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Title: The geology and geography of Northern Nigeria
Author: John Downie Falconer
Contributor: Arthur Longbottom
Henry Woods
Release date: August 12, 2026 [eBook #79349]
Language: English
Original publication: London: Macmillan and Co., 1911
Other information and formats: www.gutenberg.org/ebooks/79349
Credits: Galo Flordelis
*** START OF THE PROJECT GUTENBERG EBOOK THE GEOLOGY AND GEOGRAPHY OF NORTHERN NIGERIA ***
THE GEOLOGY AND GEOGRAPHY
OF NORTHERN NIGERIA
[Logo]
MACMILLAN AND CO., LIMITED
LONDON BOMBAY CALCUTTA
MELBOURNE
THE MACMILLAN COMPANY
NEW YORK BOSTON CHICAGO
ATLANTA SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, LTD.
TORONTO
PLATE XVII
[Illustration: INSELBERGE LANDSCAPE AROUND ABUJA.]
[Illustration: INSELBERGE LANDSCAPE IN NORTHERN YOLA.
[_Frontispiece_.]
THE
GEOLOGY AND GEOGRAPHY
OF
NORTHERN NIGERIA
BY
J. D. FALCONER, M.A., D.SC.
F.G.S., F.R.S.E., F.R.G.S.
_With Notes by the late_
ARTHUR LONGBOTTOM, B.A., F.G.S.,
_and an Appendix on the Palæontology of the Cretaceous Deposits, by_
HENRY WOODS, M.A., F.G.S.
_University Lecturer in Palæozoology, Cambridge_
WITH FIVE MAPS AND TWENTY-FOUR PLATES
MACMILLAN AND CO., LIMITED
ST. MARTIN’S STREET, LONDON
1911
RICHARD CLAY AND SONS, LIMITED
BREAD STREET HILL, E.C., AND
BUNGAY, SUFFOLK.
TO
MY OLD TEACHER AND FRIEND
PROFESSOR JAMES GEIKIE
PREFACE
The observations upon which these outlines of the Geology and Geography
of Northern Nigeria are based, were made in the course of the operations
of the Mineral Survey of the Protectorate of which I had the honour to
be the principal officer. The Survey was instituted by the Colonial
Office in 1904 upon the recommendation of Professor Wyndham R. Dunstan,
under whose general direction the work of investigation was carried out
both in Nigeria and in the laboratories of the Imperial Institute. From
1904 to 1909 five expeditions in all were made to Northern Nigeria, in
the course of which every province of the Protectorate was visited in
turn. On the first two expeditions I was accompanied by Mr. Douglas R.
Home, F.R.G.S., and on the remaining three by Mr. Arthur Longbottom,
B.A., F.G.S. On an average seven months of each year were devoted to the
actual field work, but the great extent of territory to be investigated,
coupled with the difficulties of transport, rendered it impossible to do
more in the time available than make a series of cross traverses through
the various provinces with the object of ascertaining in a general way
their geological structure and mineral resources.
In the following pages I have attempted to correlate the various
observations which were made in the course of our travels and to weave a
more or less consecutive story out of the disconnected contents of our
notebooks. I am myself, however, only too conscious that many of my
deductions may have been drawn from insufficient data and that further
knowledge may overturn many of my hypotheses. In a new land someone must
do the pioneering, and I shall be amply satisfied if my work affords
some foundation, however insecure, for the investigations of future
workers.
It is with the deepest regret that I have to record the death of my
friend and former colleague, Mr. Arthur Longbottom, since this work went
to press. He died of dysentery, after much suffering, in November, 1910,
at Naraguta, on the Bauchi plateau in Northern Nigeria, whither he had
returned on a tour of inspection on behalf of a private syndicate. We
had hoped that he would be able to continue the work which we had begun
together and that he would return to England with a fresh store of
observations which would increase our scanty knowledge of the geology
and geography of the Protectorate. His extensive experience in various
parts of Northern Nigeria would have rendered those observations of the
greatest value, but his untimely death has put an end not only to a most
promising career but to all hope of any further immediate advance in our
knowledge of the structure of this outlying portion of the Empire.
Before he left England Mr. Longbottom contributed all the paragraphs and
sections throughout the book marked with his initials (A. L.). He also
supplied the photographs from which have been prepared Plates I, II, VI,
VII, VIII, Fig. 1, and XVI. For Plate VIII, Fig. 2, I am indebted to the
Public Works Department of Northern Nigeria. Mr. Woods has kindly
contributed the Appendix on the Cretaceous Mollusca and the note upon
the Eocene fossils on p. 165.
I gratefully acknowledge a Grant towards the cost of publication of this
work from the Carnegie Trust for the Universities of Scotland. I am
especially indebted to Sir Archibald Geikie for help and encouragement
in the matter of publication. My acknowledgments are due also to various
friends for assistance and advice, and more particularly to Professor
James Geikie, Professor Wyndham R. Dunstan, Professor Knight, Dr. Horne,
and Dr. Flett.
J. D. FALCONER.
JANUARY, 1911.
CONTENTS
CHAPTER I
PAGE
PHYSICAL GEOGRAPHY 1
Introduction 1
Hydrographical Systems 3
The Niger 4
The Benue 10
The Niger Provinces:—Kabba, Ilorin, Borgu, Kontagora, Nupe 16
The Benue Provinces:—Bassa, Nassarawa, Muri, Yola 25
The Central Provinces:—Zaria, Bauchi 35
The Northern Provinces:—Bornu, Kano, Sokoto 50
CHAPTER II
CRYSTALLINE ROCKS 63
Typical Traverses 64
Borgu and the Niger Valley between Jebba and Yelwa 69
Kabba and Ilorin 75
The Central Crystalline Area 86
Southern Muri and Yola 103
CHAPTER III
PETROGRAPHY OF THE GNEISSES AND SCHISTS AND ASSOCIATED IGNEOUS 108
INTRUSIONS
The Gneisses and Schists 108
The Intrusive Rocks 123
_a._ The Older Granites 123
_b._ The Younger Intrusions 129
1. Granites and Pegmatites 129
2. Syenites and Diorites 135
3. Porphyries, Porphyrites, Diabases and Basalts 137
CHAPTER IV
CRETACEOUS ROCKS 144
The Cretaceous Rocks of Muri Province 147
The Cretaceous Rocks of Yola Province 154
The Cretaceous Rocks of Bauchi and Bornu 157
CHAPTER V
EOCENE ROCKS 164
The Sokoto and Lokoja Series 165
The Duguri, Gombe, and Kerri-Kerri Sandstones 175
The Upper Benue Sandstones 184
CHAPTER VI
SUPERFICIAL ACCUMULATIONS 194
In Hausaland and Borgu 196
In Bornu and Katagum 215
In the Middle Gongola Valley 222
In the Benue Valley 223
CHAPTER VII
TERTIARY CRUSTAL MOVEMENTS AND DRAINAGE CHANGES 227
CHAPTER VIII
TERTIARY VOLCANIC ACTION 253
APPENDIX I—Mineral Resources 265
APPENDIX II—Palæontology of the Cretaceous Deposits 273
INDEX OF AUTHORS 287
GENERAL INDEX 289
LIST OF ILLUSTRATIONS
_To face
page_
PLATE I.— FIG. 1. On the Niger above Igbo 12
FIG. 2. Lokoja and Mt. Patti 12
PLATE II.— The Confluence of the Niger and the Benue 20
PLATE III.— FIG. 1. Abuja Rock 28
FIG. 2. Abuja Rock from the Walls of Abuja 28
PLATE IV.— FIG. 1. A Granite Mount near Abuja 48
FIG. 2. The Margin of the Nassarawa
Tableland from Jaginde 48
PLATE V.— FIG. 1. Worn Volcanic Cones near Awe 60
FIG. 2. The Wase Rock 60
PLATE VI.— FIG. 1. Yendam Hill, Yola Province 76
FIG. 2. Craters at Mboi 76
PLATE VII.— FIG. 1. Craters in the Mboi Hills 90
FIG. 2. A Crater in the Mboi Hills 90
PLATE VIII.— FIG. 1. Exfoliation in the Kilba Hills 104
FIG. 2. The Gorge of the Kaduna at Zungeru 104
PLATE IX.— FIG. 1. On the Summit of the Bauchi Plateau;
the Parade Ground at Bukuru 120
FIG. 2. On the Plateau looking Westward
from Bukuru 120
PLATE X.— FIG. 1. A Worn Volcanic Hill on the Bauchi
Plateau near Zamagan 134
FIG. 2. The Twin Cones of Kereng from Panyam 134
PLATE XI.— FIG. 1. Hillocks of Drift on the Bauchi
Plateau near Vom 152
FIG. 2. Hillocks of Drift to the West
of Naraguta 152
PLATE XII.— FIG. 1. A Granite Mount near Bauchi 170
FIG. 2. „ „ „ 170
PLATE XIII.— FIG. 1. The Sandstone Hills of Ture 190
FIG. 2. Ture or Tangale Peak 190
PLATE XIV.— FIG. 1. Dozi Hill, Kerri-Kerri 210
FIG. 2. A Trench-like Valley near Lele
in Kerri-Kerri 210
PLATE XV.— FIG. 1. Cretaceous Sandstones and Shales
at Awe 224
FIG. 2. A Flat-topped Hillock of Drift
above Naraguta 224
PLATE XVI.— Upper Benue Sandstones and Conglomerates
upon a Hummocky Granitic Floor 245
PLATE XVII.— FIG. 1. Inselberge Landscape around }
Abuja }
} _Frontispiece._
FIG. 2. Inselberge Landscape in }
Northern Yola }
PLATE XVIII.— FIG. 1. The Salt Fields of Awe; Volcanic
Hills in the Background 260
FIG. 2. Akiri Saltings 260
PLATE XIX.— Cretaceous Mollusca 286
PLATE XX.— „ „ 286
PLATE XXI.— „ „ 286
PLATE XXII.— „ „ 286
PLATE XXIII.— „ „ 286
PLATE XXIV.— „ „ 286
MAPS—
_To face
page_
I.—PRIMARY WATERSHEDS AND RIVER SYSTEMS 4
II.—THE BAUCHI PLATEAU AND THE NASSARAWA TABLELAND 40
III.—AXES OF ELEVATION 236
IV.—MINERAL RESOURCES 266
V.—COLOURED GEOLOGICAL AND TOPOGRAPHICAL MAP
AND SECTIONS _At end of Volume_
THE GEOLOGY AND GEOGRAPHY
OF NORTHERN NIGERIA
NORTHERN NIGERIA
CHAPTER I
PHYSICAL GEOGRAPHY
Introduction. Hydrographical systems; primary watersheds. The Niger:
Lokoja and Mount Patti; Jebba and the Juju Rock; the rapids of the
Niger. The Benue: the great plain of Muri; sandbanks. The Niger
Provinces: the tableland of central Kabba; domes and inselberge; the
plain of Ilorin; Borgu; Kontagora; typical scenery of crystalline and
sedimentary regions; the plains of Nupe; the sandstone plateaux of the
Niger. The Benue Provinces: Bassa and Nassarawa; the river belt of
sandstones; the Karshi hills and the Abuja Rock; the central plain; the
northern tableland; the plains of Muri; volcanic hills of the middle
Benue; the Wase Rock; the Murchison hills; the plateau of southern Yola;
the lower Gongola; volcanic craters and granite inselberge. The Central
Provinces: Zungeru and the gorge of the Kaduna; the plains and hills of
Zaria; the Bauchi plateau; the Ningi hills; Angass and Kanna; the
eastern plains; the Gongola valley; the cliffs of Tongolan; the Kudu
valley; the Gateri hills; Tangale and Waja; The Peak. The Northern
Provinces: southern Bornu; the Bima hills; the Barbur volcanic plateau;
the Tagum hills; eastern Bornu; firki land and blown sand; Lake Chad;
the central plains of Bornu; the Kerri Kerri plateau; intersecting
trench-like valleys; the plains of Katagum; Kano; Mounts Dala and Kogon
Dutsi; Kazaure; Katsina; Sokoto; the Gulbin Kebbi; dallols and dissected
sandstone plateaux.
The Protectorate of Northern Nigeria comprises an area of about 255,700
square miles, of which the greater part is enclosed within the obtuse
angle formed by the Niger with its great tributary the Benue. The north-
eastern corner is occupied by Lake Chad, whence irregular lines forming
the northern and eastern boundaries between the Protectorate, the French
Sudan and the Kameruns run west and south respectively to the Niger and
the Benue. Within the oblique figure thus formed lie the provinces of
Sokoto, Kano, Bornu, Bauchi, Zaria, Kontagora, Nupe, Nassarawa and
portions of Muri and Yola. West of the Niger lie Borgu, Ilorin and
Kabba, which adjoin Dahomey and Southern Nigeria, while south of the
Benue lie Bassa and parts of Muri and Yola, marching with Southern
Nigeria and the Kameruns. The administrative headquarters of the
Protectorate are at present fixed at Zungeru on the Kaduna in western
Zaria, while the subordinate headquarters of the fourteen provinces
enumerated above are located respectively at Sokoto, Kano, Maidugari,
Bauchi, Zaria, Kontagora, Bida, Keffi, Ibi, Yola, Kiama, Ilorin, Lokoja,
and Dekina. Lokoja also, in addition to being the headquarters of Kabba
province, is from its situation at the confluence of the Niger and the
Benue the natural emporium of European commerce.
Many of the more striking physical features of the Protectorate are now
comparatively well known. The Niger with its flat-topped sandstone
hills, the Benue and the plains of Muri, the peaks of the Murchison
Hills, Lake Chad, and the sands and swamps of Bornu, have become
familiar from the writings of both early and recent explorers.[1]
Equally striking, however, are the broad, flat-bottomed valleys of the
north-west, in which diminutive streams now represent the mighty rivers
of old; the rolling sandy plains of Hausaland[2] with their sluggish
rivers, their isolated granite domes and groups of rounded hills; and
the mysterious Bauchi plateau, the home of innumerable pagan tribes,
whose precipitous walls on the south and west long marked the limit
between the known and the unknown.
_Hydrographical Systems._ (Map. p. 4).
The Bauchi plateau, which rises to a height of 4,000 to 4,500 feet above
the sea, is the hydrographical centre of the Protectorate. From it the
rivers radiate north, south, east, and west; but while at first they
flow in such diverse directions, they become ultimately included within
one or other of the two great hydrographical systems, the Niger-Benue
system and the Chad or Inland system. All the rivers leaving the
plateau, with the exception of those flowing directly north, belong to
the Niger-Benue system. The “primary watershed,” or dividing line
between the two great hydrographical systems, runs from the
neighbourhood of Daura on the northern frontier of the Protectorate
south-westward between Katsina and Kano to the twelfth parallel. Thence
it runs south-eastward to the latitude of Zaria, and then southward to
the neighbourhood of Bukuru, on the summit of the plateau. From Bukuru
it strikes approximately north-eastward to Kerri Kerri, and thence
south-eastward to the Yola border in the neighbourhood of Mubi. Within
the Chad system the only important river is the Yo, of which the rivers
Hadeija, Katagum, Delimi, and Shidya form the headwaters. The Yedseram,
the only other large river of Bornu, forms for some distance the
boundary line between Nigeria and the northern territories of the
Kameruns. In the west the primary watershed of the Niger system
coincides approximately with the boundary of the Protectorate from the
eleventh parallel to the neighbourhood of Aiedi on the eighth parallel,
whence it runs eastward to Kabba, and thence southward through the
Egbira and Kukuruku hills into Southern Nigeria. South of the Benue, the
primary watershed runs through Southern Nigeria and the Kameruns
entirely beyond the southern limit of the Protectorate.
_The Niger._
The Niger enters the Protectorate at Ilo in the north-west, and flows
within the Protectorate for a distance of four hundred and fifty miles
to Idah on the Southern Nigerian frontier. Its breadth varies in places
from a few hundred yards in the dry season to several miles during the
rains. The annual variation in depth may reach at Lokoja as much as
thirty-five feet, and when in flood the river is navigable for small
ocean-going steamers as far as Baro, the terminus of the Baro-Kano
railway, and for light stern-wheelers as far as Bajibo, twenty miles
north of Jebba. Its largest tributaries within the Protectorate join the
Niger on the left, those on the right being comparatively short and
unimportant. On the left are the Kebbi, Malenda, Kontagora, Kaduna,
Bako, Gurara, and Benue; on the right the Wessa, Oli, Teshi, Awo, Oshin,
Oyi, Oro, Kampi, Mimi, and Meadowbank. Of these the Gurara is navigable
in flood for small stern-wheelers for a distance of thirty miles from
the Niger, the Kaduna as far as Wushishi at the foot of the rapids, and
the Kebbi as far as the confluence of the Jega river.
[Illustration: NORTHERN NIGERIA
Primary Watersheds and River Systems.
MAP No. 1.
London. Macmillan & Co., Ltd.
_Face p._ 4.]
The alternation of narrow gorge and open plain is a striking feature of
the Niger valley within the Protectorate. Where the channel is narrow
and bounded by steep cliffs on either side the seasonal variation in
level gives rise to comparatively little change in the aspect of the
river. The blackened rocks in the river bed and the sandy stretches on
either shore may be lost to view, but the flood is still confined within
its walls of rock. It is where a plain tract intervenes in the course of
the river that the contrast between the Niger in flood and the Niger at
low water becomes particularly striking. The sandy bars and shallows
which in the dry season present such obstacles to navigation, as well as
the grassy banks and islands of alluvium with their terraced slopes of
sand and clay, are buried during the rains under a flood, which may
spread for miles over the plain on either side of the river.
At Idah the Niger narrows and flows swiftly between low cliffs of white
sandstone which pass northward into stretches of wooded rising ground on
either bank. Ten miles above Idah a group of flat-topped hills approach
the right bank of the river, only to turn off abruptly inland in a line
of buttresses six to seven hundred feet in height. Farther on, the low,
flattened ridges on the left bank run out in a line of rounded hills:
and beyond, the river broadens and a broken plain stretches inland to
distant groups of lofty hills. Nearing Itobe Mount Purdy stands out, a
solitary cone, in advance of the level-crested plateau beyond. From
Itobe to Igbo the Niger flows through the most picturesque portion of
its lower course.[3] The hills rapidly close in, the channel becomes
narrow and rocky, lofty granite domes and rounded hills separated by
narrow gloomy valleys rise up on either bank, and on the left the level
crest of the plateau looms behind on the sky-line (Plate I, Fig. 1).
Below Lokoja the river opens out, the hills die down on either side, and
from the left bank a broken hummocky plain stretches inland to the base
of the Bassa hills, while at Lokoja itself flat-topped Patti (Plate I,
Fig. 2), with its projecting buttresses and the red roofs of the
cantonment at its base, stands sentinel over the confluence of the Niger
and the Benue (Plate II).
An extensive alluvial plain annually flooded during the rains stretches
away to the Benue from the left bank of the Niger for long miles above
Lokoja. The right bank is occupied more or less continuously by the worn
escarpment of the plateau of which Mount Patti forms the southernmost
extremity. Above Koton Karifi the hills reappear on the left bank, and
for more than twenty miles the Niger flows between the broken walls of
what has once been a magnificent river gorge. In places for considerable
distances the flat-topped hills still rise up with precipitous slopes on
either side in all their ancient grandeur. More frequently, however, the
escarpments are rounded off and much eroded by lateral valleys, while at
times they recede to a distance, and leave between themselves and the
river a more or less extensive swampy plain, with here and there a flat-
topped remnant of the once continuous plateau. Before Baro is reached,
however, the plateau has disappeared on the right bank, and on the left
is represented only by more or less isolated flat-topped fragments. The
bend of the Niger at Egga and Kacha marks the eastward limit of an
extensive alluvial flood plain, which includes the lower courses of the
Kampi and the Bako, and stretches westward on the north bank as far as
the Kaduna. On the south bank the pinnacle peaks of Egbom[4] tower high
above the river, while a flattened ridge extends westward as far as
Pategi. Beyond the Kaduna the Niger flows through a broad and shallow
valley with extensive swampy plains on either hand, and only at Shonga
on the right bank and at Rabba on the left does the river approach the
rising ground which elsewhere bounds the valley in the distance.
At Jebba the hills close in on either side, and the Niger issues by a
double channel from a picturesque gorge which extends northward as a
deeply cut and rocky valley as far as Bussa, a distance of seventy-five
miles. For eight hundred yards above Jebba the gorge is narrow and
bounded by vertical walls of quartzite. The Juju Rock[5] and the other
islands which interrupt the channel mount steeply from the river, the
rapid current pours over sunken rocks which rise abruptly from the river
bed, and mighty whirlpools bear witness to the depth of the water.
Beyond Jebba the quartzite gives place to granite, the vertical walls
become rounded, wooded slopes, the river broadens its rocky bed and
flows in a narrow valley between two belts of broken hilly ground which
lead up to the plains of Kontagora and Borgu, five hundred feet above
the level of the river. In spite of the obstructions in the channel the
river is navigable with caution for small stern-wheelers as far as
Bajibo. Above Bajibo, however, navigation becomes precarious and
dangerous even for native canoes. The current becomes more violent;
rocks, cascades, and whirlpools more numerous, and small rapids more and
more frequent, until above Leaba the Niger flows in a rocky gorge which
leads to the base of the Great Rapid of Wuru.[6] Here the river is
formed of two branches with a rocky island between. The fall of forty
feet between the summit and foot of the rapid is distributed over some
thirteen hundred yards of river, giving a slope of at least 1 in 100 and
a current of from fifteen to eighteen miles an hour. In the rains the
granite boulders are completely submerged under a seething flood, and
even in the dry season the water is thrown up by the projecting rocks in
sheets of foam, and the roar of the rapid is heard for long distances on
either side.
A stretch of two and a half miles of comparatively smooth water, broken
only by some small rapids and cascades, separates the Great Rapid of
Wuru from the rapid of Patassi, which is composed of two sets of
cascades, each twelve to fifteen feet in height and separated by a
straight reach of some two hundred and eighty yards. Twenty miles
farther on, the third important set of rapids is reached, the rapids of
Garafiri, whose cascades and whirlpools occupy a stretch of about three
miles between the villages of Garafiri and Malali. At Bussa, the scene
of Park’s disaster, the channel is still much obstructed by projecting
rocks and islands, and the small cascades and the rapid current still
render navigation both difficult and dangerous. The right bank of the
river is here formed of low, wooded rising ground, while on the left
ranges of low hills rise in series one behind the other. Soon, however,
the hills die down and the rocks disappear from the river bed, and at
Warra the river is broad and smooth with an extensive flood plain on
either side. From Warra to Ineku the Niger is formed of two branches, of
which the more westerly preserves the calm aspect of the river at Warra,
while the more easterly becomes again obstructed by rocks and rapids.
Between Ineku and Yelwa the river is bounded by low, rounded hills
covered with scanty forest, innumerable islands rise from the river bed
supporting an abundant vegetation, while several sets of lesser rapids
obstruct the channel. At Yelwa the banks become low and swampy, the
hills begin to recede from the river, and five miles farther on, near
the village of Sakassi, the last set of minor rapids occurs in the
channel. Beyond Sakassi the whole aspect of the river changes, the
channel becomes broad and sandy and obstructed by nothing more serious
than sand bars. On the north bank a broad, swampy flood plain, with low
rising ground in the distance, bounds the river as far as the mouth of
the Kebbi, and from the Kebbi to the swamps of Ilo an open sandy plain
stretches northward between the river and the base of the plateau. On
the south bank the gently undulating plain which bounds the river
stretches westward to the Dahomey border, broken only by some scattered
groups of flat-topped hills.
_The Benue._
The Benue enters the Protectorate to the east of Yola, and after a
course of nearly five hundred miles joins the Niger at Lokoja. Its
principal tributaries are on the right bank the Gongola, Kudu, Wase,
Simanka, Ankwe, Modu and Okwa; on the left bank the Faro, Ini,
Mainarawa, Tarabba, Donga and Katsena. The Benue is navigable when in
flood throughout its whole course for steamers of light draught. The
Katsena is navigable as far as Katsena Allah, the Gongola as far as
Nafada, and the Donga and Tarabba for considerable distances up stream.
In the matter of scenery the Benue presents a striking contrast to the
Niger. There is on the Benue no picturesque alternation of narrow gorge
and open plain; no rapids trouble its surface from Lokoja to the flat-
topped hills of Yola, and even in the dry season it is only at long
intervals that rocks are exposed in its sandy bed or in its steep banks
of alluvium. The precipitous front of Mt. Patti and the level crest of
the plateau on the right bank of the Niger are visible from the Benue
for twenty miles above Lokoja, across the great alluvial plain which
marks the confluence of the two mighty rivers. Above Mozum a range of
flat-topped hills with occasional peaks follows the right bank for some
distance, but soon turns inland and leaves a broken plain between the
river and the base of the hills. A similar range, also separated from
the river by a broken plain, runs parallel with the left bank a few
miles inland. The escarpments of these ranges on either side of the
river represent the walls of a broad flat-bottomed valley which is
comparable in origin with the steep-sided gorge of the Niger above
Lokoja. Here on the Benue, however, erosion has been more intense, and
the hills have been cut farther back on either hand.
Above Umaisha a tract of broken country stretches eastward on the north
bank and gives place in the neighbourhood of Loko to alluvial swamps in
the foreground, with undulating sandy plains beyond. On the left bank,
meanwhile, the hills approach the river at Amara and pass eastward into
a hummocky plain, which is bounded to the south by a distant range of
flat-topped hills. Above Loko the Doma Hills touch the river at Udeni on
the right bank, but turn off abruptly inland with their low, buttressed
slopes overlooking an extensive swampy plain which stretches eastward as
far as Akwaneja. Beyond Akwaneja the right bank assumes a similar aspect
to that of the left. The river is bounded by low, well-wooded banks,
with here and there a projecting mass of sandstone, while a hummocky
plain stretches inland on either side to distant rising ground. The
volcanic tract above Abinsi introduces an agreeable change into the
monotony of the scenery. The worn and weathered cones and banks of lava
stand out picturesquely from the wooded plains, and on the river-side
the black basalt and the yellow sand offer a pleasing contrast to the
eye. Soon, however, the low banks resume their familiar aspect and a
featureless plain spreads out on either hand. Nearing Ibi the right bank
becomes low and swampy, and the great plain of Muri opens out beyond.
From Ibi to Lau the channel is broad and sandy, and the river winds
about amongst everchanging sand-bars and banks of alluvium. In the rains
broad stretches are flooded on either side, and for long distances on
the right bank the plain beyond becomes an almost impassable swamp,
while on the left long tongues of marshland run up the tributary
valleys. Far to the north the peaks of the Murchison Hills[7] tower high
above the plain, while to the south the rock of Bantaji guards the
swamps of the Donga.
PLATE I
[Illustration: ON THE NIGER ABOVE IGBO.]
[Illustration: LOKOJA AND MOUNT PATTI.]
Farther east, and especially between Numan and Yola, the river plain is
traversed by a large number of anastomosing channels, which in many
cases are only connected with the main stream during the rains, and in
the dry season are cut off by sand-bars into strings of shallow pools
and marshes. Alluvial mud covers the rocks for some miles inland, and
while in the rains travel on foot is impossible and the villages are
isolated one from another, in the dry season the ground shrinks and is
broken everywhere by wide cracks. This alluvial plain is perfectly
level, so that the small volcanic mounds, dotted here and there, which
in a more varied country would be overlooked, form conspicuous
landmarks. The most striking of these hillocks is Jeme Hill, below
Numan, which stands on the river bank and can be seen for twenty miles
around. (A. L.)
The Benue plain, which is so extensive in Muri province, contracts near
Lau, and thence eastward it is limited on both sides by plateau areas
which rise suddenly and steeply at distances of fifteen miles from the
river. These masses of hills with their varying outlines introduce a
pleasing change into the hitherto monotonous river journey, and contrast
sharply with the river plain. Here the plain of the Benue is like a
broad and shallow trench, bounded by sharply rising walls, whose height
is small compared with the distance between. At the same time the height
is considerable in itself, for the rise to the plateau is well over a
thousand feet. (A. L.)
These hills running eastward parallel to the river are breached by the
Gongola plain, about ten miles wide, entering from the north, and by the
plain of the Ma-Ini and Maio Belwa entering from the south. Farther on
the hills come in again. To the north they are rather scattered and
detached, with long tabular outlines, or, as in the Bagele Hills, with
terraced sides and abrupt scarps. On the other side, though flat-topped
hills do occur, the outlines are usually more varied, while south of
Yola rises the typical plateau of the Vere Hills—a solid mass fronting
boldly outwards upon the surrounding plains with continuous rocky faces
indented by a few deeply-cut and narrow valleys. Meanwhile the plain of
the Benue with its flat and marshy character continues eastward beyond
Yola and the Faro river into the German Kameruns. (A. L.)
Along the whole course of the Benue from Lokoja to Yola exposures of the
underlying rocks are very infrequent, and this remark applies equally to
the lower courses of the tributaries. The main trunks of the Benue
system have long passed the stage at which they effected any appreciable
erosion, and now serve only as the channels along which detrital
material brought down by the head-waters is transported to the Niger.
The finer portion of the material is carried in suspension, and is
responsible for the covering of alluvial mud found over those parts of
the river plain which are inundated during the annual rise. The coarser
portion is rolled along the bottom and gives rise to the sandbanks.
These are covered in flood, but for the other part of the year they are
a constant feature of the river scenery. The stream, much lessened in
volume, swings first to one side and then to the other of the wide river
bed, with the broad sandbanks occupying the bends. (A. L.)
The sand is rather coarse, principally of clean grains of quartz and
felspar, but at times with pebbles of vein quartz or occasionally of
polished limonite. Typically, a spread of sand, many acres in extent and
with its level surface only a few feet above the water, ends on the
down-stream side in a slope which plunges into deep water at perhaps
40°. While the river is falling, the formation of these sandbanks can be
well studied. The water, covering the sandbank by only a few inches, is
seen to sweep grains of sand or occasional pebbles along over the level
surface till they reach the end of the bank and roll down into deep
water, thus helping to form the next lamina. In this way the sandbank is
built up of successive laminæ inclined down stream at a steep angle to
the horizontal, and wherever a bank is cut into by a change in the
direction of the current the inclined lamination is beautifully
displayed. The lamination is of course only a particular instance of the
“current bedding” produced whenever detritus in process of deposition is
subjected to currents of air or water, but it is important to understand
that the oblique lamination of the sandbanks is brought about by the
river currents, and not by the wind as has been supposed. Any slight
rearrangement of the surface layers by wind is obliterated by the next
rise of the river. (A. L.)
_The Niger Provinces.—Kabba, Ilorin, Borgu, Kontagora, Nupe._
The orographical structure of Kabba and Ilorin is somewhat complex. The
primary watershed of the Niger basin which runs from Kabba to Aiedi and
thence westward a few miles south of the Ilorin frontier, is not defined
by any range of hills or mountains, but by the crest line of a lofty
plain which, like a low, extended arch, slopes gently northward towards
the Niger and southward towards the sea. In central Kabba the high plain
assumes, to some extent, the character of a tableland, which is bounded
to the north-east by the Lukke hills and the broken ground beyond the
Mimi valley. To the east and south-east the tableland is abruptly
limited by the worn escarpments of the Shokko Shokko, Agwede and Egbira
hills, at whose base, 500 feet below, are spread out the broad, well-
watered and well-wooded plains which stretch away eastward to the Niger,
and of which passing glimpses are caught from the river below Itobe. To
the south these undulating plains slope gently upward and inward from
the Niger and lose themselves in the narrow valleys of the Kukuruku
Hills.
Although such in brief is the orographical character of the major
portion of Kabba and Ilorin, the actual conditions of the surface and
the peculiar character of the scenery to some extent obscure the issue.
In central Kabba the undulating surface of the tableland is in places
densely wooded, in places covered with thin bush, in places open,
grassy, and park-like with cultivation near the towns and villages,
while above the plain at frequent intervals rise isolated rounded hills
or groups of hills, or even short rocky ranges, which form secondary
watersheds between the minor streams. Similarly, on the lower plains
adjoining the Niger, the attention is arrested by the dome-shaped island
mountains and picturesque clusters of rounded hills which rise abruptly
into view. The full significance of this peculiar type of scenery can
only be grasped when gazing from the summit of one of the isolated hills
themselves, or from some favourable spot on the crest of the escarpment
which separates the higher from the lower plains. Then all the minor
irregularities disappear, and the level, wooded surface of the plain can
be seen stretching up to and around and between the bases of the hills
which rise like rocky islands from a verdant sea. The hills may be
covered to the summit with angular _débris_ and scanty forest, or the
lower slopes may be littered with exfoliation fragments, amongst which a
few trees find a precarious footing, while the summit may be smooth and
bare, with a few large boulders perched picturesquely in apparently
impossible positions. The contours of the bare and treeless summits are
frequently particularly graceful with beautiful domes and cupolas rising
from gently-rounded slopes of granite or gneiss. In many cases a
precipitous path leads to the village which caps the summit, while for
long acres round the base the plain is cleared and cultivated. (Plate
XVII.)[8]
The northern part of Kabba is more rugged, the population is more scanty
and forest more abundant. The rocky valley of the Mimi is bounded to the
north by the broken, hilly ground between Jakura and Akpara, which
passes to the east and north under the flat-topped sandstone hills which
fringe the Niger. The Jakpara Hills with their conical peaks upon
rounded bases are landmarks for miles around. The white quartzite hills
in the neighbourhood of Lukke alternate with belts of soft schists which
support a luxuriant forest growth. Farther west by Moppa, Sanlu and
Takete detached groups of granite hills rise upon an undulating plain.
The thicker vegetation, however, obstructs the view, and it is only when
the landscape is surveyed from the summit of one of the higher hills
that the structure is seen to be very similar to that of southern Kabba.
To the west, Ogga and Eri are set within a cluster of rocky hills, while
to the north there is a comparatively rapid fall to the plains of the
Kampi. Between the Kampi and the Niger a steep-sided range of sandstone
hills capped by the pinnacle peaks of Egbom (p. 7) runs out westward in
a succession of rounded ridges two hundred feet above the level of the
river, and not until near Share is the river belt of sandstones again
bounded by a low escarpment. The Shappa and Orissa Hills form the only
prominent range in central Ilorin. The Shappa Hills are of granite, and
many of the peaks are gracefully rounded and most picturesque, while the
quartzites and gneisses of the Orissa hills give more rugged and
irregular contours. West of the Orissa hills lies the great plain of
Ilorin, thickly populated and well cultivated, in broad undulations
trending generally north and south, with scattered inselberge and many
kopjes of granite boulders, but with no conspicuous ranges or groups of
hills. The Jebba hills, which extend southward for some distance towards
Ilorin, pass westward into an extensive tract of hilly country, high
lying, little known and largely uninhabited, which forms a natural
boundary between Ilorin and Borgu.
The primary watershed of the Niger basin follows the frontier of the
Protectorate more or less closely from the eighth parallel to the south
of Ilorin as far as Okuta in the west of Borgu, whence it runs northward
to the latitude of Nikki before turning off westward into Dahomey. With
the watershed the high plains, along whose crest it runs, also swing
round to the west of Borgu and fall off gently towards the Niger on the
right, while along the river itself from Bussa to Jebba a narrow belt of
broken country marks the transition from the high plains of Borgu to the
gorge-like valley of the Niger.
Southern Borgu is one of the least inviting districts in the
Protectorate. The rocks are concealed almost everywhere by recent
alluvium or by surface ironstone,[9] which in places forms extensive
platforms entirely bare of vegetation. Where rocks do emerge from the
superficial accumulations they seldom form hills. The granite hills
around Kiama are an exception to this rule, and along the western
frontier a number of granite domes rise from two to three hundred feet
high. These domes, however, are separated by wide stretches of country
over which all geological processes seem to have come to a standstill.
The drainage system is very poorly defined, and for the most part the
water is carried off, not by stream courses with sand and boulders, but
along shallow, soil-covered depressions in the plain. Such streams as
there are dry up soon after the end of the rains, and the natives then
resort largely to wells dug in the surface soil. The crests of the
gentle undulations follow one another on the sky-line in endless
succession, and the journey becomes monotonous in the extreme. The
northern part of the province is diversified by a number of sandstone
hills, and some swift, clear streams, fed from springs at the base of
the hills, afford a welcome change to the jaded traveller. (A. L.)
In the provinces which lie to the east of the Niger the intimate
relation which exists between the character of the scenery and the
nature of the rocks becomes increasingly evident. In Kabba, Ilorin, and
Borgu, where crystalline rocks of a granitic or gneissic character cover
by far the larger part of the surface, the prevailing and most
characteristic type of scenery is that of rolling sandy plains with
isolated hills or groups of hills rising abruptly above the general
level. On the other hand, where sedimentary rocks, such as sandstones
and ironstones, cover the surface, as in the river belt of sandstones in
Kabba and Ilorin, the characteristic type of scenery is that of steep-
sided and flat-topped ranges and plateaux, with detached tabular and
conical hills rising from the lower plains. Over the greater part of
Kontagora province, to the north of the parallel of Kontagora itself,
granites and gneisses form the surface rock. In the west, near N’gashki,
a group of granite hills rises abruptly from the plain which stretches
away to the east, with its surface diversified in typical fashion by
occasional inselberge. A belt of slaty rocks much pierced by quartz
veins runs along the left bank of the Niger from N’gashki to Yelwa, and
gives rise to a stretch of broken country adjoining the river. The
granites and quartzites in the lower valley of the Malenda river also
give rise to much rough and stony country, which culminates in the
quartzite hills of Maibirro. Beyond Jindani and Yelwa the plain
reappears in a more typical character, and stretches northward as a
gently undulating surface covered with deep, sandy drift, from which
emerge at long intervals low, rounded, or rocky hummocks of granite or
gneiss.
