; ii. 116
Dacite, i. 230; ii. 185
Dakyns, Mr. J. R., i. 90, 229, 272; ii. 10
Dalmellington, volcanic rocks at, i. 333; ii. 62
Dalmeny type of dolerite and basalt, i. 418, 420
Dalradian rocks, probable crushed necks of, i. 75, 125;
lavas and sills of, 121;
green schists of, 124
Dalry, Ayrshire, buried volcanoes of, i. 434
Dana, J. D., ii. 189
Darwin, C., i. 27
Daubrée, A., i. 72, 404
Davies, J., i. 156, 157
Dechen, H. von, i. 46; ii. 112, 280, 333, 340, 367, 372, 381
Deformation, effects of, on volcanic rocks, i. 75, 115, 117, 119, 121,
127, 129, 162
De la Beche, H., i. 142, 143, 170, 175, 204, 205, 207, 259; ii. 9, 10,
19, 33, 95, 96, 97
Delessite, ii. 79
Denudation, influence of, on volcanoes, i. 3, 4, 8, 40, 43, 45, 46, 54,
58, 71, 73, 75, 79, 87, 100-107, 370, 433, 434, 436, 476; ii. 55, 61,
62, 179, 181, 182, 241, 245, 248, 249, 255, 257, 282, 283, 292, 316,
317, 363, 373, 407, 455
Derbyshire, toadstones of, i. 359; ii. 8
Desmarest, ii. 373
Devitrification of volcanic rocks, i. 18, 19, 78; ii. 437, 446
Devonian system, i. 257;
volcanoes of, 259
Devonshire, volcanic scenery of, i. 103;
Devonian volcanic rocks of, 259;
Carboniferous volcanic rocks of, ii. 32;
Permian volcanic rocks of, 94
Diabase, i. 151, 153 (analyses), 156, 192, 194, 204, 206, 214, 217, 235,
240, 247, 249, 273 (analyses), 278, 292, 318, 320, 330, 335, 344, 345,
351, 403; ii. 5, 136, 415
Diabase-porphyrite, i. 192, 204
Diamond found in volcanic vents, i. 62
Dick, Mr. A., jun., i. 380
Dickson, Mr. E., ii. 23
Differentiation in igneous rocks, i. 22, 27, 84, 85, 90, 91, 449;
ii. 300, 475
"Dimetian," i. 145
Dingle-beds, i. 346
Dingle, Upper Silurian nodular lavas of, i. 20, 254
Diorite, i. 78, 247, 249, 277, 278, 288; ii. 36
Dirrington Law, i. 290
Dittmar, Prof., ii. 137
Dolerite, i. 119, 134, 178, 190, 206, 230, 247, 261, 378, 403, 407, 417,
448; ii. 5, 11, 35, 49, 102, 103, 104, 125, 136, 157, 183, 271, 299,
303, 307, 319, 328
Dolgelli, i. 169, 178, 188
Donegal, Dalradian rocks of, i. 122;
dykes in, ii. 124
Drogheda, volcanic rocks near, i. 244
Duffin, W. le S., ii. 426
Dumbarton, rocks near, i. 402, 404
Dumfoyn, a volcanic neck, i. 395, 398, 400
Dumgoyn, a volcanic neck, i. 395, 397, 398, 400
Dundee, sills and bosses near, i. 292, 306
Duneaton Water, volcanic rocks of, i. 329
Dunite, ii. 309
Du Noyer, G. V., i. 245, 250, 254; ii. 272, 426
Durham, Mr., i. 275
Durness Limestone, i. 112, 121, 123, 141
Dust, volcanic, i. 13
Dutton, Capt. C. E., i. 68; ii. 267
Dykes, vitreous margins of, i. 18;
formation of, 54, 98;
in necks, 66;
filled with agglomerate, 70;
grouping of, among intrusive rocks, 77;
character of, 79;
extent of, in Britain, 80;
age of, 81;
compound, 81; ii. 59;
expulsion of lava from, i. 82; ii. 128;
connected with the surface, i. 82;
pre-Cambrian, 118;
flow-structure in, 161;
Cambrian, 156;
Silurian, 187, 216, 235, 237, 248, 249;
Old Red Sandstone, 277, 291, 338, 345;
Carboniferous, 406, 429, 460; ii. 1, 30;
Permian, 83;
amygdaloidal structure of, 85
---- Tertiary, ii. 114;
arguments for their geological age, 118, 125, 171;
geographical distribution, 121;
two types of protrusion of, 122;
nature of component rocks of, 125;
external character of, 126;
classification of basic, 129;
enclosed fragments in, 129, 131, 144;
porphyritic and amygdaloidal structures of, 128, 129, 130;
veins in, 130;
joints in, 132, 166;
microscopic characters of, 134;
chemical characters of, 137, 139;
hade of, 139;
breadth of, 139;
interruptions of, 142;
length of, 142;
persistence of mineral characters of, 144;
direction of, 145, 159;
upward termination of, 147;
known vertical extension of, 150;
evidence of movement of molten rock of, 151;
branches and veins from, 152;
connection with sills, 155;
intersecting, 158;
compound or of more than one infilling, 159;
double, treble, and multiple, 160, 318, 417, 439;
compound, with basic and acid bands, 161, 435;
contact metamorphism of, 163;
relation of, to geological structure, 166;
origin and history of, 175;
Icelandic example of, 261;
example communicating with cinder cone in Utah, 268;
connection of, with surface, 179, 269, 280;
latest protrusions of, 381, 408, 416;
of granophyre, 435, 436, 437, 439
Earth, condition of the interior of the, i. 10;
fractures in crust of the, 11
Earthquakes, influence of, on early man, i. 1;
transient effects of, 3, 8
East Lothian, trachyte lavas of, i. 24;
Carboniferous volcanic plateau of, 370, 389, 403, 409
Edinburgh, volcanic rocks near, i. 24, 102, 104, 269, 273, 276, 279,
281, 285, 287, 289, 291, 293, 311, 317, 318, 323, 364, 370, 373, 385,
386, 387, 389, 410, 420, 436, 449
Egan, Mr. F. W., i. 242; ii. 201, 423
Eifel, i. 4, 46, 58, 100
Eigg, Isle of, ii. 115;
pitchstone of, 185, 217, 242, 445;
brecciated basalt in, 189, 192;
basalt plateau of, 215, 234;
Scuir of, 217, 234, 447;
sills of, 318;
acid bosses of, 403;
acid sills of, 431;
proofs of subsidence at, 447;
enormous denudation of, 239, 447, 458
Eildon Hills, i. 375
Electric Peak, i. 79, 82, 84
Elvans, i. 249, 281
Engulphment craters, i. 58
Ennerdale, granite of, i. 236
Enniscorthy, volcanic rocks near, i. 245
Environment, influence of, on early man, i. 1
Eozoic period, i. 110
Epidiorite, i. 118, 124, 129, 184, 247, 249
Erosion, laws of, i. 101
Eruptions, transient effects of, i. 3;
old submarine, how ascertained, 48;
lacustrine, 49;
fluviatile, 49;
terrestrial, 50;
evidence of intervals between, 283, 287, 300, 442; ii. 42, 59, 203,
205, 221, 251, 254, 287
Erzeroum, old volcanoes near, i. 32
Eskdale Dyke, ii. 127, 133, 136, 137, 140, 143, 145, 146, 153
---- (Lake District), granite of, i. 236
Etheridge, Mr. R, jun., ii. 24
Etna, i. 2, 4, 10, 55; ii. 261
Eurite, i. 188
Europe, basalt plateaux of north-western, i. 51, 52; ii. 181;
pre-Cambrian disturbances of north-western, i. 117
Explosion-craters, i. 58; ii. 266
Explosions, volcanic, i. 246; ii. 196, 266, 425, 472
Extrusive rocks, defined, i. 14;
textures of, 78
Fair Head, sills of, ii. 301
Farey, J., ii. 9
Farne Islands, ii. 2
Faroe Isles, basalt plateaux of, i. 52, 102; ii. 191, 192, 194, 256;
vents in, i. 63; ii. 293;
dykes of, 122, 133;
tuffs and lignites of, 258;
sills of, 322;
absence of gabbro bosses in, 355;
subsidence of, 447;
dip of basalts in, 448;
proofs of denudation in, 458
Faujas St. Fond, ii. 109, 112
Faults, connexion of volcanic vents with, i. 69; ii. 65;
boundary, i. 294, 303, 305, 369; ii. 169;
effects of, i. 446; ii. 200;
connection with, dykes, 168