PLATE II
[Illustration: THE CONFLUENCE OF THE NIGER AND THE BENUE.]
[Illustration: It is in the east of the province, however, between
Banaga and Kotonkoro, that the landscape assumes the typical aspect of
the high plains of Hausaland. In front, behind, and for long miles on
either hand, the open, sandy plain stretches away to the horizon, with
here and there a low turtleback or heap of granite boulders, and at long
intervals a loftier dome or a group of rounded granite hills. The rivers
flow in almost imperceptible valleys in broad, sandy channels, or over
smooth and bare surfaces of rock, level with the plain, while the low
watersheds between the rivers are crossed almost unconsciously by the
traveller.
The country around Kotonkoro and to the south-west is extremely
picturesque. Numerous domes of bare rock rise from a grassy park-like
country. The plain around one of these domes passes imperceptibly into a
gentle slope of sandy wash, which leads the eye up to the spring of the
dome. As the path winds along, first one and then another of these domes
engages the attention. On approaching Mahorro two of the highest hills
have flattened summits and are found to be capped by about twenty feet
of stratified ironstone. To the north-east of Kontagora these outliers
are more numerous, though seldom of a sufficient size to mark on the
map. The country is in consequence very rough and broken and the track
becomes difficult and stony. The poor agricultural prospects, however,
are compensated for by good iron ore, and the crude native furnaces soon
come to be a familiar sight. When these outliers are finally left behind
at Libo, a belt of very rough broken country has to be crossed before
descending to the Kara river. (A. L.)
The town of Kontagora is set in an open undulating sandy plain, deeply
covered with surface drift, which stretches eastward to Moriga and the
Kara river, with its surface diversified by a few isolated sandstone
hills near Kontagora and a group of granite hills at Beri. To the south
of Kontagora begins the sedimentary series which stretches southward
into Nupe province and westward to within a few miles of the Niger. The
change in the nature of the surface rock is accompanied by corresponding
changes in the character of the scenery. Here at first are the same
rolling sandy plains, if possible more deeply covered with drift than
before; but here are no turtlebacks or lofty domes of granite forming
conspicuous features in the landscape, no rivers flowing over smooth
surfaces of rock in valleys so shallow that they are almost
indistinguishable from the plain. In place of these are low, flat-topped
sandstone hills rising singly or in groups above the general level, and
deep and narrow canyon-like valleys cut down, it may be, fifty or a
hundred feet below the surface of the plain. A typical group of hills
with their level summits of hard ferruginous grit and their slopes
covered with rounded boulders of the same occurs at Madara, a few miles
south of Kontagora, while at Kagara, Kaboji, and Sonko the streams run
in typical canyon valleys, whose presence is unsuspected upon the plain
until the chasm opens abruptly at one’s feet. The larger rivers flow in
broad, trench-like valleys, with their steep and wooded slopes covered
with rounded pebbles and boulders of ironstone and grit. To the west the
plains pass into the belt of hilly ground which leads to the Niger
valley, and which in its eastern portion shows the characteristic
tabular outlines of the sedimentary series, and in the west, adjoining
the river, the rugged irregular contours of broken crystalline hills.
The province of Nupe is somewhat unequally divided into two parts by the
Kaduna river. With the exception of the quartzite and granite hills of
Jebba and Kailema and the crystalline rocks of the eastern corner of the
province between Badegi and the Gurara river, the entire surface is
covered by sandstones, which are continuous with the sedimentary series
of southern Kontagora. West of the Kaduna, however, the high plains
become much dissected and broken up into extended tabular ridges and
irregular plateau-like masses, with the tracks leading over a succession
of low, well-wooded hills and shallow, flat-bottomed valleys bounded by
rough and stony slopes. On either side of the Kaduna rise isolated
tabular hills and short ranges separated by long stretches of open
plain, while to the east of Bida there open out the great rolling sandy
plains of Nupe, which extend from the Bako river to Badegi and southward
to the bend of the Niger. Beyond Kacha the character of the landscape
again changes: scattered hills and short ranges diversify the surface
and coalesce southward into the extended flat-topped ranges and
plateaux, whose wooded slopes dip steeply down to the narrow ledge of
crystalline rock which fringes the Niger. In the north-eastern angle of
the province beyond Badegi the sandstones give place imperceptibly to
granites and gneiss. The sandy plains of Nupe are continued eastward,
but their drift-covered surface presents now the characteristic features
of a crystalline floor. Knobs and hummocks and low ridges of gneiss
project here and there above the general level, while lofty groups of
rounded granite hills, like those of Gaung, rise abruptly and
picturesquely from the plain.
The sandstone plateaux which bound the Niger to the north of the Gurara
river are continued southward as far as Koton Karifi, and pass eastward
into Nassarawa as a dissected system of flat-topped and conical hills.
To the south of the confluence of the Niger and the Benue, the scenery
on the left bank of the Niger in Bassa province, as already described
(p. 6) is that of a hummocky plain with groups of rounded hills in the
foreground, backed and capped by lofty plateaux and conical hills. Such
a type of scenery is readily explicable as that of a dissected
sedimentary series resting upon an irregular crystalline floor of
granite and gneiss.
_The Benue Provinces. Bassa, Nassarawa, Muri, and Yola._
With the exception of the narrow strip of crystalline rocks exposed
along the Niger, the province of Bassa, so far as known, is entirely
covered by the sedimentary series whose western and northern escarpments
are visible respectively from the Niger and the Benue. These
escarpments, however, which from a distance appear continuous, are found
on nearer view to be much incised by river valleys which have been cut
back for long distances into the high ground behind. A system of lofty,
flat-topped and conical hills, extended ranges and plateau-like masses,
separated by narrow, steep-sided valleys, has thus been dissected in the
north and west out of the once continuous tableland. In the interior the
relief lessens, and the surface of the tableland becomes broken up into
a succession of tabular ranges and rounded ridges, separated by shallow
valleys well cleared for cultivation. The hills are usually thickly
wooded and the paths frequently rough and stony and covered with pebbles
and broken fragments of ironstone. In the south the Akpoto country
becomes in places particularly hilly and difficult, while the little
known Munchi country in the east is said to open out into undulating
park-like plains beyond the belt of forest which fringes the river.
For purposes of description the province of Nassarawa may be
conveniently divided into three portions, the river belt of sandstones,
the central crystalline plains and hills, and the northern tableland.
The Gurara river forms the greater part of the western and northern
boundary and drains all but the eastern portion of the elevated plains
in the north. The river belt of sandstones between the Gurara and the
Okwa in the west presents the characteristic and familiar scenery of a
dissected sandstone tableland. Beyond the Okwa river the flat-topped
hills and ranges die down into undulating sandy plains which stretch
eastward as far as the Modu river, while the detached group of hills,
which strikes inland from Udeni, is the last prominent tabular range on
the lower Benue. Farther east, the low-lying plains stretch into the
unexplored land of the Munchis, while to the north the trade route from
Loko to Ibi leads over well wooded undulating plains deeply covered with
sandy drift. The solid rock is frequently exposed in the beds of the
larger rivers and streams which have cut their channels through the
superficial accumulations; and here and there, as at Doma and Lafia,
groups of low rounded sandstone hills and projecting hummocks of grit
diversify the surface of the plain.
Central Nassarawa is, in the west, particularly broken and rugged.
Around Gwombe and Gwagwada and as far east as Buga a mass of low rounded
hills has been carved out of a series of soft micaceous schists. The
rocks are much decomposed on the surface into red gritty earth which
supports an abundant vegetation and in which the rivers and streams have
cut deep and narrow channels. To the north these rounded hills pass into
a tract of exceedingly difficult and broken country where precipitous
hills of granite and gneiss, separated by narrow rocky valleys, rise to
a height of two thousand feet above the sea. This mountainous tract, of
which the Karshi hills are the northern prolongation, forms the
secondary watershed between the river systems of the Gurara and the
Okwa. Beyond the Karshi hills a lofty and rapidly undulating plain, in
places rough and stony, and much diversified by inselberge (Plate IV,
Fig. 1) and short ranges of granite hills, stretches north-westward
towards Abuja. The Abuja Rock, a typical granite mount, streaked and
channelled by the summer rains, is a far-famed local landmark and an
object of superstitious veneration to the inhabitants of the surrounding
country. (Plate III, Figs. 1 and 2.)
Eastward from Buga to Nassarawa extends a broken plain with hummocks of
gneiss and detached groups of granite hills. At Kurudu, the Okwa river
flows through a narrow picturesque gorge between two rounded granite
hills, with the bare rock in the river bed honeycombed with potholes.
Nassarawa itself is set in the midst of gently undulating park-like
plains, well wooded and well cultivated, which extend northward as far
as Keffi and are continuous southward with the sandy plains of the river
belt. To the east they are limited by the rocky mass of the Anagoda
hills and the broken hilly ground between Keffi and the Modu river. To
the north of Keffi there opens out the great central plain of Nassarawa
which extends from the neighbourhood of Abuja in the west to the Mada
country in the east, and is limited northward by the worn escarpment of
the northern tableland. Turtlebacks and kopjes of granite and broken
hummocks of gneiss rise at intervals from the undulating surface of the
plain, while the minor irregularities are obscured, as usual, by deep
sandy drift. The larger rivers flow in broad sandy beds with long
stretches of swampland on either hand, or, where the drift is thinner,
in narrow rocky channels with belts of broken plain on either side from
which the surface drift has been in large part removed.
PLATE III
[Illustration: ABUJA ROCK.]
[Illustration: ABUJA ROCK FROM THE WALLS OF ABUJA.]
Viewed from the central plain, the margin of the northern tableland is
in places particularly picturesque. A well-marked, though much worn and
rounded escarpment (Plate IV, Fig. 2), extends continuously from the
neighbourhood of Kari in the west by way of Kukwi, Tatara, and Jaginde,
to Darroro and Assab in the east, and the skyline is variously
ornamented by domes, cupolas, conical peaks and perched blocks, with
here and there, as beyond Darroro, a jagged wall of granite rising
perpendicularly from the plain. The escarpment is steeper in the east
than in the west, and beyond Kari it is lost in the broken hilly ground
around Abuja, which falls in a succession of rocky steps to the plains
of the Gurara. The tableland itself slopes gently towards the Gurara,
and in the west, in the picturesque Gwari country, the numerous domes
and turtlebacks of granite afford ideal sites for pagan villages.
Parallel with the escarpment runs a narrow belt of broken country which
represents the worn edge of the tableland, while to the north the
undulating park-like plains, diversified by inselberge and groups of
rounded hills, wrapped closely round with sandy drift, recall in every
detail of scenery the central plain of Nassarawa below. A belt of broken
rising ground, stretching northward from Jaginde, separates the Gurara
system from the Kogin Kogom which drains the grassy plains of Morrua and
issues behind Jaginde from a narrow rocky valley cut backward for a
considerable distance into the plateau. The flat-topped granite massif
of the Kagoro hills, whose precipitous wall forms the continuation of
the escarpment from Darroro to Assab, projects also on the north above
the plateau and overlooks the lofty undulating plains of Morrua and Akul
which stretch away northward towards the Kaduna. Far away to the south
of Darroro, the peaks of the Mada hills can be seen across a broken
wooded plain: but this eastern portion of the province is as yet unknown
and unexplored.
The great plain, which forms the major part of Muri province, extends
from the base of the Murchison hills on the north to the frontier of the
Protectorate on the south, and from the Ankwe river in the west to the
Wurkum and Shebshi hills in the east. With the exception of the narrow
margin of crystalline rock in the north-west which runs parallel with
the Murchison hills, and the broader belt in the south between Takum and
Bakundi, the plain possesses throughout a floor of sedimentary rock. The
flat-topped hills, which are such a conspicuous feature of the scenery
of the lower Benue and the Niger, are here, however, entirely wanting.
The surface of the plain slopes gently towards the Benue from either
side, and into it the tributary rivers have cut a succession of shallow
swampy valleys separated by tracts of gentle rising ground, which run
approximately at right angles to the Benue. The undulating plain thus
produced is deeply covered with surface drift and relieved only by an
occasional hummock of sandstone or grit. The thin forest, which usually
covers the rising ground, gives place in the neighbourhood of the rivers
to extensive stretches of open grass land, which, in the rains, become
almost impassable swamps.
West of the Ankwe river the surface of the plain is more frequently
broken by ridges of rock, and around Awe and on either side of the Benue
to the south a number of worn volcanic cones are scattered over the
plain. (Plate V, Fig. 1.) Similar volcanic hills project above the
surface on the northern margin of the plain between Yelua and Wase. The
Wase Rock[10] itself, a pillar of trachyte, is a landmark for miles
around. (Plate V, Fig. 2.) At Namu, also, in the north-west of the
province, and at Bantaji on the Donga to the south of the Benue,
solitary hills of similar origin form striking features in the
landscape. Turtlebacks and kopjes of granite boulders are found only on
the margins of the plain where the floor is crystalline. In the north-
west the narrow belt of hummocky crystalline ground, with its inselberge
and groups of granite hills, is limited by the domes and pinnacles of
the little known Murchison hills,[11] while in the south the undulating
plains which stretch along the frontier of the Protectorate from the
Katsena river to the Tarabba present all the characteristic features of
a crystalline floor. In the north-east of the province, between Bashar
and Jeb-jeb, the plains stretch northward up the Kudu valley but are
limited to the east by the Ligri, Wurkum, and Jenoe hills, whose steep
and rocky sandstone slopes become in places quite rugged and
picturesque. The Kwana and Mumuye hills, which bound the plain to the
south of the Benue, are largely granitic and present the usual rounded
contours, while the foothills of the Shebshi range are of quartzite and
gneiss and give rise to more rugged and irregular outlines.
The Mumuye and Shebshi hills are portions of an extensive tract of hilly
country which stretches into German territory and which occupies the
southern part of Yola province.[12] The western boundary of this tract
runs almost northward, past Jalingu and Kwana, as if to approach the
Benue at Lau, but instead turns sharply at right angles. Running
eastward from near Kwana there is a practically continuous wall of rock,
and at Bamga a steep and almost precipitous climb brings the traveller
from Lau to Yola on to the rim of the plateau area. From this position
he looks out over the Benue plain several hundred feet below. The river
is far away to the north, while beyond that again, just discernible on a
clear day, is the further wall of hills stretching continuously from the
Wurkum country to the Gongola. Turning south, he sees the tumbled
Shebshi hills as a faint silhouette in the distance, while in the
foreground, slightly below him, stretch sandy uplands with a gentle
eastward slope. The track drops down upon the uplands and then proceeds
just within the rim of the plateau towards Yola. For the first day this
rim is made up of a chain of rounded hills. Then a number of sandstone
outliers arrests the attention. (Plate VI, Fig 1.) The sandstone is
massive and breaks down along vertical faces. The fallen blocks, in
assuming their position of rest, smooth off the irregularities of the
granite base, and from the truncated cones thus produced, the cliff-
sided outliers rise and dominate the landscape, like great fortresses.
From this point the plateau descends gently to the plain of the Maio
Belwa and Ma-ini rivers. (A. L.)
Opposite Lau the hilly ground to the north has similar outlines to those
of the crystalline plateau to the south. Farther east, behind Wuduku,
its appearance changes and the hills are composed of sandstone. Here the
ridge seems to be a bold escarpment limiting the unknown Yam-yam country
in precipitous cliffs, but with a dip slope to the south which plunges
down below the marshy levels of the Benue. Further eastward again there
is another change and the outlines of the hills become regular and flat.
Where the plain of the Gongola enters the Benue valley, they turn at
right angles and run northward parallel to the river. At the angle which
separates the two plains the mass is seen from a distance to be made up
of horizontal beds, with seven or eight terraces on its sides. These
hills, which run up to the west of Kombo, are still, however, unentered
and unexplored. (A. L.)
The plain of the lower Gongola is a repetition of the Benue plain on a
somewhat smaller scale. The floor of the trench is about ten miles wide
and the height of the walls perhaps a thousand feet. On travelling up
the Gongola, Kwaba hill is conspicuous two or three days before it is
actually reached. It is a massive hill of granite with numerous
satellites, of which some are of granite while others are outliers of
horizontal sandstones, bounded by vertical cliffs. The successive beds
are exposed in their sides like string-courses of masonry, the Gongola
outliers differing in this respect from those of southern Yola, which
have been carved out of unbedded sandstones. (A. L.)
The plateau of the Vere hills has been already mentioned (p. 14). It is
completely separated from the Karin mountains, Alantika mountains, and
other hilly ground to the south by broad and level plains. The essential
feature of the scenery is the contrast between the plain and the plateau
masses. They rise up from one to two thousand feet higher than the
plain, with no transitional foothills or slopes. The base is concealed
by sandy wash, and at intervals streams, issuing from the plateau by
narrow ravines, trench down through the compacted wash to form steep-
sided nullahs. (A. L.)
Opposite to the Vere hills and across the Benue stand the Bagele hills.
A little further north is the Hossere Furo. Both groups are of
sandstone, rising up from the smooth grassy plain. The rise, however, is
not immediate, as it is in the Vere hills, but through basal slopes and
foothills toward the higher summits. North-eastern Yola consists of a
high-level plain, elevated considerably above the plain of the Benue,
but passing into it more gradually than do the plateaux to the south.
There is, however, one step of about 200 feet running eastward from Song
to the frontier, while the belt of country twenty miles broad to the
north of it forms rather a curious contrast to the remainder of north-
eastern Yola. Possibly there has been a slight local uplift. Granite is
seen over almost the whole surface breaking down into coarse sand and
giving rise to a hummocky country difficult to travel. The variations in
level are only small, and taken as a whole it is still a plain, but the
breaking down of the granite is going on actively over the whole
surface. Parallel to this country and almost in alignment with it, is an
interesting string of craters and flows of basalt. (Plate VI, Fig. 2,
VII, Figs. 1, 2.) The eastern extremity of this line appears to be at
Song, but westward it extends across the Mboi hills into the Yungeru
country, to what distance is not yet known. The remainder of north-
eastern Yola, with its rocky islets set in extensive thinly wooded
plains, is similar in character to that of so many other parts of the
Protectorate. Some of the inselberge, especially in the Kilba hills
(Plate VIII, Fig. 1.), afford excellent examples of exfoliation. (A. L.)
_The Central Provinces.—Zaria and Bauchi._
The province of Zaria lies almost entirely within the drainage basin of
the Kaduna river, only a narrow belt of country in the south and south-
west being drained by the rivers Gurara and Bako. With the exception of
the uppermost springs of the Kaduna and the lower navigable portion of
its valley below Wushishi, the whole course of the river is included
within the province. At Wushishi the river narrows and its channel
becomes much obstructed by rocks and rapids, as it winds through the
belt of hills which separate the rocky plains of western Zaria from the
lower plains of Nupe. At Zungeru the Kaduna flows in the dry season in a
narrow picturesque gorge between white walls of quartzite, which in the
rains are buried under a rushing foaming torrent. (Plate VIII, Fig. 2.)
From the bend of the river above Girku in the east of the province, the
Kaduna flows south-westward over stretches of drift-covered plain and
through scattered groups of hills with its channel marked by frequent
rocks and rapids, while beyond Girku the river meanders peacefully over
the lofty plains which stretch eastward to the boundary of the province.
The larger tributaries of the Kaduna on the north bank, the Kara,
Koriga, Tubo, and Shika, take their rise upon the high plains of
Hausaland, along whose crest runs the primary watershed between the
systems of the Niger and Lake Chad, and, like the Kaduna itself, they
flow in their middle and lower courses over broken plains and between
groups of hills which separate the minor streams. The Leri river in the
east rises on the high plains of the Bauchi border, while the Dunia and
Serakin Pawa, the principal tributaries on the left bank, as well as the
headwaters of the Gurara, take their rise amidst the scattered hills
upon the lofty plains of the south-east. The Bako river rises in the
hilly ground around Fuka and, with the Gurara, drains the extreme
southern portion of the province.
The hills in Zaria province are of two types; (1) ranges or belts of low
rounded and somewhat broken hills, composed largely of the more
resistant schists, or of a mixture of granite and schist; (2) kopjes,
inselberge and small detached groups of rounded granite hills, with here
and there an extensive elongated mass of granite with a more or less
flattened tabular summit and steep though worn and rounded slopes rising
abruptly from the plains which encircle its base. To the first type
belong the tract of hilly country in the west of the province which
stretches from Mikonkeli to the south of the Kaduna as far as Kagara to
the north, the belt of hills between Gwari and Koriga in the north of
the province, the broken stony country around Fuka in the south, and the
line of hills in the centre of the province from Dorka to Kaserimi. To
the second type belong the solitary turtlebacks and detached masses of
granite hills which so abundantly diversify the plains, while the
Kudaru, Ruruma, and Kerko hills are somewhat worn examples of the steep-
sided granite massifs, whose flattened though hummocky summits form the
retreat of numerous pagan tribes.
Over the greater part of the province the configuration of the surface
presents a very close resemblance to the characteristic scenery of the
plains of Kabba (p. 17). Between and around the island mountains and
groups of hills and tracts of hilly country, spread undulating thinly-
wooded drift-covered plains. The towns and villages are set
picturesquely on the slopes and summits of the hills or in patches of
thick forest in the midst of the plain. For long distances round the
bases of the hills the plains are cleared and cultivated, and from the
summit of any of the isolated mounts the peculiar character of the
landscape impresses itself, as before, upon the observer. Here and there
the solitary hills raise their rounded crests in lonely grandeur, while
around and between them the rivers pick their sluggish course over the
monotonous plain. The lofty drift-covered levels of the south-east are
largely uninhabited and cultivated only by the pagans who daily descend
from their rocky strongholds in the Ruruma Hills or on the margin of the
Bauchi plateau. Only in the north-east between Girku, Leri, and Zaria do
the plains assume the typical aspect of Hausaland (p. 21). The gently
undulating surface is deeply buried in sandy drift, and in places
floored with surface ironstone: extensive stretches of the grassy plains
become swampland in the rains: the landscape is diversified only at long
intervals by occasional turtlebacks and groups of granite hills, while
well-walled towns and villages are set here and there upon the open
plains or built round the bases of the solitary hills themselves, which
serve as watch-towers in times of danger.
The major part of the province of Zaria may be conceived as a shallow,
elongated basin, draining westward, and shut in on the north and north-
east by the high plains of Hausaland and on the south-east by the Amo,
Piti, and Chawai hills, the lofty plains of Katab, and the tableland of
Nassarawa. The greater part of Bauchi province, on the other hand, may
be looked upon as an undulating plain sloping gently towards the north-
east. The south-western portion of the province is occupied by the great
plateau of Bauchi, which is limited on the north and south by well-
defined walls of rock. (Map p. 40). The plateau, however, is not a true
tableland in the sense of an elevated plain marked off on all sides by
precipitous walls from the lower plains around. Like the tablelands of
Kabba, Nassarawa, and Southern Yola, which are at most inclined plains
bounded by escarpments on one side only, the Bauchi plateau from its
summit at Bukuru slopes gently eastward and merges gradually into the
plains of the Gongola, while on the west between the northern and
southern escarpments it is continuous for a short distance between Kwoll
and Assab with the highest portion of the tableland of Nassarawa. The
intimate relation which exists between the Bauchi plateau and the
tableland of Nassarawa is further demonstrated by the continuation of
the escarpment of the tableland beyond Assab to form the southern wall
of the plateau. From Assab the escarpment, here 1,500 to 2,000 feet in
height, runs south-eastward to the neighbourhood of the ninth parallel
before swinging round to the north-east and losing itself amongst the
rocks of Angass, where the single escarpment appears to be replaced by a
succession of rocky steps leading down to the tract of broken country
between Angass and Kanna, which is drained by the headwaters of the Wase
river.
The northern margin of the plateau is more irregular than the southern
and marked by projecting spurs which have now assumed the character of
mountain ranges. In the west the escarpment begins in the neighbourhood
of Kwoll and Miango and stretches westward as a somewhat worn and
rounded cliff, 1,000 feet in height, separating the lofty plains of
Rukuba from the lower plains of Zaria. Beyond Chawai and Piti the
escarpment is continued as the western slope of the Amo, Jengre, and
Gura hills, which run out upon the broken plains to the north, while the
eastern slope of the hills again assumes the character of an escarpment
and bounds the valley of the Delimi as far as the gorge of the river
below Naraguta. To the east of the Delimi, however, the single
escarpment is replaced by at least two steps which interpose the broken
plains of Toro and the summits of the Kwandokaya hills between the
plateau and the northern plains, while to the south-east of Polchi the
escarpments run out upon the plains of the upper Gongola in groups of
picturesquely rounded hills. There is some reason also to believe that
the plateau is stepped or broken in the neighbourhood of Basak in the
Rukuba country, but this point must await further investigation.
The surface of the plateau reaches its highest elevation of about 4,100
feet in the neighbourhood of Ngell and Bukuru, whence the headwaters of
the Delimi, Sungo, Gongola, and Kaduna radiate approximately north,
south, east, and west. Of these rivers the Delimi ultimately joins the
Yo and falls into Lake Chad, the Gongola and Sungo join the Benue, and
the Kaduna the Niger. Thus the few square miles in the neighbourhood of
Ngell and Bukuru may rightly be claimed as the hydrographical centre of
the Protectorate (p. 3), and even the most casual observer cannot fail
to be impressed here with that peculiar character of the primary
watershed which has already been remarked in the case of the western
tributaries of the Niger and the northern tributaries of the Kaduna.
There is no group of rocky mountains, no rugged peak at the
hydrographical centre of the Protectorate, no lofty ranges forming the
watersheds between the radiating rivers: only an undulating, swampy,
alluvial plain, diversified by a few short, broken ridges and low kopjes
of granite boulders. Into these plains, however, the rivers which flow
north, west, and south are rapidly carving narrow rocky valleys, and
their descent to the lower plains is marked in every case by picturesque
waterfalls and narrow gorges cut back for considerable distances into
the plateau itself. The wooded gorge of the Delimi below Naraguta, the
steep waterfalls of the Kaduna between Kwoll and Miango, and the deep
rocky channel of the Kogin Rim above Assab afford striking evidence of
the activity of the rivers. Only in the east where the plateau inclines
gently towards the Gongola do the streams flow in sandy beds between
deep banks of alluvium, with broad stretches of swamp on either bank
wherever the slope is insufficient to drain the summer floods.
[Illustration: THE BAUCHI PLATEAU and the NASSARAWA TABLELAND.
MAP No. 2.
London. Macmillan & Co., Ltd.
_Face p._ 40.]
The Bauchi plateau, in the more restricted sense, may be taken to
include all those portions of the elevated plains which rise above 3,500
feet. Upon this definition the plateau is bounded on the south by the
continuous escarpment which stretches from Sura to Assab, and on the
west by a line drawn northward from Assab to Kwoll and Miango. Beyond
Miango the plateau is again limited by an escarpment as far as the
Jengre Hills, whence the boundary runs southward to Naraguta and thence
eastward along the higher escarpment behind Toro. The eastern boundary
of the plateau is formed by a line drawn approximately from south to
north from Kereng to Toro. The most striking feature of the greater part
of the plateau as thus defined, with the exception of the Rukuba country
to the north, is the bare and treeless aspect of the surface, which
assumes the character of undulating grassy downs extensively cultivated
in the neighbourhood of the towns and villages (Plate IX, Figs. 1, 2).
Only amidst the clusters of rocky hills which diversify the surface do a
few wind-worn trees bear witness to the former extent of the forest,
while round the bases of the hills and among the boulders on their
slopes are set the rectangular compounds of the pagan villages, whose
dark green hedges of prickly cactus add a pleasing touch of colour to
the mellow landscape. The low hills which rise from the plateau have
also a peculiarity all their own, which is to be directly correlated
with the greater diurnal variation in temperature. The turtlebacks and
domes and rounded hills of the lower plains are here conspicuously
absent, and in their place appear heaps of boulders and jagged ridges of
granite, with picturesque peaks and pinnacles and perched blocks
adorning their crests. Here and there, also, in the Rukuba and Kibyen
country in the west and in Sura in the south-east, more or less worn and
rounded volcanic cones rise above the general level of the plateau
(Plate X, Fig. 1). The craters of Kereng (Plate X, Fig. 2) in the south-
east, however, are remarkably well preserved. The surface of the plateau
itself is, moreover, deeply covered with drifted alluvium and provided
in places with a crust of surface ironstone through which the rivers and
streams have in many cases cut to the rock below. Above Assab the drift
has been largely removed from the surface, and between Rim and Woran
numerous grassy, flat-topped hills of red, gritty earth, capped by a
hard ferruginous crust, rise in places to a height of at least 300 feet
above the level of the plateau margin. These hillocks recall in
appearance the tabular sandstone hills of the Niger valley, and bear
striking witness to the former extent and thickness of the superficial
accumulations (Plate XI, Fig. 1). The broken, rocky character of the
marginal belt of plateau on the north in the neighbourhood of Zammagan,
Basak and Naraguta is probably also largely due to the removal of the
covering of drift which formerly obscured many of the irregularities of
the surface. Isolated hillocks of drift indeed are common throughout
Rukuba to the west of Jos and Naraguta (Plate XI, Fig. 2), while at
Basak they may be seen capping the very summit of the cliffs.
Beyond the limits of the plateau proper, a thinly wooded drift-covered
plain, in places somewhat broken and hummocky and diversified in the
usual manner by inselberge and scattered groups of granite hills and
drained by the headwaters of the Gongola, stretches eastward to the
meridian of Bauchi. Similarly, beyond the worn and rounded escarpments
and projecting spurs which form the northern margin of the plateau, an
undulating wooded plain, drained by the Delimi and its tributaries,
extends north-eastward towards the Kano border. The western watershed of
the Delimi valley runs along the hummocky drift-covered plain which
stretches northward between the Gura hills and the detached mass of the
Liruei-Kano hills, and thence through the rocky hilly country round
Tirkunya into the Ningi hills, which form the most striking feature of
the north-western portion of the province. This elongated mass of hills,
which stretches from Bura in the west to Ari in the east, is a typical
instance of those steep-sided and more or less tabular granite massifs,
of which examples have already been noted in the Ruruma hills of Zaria,
the Kagoro hills of Nassarawa and the Vere hills of Yola. The Ningi
massif reaches a higher elevation in the west than in the east, its
slopes are much rounded and incised by narrow valleys, its summit is
broken and hummocky with a scanty soil of disintegrated granite, while
wooded drift-covered plains stretch outward from its base towards Zaria
on the west, towards Kano and Katagum on the north, and towards the
Delimi valley on the east and south.
The surface of these wooded plains rises gently in the north to form the
low watershed between the valleys of the Delimi and the Shidya. The
headwaters of the Shidya rise in the tract of broken hilly country which
lies to the north and north-east of Bauchi, and forms in part the
northern watershed of the Gongola basin. Around Bauchi itself domes and
turtlebacks and groups of rounded hills, rising from the stony and
drift-covered plains, are particularly numerous and often remarkably
picturesque in their outlines, the lion couchant being one of the
commonest forms which the bare granite hills assume. (Plate XII, Figs. 1
and 2.) South of the Gongola the plains slope gently upward to the base
of the Jarawa and Kanna hills, becoming more and more broken and stony
as they approach the watershed of the Wase river. The granite massif of
the Wadai hills between Angass and Sura is only known along the northern
margin, while Angass[13] itself is a particularly broken and rugged
district separated from the Murchison Range by a series of rocky valleys
still largely unexplored. Between Angass and Kanna the high drift-
covered plains of Bauchi pass into a tract of broken country which is
drained by the headwaters of the Wase river and falls rapidly to the
plains of Yergum.[14] The line of fracture to the west which forms the
margin of the plateau appears to be replaced here by a simple flexure or
perhaps by a succession of worn and broken steps. Farther east rise the
Kanna and Kantana hills, with their projecting spur towards Kwankyam,
whose steep though worn escarpments interpose an ascent of 700 to 1,000
feet between the lower and higher plains. The western hills, which are
prolonged northward into the Jarawa country, are composed of crystalline
rocks, and the more prominent peaks possess the usual rounded outlines
of granite hills. The Kantana hills to the east, however, are
sedimentary in origin, and from a distance show the familiar tabular
outlines of horizontal sandstone hills, the narrow plateau on top being,
however, much eroded and incised. Eastward the escarpment appears to be
much stepped, and in the neighbourhood of Tukkur wheels northward to
form the eastern slope of the Jarawa hills which run out upon the plain
of the Gongola.
The eastern portion of the province is almost entirely covered with
sedimentary rocks. In the centre, a little to the south of the parallel
of Bauchi, broad plains stretch eastward from Goram and Lafiangale to
Deba Habe and the Gongola, bounded on the north by the broken sandstone
plateaux which are enclosed within the bend of the Gongola, and on the
south by the slopes of the Kudu valley, the rounded hills of Gateri and
the picturesque peaks of Tangale. In the west the plains are deeply
covered with sandy drift, through which the headwaters of the Kudu have
cut narrow canyon valleys into the sandstones below. Farther east the
summer rains render the plains for long distances almost impassable
swamps which drain imperceptibly east or west, and leave in the dry
season a gently undulating surface of black, gritty clay, baked and
fissured by the summer sun. To the north of the central plain and within
the bend of the Gongola lies a region of detached tabular masses of
elongated or irregular outline, together with groups of worn and rounded
sandstone hills, separated by stretches of sandy plain, presenting all
the familiar scenic features of the dissected sandstone plateaux of the
Niger. The narrow valley of the Gongola, between Deu and Gombe, bounded
by flat-topped ranges on either side, presents a marked contrast in
scenery to the upper course of the river above Baddera, where it threads
its way over the high plains of Bauchi between turtlebacks and domes of
granite. Below Gombe, however, the valley opens out and the river flows
as far as Nafada through an open sandy plain, diversified by occasional
flat-topped sandstone hills. Between Nafada and Ashaka the channel winds
between detached hills and short tabular ranges which rise from an
undulating, thinly-wooded plain, while below Ashaka the river enters the
deeply drift-covered plain which stretches from the base of the Tilde
and Dukul hills on the west to the foot of the escarpment of the Barbur
plateau in Bornu on the east. As far as Jurara and Deba Filani the plain
is thickly populated, well-cleared and cultivated; southward, however,
within the Habe country, the scattered villages are separated by longer
stretches of bush, while the plain is diversified by hummocks of
sandstone and worn hillocks of basalt with the intervening
irregularities of the surface deeply buried in drift. Beyond Deba Habe,
the plain of the Gongola forms the eastward continuation of the central
plain of Bauchi, whose sandy and swampy surface stretches southward to
the base of the Tangale hills.
The south-eastern portion of the province is occupied in part by the
upper valley of the Kudu river and in part by the extensive mass of
hills which, with the exception of the outer margin around Gateri,
Tangale, and Waja, is still largely unentered and unexplored. A series
of narrow valleys, terminated by canyon-like gorges, have been cut by
the headwaters of the Kudu into the elevated plains of Pali and
Lafiangale. The difference in level between the upper plains and the
river bed is greatest in Dugari in the west, where the streams flow in
narrow flat-bottomed valleys, bounded by perpendicular walls of white
sandstone three to five hundred feet in height. From the cliffs above
Tongolan the scenery recalls in miniature the chasms, amphitheatres, and
canyons of Colorado, with their promontories and projecting buttresses,
steep slopes and vertical ledges, while south-eastward towards Yuli the
remnants of a lower plateau mark a former pause in the activity of the
river. Farther east, by Panguru and Putu, the relative height of the
upper plains decreases, the sandstones become broken and inclined, and
the tributary valleys are bounded, not by cliffs, but by rounded, drift-
covered slopes leading upward to the higher undulating plains. The
folded sandstones of Gateri and Ligri give rise to a group of rounded
hills deeply incised by narrow valleys and backed by the ridges and
peaks of the unexplored Wurkum and Pero hills, which stretch north-
eastward towards the Gongola. From Gateri northward to Putu the track
leads over somewhat broken country, while eastward from Putu to Tangalto
stretch open, undulating, swampy plains, diversified only by occasional
conical stumps of basalt.
PLATE IV
[Illustration: A GRANITE MOUNT NEAR ABUJA.]
[Illustration: THE MARGIN OF THE NASSARAWA TABLELAND FROM JAGINDE.]
The tract of country which lies between Tangale and the Gongola is one
of the most picturesque within the Protectorate. The hills rise in
detached groups from an irregular hummocky plain, and form the advanced
guard of the belt of lofty highlands which intervene between the
northern plains and the Benue. While the whole district may be spoken of
generally as Tangale, the Tangale tribe itself occupies a very
restricted area and is lodged within only the westernmost group of
hills, the successive groups to the east and south being inhabited by
the kindred but mutually hostile tribes of Chongwom, Ture, Awok, Tula,
and Waja. Tangalto and Tall, the strongholds of the Tangale tribe, are
set on the slopes of rounded granite hills, picturesquely pierced by
worn pillars of trachyte and basalt, and separated by a narrow
undulating plain from the neighbouring group of granite hills inhabited
by the pagans of Chongwom and Kaltungo. Beyond Chongwom the character of
the scenery entirely changes. In place of the rounded crystalline hills
there rise precipitous and more or less tabular masses of well-bedded
sedimentary rock, forming natural fortresses in the midst of an
undulating park-like plain, diversified by knobs and hummocks and
standing rocks of sandstone (Plate XIII, Fig. 1). At times the summit
ridges are gently curved and the hills are bounded on one side by a
steep escarpment and on the other by a dip slope sinking gradually to
the plains below, while the whole landscape is dominated by the Ture or
Tangale Peak, a picturesque pillar of trachyte rising from a worn and
flattened basal cone of sandstone (Plate XIII, Fig. 2). Eastward, around
Jelengu and Reme, somewhat similar pillars of volcanic rock project here
and there above the sandstone hills, while the worn escarpments of Waja
appear to be continuous with those which form the western boundary of
the lower Gongola valley. The country which lies to the south of Tangale
and Waja is quite unknown. From the upper slopes of the Ture Peak, a
second distant range of hills can be seen running parallel with the
jagged ridge in the foreground, and it is probable that, instead of a
mass of hills or a continuous plateau, there is within this unexplored
land a succession of broken valleys and rocky ridges, forming one of the
last strongholds of paganism within the Protectorate.
_The Northern Provinces.—Bornu, Kano, and Sokoto._
The southern portion of Bornu, which lies immediately east of the
Gongola, presents, so far as the character of its physical features is
concerned, a greater affinity with the central provinces of Bauchi and
Zaria than with the northern Hausa States or with the northern portion
of Bornu itself. The valley of the Gongola to the east of the river is
more broken and hummocky than it is to the west. The sandstones are much
faulted and variously inclined and form belts of rocky plain between the
swampy valleys of the tributary streams. Here and there, as at Wuyo and
Gulani, a circle of jagged ridges forms a natural battlement round a
pagan town, or, as at Balbea, a flattened sandstone hill forms a natural
stronghold in the midst of the plain. More striking, however, are the
level crests, the dip slopes and the steep escarpments of the Bima and
Wadi hills, which form the landmarks of the Gongola plain from Ashaka to
Wuyo. From Gulani north-westward to Ashaka stretches an extensive
undulating plain, broken only by an occasional ridge of sandstone, with
black swamp earth covering all the minor irregularities of the surface
and creeping up the slopes of the sandstone hills themselves. Eastward
the broken plain of the Gongola is bounded by the worn escarpment of the
Barbur volcanic plateau which becomes higher towards the north, and
between Bilaraba and Dukshi is fringed with a series of flat-topped and
conical outliers of basalt. To the east of Gulani and Balbea the ascent
to the plateau is over basal sandstones capped by black volcanic rock.
Farther north, however, in the neighbourhood of Dukshi, the escarpment
is entirely composed of basalt and rises from an irregular crystalline
floor. The summit of the plateau is broken and rocky in the west, deeply
trenched by the tributary streams of the Gongola, and diversified by
numerous worn and rounded cones of ash and lava. Eastward towards
Buratai the plateau descends by a series of low rocky steps separated by
drift-covered levels to the plains of Hong and Ajigin, whose deep
covering of dark gritty clay, hard and fissured in the dry season, forms
an almost impassable swamp in the rains. In the south-east, towards Biu
and the little-known Burra country, the volcanic cones become higher and
more conspicuous, while the surface of the plateau, according to native
information, becomes much dissected and broken up into narrow plains and
rocky valleys.
Two tracts of hilly country, separated by the swamps of Hong and Ajigin,
lie respectively to the north-west and north-east of the volcanic
plateau. The worn and flattened hills of sandstone and ironstone, which
stretch westward from Buni and Ligdar towards Gabai, pass northward into
the high, drift-covered plains of Mutwe and Gujba, which form the
watershed between the Gongola and Lake Chad. In the east the Tagum and
other groups of flat-topped hills around Lua, Ndufa, and Korode
represent the remains of a sandstone plateau once continuous with the
hills of Buni and Ligdar across the swamps of Ajigin. To the north and
north-east of the Tagum hills stretch the great wooded plains of Bornu,
with their gently undulating surface sloping imperceptibly towards Lake
Chad, and with not a single hill nor even a hummock of boulders to
relieve the monotony of the scenery. To the south-east of the Tagum
hills the sandstones run out upon a crystalline floor which stretches
southward into Yola province, with its surface diversified only at long
intervals by island mountains and groups of granite hills.
The plains of Bornu, which stretch northward from the rocks of Chibuk
and the Tagum hills to Lake Chad and the river Yo, are in the east so
imperfectly drained that extensive tracts of their surface become marshy
and swampy during the rainy season. The swamps reach their greatest
development in the valley of the Yedseram and in the immediate
neighbourhood of Chad, where the whole country is flooded after the
rains. Barth[15] has described in a sufficiently vivid manner the
featureless and cheerless aspect of the swampy plains to the south and
south-east of Chad. Along the caravan road from Maidugari to Kukawa
transverse stretches of swamp land (firki) alternate with belts of
slightly higher ground, hard and smooth on the surface, or thinly coated
with loose yellow sand or dark grey loam. The character of the
vegetation reflects immediately the changes in the nature of the surface
soil. In the dry season, after the grass is burnt, the cracked and
fissured surface of the firki land is absolutely bare and treeless, the
hard and smooth surfaces of sandy clay are covered with thin mimosa
bush, while the stretches of loose sandy loam support a more abundant
growth of trees and scrub. The ancient dunes and the undulating sandy
plains which bound the northern basin of Chad support an open forest
growth, while recent accumulations of blown sand are now found only
within the towns themselves, or in places which have been cleared of
forest for purposes of cultivation.
The Maidugari river, which rises on the Yola border and loses itself in
swamp a few miles to the north of Maidugari, has cut deeply into the
superficial sandy drift in the neighbourhood of Maidugari and Maifoni
and formed a comparatively narrow valley with steeply-rounded slopes.
From Maifoni a gently rolling plain stretches westward to Gujba, with
occasional thick-stemmed kukas rising prominently above the low growth
of scrubby trees and prickly mimosa which covers the surface, and with
here and there a dry “tubka” or shallow swampy hollow forming an open
space within the forest. At Murguba, and again at Gujba, the plain is
broken by shallow water-courses, bounded by low and ill-defined ridges
of rising ground. These extended swampy hollows are flooded during the
rains, and in the dry season marked by a line of wells and occasional
pools separated by sandy bars. Beyond Gujba the rolling plains become
again marked by a rapid alternation of sandy rising ground and clayey
shallows draining southward in the rains. Between Chumga and Abakri,
however, rock begins to appear in places on the surface of the drift-
covered undulations which separate the swampy valleys, while a few
isolated flat-topped or rounded hills are scattered over the plains.
From Abakri can be seen to the west the long flattened and rounded
ridges and tabular hills, half buried in sandy drift, which introduce
the Kerri Kerri plateau. Daura and Durua are set within this broken
country, each at the base of a low flat-topped range of ironstone hills,
while the track leads over a succession of stony ridges and shallow
cultivated valleys to the continuous plateau surface which is reached
about half-way between Durua and Momdo. The plateau itself is no higher
than the summits of the marginal hills, which with their drift-filled
valleys represent merely the dissected eastern front of the tableland.
The surface of the plateau, at first thinly wooded and in places
somewhat stony, passes westward into the open sandy undulating plains
around Potiskum, which fall away gently to the north and west into the
valley of the Kogin Shidya. Here and there a low hill or a rounded ridge
of sandstone or ironstone rises above the general level, with the sandy
drift which obscures all the minor irregularities of the surface deeply
banked against its slopes.
The Kerri Kerri plateau, like the tableland of Nassarawa and the
elevated plains of Bauchi, is not a true plateau in the strict sense of
the term, but rather an inclined plane, bounded by more or less worn
escarpments on the south and passing gradually northward into the plains
of Ngizem. The escarpments which stretch from the neighbourhood of Daura
south-westward to the provincial boundary reach their greatest elevation
behind Dozi, Lewe, and Gamari to the north-west of Nafada, where
precipitous cliffs of white sandstone 500 to 600 feet high rise abruptly
from the sandy plains which stretch southward to the Gongola. The
escarpment, however, is in no sense a continuous one, for, as already
noted in the neighbourhood of Daura, the margin of the plateau has been
much cut up and dissected, especially in the east, into a system of more
or less detached flat-topped ranges and tabular masses separated by
narrow valleys or by short stretches of open plain.
Westward where the escarpment is higher, as in the neighbourhood of Dozi
and Kadi, the cliffs are furnished with the characteristic buttresses
and indentations of horizontal sandstone rocks, while the margin of the
plateau is fringed with a series of detached tabular and conical
outliers of varying size and elevation (Plate XIV, Fig. 1), the larger
of which form natural strongholds for pagan villages.[16] The most
remarkable feature, however, about the margin of the plateau is that,
behind the fringing belt of detached ranges and outlying groups of
hills, the main mass of the plateau has been grooved or incised by a
series of narrow flat-bottomed trench-like valleys, bounded by
perpendicular walls of white sandstone and notched and indented by
lateral gullies (Plate XIV, Fig. 2). For long distances the narrow
valleys preserve a very constant breadth, while frequently they turn
abruptly at right angles or are joined by lateral valleys of similar
character which unite the whole series into an interlacing system
through which it is possible to travel from Daura to Zai without once
ascending to the level of the plateau above. At their upper ends the
valleys rapidly contract and run out upon the plateau as steep and rocky
gullies. Their floors, like the surface of the plateau above, are well
cleared for cultivation and studded with scattered trees, while the
villages are set aloft on the margin of the cliffs overlooking the wells
in the valley below.
Beyond the western boundary of Bornu the escarpments die down, and the
drift-covered surface of the plateau falls gently to the swampy valley
of the Kogin Shidya. The last trace of the sandstones of Kerri Kerri is
found in the low hills of conglomerate and grit around Hardawa, while
twenty miles to the north according to native information the sandstones
disappear in the neighbourhood of Lise under the sands and swamps of
Katagum. Beyond Hardawa begin the undulating plains of Hausaland (pp.
21, 38), which stretch westward, to the north of the Ningi massif,
through the heart of Kano and the north-eastern portion of Zaria into
Kontagora and Sokoto. Here and there a heap of granite boulders or an
isolated group of rounded hills, as at Shira, Fagam and Kila, rise out
of the red sandy drift which is deeply banked against their slopes, and
which elsewhere effectually buries the irregularities of the crystalline
floor. Occasionally a smooth and rounded surface of gneiss or granite
outcrops level with the plain, while the rivers which flow in broad,
shallow and ill-defined valleys, indistinguishable at a distance from
the rest of the plain, have here and there succeeded in cutting their
channels through the sandy drift to the solid rock below. Where
uncultivated the plains are covered with prickly mimosa bush, or with
trees well spaced or in bushy clumps with open grass around. The farm-
land, however, is well cleared and comparatively bare with a few large
trees scattered here and there amongst the furrows, while the fallow is
thickly covered with tall aromatic grass and low scrubby trees of a few
seasons’ growth.
From Jemaari north-eastward to Katagum, and thence westward to the
immediate neighbourhood of Kano, not a single rock is anywhere exposed,
and not a single block or boulder or even a pebble of quartz is to be
found in the sandy loam which covers the surface. North of Jemaari the
track leads through gently undulating country presenting the usual
alternations of farm-land and thinly wooded plain. Between Itas and
Murmur the surface becomes in places quite hummocky with ancient dunes
in the form of low rounded ridges of loose sandy drift, extended
approximately east and west, rising in succession upon the level plain
and bearing witness to the former prevalence of desert conditions.
Around Katagum the drainage is so imperfect that long stretches of the
plain are under water in the rains, while westward beyond Kwotolu the
surface resumes its undulating aspect with its alternation of sandy
rising ground and clayey shallows. Here and there, as at Koya, a rounded
ridge of loose sand, now covered with grass and scattered trees, affords
evidence of the former activity of the wind, while in places in the
hollows are small sheets of fresh water forming lakes which are more or
less permanent throughout the year.
Around Kano and north-westward towards Katsina the open undulating plain
presents all the characteristic features of a crystalline floor. For
long distances round the towns the red sandy loam which covers the
surface has been cleared of all but the larger trees and subjected for
generations to intense cultivation. Here and there around Kano smooth
surfaces of rock appear level with the plain, or rise into low rounded
turtlebacks or heaps of granite boulders. The two prominent tabular
hills of weathered diorite, Dala and Kogon Dutsi,[17] within the walls
of Kano, have a peculiar interest of their own which will be related in
the sequel (p. 201). Northward between Kano and Kazaure the undulating
sandy plain is diversified only by the low flat-topped hillocks of
earthy grit and conglomerate in the neighbourhood of Sai and Ingogo and
by the broken ridges of amethystine quartz which rise from the plain
immediately south of Kunchi. Kazaure is set in the midst of rocky ridges
of blue quartzite with red sandy drift filling up the valleys and banked
against the slopes of the hills. Around Rauni rise low extended
flattened ridges, which appear to be composed throughout of dark red
vesicular ironstone. At Tummas, half way to Ingawa, low granite knobs
and heaps of boulders project above the surface, but from Ingawa north-
westward to Katsina the undulating plains are featureless, with no
prominent granite hills or kopjes to break the monotony of the
landscape. Smooth surfaces of rock are here and there exposed level with
the plain or in the channels of the streams, but otherwise the deep
sandy drift which covers the surface effectually obscures all the minor
irregularities.
Around Katsina both sandstones and granites are exposed upon the surface
of the plain, but it is not until the neighbourhood of Kurefi is reached
that a decided change takes place in the character of the scenery. From
Kurefi to Runka the track leads through rocky granite country which
assumes its wildest aspect in the neighbourhood of Tummo, where the
landscape, as far as one can see, is made up of an assemblage of low
rounded hills, domes, tors, heaps of boulders, perched blocks and smooth
surfaces of granite, with no particularly high or prominent peaks. On
the margin of the mass near Kurefi and Runka the aspect of the country
is smoother; the turtlebacks and rounded hills are separated by narrow
stretches of plain, and often half-buried under the sandy drift which is
banked up against their slopes. From Runka south-westward to Kotorkoshi,
and thence north-westward to Karakai, the landscape assumes its familiar
aspect of a gently undulating thinly-wooded surface, diversified by
isolated groups of granite hills which, at Duru and Kotorkoshi, rise
abruptly and picturesquely from the drift-covered plain. A belt of
broken hilly country stretches N.N.E. and S.S.W. between Karakai and
Maradu, parallel with the strike of the quartzites, phyllites, and
gneisses out of which the hills are carved. Immediately beyond Maradu
knobs and hummocks of volcanic agglomerate project from the surface of
the plain, while northward the track leaves the hummocky crystalline
floor in the neighbourhood of Gora and follows the level swampy plain of
the Gulbin Gandi as far as Dankaiwa. At Dankaiwa, in the cliff which
faces the river, is the first exposure of the red earthy sandstones of
Sokoto, while at Dampo, a little farther on, the first flat-topped
sandstone hill rises above the sandy plain. From Dampo the track again
runs along the swampy valley of the river as far as Rara, whence it
leads across undulating park-like plains to the broken sandstone hills
of Sokoto.
The northern and western portions of Sokoto province, which consist of a
dissected plateau of horizontal sandstones, possess in general all the
familiar scenic features of such a formation as already described from
other parts of the Protectorate. The apparent differences are
differences more in degree than in kind. Like the Niger above Lokoja,
the Benue in its lower and upper course and the Gongola above Gombe, the
Gulbin Kebbi or Sokoto river flows in a broad flat-bottomed trench-like
valley between two steep and worn escarpments, which lead up to the
plateau on either side. The breadth of the valley of the Gulbin Kebbi
is, however, out of all proportion both to the height of the bounding
walls and to the volume of the river itself. The floor of the valley is
usually several miles in breadth, while the cliffs on either side rarely
reach 200 feet in height. In the rainy season a sluggish flood covers
the bottom of the valley, but in the dry season the river is represented
by one or more diminutive streams, which disappear from time to time in
stretches of swamp. The continuity of the flat-topped escarpments is
much broken by lateral valleys, and here and there the vertical walls
are replaced for long distances by steeply-rounded rising ground. The
tributaries of the Gulbin Kebbi, where they flow through sandstone
country, are likewise bounded by worn and broken escarpments. Similar
broad flat-bottomed valleys with diminutive streams are common also
within the French Sudan to the north of Sokoto, and are there generally
known as “dallols.” Of them all, however, the Gulbin Kebbi possesses the
greatest volume of water, which is contributed practically entirely by
its numerous southern affluents which rise on the high and well-watered
plains on the borders of Kano, Zaria, Kontagora and Sokoto.
PLATE V
[Illustration: WORN VOLCANIC CONES NEAR AWE.]
[Illustration: THE WASE ROCK.]
The plateau areas between the larger valleys are themselves much
dissected, especially in the east and north, by the smaller tributary
streams. In the west there are some fairly extensive stretches of
continuous tableland, whose gently undulating and thinly-wooded surface
is diversified by occasional flat-topped and conical hills, the relics
of a former plain which has now been almost entirely removed in the
course of denudation. To the north and north-east of Sokoto the
irregularities of the dissected plateau are softened in contour by
accumulations of blown sand which covers the floors of the smaller
valleys and creeps up their lateral slopes, rounding off the steeper
cliffs of the flattened ridges on either side.
[Footnote 1: Mungo Park; Denham and Clapperton; R. and J. Lander; Laird
and Oldfield; Allen and Thomson; Baikie; Crowther; Barth; Vogel;
Passarge; Lenfant; Lugard; Wallace; Boyd-Alexander.]
[Footnote 2: “Hausaland,” in the general sense of the term, may be taken
to include the provinces of Sokoto, Kano, Kontagora, Zaria and Bauchi.]
[Footnote 3: _Cf._ Laird and Oldfield, _Expedition to Africa_, 1837, p.
136. The hills between Itobe and Igbo are here erroneously described as
the eastward prolongation of the Kong Mountains.]
[Footnote 4: _Cf._ Vandeleur, _Geographical Journal_, X, 1897, p. 349.]
[Footnote 5: _Vide_ Lenfant, _Le Niger_, 1905, illus., pp. 72, 73.]
[Footnote 6: Lenfant, _op. cit._, p. 78 _et seq._]
[Footnote 7: Boyd-Alexander, _From the Niger to the Nile_, 1908, p. 47.]
[Footnote 8: An excellent illustration of this peculiar type of scenery
(Inselberglandschaft) is given by Bornhardt, _Deutsch-Ost-
Afrika—Géologie_, Bd. VII, 1900, Taf. 3, p. 30.]
[Footnote 9: Erroneously designated lava by the earlier explorers.]
[Footnote 10: _Cf._ Boyd-Alexander, _From the Niger to the Nile_, 1908,
p. 86.]
[Footnote 11: _Cf._ Boyd-Alexander, _op. cit._, p. 46.]
[Footnote 12: _Cf._ Whitlock, “The Yola-Cross River Boundary
Commission,” _G. J._, XXXVI, 1910, p. 426.]
[Footnote 13: _Cf._ Boyd-Alexander, _op. cit._, p. 98.]
[Footnote 14: _Ibid._, p. 94.]
[Footnote 15: Barth, _Central Africa_, 1857, II, p. 355.]
[Footnote 16: _Cf._ Boyd-Alexander, _From the Niger to the Nile_, pp.
112, 164.]
[Footnote 17: _Cf._ Barth, _Central Africa_, 1857, p. 96.]
CHAPTER II
CRYSTALLINE ROCKS
Distribution of the crystalline rocks. Typical traverses. The harder and
softer gneisses. The Jebba quartzites: the phyllites and slaty schists
of the Niger valley; the sheared trachytes of Bussa; the Ngashki
phyllites; replacement of the softer gneisses by the harder banded
types. The gneisses and schists of Ilorin; the quartzites of the Orissa
hills; the stanniferous pegmatites of Eri. The muscovite schists of
Northern Kabba; the quartzites of Lukke; the crystalline limestone
between Jakura and Wa; the mica schists of Sequarre; the magnetite and
hæmatite schists of Eastern Kabba; the Igbo limestone. The phyllites and
schists of Sokoto province; the volcanic agglomerate of Maradu; the blue
quartzite of Kazaure; the diorites of Kano; Mounts Dala and Kogon Dutsi;
the gneisses and schists of north-eastern Kontagora; the Zungeru belt of
phyllites and schists; auriferous quartz reefs; the quartzites and
schists of Eastern Zaria; the gneisses and granites of Abuja; the
tourmaline rocks of Darroro; the mica schists of Gwombe; the banded
gneisses of Bauchi province; the granites of the Murchison Hills; the
later intrusions. The inliers of Arofu; the granites and gneisses of
southern Muri; the Vere Hills; exfoliation in the Kilba Hills; the
crystalline rocks of Bornu.
While crystalline rocks undoubtedly everywhere form the foundation of
the Protectorate, there are extensive tracts over which the later
sedimentary rocks obscure the underlying granites, gneisses, and
schists. The largest area over which these crystalline rocks are exposed
is that which occupies the centre of the Protectorate and extends from
the Niger in the west to beyond Bauchi in the east, and from Nassarawa
in the south to the neighbourhood of Katsina in the north. The major
portion of the provinces of Kabba, Ilorin and Borgu to the west of the
Niger also possess a crystalline floor, while similar rocks cover a
narrow strip on the southern border of Muri and Yola, as well as a
considerable tract of country to the east of the Gongola, lying partly
in Yola and partly in Bornu.
_Typical Traverses._
The following typical traverses show the character of the exposures
along the tracks.
I. Filelin (near Lokoja) to Ohoto:
9.20 A.M.— Left Filelin.
9.34 „ Blocks of ferruginous sandstone and grit.
9.38 „ Masses of biotite pegmatite; large felspars often
graphic.
9.40 „ Fine-grained, non-foliated reddish biotite granite with
streaks and veins of graphic biotite pegmatite. The
granite at places on the margin follows the foliation of
the adjoining biotite gneisses, in places crosses the
foliation, or is in part injected into the gneiss. The
biotite gneiss is often folded and distorted; in places
there is an alternation of dark bands of biotite gneiss
with lighter coloured felspathic layers. The foliation
is much disturbed, but in a general way E.N.E. and
vertical. The gneiss is cut by granite and pegmatite
veins and dykes.
9.45 „ Reddish granite continued. In places the granite shows
distinct banding and alternation of coarse and fine
layers; sometimes banded with pegmatites.
9.50 „ Same pink granite; but here an irregular mixture of
coarse and fine material with in places knots of coarse
granite in a fine grained matrix.
10.0 „ Small granite knobs on right.
10.3 „ Sandstones.
10.10 „ Biotite gneiss, strike N.E.–S.W., foliation dip 40° N.,
cut by coarse pegmatites as before; in places
interfoliated bands of segregation pegmatite.
10.15 „ Soft biotite gneisses.
10.30 „ Large blocks of pink granite and pegmatite.
10.40 „ Granite piercing hard white biotite gneiss with knots of
amphibolite. Strike of gneisses N. 35° E., foliation dip
N.W. There is a sharp junction between the granite and
gneiss, but no chilled margin.
10.50 „ Granite.
11.0 „ No exposures.
11.30 „ Extensive mass of coarse-grained biotite gneiss, crossed
by pegmatite veins and dykes of coarse red granite with
large felspars.
11.35 „ Biotite augen-gneiss with felspars sometimes two inches
long; crossed by veins of fine red granite or aplite.
11.40 „ No exposures.
12.30 P.M.— Bare surface of augen-gneiss level with the plain.
12.40 „ Dyke or sheet of fine grained reddish granite showing
distinct foliation.
12.45 „ Fine grained and finely banded biotite gneisses crossed
and injected by granite veins.
12.50 „ Sandstones.
1.15 „ Ohoto.
II. Jakura to Wa (north side of Mimi Valley).
6.30 A.M.— Left Jakura hill; on brow of hill quartz schist, strike
N. 10° W., foliation vertical; at foot of hill fine
grained pink granite with pegmatite, passing into
friable pink granite with large crystals of graphic
quartz and felspar in a finer matrix.
6.55 „ Soft biotite gneisses, strike N.N.E., dip W.N.W.
6.58 „ Band of amphibolite, dip unascertainable, followed by
soft biotite gneisses. Pebbles of hornblende epidote
schist.
7.12 „ Bare exposure of red pegmatite dyke cutting fine grained
felspathic gneiss, with in places a sharp junction and
in others not.
7.20 „ Quartz schist.
7.35 „ Magnetite hornblende schist, strike N. 10° W., foliation
dip practically horizontal.
7.40 „ Quartz hæmatite schist, dip unascertainable.
8.0 „ Massive and granular quartzite and quartz schist.
8.15 „ Biotite gneiss.
8.30 „ Quartz-muscovite schist, strike N.N.E., dip low to
W.N.W.
8.35 „ Pebbly muscovite schist, strike N.–S., foliation
vertical or highly inclined.
8.45 „ Pegmatites.
8.50 „ White marble forming rounded knobby hills cut by
pegmatite veins and enclosing streaks and veins of
quartz-epidote-calcite-idocrase rock on right hand; on
left the marble is foliated, with folia of pyrites and
knots of quartz; strike of foliation N. 10° E., dip 10°
W.
9.5 „ Marble ends.
9.8 „ No exposures; grass land.
9.15 „ Quartzites.
9-35 „ Soft biotite gneiss; foliation generally vertical;
strike N.N.E., dip vertical or high towards W.
9.50 „ Garnetiferous biotite gneiss with quartz segregations;
folia crumpled and contorted; strike generally N.–S.
10.10 „ Garnetiferous biotite gneiss as before; folia
practically horizontal.
10.25 „ Soft micaceous schists pierced by granite; strike N. 35°
E., dip S.E.
10.35 „ Pink granite in places foliated; graphic pegmatites on
slope of Wa hill.
10.45 „ Wa. On top of hill white mica schists with magnetite
knots, strike N.–S., dip 20° W.; cut by
quartz-muscovite-magnetite pegmatites.
III. Olle to Sura (Central Kabba).
6.20 A.M.— Left Olle.
6.25 „ Quartz schists and quartz-muscovite schists, strike N.
40° E., dip 55° W.
6.27 „ Soft fine-grained quartz-muscovite schists, strike N.
10° E., dip 70° W.
6.35 „ Muscovite schists, strike N. 30° W., foliation vertical.
6.40 „ Thick dyke of graphic quartz and felspar, with crystals
of tourmaline.
7.5 „ Fine grained white granite.
7.25 „ Coarse white granite in river bed.
7.30 „ No exposures.
8.10 „ Micaceous gneiss, strike N. 35° E.; much folded and
contorted with streaks and knots of pegmatite, probably
proceeding from a thick intrusive mass of quartz-felspar
pegmatite which adjoins the gneiss.
8.35 „ Muscovite schists, strike N.–S., foliation vertical.
8.40 „ Track leads along valley between hills of white
quartzite; loose blocks of compact and granular
quartzite.
9.0 „ Summit of pass; blocks of white quartzite.
9.5 „ Muscovite schists, strike N.–S., foliation vertical.
9.20 „ Lukke.
9.30 „ Soft muscovite schists and quartzites.
9.40 „ Muscovite schists, strike N. 10° W., foliation vertical.
9.50 „ Broken fragments of quartzite, quartz-muscovite
pegmatite, and quartz-tourmaline pegmatite on road;
intrusive masses of graphic felspar and quartz with
crystals of muscovite, associated with exposures of
coarse muscovite granite.
10.10 „ Biotite gneisses much crumpled and folded, exposed in
bed of stream, strike N.W.
10.15 „ Quartzites and muscovite pegmatites.
10.20 „ Intrusive mass of muscovite granite with large crystals
of graphic quartz and felspar in a fine grained granitic
matrix; banded in places with coarse and fine layers.
10.40 „ Exposures of quartz-muscovite pegmatite.
11.0 „ Soft micaceous schist, strike N. 10° W., foliation
vertical.
11.5 „ In bed of river and for 500 yards beyond, garnetiferous
mica schists with quartz lenticles, strike N. 10° W.,
dip 15° W.
11.30 „ Schistose biotite granite, with schistose pegmatite
veins forming Sura hill.
11.40 „ Sura.
IV. Agwede to Kabba.
6.30 A.M.— Left Agwede hill. On top, white granite usually showing
faint foliation and frequently enclosing portions of
dark mica schist; on slope, whitish ribboned gneisses,
often folded and contorted, strike N.E. to E., foliation
vertical.
6.40 „ Dyke of graphic pegmatite in gneiss; sharp junction.
Frequent bands of schistose pegmatite in the pale
coloured banded gneisses; in places the gneisses are
hornblendic with knots of fine-grained hornblende gneiss
or schist.
7.0 „ Ascent to plateau over contorted gneisses.
7.20 „ Same contorted gneisses passing into and invaded by
white schistose granite; wooded plain, few exposures.
8.20 „ Occasional exposures of similar contorted and granitised
gneiss; strike E. 10° N. to E. 10° S.
9.5 „ Faintly banded and indistinctly foliated biotite gneiss.
9.30 „ Odouape.
10.30 „ Level surface of composite gneiss.
11.30 „ Much crumpled and folded biotite gneiss; strike N.–S.
Followed by garnetiferous biotite gneiss.
11.45 „ Composite gneiss; foliation vertical; strike N. 10° W.
12.0 „ Kabba.
V. Izon to Abuja (Nassarawa province).
6.25 A.M.— Left Izon. Open grassy or thinly-wooded plains for two
hours; no exposures.
8.20 „ Hills; ascent of 300 feet; biotite and hornblende gneiss
and amphibolite; epidote-hornblende gneisses; in places
twisted and contorted and extensively invaded and
injected by masses of biotite granite; strike varies
between N. 10° E. and N. 10° W.; foliation mostly
vertical.
9.0 „ Open plain; thick granite wash.
9.30 „ Hills; granite invading hornblende epidote gneisses.
9.42 „ Abuchi. Amphibolite veined with granite.
10.0 „ Narrow plain; thick drift.
10.30 „ Hills; soft micaceous gneisses, strike N. 10° W.,
foliation vertical or high to E.
10.35 „ Hard biotite gneisses, strike N.–S., foliation linear
and vertical.
10.45 „ Ribboned and banded biotite gneisses with quartz veins
parallel to or crossing the foliation; strike N.–S.;
foliation vertical.
11.0 „ Abuja granite with foliated margin; vertical crush lines
in granite striking N.–S.
11.15 „ Abuja Residency.
VI. Boi to Ronn (Southern Bauchi).
6.30 A.M.— Left Boi; hornblende epidote gneisses broken and invaded
by granite; in places much folded and crumpled.
6.40 „ Felspathic gneiss, strike E.–W., dip high to S.
7.20 „ Complex of quartz epidote rock, amphibolite, epidote
hornblende schist, and gneiss, in places much
granitised; general strike E. 10° N.
7.45 „ Hummocks of banded gneiss; open country; few exposures.
8.40 „ Hornblende epidote gneisses.
8.45 „ Goshin Duchi. Hills of biotite granite behind town.
9.3 „ Quartz schists, fine grained gneisses, epidote
hornblende and quartz hornblende rocks, much granitised
in places veins and dykes of pegmatite and basalt.
9.20 „ Blocks of basalt and porphyry.
9.35 „ Hills of fine grained biotite granite.
10.15 „ Granite rocks, in places schistose.
10.45 „ Rocky, hilly country; banded and contorted gneisses
micaceous and hornblendic; general strike E. 10° S.
11.10 „ Ronn.
It is unnecessary, however, to quote the details of particular traverses
at greater length, especially as such descriptions serve only to show
the character of the phenomena to be correlated. It is sufficiently
evident that there are within the Protectorate two fairly well-defined
series of crystalline rocks, a less metamorphosed series of quartzites,
schists and gneisses representing an original sedimentary succession,
and a more highly metamorphosed series of banded and contorted gneisses,
whose relation to the less metamorphosed group will be discussed in the
sequel. The less metamorphosed gneisses present in the field somewhat
granular and incoherent outcrops and may be conveniently termed the
“softer” gneisses, while the “harder” or more highly metamorphosed
gneisses are readily distinguishable by the smooth and compact character
of their exposed surfaces. The softer gneisses appear to disintegrate
readily into a gritty sand under present conditions, while the harder
gneisses exfoliate and pass into angular débris. Both series are
extensively invaded by granitic and other igneous intrusions.
_Borgu and the Niger Valley between Jebba and Yelwa._
In the province of Borgu the superficial accumulations are so extensive
and the exposures so few and so widely scattered that the general
character of the underlying rocks is somewhat difficult to ascertain. It
is probable, however, that, like the neighbouring region of Nikki,[18]
there is a floor of well foliated and granitoid gneisses of varied
origin, much pierced and invaded by veins of granite, pegmatite, quartz
porphyry and quartz. Tremolite schists are known from Mori, and sheared
porphyries from Kanikoko. The occasional inselberge are composed
invariably of grey porphyritic granite. At Jebba the gorge of the Niger
is carved out of a series of sheared quartzites and pebbly grits with
the shear planes at times well spaced and the deformed pebbles still
distinctly visible. Secondary mica has been sparingly formed along the
planes of movement, together with knots and streaks of tourmaline and
tourmaline quartz. The predominant strike is N.–S., and the prevailing
dip of the foliation 65° W. The well-jointed Juju Rock is composed of a
laminated quartzite schist, which splits readily into thin curving
slabs, whose surfaces are coated continuously with white mica and show a
well-defined strain-slip corrugation. Westward the Jebba quartzites pass
into highly sheared quartz schists, quartz mica schists and granulitic
gneisses, which are followed by crumpled and contorted muscovite biotite
schists and gneisses and by granite at the river Awo. The quartzites and
granulitic gneisses are produced along the strike to form the range of
low wooded hills which stretches southward from Jebba towards Ilorin. To
the east of Jebba the quartzites are abruptly limited by hard banded and
composite gneisses pierced by masses of porphyritic biotite granite.
To the north of Jebba the Kailema granite, over which the track leads
from Jebba to Bajibo, is a massive hornblende granite, porphyritic with
white and pink felspars and somewhat finer-grained on the margin. The
granite has evidently pierced a series of hornblende, epidote and talc
schists and micaceous gneisses which outcrop imperfectly to the east.
Still farther north, at Wuru, the channel of the Niger is occupied by a
hard and coarsely foliated epidote hornblende gneiss, much pierced by
irregular veins of schistose granite and pegmatite. The foliation planes
of the gneiss are vertical and strike approximately N. 20° E. On the
left bank the gneiss is succeeded by thinly bedded quartzites and quartz
schists with scattered knots, lenticles and veins of quartz and finely
fibrous tourmaline. Intercalations of much corrugated and wrinkled mica
schist bear witness to differential movements along certain lines. The
whole series, which strikes N.–S. and dips vertically or at high angles
to the east or west, has apparently suffered a rapid and repeated
folding. These rocks are followed by a series of fine-grained green and
yellow quartzites and well-cleaved slaty schists with numerous quartz
reefs, which pass eastward into a complex of hornblende gneisses,
epidote hornblende gneisses, amphibolites and fine grained schistose
granites which disappear at Kumboji under the sandstones of southern
Kontagora.
At Bussa on the right bank of the Niger, near the Residency, there are
exposures of much decomposed shaly phyllites, vertical or highly
inclined to the east and striking N. 10° E. These are followed on the
left bank by a thick series of little altered grey and greenish granular
quartzites, which in places become pebbly and conglomeratic and enclose
bands of greenish trachyte, also showing little or no sign of shearing.
Further exposures of black and coarsely-sheared trachytes are found in
the swamps between Bussa and Libata. Eastward the quartzites are much
invaded by masses of dark green diorite and by thick veins of tourmaline
quartz, the latter showing every variation between a banded tourmaline
quartz rock and a felted mass of finely fibrous tourmaline. Nearer Auna
these rocks are succeeded by well-foliated augen gneisses which are
overlaid by the sandstones of southern Kontagora.
To the north of Bussa phyllites are again exposed in the low rounded
hills to the north-west of Ngashki, where they strike N.–S., and possess
a high dip of 65° to the east. Locally, however, the strike may be
directed as much as 35° to the east of north, indicating that the rocks
do not pursue a constantly rectilineal direction from south to north,
but rather possess a zig-zag character with short and sharp deflections
towards the east. The phyllites, which are mostly grey in colour, but in
places mottled with large irregular whitish spots on a grey ground, pass
eastward into soft grey sericitic schists with crystals of magnetite and
pyrites. The whole series is pierced by quartz tourmaline veins, in
which all proportions are to be found, from almost pure quartz through
banded quartz tourmaline rocks to masses of pure tourmaline of the
finely fibrous variety. Sills of much decomposed vesicular dolerite are
common also amongst the phyllites. Farther east intercalations of
micaceous gneiss become abundant within the schists, and a red fine-
grained granulitic gneiss intervenes between them and the massive
biotite granite of Abara, with its large crystals of felspar. To the
north of Ngashki, phyllites and green slaty schists with lenticular
streaks and veins of quartz and tourmaline quartz occur near Lokofe. A
good exposure in the river Malenda shows pebbly lustrous schists
striking N.–S., and dipping steeply towards the east. At Yelwa a narrow
strip of phyllites and slaty schists on the left bank of the Niger,
passes westward into knotted mica schists and biotite gneisses in the
river bed and eastward into the granitic gneiss of Bin Yauri. In the
country which lies to the east of Lokofe and Yelwa, and especially in
the neighbourhood of Tabachi and Maibirro, green slaty schists of
varying hardness are frequently exposed, usually associated with harder
gritty bands and much invaded by veins of quartz and tourmaline quartz,
and irregular intrusions of granite. In places also there may be found
intercalations of hard, gritty, carbonaceous and little-metamorphosed
slaty beds. At Chuggu, moreover, between Tabachi and Maibirro, there is
a parallel belt of biotite gneisses, hornblende gneisses, and epidote
hornblende gneisses, while at Maibirro the slaty schists, here
associated with thinly-bedded quartzites, quartz schists and quartz
muscovite schists, striking N. 35° E., with a dip to the north-west of
65°, are succeeded in the river Malenda by well-banded biotite gneisses
much veined with felspathic and granitic material. The actual junction
of the banded gneisses with the quartz schists is, however, not exposed.