---- of Tertiary basalt-plateau, ii. 452
Felsite (Felstone), Torridonian, i. 120;
Uriconian, 130, 133;
of Malverns, 134;
Cambrian, 151, 160, 161, 164, 165, 167, 168;
Silurian, 184, 199, 205, 206, 207, 210, 212, 218, 231, 232, 246, 247,
252, 255;
Old Red Sandstone, 276, 277, 291, 293, 321, 327, 335, 346;
Carboniferous, ii. 36, 49;
Permian, 85;
Tertiary, 174, 369, 424, 446
Felsitic breccia, ii. 195
---- type of devitrification, i. 19
Felspar, ejected crystals of, i. 181; ii. 58, 79;
large porphyritic crystals of, in dykes, 129, 135
Fife, Old Red Sandstone volcanic rocks of, i. 307;
Carboniferous volcanic rocks of, 428, 429, 430, 433, 437, 448;
Permian volcanic rocks of, ii. 56, 69
Fingal's Cave, i. 25; ii. 210
Fisher, Rev. O., i. 98
Fishguard, volcanic rocks at, i. 205
Fissure type of volcanoes, i. 42, 52; ii. 108, 115, 267
Fissures, volcanic, i. 42, 52, 53, 54, 425; ii. 141, 145, 159, 176;
filled with agglomerate, i. 70;
filled with dykes, 81, 118;
compound, 82, ii. 159;
pre-Cambrian, i. 118, 119;
Carboniferous, 425;
Tertiary, ii. 141, 159, 176, 425;
modern of Iceland, 262;
cause of, 177
Fleming, John, i. 268
Flow-structure, i. 21, 157, 160, 161, 162, 184, 210, 232, 246, 248, 255,
315, 321, 327, 346; ii. 129, 152, 190, 191, 332, 369, 392, 402, 424,
437, 441
Foot, F. J., i. 316
Forbes, D., ii. 370
---- Edward, ii. 66, 113, 114, 198
---- J. D., ii. 112, 333, 372, 381
Forellenstein, ii. 332
Forest of Wyre coal-field, ii. 102
Forfarshire, volcanic rocks of, i. 285, 299;
flagstones of, 299
Forster, M., ii. 113
Forth-basin, Carboniferous system of, i. 361;
Carboniferous plateaux of, 370;
Carboniferous puys of, 416, 427, 429, 430, 432, 434, 437, 440, 446, 462;
Permian volcanoes of, ii. 55, 67
Foster, Mr. C. le Neve, ii. 10
Fouqué, Prof., i. 18, 21; ii. 134
Fox, Mr. Howard, ii. 36
Fox Strangways, Mr. C., i, 135
Fragmental volcanic rocks, i. 14;
only arise from explosions which reach the surface, 57
(_see_ Agglomerates, Conglomerates, Tuffs)
France, Tertiary volcanoes of Central, i. 4, 10, 29, 41, 45, 49, 58, 60,
70; ii. 31, 271, 281, 373;
Carboniferous volcanic action in, i. 357
Frankland, Prof. E., i. 273, 278
Fundamental complex of oldest gneiss, i. 114, 115
---- gneiss, i. 115
Gabbro, granulitic, ii. 329;
banded structure of, i. 116; ii. 329, 354, 357, 476;
coarse-grained massive, 330;
pale varieties in veins, 330;
gneiss-like aspect of, 342, 254, 358
---- of Carrock Fell, i. 91;
Silurian, 195, 206, 247;
Devonian, 262;
Tertiary, 84, 90, 116; ii. 307, 308, 309, 319, 327, 334, 349, 355, 358,
391, 406, 407
Gairloch, peculiar pre-Cambrian rocks of, i. 115, 117
Galapagos Islands, i. 27
Gallaston type of dolerite and basalt, i. 418
Galloway, granites of, i. 93, 95, 272, 277, 290, 331
Garabol Hill, differentiation at, i. 90
Gardiner, Miss, i. 95
Gardiner, Mr. C. J., i. 256
Gardner, Mr. Starkie, ii. 196, 198, 212
Garlton Hills, i. 102, 370, 377, 378, 379, 380, 386, 390, 405, 412
Garnet found in volcanic vents, i. 62
Garth Grit, i. 177, 185, 208
Gases dissolved in the volcanic magma, i. 13, 15, 72, 97, 99
Geikie, Prof. J., i. 277, 306, 308, 331, 336, 339, 340, 369, 375, 426;
ii. 57, 191, 259, 322
Genèvre, Mont, i. 194
Geological action, supposed former greater intensity of, i, 139
---- contrasts, i. 103
---- history, i. 109, 113
---- Survey of Great Britain, i. 113, 115, 118, 119, 121, 122, 123, 124,
125, 126, 129, 130, 133, 135, 142, 143, 144, 145, 159, 160, 166, 170,
171, 175, 176, 179, 181, 182, 183, 186, 187, 188, 190, 196, 198, 201,
204, 205, 207, 208, 212, 214, 215, 216, 217, 218, 219, 220, 221, 225,
227, 228, 232, 233, 238, 239, 240, 242, 243, 244, 245, 250, 251, 254,
259, 270, 275, 278, 294, 299, 306, 307, 308, 314, 315, 317, 318, 325,
329, 331, 336, 339, 340, 344, 346, 349, 350, 352, 364, 369, 372, 373,
375, 397, 403, 404, 406, 407, 411, 423, 425, 434, 449, 462, 475, 476;
ii. 3, 4, 9, 10, 12, 13, 16, 17, 20, 23, 33, 36, 37, 42, 43, 46, 48, 49,
56, 58, 65, 66, 68, 94, 95, 102, 103, 118, 121, 125, 127, 144, 145, 148,
162, 164, 170, 174, 175, 190, 192, 199, 201, 203, 253, 272, 277, 292,
347, 384, 391, 420, 422, 423, 426, 428, 433, 435, 446, 449
Giant's Causeway, ii. 80, 109, 186, 188, 192, 206
Gilbert, Mr. G. K., i. 87; ii. 362, 363
Girvan, i. 192, 200
Glaciation, absence of, in Devonshire, i. 261
Glass in volcanic rocks, i. 18, 33, 60, 78, 180, 211, 216, 230, 232, 235,
316; ii. 85, 120, 126, 133, 135, 137, 184, 204, 247, 272, 285, 316, 317
Globulites, ii. 135
Gloucestershire, Silurian volcanoes of, i. 238
Gneiss, analogies of, with igneous rocks, i. 93; ii. 476;
oldest, i. 110, 115
Godwin-Austen, A. C., i. 259, 262
Goodchild, Mr. J. G., i. 229, 449; ii. 150
Grainger, Rev. Dr., ii. 198
Grand Sarcoui, ii. 373, 374, 381
Granite, bosses of, i. 88, 90, 93;
plutonic and volcanic, 89;
metamorphism by, 95;
altered by dykes, ii. 164;
pre-Cambrian, i. 119;
post-Arenig in Highlands, 126, 310;
in Cambrian rocks, 155;
in Silurian rocks, 200, 229, 236, 238, 249;
of probably Old Red Sandstone age, 272, 277, 290, 331, 337;
Tertiary, ii. 366, 418, 420
Granitite, i. 188, 277, 290, 337; ii. 367
Granophyre, alteration of rocks by, i. 95, 96;
scenery of, 105;
solvent action of, 82, 84, 85, 96, 99; ii. 163, 392, 415, 422, 433;
brecciated, 382;
spherulitic, 381;
bedded structure of, 381, 403, 404;
apt to be intruded at the base of a volcanic series, 403;
shattering of rocks invaded by, 405, 411, 413, 416, 439;
veins of, 409, 410, 432, 437;
Silurian, i. 214, 215
---- Tertiary, i. 339; ii. 368, 395, 408, 430;
boundaries of, 382, 409;
relation to older vents, 280, 384, 399;
relation to plateau-basalts, 386, 396, 402, 404;
relation to gabbro, 391, 402, 404, 410;
relation to basic dykes, 395;
proof of liquidity of, 413;
sills of, 430, 436, 437;
dykes of, 435
Granophyric structure, i. 20; ii. 366
Graphite in Tertiary volcanic series, ii. 198
Graptolites, i. 174, 196, 197
Grauwacke or Devonian rocks, De la Beche on, i. 259, ii. 33
Graves, Lieut. T., ii. 451
Great Glen of Scotland, i. 121
Greeks, influence of volcanoes on, i. 1
Green, A. H., i. 133, 134, 163; ii. 10, 12
Greenland, Tertiary basalts of, ii. 182
Greenly, Mr. E., i. 129, 214
Greenock, Lord, i. 363
Green schists of the Scottish Highlands, i. 122;
of Anglesey, 129
Greenstone, i. 183, 187, 206, 217, 219, 249, 259, 261; ii. 34, 35, 37,
103, 104, 355
Greenstone-ash, i. 219
Griffith, Sir R., ii. 299, 422
Gunn, Mr. W., i. 114, 298, 311, 336, 369, 407, 410; ii. 58, 172, 420