The rocks of the Niger valley, between Jebba and Yelwa, are thus of many
and varied types. The quartzites and phyllites, the slaty schists and
micaceous gneisses, and the green hornblendic and epidotic beds
represent the sandstones, shales, grits and arkoses of the sedimentary
series, while the sheared trachytes and porphyries, the schistose
granites and the coarse-grained amphibolites represent the original
igneous intrusions within the sedimentary succession. The whole series
strikes fairly constantly N.–S., and forms a practically continuous belt
between Jebba and Yelwa. In most places there are indications that the
rocks have suffered a repeated folding into fairly sharp anticlines and
synclines with vertical axial planes, the folding being apparently more
rapid in the phyllites and schists than in the quartzites and gneisses.
There has unfortunately been no opportunity of tracing the belt in
detail along the strike, and little is known of the lateral variation or
repetition of the individual members or of the original sequence of the
sedimentary succession. There is no reason to believe that there has
been any increase or decrease in the intensity of the metamorphism
throughout the belt from south to north, and no indication has been
found of the occurrence of any thermal metamorphism in the neighbourhood
of the later granitic intrusions.
The most remarkable feature, however, about this belt of schistose rock
within the Niger valley is the manner in which the less metamorphosed
series of quartzites, phyllites and schists is bounded abruptly and
locally replaced by a more highly metamorphosed and much granitised
series of well-foliated and often banded gneisses. At Maibirro, for
example, the schists are abruptly succeeded by banded gneisses of a type
which is nowhere found between Maibirro and the Niger. Similarly at Wuru
and Jebba, the quartzites give place unexpectedly to composite
granitised gneisses, which must have originated under very different
conditions from those which brought about the metamorphism of the
phyllites and schists. This is a peculiarity of the fundamental
architecture which is repeated in many other parts of the Protectorate.
_Kabba and Ilorin._
Over the greater part of the province of Ilorin the surface
accumulations are so extensive that the nature of the underlying rocks
can be judged only from small and scattered exposures. So far as known
there are no belts of phyllites and quartzites upon the western or
central plains comparable with that of the Niger valley above Jebba. The
predominant rock in central Ilorin is a fairly coarse-grained, granular
and felspathic muscovite biotite gneiss, sometimes an augen-gneiss with
large eyes of felspar, which disintegrates readily into a felspathic
sand. With it are associated in places thin bands of quartzite, quartz
schist, quartz muscovite schist, mica schist and gneiss, often
garnetiferous, amphibolite and schistose granite, which, however, are
neither sufficiently extensive nor sufficiently resistant to give rise
to any prominent feature upon the surface. The whole series which
strikes approximately N.–S. is much veined by muscovite and tourmaline
pegmatites and invaded by masses of granite usually of a more or less
foliated character. The larger masses rise into turtlebacks or heaps of
boulders, and in their neighbourhood the gneisses have often been so
intimately veined and traversed by the granite as to produce a composite
or banded rock. At Share hard biotite gneisses with linear foliation are
extensively developed and associated with hornblendic gneisses and
amphibolites and cut by tourmaline pegmatites.
In the Orissa hills, which lie to the east of the great plain of Ilorin,
the softer gneisses enclose for the first time a prominent series of
quartzites, quartz schists, quartz mica schists and granulitic gneisses
which, with their general meridional strike and high dip to the west,
are very similar to those which form the low hilly ground to the south
and west of Jebba. The Shappa hills which succeed the Orissa hills to
the south are composed in great part of biotite and hornblende granites,
coarse-grained, fine-grained and porphyritic, which appear to be
intrusive into a series of hard biotite and hornblende gneisses, in
places well-banded and much veined with pegmatites, which occupies the
south-eastern corner of the province. Around Eri the softer gneisses are
again well developed and extensively exposed, the series here consisting
of thin quartzites, felspathic gneisses and much contorted and
occasionally garnetiferous micaceous gneisses and schists, with
interbedded hornblende schists and irregular masses of coarse-grained
amphibolite, probably of intrusive origin. The whole series is much
pierced by veins of quartz, tourmaline quartz and muscovite pegmatite,
the latter in places carrying tinstone.
PLATE VI
[Illustration: YENDAM HILL, YOLA PROVINCE.]
[Illustration: CRATERS AT MBOL.]
The softer gneisses and schists are found everywhere in northern Kabba
between Eri and the Ilorin border and the sandstone plateaux which
fringe the Niger. The usual succession of thinly bedded micaceous and
felspathic gneisses and hornblende schists is varied by the
intercalation of several belts of platy muscovite schists and quartz
muscovite schists, whose deeply decomposed surface usually supports a
luxuriant forest growth. Within the area mentioned there are at least
four of these belts, more or less well-defined and continuous along
their strike from north to south. The first runs southward from the
neighbourhood of Idofin by way of Kponyon, Onore, Ote, and Omwo into
southern Kabba: the second strikes southward from Okoro and Iye,
skirting Sanlu to the east and Moppa to the west: the third runs from
Shale and Oferre by way of Lukke, Olle, and Appe to the neighbourhood of
Odouape, and the fourth is exposed along the track leading from Wa to
Jakura on the north side of the Mimi valley. While the fissile quartz
muscovite schist may be taken as the index rock of the various belts,
each belt is in itself a complex of muscovite schists, quartz muscovite
schists, quartz schists, quartzites, biotitic and hornblendic schists
and other rocks and the proportions of these rocks vary not only in the
successive belts but at different points within the same belt. In the
third belt for example there is along with the muscovite schist a great
development of quartzites and quartz schists around Lukke, where they
form prominent rocky hills (_v._ Traverse No. III. p. 66). Farther north
several bands of chlorite and talc schist are found in the same belt in
the neighbourhood of Iye, while in the fourth belt between Wa and Jakura
there are intercalations of such other rocks as quartz magnetite schist,
magnetite hornblende schist, and crystalline limestone (_v._ Traverse
No. II. p. 65). The limestone is a white coarse-grained saccharoid
marble which may be traced for at least 1,000 feet across the strike
without affording any indication of repetition by folding. On the margin
the marble is distinctly schistose with the foliation planes striking N.
10° E. and dipping at a low angle towards the west. The rocks in the
neighbourhood are somewhat irregularly folded with the foliation planes
as a rule parallel with the bedding. Their junction with the marble is
not exposed, but if the marginal foliation of the latter be also
parallel with the bedding, the actual thickness of the original
limestone must have been very much less than 1000 feet. The marble is
very pure, consisting almost entirely of calcite with occasional streaks
and segregation bands of quartz-epidote-calcite-idocrase rock
representing the original impurities within the limestone. Minute spots
of graphite are abundant throughout and on the margin streaks and knots
of quartz and pyrites are common along the foliation planes. In places
the marble is irregularly crossed by later pegmatite veins.
The tracts which intervene between the various belts are occupied by a
variety of soft micaceous and hornblendic schists and differ from the
belts already described only in the comparative rarity of muscovite
schists and quartzites. The absence of sufficiently detailed
investigation, however, makes it impossible to say to what extent the
various belts described above represent repetitions of one and the same
belt, or to what extent they represent the original sequence of the
sedimentary succession. While the average strike of the whole series is
meridional, its undulating or zigzag character is evident from the
occurrence of individual exposures showing every angle between north and
east. The different members of the series, while frequently vertical,
may dip at various angles to the east or west or even in places approach
the horizontal. Individual bands are frequently puckered and contorted
and the whole series is invaded in places by extensive masses of biotite
granite which rise into turtlebacks and groups of rounded hills. The
granites are usually foliated on the margin and sometimes throughout and
frequently show at their junction with the gneisses and schists a narrow
zone of injected rock. Small detached exposures of the harder banded
gneisses, elongated upon a parallel strike, are distributed irregularly
throughout the softer series. The gneisses and schists are abundantly
pierced by muscovite and tourmaline pegmatites and veins of quartz and
tourmaline quartz, the latter in the neighbourhood of Iye being minutely
banded as in the case of some of the tourmaline quartz veins at Bussa
and Ngashki. With the pegmatites appear to be connected some small
intrusive masses of non-foliated granite which become more numerous in
the eastern portion of the area under consideration (_v._ Traverse No.
I. p. 64).
The striking feature of the architecture of the high plains of central
Kabba is the manner in which the softer gneisses of the north with their
intercalated bands of schists and quartzes are invaded, and in part
replaced, by masses of granite and of hard banded and composite
gneisses. The first belt of schists can be traced southward with
intervals from Omwo to the neighbourhood of Arigidi and Sequarre, where
the soft micaceous schists carry much staurolite. Harder gneisses
however appear to bound this softer belt irregularly to the west and
east. The prolongation of the second belt of schists in northern Kabba
is probably represented by two narrow strips of quartzite enclosed in
the harder gneisses between Kabba and Lupa. The third belt appears to be
somewhat abruptly replaced in the neighbourhood of Odouape by a complex
mass of granites and hard composite and banded gneisses which extends
westward across the high plains of Kabba from Agwede and Shokko Shokko
to Ikinre and Omwo (_v._ Traverse No. IV., p. 67), and is apparently
prolonged southward into the Egbira hills. The fourth belt appears to
suffer much lateral variation, and is represented on the main road
between Lokoja and Kabba only by some thin bands of muscovite schist
enclosed within a series of micaceous and felspathic gneisses.
In southern Kabba the few facts at our disposal seem to indicate the
almost complete absence of quartzites and muscovite schists and a
repeated alternation of the softer and harder gneisses, the whole series
being pierced by pegmatite veins and intrusive masses of granite.
Quartzites and quartz schists reappear, however, in the Kukuruku hills
on the borders of Northern and Southern Nigeria.[19]
The lower plains which stretch eastward to the Niger from the
escarpments of the Egbira and Agwede hills are occupied very largely by
gneisses of the softer series. Quartzites, quartz schists and quartz-
muscovite schists occur within the series, but their distribution over
the plains is so irregular, and their prolongation along the strike so
inconstant, that they cannot in any way be correlated with the
quartzites and schists of either the third or fourth belts in northern
Kabba. It would seem, indeed, that instead of the regular alternation to
be found in the north, there is over these eastern plains a greater
mixing of types and a greater variation in character of the individual
members when traced along the strike. Quartz-magnetite and quartz-
hæmatite schists are fairly common, typical occurrences being found in
the neighbourhood of Bangede and around Okuruku and Anamu near Okeli,
and on the plains to the west of the Againyi hills, as well as at the
foot of their western slope. Amongst the granites and granitoid gneisses
which form the escarpment of the Egbira hills, bands of felspathic
gneiss and micaceous schist are frequently enclosed, and on the plains
the gneisses and schists are crossed in the usual manner by pegmatite
veins and pierced by masses of more or less foliated granite, which form
the prominent isolated hills which diversify the surface of the plain.
Here and there also may be found a small exposure of composite or banded
gneiss, but it is only in the immediate neighbourhood of the Niger that
the harder gneisses attain any extensive development. There they form a
narrow belt, marked by prominent intrusions of granite, which stretches
southward from Lokoja along the right bank of the river, and adds
picturesqueness to the Niger valley between Igbo and Itobe (p. 6). At
Lokoja the harder gneisses are exposed in the river bed, and the softer
gneisses and schists at the foot of Mt. Patti. The average strike is
N.E. to S.W., and the whole series is much pierced by veins of granite
and graphic pegmatite, the latter containing frequent knots of
magnetite.
To the east of the Niger the softer gneisses reappear in the narrow
strip of crystalline rocks in Bassa province which fringes the left bank
of the river. Typical exposures are found in the neighbourhood of Igbo
and Itobe, where the gneisses are associated as usual with early
foliated or partially foliated granites and with later intrusions of
muscovite and tourmaline granites and pegmatites, the tourmaline
occurring as a rule in large and well-formed crystals. At Igbo the
micaceous gneisses enclose a number of narrow bands of fine-grained,
crystalline limestone, each 3 to 4 feet in thickness. The limestone
varies much in colour and hardness, the purest varieties being blue,
while weathering and impurities produce grey and brown tints. Much of
the limestone is gritty, but the commonest accessory is a pale-coloured
tremolite in rosettes and felted veins of fibrous crystals and in
scattered well-formed prisms.
The crystalline rocks of Kabba and Ilorin thus present considerable
resemblances in individual types to those of the Niger valley above
Jebba. The most striking differences are the occurrence of such new
types as crystalline limestone and magnetite and hæmatite schist in
eastern Kabba and Bassa, and the apparent absence in the southern
provinces of the characteristic phyllites and slaty schists of the Niger
valley to the north. The place of the phyllites, however, is probably
taken by the muscovite and quartz-muscovite schists, which form such
prominent members of the metamorphic series in northern Kabba. If so,
there would seem to have been a slight increase in the intensity of the
regional metamorphism in these southern provinces as compared with the
Niger valley above Jebba. Of thermal metamorphism, on the other hand,
there is little trace. The schists and gneisses in the neighbourhood of
Sequarre in western Kabba and in several localities in northern Kabba
and Ilorin contain a considerable quantity of staurolite and other
metamorphic minerals. Their distribution, however, appears to possess a
linear character which is somewhat inconsistent with a contact origin,
while sufficiently extensive igneous intrusions, to whose influence
their local development might be ascribed, have nowhere been definitely
recognised in the vicinity. The numerous intrusive masses of both
foliated and unfoliated granite throughout these provinces appear to
have induced little or no contact metamorphism in the surrounding
gneisses and schists. An exception to this rule may perhaps be made in
the case of the tourmaline granite behind Igbo on the Niger, to whose
influence may possibly be ascribed the extensive development of
tremolite in the neighbouring crystalline limestones. At the same time
it should be remembered that the occurrence of tremolite in metamorphic
limestones cannot be taken as an invariable indication of contact
action.
The irregular association of a less metamorphosed series of quartzites,
schists, and gneisses with a more highly metamorphosed series of well-
foliated, banded and contorted gneisses, exhibited by the crystalline
rocks of the Niger valley above Jebba, forms likewise a striking feature
of the crystalline rocks of Kabba and Ilorin. The banded gneisses may be
either biotitic or hornblendic and the lighter and darker bands may
alternate so regularly and rapidly as to give rise to a striped or
ribboned appearance even in the hand specimen. They are for the most
part of a medium and even grain and never porphyritic, although
sometimes they enclose lenticular masses of finer grained biotite gneiss
or of amphibolite. As a rule they are much invaded by granite and
frequently rendered composite by thin granitic intrusions. The
foliation, which may be parallel or oblique to the striation, is their
least conspicuous feature. Pegmatite veins are usually abundant and
frequently foliated. As already described (p. 74) these hard banded and
contorted gneisses are found irregularly replacing the softer gneisses
both along and across the strike.
So far as known there are no schistose rocks in Kabba and Ilorin
comparable to the sheared trachytes of the Niger valley and the sheared
porphyries of Borgu. Rare occurrences of thin basaltic veins have been
observed, but as a rule the later intrusions in Kabba and Ilorin take
the form of small masses of red or white aplitic granite or of dykes and
veins of muscovite and tourmaline pegmatite or of graphic granite. These
later granites have usually a peculiar granular and incoherent character
and never possess chilled margins. Frequent occurrences have been noted
of the granular granite acting as a kind of ground mass to large
scattered porphyritic masses of graphic quartz and felspar (_v._
Traverses Nos. I, II, III, pp. 64–66). Frequently also the granites show
an ill-defined banding caused by an alternation of coarser and finer
grained layers. It is possible that the muscovite and tourmaline
pegmatites, the veins of graphic granite, the quartz reefs, the banded
tourmaline quartz rocks, and the small intrusive masses of acid granite
may all possess a certain genetic connection. Only at Igbo, however,
does there occur an extensive mass of reddish muscovite tourmaline
granite which to some extent unites the characters of all the dyke rocks
and represents in a general way the original magma from which the
various dykes and veins and intrusive masses have been differentiated.
The abundance of tourmaline throughout Kabba and Ilorin led to the hope
that tinstone would in places be associated with it, but only at Eri in
eastern Ilorin have the pegmatites proved stanniferous. The quartz reefs
also have nowhere been found to yield more than the minutest trace of
gold.
4. _The Central Crystalline Area._
The peculiarities of structure which have been remarked in the western
provinces are continued throughout the larger crystalline area in the
centre of the Protectorate. To the east as to the west of the Niger
there has been an irregular and local replacement of a series of softer
gneisses, quartzites, and schists by a complex of harder composite and
often well-banded gneisses. There is also in the western part of the
central area within the softer gneissic series a well-defined grouping
of the quartzites and phyllites or micaceous schists into regular and
definite belts which preserve their identity for considerable distances
along the strike. In the east, however, this regular grouping appears to
be absent, the quartzites and mica schists being distributed irregularly
and in short and narrow bands amongst the felspathic and micaceous
gneisses.
In the east of Sokoto province there are at least two broad and well-
defined belts of phyllites and indications of a third. The first is
found between Bukwium and Anka near the Kontagora border, and is well
exposed in the Anka river. The strike of the phyllites varies from N.–S.
to N. 20° E. Surface creep here and there makes the dips uncertain, but
the inclination is usually high, and is sometimes to the west, sometimes
to the east. This fact probably accounts for the great width of the
belt. It is not known whether the phyllites extend to the east of Anka,
but after crossing them at right angles to the strike for thirty miles
to Bukwium they are still in full force, and the western limit is
unknown. There are numerous quartzite bands, especially about Anka and
to the south of it. The phyllites are soft, fine-grained and dark-
coloured, and are traversed by abundant quartz veins, some of which
carry pyrites. It is interesting to note that the phyllites are only
exposed in the neighbourhood of the river. Away from the river, on the
level plain, there is no indication of them, and this fact is of
significance when considering the possible extension of the phyllite
belts. There is a numerous and interesting series of igneous rocks
within the phyllite area, including such types as hornblende granite and
syenite, pegmatite and quartz porphyry. These rocks, as a rule, show no
sign of crushing, and are probably later intrusions. Some of the
porphyries, however, show an imperfect foliation, and appear to have
been exposed to some of the later crustal movements. Bands of
amphibolite and veins of tourmaline quartz also occur within the
phyllitic series. (A. L.)
The second group of phyllites and quartzites forms a belt of low hilly
country striking N.N.E.–S.S.W. across the Sokoto-Kano caravan route
between the villages of Maradu and Karakai, and continues northward at
least as far as Bazai. The series attains across the strike a breadth of
about thirteen miles, and consists of a succession of slaty schists and
knotted phyllites and quartzites, much pierced by quartz veins and thin
bands of schistose granite, and dipping fairly constantly in the west at
70° E.S.E., but gradually heeling over in the east to the W.N.W. Towards
Karakai the quartzites predominate and contain much tourmaline, and many
samples may be found showing quartz and tourmaline or quartz, slate and
tourmaline in alternating ribbon-like bands. The third belt is very
imperfectly exposed on the borders of the province between Woneka and
Duru, and is represented only by some scattered outcrops of slaty
phyllite and mica schist.
Between Maradu and the margin of the Sokoto sandstones to the west, the
softer gneisses are exposed upon the sandy plains in a very fragmentary
manner, and appear to consist of micaceous and hornblendic gneisses and
schists, and of hornblende epidote gneisses and amphibolites with
numerous small granitic intrusions. The rocky knobs and kopjes two miles
west of Maradu are formed of a coarsely banded volcanic agglomerate,
dipping eastward at angles of 30°–40°. The agglomerate may be traced to
within a mile of the village, where it gives place to a thinly bedded
series of red ashy sandstones and ash beds. East of the rest camp, these
beds strike N.N.E. and dip at 40° E.S.E. while west of the camp they
strike E.N.E. and dip at 30° S.S.E. The ash is exposed in, and to the
west of, the village, but to the east its place is taken by a much
broken and shattered granitic rock. The actual junction, however, was
not observed. Such evidence as there is points to the former existence
of a volcanic neck or vent in this locality.
Between Karakai and Woneka the surface accumulations obscure the rock
for long distances, especially to the west and east of the granite hills
of Kotorkoshi. Around Bungudu, however, there are numerous exposures of
a coarse-grained composite gneiss which probably extends eastward to the
boundary of the province. Beyond Duru a grey biotite granite with a
distinctly foliated margin forms the broken hilly country between Runka
and Kurefi, but between Kurefi and Sani the granite gives place to a
group of dark-coloured biotite gneisses before the crystalline rocks
disappear under the sandstones of Katsina. Knobs and hummocks of
granite, however, protrude through the sedimentary rocks round the walls
of the town, and Barth[20] has reported the occurrence of similar mounds
and ridges immediately to the north. To the south-east of Katsina the
surface rock appears to be everywhere a reddish granite as far as Duru,
but beyond Duru the superficial accumulations so effectually cover up
all but the occasional kopjes and boulders of granite that the character
of the underlying rocks is difficult to ascertain. From the scattered
fragments of quartzite, gneiss and amphibolite, and occasional granite
boulders in the surface drift, it is probable, however, that for a
considerable distance to the west of Kazaure the floor is formed of the
sedimentary gneisses with the usual granitic intrusions. The hills
around Kazaure are composed of hard blue quartzite much pierced by later
veins of white quartz, and striking N.N.E.–S.S.W. with a dip of 30° to
the west. This belt can be traced southward to the neighbourhood of
Kasuan Kuka, where the quartzites become more micaceous and where some
rocky exposures of granular amethystine quartz probably represent the
intrusive quartz veins of Kazaure. From Kasuan Kuka to Kano, granites
and an occasional quartz vein are the only rocks exposed, while the deep
drift effectually prevents the discovery of the relationship between the
surrounding rocks and the decomposed diorites of the two flat-topped
hills within the walls of Kano. A few miles east of Kano the crystalline
rocks disappear under the sands and clays of Katagum and Bornu. There is
every reason, however, to believe that they are continuous to the north-
east with the granites, gneisses and quartzites of Machina, Zinder and
Mounio.
The two detached hills, Dala and Kogon Dutsi, the landmarks of Kano,
with their precipitous slopes and tabular summits, 150 feet above the
level of the plain, present from a distance a decidedly sedimentary
appearance. Clapperton,[21] indeed, described them as consisting of
“argillaceous ironstone mixed with pebbles and a rather soft kind of
marl,” while Dala, the eastern mount, “is of clay-ironstone and
conglomerate lying upon a bed of soft light clay apparently mixed with
vegetable remains.” Closer investigation has shown, however, that both
hills are of crystalline origin, Dala being composed entirely of earthy
decomposition products with a gritty ferruginous cap, while Kogon Dutsi
still retains cores or kernels of undecomposed diorite in the midst of
the friable earthy material which forms the greater part of the hill.
The felspar and hornblende of the original diorite have decomposed in
the lower part of Dala into a whitish clay streaked and spotted with
dark green chloritic material. In the upper part the colouration is more
uniformly white or yellow, the bulk of the iron having been leached out
to form the red ferruginous cap. That the latter is itself composite,
however, is evident from the occurrence of gritty grains, subangular
fragments of quartz and rolled pebbles of ironstone in the vacuolar and
concretionary crust. Kogon Dutsi presents a similar composition, with
the exception that the material becomes coarser and more granular in the
neighbourhood of the kernels of undecomposed rock. The extensive
decomposition of the crystalline rocks to which the earthy hills of Dala
and Kogon Dutsi bear witness will be found in the sequel (p. 201) to be
an important factor in the evolution of the characteristic scenery of
the high plains of Hausaland.
PLATE VII
[Illustration: CRATERS IN THE MBOI HILLS.]
[Illustration: A CRATER IN THE MBOI HILLS.]
Around Kontagora and beyond the northern limit of the sandstones which
occupy the southern portion of the province of the same name, there are
found hard biotite gneisses in places well striated with pink felspathic
and dark biotitic bands and crossed by numerous pegmatite veins. To the
east the crystalline floor is deeply covered with drift, and outcrops
are comparatively rare. In the Beri river, however, there is a good
exposure of soft felspathic biotite gneisses, with knots and bands of
pegmatite, while at Beri a pink granite forms a compact group of rocky
hills. The granite possesses a distinctly foliated margin, and is
followed to the east by hard hornblende and biotite gneisses much
pierced by granite veins, which may be traced to within six miles of
Bobi before they are replaced by the softer micaceous series. On account
of the extent of the superficial accumulations, however, these softer
gneisses are well exposed for the first time only in the rocky bed of
the Kara river which forms the boundary between the provinces of
Kontagora and Zaria.
Though the structure of the north-eastern part of Kontagora province is
complicated in detail, it may be described in general terms as a
succession of gneiss and quartzite bands alternating with one another.
They strike a few degrees east of north, and probably pass to the east
of the Bukwium-Anka phyllites. The quartzites are frequently tourmaline-
bearing, and one band west of Kotonkoro carries scales of graphite. This
fact is interesting in connection with the graphitic schists reported
from a neighbouring part of Zaria.[22] The gneisses are usually biotite
gneisses, sometimes markedly banded, at other times indistinctly so.
Schistose rocks are seen in places, quartz schists being prominent to
the south of Mahorro, where talc schists are also seen. The gneiss and
quartzite country is traversed by pegmatite and quartz veins, but
intrusions of unfoliated granite are rather uncommon. Most of the
turtle-backs are foliated throughout, differing in this respect from
those of many other parts of the Protectorate. Where the Sokoto road
leaves the province, the boundary is marked by a high sharp ridge of
amphibolite. (A. L.)
A tract of broken hilly country, almost entirely carved out of the
softer gneissic series, occupies the south-western corner of Zaria
province, and a diagonal traverse across the hills from Moriga on the
Kara river by way of Garun Gabbas and Zungeru to Kato and Mikonkeli
presents a particularly instructive section. The hills which begin about
ten miles south-east of Moriga consist between Inga and Garun Gabbas of
a succession of quartz schists, quartz muscovite schists, micaceous and
hornblendic schists, felspathic gneisses and amphibolites, pierced by
numerous quartz reefs and veins and by a few dykes of basalt and
porphyry, the whole series possessing an average strike of N. 10° E.
Between Garun Gabbas and the Kaduna river ridges of quartz and quartzite
rise conspicuously at intervals out of the pebbly drift which covers the
surface. In the railway cutting on the right bank of the Kaduna near
Zungeru, quartzites and quartz schists are associated with bands of a
hard and minutely foliated granulitic gneiss containing knots and
lenticles of quartzo-felspathic and chloritic material. In the river bed
below the railway bridge the most conspicuous rock is a well-bedded
white quartzite striking N. 30° E. and dipping at an angle of 64° to the
south-east (Plate VIII, Fig. 2). The quartzite varies much from a
massive variety with knots of pyrites and scattered flakes of white mica
through a streaky lustrous micaceous quartzite with linear foliation
planes, into a well-foliated quartz muscovite biotite schist. The
different varieties are often arranged in alternating parallel bands and
the whole series is pierced by occasional narrow veins of quartz. The
trench which at the Kaduna bridge forms the low water channel of the
river has been carved out of a band of hornblende schist intercalated
along the strike of the quartzites. The schist, which in places is
composed almost entirely of hornblende, but in others interfoliated and
streaked with felspar, has been more easily worn away than the
quartzite, and grooved and pot-holed by the action of the river. The
quartzites and quartz schists, with intercalated sills of hornblende
schist, continue eastward to the adjoining channel of the Dago, where
they are replaced by finely striated biotite gneisses. At Zungeru
micaceous and felspathic gneisses are mingled with soft, thinly bedded
and repeatedly folded hornblende mica epidote gneisses or schists. The
whole series is pierced in the usual manner by veins of quartz,
pegmatite and granite and strikes in a general way N. 10° E. with a dip
of 60° to the east. Immediately east of the railway station, however,
these rocks give place to well-foliated granulitic gneisses with knots
of felspar, striking N. 20° E. and dipping at 45° E. These gneisses,
which probably represent more completely altered portions of the
sedimentary series, are in places well striated, with the bands wavy and
contorted and crossed irregularly by veins of quartz. They are followed
in the vicinity of Kwarra-town by coarsely schistose grits which are
among the least altered rocks within the Protectorate. Between Zungeru
and Gidan Motu there are numerous exposures of green hornblendic beds
similar to those at Zungeru. They become, however, more fissile and
contorted towards the east. The strike oscillates between N.N.E. and
N.N.W., and the dip varies between vertical and high angles to the east
or west, while at Gidan Motu there is an intercalated belt of platy
biotite gneisses which strike N. 60° W. and dip at 60° S.S.W. Between
Gidan Motu and Kato green slaty schists and phyllites are extensively
developed and steeply folded at Kato into sharp anticlines and synclines
which possess an average strike of N. 20° W. The phyllites enclose much
quartz in lenticles and veins and contain abundant knots and spots of
pyrites. Beyond Kato the phyllites are succeeded by soft mica schists,
which in turn give place to micaceous and felspathic gneisses. The
latter around Mikonkeli, Minna and Paiko are replaced by hard composite
and in places banded rocks and invaded by extensive masses of granite
which usually show a distinct marginal foliation.
Southward the Zungeru belt of quartzites and phyllites disappears under
the sandstones of Nupe, but any correlation with the quartzites and
schists of Kabba cannot yet be attempted. Northward the belt has been
followed as far as Birnin Gwari and from its pronounced development in
the neighbourhood of that place, it is probable that it extends
considerably farther north again. For twenty or thirty miles north of
Tegina there are a number of granite hills strung together on a roughly
north and south line, the granite having apparently been intruded along
the strike of the schistose rocks. The granites become foliated locally.
Between Bugai and Koriga there is a twenty mile belt of phyllites and
subordinate mica schists which runs northward to Birnin Gwari, where the
phyllites form a number of sharp north and south ridges and are
excellently exposed, both in the sides of the ridges and in the
intervening streams. The phyllites are traversed by many quartz veins
and gold is widely disseminated in the streams. At Koriga there is an
extensive development of hornblende gneiss. To the east are mica
schists, and graphitic schists are reported from the neighbourhood.[23]
The rocks which rise above the plain in the vicinity of Zaria are all of
granite. (A. L.)
So far as known, the Zungeru belt is the last prominent group of
phyllites which can be traced within the Protectorate. To the east their
place is taken by muscovite and biotite schists, which are to be
regarded as more completely reconstructed representatives of the
original shales, the metamorphic action having apparently been somewhat
more intense in eastern and southern Zaria and Nassarawa. Several belts
of quartzites and schists are enclosed in the usual manner within a
complex of softer and harder gneisses to the east of Minna and
Mikonkeli. At Kuta, for example, there is a belt of quartz muscovite
schist passing on the one hand into knotted micaceous schists and on the
other into quartzites and carrying much sillimanite and magnetite and
large lenticles and knots of quartz and tourmaline quartz. The whole
series strikes approximately N. 30° E. and dips at 45° N.W. Again
between Chiri and Guni there is much soft micaceous schist rising into
low rounded and well-wooded hills, striking generally N.–S., well
frilled in places, with a high content in ilmenite and numerous knots
and veins of tourmaline quartz. Farther east the hills which run from
the neighbourhood of Dorka to Kaderri and Kaserimi are composed very
largely of quartzites, quartz schists, quartz muscovite schists and
micaceous gneisses, striking N. 10° W., dipping at an angle of 60° W.,
and pierced by quartz veins, muscovite pegmatites and granites. Between
these more prominent belts occur numerous smaller exposures of schists
and quartzites and of both the softer and the harder gneisses. To the
east of Kaserimi and Duchi and in the valley of the Kaduna the surface
is so deeply and extensively covered with drift that little rock beyond
the marginally foliated biotite granites of the isolated hills is
available for investigation. There are, however, indications from
occasional fragments in the drift and rare exposures in the stream
courses that a considerable extent of the buried surface is composed of
a softer gneissic series. To the east of the Kaduna, the Ruruma and
Kerko hills which rise so prominently from the south-eastern plains are
composed throughout of a coarse-grained biotite granite, locally
carrying hornblende in addition and in places distinctly foliated on the
margin. The junction with the gneisses is only rarely seen on account of
the thick masses of drift which envelop the base of the hills. At Guri
on the eastern slope of the Ruruma hills there are exposures of finely
laminated hornblende epidote gneisses, while the granites and the
marginal rocks are pierced by numerous dykes of basalt, dolerite and
quartz porphyry.
The high plains of south-eastern Zaria are occupied by mica schists and
gneisses, with granite hills in one or two localities. Along the eastern
border of the province the Rukuba plateau ends in a ragged edge of
biotite granite which veins the gneisses at its foot. The country around
Jengre and Limoro is interesting on account of the occurrence of masses
of soda granite and especially on account of numerous intrusions of dyke
rocks, felsites often with flow structure, porphyries and quartz
porphyries. Tinstone is frequently met with in the streams of the
district. Similar dyke rocks are exposed around the base of the Kudaru
hills to the north-west of Leri and there, in addition, dykes and
veinlets of basalt penetrate the granite. These hills are made up of
hornblende granites with syenitic modifications, and locally, towards
the boundary, the granite takes on a distinctly foliated structure, with
rounding of the porphyritic felspars as in augen-gneiss. (A. L.)
In Nupe, between the Gurara river and the sandstones of the Niger
valley, the crystalline series is introduced by muscovite biotite
gneisses, quartzites, hornblende and talc schists and amphibolites,
pierced by extensive masses of granite around Gaung, Lefu and Paiko. In
the bed of the Gurara, between Gaung and Izon, soft gneisses and
hornblende epidote rocks alternate with hard and well-foliated gneisses
striking a little west of north, and varying in dip from vertical to 40°
or 50° E. Five miles beyond Izon, the Abuja road ascends over a crumpled
and contorted series of biotite, hornblende, and epidote hornblende
gneisses much injected by masses of biotite granite. Beyond Abuchi a
second ascent over soft micaceous gneisses, striking N. 10° W. and
vertically foliated, leads up to the striated, ribboned and banded
gneisses which have been invaded by the biotite granite of Abuja. The
banded gneisses strike N.–S., and are much granitised in the immediate
neighbourhood of the intrusive rock, while the granite itself is
marginally foliated and crossed by crush lines at the Izon gate (_v._
Traverse No. V, p. 68). Partially foliated granites and banded and
composite gneisses continue to within five miles of Kao, whence a belt
of soft micaceous gneisses and quartz muscovite schists, striking N. 20°
E., stretches eastward to the neighbourhood of Kari. At Kari the
escarpment of the northern tableland is formed of soft biotite gneisses,
much pierced by veins of quartz and tourmaline quartz. Eastward by way
of Kukwi, Anta, Tatara and Jaginde the escarpment shows much granite,
associated both with banded gneisses and with softer micaceous and
hornblendic rocks. The gneisses and granites are extensively invaded by
muscovite and tourmaline pegmatites, and by irregular intrusions of
basalt and dolerite. Between Jaginde and Darroro quartzites, quartz
schists and mica schists outcrop along the escarpment, and at Darroro a
similar series of thin quartzites and gneisses, striking generally
N.–S., is pierced by extensive masses of quartz-muscovite-tourmaline
pegmatite, and capped by the basalts of the Morrua plateau. The
pegmatite is rudely banded parallel with the strike of the gneisses, the
various bands being distinguished by their richness in tourmaline. All
proportions may be found between quartz muscovite and quartz tourmaline
rock, the tourmaline in the latter being either in large well-formed
prisms, or in thin alternating bands of felted fibrous crystals. North-
east of Darroro the vertical granite cliffs of the Kagoro massif form
the margin of the Bauchi plateau, while in the neighbourhood of Assab
the later basalts much obscure the crystalline rocks of the lower
plains. At Assab the escarpment is largely composed of biotite granite,
in places porphyritic, and apparently intrusive into a series of
micaceous gneisses which strike N. 10° E. and dip at an angle of 40° to
the west.
The northern tableland and the central plains of Nassarawa are composed
very largely of the softer gneisses with scattered outcrops of harder
composite rocks. Quartz muscovite schists occur in a broad belt on the
tableland in the vicinity of Aribi, and calc-silicate rocks between Kwai
and Jaginde. The composite and banded gneisses reach their greatest
development in the Karshi hills between Abuja and Keffi. The Mada hills
and the greater part of eastern Nassarawa are still unexplored. In
southern Nassarawa a broad belt of quartz muscovite schists, mica
schists, hornblende schists and chlorite schists extends between Gwombe
and Buga, the mica schists in the neighbourhood of Zagabutu being
particularly rich in staurolite, garnet and imperfectly crystallised
knots of andalusite, presumably of contact origin. The whole series has
been steeply folded on a meridional axis and then coarsely corrugated
transversely, the result being that individual exposures may strike at
any angle between E.–W. and N.–S. The average strike is inclined around
Gwombe to the north-west, and around Buga to the north-east. To the east
of Buga the schists give place to the usual varied succession of softer
gneisses, pierced in the usual manner by veins of quartz and tourmaline
quartz, and associated from time to time with banded and composite rocks
and detached masses of granite. Numerous dykes of basalt and quartz
porphyry appear to be associated with the granites of the Kurudu and
Anagoda hills, which lie respectively west and south-east of the town of
Nassarawa.
The crystalline rocks of Bauchi province present similar characters to
those of other parts of the Protectorate. The broad belts of schists and
quartzites found in other provinces are here, however, conspicuously
absent, such types being represented only by thin bands intercalated
within the softer gneisses. Banded and composite rocks, on the other
hand, appear to be present in somewhat greater proportion than
elsewhere, while domes and turtlebacks of foliated or partially foliated
granite (Plate XII, Figs. 1, 2) are in places particularly abundant. The
softer and harder rocks are, however, so intimately mingled that a
detailed description of the succession is impossible. Excellent
exposures of the harder series are to be found in the Kogin Delimi below
Naraguta in the west of the province, and of the softer gneisses in the
Jarawa hills to the south of Bauchi, and of both in the broken rocky
country between Kanna and Angass in the southern portion of the province
(_v._ Traverse No. VI, p. 68). The strike of the gneisses, while very
variable, appears to have a greater tendency to assume an E.–W.
direction in Bauchi than in other provinces, a tendency which is
probably due to the greater intensity of the transverse forces which
have elsewhere corrugated the prevailing meridional strike of the
gneisses and schists. This feature is particularly marked in the
neighbourhood of the Ningi massif, which is elongated approximately
along the strike of the surrounding softer and harder gneisses.
Another notable feature of the province of Bauchi is the number and
variety of the later igneous intrusions. Pegmatites, quartz reefs and
veins of tourmaline quartz are as abundant in Bauchi as elsewhere, but
the attention is somewhat diverted from these commoner intrusive types
by the presence of an abundant series of dyke rocks, and of extensive
masses of non-foliated granite and syenite. The pegmatites are in places
particularly rich in ilmenite, rutile and chalcedonic quartz; the dyke
rocks present almost every variety between basalt and quartz porphyry;
and many of the granites are especially rich in soda, and in places show
chilled margins at their junction with the gneisses. The summit of the
plateau around Bukuru is occupied by one of these extensive granitic
intrusions, which is of peculiar interest from the fact that in places
it is veined and impregnated with tinstone and other minerals. A similar
granite, also carrying tinstone, forms the greater part of the
Kwandokaya hills to the north of Toro. Many intrusions of syenite are
found around and to the east of the town of Bauchi. The Ningi massif is
largely composed of a soda granite, with a well-chilled margin,
accompanied by numerous dykes of granite and porphyry, and in places
carrying tinstone. Similar granites are found at Kila and Fagam on the
borders of Bauchi and Kano, at Shira in Katagum to the north, and at
Gurkawa on the Muri border to the south. The Murchison Hills are
believed to be largely granitic, but they and the southern margin of the
plateau are still practically unexplored. A small detached area of
granite and gneiss, pierced by numerous pipes of basalt and trachyte, is
found around Tangalto and Chongwom in the south-east of Bauchi province.
_Southern Muri and Yola._ (A. L.)
The strip of crystalline rocks which runs along the southern boundary of
Muri and Yola has an average width of forty miles and is separated from
the central area by the sedimentary rocks of the Benue plain. The
crystalline floor appears at the surface, however, in a number of small
inliers around Arofu in western Muri, and thus quite near to the river.
Five miles east of Arofu a small area of fresh biotite-granite about
fifty yards in diameter peeps from beneath the limestone. Immediately
north of the town itself there is a knoll of granitoid gneisses striking
north and south. On proceeding southward along this line of strike
limestone is passed over for three miles, but then the gneiss appears at
the surface again, and this time forms a ridge running southward for at
least some miles. It may well be that other inliers of crystalline rocks
occur in the unexplored Munchi country.
Farther south the transition from sedimentary to crystalline rocks is
not accompanied by any feature in the drift-covered plain. Though the
exposures are not good, it appears that biotite-gneisses and schists
with the customary northerly strike are penetrated by granite veins. The
rocky masses which lie south and south-west of Takum are probably
largely or entirely of granite. The bulk of the material which forms the
scattered hills around and to the north of Takum is also granite, but
here it is inextricably mixed up with gneiss which is met with in patchy
fashion from top to bottom of the hills. The foothills of Shebshi are
schistose grits and gneisses invaded by granites in the usual
manner.[24]
PLATE VIII
[Illustration: EXFOLIATION IN THE KILBA HILLS.]
[Illustration: THE GORGE OF THE KADUNA AT ZUNGERU.]
Only the northern part of the Mumuye-Manna plateau has been traversed.
The rock there is almost always biotite-granite, but it is by no means
always of the same variety. Occasional quartz veins are seen on the
plateau and also on the lower ground between Numnai and Panteti, where
the quartz carries pyrites. Intrusions of quartz-porphyry are often seen
cutting through the granites, _e.g._, east of Numnai, around Shega Mundi
hill, around Panti Baturi and at Yendam Hill. The granites only
occasionally carry included blocks and fragments of gneiss, but on
descending to the plain biotite-gneiss is met with striking N. 10° E.,
and the plain from Chukol eastward to the Vere Hills is occupied by
biotite-gneiss, with some intrusions of granite. The Vere Hills
themselves are made up almost entirely of granite, though to a small
extent of gneiss also. Biotite-gneiss is frequently seen round the base
of the plateau, generally much penetrated by granite or quartz-porphyry.
Thus, speaking generally, the plains of southern Yola are mainly of
gneiss and the plateau areas mainly of granite.
To the north of the Benue plain again, the country around Song, Goila
and Korubi is a biotite-granite area with some conspicuous dykes of
quartz-porphyry running for miles across the country. Near Zummu
porphyry rises just above the soil of the plain and exhibits flow-
structure, while between Zummu and Holma a felsite dyke is seen cutting
through the granite. The hills around Holma are built of a well-jointed
biotite-granite, and the sides of the hills are concealed by the loose
blocks split off along the joints. The granite of the Kilba Hills, near
Pella, is a very fresh, coarsely-crystalline biotite-granite, and
locally, where the Kilba Hills rise out of the plain to the west, the
granite takes on an augenstructure. In consequence of exfoliation (Plate
VIII, Fig. 1) most of these hills rise up as smooth bare rocks, but some
are made up of heaps of blocks, and it is not possible to explain the
contrast as due to a difference in material. Between the hills around
Holma and those around Pella there is a plain of gneiss about fifteen
miles broad. The gneiss has been formed by the injection of a micaceous
gneiss or schist with innumerable strings and veins of granite. The
exposures are excellent throughout, but the gneiss never forms hills.
The country north of Pella is comparatively uninteresting. It consists
of an extensive plain with isolated hills and rocks of biotite-granite.
The nature of the rocks underlying the plain is practically unknown. The
small area of the Kwaba Hills is composed of biotite-granite with some
quartz-porphyry. (A. L.)
Crystalline rocks, largely of a gneissic character, are exposed also in
southern Bornu around Kwoia to the east of the Gongola and along the
base of the escarpment on the north-western margin of the Burra volcanic
plateau. In eastern Bornu the granite rocks of Chibuk and Kobshi are the
first prominent outcrops of crystalline rocks to the south of Maidugari.
Barth,[25] however, mentions an isolated granitic cone, Mt. Deladeba, to
the N.N.E. of Isge, while Courtet,[26] Gentil and Freydenberg[27] note
the occurrence of knobs of crystalline rock between Lake Chad and Lake
Fittri. It is highly probable indeed that the thick alluvial
accumulations to the east and south-east of Chad and in the lower Shari
valley rest for the most part upon a crystalline floor, provided with
scattered relics of Tertiary sandstone. (J. D. F.)
[Footnote 18: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 325.]
[Footnote 19: Parkinson, _Q. J. G. S._ (63), 1907, p. 317.]
[Footnote 20: Barth, _Central Africa_, 1857, Vol. II, pp. 41, 42.]
[Footnote 21: Denham and Clapperton’s _Narrative_, 1826, pp. 50, 57.]
[Footnote 22: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]
[Footnote 23: Dunstan, _Colonial Reports—Miscellaneous_, No. 26, p. 15.]
[Footnote 24: See also Downes, _Yola and Cross River Boundary
Delimitation_, 1910, p. 35.]
[Footnote 25: Barth, _Central Africa_, 1857, Vol. II, p. 369.]
[Footnote 26: Courtet, _C. Rd. Ac. Sc._, 140, p. 160.]
[Footnote 27: Gentil and Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII,
1908, p. 44.]
CHAPTER III
PETROGRAPHY OF THE GNEISSES AND SCHISTS AND ASSOCIATED IGNEOUS
INTRUSIONS.
Subdivision of the gneisses and schists; predominant meridional strike.
Gneisses and schists of sedimentary origin; variation in degree of
metamorphism; phyllites and slaty schists; green schists; quartzites;
quartz schists and granulitic gneisses; crystalline limestones;
micaceous and felspathic gneisses; calc silicate rocks; epidote
hornblende rocks; magnetite and hæmatite schists; sheared trachytes of
Bussa; altered dolerites of Ngashki; amphibolites and talc schists;
granulites. Petrography of the banded Archæan gneisses: relationship of
the two groups of gneisses. The gneisses and schists of the Sahara and
of West Africa. Age of the sedimentary gneisses. Intrusive rocks: the
older granites; variation in degree of reconstruction; brecciation and
injection structures; the younger granites; aplitic and tourmaline-
bearing granites and pegmatites; tourmaline quartz veins; stanniferous
pegmatites; schorl rocks; soda granites; biotite and riebeckite
granites; tinstone associated with soda granites; biotite and augite
syenites; augite diorites; riebeckite quartz porphyries; ægirine
granophyre; augite syenite porphyries; alkali syenite porphyries;
diorite porphyrites; mica porphyrites; hornblende porphyrites; diabases
and basalts. Granites and dyke rocks of the Sahara. Age of the soda
granites; successive periods of intrusion.
THE GNEISSES AND SCHISTS.
For descriptive purposes it has been found convenient to separate the
crystalline rocks of the Protectorate into two groups, (1) a group of
quartzites, phyllites, schists, and soft felspathic and micaceous
gneisses, and (2) a group of hard well-foliated, much granitised and
frequently banded and contorted micaceous and hornblendic gneisses. The
first group occupies by far the greater area within the Protectorate,
and presents in the west a well-marked alternation of belts in which
quartzites, phyllites and schists predominate, with belts of which the
prevailing members are felspathic and micaceous gneisses. The second
group has no definite distribution within the Protectorate, but appears
to replace locally and irregularly the members of the first group
throughout the whole crystalline area. The members of both groups appear
to be irregularly folded along axial planes, which are usually vertical
or highly inclined, but it is probable that detailed investigation would
reveal a regular system of major folds, accompanied by much local and
minor crumpling. The folding, which is typically meridional, has been
affected by equatorial stresses in such a way as to produce a transverse
twisting or corrugation of the predominant strike. The result is that
individual exposures may vary in direction of strike from N.–S. to
E.–W., while in places, as in Bauchi province, the strike may become
regionally equatorial. A similar equatorial divergence of the prevalent
meridional strike has been noted by M. Hubert[28] in eastern Dahomey,
but his suggestion that the equatorial direction becomes predominant in
Borgu and the Niger valley has been found to be unsupported by local
evidence. It is impossible to believe, moreover, that there can be any
connection, as suggested by M. Chudeau,[29] between this locally
divergent strike of the gneisses and the distribution of the overlying
Cretaceous and Eocene rocks.
The sedimentary origin of the first group is fairly certain. The pebbly
quartzites, quartz schists, schistose grits, mica schists, slaty schists
and phyllites represent corresponding varieties of sandstones, grits and
shales, and frequently exhibit traces of their sedimentary origin in the
deformed pebbles and clastic grains which they contain. The granular
felspathic and micaceous gneisses on the other hand are more highly
reconstructed, and afford in themselves little or no evidence of a
clastic origin. Their constant association and alternation with the
quartzites and schists make it probable, however, that they represent
the arkoses and greywackes of the original sedimentary series. Many of
the green hornblendic and epidotic beds probably also represent original
gritty accumulations of chloritic and epidotic character, while the
quartz magnetite schists of eastern Kabba correspond to ferruginous
grits or gritty ironstones of sedimentary origin. The fine-grained
crystalline limestones of Igbo, and the course granular marble between
Jakura and Wa, similarly represent original clastic calcareous rocks
within the sedimentary succession. To what extent the recurrence of
belts of quartzites, phyllites and schists in the western part of the
Protectorate represents the repetition of one or more belts is quite
unknown. The phyllites and schists are moreover everywhere
unfossiliferous and the age of the original sedimentary rocks is
consequently undetermined. The foliation of the reconstructed rocks is,
as a rule, parallel with the bedding, but the more micaceous schists
frequently show a more or less well-defined and oblique strain-slip
corrugation.
The alternation of belts of less and more highly metamorphosed rocks in
the west of the Protectorate coincides to some extent with an
alternation of belts in which shear structures predominate, with belts
in which reconstruction is prevalent. The various belts appear to pass
insensibly into each other, but it has been found impossible in the
course of a rapid survey to decide either to what extent shearing may
have everywhere preceded reconstruction, or to what extent the varying
degree of reconstruction in alternate belts may represent a
corresponding variation in the intensity of the accompanying folding.
All that can be said with certainty is that the sedimentary series as a
whole has been subjected to a highly complex folding accompanied by much
shearing and reconstruction, and that while over the greater part of the
Protectorate the stresses have been for the most part relieved by, or
translated into, a molecular and mineralogical rearrangement, there are
certain belts in which relief has been primarily obtained by mechanical
readjustment or shearing. Moreover, as in the reconstructed areas there
are certain beds which appear to be more coarsely crystalline and less
distinctly foliated than others, so in the shear belts there are certain
beds which have assumed the character of granulitic gneisses, while
others in the immediate neighbourhood have retained to a very large
extent their original clastic and gritty character. Most of the
reconstructed gneisses exhibit under the microscope faint strain
phenomena which, with the incipient strain-slip foliation of the more
micaceous schists, are probably to be ascribed to regional stresses
affecting the whole series after the period of maximum reconstruction.
Grey and purple sericitic phyllites with knots of magnetite and pyrites
are extensively developed in the neighbourhood of Ngashki in the Niger
valley, around Anka and Bukwium on the borders of Sokoto and Kontagora,
and in the Zungeru belt in eastern Zaria from Kato to Birnin Gwari and
Koriga. In Kabba and Nassarawa and southern Zaria their place is taken
by fine-grained quartz muscovite schists with abundant magnetite. With
the phyllites are associated in the Niger valley and Sokoto province
green quartzose slaty schists (1195, 1325),[30] whose colouration is due
to recrystallised chloritic and hornblendic material. Mr. Parkinson[31]
has described similar slaty schists from Southern Nigeria. In the
neighbourhood of Zungeru these green schists become softer and show
under the microscope (1563–4) ragged porous or fibrous crystals and
crystal aggregates of reconstructed hornblende in a matrix of granulitic
quartz, felspar and magnetite, which encloses also scattered grains of
hornblende, biotite, chlorite and epidote, as well as relics of original
clastic grains of quartz and felspar. These rocks present a considerable
resemblance to the “green beds” of the Scottish Highlands.[32] True
quartzites are comparatively rare and only at Kazaure do pure siliceous
rocks attain any extensive development. Elsewhere the rocks which
present the characteristic appearance of quartzite in the field are
usually more or less felspathic and micaceous, and include schistose
grits, micaceous quartzites, quartz muscovite schists and granulitic
gneisses which show all the familiar stages in the deformation of
felspathic grits and sandstones. Some of the white and pink granulitic
gneisses, such as those of Zungeru, may, however, be ultimately proved
to have been originally of an eruptive character. A characteristic
feature of these rocks is the development of tourmaline and sometimes of
pyrites along the shear and foliation planes as the result of later
pneumatolytic action. At Kuta a belt of coarse quartz muscovite schist
contains abundant sillimanite in sheaf-like aggregates.
Mica schists in which biotite predominates are common throughout and
frequently carry garnet, magnetite and staurolite in conspicuous
crystals. In the Igbo limestones the original impurities have
recrystallised as tremolite, diopside, scapolite (122), sphene, biotite,
felspar, quartz and pyrites, while the streaks of silicates in the
limestone between Jakura and Wa contain diopside, epidote, idocrase,
felspar, and quartz. In the latter limestone also, scales of graphite
are common throughout and pyrites on the margin. Amongst the
reconstructed gneisses, which show a remarkable and rapid variation in
texture, the paler-coloured felspathic rocks are usually rich in
orthoclase and microcline, and occasionally muscovite, while the darker-
coloured varieties, in which biotite predominates, frequently carry
garnet, sillimanite, staurolite and in places also sphene, tremolite and
carbonates. Andalusite-bearing rocks are known from certain localities,
but kyanite gneisses and schists have not been definitely recognised in
place. The local occurrence of kyanite on the river sands, however,
points to its presence within the neighbouring rocks. Calc silicate
types are represented by the tremolite schists of the Nassarawa
tableland (668, 689), by a diopside tremolite schist from Borgu (1221),
a diopside and wollastonite-bearing rock with sphene from Maibirro in
the Niger valley (1192), a biotite tremolite schist with calcite from
Olle in northern Kabba (879), and a diopside and scapolite-bearing rock
with sphene and green spinel from Atuka, near Okuruku, in eastern Kabba
(843). The epidote hornblende gneisses of Boi in southern Bauchi (976)
contain much diopside and sphene, and are probably of sedimentary
origin, while relics of clastic felspar are common in an epidote rock
from Goshin Duchi a few miles east of Boi (983). A similar clastic
origin is presumed for many of the epidosites and epidote hornblende
rocks associated with the sedimentary gneisses in other parts of the
Protectorate. The quartz magnetite and quartz haematite schists include
coarse granular and fine-grained granulitic types (844, 1370), the
latter presenting considerable resemblance to the itabirites of the
United States. Their sedimentary origin is presumed throughout and only
in the case of the massive quartz magnetite rock of Okuruku (148), which
is associated with a magnetite-bearing epidiorite, is there any
probability of an igneous origin.
Two stages of alteration can be recognised among the sheared trachytes
of Bussa (p. 72). The least altered types (1164, 1167) show fracturing
of the phenocrysts of felspar and the replacement of the original ferro-
magnesian minerals by fibrous uralite, chlorite, epidote and calcite.
The more altered types (1166) are characterised by the production of
abundant white mica in the ground-mass and the development of biotite,
magnetite and carbonates from the original ferro-magnesian material.
Original vesicles of quartz and calcite in these rocks have been
deformed and the quartz replaced by a mosaic of angular grains. A
sheared quartz-porphyry from Kanikoko (1206) exhibits deformation and
granulation of the phenocrysts of quartz and felspar and the abundant
development of white mica in the ground-mass. In the altered dolerites
of Ngashki (1175, 1176) the original structures are almost entirely
obscured by the extensive production of granular epidote, calcite,
uralite, chlorite, magnetite, felspar and quartz. Knots of tourmaline
are common in these altered rocks. The types grouped as amphibolites
range from epidiorites to hornblende schists and amongst them may be
recognised all the familiar stages of saussuritisation and
recrystallisation of original tonalitic, dioritic and diabasic rocks.
It is probable, however, that amongst the amphibolites are included
rocks which were intruded at many different periods during the
metamorphism of the gneisses and that to difference of age as well as to
local intensification of the metamorphism is to be ascribed the variable
degree of construction which the rocks have severally undergone. The
chlorite and talc schists are to be regarded as basic intrusions which
had suffered considerable decomposition before being subjected to
metamorphism.
Amongst highly reconstructed rocks of doubtful derivation, associated
with the sedimentary gneisses, are a garnetiferous quartz enstatite
biotite granulite from Atuka near Okuruku in southern Kabba (841) and a
garnet granulite from the adjoining village of Kuturu (168) composed of
labradorite, quartz and garnet with a distinct centric structure. The
garnet is in spongy masses associated with a small quantity of green
pyroxene, sphene, apatite and iron ore. A pyroxene granulite, from
Againyi (823), consists of a porous intergrowth of pale green pyroxene
and green hornblende, the former predominating, in a matrix of clear
labradorite. Sphene and ilmenite are also present in irregular
intergrowths as well as a little accessory epidote and apatite. Another
variety (832) contains no amphibole and consists of pale green pyroxene
and garnet in a matrix of labradorite and quartz. A third type from the
same locality (821) is an epidote granulite consisting of predominant
epidote and garnet with subordinate green pyroxene, sphene and ilmenite
in a matrix of labradorite. A pyroxene granulite from Nassarawa province
(854) shows ragged crystals of diallage with quartz inclusions and
rounded garnets with inclusions of both quartz and diallage in a matrix
of coarsely granular quartz. A hornblende granulite from the same
province (859) consists of rounded spongy garnets and radiate groups of
actinolite fibres with a little accessory ilmenite in a typical fine-
grained quartzo-felspathic mosaic. A remarkable biotite granulite from
Eri in eastern Ilorin (1056) contains in addition to biotite and
granular garnet as the predominate ingredients much sillimanite,
staurolite and andalusite in a matrix of untwinned felspar and quartz.
The harder gneisses of the second group are subordinate in amount and
irregularly distributed throughout the micaceous and felspathic gneisses
of the first group. They consist of a series of much granitised medium
and even-grained biotitic and hornblendic gneisses which usually show a
lenticular or linear foliation but frequently also a ribboned or banded
and contorted character. In some cases the banding is broad and only to
be distinguished in the field, while hand specimens present quite a
homogeneous character. All variations may be found between acid aplitic
and basic hornblendic bands, and knots and lenticular masses of more
basic material are common within the more acid types. As a rule the
banding is much interrupted by pale coloured granitic material into
which the lighter coloured bands seem to pass imperceptibly. A striking
feature is the presence of a series of early pegmatite veins folded and
foliated with the gneisses. There is in fact every indication that these
banded rocks are of a primarily composite character and represent either
an originally sedimentary or a differentiated igneous succession invaded
and injected by acid granitic material and pegmatite veins in the deeper
portions of the crust and subsequently subjected to a later folding,
foliation and recrystallisation. The foliation is in general vertical
and parallel with the banding. Under the microscope the striated and
banded gneisses exhibit all the characteristic features of typical
archæan rocks (801, 846). They are composed of a granular aggregate of
quartz, felspar, hornblende or biotite with sphene, apatite and iron
ores as accessories. The felspars are usually allotriomorphic and vary
from labradorite in the more basic to orthoclase and microcline in the
more acid rocks. The biotite occurs in tabular flakes singly or in
aggregates with the basal plane well-developed but with ragged margins
in the prism zone, while the hornblende forms spongy growths or
irregular plates with embayed margins and inclusions of quartz. Strain
shadows are common in the quartz and felspar but only traces of
cataclastic granulisation or of secondary micropegmatite are as a rule
recognisable. Van Hise[33] is of opinion that the strain phenomena and
the vermicular quartz to be observed in these thoroughly reconstituted
gneisses are due to the partial lagging of reconstruction behind
deformation. It is equally probable, however, that the strain effects
and micropegmatitic structures have been induced after the period of
maximum reconstruction by later strains of lesser magnitude.
The mutual relations of the two groups of gneisses are difficult to
determine. The actual contact between the two has nowhere been found
clearly exposed and its character must therefore be to a certain extent
conjectural. The apparent irregular replacement of the softer gneisses
by members of the harder group when traversed across the strike and the
apparent extension into the softer gneisses of tongue-like masses from
the larger areas of harder rock is certainly suggestive of an intrusive
character as advocated by Mr. Parkinson[34] in Southern Nigeria and
Liberia. Upon this hypothesis the harder gneisses would appear to
correspond to an intimate mixture of granitic and dioritic material,
whose intrusion took place during the folding and reconstruction of the
sedimentary gneisses. It is difficult, however, to understand why these
rocks should present in their even-grained and closely foliated and
banded character such an entirely different aspect in the field from
that of even the earliest recognisable granitic intrusions within the
sedimentary gneisses and why they alone should possess an early set of
folded and foliated pegmatite veins in addition to the later pegmatites
which are common to both groups of gneisses. All the phenomena indeed
are more readily explicable on the assumption that the two groups differ
in age, that the sedimentary gneisses were originally deposited
unconformably upon an earlier and probably archæan gneissic or igneous
complex and that the latter series was refoliated and recrystallised
during the reconstruction of the former. The peculiar distribution of
the harder gneisses and their intimate alternation with the softer
series would then be due in part to the original unconformability
between the two groups and in part to the irregular character and the
varying amplitude of the later folding to which both were subjected. The
abrupt replacement of the sheared but little altered quartzites of the
Niger valley by the harder gneisses at Jebba, Wuru and Maibirro probably
indicates also a certain amount of faulting and thrusting accompanying
the folding. On account of the subordinate and ill-defined character of
the distribution of the harder rocks no attempt has been made to
distinguish upon the map between the archæan and the later sedimentary
gneisses.
PLATE IX
[Illustration: ON THE SUMMIT OF THE BAUCHI PLATEAU.
The Parade Ground at Bukuru.]
[Illustration: ON THE PLATEAU LOOKING WESTWARD FROM BUKURU.]
In the Sahara to the north, M. Chudeau[35] has described the occurrence
of archæan and sedimentary gneisses whose mutual relations are similar
to those which prevail in Northern Nigeria. The archæan areas are of
comparatively small extent, and are primarily distinguished, according
to M. Chudeau, by the presence of numerous domes and turtlebacks of
granite. This method of distinguishing between the two groups, however,
cannot be applied in Nigeria, where intrusive granites are equally
common amongst both the earlier and the later gneisses. The metamorphic
rocks of sedimentary origin attain a great development in the Sahara[36]
and include quartzites, phyllites, schists, gneisses and crystalline
limestone similar in type to those of Nigeria, and folded in a similar
complex manner along axes whose direction is predominantly meridional.
The decrease northward of the intensity of the metamorphism, as
advocated by M. Chudeau, requires much further investigation in view of
the local occurrence of little altered rocks within the Protectorate. M.
Chudeau believes in the original unconformability of the later gneisses
upon the archæan and points out, with M. Haug,[37] the analogy between
the system of folding of the gneisses of the Sahara and the Sudan, and
the Caledonian folding of the crystalline rocks of Scotland and
Scandinavia. He demonstrates the pre-Devonian age of the sedimentary
gneisses, and assigns them provisionally to the Silurian on the strength
of their supposed continuity with the graptolite-bearing shales of
Tindesset[38] and Hassi el Kheneg.[39] The stratigraphical evidence
available, however, seems rather to point to an unconformability between
the shales and the schists, and M. Flamand[40] prefers to assign a pre-
Cambrian age to the whole series of crystalline schists and gneisses.
In Dahomey, M. Hubert[41] has described a similar complex of archæan and
sedimentary gneisses in which the latter clearly predominate. His
separation as Silurian of the quartzites and schists of Atacora from the
other metamorphic rocks appears, however, to be unjustifiable.
Quartzites, gneisses and schists of sedimentary origin associated with
granites and archæan rocks, are found also in West Africa, in French
Guinea and the Fouta Djalon,[42] in Liberia,[43] the Ivory Coast,[44]
the Gold Coast,[45] Togoland,[46] and Southern Nigeria,[47] and in
Central Africa to the south and south-east of Chad,[48] in the
Kameruns,[49] and in the French Congo.[50] Similar rocks attain also a
remarkable development to the south of the equator.[51]
The comparison which M. Chudeau has drawn between the strike of the
gneisses and schists of the Sudan on the one hand, and of the Scottish
Highlands on the other, may be further extended not only to the
petrographical character of the constituent groups of the gneissic
complex in West Africa, but also to the respective relations of the
various groups to each other, and to the intrusive rocks which they
enclose.[52] Dr. Voit[53] has, moreover, arrived at similar conclusions
in the course of his investigation into the structure of the crystalline
complex of the northern Transvaal, the constitution and mutual
relationships of the Swaziland system and the older granites and
gneisses being closely paralleled in the rocks of Nigeria.
THE INTRUSIVE ROCKS.
Many of the thin bands of well-foliated augen and granitoid gneiss
enclosed within the metamorphic series undoubtedly represent original
granitic intrusions, but these are so intimately bound up with the
adjoining gneisses and schists, that no discrimination can yet be
attempted. Attention has been more particularly directed in the field to
those granitoid rocks which form the prominent inselberge and isolated
groups of hills, and amongst them it has been found possible to
distinguish an older from a younger series of intrusions. The older
intrusive rocks consist very largely of granite, while the younger
intrusions include granitic, syenitic and diabasic types with numerous
associated dyke rocks. To what extent the epidiorites, amphibolites, and
granulites of the gneissic series represent more basic intrusions
associated with the older granites is quite unknown.
(_a._) _The older Granites._
The older granites are never sharply marked off from the adjoining
gneisses, and invariably show a certain amount of foliation in the
field, and of cataclastic deformation and reconstruction under the
microscope. They appear to have been intruded at different times within
the period of the evolution of the gneisses, and the amount of
reconstruction which they themselves have undergone may be taken as a
general indication of their age. Vermicular micropegmatite is most
extensively developed in those schistose granites which have stopped
short of complete reconstruction and which still afford abundant
evidence of cataclastic deformation. It may also occur, however, in
small amount in thoroughly foliated and reconstructed granites, in which
it is assumed to have been developed, as in the case of the banded and
striated gneisses (p. 118), by minor crustal strains, after the period
of maximum reconstruction.
The most highly altered types form extensive lenticular or phacolitic
masses of well-foliated and fairly coarse-grained biotitic and
hornblendic gneisses (849, 481), containing streaks and patches of
darker-coloured and finer-grained basic material. Under the microscope
they possess a coarsely granular and thoroughly reconstructed character,
and in the mutual relations of their constituents present a considerable
resemblance to the banded gneisses of similar composition. The quartz
and felspar show much later straining with incipient granulisation and
traces of micropegmatite. These rocks have participated in all the
changes which the adjoining gneisses have undergone, and were apparently
intruded before the period of maximum reconstruction of the latter.
The least altered of the older granites (806, 1560) are typically
distinguished by the irregular batholitic character of the intrusions
and the partial and frequently only marginal character of the foliation.
They sometimes have a tendency to become finer-grained towards their
junction with the gneisses, and they usually enclose portions of the
adjoining rocks and vein and inject with granitic material the marginal
gneisses and schists. The inclusions may be of either the harder or
softer gneisses according to the character of the rocks which the
granite has invaded. In places a parallel arrangement of the felspars
appears to have been induced through streaming during crystallisation.
The partial foliation is parallel in a general way with that of the
surrounding gneisses and schists. The granites vary much in character
from biotitic and hornblendic rocks with syenitic modifications to
muscovite and microcline granites and aplites. The quartz and felspar
are strained throughout and cataclastic granulitisation is common on the
margins of the crystals. Vermicular quartz is most abundant in the
marginal and more distinctly foliated portions. These rocks would appear
to have been intruded after the period of maximum reconstruction of the
gneisses, but before the final cessation of stress and strain. In
places, as at Abuja, crush lines cross the granites, and in their
immediate neighbourhood the rocks show a higher degree of reconstruction
and a greater development of micropegmatite in the matrix.
These later members of the older granites appear to have produced little
or no contact metamorphism in the surrounding rocks. Brecciation and
injection structures on the other hand are very frequent. The hard
banded gneisses on the margin of the Abuja granite, for example, are
abundantly injected with quartz and quartzo-felspathic material.
Similarly at Zwoll, in Bauchi province, a porphyritic biotite granite
has brecciated a series of banded and contorted biotite and hornblende
gneisses, and many of the enclosed fragments are partially absorbed or
minutely injected with granitic material. The Okuruku granite in
southern Kabba, on the other hand, has invaded and brecciated members of
both the harder gneisses and the softer series of biotite schists,
magnetite schists and amphibolites. In the neighbourhood of the
inclusions the granite becomes knotted and richer in biotite,
hornblende, sphene and other minerals, while the inclusions themselves
are irregularly veined and minutely penetrated by quartzo-felspathic
material. In every case it would appear that the rocks, which the
granites have brecciated and invaded, possessed their gneissic or
schistose character before the actual intrusion of the granites, and
that they, as well as the larger inclusions, suffered little further
change as the result of the stresses and strains which brought about the
partial foliation of the granites. The gneisses as a whole show strain
phenomena, but only traces of granulitisation and of secondary formation
of micropegmatite, while the granites show extensive marginal formation
of micropegmatite, together with intense cataclastic deformation. It
would seem indeed that these granites were intruded after the period of
maximum reconstruction of the gneisses, but before the final cessation
of stress and strain, and that while the gneisses which had originated
under conditions of maximum strain were comparatively stable under
similar strains of diminishing intensity, the granites which had
crystallised from igneous fusion were in a condition of unstable
equilibrium with respect to the stresses and strains to which they were
subjected after crystallisation. The gneisses, as it were, possessed a
certain amount of elasticity and transmitted the later strains with
little resultant deformation, while the rigid masses of granite arrested
those strains and suffered in consequence a partial deformation. The
production of a strain-slip foliation in some of the more micaceous beds
is probably to be referred to the same period of differential
deformation of the granites. These later strains became locally
intensified along crush lines, and in the case of the Abuja granite
produced granulitisation both of the granite and of the felspathised
gneisses in the neighbourhood of the crush line.
In addition to the two extreme types of the older granites there are
many elliptical masses of similar material, elongated in a direction
parallel with the strike of the gneisses, which show a decided foliation
throughout, sometimes more and sometimes less intense (48, 1223). As a
rule they are indistinctly marked off from the adjoining rocks and
preserve their normal texture up to the margin of the intrusion. The
granites are predominantly biotite granites, more rarely muscovite
biotite granites, and the felspars are frequently porphyritic. Under the
microscope they show a higher degree of reconstruction than the least
altered granites, but a less complete reconstitution than the most
highly altered types. The felspars vary from oligoclase to microcline,
and the plagioclases often show basic interiors and zoned margins. Both
the felspar and the quartz show abundant strain phenomena, and the
felspars are usually partially surrounded by a secondary fringe of
vermiculate micropegmatite in optically discontinuous grains. Frequently
also the felspars are corroded or irregularly penetrated by quartz or
micropegmatite, while the matrix consists largely of granular and
partially recrystallised felspar, muscovite, microcline and quartz. The
darker patches within these granites may be taken to represent portions
of the adjoining rocks caught up during intrusion and partially
absorbed, recrystallised and foliated with the granites. These rocks may
possibly be correlated in the period of their intrusion with the least
altered granites and regarded as the smaller and therefore more readily
reconstructed representatives of the larger partially foliated masses.
On the other hand, in the absence of the inclusions of adjoining
gneisses which form the characteristic feature of the least altered
granites, it is possible to regard them as of somewhat earlier origin
and as having participated to a considerable extent in the general
reconstruction of the gneisses themselves.
It seems probable indeed that the crustal movements which brought about
the metamorphism of the gneisses were of a somewhat spasmodic character
and spread over a very considerable interval of time and that the
intrusion of granitic and other igneous material took place at intervals
throughout the whole period. No hard and fast line can therefore be
drawn between the highly reconstructed and gneissose granites and the
imperfectly or only partially foliated types, a complete series of
intermediate varieties being apparently possible between the two extreme
members of the series.
(_b_) _The Younger Intrusions._
1. Granites and pegmatites.
Amongst the later granites, which present for the most part a batholitic
habit, two types may be distinguished, differing in mineralogical
composition and probably also to some extent in age. The rocks of the
first type never show chilled margins, and were evidently intruded while
the adjoining gneisses still possessed a comparatively high temperature.
They include non-foliated aplitic granites and pegmatites usually rich
in muscovite and tourmaline. The second type includes riebeckite and
biotite granites which are sharply marked off from the adjoining rocks
by felsitic and fine-grained margins. The rocks of the first type
usually exhibit under the microscope a faint straining of the quartz and
felspar and were probably intruded before the final cessation of the
crustal movements to which the reconstruction of the gneisses and the
foliation of the earlier granites were due. The members of the second
type, on the other hand, are practically free from cataclastic phenomena
and evidently assumed their present position after the cessation of
movement within the crust. There is no reason, however, to assume any
great difference in age between the two types or any marked
discontinuity in magmatic conditions. The same magma, of a somewhat acid
and alkaline character, was probably present in the depths throughout
the whole period of intrusion, and the varying character of the products
may be taken as affording some clue to the nature of the differentiation
within the magma itself.
The alkaline granites of the first type are pale-coloured granular rocks
composed almost entirely of quartz, microcline, orthoclase, perthite,
and acid plagioclase. Biotite and other ferro-magnesian minerals are
comparatively rare, while muscovite and tourmaline are frequently
abundant and in large primary crystals. The quartz and felspar have a
tendency to pegmatitic intergrowth, and sometimes masses of graphic
pegmatite occur porphyritically in a normal quartzo-felspathic matrix.
The tourmaline and muscovite may occur also as at Igbo in two
generations, in large crystals and in the ground mass. Frequently also
these granites possess a tendency to a coarsely banded structure, the
banding being due either to variation in size of grain of the quartz and
felspar or to variation in the abundance of tourmaline and muscovite in
successive bands. These granites rarely form intrusions of any size, the
largest known being the tourmaline granites of Igbo and Darroro, but
even they are insignificant in comparison with the alkaline granites of
the second type.
The pegmatite dykes and veins which are so abundant throughout the
gneissic series evidently belong to the same period of intrusion. Like
the aplitic and tourmaline-bearing granites they never exhibit fine-
grained margins at their junction with the gneisses. They vary much in
composition, and include graphic granites, quartz-felspar pegmatites
with biotite, magnetite or ilmenite, quartz-felspar pegmatites with
muscovite or tourmaline or both, and quartz-muscovite pegmatites with or
without tourmaline. At Eri there are stanniferous pegmatites carrying
quartz, muscovite and cassiterite, but no tourmaline. The massive
quartz-reefs which frequently enclose portions of the surrounding
schists and carry tourmaline, pyrites and sometimes gold, are also to be
referred to the same period of intrusion. Inseparable from them and from
the pegmatites are the smaller veins of quartz and tourmaline quartz and
the schorl rocks of a foliated or striated character which occur as a
rule in association with the veins of tourmaline quartz.[54] The schorl
rocks with a gneissic habit in which the quartz is minutely granulitic
and the tourmaline is in small crystals evidently recrystallised and
full of quartz inclusions, are probably altered gneisses and schistose
granites in which the felspars have been replaced by quartz and
tourmaline. Traces of decomposed felspathic material are to be found in
every specimen, but no pseudomorphs in tourmaline of felspar or of other
minerals have been definitely recognised. The striated schorl rocks in
which the darker bands are formed of finely fibrous tourmaline, appear
to arise also as modifications of the adjoining rocks in the vicinity of
the tourmaline quartz veins. Sometimes a foliated rock may be traced
through a striated type into a massive tourmaline rock on the margin of
the vein in which the original structures have entirely disappeared, and
in which the original minerals, including the quartz, have been entirely
replaced by finely fibrous tourmaline. In the quartz veins, pegmatites
and granites, on the other hand, the tourmaline is in well-formed
crystals of primary origin. It would seem, therefore, that the
pneumatolytic or metasomatic formation of the schorl rocks was brought
about by emanations from the quartz veins and pegmatites before the
final consolidation of the latter. The extensive development of
tourmaline and pyrites in the quartzites and schists of certain areas is
also to be ascribed to the circulation of mineralising vapours and
solutions throughout the adjoining rocks during the same period. The
remarkable absence of tinstone in the tourmaline pegmatites, together
with its appearance in pegmatites at Eri which do not carry tourmaline,
seems to point to the stanniferous pegmatites having originated at a
somewhat later date than the normal tourmaline-bearing pegmatites. This
suggestion is supported by the occurrence of tinstone in association
with the soda granites of Bauchi province.