Gypsum deposits, ii. 54
Hade of dykes, ii. 139
Hæmatitic iron-ore, ii. 197
Hall, Sir James, i. 72, 363
Hälleflinta, i. 131, 167
Hardman, E. T., ii. 365, 449
Harker, Mr. A., i. 90, 91, 93, 95, 96, 99, 188, 209, 210, 211, 212, 213,
214, 217, 218, 222, 227, 228, 230, 231, 232, 235, 236, 237, 238, 290;
ii. 124, 125, 126, 129, 130, 139, 144, 146, 160, 162, 163, 164, 174,
185, 189, 190, 223, 224, 247, 269, 281, 284, 285, 309, 310, 318, 320,
334, 339, 347, 348, 368, 382, 384, 385, 387, 389, 392, 407, 408, 409,
413, 415, 433, 434, 437, 441, 446
Harkness, R., i. 228; ii. 56
Harlech anticline, i. 159, 179, 187;
group, 176
Hatch, Dr. F. H., i. 183, 184, 187, 188, 229, 230, 246, 247, 248, 249,
261, 277, 278, 306, 377, 380, 381, 417, 419, 420; ii. 57, 96, 184, 274,
276, 299, 319, 332, 367, 368, 369, 370, 388, 398
Haughton, Prof. S., i. 346; ii. 422
Hawaii, lava-fountains of, i. 12;
differentiation in lavas of, 27;
lava-cauldron of, 58
Haworth, Mr. E., ii. 96
Hay Cunningham, R. I., i. 269, 317, 363, 372, 373, 449, 451; ii. 237,
238, 244
Heaphy, Mr. C., i. 432
Hebrides, basalt-sheets of, i. 24, 47, 52, 102;
acid rocks of, 95, 102;
gabbros of, 84, 90, 102;
scenery of, 105;
pre-Cambrian rocks of, 112, 114, 117, 121;
Cambrian land of, 141;
early observations on the Tertiary volcanic rocks of, ii. 109, 110, 111;
dykes of, 118, 146, 158, 174;
basalts of, 181, 186, 215;
pitchstone lava of, 238, 246;
plateau-scenery of, 249;
Tertiary rivers and lakes of, 217, 228, 231, 234, 252;
vents of, 274;
basic sills of, 304;
gabbro intrusions of, 327;
acid intrusions of, 364, 379, 430, 437;
dislocations of, 452;
denudation of, 455
Heddle, Dr., i. 274, 302; ii. 78, 79, 246, 307, 406
Helland, Prof. A., ii. 191, 261, 263, 264
Henderson, Mr. J., i. 449
Henry Mountains, laccolites of, i. 86; ii. 362
Henslow, J. S., ii. 22, 224
Heterogeneity in volcanic magmas, i. 85, 90; ii, 190, 334
Hett Dyke, ii. 1, 7, 147
Hibbert, S., i. 46
Hicks, Dr. H., i. 126, 145, 154, 158, 159, 166, 206
Hill, Mr. J. B., ii. 140
Hill, Rev. E., i. 135
Hinde, Dr. G. J., i. 198; ii. 35
Hinxman, Mr. L., i. 114, 344; ii. 121
Hobson, Mr. B., i. 260; ii. 23, 27, 96, 99
Holden, Mr. J. S., ii. 204
Holl, H. B., i. 133, 134, 170
Holland, Mr. P., i. 177, 178, 179; ii. 23
Hollybush Sandstone, i. 133, 170
Holocrystalline structure, i. 78; ii. 136, 184
Hopkins, W., ii. 177, 179, 268
Hornblende, ejected crystals of, i. 178, 181; ii. 49, 51, 58, 79
Hornblende-schists formed from basic igneous rocks, i. 75, 114, 118, 119,
124, 129
Horne, Mr. John, i. 114, 196, 199, 200, 344, 345, 375; ii. 23, 144, 292
Hornito of a lava-stream, i. 55; ii. 264
Hornstone, i. 131, 136, 277, 278 (analyses), 324
Houston Marls, i. 423, 436, 440, 444, 466
Howard, Mr. H. T., i. 207
Howell, Mr. H. H., i. 294, 307, 364
Hoy, Island of, i. 350
Hughes, Prof. T. M'K., i. 126, 144, 160, 161, 166, 168, 220, 222, 223, 227
Hull, Mr. E., ii. 42, 95, 103, 272, 421, 426, 449
Hurlet Limestone, i. 360, 366, 394, 410, 415, 444, 452, 456, 467, 470, 474
Huronian rocks, i. 111
Hutchings, Mr. W. M., i. 227, 230, 233
Hutton, James, i. 363; ii. 9, 110
Hutton, W., ii. 3
Hyperite, i. 279
Hysgeir, pitchstone of, ii. 246
Iceland, Tertiary basalts of, ii. 182, 260;
Tertiary gabbros and liparites or granophyres of, 261;
continuity of volcanic phenomenon of, 261;
lava-fields of, i. 24, 42, 53, 100; ii. 260;
lava-domes of, i. 10; ii. 265;
fissures of, i. 70; ii. 262, 271, 454;
dykes of, 122, 261;
cinder cones of, 264, 271;
subsidence of, 447
Idaho, lava-fields of, ii. 267
Iddings, Prof., i. 28, 29, 30, 78, 79, 82, 84, 90; ii. 128, 178
Idiomorphic crystals, i. 21, 417, 420; ii. 40
Index Limestone of the Scottish coal-fields, i. 360, 444, 452
India, fissure-eruptions of, i. 10;
volcanic plateau of, ii. 180
Intermediate volcanic rocks, silica-percentage of, i. 14
Intersertal structure, i. 417; ii. 136
Intrusive rooks, defined, i. 14;
occasional cellular character of, 16;
flow-structure in, 22, 161;
varieties of, 77;
textures of, 78, 449; ii. 274, 360, 392;
in sheets, sills, and laccolites, i. 83;
melting of rocks by, 82, 84; ii. 129, 163, 392;
consolidation of, i. 84;
banding of, 84, 450; ii. 329, 342;
heterogeneity of, i. 85; ii. 344;
metamorphism by, i. 94, 451;
influence of surrounding rocks on, 95;
conditions of their intrusion, 97;
columnar structure in, ii. 187, 291, 301
---- Pre-Cambrian, i. 116;
Cambrian, 156;
Silurian, 187, 195, 206, 216, 235, 237, 248, 249;
Devonian, 261;
Old Red Sandstone, 277, 291, 321, 335, 338, 343, 345;
Carboniferous, 406, 408, 420, 446; ii. 1, 21, 30, 33, 40, 48;
Permian, 58, 64;
Tertiary, 270, 298
Ireland, submarine eruptions of, i. 48;
Dalradian rocks of, 122, 123, 126;
Arenig rocks in, 123;
Silurian volcanic rocks of, 239, 251;
granites of, 290;
Old Red Sandstone volcanic rocks of, 346, 348;
Carboniferous volcanic rocks of, 359; ii. 37;
early observers among the Tertiary volcanic rooks of, 109;
Tertiary basalt plateau of, 364, 370, 371;
gabbros of, 359;
acid rocks of, 420
Iron-ore, pisolitic (Arenig), i. 181, 208;
Tertiary, of Antrim, ii. 204
Irvine, Mr. D. R., i. 294, 299
Irving, Rev. A., ii. 95
Isogeotherms, shifting of, i. 98
Italy, old volcanoes of, i. 4; ii. 474, 477
Jack, Mr. R. L., i. 294, 308, 369, 375, 396, 404; ii. 57, 145
Jameson, Robert, i. 268, 269, 317, 363; ii. 109, 161, 244, 333, 355, 364
Jan Mayen, ii. 182
Jedburgh type of dolerite and basalt, i. 418
Jennings, Mr. C. V., i. 176, 177, 179, 180, 181, 184, 185, 186, 187, 188
Johnston-Lavis, Dr., ii. 261
Joints in dykes, ii. 132
Judd, Prof. J. W., i. 157, 275; ii. 115, 116, 134, 137, 162, 185, 209,
211, 245, 247, 267, 274, 278, 280, 303, 307, 309, 315, 316, 319, 322,
328, 329, 332, 333, 349, 356, 360, 372, 388, 410, 439
Jukes, J. B., i. 143, 171, 175, 208, 218, 219, 245, 246, 250, 254, 316;
ii. 10, 20, 42, 47, 49, 101, 103, 105
Jurassic period, physical conditions of the, ii. 108, 182
Kelly, J., i. 314
Kenmare, Old Red Sandstone volcanic rocks of, i. 350
Keratophyre, i. 247
Kerrera, Isle of, i. 342
Kersantite, i. 261
Keswick, i. 229
Kildare, Chair of, Bala volcanic rocks at, i. 245, 256
Killarney, nodular lavas of, i. 20, 272, 346
Kilpatrick Hills, i. 385, 388, 403, 410
Kilroe, Mr. J. R., i. 251, 253, 315
Kilsyth type of dolerite and basalt, i. 418
Kinahan, Mr. G. H., i. 349; ii. 45, 49, 426
Kincardineshire, volcanic necks of, i. 281, 286, 293, 299;
Old Red Sandstone of, 301
King, Mr. Clarence, i. 27
King's County, volcanic necks of, ii. 37
Kippie Law type of basalt, i. 418
Kirkby, Mr. J., ii. 106
Kirwan, R., ii. 110
Knockfeerina, Old Red Sandstone volcanic rocks of, i. 349
Knocklayd, ii. 200
Kynaston, Mr. H., i. 343
Labyrinthodonts, i. 356
Laccolites, i. 77, 83, 86, 88, 98, 99, 190; ii. 363
Lacroix, Prof., i. 96
Lacustrine volcanic eruptions, i. 49
Lagorio, Dr. A., ii. 137
Lake, Mr. P., i. 177, 179
Lake-District, i. 227, 290;
Vesuvian cone of, i. 42, 45
"Lake Caledonia," i. 272, 294, 296
"Lake of Lorne," i. 341
"Lake Orcadie," i. 266, 271, 343, 350
Lakes, eruptions in, i. 49;
crater, 58;
of Old Red Sandstone, 264
Lambay Island, conglomerates of, i. 244
Lammermuir, granites of, i. 290, 340
Lamplugh, Mr. G. W., i. 32, 220; ii. 23, 28
Lamprophyre, i. 291, 293
Lanarkshire, i. 291, 368, 416
Land, sculpture of the, i. 101, 102
Landslips, ii. 200, 287
Lankester, Prof. E. Ray, i. 310
Lapilli, volcanic, i. 33, 34, 61, 151
Lapworth, Prof. C., i. 130, 132, 137, 171, 172, 189, 190, 196
Largs, volcanic vent near, i. 56, 396, 397, 401
Lasaulx, Prof. von, ii. 365, 371, 426
Laurentian gneiss, i. 110
Lavas, classification of, i. 14;
flow-structure of, 16, 21;
vesicular structure of, 17;
glass in, 18;
devitrification of, 19;
bedding of, 24;
effect of water on molten, 25, 334;
sack-like or pillow-structure of, 26, 184, 193, 201, 240, 244, 252;
seldom occur in solitary sheets, 26;
variations in structure in, 27;
sequence of, in eruptions, 28, 92, 377, 386;
crusts of, disrupted in volcanic explosions, 58, 59, 60; ii. 189;
alternations of acid and basic, i. 28, 61, 152, 157, 165, 207, 213,
284, 318; ii. 236, 266;
contrasted with intrusive rocks, i. 78;
sandstone veins in, 283, 300, 303, 320, 327, 333, 337; ii. 59, 98;
shattered or agglomerate structure of, 99;
metamorphism of, i. 231, 240, 338; ii. 272, 276, 337, 339, 340, 347,
355, 379, 386, 397, 399, 400, 404, 413
---- Cambrian, i. 152, 168
---- Silurian of Merionethshire, i. 183;
Scotland, 191;
Builth, 203;
Pembrokeshire, 205;
Caernarvonshire, 207;
Berwyn Hills, 218;
Anglesey, 219;
Lake District, 227;
Gloucestershire, 238;
Ireland, 239, 254
---- Lower Old Red Sandstone, i. 273, 281, 294, 317
---- Carboniferous, i. 377, 384, 417, 436, 440, 443; ii. 8, 18, 34, 45
---- Permian, ii. 68, 96
---- Tertiary, ii. 183;
types of, 186;
banding of, 189;
thickness of, 192;
lenticular character of, 193;
of Antrim, 199;
irregular bedding of the vitreous, 243
---- modern Icelandic eruptions of, ii. 261
Lava-domes, ii. 265
Lava-plug of volcanic funnels, permanence of, i. 40, 41, 55, 73, 76, 430
Lawson, Prof. A. C., i. 82
Leaf-beds, ii. 198
Lebour, Prof., i. 336; ii. 2, 3, 5, 7
Leckstone, i. 419, 442, 443
Lecoq, H., i. 45; ii. 373
Leinster granite, ii. 245, 249, 290
Lewisian Gneiss, i. 81, 110, 111, 113, 118
Liddesdale, Carboniferous volcanic vents of, i. 55, 416, 425, 440, 475
Life, earliest traces of, i. 140
Lignite in Tertiary volcanic series, ii. 198
Limburgite, i. 377, 408, 417, 420, 448; ii. 40, 46
---- type, i. 418
Limerick, Carboniferous volcanic rocks of, i. 421, 430; ii. 41;
Old Red Sandstone volcanic rocks of, i. 348
Limestone, metamorphism of, i. 72, 451; ii. 14, 22, 164, 280, 383
Lindley and Hutton on Eigg conifer, ii. 238
Lingula Flags, i. 144, 177
Linlithgowshire (_see_ West Lothian)
Lintrathen, porphyry of, i. 277, 292, 311
Lion's haunch type of dolerite and basalt, i. 418
Lithomarge, ii. 197, 204
Lizard, rocks at the, i. 194
Llanberis, Pass of, i. 159, 163
---- group, i. 144
Llandeilo group, i. 175, 196, 242;
volcanic rocks of, 186, 202, 221, 227, 241
Llandeiniolen, i. 160
Llandovery group, i. 175, 196;
possible volcanic rocks of, 238
Llangadock, i. 205
Llangefni, i. 220
Llanllyfni, i. 161
Llanwrtyd, i. 204
Lleyn Peninsula, i. 208, 209, 213, 215
Lloyd Morgan, Prof., i. 145, 147, 154
Llyn Padarn, i. 157, 159, 160
Loch Carron, pre-Cambrian rocks of, i. 115, 117
Loch Lomond, dykes at, ii. 180
Loch Tay Limestone, i. 122, 124, 125
Lomas, Mr. J., ii. 191, 322
Longulites, ii. 135
Lonsdale, W., i. 257
Longmyndian rocks, i. 111, 129, 132
Lorne, volcanic rocks of, i. 102, 271, 281, 341
Lough Mask, Silurian volcanic rocks of, i. 251
---- Nafooey, Silurian volcanic rocks of, i. 251
---- Neagh, subsidence of site of, ii. 201, 205;
history of, 448
Ludlow group, i. 175
Lycopods, fossil, i. 174
Maare, volcanic, i. 58; ii. 287, 288, 296
Macconochie, Mr. A., ii. 58
Macculloch, John, i. 95, 269, 270; ii. 22, 111, 113, 123, 140, 154, 156,
159, 172, 174, 175, 213, 217, 231, 237, 244, 251, 280, 293, 304-307,
310, 314, 315, 327, 349, 364, 371, 403, 406, 408, 409, 418
Macknight, Dr., i. 268, 317
Maclaren, Charles, i. 269, 317, 325, 363, 372, 373, 451, 462; ii. 67
M'Henry, Mr. A., i. 32, 240, 242, 244, 314, 347; ii. 201, 204, 272, 293,
426, 427, 428, 429
M'Mahon, General, i. 260; ii. 35, 36
Magma, volcanic, explosive energy of, i. 13;
gases and vapours dissolved in, 13, 72, 97, 99;
differentiation of, 22, 84, 91;
solvent action of, 82, 84, 85, 99; ii. 392, 415, 422, 433;
heterogeneity of, i. 85, 90, 91; ii. 344, 360, 476;
metamorphic action of, i. 94;
alteration of, by incorporation of foreign material, 96;
ii. 386, 390, 392;
conditions for the injection of, i. 97, 99
Magnesian Limestone, ii. 54
Malvern, pre-Cambrian volcanic rocks of, i. 133;
Cambrian volcanic rocks of, 169
Man, Isle of, Carboniferous volcanic rocks of, ii. 22
Manod, i. 184
Marl Slate, ii. 54
Marl, volcanic, i. 423, 436, 440, 444, 466
Marr, Mr. J. E., i. 227, 228, 230, 231, 232, 236, 237, 238, 290; ii. 189
Matlock Bath, ii. 13, 22
Mediterranean, earthquakes and volcanoes of, i. 1
Melaphyre, i. 131
Mello, Mr. J. M., ii. 22
Melrose, rocks near, i. 397, 398, 400, 425
Melting of rocks by igneous intrusions, i. 82, 84, 85, 96, 99; ii. 163,
392, 415, 422, 433
Menai Strait, i. 159
Menevian group, i. 114
Merse, volcanic plateau of the, i. 375
Metamorphism of tuffs, i. 157;
of lavas (_see_ under Lavas)
---- by lavas, i. 27;
by sills and bosses, 87, 94, 95, 216, 236, 331, 338, 431; ii. 7, 22,
36, 131, 148, 163, 299, 300, 310, 337, 339, 340, 347, 355, 356, 357,
358, 362, 378, 383, 386, 397, 399, 400, 404, 413
---- in vents, i. 67, 71, 82, 93, 399, 404; ii. 39, 78, 292
---- around vents, i. 72, 349, 350, 352, 399, 404, 432; ii. 76, 272, 273,
276, 280, 292
---- regional, i. 121, 123; ii. 35
Mica, ejected crystals of, in volcanic breccias, ii. 49, 58, 79, 80
Mica-porphyrite, i. 277, 338
Michel Lévy, M., i. 18, 21, 46, 88; ii. 373, 374
Microgranite, i. 131, 215, 235, 249; ii. 367, 437