The alkaline granites of the second type are typically distinguished by
possessing fine-grained porphyritic and felsitic margins which indicate
a considerable difference in temperature between the granites and the
surrounding gneisses at the period of intrusion. To this group are
referred the biotite granite of the Anagoda hills in S. Nassarawa, the
riebeckite granite of the Gurkawa Hills in northern Muri, the biotite
granite of Bukuru and Ngell, and the riebeckite granites of the
Kwandokaya Hills and of the Ningi-Burra massif in Bauchi province, the
riebeckite granite of Fagam, Gadama and Galambi on the borders of Bauchi
and Kano, the riebeckite granite of Shira in the sub-province of
Katagum, and probably also the Kailema granite near Jebba and the
biotite granite of Abara near Ngashki in western Kontagora. The alkaline
character of the original magma is reflected in the nature both of the
felspars and of the ferro-magnesians. The felspars are much kaolinised
and consist chiefly of tabular crystals of untwinned orthoclase probably
rich in soda, and of streaky perthitic intergrowths of orthoclase and
albite. Where enclosed in quartz, the terminations of the crystals are
frequently idiomorphic. Microlites of riebeckite are common inclusions
and small plagioclases of an earlier growth are frequently enclosed in
the later felspars and in the quartz. The quartz forms aggregates of
irregular grains showing faint strain shadows and has a great tendency
to form coarsely pegmatitic intergrowths with the felspar. Where biotite
alone is present, it is of a normal greenish-brown colour. Riebeckite,
however, is the most characteristic ferro-magnesian, and occurs for the
most part in irregular plates moulded on the felspars and sometimes
enclosing small crystals of felspar in poikilitic fashion. Occasionally,
as in the Shira granite, the riebeckite builds elongated crystals more
or less idiomorphic in the prism zone and frequently intergrown with
ægirine. Crystals of riebeckite over an inch in length are common in
knots in the Kila granite, while on the other hand the margin of the
Gurkawa granite is stippled with minute porous crystals of the same
mineral. Associated with the riebeckite in places are other blue and
brown soda amphiboles[55] as well as green ægirine and a golden-coloured
mica, while zircon, apatite and iron ores are constant but scanty
accessories. Pyrites is sometimes abundant in the porphyritic and
felsitic marginal rocks.
A remarkable feature of some of these later granites is the occurrence
in them of tinstone and numerous sulphide ores. It would appear that in
places these later granites have been broken and fissured, and that
along these fissures vapours and solutions have risen which have
chloritised and mineralised the adjoining rock and in part refilled the
fissures with ore-bearing pegmatite and vein stuff. In the Bukuru
granite the cassiterite is associated in the neighbourhood of Ngell with
pyrites, chalcopyrite, tetrahedrite, blende and galena. Topaz and brown
zircon are found accompanying the cassiterite in the tin-bearing
alluvium at Naraguta and elsewhere, and are probably also of
pneumatolytic origin. The granites of the Kwandokaya hills, of the
Jengre and Limoro hills, of the Ningi-Burra massif, and of the Gadama
and Fagam hills also yield in places a tin-bearing drift, and have
probably suffered, like the granite of Bukuru, a local fissuring and
mineralisation. Tourmaline has not been definitely recognised as a
product of these later emanations.
Similar soda granites with microgranitic and felsitic margins have been
described by MM. Foureau, Gentil and Lacroix[56] from Zinder, by M.
Chudeau[57] from Mounio and Goure, by M. Garde[58] from Machina and by
M. Hubert[59] from Dahomey, while Captain Freydenberg[60] has noted the
occurrence of a soda syenite with granitic modifications at Melfi to the
south-east of Chad.
PLATE X
[Illustration: A WORN VOLCANIC HILL ON THE BAUCHI PLATEAU NEAR ZAMAGAN.]
[Illustration: THE TWIN CONES OF KERENG FROM PANYAM]
2. _Syenites and Diorites_
These rocks, which are by no means abundant, occur as small bosses or
intrusive masses within the gneisses and schists. As a rule their
junction with the surrounding rocks is not exposed, and their
relationship to the later granites has not been definitely ascertained.
They should probably be regarded, however, as somewhat basic
modifications or differentiation products of the later granitic magma.
Slight strain effects are usually apparent in the quartz and felspar.
The low hills around the town of Bauchi are composed of a coarse-grained
augite syenite, in which orthoclase felspar predominates in large
twinned crystals with a peculiar brownish tint and a resinous lustre.
Under the microscope (49) subordinate oligoclase is present in smaller
crystals, while the knots of darker minerals are composed of
intergrowths of violet-coloured augite, green hornblende, brown biotite
and iron ores. Quartz is interstitial and occurs also in secondary
vermicular growths on the margins of the felspars. The accessories are
zircon and apatite, and the whole rock is much stained with secondary
limonite. A rock of similar appearance from Kende to the east of Bauchi
(69) is somewhat more basic and may be termed a quartz augite-diorite.
The predominant felspar is a basic andesine, while the ferro-magnesians
are pale-coloured augite and green hornblende in irregular plates and
intergrowths. The quartz is interstitial or intergrown with the felspar
in pegmatitic fashion. A similar rock from Kanna (1677) contains much
biotite in addition to the augite and hornblende. The rock of Kogon
Dutsi (1338) within the walls of Kano is a basic augite diorite without
quartz, consisting predominantly of acid labradorite and pale-coloured
augite. Both the augite and felspar have a tendency to schillerisation
and the augite is accompanied by, and in part intergrown with, green
hornblende, brown biotite and iron ores, while hypersthene is a scanty
accessory. An augite-diorite from eastern Kontagora (1302) contains a
little interstitial quartz and felspars varying from oligoclase to acid
labradorite. The ferro-magnesian minerals are much intergrown and
include pale-coloured augite, green hornblende and brown biotite.
3. _Porphyries, Porphyrites, Diabases and Basalts._
The later sills and dyke rocks are most abundant in the central
provinces and especially in Bauchi, eastern Zaria and northern
Nassarawa, but are somewhat sparingly distributed throughout the
remainder of the Protectorate. They include quartz porphyries,
orthoclase porphyries, mica and hornblende porphyrites, diabases and
basalts. In the central area the high content in soda which
characterises the more acid types undoubtedly indicates a genetic
connection between the later soda granites and the dyke rocks, although
investigation has not yet proceeded far enough to allow of the
recognition of the major foci to which the dyke rocks may be referred.
Amongst the quartz porphyries micro-granitic, granophyric and
felsophyric types are equally common. In the central provinces
riebeckite is frequent both among the phenocrysts and in the ground
mass, while elsewhere biotite is the commonest coloured constituent. An
ægirine granophyre occurs at Chikobo (1417) on the borders of Bauchi and
Zaria. Orthoclase porphyries are commonest in the central area and
possess typically a trachytic ground mass, of which riebeckite and
ægirine are frequent constituents. Augite syenite porphyries are known
from the Kudaru Hills in eastern Zaria (1397) and from the vicinity of
Limoro on the borders of Bauchi and Zaria (1413). The latter occurrence
contains hornblende and biotite in addition to the augite, as well as a
small amount of quartz in the ground mass. An extensive mass of
granophyric diorite porphyrite occurs in western Kontagora between Auna
and Bussa (1162). The felspars are turbid and much epidotised and
surrounded by a fringe of more acid material which passes outwards into
the granophyric matrix, while smaller irregular crystals of green augite
and ilmenite and abundant minute plates of green mica are also enclosed
within the pegmatitic material. A peculiar rock from Toro in Bauchi
province (958) may be termed a quartz diorite porphyrite, and presents a
certain affinity with the basic members of the charnockitic series of
the Ivory Coast.[61] The phenocrysts have a tendency to aggregate in
groups and include rounded quartz grains, tabular crystals of andesine
and labradorite with inclusions of granular augite, elongated crystals
of hypersthene and irregular plates of biotite much corroded by the
ground mass and enclosing grains of augite and apatite. The ground mass
is composed of granular green augite, felspar and quartz, with much
magnetite and apatite. The felspar of the ground mass is frequently
untwinned and more acid than the phenocrysts, while the quartz forms in
places micropoikilitic plates enclosing the other ingredients of the
ground mass. As alkali-syenite porphyries may be grouped the bluish-grey
trachytoid rocks of Kanikoko (1217–1219). The phenocrysts include
tabular plates of sanidine, microcline, microperthite and microcline-
microperthite and fairly well-formed crystals of diopside and enstatite,
much intergrown with brown mica and bluish amphibole. The ground mass
consists of microlites of alkali felspar and granules of the ferro-
magnesian minerals. Typical mica porphyrites with phenocrysts of biotite
and zoned andesines occur at Barazara (1274), and between Dan Gerumfa
and Gwashi (1276) in eastern Kontagora. A hornblende porphyrite between
Naraguta and Tilde (956) is inconspicuously porphyritic and composed of
allotriomorphic green hornblende with subordinate biotite and augite in
a matrix of acid plagioclase, orthoclase and quartz. A hornblende
porphyrite from Borgu (1220) contains large poikilitic crystals of
augite and hornblende in a granular matrix of labradorite, augite,
hypersthene, hornblende and iron ore. A somewhat similar rock from
Bauchi province (1602) contains large poikilitic hornblende crystals in
a matrix of granular augite, magnetite and lath-shaped crystals of
labradorite. A labradorite porphyrite from Toro in Bauchi province
contains large tabular phenocrysts of acid labradorite in a ground mass
of green augite, brown biotite, secondary green hornblende and lath-
shaped crystals of acid plagioclase. The diabases and basalts of the
central area are particularly numerous round the margin of the Bauchi
plateau and the Nassarawa tableland, and probably include intrusions of
many different ages. Some of the coarser-grained diabases present
affinities with gabbros in the idomorphism of their augite crystals (47,
1695). Others exhibit a typical ophitic structure (962, 985), while
others are predominantly granulitic (690). A hornblende diabase with
pale green augite, greenish-brown hornblende and deep red biotite occurs
in the Ruruma hills (1466) in eastern Zaria. Biotite also accompanies
the augite in an ophitic diabase from the Kudaru hills (1390) in north-
east Zaria. Uralite diabases are fairly common, but olivine diabases
(747) are conspicuously rare. The basalts present few features of
interest and are typically compact, non-porphyritic rocks with a
holocrystalline, ophitic or granulitic structure and free from olivine.
MM. Gentil[62] and Chudeau[63] have described the occurrence of
intrusive granites, pegmatites, porphyries, lamprophyres, diabases and
gabbros, throughout the crystalline areas of the Central Sahara, and Dr.
Passarge[64] has recorded similar intrusive rocks from the northern
gneissic region of Adamawa. Ægirine rhyolites and alkaline trachytes of
an intrusive character are known also from Zinder, Gabana, Hadj el
Hamis,[65] and M’Burao.[66] M. Chudeau believes that the alkaline
granites of Mounio with their associated hypabyssal types pierce the
sandstones of the Tegama, and would therefore refer all the known
occurrences of soda granite in Central Africa to a late Cretaceous or
Tertiary age. In Nigeria, however, there is little or no evidence to
support this view. The soda granites are nowhere found piercing
sedimentary rocks within the Protectorate, and while it is possible that
they may have been originally intruded into rocks of Cretaceous age
which have now been removed, it is remarkable that, with the exception
of rare sills of dolerite and the Tertiary pipes of basalt, no intrusive
dykes or masses of hypabyssal or plutonic material are to be found
within what remains of either Cretaceous or Tertiary rocks within the
Protectorate. Such intrusive rocks, however, are particularly numerous
within the crystalline areas, and especially in the neighbourhood of the
larger masses of soda granite, which are themselves frequently pierced
by marginal dykes of basalt and porphyry. In view of the abundance of
soda types among the dyke rocks, moreover, it is hard to escape the
conclusion that the later non-foliated alkaline granites of the second
type (p. 132) with the associated syenites, diorites and dyke rocks of
acid, intermediate and basic composition, all possess a certain genetic
relationship, and should be properly ascribed to one and the same period
of igneous activity. All the facts, indeed, are more readily explicable
on the assumption of the pre-Cretaceous age of this period of intrusion
within the Protectorate. The stratigraphy of Mounio and Koutous is
somewhat doubtful, and M. Chudeau’s section[67] of the faulted margin of
the Goure granite does not indicate an actual transition from the mica
schists and quartzites into the sandstones and clays of the Tegama. M.
Garde,[68] moreover, while indicating the occurrence of crystalline
gneisses to the north of Goure does not appear to confirm M. Chudeau’s
interpretation of this important section. It should be remembered,
however, that even although further investigation may prove the truth of
M. Chudeau’s correlation, there is no reason to believe that the
intrusion and effusion of igneous rocks rich in alkalis has been
confined, as M. Chudeau would suggest, to one particular period in the
history of the Sudan.
It is interesting to trace in a general way, and subject to the
limitations indicated above, the successive changes in the character of
the igneous intrusions from the earlier to the later granites and dyke
rocks on the hypothesis of a continuity in magmatic conditions, or at
least of a periodic renewal of activity in the same igneous focus. The
successive intrusions may be tabulated as follows:—
1. Earliest well-foliated granites of intermediate composition with
numerous associated amphibolites and epidiorites.
2. Partially foliated granites of both acid-alkaline and intermediate
character, with partially sheared dyke rocks of acid, intermediate and
basic composition.
3. Aplitic granites and pegmatites, rich in muscovite and tourmaline
with associated reefs and veins of quartz and tourmaline quartz.
4. Alkaline granites rich in soda with associated syenites, diorites,
and dyke rocks.
If the normal sequence of events within the period of activity of any
igneous focus be taken to be the successive manifestation of volcanic,
plutonic and hypabyssal activity, there may perhaps be recognised in the
successive intrusions indicated above four periods of igneous activity,
each of which is now represented by the products of the plutonic and
hypabyssal phases only, while the products of the accompanying volcanic
phases, if originally present, have been removed in the course of
denudation. The tendency to the production of rocks rich in alkalis
should be remarked as a notable feature of the various periods of
intrusion; and in this connection it is interesting to note that Messrs.
Horwood and Wade[69] have likewise emphasized the alkaline character of
many of the earlier granitic intrusions to the south of the equator.
[Footnote 28: Hubert, _Mission Scientifique au Dahomey_, 1908, p. 448.]
[Footnote 29: Chudeau, _Sahara Soudanais_, 1909, p. 18.]
[Footnote 30: The numbers in brackets refer to microscopic preparations
in the Imperial Institute Collection, NN. F.]
[Footnote 31: Parkinson, _Q. J. G. S._, Vol. LXIII, p. 315.]
[Footnote 32: _Mem. Geol. Sur. Scotland_, Expl. of Sheet 55, 1905, p.
13.]
[Footnote 33: Van Hise, _Treatise on Metamorphism_, 1904, p. 696.]
[Footnote 34: Parkinson, _Q. J. G. S._, Vol. LXIII, 1907, p. 308; Vol.
LXIV, 1908, p. 313.]
[Footnote 35: Chudeau, _Sah. Soud._, p. 2.]
[Footnote 36: Gentil in _Doc. Sc. Miss. Sah._, 1905, p. 721.]
[Footnote 37: Haug, _C. Rd. Ac. Sc._, 7 Aug., 1905.]
[Footnote 38: Haug in _Doc. Sc. Miss. Sah._, 1905, p. 753.]
[Footnote 39: Flamand, _C. Rd. Ac. Sc._, 140, p. 954.]
[Footnote 40: Flamand in Voinot, _Bull. Com. Afr. Fr._, 1908, p. 218.]
[Footnote 41: Hubert, _C. Rd. Ac. Sc._, 146, 3 Feb., 1908; _Miss. Sc. au
Dahomey_, 1908, p. 253.]
[Footnote 42: Chautard, _Le Fouta Djalon_, 1905.]
[Footnote 43: Parkinson, _Q. J. G. S._, 1908, LXIV, p. 313.]
[Footnote 44: Chevalier, _La G._, XVII, 1908, p. 201.]
[Footnote 45: Lenz, _G. M._, 1877, p. 27; Knox, _Geology of Africa_,
1905, p. 93.]
[Footnote 46: Von Ammon, _Geologie von Togo_, _M.H. Geog. Gessell.
Mün._, 1903, I, p. 393.]
[Footnote 47: Parkinson, _Q. J. G. S._, 1907, LXIII, p. 308.]
[Footnote 48: Courtet in Chevalier, _L’Afr. Cent. Fr._, 1908, p. 646.]
[Footnote 49: Passarge, _Adamaua_, 1895, p. 312; Esch, _Geol. von
Kameruns_, 1904, p. 23.]
[Footnote 50: Barratt, _Ann. des Mines_, VII, 1895.]
[Footnote 51: _Cf._ Horwood and Wade, _G. M._, V, 6, 1909, p. 455.]
[Footnote 52: “The Geology of the North-West Highlands of Scotland,”
_Mem. Geol. Sur._, 1907, pp. 75, 214, 595, 607.]
[Footnote 53: Voit, _Trans. Geol. Soc. S. A._, 1905, VIII, pp. 106,
141.]
[Footnote 54: _Cf._ Flett, “Geology of Land’s End District,” _Mem. Geol.
Sur. Eng._ 1907, p. 25.]
[Footnote 55: _Cf._ Gentil et Freydenberg, _Bull. Soc. Geol. Fr._, 4th
Ser., VIII, 1908, p. 44.]
[Footnote 56: Gentil, _C. Rd. Ac. Sc._, 8 Aug., 1904; Foureau and
Gentil, _C. Rd. Ac. Sc._, 2 Jan., 1905; Lacroix, _C. Rd. Ac. Sc._, 1
May, 1905; Gentil, _Doc. Sc. Miss. Sah._, 1905, p. 697.]
[Footnote 57: Chudeau, _Sah. Soud._, 1909, p. 265; _C. Rd. Ac. Sc._ 1
July, 1907.]
[Footnote 58: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 59: Hubert, _C. Rd. Ac. Sc._, 145, 1907, p. 764.]
[Footnote 60: Freydenberg, _Chad et Shari_, 1908, p. 107; Gentil et
Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII, 1908, p. 44.]
[Footnote 61: Lacroix, _C. Rd. Ac. Sc._, 150, 1910, p. 18.]
[Footnote 62: Gentil, in Foureau’s _Miss. Sahar._, 1905, p. 720.]
[Footnote 63: Chudeau, _Sah. Soud._, 1909, p. 256.]
[Footnote 64: Passarge, _Adamawa_, 1895, p. 382.]
[Footnote 65: Garde, _C. Rd. Ac. Sc._, 5 July, 1909 (149), p. 43; _cf._
Gentil and Freydenberg, _Bull. Soc. Geol. Fr._ (4), VIII, 1908, p. 44;
_cf._ also Suess, _The Face of the Earth_, Vol. IV, _Oxford Trans._,
1909, p. 283.]
[Footnote 66: Hubert, _C. Rd. Ac. Sc._, 1904 (139), p. 378.]
[Footnote 67: Chudeau, _Sah. Soud._, 1909, p. 267.]
[Footnote 68: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 69: Horwood and Wade, _Geol. Mag._ V, 6, 1909, p. 455.]
CHAPTER IV.
CRETACEOUS ROCKS.
Distribution: relation to crystalline rocks. The Cretaceous rocks of the
Benue valley: the succession around Awe; the position of the salt-
bearing beds; the Arofu and Akwana limestones and galena veins; the
limestone-shales and brine springs of eastern Muri; the Muri sandstones;
the Gateri grits; the limestones and galena veins of the Kudu valley.
The Cretaceous rocks of Yola and of the Gongola valley: the Bima
sandstones; the Gongila limestone; the rocks of Gulani and Wuyo; the
upper sandstones of Ture and Waja. Classification of the Cretaceous
rocks. The Cretaceous rocks of the French Sudan, Southern Nigeria and
the Kameruns.
No rocks of Devonian or Carboniferous age or of any period preceding the
Cretaceous[70] have been discovered within the Protectorate. The
Cretaceous rocks, moreover, are themselves represented only by the upper
members of the series, no deposits of earlier age than the Turonian
having been anywhere definitely recognised. The Upper Cretaceous rocks
are confined within the Protectorate to the Benue valley between long.
8°·30′ E. and long. 12°·15′ E., and to the valley of the Gongola between
its confluence with the Benue and lat. 11°·15′ N. They form the greater
part of the plains of Muri and Yola, and of the narrow plain of the
Gongola in Bauchi and Bornu, but only around Ligri and Gateri and in
Wurkum, Tangale and Waja do they rise into ranges of rounded or tabular
hills. The rocks consist of a well-bedded series of red and white
sandstones and grits, carbonaceous and calcareous shales, ironstones and
limestones, usually somewhat faulted and gently folded into low
anticlines and shallow synclines, and in places pierced by intrusive
masses of dolerite or basalt. The original thickness is unascertainable,
partly on account of the folding and faulting to which the rocks have
been subjected and partly on account of the thick covering of drift
which obscures the rocks for long distances and renders difficult or
impossible the correlation of the various exposures. In the west the
Cretaceous rocks are believed to pass unconformably under the sandstones
of the Lokoja series in the neighbourhood of Udeni, but the actual
junction has nowhere been observed. How far westward they extend under
the Lokoja sandstones is also undetermined. An exposure of white
sandstones and carbonaceous shales in the bed of a stream near Akwacha
in Bassa province seems to indicate that they extend for a considerable
distance west of Loko, but that they do not reach the Niger is
abundantly evident from the absence of any trace of them in the
neighbourhood of the river below Lokoja.
To the north and to the south of the plains of Muri, the Cretaceous
sandstones give place imperceptibly to a crystalline floor. The actual
contact, however, has been found wherever examined to be so much
obscured by drift that it is impossible to say whether any part of the
margin is bounded by faults or not. In the east the Cretaceous rocks
pass under the upper Benue sandstones and, so far as known, do not
reappear anywhere east of Yola. Similarly in the Kudu valley they
disappear under the sandstones of Duguri and in the Gongola valley under
the Gombe sandstones to the west and north. To the east of the Gongola
between Bilaraba and Dusua they rest upon a hummocky crystalline
surface, while farther south they pass behind Balbea under the Barbur
basalt plateau. To the west of Kwoia, however, their junction with the
crystalline rocks, although somewhat badly exposed, has every appearance
of being fractured, while on the lower Gongola the relations between the
Cretaceous and the crystalline rocks to the east are such as to suggest
most forcibly an extensive faulting along the contact. Within the area
covered by the Cretaceous rocks inliers of granite and gneiss are
exposed around Arofu on the middle Benue, around Tangalto and Chongwom
in southern Bauchi and in various places upon the narrow plain of the
Gongola and, while in some cases the crystalline rocks exposed upon the
plain of the Gongola have undoubtedly been brought to the surface by
faulting, it is at the same time fairly certain that the earlier
Cretaceous rocks were deposited upon a highly irregular crystalline
floor, the more elevated portions of which have been exposed at the
surface in the course of denudation. For the same reason also the
apparent base of the Cretaceous in the neighbourhood of these inliers of
crystalline rock cannot be regarded as the actual base of the series,
which is at most very imperfectly known.
_Muri Province._
In Muri province a series of limestones and shales definitely known to
be of Upper Cretaceous and Turonian age[71] is exposed in western Muri
around Awe to the north and around Arofu and Akwana to the south of the
Benue. In eastern Muri, the same series is found at Kumberi on the right
bank of the Benue a few miles above Amar. At Awe a sharp little
anticline runs E.N.E. along the north side of the town and brings up
coarse white felspathic grits and sandstones with two bands of
carbonaceous shales (Plate XV, Fig. 1). It begins to the north-east of
the town as a gentle arch with its axis dipping gently westward. The
intensity of the folding increases rapidly in that direction until the
anticline passes into a fault with the downthrow towards the north, but
farther west the fold reappears and speedily flattens out. The northern
limb of the fold is much more steeply inclined than the southern and the
beds are even in some places overturned. At the same time the sandstone
outcrop takes the form of a bow with the concave side to the north, the
whole structure being evidently produced by a local intensification of
the folding. (A. L.)
To the north and south of Awe, there are synclinal areas in which rocks
higher in the series have been preserved. From the various exposures in
the neighbourhood the following approximate sequence can be made out:
Soft white quartzose sandstones, often with calcareous cement.
Thinly-bedded light-coloured calcareous shales and sandstones with bands
of impure shelly limestone.
Greenish shales with ironstone bands and nodules.
White grits and sandstones and carbonaceous shales with brine springs.
Base not exposed.
(A. L.)
Grits and sandstones similar to the lowest beds seen at Awe, with brine
springs but without carbonaceous shales, come up to the surface again
along a line running from Akiri to Azara and Ribi, but north of that
again comes another synclinal area, for between Azara and the Ankwe
river exposures of shales and limestones are met with. Over the whole of
this part of Muri indeed the beds appear to be thrown into gentle
anticlines and synclines whose axes run approximately E.N.E. A lower and
essentially sandy division, often yielding brine, appears along the
cores of the anticlines as at Azara, Akiri and Awe, while an upper,
essentially shaly division, is preserved in the intervening synclinal
areas. (A. L.)
Limestones are known to occur in the volcanic tract on the Benue, south-
west of Awe, while at Tunga wharf, immediately south of Awe, a river
section exposes the lower sandstones striking E.N.E., and dipping
northwards at 27°. At Arofu a small inlier of granite to the east of the
town is seen to be five-sixths surrounded by silicified limestone with
the remainder of its perimeter concealed by marshy ground. An excavation
proved the limestone to rest directly on the granite and we are
therefore compelled to conclude that this granite forms a knoll
surrounded and at one time overlaid by the limestone. Whether similar
relations hold good between the other inliers of crystalline rocks in
the neighbourhood and the limestone is not certain. The outcrop of the
limestones is a broad one, about half a mile across in the vicinity of
Arofu and still more to the south of Akwana. The broad outcrop is
accounted for partly by the gentle dip, partly perhaps by gentle
undulations of the strata, but partly also by the thickness of the
limestone. What this is it is impossible to say in the present state of
our knowledge, but it is certainly much greater than that of the
limestone bands known in the neighbouring parts of the province. (A. L.)
The saltings of Arofu and Akwana bear witness to a salt-bearing series
underneath the limestones although that series itself is nowhere
exposed. Above the limestones comes a series of shales, shaly sandstones
and ironstones with a few thin intercalated limestone bands. This shaly
series outcrops between Arofu and the river and runs a little south of
west to beyond Akwana. The dip is about 10° and almost north. At Shaii
however, twelve miles east of Arofu, similar shales dip at 25° towards
N. 40° E., so that the folding is somewhat variable. Around Arofu the
limestones are seldom seen in an unaltered condition, for almost all the
rock exposed is silicified. Thus the country is crossed by innumerable
dyke-like masses of silicified limestone standing up several feet above
the soil. In the rising ground south of Akwana the silicified bands give
rise to several small cascades. Knots of galena are common in the
silicified limestone and in places as at Arofu there are veins of galena
and fluorspar. The original structures of the limestone which was partly
foraminiferal but very largely oolitic and pisolitic have been
excellently preserved during the change. At Arofu, however, a compact
unaltered variety contains imperfect remains of ammonites. It is
possible that these limestones which lie to the south of the Benue may
be on a somewhat different horizon from those of Awe, but the presence
of the underlying salt-bearing series indicates that the general
succession is the same. (A. L.)
At Kumberi some miles above Amar there occurs the following excellent
riverside section of the limestone-shale series, in which the
subordinate character of the limestone bands is very evident:
Occasional sandstone seen in the river bank:—
About 100 ft.—Fine-grained sandstone, mudstone and shales alternating
as below.
12 „ Sandstone.
About 50 „ Shales.
2 „ Good white limestone with numerous teeth, bones, and
phosphatic casts of lamellibranch and gasteropod shells.
Ammonites.
6 „ Shales.
1½ „ Shelly limestone.
8 „ Shales.
2 „ Friable sandstone.
10 „ Shales with mudstone bands.
15 „ Blue-gray shales.
20 „ Shales with mudstone bands.
2 „ Mudstone with large calcareous concretions round
ammonite shells.
5 „ Shaly sandstone.
10 „ Friable sandstone.
2 „ Sandy shales.
10 „ Soft, friable sandstones with shaly partings.
About 200 „ Fine-grained and soft sandstones, mudstones, and shales
alternating as above.
Occasional sandstones in river bank.
Total: 450 to 500 ft. Limestone : 3½ ft.
Strike uniformly N. 50° E.; dip, 30°–35° S.E.
(A. L.)
Farther east again scattered fragments of limestone indicate the
presence of the limestone-shale series in a synclinal between Bomanda
and Muri. An anticlinal axis, running a few hundred yards south of
Bomanda, brings up a lower series of soft thinly-bedded calcareous grits
with partings of green calcareous marl along the crest of the arch.
Brine springs like those of Awe rise through the northern limb. At Muri
the lower reddish grits are seen in full force and dip southwards at
about 60°. A careful estimate of the thickness exposed worked out at
over 1,500 ft. though neither the base nor the upper limit was seen. To
the north fragments of limestone are found in the swamps of the Kudu
valley to the west of Jebjeb and Gateri but the associated rocks have
nowhere been observed. (A. L.)
The white grits and sandstones and carbonaceous shales of Awe and the
calcareous grits and marls of Bomanda represent the passage beds between
the upper limestone-shale series and the lower sandstones exposed at
Akiri, Ribi and Tunga wharf in the west and at Muri town in the east of
the province. The weak brine which rises in springs through the passage
beds at Awe and Bomanda and even through the limestones at Akwana
appears to take its origin in the upper part of the lower sandstone
series. Brine springs and surface incrustations are found also where the
lower sandstones outcrop at Jebjeb, Jebero, Jenoe, and Langa in eastern
Muri, at Akiri, Azara, Ribi, Kanje and Abune in western Muri, and at
Doya and Keana on the borders of Nassarawa. These localities occur
immediately round the margins of the more marked synclinal areas
containing the remains of the limestone-shale series, a fact which
warrants the assumption that the upper part of the sandstones underlying
the limestones is impregnated with salt and was originally accumulated
under arid conditions. Indications of the salt-bearing strata are also
found at Lafin Gisheri between Tunga and Ibi and at a place of the same
name between Shishi and Wurio to the east of Ibi. In these localities we
are justified in assuming that the rocks immediately below the surface
belong also to the upper part of the lower sandstone series.
To the north of the salt area in western Muri well bedded sandstones are
exposed in many places between Lafia in Nassarawa province, and Wase and
Bashar to the north of Ibi. The prevailing type of rock is a reddish
grit or sandstone, frequently brick red, with knots and spots of white
felspathic material, everywhere unfossiliferous, and nowhere yielding
any indication of salt impregnations. At Bashar thin beds of white and
red clay are interstratified with the sandstones. The rocks are usually
inclined at a low angle to the horizontal and appear to be thrown into a
series of gentle undulations. From their position with respect to the
salt-bearing beds and the limestone-shale series, it may be assumed,
although continuous exposures are wanting, that these rocks lie
conformably at or near the base of the sedimentary succession in western
Muri. Their thickness is unknown, but if, as seems probable, they are to
be correlated with the sandstones of Muri town which underlie the
limestones in the eastern part of the province, they must be, according
to Mr. Longbottom’s determination (p. 151), at least 1,500 ft. thick,
and they may well be more. The whole series underlying the passage beds
of Awe and Bomanda and impregnated with salt in the upper part may
conveniently be termed the “Muri sandstones.”
PLATE XI
[Illustration: HILLOCKS OF DRIFT ON THE BAUCHI PLATEAU NEAR VOM.]
[Illustration: HILLOCKS OF DRIFT TO THE WEST OF NARAGUTA.]
Upon the drift-covered plains of southern Muri are found occasional
detached exposures of sandstones and grits, the majority of which
probably belong to the lower sandstone series. The presence of the “Muri
sandstones” underneath the limestones of Arofu has not been demonstrated
but, as already stated, the occurrence of brine springs at Akwana
affords a considerable presumption in favour of their occurrence.
Whether they outcrop in the unexplored Munchi country to the south and
south-west of Akwana is, however, quite unknown.
In the north-eastern part of the province the “Muri sandstones” become
more steeply folded, and in the Jenoe, Ligri, and Gateri hills appear to
be underlaid by a series of very coarse-grained white felspathic grits
or arkoses. The axes of the folds strike in a general way a few degrees
north of east, and the folding appears to be most intense in the
immediate neighbourhood of Gateri where the grits are much hardened, and
in places set on end. On the western slopes of the Gateri hills are
white and red grits and sandstones, the representatives of the “Muri
sandstones,” dipping generally westward and apparently underlying the
fragmentary limestones of the Kudu valley. In the Gateri hills,
moreover, there are indications, as at Awe, that the axes of the folds
are not horizontal, but tilted gently to the west, and the distribution
of the upper limestones in Muri province in elliptical areas or
irregular centroclinals surrounded by the lower sandstones clearly
suggests a later transverse corrugation of the whole series of gently
folded rocks. It is probable, therefore, that the limestones of the Kudu
valley, although originally occupying a much higher place in the series
than the Gateri grits, have reached their present relatively low
position, not so much as the result of faulting, but rather as the
result of the irregular folding to which the whole series has been
subjected. At the same time the occurrence of galena veins in the swamps
to the west of Jebjeb indicates that the rocks of the Kudu valley, like
those of Awe and Arofu, have undergone a certain amount of shattering
and fissuring contemporaneously with the folding. Amongst the fissure-
veins may also be included the broad dykes of limonitic ironstone which
form conspicuous hillocks around Akiri in western Muri.
_Yola Province._ (A. L.)
The limestone-shale series runs up the plain of the Benue from Lau
almost to Yola, and may even be found eventually farther east than Yola.
This series occupies the lower ground and nowhere forms hills. In
addition to it there are sandstones of various kinds, sometimes
occupying the plain and sometimes building up hills above the plain. The
sandstones of the hills are approximately horizontal and rest upon the
lime-stone-shale series. They will be referred to as the “Upper Benue
sandstones.” The question of their conformability or otherwise to the
limestone-shales will be discussed later (p. 188).
The limestone-shale series, being soft and easily denuded, is very
poorly exposed, and is generally hidden by the river alluvium.
Calcareous sandstone and greenish yellow shale are seen near Lau, and
south-west of Kunini large blocks and slabs of shelly limestone a few
inches thick lie scattered about over the silt. An excavation at Kunini
proved that the limestone is in thin bands, 12 or 18 inches thick,
intercalated between shales. The series is thrown into small anticlines
and synclines. Of one small anticline noticed the strike of the axis was
N. 10° E., one limb being inclined at 20°, the other at 60°. The shales
are seldom seen but they give rise to a characteristic soil which is
easily recognised and which leads to the discovery of limestone lumps
and fragments on further search. Low-lying country of this character,
with numerous slabs and lumps of limestone lying about on the surface
stretches up the Benue to the vicinity of Namtari.
Similar areas of limestone-shales are found on the Gongola between Kiri
and Shillen and between Mada and Kombo. The latter area runs up the
plain of the Maio Hawal, but for what distance is unknown. In both areas
the limestone is shelly and very fossiliferous and there are no sections
showing the shales or giving the dips. The only natural section of this
series which was met with in Yola province was at Numan. Just to the
east of the village, white fine-grained sandstone crops out and is
followed by still finer sandstones and mudstones on which the village
itself is built. No limestone appears in the section and only a little
shale. The dip is 10° towards N. 55° W. The mudstone is usually reddish,
but the fresher parts are greenish and glauconitic-looking. Phosphatic
nodules are found in this material and also in lumps of shelly limestone
set up by the natives in Dong. A small inlier of Cretaceous limestone is
exposed also between Jibaru and Murki in eastern Yola to the north of
the Bagele hills.
The thick grits and sandstones which emerge from beneath the limestone-
shale series in Muri province are probably responsible for the bold
escarpment behind Wuduku, but in the absence of detailed examination it
is impossible to say if these infra-limestone grits were ever deposited
in Yola province, and if, being so deposited, they crop out between the
limestone-shale series and the crystallines or are faulted out. The
problem, a simple one in itself, is enormously complicated by the fact
that the “Upper Benue Sandstones” rest upon the limestone-shales and
conceal their boundary with the crystallines. Consequently, the relation
of the Cretaceous rocks to the crystallines is at present unknown.