Microlites of igneous rocks, i. 18, 21, 33; ii. 135, 275
Micropegmatitic structure, i. 20, 418, 449; ii. 5, 368, 437
Microscopic examination of rocks, i. 18, 21
Midlands, eruptive rocks of English, ii. 100
Midlothian, Carboniferous volcanic plateau of, i. 373, 385, 387;
sills of, 446
Miller, Hugh, ii. 237
---- Mr. H., i. 336
Mills, Abraham, ii. 109
Millstone grit, i. 358, 360, 366
Minette, i. 277, 278, 291, 293
Minto Crags, i. 375, 397
Mitchell, Rev. Hugh, i. 301
Moel Siabod, i. 175
---- Wyn, i. 175, 176, 184, 185
Monckton, Mr. H. W., i. 449; ii. 224
Montana, lava-fields of, ii. 115, 267
Montrose, volcanoes of, i. 299
Moray Firth, basin of, i. 271, 343
Morton, Mr. G. H., i. 189
Morven, basalt-plateau of, ii. 208
Mountain-chains, origin of, i. 11, 12, 98
Mourne mountains, granite of, i. 93; ii. 124, 366, 367, 420
Muck, Isle of, ii. 215
Mud-lava, ii. 85
Mudstone, volcanic, i. 423, 436, 440, 444, 466; ii. 86, 222, 258
Mull, branching amygdales of, i. 17;
perlitic glass from, 19;
pale lavas of, ii. 184, 213;
basalt of, 188, 192, 193;
breccias of, 196;
plateau of, i. 24, ii. 208;
non-volcanic breccias in, 196, 211;
flint gravel in, 211;
leaf-beds of, 212;
vents in, 274, 278;
gabbro of, 355;
acid bosses of, i. 20; ii. 395;
acid sills of, 430;
acid dykes and veins of, 443;
enormous denudation of, 457, 461
Murchison, R. I., i. 113, 121, 129, 142, 173, 175, 189, 204, 205, 207,
238, 257; ii. 56, 95
Mynydd-mawr, i. 209, 211, 216
Mythology, influence of earthquakes and volcanoes on, i. 2
Nant Francon, i. 161
Naples, puys of, i. 100, 429
Napoleonite, i. 22
Necker, L. A., ii. 112, 123, 139, 140, 146
Necks, volcanic, i. 56;
of fragmentary materials, 56;
of non-volcanic detritus, 57, 289, 343, 426;
of agglomerate, 58; ii. 276;
internal stratification in, i. 63; ii. 80, 294;
with central lava-plug, i. 64, 430;
with dykes and veins, 66, 430; ii, 291;
of lava-form material, i. 67, 430; ii. 271;
parasitic, i. 69;
connection of, with cones, 70, 435; ii, 70, 89, 277, 281, 290;
metamorphism of, i. 67, 71, 82, 93, 399; ii. 39, 78;
metamorphism of rocks around, i. 72, 349, 350, 352, 399, 404, 432;
ii. 76, 272, 273, 274, 276, 280, 292;
inward dip of rocks towards, i. 73, 352; ii. 80;
connection of, with bosses, i. 93; ii. 276, 284;
entombment and exposure of, i. 434;
connection of with valleys, 272, 366, 375; ii. 61, 65;
relation between their size and the character of the agglomerate, 76;
connection of, with sheets of tuff or lava, 70, 89, 277, 284
---- Silurian, i. 215, 235;
Old Red Sandstone, 277, 288, 293, 311, 318, 323, 328;
Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47;
Permian, ii. 62, 67;
Tertiary, ii. 202, 270, 276
Neptunist and Plutonist controversy, i. 363; ii. 67, 95, 110, 112
New Mexico, necks in, i. 68
Newry granite, i. 290
Nicholson, Prof. Alleyne, i. 228, 229
Nicol, James, i. 311, 369
---- W., ii. 238
Nigrine, ii. 79
Nithsdale, Permian volcanic rocks of, ii. 58, 60, 62, 65
Nodular structure of lavas, i. 20, 162, 204, 206, 207, 211, 232, 247,
255, 274, 346
Nolan, Mr. J., i. 240, 251, 315; ii. 423, 424, 425
Non-volcanic debris, among volcanic rocks, i. 31, 57, 289, 313, 345, 381,
399, 402, 422, 426, 437; ii. 18, 27, 28, 58, 64, 76, 78, 99, 195, 196,
281, 423;
indicates comparatively feeble eruptions, i. 57, 289, 345, 426, 438;
ii. 293;
points to earliest eruptions of a vent, ii. 76
Nordenskjöld, Mr. O., i. 120
North Berwick Law, i. 371, 373, 403
North, Mr. Barker, ii. 244
Norway, eruptive rocks of, i. 28
Nuneaton, i. 171
Obsidian, i. 18, 19; ii. 370
Ochil Hills, i. 274, 276, 277, 279, 281, 286, 287, 288, 293, 303, 308, 311
Oil-shales of the Lothians, i. 361, 362, 462
O'Kelly, J., i. 349; ii. 49
Oldham, T., ii. 299
Old Red Sandstone, lines of vents in, i. 69;
granite protrusions of, 236, 272, 277, 290, 337;
of County Waterford, 251;
distribution in Britain, 257;
an exceptional stratigraphical type, 258;
conditions of its deposit, 259, 263, 297;
original scenery of, 265;
vegetation of, 265;
isolation of the water-basins of, shown by fossil evidence, 265;
classification of, 260;
history of the investigation of, 268;
volcanic centres in, 271;
nature of volcanic products in, 273;
structure of lavas and tuffs of, 281;
volcanoes of, 259, 263, 294, 303, 323, 325, 337, 341, 343, 346, 348, 352;
subdivisions of, 297;
thickest conglomerates of, 301;
composition of conglomerates of, 302, 315, 341;
unconformabilities in, 267, 328, 333;
Upper division of, 348, 375, 383; ii. 42
Olenellus-zone, i. 112, 130, 132, 140, 144
Olenus-zone, i. 144
Olivine, i. 154, 418, 420; ii. 58, 135
Omagh, i. 315
Ophitic structure, i. 21, 417; ii. 136, 184, 274
---- type of dolerite, i. 418, 421
Oregon, crater lake in, i. 58
Orkney Isles, i. 271, 344, 350; ii. 121
Orthoclase, ejected crystals of, ii. 79
Orthophyre, i. 273, 276, 277, 308
Osann, A., ii. 191
Oyenhausen, C. von, ii. 112, 280, 333, 340, 367, 372, 381
Palæopicrite, i. 261
Palæozoic systems, i. 139;
volcanic rocks resemble modern, i. 30
Palagonite, i. 33, 61, 151, 180, 246, 422, 423; ii. 44, 46, 57, 223
Paradoxides-zone, i. 144
Paramorphism, i. 249
Peach, Mr. B. N., i. 114, 128, 147, 168, 191, 192, 195, 196, 197, 198,
199, 200, 216, 240, 277-294, 307, 308, 329, 331, 344, 345, 369, 375,
425, 426, 476; ii. 133, 145
Pebidian, i. 145
Pegmatite, i. 20, 119, 127, 418, 449; ii. 5, 368, 437
Pembrokeshire, volcanic rocks of, i. 145, 159, 205
Penmaen-mawr, i. 209, 215
Pennant, T., ii. 109
Pennine chain, ii. 8
Pentland Hills, volcanic series of the, i. 102, 269, 273, 276, 279, 281,
285, 287, 289, 291, 311, 317
Perlite, i. 130
Perlitic structure, i. 19, 196, 199, 206, 211, 216, 232, 274
Permian system, geographical conditions accompanying the deposition of,
ii. 53, 97;
subdivisions of, in S. W. England, 94;
volcanic phenomena of, i. 46; ii. 55;
lavas and tuffs of, 57, 58;
vents of, 62, 67, 70, 96;
sills of, 91, 100
Permo-carboniferous strata, ii. 54
Petersen, Dr., i. 275, 336
Phillips, J., i. 133, 170, 205, 238; ii. 3
---- J. A., i. 260, 261
---- W., i. 171; ii. 95
Phonolite, i. 380 (analysis); ii. 375
Phyllite, i. 162, 222
Picrite, i. 377, 417, 420, 448, 450; ii. 57
---- type, i. 418
Pillow-structure in lavas, i. 26, 184, 193, 201, 240, 244, 252;
ii. 189, 259
Pitchstone, i. 18, 19, 130; ii. 134, 174, 204, 238, 242, 246, 370,
437, 444
Plagioclase, ejected crystals of, ii. 79
Plants fossil, in tuffs, i. 392; ii. 113, 198, 212, 222
Platania, G., i. 26
Plateau-type of volcanoes, i. 42, 100, 308, 341, 364
Plateaux, Carboniferous, of Scotland, i. 364;
distribution of, 367;
composition of, 377;
structure of, 383;
lavas and tuffs of, 383;
vents of, 394;
dykes and sills of, 406;
close of eruption of, 410;
Tertiary, ii. 181;
formation of modern Icelandic, 265
Player, Mr. J. H., analyses by, i. 377, 381; ii. 138, 330
Playfair, John, i. 363; ii. 110
Plinthite, ii. 197
Pliocene (supposed) of Lough Neagh, ii. 449
Plutonic operations of volcanoes, i. 77;
granite, 88
Plutonists and Neptunists, i. 363; ii. 67, 95, 110, 112
Pomeroy, volcanic series near, i. 315
Porphyrite, i. 190, 193, 207, 229, 240, 252, 273, 274, 377, 379
Porphyritic structure, i. 19, 274; ii. 128
Portlock, J. E., ii. 110, 111, 113, 199, 201, 299, 364
Portraine, conglomerates at, i. 244
Portrush, shells in supposed basalt at, ii. 110, 299
Potstone, i. 125
Pre-Cambrian rocks, i. 111, 121, 126
Pressure, experimental proof of effects of, i. 24
Prestwich, Sir J., ii. 103
Propylites, ii. 185, 388
Proterobase, i. 247
_Pterygotus_, i. 265
Pumice in tuffs, i. 244, 422; ii. 17, 27, 28, 32, 44, 46, 286, 288;
in volcanic necks, i. 60, 180; ii. 17, 39, 195
Pumiceous structure, i. 15, 33, 34, 60
Puys, as a type of volcano, i. 10, 44, 100, 414;
probable subærial nature of some, 432;
Carboniferous, 308, 364, 414, 424, 463; ii. 13, 28, 34, 47;
Permian, ii. 62;
Tertiary, 271, 276
Puy de Chopine, i. 32; ii. 374
---- Dôme, ii. 373
---- Montchar, i. 32
---- Pariou, i. 66, 70; ii. 31, 281
Pyroclastic detritus, i. 31, 58, 61
Pyromeride, i. 211
Pyrope, ii. 58, 79
Pyroxene, ii. 135
Quartz-porphyry, i. 19, 156, 159, 160, 161, 165, 277, 291, 314; ii. 96,
369, 420, 423, 430, 431
Quartz-trachyte, ii. 371
Quartzite, i. 112, 170
Raasay, basalt of, ii. 192;
neck-like breccias in, 293;
acid sill of, 430
Raddling or red-staining of rocks, i. 250, 261
Radiolarian cherts, i. 123, 167, 169, 173, 174, 184, 196, 201, 244
Rain-pittings in strata, i. 342
Raisin, Miss, i. 161, 163, 164, 165, 210
Ramsay, A. C., i. 126, 142, 143, 144, 145, 158, 159, 168, 175, 176, 177,
178, 179, 180, 182, 183, 204, 205, 208, 210, 212, 214, 223, 237, 364
Ratho type of dolerite, i. 418, 421
Red Head, section at, i. 300
Red Hills, Skye, scenery of, i. 105; ii. 379
Reed, Mr. Cowper, i. 205
Reid, Mr. Clement, ii. 449, 450
Renard, Prof. A., i. 148, 149
Renfrewshire, Carboniferous volcanic rocks of, i. 368, 385, 397, 400, 404,
408, 416, 430, 447
Reyer, Prof. E., ii. 474
Reynolds, Mr. S. H., i. 177, 179, 256
Rhobel Fawr, i. 177, 178, 186
Rhyolite, i. 19, 22, 24, 131, 161, 165, 167, 168, 178, 204, 210, 231, 232,
255, 276, 278; ii. 185, 205, 371, 424, 437
Rhyolitic conglomerate, ii. 195, 206, 429
Richardson, Rev. W., ii. 110
Richthofen, F. von, i. 28; ii. 115, 116
Rivers made to shift their channels by volcanic eruptions, i. 42, 49;
of the Tertiary volcanic period, ii. 217, 228, 231, 234, 456
Rocks, oldest known, i. 110
Roscommon, volcanic rocks of, i. 316
Rosenbusch, Prof. H., ii. 136, 137
Ross, Mr. Alexander, ii. 406, 409
Rothliegende, ii. 95
Roxburghshire, Carboniferous vents of, i. 55, 403, 404
Rubers Law, i. 375, 380, 404
Rum, i. 112;
basalt-plateau of, ii. 215;
gabbros of, 332, 349;
acid bosses of, 403;
acid sills of, 431;
pitchstone of, 445
Rutley, Mr. F., i. 131, 133, 170, 207, 210, 227, 231, 232, 238, 260;
ii. 23, 35
Sahlite found in volcanic vents, i. 62
Saline Hill, volcanic vents of, i. 433, 435, 440
Sanday, basalts of, ii. 215;
conglomerates of, 226
Sandstone altered into quartzite, i. 72, 349, 350, 404, 432, 451;
ii. 76, 164;
veinings of, in lava, i. 283, 300, 303, 320, 327, 333, 337;
ii. 59, 98
Sandwich Islands, lava-cones of, i. 10
Sanidine ejected from volcanic vents, ii. 58, 79
Sanquhar, Silurian volcanic rocks at, i. 192, 195, 199;
Permian volcanic rocks at, ii. 62
Santorin, ii. 134
Saponite, ii. 79
Scenery, origin of, i. 8, 100
Schalstein, i. 262; ii. 36
Schists, primeval, i. 110, 114, 118, 119;
produced by deformation of igneous rocks, 75, 114, 118, 119, 121, 162,
240, 249, 252, 261
Schmidt, Dr. C. W., ii. 266
Scoriaceous structure, i. 15, 16, 282, 327, 339
Scorpions, fossil, i. 174, 356, 466
Scotland, lines of fault in, i. 11;
Vesuvian cones of, 42;
plateaux of, 43;
puys of, 46;
submarine lavas of, 48;
Carboniferous vents of, 55;
volcanic scenery of, 104;
pre-Cambrian rocks of, 111;
Cambrian rocks of, 112;
Lewisian gneiss of, 114;
Dalradian rocks of, 121;
Arenig rocks of, 123, 191;
Old Red Sandstone of, 266, 273, 281, 291;
Carboniferous geography of, 356;
Carboniferous volcanoes of, 359;
Carboniferous plateaux of, 367;
Carboniferous puys of, 414;
Permian volcanoes of, ii. 55;
Tertiary dykes of, 122;
Tertiary basalt-plateaux of, 208, 274, 304;
Tertiary gabbros of, 327;
Tertiary acid rocks of, 379
Scrope, G. P., i. 27, 82, 45, 116; ii. 373, 374, 381
Sedgwick, A., i. 142, 166, 175, 218, 227, 257; ii. 1, 2, 3, 5, 113,
139, 153, 157
Segregation (_see_ Differentiation)
Segregation-veins, i. 84, 92; ii. 66, 130, 300, 303
Selwyn, Mr. A. C. R., i. 143, 175, 208, 221
Semi-opal, ii. 79
Sepulchre Mountain, i. 79
Serpentine, i. 195, 293
Shale, alteration of, i. 72, 451; ii. 164
Shap, granite of, i. 236, 238, 271, 290
Sheets, intrusive (_see_ Sills)
Shelve, i. 176, 190
Shetland, i. 271, 289, 292, 293, 345
Shiant Isles, ii. 307
Shineton Shales, i. 144
Shore-lines, traces of ancient, i. 295, 305
Shropshire, ancient volcanic rocks of, i. 129, 189;
latest eruptive rocks of, ii. 101
Sicily, i. 26
Sidlaw Hills, i. 286, 294, 303
Sills, vitreous margins of, i. 18;
tectonic relations of, 77, 83, 451;
origin of name, 83;
differentiation (segregation) in, 81, 450; ii. 476;
ordinary stratigraphical position of, i. 85;
considered as parts of incompleted volcanoes, 86;
metamorphism by, 87, 94, 451; ii. 299, 303, 310;
conditions for injection of, i. 97, 98, 99, 458;
columnar structure of, ii. 187, 291, 301, 306, 308, 319;
amygdaloidal structure in, 299, 312;
banding of, 309;
split by later sills, 310, 316;
extreme subdivision of, 311;
slaggy surface in some, 312;
give off veins, 313;
double and multiple, 318, 434;
connection with vents, 322
---- pre-Cambrian, i. 118, 124;
Cambrian, 155, 170, 171;
Silurian, 187, 195, 206, 216, 237, 249;
Devonian, 261;
Old Red Sandstone, 277, 291, 321, 335, 343, 345;
Carboniferous, 408, 446, 472; ii. 2, 21, 30, 48;
Permian, 64, 66;
of Midlands, 102, 103;
Tertiary, (1) Basic, 298;
(2) Acid, 366, 430
Silurian system, i. 173;
vegetation of, 174;
geography of, 263;