The Cretaceous rocks floor the trench of the upper Benue valley, while
the plateau areas to the south and, east of the Gongola, to the north
also, are of crystallines. It is improbable from the characters of the
limestone-shale series that these beds were formed in a narrow inlet
running up into a land mass. On the contrary they are fine-grained
marine sediments which were probably deposited in open water. Their
present position, on the floor of the valley, at a much lower altitude
than the crystalline plateaux north and south must be explained as the
result of tectonic movements. The wrinkled character of the series is
consistent with this explanation, and in this connection sections in the
neighbourhood of Kombo are suggestive. There the broad channel of the
Gongola, usually half a mile wide, becomes suddenly constricted to less
than a hundred yards by a ridge of grits dipping at 50° towards S. 30°
E. These grits pass under limestone-shales to the south-east, and must
therefore be correlated with the Awe and Bomanda grits. When followed
eastward, however, for only a few miles, this prominent ridge of grits
disappears, and the limestone-shales are seen to approach to within at
least a few yards of the granite. This strongly suggests that the
limestone-shale belt running up the Maio Hawal is faulted down to abut
against the granite. In a similar way, it is believed that the Upper
Benue valley, to the east of its confluence with the Gongola, was
initiated as a tectonic valley. (A. L.)
_Bauchi and Bornu._
To the north as to the west of Gateri, a series of white sandstones
intervenes between the Gateri grits and the limestones which outcrop in
the swamps between Putu and Tangalto. These limestones are exposed in a
very fragmentary manner, but the presence of a narrow belt of the lower
white salt-bearing sandstones in the midst of the limestones half-way
between Putu and Tangalto indicates here as elsewhere the occurrence of
the usual succession and an irregular folding of the whole series.
Approaching Tangalto the Cretaceous rocks run out upon the crystallines
as white sandstones and quartzose conglomerates whose horizon probably
coincides with those of the Ture grits to the east. In the hilly country
to the east of Tangalto and on the plains to the north, the inclination
of the Cretaceous rocks is in part due to a gentle folding, and in part
to an extensive faulting which has in places on the Gongola plain
brought up even the base of the series and the underlying crystalline
rocks to the same level as the limestones. Around Ture (Plate XIII, Fig.
1) and Awok, the varied dip of the well-bedded sandstones indicates a
gentle curving into low domes and shallow basins, while a similar
structure is very evident at Gulani, to the east of the Gongola, where
the town is set within a circle of low hills which possess a well-marked
centroclinal dip. On the other hand, the Wadi, Shinga, Bima and Gunna
hills, are separated by faults, and owe their greater northerly
inclination to the effect of a larger fault which is well seen at Gunna
on the left bank of the Gongola, and on the track between Wadi and Wuyo,
where the basal sandstones of the Bima hills with the underlying
crystalline rocks are brought up to the surface of the plain. The
faulting, like the folding, appears to be in two well-marked directions
at right angles to each other and approximately N.N.W., and E.N.E., and
it is probable that there is here, as in the neighbourhood of Awe, some
intimate relation between the faults and folds, for whose complete
demonstration much more detailed investigation would be required.
If the Gunna fault be taken as showing the base of the Cretaceous, the
succession in the Gongola valley begins with a series of sandstones and
grits at least 1,000 ft. thick. The basal beds are green and purple
earthy grits, composed of the debris of crystalline rocks and containing
large rounded fragments of granite and pegmatite. These are followed by
alternating bands of red, white and purple grits, which pass into the
white and red sandstones and grits of the Bima and Wadi hills. The
marginal rocks, which overlie the crystallines between Bilaraba and
Dusua are white, red, and purple grits, much broken and current bedded
and probably belonging to the same horizon as the lower sandstones of
the Bima hills. These “Bima sandstones” are succeeded after an unknown
interval by the Cretaceous limestones which outcrop in many places upon
the plains to the east and to the west of the Gongola, and are
particularly frequent over the tract of country which lies between Dukul
and Deba Habe to the west, and Wuyo and Gulani to the east of the river.
In one locality, a mile to the south of the village of Gongila, between
Nafada and Ashaka, the limestone is clearly shown resting upon the lower
series of white and red sandstones and grits. As a rule, however, the
rocks associated with the limestone, are badly exposed and obscured by
drift, while the shales which presumably accompany the limestones have
nowhere been observed. At Gulani, the limestones pass conformably under
a series of coarse white and red grits and white, yellow and brick red
sandstones with intercalated bands of clay and flaggy ironstones. With
these upper rocks the red and white sandstones, clays, and ironstones of
Wuyo present much lithological resemblance and probably belong to the
same horizon. Similarly at Reme in the extreme south-east of Bauchi
province the limestones are overlaid by a well-stratified succession of
white and red sandstones, grits, and flaggy ironstones, which are at
least 800 ft. thick, and form the escarpments of the flat-topped hills
of Waja. The whole series becomes markedly ferruginous towards the
summit, and while the contact with the limestones below is not actually
exposed, there is no reason to believe that it is other than
conformable. These rocks in Waja represent the highest known members of
the Cretaceous series, and it is they which are continued westward by
Tula, Awok, and Ture, and run out upon the crystallines to the east of
Chongwom. Around Awok and Ture these upper sandstones are distinctly and
variously inclined, but in Tula and Waja the dips are so gentle as to be
almost imperceptible. The white grits and sandstones are usually
unfossiliferous, but in the neighbourhood of Ture and the Peak (Plate
XIII, Fig. 2), they contain much silicified wood from the presence of
which a subsidiary argument may perhaps be drawn in favour of their
Cretaceous age. The Gongola valley to the east and north-east of Waja
retains the faulted character which it exhibits to the north, while the
country which lies between the escarpments of the Waja hills on the
north, and the Yam Yam hills fronting the Benue on the south, is quite
unknown and may contain representatives of both granitic and sedimentary
rocks.
From the various exposures described, the Cretaceous series within the
Protectorate may be tabulated as follows:
----------+-----------------------------+---------------------------
| Muri and Yola. | Bauchi and Bornu.
----------+-----------------------------+---------------------------
Upper | |Sandstones, grits, clays,
grits and | |and ironstones of Waja,
sandstones| |Ture, Gulani and Wuyo.
| |
Limestones| Turonian limestones and |Turonian limestones of the
| shales of the Benue valley |Gongola valley.
| |
|{ Passage beds (white |
|{ sandstone and carbonaceous |
|{ shales at Awe: calcareous |
Lower |{ grits and green calcareous |
grits and |{ marls at Bomanda). |
sandstones|{ |
|{ Muri sandstones (upper part|Bima sandstones and grits.
|{ salt-bearing). |
|{ |
|{ Gateri grits. |
----------+-----------------------------+---------------------------
No evidence of contemporaneous volcanic activity has been found within
the Cretaceous rocks of the Protectorate. Intrusive sills of dolerite,
however, have been observed within the Cretaceous sandstones in the
neighbourhood of Awe, Bashar and Serakin Kudu in Muri province, but
they, together with the numerous pipes and necks of basalt and trachyte
which pierce the Cretaceous rocks in the valleys of the Benue and
Gongola, are probably to be referred to some phase of Tertiary volcanic
activity (see Chapter VIII).
In the French Sudan limestones of Turonian age have been described from
the Damergou[72] to the north of Zinder, from Bilma to the north of Lake
Chad,[73] and from Tabankort, Tabrichet and Mabrouka[74] to the south
and west of the Adr’ar’ des Ifor’as. The occurrence in the Damergou of
gypsiferous clays in association with Cretaceous limestones has not been
paralleled in Northern Nigeria. On the other hand the presence of a
salt-bearing series within the Cretaceous has not hitherto been
recognised in the Sahara. Any correlation of the lower grits and
sandstones of Northern Nigeria with the Lower Cretaceous of the Tegama,
as established by M. Chudeau,[75] would, however, be premature, in view
of the absence of determinative fossils in both and the marked
lithological differences between the two. It seems better for the
present to avoid any definite subdivision other than that indicated
above and to refer the whole series of grits, sandstones, clays,
ironstones and limestones provisionally to the Upper Cretaceous. To the
south of the Protectorate rocks of a similar age have been described by
Mr. Parkinson[76] and Dr. Esch[77] from Southern Nigeria and the
Kameruns respectively.
[Footnote 70: See Chudeau, _Sah. Soud._, 1909, pp. 47, 11, for pre-
Cretaceous rocks of the Sahara.]
[Footnote 71: See Appendix II, _Palæontology_, p. 273.]
[Footnote 72: De Lapparent, _C. Rd. Ac. Sc._, 135, 1903, p. 1298;
Chudeau, _Sah. Soud._, 1909, p. 89.]
[Footnote 73: De Lapparent, _C. Rd. Ac. Sc._, 132, 1901, p. 388.]
[Footnote 74: Chudeau, _Sah. Soud._, 1909, p. 91.]
[Footnote 75: _Ibid._, p. 76.]
[Footnote 76: Parkinson, “The Geology of the Oban Hills,” _Q. J. G. S._,
63, 1907, p. 314.]
[Footnote 77: Esch, _Geologie von Kamerun_, 1904, p. 4.]
CHAPTER V
EOCENE ROCKS
The Sokoto and Lokoja series: the nummulitic limestone of Sokoto; the
upper ferruginous beds; the pisolitic clay of Giro; the white and pink
clays of Kurukuru; the partial colouration of the lower sandstones and
clays; the Lokoja sandstones and ironstones; the pisolitic ironstones of
Mount Patti; outliers of the Lokoja series. The Duguri sandstones and
grits: the clays of Pali, Kirifi and Deu; the Gombe sandstones; the
ironstones of the Tagum hills; the grits and ironstones of Kerri Kerri;
the age of the Duguri, Gombe and Kerri Kerri sandstones. The Upper Benue
sandstones: their relation to the Cretaceous and crystalline rocks of
Yola; age of the Upper Benue sandstones; outliers of Eocene sandstones
in Muri province; Tertiary history of the Upper Benue valley. The Eocene
rocks of the Upper Niger and the Kameruns. The stratigraphy of the Chad
area.
The dawn of the Eocene was marked by regional upheaval, by the folding
and fracturing of the Cretaceous rocks and by subsequent intense
erosion, as the result of which the underlying crystalline floor was
again laid bare over the greater part of the Protectorate. A period of
depression, however, ensued, during which the sandstones, clays,
limestones and ironstones of the middle Eocene were extensively and
unconformably accumulated both upon the crystalline rocks and upon the
worn and denuded edges of the Cretaceous. The existence of this hitherto
unsuspected unconformity between the Eocene and the Cretaceous
introduces a new element of complication into the history of the Sudan
and the southern Sahara.[78] In the opinion of M. Chudeau[79] deposition
was continuous at least from the lower Turonian to the close of the
Eocene, while the early Eocene was marked by a widespread transgression
of the waters over the Niger valley. The presence of the unconformity,
however, makes it evident that the known distribution of the Cretaceous
cannot in any way be taken to indicate the former limits of the
Cretaceous sea.
The Eocene rocks within the Protectorate may be conveniently classified
into three groups:
1. The Sokoto and Lokoja series.
2. The Duguri, Gombe and Kerri Kerri sandstones.
3. The Upper Benue sandstones.
1. _The Sokoto and Lokoja series._
The occurrence of fossiliferous limestones of Eocene age at Kalfu,
Tamaske, Garadimi and Toronkawa to the north of Sokoto has been known
since 1903.[80] The limestones have been found, however, to extend much
farther south and to be associated with sandstones, clays and ironstones
which are continuous with those in the Niger valley from Ilo to Lokoja.
One of the finest sections of the limestone series is found in the
cliffs facing the river beyond Wurnu to the north-east of Sokoto where
the following succession may be observed:
10 ft.—Dark red ferruginous grit on top.
10 „ Ferruginous oolite.
30 „ Soft, chalky, fossiliferous limestone with phosphatic nodules
and rounded lumps of hard and somewhat phosphatised limestone.
10 „ Calcareous clay.
80 „ Blue shale, much obscured by landslips.
20 „ Compact yellow calcareous clay, with efflorescence of gypsum.
40 „ Red, yellow, green and black gypseous shales and clays.
50 „ Black carbonaceous shales with radiating crystals of gypsum
alternating in thin bands with yellow ferruginous clays with
knots of ironstone; efflorescences of alum and gypsum.
At Wurnu the same succession is represented by red sandstones above and
white, yellow and blue clays and shales below, with fragments of
limestone at the foot of the slope indicating a thin band of limestone
intercalated probably near the summit of the shales. Around Sokoto there
is much earthy ferruginous oolite on top passing downwards as a rule
into well-bedded red sandstones. Below the Wurnu gate, however, there is
an exposure of about thirty feet of blue and white calcareous shales and
clays with thin bands of hard limestone 3–4 inches thick, numerous
fossils and occasional fragments of gypsum. At Alkamu, near Sokoto, a
chalky limestone with nodules and echinoids, associated with calcareous
shales and overlaid by earthy oolite, encloses rounded lumps of harder
nummulitic and crinoidal limestone which look as if they had been
originally washed into place. This appears to be the first record of the
occurrence of nummulitic limestone in West Africa. A similar hard
nummulitic limestone is found as far south as the neighbourhood of Jega
and more detailed investigation would probably reveal its presence in
many localities between Sokoto and Jega. The limestones, however,
nowhere form thick and massive beds, but appear to occur always in thin
bands or as nodules in calcareous shales or clays. The occurrence of
gypseous shales below the limestones at Wurnu and Sokoto indicates the
earlier presence of arid conditions, but whether or not the various
exposures of limestone occur on one and the same horizon has not been
determined.
Neither limestones nor calcareous shales are known from the immediate
neighbourhood of the Gulbin Kebbi below Sokoto.[81] There are, however,
numerous indications in the cliffs which bound the valley of a lower
lighter-coloured earthy series overlaid by a succession of gritty and
ferruginous beds. At Tozo white clays and earthy sandstones with thin
bands of limonite are covered by oolitic ironstone which is overlaid in
turn by hard red sandstone and pebbly ferruginous grit between Tozo and
Kantami. The escarpments of Gulma are composed of soft white earthy grit
below, becoming redder and more sandy towards the top and capped by a
layer of hard ferruginous sandstone. At Birnin Kebbi the lower massive
white and yellow earthy sandstones become mottled red and pass into dark
red ferruginous sandstones on top. At Giro the lowest rock exposed is a
yellowish pebbly grit with intersecting silicified ribs which passes in
places into a friable white pebbly marl. This is followed by a thick
(100 ft.) bed of white clay, frequently stained red along the joints and
irregular cracks, almost every specimen of which shows a pisolitic or
concretionary structure. Traced southward this bed becomes much
silicified in its upper part, with retention of the original structure,
and all stages may be traced between the normal pisolitic clay and a
pure silicified and somewhat opalescent pisolite. Northward and eastward
red and yellow sandy beds and friable pebbly grits become intercalated
in the clay, while fragments of a fine-grained and very ferruginous
sandstone afford indications of the former presence of ferruginous beds
on top. Fragments of a similar silicified clay are found as far south as
Marake where the prevailing rock is a brick red earthy sandstone, in
places spotted with felspathic knots. At Kurukuru, near Ilo on the
Niger, soft white and pink clays are again prominently exposed under a
cap of ferruginous oolite. A pisolitic clay very similar to that of Giro
has been described by M. Hubert[82] from Gaya on the Niger.
The sedimentary rocks run out upon the crystallines as friable pebbly
yellow sandstones at Shunga to the east of the Niger, as a hard yellow
silicified sandstone at Bakinrua in Borgu to the west of the Niger, and
farther south as ferruginous sandstones and conglomerates at Konkwesso
and Kabe, and soft white earthy sandstones on the Niger to the west of
Warra. On the plateaux throughout Sokoto province the prevailing rock
belongs to the upper ferruginous series of hard red sandstones, grits
and ironstones, the softer and lighter-coloured rocks below being
exposed for the most part only in the cliffs which bound the river
valleys. The occurrence of outliers of still higher beds upon the
surface of the plateaux indicates that the actual summit of the series
has probably nowhere been observed.
There can be little doubt that the partial colouration of the lower
beds, and to some extent also the hardening of the upper sandstones, is
due to some considerable extent to the percolation of iron-bearing
solutions from above downwards. There is equally little doubt that the
oolitic ironstones are of primary and not of secondary or metasomatic
origin and that the deposition of the ferruginous material took place
during the latest stage in the history of Eocene sedimentation. All
gradations indeed can be traced between a pure oolitic ironstone and a
ferruginous sandstone with scattered oolitic grains of limonite. M.
Chudeau[83] insists that even where enclosed between thick beds of clay,
the oolitic ironstones are in every case the product of the
decalcification of limestone. He has himself, however, supplied a
sufficient answer to his own argument in calling attention to the fact
that the limestones where now exposed never show any trace of even a
superficial alteration into ironstones and in admitting, as he does, the
truth of M. Hubert’s[84] contention that the oolitic ironstones of the
Niger valley are nowhere accompanied by or traceable into limestones of
a similar character. The flints of Ansongo,[85] moreover, like the
nodules of Sokoto, indicate nothing more than the local occurrence of
limestone within the Eocene accumulations.
PLATE XII
[Illustration: A GRANITE MOUNT NEAR BAUCHI.]
[Illustration: A GRANITE MOUNT NEAR BAUCHI.]
The sandstones of southern Kontagora and of the Niger valley below
Jebba, which may be conveniently grouped as the “Lokoja series” from the
locality of their greatest development, represent the southward
continuation of the Sokoto sedimentary series and more particularly of
the sandstones, grits, conglomerates and ironstones of the southern
portion of the series, lying below the confluence of the Gulbin Kebbi
and the Niger. The break in continuity at Warra is more apparent than
real and is due merely to the fact that the Niger has entirely removed
the sedimentary rocks from its channel and thus exposed the underlying
crystallines. A well-marked difference in character may be traced
between the Eocene rocks of Sokoto province and their southern
representatives, a difference, however, which is entirely due to a
dissimilarity in the earlier conditions of sedimentation. In the Sokoto
series there is a well-marked succession of marine and deeper water
accumulations by shallow water and lacustrine accumulations. Amongst the
southern sandstones, however, there is no trace of a marine series, no
trace of limestones or of carbonaceous or calcareous shales and even
clays are conspicuous by the rarity of their occurrence. The series
throughout, which attains at Lokoja a thickness of 1,000 feet, is a
shallow water accumulation of sandstones, grits, conglomerates and
ironstones, comparable to some extent with the upper shallow water
portion alone of the Sokoto series. The resemblance indeed between the
uppermost members of both series is heightened by the prevalence in both
of oolitic and pisolitic ironstone, and it seems probable that, in spite
of their earlier dissimilarity, the conditions towards the close of
sedimentation in Eocene times were identical over the whole area from
Sokoto to Lokoja. A further resemblance between the Lokoja series and
the rocks of the lower valley of the Gulbin Kebbi is the frequent
staining of the lower members of the series by descending ferruginous
solutions. Wherever the lowest rocks of the Lokoja series are exposed
they are almost invariably light-coloured sandstones and conglomerates.
The colourisation begins along the joints and fissures and spreads
inward as an irregular pink or reddish mottling. Traced upward the rocks
gradually become uniformly coloured and the tints gradually deepen until
the rocks assume the characteristic dull dark red or chocolate colour of
the uppermost members of the series. The staining in places, owing
probably to local differences in texture, has reached a greater depth
than in others with the result that at certain points within the series
the lower lighter-coloured rocks appear to attain a less development
than in others. There is, however, to be observed in places towards the
upper portion of the series an alternation of more and less ferruginous
beds underneath the richly ferruginous cap of chocolate-coloured
sandstones and ironstones, while here and there intercalations of
ferruginous beds are to be found even within the lower lighter-coloured
members of the series. In northern Borgu, moreover, it is common to find
the basal conglomeratic beds richly ferruginous, a fact which is
probably to be explained as due to the gradual extension of the marginal
swamps during the advent of shallow water conditions.
The outliers of the Lokoja series which lie to the north and north-east
of Kontagora belong to the uppermost portion of the series and consist
for the most part of oolitic and pisolitic ironstones. Immediately south
of Kontagora, soft white sandstones and earthy grits are exposed in the
gorge of a stream at Kagara. Again at Sonko in a narrow canyon valley a
fine-grained hard ferruginous sandstone is intercalated between beds of
yellow gritty clay. Between Sonko and Jebba, there is a great
development of ferruginous sandstones, ironstones and grits which extend
also over a large portion of western Nupe. In western Kontagora, yellow
grits and sandstones occur round Auna, while at Shaffini yellow and
reddish pebbly sandstones form the base of the hills of ferruginous grit
which rise above the plain. At Kumboji the rocks are red pebbly
sandstones and ferruginous grits, while at Chika a lower bed of yellow
sandstone is succeeded by red grit and ferruginous conglomerate.
On the Niger below Jebba, ferruginous sandstones and gritty ironstones
are exposed at Rabba on the left bank, and red sandstones and pebbly
grits at Shonga on the right. At Share white pebbly sandstones and
conglomerates with thin bands of clay occur below the ferruginous grits
which form the summit of the tabular hills. At Padda, to the east of
Pategi, the basal beds are white conglomerates and sandstones, knotted
with fragments of felspar and passing upward into the alternating series
of white and reddish sandstones which rise into the pinnacle peaks of
Egbom. On the banks of the Kaduna above Wuyo, white earthy sandstones
are frequently exposed passing upward in the isolated hills through
alternations of white and red sandstones into hard ferruginous grit and
ironstone. Throughout southern and western Nupe, the rocks, where
exposed, are invariably red sandstones or ferruginous grits. Between
Kacha and Baro, however, the lower beds are white or mottled red and
pass upward into a succession of red and white sandstones and grits. A
similar alternation occurs in the base of the outlying Jakpara hills in
northern Kabba which are capped by red ferruginous grit.
From Baro southward along the Niger and eastward into Nassarawa and
Bassa, oolitic ironstones and earthy limonites associated with
ferruginous sandstones and grits attain a greater development while the
colouration of the lower beds is in general much more complete. At Baro
a white or reddish sandy clay with pebbles of quartz and kaolinised
felspar is exposed at the base of Mt. Gidda. This is followed by a thick
mass of oolitic ironstone covering the greater part of the slope, in
places much decayed and ribbed, banded and knotted with concretionary
limonite. The upper portion of the hill is composed of red sandstone and
ferruginous grit, the uppermost beds being well banded with coarser
material, and enclosing pebbles of quartz and felspar. At Lokoja the
lowest rock exposed at the base of Mt. Patti is a soft white felspathic
sandstone with pebbles of quartz, kaolinised felspar and pegmatite which
occasionally accumulate into conglomeratic bands. The greater part of
the hill, however, is composed of white sandstones and grits
indistinctly bedded and stained irregularly, and in places to a
considerable depth, with iron ore. In the upper part of the hill the
white sandstones give place to ferruginous sandstones with concretionary
knots of earthy limonite and intercalated bands and irregular lenticular
patches of oolitic ironstone. The uppermost 150 feet are made up of
oolitic, pisolitic and earthy ironstones enclosing irregular masses of
hard lustrous concretionary limonite, while the summit is capped by a
pebbly ironstone composed of small rounded and elliptical pebbles of
compact ironstone cemented by earthy limonite containing a few grains of
quartz.[86]
The prolongation of the Lokoja series into Kabba, Nassarawa and Bassa is
accompanied by much variation in the extent and thickness of the lower
lighter-coloured rocks, and in the proportions of ferruginous sandstone
and oolitic ironstone in the upper part of the series. There are also
indications to the north and east of Lokoja of the ironstones occupying
in places a much lower horizon than they do in Mt. Patti, as for example
in Mt. Gidda where the ironstones are found only a little way above the
river level and at Amara on the Benue where a ferruginous conglomerate
very similar to that on the summit of Mt. Patti is exposed in the bank
of the river. On the Benue, the Lokoja series runs out unconformably and
presumably upon Cretaceous rocks in the Doma hills behind Udeni which
present the usual succession of white sandstones below and red
sandstones above. In Bassa the Lokoja series, although not actually
traced, is believed to be continuous with the white and red sandstones
and clays with plant remains exposed in the neighbourhood of Idah and
Igore on the borders of Southern Nigeria. A small outlier of red and
white sandstones and earthy limonite, near Ojerami and Akwi in southern
Kabba, bears witness moreover to the former extent of the Lokoja series
to the west of the Niger.
(2). _The Duguri, Gombe, and Kerri Kerri sandstones._
The post-Cretaceous rocks of eastern Bauchi and Bornu, while more or
less continuous from south to north, may be conveniently divided into
three groups, the Duguri sandstones and grits, the Gombe grits and
clays, and the Kerri Kerri grits and ironstones. The series throughout
is a well-bedded succession of grits, sandstones, clays and ironstones,
usually lighter-coloured below, and dark red and ferruginous above.
Although everywhere unfossiliferous the series presents many analogies
with the Eocene rocks of the Niger valley, and it is largely upon the
strength of those analogies that these eastern rocks are assumed to be
also of Eocene age. Their stratification throughout is practically
horizontal, though in a few localities a gentle dip may be observed.
Their junction with the underlying Cretaceous is believed to be
unconformable, but the actual contact has nowhere been observed. Their
total thickness, from the level of the Gongola at Nafada to the summit
of the Kerri Kerri plateau, is about 1,000 ft., and there is no reason
to believe that they elsewhere exceed this figure to any great extent.
In Duguri, the basal beds are not exposed, but in the cliffs behind
Tongolan there is an excellent section of 400 feet of white and purple
grits capped by hard red ferruginous sandstone, and banked against the
crystalline rocks to the west. Between Tongolan and Yuli there is an
alternation of white and red sandstones with occasional beds of gritty
clay, while at Samia a series of white sandstones and clays overlaid by
red sandstones and grits is exposed in the cliffs which bound the valley
of the Yuli river. The boundary between the Cretaceous and Eocene is
very approximate, and has been drawn provisionally a little east of
Samia, where the horizontal sandstones appear to give place to
sandstones possessing a distinct inclination to the west and probably
belonging to the inclined Cretaceous series. Southward, the white grits
of Duguri appear to be largely replaced by red ferruginous sandstones,
which are well exposed around Tukkur to the east of Kantana. On the
plateau at Kantana, a thin series of red sandstones with bands and
streaks of flaggy limonite, underlaid by white sandstones, pebbly grits,
and conglomerates, rests directly upon the crystalline rocks, while the
presence of outliers of hard red sandstone on the summits of the hills
around Kanna affords some indication of the originally greater westward
extension of the series. The correlation of the various rocks in Duguri
and the Kanna district is rendered difficult both by the rapid lateral
variation in character of individual beds and by the occurrence of much
faulting along the eastern margin of the plateau. The distinction of a
lower and lighter-coloured series from an upper ferruginous series is
less marked in Duguri than elsewhere, the lower series being apparently
in great part an irregular succession of white and red sandstones and
grits.
Over the central plains of eastern Bauchi the prevailing rock is a soft
reddish grit in places quite pebbly, in others passing into white and
purple sandstones. Exposures are found mostly in the deep and narrow
channels which the streams have cut below the surface of the plain. At
Pali bands of white gritty clay in every respect similar to the clays of
Kurukuru on the Niger (p. 168) are intercalated amongst the sandstones,
some of them containing knots and lenticular patches of purer kaolin. At
Panguru rounded masses of earthy and oolitic ironstone are found in the
surface alluvium, but it is doubtful whether they are concretions in
situ or rolled fragments of an earlier deposit. A thin series of soft
white and red sandstones and earthy limonites capped by hard red grit
and possessing a dip of 10° S. and belonging probably to the upper part
of the series is exposed immediately south of Putu. At Kirifi and Deu on
the Gongola, thick beds of white clay similar to the clays of Pali are
exposed below harder beds of red sandstone and red ferruginous grit.
Similarly at Keffin Serakin Yaki, to the west of the Gongola, banks of
white clay occur as large lenticular patches in reddish grit. Between
Deu and Gombe white sandstones and clays are frequently exposed in the
lower part of the cliffs which bound the river valley, the upper part
being invariably red sandstone or hard ferruginous grit. At Gombe a
characteristic section is exposed in the hill behind the Residency. A
coarse white clay with gritty and sandy bands in its upper part and
somewhat stained with iron forms the lower slopes of the hill, while on
top are dark red sandstones and pebbly grits passing in places into
flaggy or oolitic ironstones. The Gombe grits and clays are continuous
within the bend of the Gongola with the grits and ironstones of the
escarpments behind Tilde and Dukul which bound the narrow plain of the
Gongola in the extreme east of Bauchi province. These eastern
escarpments, however, are much higher than the escarpments of the west
and the upper ferruginous series attains in them a much greater
development. At Dukul the thinly bedded soft white sandstones which
outcrop at the base of the escarpment are covered by a thick series of
hard red grits with bands of flaggy limonite. Around Tilde and Ribadu
the ferruginous grits alone are exposed, but at Birri to the north the
lower series is again exposed as thinly bedded white sandstones under
the usual ferruginous cap. To the east of the Gongola, the Gombe series
is continued in the low hills of ferruginous grit, conglomerate, and
ironstone around Ligdar and Buni, and farther east in the sandstones and
ironstones of the broken hills which stretch from Ndufa to Lua in
southern Bornu. Around Ndufa yellow and brick-red sandstones are exposed
below with ferruginous grits above, containing in places scattered knots
of hæmatite and streaks of lustrous pisolite. At Lua the Tagum hills are
entirely composed of ferruginous grit enclosing in the upper layers
large knots, lenticles, and irregular masses of hard pisolitic limonite
similar in every respect to the lustrous pisolite of Mt. Patti at
Lokoja. Immediately south of Koje the lower rocks run out upon the
crystallines in alternating bands of yellow sandstone, coarse grit, and
fine white clay somewhat stained with iron ore.
In the Kerri Kerri country in western Bornu to the north of the Gongola,
the post-Cretaceous rocks are magnificently exposed in the southern
escarpments of the Kerri Kerri plateau. The stepped cliffs of Kadi
consist of two thick and imperfectly stratified beds of soft white
earthy grit, passing in places into a coarser pebbly grit and separated
by a thin bed of red ferruginous sandstone with streaks of limonite
which forms the floor of the step. The summit of the upper bed is
reddened and capped by a thin layer of concretionary ironstone. The
detached tabular hills which fringe the plateau are either of the same
height as the plateau or cut down to the level of the intercalated bed
of hard red sandstone (Plate XIV, Fig. 1). In many cases the detached
hills are stepped like the escarpment of the plateau, and at times the
upper grits are represented merely by a conical peak set upon a platform
floored by the ferruginous band which separates the upper and lower
grits. Farther east the prominent tabular hill behind Fika has a very
similar structure. The base is formed of white and brick-red grits upon
which is set a thick bed of white earthy grit vertically jointed and
bounded by precipitous cliffs. A narrow ledge of hard red sandstone
separates this lower grit from an upper bed of similar character
somewhat stained with iron, whose gentler slopes are covered with broken
fragments of the red ferruginous grit which caps the summit. Beyond Fika
the Potiskum road leads as far as Boza through the trench-like
intersecting valleys so characteristic of the margin of the plateau with
the rocks well exposed in the vertical cliffs on either side (Plate XIV,
Fig. 2). Almost invariably soft reddish grits are found below and white
earthy grits with bands and streaks of white and pink clay above, the
whole being capped at Boza by a series of coarse red ferruginous grits.
The stratification which is indistinct in the lower part becomes more
marked towards the summit of the series and affords indications in
places of a very gentle inclination towards the north. The upper
ferruginous grits frequently rise into low hills and mounds on the
surface of the plateau between Farsawa and Potiskum and near Gojin they
contain abundant pisolitic knots of limonite, whose presence renders the
grits very similar in appearance to those of the Tagum hills in southern
Bornu.
To the west, the Kerri Kerri grits run out upon the crystalline rocks in
the neighbourhood of Hardawa, as hard white conglomerates and grits,
while to the north and east they are gradually lost under the recent
accumulations of blown sand and earthy drift. Between Potiskum and
Gujba, the surface rocks belong mostly to the upper ferruginous series,
the lower red and white grits being exposed only in the base of the
ironstone hills of Durua. The ironstones are very variable in character
and include richly ferruginous grits with streaks of limonite, gritty
pisolitic ironstones, pure lustrous pisolitic ores and flaggy and earthy
limonites, the latter containing in places much decayed and
unrecognisable plant remains. East of Abakri outcrops become rare and
the last trace of the Kerri Kerri rocks is found at Girboa in some
scanty exposures and broken fragments of ferruginous grit. In central
Bornu the only known occurrence of rock of any kind is the outcrop of
sandstone recorded by Barth in the banks of the Maidugari river.[87]
It should be remembered that the assumption of an Eocene age for the
Duguri, Gombe, and Kerri Kerri grits, clays, and ironstones rests
entirely upon analogy. They exhibit frequent resemblances to the Eocene
rocks of the Niger valley and more particularly to the Lokoja series and
to the southern portion of the Sokoto series. They are throughout
shallow water accumulations, for the most part distinctly bedded, still
practically horizontal and presenting, like the rocks of the Niger
valley, a succession of a lower lighter-coloured series by an upper and
more richly ferruginous series. The lithological resemblances are also
particularly striking. The earthy grits of Kerri Kerri can be exactly
paralleled at many points in the Niger valley and in the valley of the
Gulbin Kebbi, although in Kerri Kerri they attain a much more extensive
development. The clays of Pali, Kirifi and Deu are similar in every
respect to the clays of Kurukuru on the Niger, while the peculiar
pisolitic ironstones and lustrous limonites of Kerri Kerri and the Tagum
hills are exactly similar to those of Mt. Patti at Lokoja.
The assumption of an unconformable contact of the Duguri, Gombe, and
Kerri Kerri rocks with the underlying Cretaceous rests upon the
practically constant horizontality of the former and the practically
constant folding and faulting of the latter. Certain broad resemblances,
however, can be detected between the uppermost and least inclined
Cretaceous rocks of Ture and Waja and the sedimentary series of Duguri,
Gombe, and Kerri Kerri. Like the latter the rocks of Waja are of shallow
water origin and show a gradual transition from lighter-coloured
sandstones below to hard red ferruginous sandstones and ironstones
above. They are, however, as a rule more distinctly bedded and inclined
and much harder and more compact than the Gombe and Kerri Kerri series,
while in lithological character they differ in the absence of
intercalated clays in the lower part and in the absence in the upper
part of the oolitic and pisolitic ironstones so characteristic of the
ferruginous series in the Kerri Kerri and the Tagum Hills.
There is little doubt that in structural and lithological characters the
rocks of Duguri, Gombe, and Kerri Kerri present a greater resemblance to
the Eocene rocks of the Niger than to the Cretaceous rocks of Waja. The
unfortunate absence of palæontological evidence and of any decisive
contacts with known Cretaceous rocks is however to be greatly regretted.
On the whole it seems improbable that rocks so distinctly inclined and
so evidently undulating as the Cretaceous of the Gongola valley should
give place so rapidly in the west and north to such an extensive series
of horizontal sandstones and ironstones as is to be found in eastern
Bauchi and Bornu. The entire absence from the latter moreover of the
pipes and necks of basalt, so abundant in the Cretaceous of the Gongola
valley, is subsidiary evidence in favour of a difference of age. It
would seem indeed that, as in the case of the Lokoja sandstones and the
Cretaceous rocks of the Benue, all the facts are much more readily
explicable on the assumption of an unconformity between the Cretaceous
and the Eocene in Bauchi and Bornu.
The possibility of contemporaneous volcanic activity during the
deposition of the Eocene sandstones and ironstones is discussed in
Chapter VIII.
3. _The Upper Benue sandstones._ (A. L.)
Upon the Cretaceous rocks which occupy the Benue plain in Yola province
there rests a thick series of sandstones which will be referred to as
the “Upper Benue Sandstones.” The relation between these sandstones and
their basement is extremely interesting and yields valuable information
as to the history of this part of Nigeria.
The southern boundary of the sandstones is a very irregular one. Thus on
passing from Gidan Jauro to Bamga the path proceeds over sandstones for
a time, then passes on to granite for a short distance, then on to
sandstone (an arkose) again and finally on to the crystallines. North of
Bamga the sandstone partly wraps round a small hillock of granite. But
in the cliff-like sides of the Yendam Hills the evidence that the
sandstones have buried up an old and hilly land-surface is very clear.
Thus in Plate XVI a hill some hundred feet high is seen to be composite.
The bulk of the hill (off the plate to the left) is granite, but its
base is hidden by conglomerates, which pass upwards into pebbly
sandstones. The latter are banked up against the granite and overtop it,
thus forming a tabular hill. Many other sections in the district show a
similar relation between sandstones and crystallines. (Plate VI, Fig.
1.) There is here no question of a faulted margin, but evidence on all
sides that an older and hilly surface has been buried up in sandstones.
If now one stands amongst these hills, there is the Benue plain to the
north, floored by the Cretaceous limestone-shales, with the “Upper Benue
Sandstones” resting upon them; around are the composite hills with the
sandstone banked up against the granite, while to the south is the
crystalline plateau of southern Yola, with the highest of its hills
capped by sandstone. The accompanying section[88] is approximately
correct, though the maps will not permit of great accuracy.
Passing on towards Yola, there are crystallines at Bikola and again
beyond Jelima, but in the lower ground between the two are pebbly
sandstones and conglomerates running up from the plain into the
crystalline area as a tongue. Here the present stream course seems to
occupy the same position as an older drainage channel existing before
the deposition of the detrital rocks. The mass of the Gumba hills is
almost all of granite. The southern part, entirely of granite, rises to
1,886 ft. above sea-level, but the cone of the northern peak, 1,727 ft.
high, is of sandstone which, as in the case of the Yendam hills, must
have been banked against the granite at the time of deposition.
The observations made may be summarised by saying that if the “Upper
Benue Sandstones” were stripped off, so as to lay bare the underlying
basement, no great change in the physical geography would result. Some
detached hills and outliers would disappear, but the essential features
of the Benue plain with its Cretaceous rocks and the crystalline plateau
of southern Yola would still remain. The sandstones evidently
accumulated upon lower ground to the north, at first abutting against
the granite hills of the south but eventually rising above them. The
physical features were buried up and have been exhumed by later
denudation with great modification in detail but with the essential
characters persisting throughout.[89]
The sandstones are found to run up the Benue plain past Yola to Garua
and they also extend some little way north and north-west of Garua. The
main spread, however, appears to turn southward, and according to German
writers,[90] they are found between Djerim and Rei-Buba on the uppermost
Benue, and also on the hilly area to the west.