volcanoes of, 175;
classification of, 175;
two volcanic series of, 177
---- volcanoes in Shropshire, i. 189;
in Scotland, 191;
at Builth, 203;
in Pembrokeshire, 205;
in Caernarvonshire, 207;
in the Berwyn Hills, 218;
in Anglesey, 219;
in the Lake District, 227;
in Gloucestershire, 238;
in Ireland, 239, 254
Skae, H. M., i. 294, 299, 306, 375; ii. 57
Skiddaw, i. 228;
granite of, 236
---- Slate, i. 229
Skomer Island, i. 207
Skye, spherulitic dykes and sills of, i. 20;
ophitic structure from, 20;
basalt-terraces of, 24;
metamorphism by granophyre of, 95, 96;
volcanic scenery of, 103, 105;
dykes of, ii. 123, 124, 129, 139, 140, 146, 150, 152, 154, 160, 162,
164, 165, 173, 269;
bedded basalts of, 192, 249, 269;
tuffs of, 251;
connection of dykes and superficial lavas in, 269;
vents in, 280;
sills of, 304;
gabbro bosses of, i. 116; ii. 334;
acid bosses of, 379;
acid sills of, 431;
acid dykes of, 437;
pitchstone veins of, 445;
subsidence of, 447
Slaggy structure, i. 16, 33, 59, 282, 327, 339; ii. 98, 187
Slane, volcanic rocks near, i. 244
Slate-tuffs, i. 180, 213, 234
Slemish a volcanic neck, ii. 271
Slieve Foye, ii. 421
---- Gallion, ii. 200
---- Gullion, ii. 422
Small Isles, basalt-plateau of, ii. 215;
vents of, 288;
sills of, 318;
acid bosses of, 403;
acid sills of, 431
Small, Mr. E. W., i. 207
Smaragdite found in volcanic vents, i. 62
Snowdon, volcanic rocks of, i. 20, 42, 47, 102, 175, 208, 209, 210, 211,
212, 213, 218, 226
Soda-felsites, i. 183, 196, 247
Solfataric action, i. 71; ii. 185, 205, 388
Sollas, Prof., i. 96; ii. 175, 293, 415, 421, 422
Solway, Carboniferous volcanic plateau of, i. 375, 385, 413
Somerset, volcanic rocks of, ii. 32
Somma, denudation of, i. 3, 100
Spheroidal structure of dolerite, i. 456
Spherulitic structure, i. 19, 20, 95, 120, 130, 155, 162, 184, 211, 232,
235, 346; ii. 369, 381, 392, 432, 435, 437, 441, 446
Spilosite, i. 262
Springs, mineral, connected with volcanic action, i. 390, 445
St. Abb's Head, i. 338
Staffa, i. 25;
first notice of, ii. 109;
columnar basalts of, 186, 188, 210;
basalt conglomerate of, 195
Staffordshire, latest eruptive rocks of, ii. 101, 103
St. Andrews, old volcanoes near, ii. 71, 73, 87
St. David's, Cambrian volcanic rocks of, i. 145
---- Head, i. 205
Steam in volcanic action, i. 13, 15, 16, 71
Stecher, Dr., i. 421, 451; ii. 165
St. Kilda, dykes of, ii. 173, 416;
gabbro of, 358;
general account of geology of, 405;
granophyre of, 408
Stocks, or bosses, i. 78, 88
Strahan, Mr. A., i. 171; ii. 10, 12, 23, 28, 32
Strathaird, ii. 123, 140, 164, 269
Strathbogie, i. 344; ii. 121
Strathmore, i. 304
Stromboli, i. 4
Sublimations, traces of ancient, i. 445
Submarine eruptions, i. 48
Sub-ophitic structure, i. 417
Subsidence and volcanic action, i. 295, 297, 444, 463; ii. 42, 205,
447, 470
Subterranean igneous injections, i. 77 (_see_ Bosses, Dykes, Sills)
Suess, Prof. E., ii. 474
Sun-cracks, i. 342
Sweden, Archæan volcanic rocks of, i. 120
Syenite, ii. 366
Symes, Mr. R. G., i. 311, 343, 369; ii. 201, 428
Tate, G., ii. 3, 113
---- R., ii. 204
Tatlock, Mr. R. R., i. 273, 278
Tawney, E. B., i. 157
Teall, Mr. J. J. H., i. 90, 114, 116, 117, 118, 119, 120, 156, 192, 194,
200, 207, 210, 275, 277, 290, 311, 336, 338, 346, 407, 449; ii. 2, 3,
5, 7, 11, 32, 44, 113, 131, 134, 135, 137, 138, 140, 144, 149, 292, 293,
329, 367, 368, 369
Teesdale, i. 228
Termier, M. P., ii. 375
Terrestrial volcanic eruptions, i. 50
Tertiary Volcanic Series, ii. 181
Subaerial character of eruptions, ii. 103, 198
Scenery of, ii. 255, 256, 349, 391, 405, 408
The Plateaux, ii. 183, 249;
lavas of, 183, 218, 236, 256;
thickness of individual sheets, 192, 206, 254, 257;
lenticular character of lavas of, 193, 257;
greatest depth of, 210, 211, 213, 260;
tuffs and clays of, 194, 202, 204, 211, 222, 225, 251, 258, 277,
284, 287;
non-volcanic fragments in, 196, 211, 213, 219;
lignites of, 198, 203, 208, 213, 251;
gravels and conglomerates of, 198, 212, 238, 256;
coal of, 213, 251, 256, 287;
leaf-beds of, 204, 212, 222, 225, 288;
carbonaceous nature of the upper parts of intercalated sediments in,
223, 226, 227, 229, 232, 251, 288;
evidence for intervals between the eruptions in, 203, 205, 208, 221,
228, 240, 245, 251, 254, 288;
no evidence of great central vents in, 208, 214, 255, 258, 260, 267;
faulted condition of, 200, 208, 209, 452;
subsidences of, 205, 208, 209, 214, 447;
ancient river channels of, 217, 228, 231, 234, 456;
volcanic cones of, 202, 218, 230, 277, 281, 285;
paralleled by the modern Icelandic eruptions, 260;
vents of, 202, 218, 230, 270, 276
The Basic sills, ii. 298, 304
The Gabbro bosses, ii. 327, 349, 355, 358;
history of the gabbro intrusions, 359
The Acid rocks, ii. 364;
petrography of, 366;
history of their investigation, 371;
analogies with trachytes of Central France, 373;
intruded at base of the gabbros or of the bedded basalts, 337, 353,
357, 431, 432, 444;
bosses of Skye, 378;
of Mull, 395;
of Small Isles, 405;
of St. Kilda, 405;
of Arran, 418;
of Carlingford, 420;
of Slieve Foye, and Barnavave, 421;
of Slieve Gullion, 422;
of Antrim, 426;
acid sills, 430;
acid dykes and veins, 437
Metamorphism of the basalts, ii. 272, 276, 337, 339, 340, 347, 355, 356,
357, 358, 362, 378, 383, 386, 397, 399, 400, 404, 413
Texture, varieties of, in igneous rocks, i. 78, 449; ii. 5, 299, 360
Tholeiites, ii. 137, 158
Tholeiite type of basalt, i. 419, 421
Thornhill, volcanic rocks of, ii. 60
Thoroddsen, Th., ii. 261, 262, 263, 264, 265, 266, 278
Thrust-planes, i. 229
Time in geological history, ii. 107, 461, 465
Timmins, J. H., i. 133
Tinto, i. 278, 288, 329
Titterstone Clee Hill, ii. 101
Toadstones of Derbyshire, i. 359; ii. 8
Topley, W., i. 147; ii. 3, 5, 7
Torridonian rocks, i. 111, 112, 113, 120; ii. 350
Tortworth, volcanic rocks at, i. 238
Tourmakeady, volcanic rocks of Bala age at, i. 251
Townson, R., i. 363
Trachyte, i. 183, 230, 246, 273, 276 (analysis), 379 (analysis), 386, 403,
407, 421; ii. 36, 47, 96, 138, 152, 184, 236
Traill, Mr. W., ii. 175, 421, 422
Traprain Law, i. 372, 380, 403, 405
Traquair, Dr. R. H., i. 266
Tremadoc group, i. 144, 177
Trevelyan, W. C., ii. 3
Triassic eruptive rocks, i. 29;
geography, ii. 108
Trichites, i. 19; ii. 136
Troctolite, ii. 332
Tuffs, i. 31;
association of, 33;
composition of, 34;
alternations of, 34, 61;
blending of, with non-volcanic sediment, 35, 437;
fossiliferous, 36;
without lava, 36;
necks of, 58;
relation of, to lavas, 61;
metamorphism of, ii. 224
---- pre-Cambrian, i, 125, 135;
Cambrian, 147, 151, 155, 163, 165, 167;
Silurian, 178, 189, 190, 195, 205, 209, 212, 213, 222, 224, 229, 232,
241, 245, 246, 254, 255;
Devonian, 262;
Old Red Sandstone, 279, 281, 289, 337, 339, 351;
Carboniferous, 381, 384, 387, 399, 422, 427, 429, 432, 436, 466;
ii. 11, 18, 24, 36;
Permian, 57, 58;
Tertiary, 194, 197, 202, 204, 211, 222
Tyrol, Triassic eruptive rocks of, i. 29
Tyrone, Old Red Sandstone of, i. 314
Ulster, Old Red Sandstone volcanic rocks of, i. 314
Ultra-basic rocks, i. 14, 118, 377, 417
Unconformability, deceptive case of, i. 163
Urgneiss, i. 110
Uriconian volcanic rocks, i. 129
Ussher, Mr. W. A. E., i. 260, 262; ii. 35, 95
Utah, laccolites of, i. 86;
volcanic regions of, ii. 115, 267
Valleys, tendency of vents to appear in, i. 272, 368, 376; ii. 61, 65, 96
Vapours, action of volcanic, i. 13, 15, 16, 17, 31, 57, 72, 78, 97, 99,
180, 289, 426
Variolitic structure, i. 21, 206, 235
Veins, intrusive, i. 66, 77, 79, 98, 426, 429; ii. 311, 313, 400, 410,
432, 437
Velay, volcanic rocks of, i. 26, 27, 29, 60; ii. 271, 373, 375
Vents, volcanic, i. 53;
ground-plans of, 54;
size of, 55;
filled with non-volcanic detritus, 57;
ejected crystals found in, 62;
agglomerates of, 62; ii. 13, 28, 47, 61, 69, 276, 280, 284, 288, 289;
stratification in, i. 63; ii. 80;
metamorphism in, i. 67, 71;
metamorphism of rocks around, 72, 349, 350, 352, 399, 404, 432; ii. 76,
272, 280;
connection of, with geological structure-lines, i. 68;
occurrence of, in lines and in groups, 69;
double and multiple, 69;
possible indications of length of activity of, 72;
inward dip of strata around, 73, 352; ii. 76, 295;
stages in history of, i. 74;
tendency of, to rise in lines of valley, 272, 368, 376; ii. 61, 65,
96, 468;
criteria for the relative ages of, 270;
connection of, with later eruptive bosses, 280, 384, 399, 400
---- Silurian, i. 209, 214, 234;
Old Red Sandstone, 272, 287, 298, 305, 323, 328, 337;
Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47;
Permian, 61, 69;
Tertiary, 202, 270, 294, 400
Vesicular structure of lavas, i. 15; ii. 187
Vesuvius, denudation of, i. 3;
as an active volcano, 4;
as a type of volcano, 10, 39, 53, 100; ii. 108, 115, 261, 266
Vicary, Mr. W., ii. 95
Vogesite, i. 277, 293
Volcanello Island, i. 70
Volcanic action, permanent traces of, i. 4;
of present time elucidates that of the past, 5;
submarine, 5;
transient effects of, 8;
chief factors in, 10;
explosive energy of, 13, 99;
uniformity of, in geological time, 13, ii. 470;
metamorphism by, i. 67, 71;
underground phases of, 77;
proofs of gradual quiescence of, 155, 157, 166;
connected with subsidence, 295, 297; ii. 205, 444, 463, 470;
repetition of, in the same region, i. 368, 375, 377; ii. 42, 69,
94, 467;
developed along continental borders, 466;
persistence of, in Britain, i. 7; ii. 466;
connection of, with lines of geological structure, 468;
connection of, with terrestrial disturbance, 469;
gradual decline of, during Palæozoic time, 471;
quiescence of, during Mesozoic time, 472
Volcanic cycles, i. 27, 92
---- products, general characters of, i. 14;
persistent uniformity of, 30, 46;
thickest mass of, in Britain, 229 (_see_ Agglomerate, Lava, Tuff)
Volcano, Island of, i. 4, 24
Volcanoes, their influence on mythology, i. 1, 2;
denudation of, 3;
number of extinct, 4;
ancient, of Britain, 6;
influence on scenery, 8, 100, 102;
defined, 10;
types of, 10, 39; ii. 471;
determination of relative dates of, i. 46;
their geographical condition in old times, how ascertained, 48;
parasitic, 69;
contemporaneous denudation of, 73, 100;
connected with granite, 89;
incompleted, 86, 93, 99
Vom Rath, G., ii. 474
Wacke, i. 157
Walcott, Mr., i. 30
Wales, pre-Cambrian rocks of, i. 126, 142;
early geological work in, 142;
Cambrian volcanoes of, 145, 159;
volcanic scenery of, 176;
Silurian volcanoes of, 176, 202, 205, 207, 218, 219;
Old Red Sandstone of, 257, 259
Waller, Mr. T. H., i. 171, 278
Ward, J. C., i. 227, 228, 229, 230, 231, 233, 234, 235, 236, 237; ii. 23
Warwickshire, Cambrian rocks of, i. 137, 171
Waterford, volcanic region of, i. 247
Watts, Mr. W. W., i. 131, 132, 135, 137, 189, 190, 191, 243, 276, 278,
336, 347, 417, 421, 423; ii. 40, 42, 43, 45, 57, 96, 184, 204, 224,
272, 424, 425
Weaver, T., i. 238
Wenlock group, i. 175;
volcanic rocks of, 552
Wernerian School, ii. 109
West Lothian, volcanic rocks of, i. 47, 55, 415, 433, 437
Whin Sill of England, i. 83, 85, 97, 449; ii. 2
Whitehurst, J., ii. 9, 109
White trap, i. 96, 426, 449, 456; ii. 65, 87, 103, 165, 252
Williams, Mr. G. J., i. 179, 185, 186, 188
Williamson, W. C., i. 392
Wilson, Mr. J. S. Grant, i. 148, 149, 153, 276, 344, 375, 379, 380;
ii. 137, 164
Wilson, Mr. A., ii. 49
Winch, N. T., ii. 113, 147
Witham, H. T. M., ii. 113, 238
Wood, N., ii. 113
Woods, Mr. H., i. 204
Woodward, Dr. Henry, ii. 449
Woodward, Mr. H. B., ii. 32, 435, 453
Worcestershire, latest eruptive rocks of, ii. 101
Worth, Mr. R. N., ii. 99
Wrekin, i. 130
Wright, J. R., ii. 102
Wunsch, E., i. 369, 392
Würtemberg, puys of, i. 46
Wyoming, lava-fields of, ii. 115
Yates, J., i. 171
Yellowstone Park, volcanic phenomena of, i. 29, 31
Y-foel-frâs, i. 209, 214
Y Glyder-Fach, i. 209
Yoredale group, ii. 9, 13, 17
Young, Mr. John, i. 369, 392
Young, Prof. John, i. 294, 308
Zircon, found in volcanic vents, i. 62
Zirkel, Prof., ii. 327, 329, 334, 356, 364, 370, 372, 379, 430
THE END
_Printed by_ R. & R. Clark, Limited, _Edinburgh_.
THE CAMBRIDGE NATURAL HISTORY.
Edited by S. F. Harmer, M.A, Fellow of King's College, Cambridge,
Superintendent of the University Museum of Zoology; and A. E. Shipley,
M.A., Fellow of Christ's College, Cambridge, University Lecturer on the
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WORMS, LEECHES, etc.,
VOLUME II.
FLATWORMS. By F. W. Gamble, M.Sc., Vict., Owens College.
NEMERTINES. By Miss L. Sheldon, Newnham College, Cambridge.
THREAD-WORMS, Etc. By A. E. Shipley, M.A., Fellow of Christ's
College, Cambridge.
ROTIFERS. By Marcus Hartog, M.A., Trinity College, Cambridge, D.Sc.
Lond., Professor of Natural History in the Queen's College, Cork.
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Transcriber Note
Minor typos were corrected. A paragraph break was inserted in the
last paragraph on pages 271 and 334 to accommodate placement of
illustrations. To prevent images from splitting paragraphs, some text
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