Bearing in mind this distribution of the sandstones and their field-
relations to the underlying basement, there is seen to be an intimate
connection between the sandstones and the river, and it appears fairly
certain that the sandstones were accumulated in the valley or valley-
plain of an “Older Benue.” The outliers of the Yendam Hills and the
Bagele Hills at Yola, which rise a thousand feet or so above the river,
bear witness to the former thickness of these accumulations, now so
largely removed.
The Upper Benue Sandstones cannot yet be subdivided even locally. No
clay or shale bands have been found which would serve as horizons. The
material does not vary much. The bulk is very felspathic and has
evidently been formed out of granitic debris. At Bamga it is not always
easy to distinguish between the arkoses of the sandstone series and the
compacted granite sand at the base of the hills. The base of the series
may become conglomeratic and the sandstone is liable to be pebbly even
in the highest layers. Most of the rocks are white or slightly stained
with iron oxide, but there are also intercalated layers of very
ferruginous sandstone which occur as definite bands a few inches thick,
traceable across the entire section exposed. The ferruginous character
of these bands was determined at the time of deposition, possibly by
local concentration of the heavier minerals. In addition to such bands
there are in places much thicker masses of highly ferruginous grit
occurring at what is locally the base of the series.
Much of the sandstone is massive, without bedding planes, and it tends
to form caps to the hills with vertical sides, fifty feet or more in
height. Cross bedding is beautifully exhibited in places in the lower
part of the series, especially in the Bagele Hills, around Tagumbilli,
and at Lamurde. The laminæ succeed each other in a perfectly regular
manner through considerable thicknesses of material, one set of laminæ
being truncated and overlaid by another set with a different
inclination. Cross bedding is frequently met with in the Cretaceous
grits of the Protectorate, where the laminæ are oblique to the major
bedding planes. The cross-bedded sandstones of the Upper Benue, however,
are built up of sets of laminæ, indistinctly marked off from those above
and below, and without any true bedding planes at all. The structure
resembles in every way the cross bedding of the Benue sandbanks, and in
size of grain, in the presence of occasional pebbles and even of pebbly
layers, and in the material itself, a coarse felspathic sand, the
sandstones closely resemble the sandbanks of the present day (p. 15).
The oblique lamination has been likened to that of dunes, but the coarse
size of grain and the presence of pebbles indicate running water rather
than wind as the immediate cause.
The flat tops of the sandstone hills and the terrace-like features on
their sides show that the Upper Benue sandstones have not been folded.
They rest on the Cretaceous limestone-shale series, which is folded in
Yola province as it is in Muri. This suggests that the Upper Benue
sandstones rest unconformably on the Cretaceous and are therefore post-
Cretaceous in age. The actual boundary between the sandstones and the
Cretaceous was only met with in one place, between Murki and Jibaru,
about twenty-five miles north of Yola. There a coarse, dark red grit
rests directly on Cretaceous limestone without any transition beds
between the two. Unfortunately the dip of the limestone is not known,
and therefore the evidence of the contact is not conclusive. The
immediate change from limestone to unbedded ferruginous grit is easily
understood if the overlying series is unconformable to the lower one,
but it is not easy to account for it on the view of a continuous
sequence. This section is regarded as strongly supporting the argument
drawn from the unfolded character of the sandstones.
Though the reliance to be placed on lithological character is uncertain,
so far as it goes the evidence is all in favour of correlating the Upper
Benue sandstones with the Lokoja series. It is true that the oolitic
ironstones so frequently met with in that series are not known from the
Upper Benue area, but the soft friable sandstones and the ferruginous
character of much of the material strongly remind one of the Lokoja
series.
The above conclusions, and certainly the map of this area, will require
modification in details, for the following reason. Two groups of
sandstone are known in the Benue area, one below the limestone-shales
and therefore Cretaceous, the other resting upon the limestone-shales,
in all probability unconformably, and therefore post-Cretaceous. In the
absence of fossils from either group of sandstones, the only method of
determining the age of any particular exposure is to prove continuity
either with the sandstones passing below the limestone-shales or with
the sandstones resting upon the limestone-shales. This would require
careful work under the most favourable conditions, and is out of the
question in a preliminary survey. Thus in the Yola province it may
ultimately prove that Cretaceous (infra-limestone) sandstones have in
places been mapped with the Upper Benue sandstones.
In a few places in Muri Province there are sandstones having
lithological resemblances to the post-Cretaceous series. Near Mutum Biu,
for example, there is a porous, very ferruginous grit, breaking up into
small fragments, each coated with a skin of limonite—characters exactly
paralleled, in many localities, by the Lokoja series. Again at Loko,
sixteen miles north of Katsena Allah, there is a soft unbedded pebbly
sandstone which is much more like the Upper Benue sandstones than the
Cretaceous. It may yet prove that on the widespread Cretaceous rocks of
Muri there are here and there patches of post-Cretaceous sandstone,
serving to link up the Upper Benue sandstones with the Lokoja series.
In spite of such future modifications, it is believed that the history
of the Upper Benue can be already sketched out as follows:
1. Crustal movements (folding and perhaps faulting) dropped down the
Cretaceous rocks below the crystalline basement along the Lau-Yola line.
2. The soft character of the Cretaceous rocks marked them out for rapid
denudation, and an “Older Benue” was established along this line with
crystalline plateaux north and south.
3. For some reason the detritus carried by the river accumulated, and on
consolidation it formed the Upper Benue sandstones. The unstratified
character of the material and the absence of alluvial mud or clayey
bands indicates a rapid accumulation, probably under somewhat different
conditions from those now prevailing. The sands were banked up against
the granites of the south and buried up at least the foot hills.[91]
4. Later denudation has largely removed the sandstone and has exposed
the Cretaceous floor again in many places. (A. L.)
PLATE XIII
[Illustration: THE SANDSTONE HILLS OF TURE.]
[Illustration: TURE OR TANGALE PEAK.]
The great development of rocks of Eocene age in the region of the Upper
Niger has been sufficiently emphasised by M. Chudeau.[92] The Niger
sandstones of Dahomey[93] are undoubtedly continuous with the western
members of the Sokoto series, and are properly ascribed by M. Chudeau to
the Eocene. The sandstones of the Upper Benue valley in the northern
territories of the Kameruns described by Dr. Passarge,[94] are likewise
continuous with the Upper Benue sandstones of N. Nigeria, and therefore
most probably also of Eocene age. From the Southern Kameruns Dr.
Esch[95] has described Cretaceous and Eocene rocks whose mutual
relations are very obscure. The locally folded character of the
Cretaceous, however, suggests an unconformity at the base of the Eocene.
In Southern Nigeria Mr. Parkinson[96] has indicated the occurrence of
both Cretaceous and post-Cretaceous rocks whose relations are at present
unknown. The clays and lignites of Asaba[97] may possibly, however, have
been accumulated contemporaneously with the upper shallow-water portion
of the Eocene of Northern Nigeria.
M. Chudeau’s[98] admirable attempt at the construction of a map of the
central Sahara is somewhat marred by his admittedly audacious method of
deducing the solid geology of certain areas from the average depth of
the wells. A striking example of the inaccuracy thus introduced is the
relegation by M. Chudeau of the Eocene rocks of Sokoto to the Lower
Cretaceous. Another fruitful source of error is the failure of M.
Chudeau to recognise, doubtless through the want of sufficiently
suggestive exposures, the possibility of an unconformity between the
Cretaceous and the Eocene in the Sudan. There is every reason to believe
that the erosion which followed the post-Cretaceous elevation was active
over the whole of the Sudan, and that the greater part of the hummocky
and irregular surface thus produced was again under water during the
period of the middle Eocene depression. Moreover, the deposits then
accumulated were in turn subjected to erosion during a subsequent
movement of elevation, with the result that the early Eocene surface of
crystalline and Cretaceous rocks was again laid bare over considerable
areas. In view, therefore, of the former probable extension of the soft
sandstones and ironstones of Kerri Kerri and the Tagum Hills over the
plains of Bornu, it is permissible to doubt the accuracy of much of M.
Chudeau’s suggested stratigraphy of the Chad area. It seems more
reasonable to regard the sandstones of Kanem and Manga as Eocene rather
than Lower Cretaceous, and to look upon them as having been deposited,
as in the case of the Eocene rocks of the Gongola valley, upon an
earlier surface of Upper Cretaceous and crystalline rocks now buried
beneath the superficial accumulations of Bornu. The rocks of Koutous and
Alakhos, Ouame and Tirminy[99] present, moreover, characteristics which
ally them more with the Eocene rocks of the south than with the Turonian
of the Damergou, and it is possible that even the southern escarpment of
the Tegama itself[100] where it overlooks the Damergou should be
referred to the Eocene. The red, white, and yellow sandstones of Ennedi
and of Snoussi on the Upper Shari are probably also of the same age, and
in no way related (as suggested by M. Courtet[101]) to the Karroo beds
of South Africa. (J. D. F.)
[Footnote 78: _Cf._ Suess, _The Face of the Earth_, Vol. IV, Oxford
Trans., 1909, p. 89.]
[Footnote 79: Chudeau, _Sah. Soud._, 1909, p. 104.]
[Footnote 80: De Lapparent, _C. Rd. Ac. Sc._, 136, 1903, pp. 1, 118;
Bather, _Geol. Mag._, [5], 1, 1904, p. 292.
The Eocene fossils from Sokoto and Jega are similar to those collected
by Col. Elliot and Capt. Lelean from the Sokoto frontier, of which
accounts have been published by Dr. F. A. _Bather_ (_Geol. Mag._, 1904,
p. 292, Plate XI) and by Mr. R. Bullen Newton (_G. J._, XXIV, 1904, p.
522), who regard them as of Middle Eocene age. The fossils consist
principally of echinoids and internal casts of mollusca. The latter are
not sufficiently well preserved for exact determination.
From Sokoto numerous examples of _Hemiaster sudanensis_ (Bather were
obtained, and also specimens of Lucina and gasteropods. At Jega
_Hemiaster sudanensis_ is also abundant, and a portion of _Plesiolampas_
(probably _P. Sahara_, Bather) was obtained. The mollusca belong to the
genera _Nautilus_, _Turritella_, _Voluta_, _Cardium_, _Lucina_,
_Ostrea_, etc.
(H. W.)]
[Footnote 81: Fossiliferous limestone has, however, been reported from
Yelu at the confluence of the Dallul Mauri and the Niger valley.]
[Footnote 82: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 384.]
[Footnote 83: Chudeau, _Sah. Soud._, 1909, p. 274.]
[Footnote 84: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 112.]
[Footnote 85: Chudeau, _op. cit._, p. 275.]
[Footnote 86: _Cf._ Gurich, _Zeit. Deut. Geol. Gesell._, XXXIX, 1887, p.
122.]
[Footnote 87: Barth, _Central Africa_, 1857, II, p. 368.]
[Footnote 88: See Section IV on margin of coloured map.]
[Footnote 89: The crystalline rocks at the base of the Bagele hills,
mentioned by Barth (_Central Africa_, Vol. II, p. 478) may be taken to
represent the outcrop of a portion of the earlier hummocky floor which
was buried under the sandstones of Yola.]
[Footnote 90: Passarge, _Adamaua_, 1895, p. 363.; Edlinger, _Niger-Tsad
Expedition_, 1904, p. 154.]
[Footnote 91: _Cf._ Passarge, _Adamaua_, 1895, p. 390. Passarge did not
recognise the presence of Cretaceous rocks in the Upper Benue valley.]
[Footnote 92: Chudeau, _Sah. Soud._, 1909, p. 97.]
[Footnote 93: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 399.]
[Footnote 94: Passarge, _Adamaua_, 1895, p. 363.]
[Footnote 95: Esch, _Geologie von Kamerun_, 1904, p. 11.]
[Footnote 96: Parkinson, “The post-Cretaceous Stratigraphy of S.
Nigeria,” _Q. J. G. S._, 63, 1907, p. 309.]
[Footnote 97: Parkinson, _op. cit._]
[Footnote 98: Chudeau, _Sah. Soud._, 1909, p. 85; and map.]
[Footnote 99: Chudeau, _Sah. Soud._, 1909, p. 81 _et seq._]
[Footnote 100: Barth, _Central Africa_, 1857, Vol. I, p. 531.]
[Footnote 101: Courtet, _C. Rd. Ac. Sc._, 140, 1905, p. 162.]
CHAPTER VI
SUPERFICIAL ACCUMULATIONS
The general absence of a deeply decomposed crust upon the rocks of the
Sahara and the Sudan. The drift of Hausaland and Borgu; kankar; origin
of the plains of Hausaland; the surface ironstones; origin of the drift;
Mounts Dala and Kogon Dutsi; the surface drift of the Bauchi plateau;
conditions of formation of the surface ironstones; periods of maximum
and minimum erosion; periods of elevation and depression of the crust;
the recent arid period. The drift of Bornu and Katagum; the absence of
surface ironstones; the “firki”-land of eastern Bornu; the shelly marls
of Kukawa; the conditions of formation of the drift of Bornu; the early
existence of swampy and lacustrine conditions in Bornu; regional
oscillations of level in the Chad area; erosion of the floor of the
present lake; the cuvettes of Manga. The drift of the Gongola valley.
The drift of the Benue valley. The ferruginous crusts of the flat-topped
hills of the Lower Niger and Benue.
No fossiliferous deposits of later age than Eocene have been discovered
within the Protectorate. M. Chudeau[102] quotes the occurrence of
Miocene shales at Boutoutou[103] to the north of Gober and concludes
that the sea did not finally quit Central Africa until the close of the
Miocene. If, however, any deposits of similar age were ever accumulated
within the Protectorate, they appear to have been subsequently removed
and replaced by unconsolidated accumulations, most probably of late
Pliocene age, which now in places cover Eocene, Cretaceous, and
crystalline rocks alike. These accumulations, which vary considerably in
composition in different parts of the Protectorate, are of continental
and in part of lacustrine origin and represent the products of
weathering and disintegration of the crystalline and sedimentary rocks,
sorted and rearranged by the action of rain, river, and wind. Their
superficial portion now forms the surface soil which except in swampy
localities is characterised by the small quantity of humus which it
contains. M. Chudeau[104] has remarked upon the absence of any deep
decomposition of the rocks in situ in the Sahara and Sudan to the north
of lat. 11°, and his remarks may with justice be extended to the greater
part of Northern Nigeria, where the original products of decomposition
have rarely remained in place but have travelled outwards for a variable
distance from the parent rocks and in places accumulated to very
considerable depths. This drifted or travelled character of the surface
alluvium may be typically observed round the margins of the larger
groups of granite hills. The disintegrated debris rapidly accumulated
during torrential rains at the mouths of the narrow valleys or at the
base of the weathered slopes is gradually broken up and spread out upon
the plain during succeeding periods of less intense fluviatile action.
1. _Hausaland and Borgu._
The red sandy loam which covers the greater part of the crystalline
plains of central Hausaland and Borgu, while predominantly of local
derivation, possesses for the most part a drifted or travelled
character, and in composition presents relatively little affinity with
the rocks which anywhere immediately underlie it. A rude stratification
may at times be observed within the drift, but for the most part it
belongs to the category of ill-defined alluvium and surface wash.[105]
Red earth and subangular quartz grains in varying proportions naturally
make up the bulk of the deposit, but it contains also more or less worn
and weathered crystals of all the common rock-forming minerals together
with pebbles and fragments of quartz, quartzite, granite, gneiss,
amphibolite, and other rocks which are naturally most abundant where the
drift is thinnest. Calcareous concretions similar to the kankar of India
are common in places where the drift is predominantly earthy. They have,
however, nowhere been observed forming any continuous sheet of sub-
surface limestone, while, contrary to M. Hubert’s suggestion,[106] their
distribution appears to have little or no relation to the occurrence of
crystalline limestones within the schistose series. This composite
drift, while evidently resulting from the rearrangement of weathered and
disintegrated material, can frequently be seen covering up and wrapping
round smooth and little weathered surfaces of rock in the midst of the
open plain. The smaller inselberge moreover appear to have little
influence upon the composition of the drift, which preserves its
heterogeneous character up to the base of the dome, and even encloses on
the margin sub-angular blocks and boulders of granite detached at an
earlier period from the surface of the dome itself. In places, as on the
high plains of Zaria and Bauchi, a very considerable thickness of drift
has accumulated, and where locally it has been in part removed the rocky
floor below is hummocky and irregular and as a rule remarkably free from
any evident decomposition. It is highly probable, indeed, that the sandy
loam with its heterogeneous composition and its subangular pebbles of
various rocks was slowly accumulated upon an irregular surface and
spread out and levelled by wind and rain.[107] That it was accumulated,
moreover, under very different conditions from those which now prevail
is evident from the fact that it is nowhere still in process of
deposition upon the open plains of Hausaland. On the contrary it is now
undergoing erosion, rearrangement, and gradual removal by the rivers and
streams which in many parts of their course have not yet succeeded in
exposing the rock below. Its presence also to a remarkable depth upon
the watersheds and the loftiest plains makes it highly improbable that
such regional accumulation could have taken place at the present
elevation of the central provinces. It would seem rather that there has
been within comparatively recent times a rearrangement of the drainage
system of the Protectorate, brought about by a crustal movement of
sufficient intensity to transform the plains of Hausaland from low-lying
areas of accumulation into high-lying areas of erosion.
Such an hypothesis serves also to explain the present typical aspect of
the plains themselves when it is remembered that their gently undulating
character is due not so much to the smooth and even surface of the
underlying rocks as to the covering of drift which has obscured all the
minor irregularities of the crystalline floor below. There can be little
doubt that the plains of Hausaland arose primarily as approximately
base-levelled plains of erosion. There is, however, no reason to believe
that the actual surface of the underlying rocks coincides in more than a
general way with that of the open rolling plains of to-day. All the
indications indeed point to quite the opposite conclusion. The base-
levelled plains appear to have suffered some slight regional elevation
and erosion, and the consequent hummocky and irregular surface has been
in turn levelled up and buried in drift. The plains as we know them,
therefore, while primarily plains of erosion, are in their more obvious
characteristics to some considerable extent plains of accumulation.[108]
The processes of erosion and accumulation moreover, to which they owe
their present aspect, are for the most part the same as those which are
now active in these regions. The isolated hills and the bare surfaces of
rock frequently show evidence of abundant pot-holing and of extensive
fluviatile action. Messrs. Andrews and Bailey[109] have suggested that a
sub-aerial plain of erosion may in time, by means of dry weathering and
run-off, replace a tract of hilly country at an altitude of even more
than 3,000 ft., without the help of river erosion or planation. Whatever
local influence these processes may have, however, the possibility of
such a result upon a regional scale is exceedingly doubtful. The
efficacy of dry weathering indeed, as an agent of extensive erosion, is,
in spite of the able championship of Professor Walther,[110] becoming
more and more discounted with the progress of research in the
Sahara.[111]
A characteristic feature of the red sandy loam on the high plains of
Borgu, Zaria, and Bauchi is the occurrence in places of a ferruginous
cap or crust, which frequently forms a continuous floor many acres in
extent. The ferruginous cap has arisen through the cementation of the
quartz grains and pebbles into a hard resistant mass by means of
hydrated ores of iron originally deposited in the subsoil above ground
water level. The characteristic cellular or vesicular character and hard
lustrous surface of the ironstone are due to the leaching out of the
uncemented material and the rearrangement and partial dehydration of the
iron ores upon exposure at the surface. Wherever, therefore, the
ferruginous cap is now exposed, it may be assumed that the soil
originally overlying it has been removed in the course of denudation. In
southern Bauchi and Zaria[112] the vesicular ironstone may at times be
found forming low platforms raised a few feet above the general level of
the plain, which is itself in places provided with a similar floor of
ironstone. At other times where erosion has been more intense it caps
low flat-topped hillocks of drift whose slopes are littered with fallen
blocks and boulders of gritty ironstone. Rolled pebbles of concretionary
limonite and subangular fragments of an earlier ferruginous crust are
frequently also characteristic constituents of the surface soil. It is
evident, therefore, that in addition to the period of formation of the
ferruginous crust which now in places forms the floor of the plain,
there has been at least one earlier period of formation of gritty
ironstone within the drift. It may therefore be presumed that the drift
was originally of sufficient thickness to cover the highest platform of
ironstone of which traces still remain, and that the ironstone platforms
of lower levels were formed during subsequent intervals in the later
erosion of the drift. In places fragments or masses of pebbly ironstone
of heterogeneous composition can be seen adhering to bare and
unweathered surfaces of granite, sandstone, and other rocks. Such
occurrences are to be regarded as relics of earlier sheets of ironstone
formed, it may have been only locally, when the drift was thicker and
the ground-water at a higher level than it is at present.
The clue to the origin of the drift is found within the walls of Kano.
The two flat-topped hills, Dala and Kogon Dutsi, each 150 feet in
height, are composed almost entirely of soft and thoroughly decomposed
rock, stained red towards the summit and capped by a layer of vesicular
ironstone (p. 90). The presence of rolled fragments of an earlier
ironstone in the ferruginous cap indicates that the flattened summits
once formed the floor of the plain and were themselves probably covered
by an earlier sheet of drifted alluvium. These hills bear witness,
however, not only to the extensive decomposition _in situ_ to which the
crystalline rocks were once exposed, but also to the intensity of the
subsequent erosion which has left them as solitary relics of the earlier
surface of the plain. It is possible, indeed, that the formation of such
a sheet of weathered rock possessed a regional character, and that in
its later erosion is to be found the explanation of the general absence
of any deep decomposition of the rocks _in situ_ not only within the
Protectorate but also over the whole surface of the central Sahara and
Sudan. To the erosion of this weathered crust is also to be ascribed the
formation of the hummocky and irregular surface upon which the drift was
afterwards spread out, and it is probable that the greater part of the
drift itself was derived from the further erosion of those portions of
the original weathered surface which still remained above the level of
accumulation. No facts have been ascertained with regard to the
occurrence of true lateritic material either in the original sheet of
weathered rock or in the drifted alluvium. There is some reason to
believe, however, that the surface ironstones in places contain
considerable quantities of free alumina.
It is possible that before accumulation began the weathered rock had not
been entirely removed from the surface of the plains, and that portions
of the original sheet of decomposed material may therefore be preserved
in places underneath the drift. Moreover worn surfaces of the original
weathered rock may have been left practically devoid of any covering of
drift, and these surfaces, as around Aribi and Gantam on the Nassarawa
tableland, may have been in course of time provided with a typical crust
of gritty and vesicular ironstone. The floor of the plain of Kano around
the two prominent hills probably represents in this manner a platform
cut in the earlier sheet of weathered rock and covered with a
ferruginous crust. A similar origin should perhaps be ascribed to the
plains of Rauni which are characterised by the great depth of their
surface ironstones and the presence of hillocks of similar composition
which recall the buttes of Djougou in Dahomey.[113] Between Kano and
Katsina, moreover, the amount of drift varies much from place to place,
and appears to have been locally very largely removed and replaced by
recent products of weathering and considerable accumulations of blown
sand and clay which have probably arisen to some extent through the
rearrangement of the earlier drift under the influence of the winds.
These recent accumulations are sometimes found resting upon older
surfaces of cellular ironstone, but are never themselves covered with a
ferruginous crust.
The red earthy drift which forms the detached flat-topped hills upon the
summit of the Bauchi plateau (Plate XI, Figs. 1 and 2), while located
for the most part upon a crystalline floor, may possibly have originated
to a considerable extent through the disintegration of the fragmental
material of the volcanic cones in the neighbourhood, and through the
rearrangement of the original weathering products which may be presumed
to have formed upon the lavas of the Morrua plateau. There are, however,
few remaining traces of any earlier weathering of the crystalline and
igneous rocks preserved underneath the superficial accumulations. For
the most part the latter rest upon smooth and comparatively unweathered
surfaces of rock (Plate XV, Fig. 2), while their travelled origin is
indicated not only by the presence of quartz grains and granitic debris
in the ferruginous cap, but by the occurrence of tinstone in the drift
throughout the whole area. The tinstone is naturally most abundant
around Ngell in the neighbourhood of its centre of dispersion, but
traces of it are to be found even on the hills of Rukuba from which the
drift itself has now been almost entirely removed. There is good reason
to believe, indeed, that the Bauchi plateau up to a comparatively recent
date was continuous with the central plains of Hausaland, and that it
participated in all the oscillations of conditions to which the latter
were subjected. It is probable, therefore, that the plateau was at first
covered with a sheet of decomposed rock of variable thickness, by the
subsequent erosion of which the irregular surface was produced upon
which the drift was accumulated and spread out. It is obvious, however,
that the drift itself with its crust of compact or vesicular ironstone
is now undergoing erosion, and that the present tin-bearing alluvium is
derived for the most part from the rearrangement of the drift and only
to a small extent from the denudation of the underlying rocks which is
now again in process. It is probable that, during the earlier stages in
the erosion of the plateau drift, accumulation continued upon the plains
of Zaria and Nassarawa round the base of the plateau, but any such
action is practically negligible under the present _régime_.
M. Hubert[114] in Dahomey, and Dr. Passarge[115] in Adamaua, assign only
a very subordinate place to the formation of surface ironstones upon
drifted alluvium. They believe that the ironstones have almost always
formed within material which has arisen as the result of the
decomposition _in situ_ of the rocks immediately underlying.[116] M.
Hubert’s insistence upon this point of view has led him to suggest that
the crystalline rocks of the plains of Borgu were levelled before being
decomposed and covered with surface ironstones, and that the plains
themselves have preserved their present character and elevation since
the close of the Eocene. It may be granted that the crystalline rocks of
Nigeria were at one time deeply decomposed, but Mounts Dala and Kogon
Dutsi bear unmistakable testimony to the subsequent intense erosion of
this former crust of weathered rock. In southern Bauchi and Zaria,
moreover, extensive sheets of surface ironstone can frequently be seen
capping typical gritty alluvium, and there can be little doubt that
within the Protectorate as a whole, the formation of surface ironstone
upon material actually weathered _in situ_ is the rarer phenomenon. For
the most part the ironstones rest upon and have been formed within
drifted and travelled superficial material which has been accumulated,
near or at base-level, upon an irregular and hummocky surface, and
spread out in such a manner as to obscure all the minor irregularities
and to produce the present smooth and even character of the plains of
Hausaland. M. Chautard[117] has noted the similar extensive formation of
surface ironstones upon the ancient alluvial plains of Upper Guinea and
the Futa Djalon.
The actual conditions of formation of the surface ironstones are very
problematical. Their association with the older alluvium affords some
reason for believing that their extensive formation in the past has been
accomplished under somewhat different conditions from those which now
prevail. It seems probable also that the presence of surface ironstone
as a ferruginous cap upon drifted alluvium is an analogous phenomenon to
its occurrence upon material which has arisen, as in Mounts Dala and
Kogon Dutsi, through the decomposition _in situ_ of the rocks below.
Drs. Hatch and Corstorphine[118] believe that the surface ironstones of
South Africa have been formed very largely as bog iron ores by the
oxidation of ferruginous solutions or by the deposition of the iron
salts during the evaporation of pools of stagnant water. Captain
Freydenberg[119] has noted the accumulation of ferruginous clay on the
northern shores of Chad, but extensive sheets of ferruginous material
comparable to the platforms of surface ironstone have nowhere been
observed in process of formation upon the surface of recent alluvium or
of æolian accumulations under the present _régime_ of periodic rains.
The surface waters accumulated during the summer rains are for the most
part rapidly and completely drained, and any probability of the
formation at high altitudes within the Protectorate of pan or bog iron
ores upon an impervious subsoil would appear to be prohibited by the
general open and porous character of the drift. Moreover wherever true
swamps, subject to partial or complete annual desiccation, are to be
found, surface ironstones are typically absent, a waterlogged condition
of the subsoil being apparently unfavourable to the deposition of the
iron ores. M. Lacoin[120] believes that in alluvial deposits such as
those of the Shari and Oubangui an enrichment in iron takes place in the
lower beds at the expense of the upper, and that the banks of
ferruginous material, formed by sub-surface evaporation and oxidation
after the manner of the “alios” of Landes,[121] are afterwards exposed
by the removal of the overlying accumulations. Such an hypothesis,
however, is equally open to the objection that the surface ironstones of
Nigeria are most typically found where the sub-surface drainage is most
perfect. Professor Walther[122] believes that the surface ironstones are
entirely the product of arid conditions, and that their origin is
similar to that of the protective crust of the Libyan desert. While,
however, there can be little doubt that the hardening of the surface of
the ironstone and the production of the characteristic varnish is the
result of the partial dehydration of the iron ores upon exposure to
conditions of excessive evaporation, it is difficult to believe that the
occasional rains and dews of an arid climate could suffice to produce
any extensive superficial accumulation of the iron ores. MM.
Chautard[123] and Hubert,[124] in view of the sporadic distribution of
the ironstones, would extend to their formation Sir T. H. Holland’s
hypothesis[125] of the bacterial production of laterite under humid
conditions, an hypothesis, however, which still awaits experimental
verification. Dr. Passarge[126] is of opinion that surface ironstones
are formed under semi-arid conditions where the aridity causes the rapid
evaporation of the occasional rains, while the high temperature promotes
rapid chemical weathering of the rocks, and the excess of ozone and
oxygen compounds in the rains, together with the absence of reducing
substances in the soil, brings about the rapid oxidation and
precipitation of the iron ores. Mr. Maufe[127] believes that the
formation of surface ironstones (muram) in East Africa is an immediate
consequence of deforestation. There is some reason to believe, however,
that even underneath a covering of vegetation the slow precipitation of
iron ores in the subsoil may give rise to the formation of ferruginous
concretions which may in time coincide to form a continuous layer. Where
the streams have trenched the deeply decomposed surface of certain belts
of schists in Kabba and Nassarawa which support an abundant vegetation,
it may be observed that the iron shows a decided tendency to accumulate
in the upper portion. The solution of the iron is brought about by the
descent of water charged with carbon dioxide and with organic acids,
arising primarily from the decomposition of the superficial vegetation,
while the subsequent rise and redeposition of the iron ores above ground
water level is accomplished during periods of less relative humidity. It
is conceivable that a similar action may take place within alluvial
deposits when covered with a rich vegetation, and that a subsequent
change in conditions may bring about the more rapid evaporation of the
ferruginous solutions, the removal of the vegetation and of the surface
soil, and the hardening and polishing of the crust into a typical
surface ironstone. The present irregular distribution of the ironstones
may have been conditioned by the amount of subsequent erosion as well as
by the original composition of the drift, the content in iron of the
circulating surface waters, the abundance of the vegetation and the
character of the drainage, the free access of oxygen to the subsoil
being a necessary condition of their formation.
Dr. J. M. Maclaren,[128] Mr. F. P. Mennell[129] and Mr. J. M.
Campbell[130] have suggested an analogous origin for certain laterites
and ironstones of India, Rhodesia and Upper Guinea respectively, and
postulate for their formation an alternation of wet and dry seasons
similar to the present _régime_. Messrs. Mennell and Campbell are of
opinion, moreover, that the concentration of the ores upon definite
horizons to form the floors of successive platforms or terraces may be
explained as the result of a spasmodic lowering of the ground water
level under climatic conditions similar to those which now prevail.
While, however, the erosion of the platforms themselves within the drift
may certainly be ascribed to this cause, it is much more probable that,
at least in the case of Nigeria, the spasmodic activity in the
deposition of the iron ores has been brought about by an alternation of
humid and arid conditions, the formation of ironstone being most marked
during periods of gradual desiccation. It is well known that the present
_régime_ of periodic rains within the Sudan represents a return to more
humid conditions after a period of aridity, and there is some reason to
believe that there have been many such climatic oscillations in the
past. Nothing is more probable, therefore, than that certain periods of
desiccation should immediately follow the formation of certain platforms
of erosion within the drift, and that to the incidence of these periods
of desiccation is to be ascribed not only the maximum deposition of the
iron ores but also the removal of the overlying alluvium and the
hardening of the surface of the ironstones. It is probably correct to
assume that an annual alternation of rains and droughts similar to the
present _régime_ is favourable to the deposition of iron ores in the
subsoil, and there is every reason to believe that their accumulation is
still going on where conditions favourable to their formation are to be
found. Under present conditions, however, the amount of iron salts
dissolved in the ground water must be relatively less than it would be
at the close of a period of maximum humidity, and it may therefore be
presumed that in Northern Nigeria the present is a period of
comparatively slow accumulation of iron ores. There is indeed, as
already remarked, little or no evidence of any extensive formation of
surface ironstone, since the beginning of the present _régime_.
The ironstones must be exposed actually at the surface before they can
assume their characteristic lustrous carapace, and the removal of the
vegetation and of the surface soil can be accomplished only by a
climatic change. Mr. Campbell[131] in Upper Guinea, and Mr. Maufe[132]
in East Africa, would assign the deforestation very largely to the
influence of man, and the subsequent removal of the soil to the agency
of the tropical rains.[133] It is difficult to believe, however, that
the characteristic ironstone platforms of Hausaland can possess such a
comparatively recent origin, and there can be little doubt that the
highest platforms of which traces still remain should be referred to at
least the later Tertiary period. Once formed, the hard resistant crust
may be covered by later alluvial or æolian accumulations or by the
products of disintegration and re-arrangement of the ironstone itself,
upon the return of more humid conditions. Wherever covered by later
deposits, however, the ironstone is softer and lacks its characteristic
varnish. Moreover, where a scattered vegetation is now found creeping
over a surface of lustrous ironstone, it is frequently a difficult
matter to decide how much of the scanty soil is a recent accumulation,
and how much of it represents the original covering of alluvium which
had not been entirely removed.
PLATE XIV
[Illustration: DOZI HILL, KERRI-KERRI.]
[Illustration: A TRENCH-LIKE VALLEY NEAR LELE, IN KERRI-KERRI.]
The formation of the surface ironstones of the plains of Hausaland is
thus by no means readily explicable. Their origin is probably due, in
part, to many different causes, and their fragmental occurrence at
various levels, sometimes upon weathered rock, sometimes upon
unweathered rock, and sometimes upon drifted alluvium, points to
repeated formation and repeated erosion at several different periods. It
is believed that, if thoroughly investigated, their distribution and the
distribution of the surface drift would afford some clue to those
repeated climatic and crustal changes to which within comparatively
recent times the surface of the central Sudan has been subjected. The
evidence already detailed appears to be sufficient to establish the
following succession of events upon the plains of Hausaland:—
(1) A period of minimum erosion during which the superficial rocks were
deeply decomposed to a variable depth and covered with a ferruginous
cap.
(2) A period of maximum erosion during which the earlier sheet of
decomposed rock was largely removed, with the exception of such relics
as Mounts Dala and Kogon Dutsi.
(3) A period of minimum erosion and accumulation during which the
surface drift was spread out upon the plains.
(4) The present period of erosion of the drift during which, in places,
successive platforms have been cut in the drift and floored with
concretionary ironstone.
The cause of the alternation of these periods of accumulation and
erosion is probably to be sought in crustal changes of a regional
character. If a condition of low relief be granted to begin with, a
general depression of the surface to the neighbourhood of base level may
be postulated during the periods of accumulation, and a corresponding
elevation of the surface above base level during the periods of erosion.
The various periods indicated above may therefore be termed
respectively:—
(1) The first period of depression.
(2) The first period of elevation.
(3) The second period of depression.
(4) The second period of elevation.
The history of the earlier periods is somewhat obscure and it is
possible that there may have been many minor oscillations of the crust
during the first period of depression and the first period of elevation.
It is possible also that the first period of depression may have been
represented merely by a general base levelling of the surface through
erosion, and that the original sheet of weathered rock may have been
formed in part underneath sedimentary rocks which were afterwards
removed. The flat-topped hillocks of consolidated granitic grit and clay
between Kano and Kazaure may perhaps represent contemporaneous
accumulations during the first period of erosion, which were themselves
subsequently exposed to denudation. There is some reason to believe,
also, that there has been within the various periods a minor oscillation
in climatic conditions. These minor oscillations would naturally be most
clearly recognisable within the second period of elevation which is
continuous with and includes the present. There is little doubt that the
formation of the surface ironstones was favoured by the successive
occurrence of periods of maximum evaporation, and it is highly probable
that these periods assumed at times a typically desert character, and
that the partially eroded surface of the drift was covered up at
intervals by æolian accumulations. One such period at least immediately
preceded the present more humid conditions, and is clearly recognisable
in the worn and rounded dunes of the dead _erg_ of northern Hausaland.
The dunes are now grassed over and covered with scattered trees, but
their origin is still sufficiently indicated in their composition and
general outlines as well as in the characteristic banking of the sand
against rocky exposures. The surface soil and the recent alluvium of the
plains of northern Hausaland probably contain much wind-blown material,
and the swampy character of much of the surface of the plains during the
rainy season is probably to be in part explained as the result of the
interference of the accumulations of this recent arid period with the
earlier system of drainage. At the same time, however, it should be
remembered that the erosion of the drift has in most places not yet
become very intense and that the average slope of the plains of
Hausaland is as a rule so gentle that a considerable volume of water
must accumulate before a perceptible flow can be established. Much
swampland is found on the high plains around and to the east of Zaria,
where Dr. Baikie[134] first recognised the drifted character of the
alluvium.
2. _Bornu and Katagum._
A striking feature of the plains of Bornu and Katagum and of the Gongola
valley between Nafada and the Yola border is the absence of any
superficial crust of vesicular ironstone. The drift which may reach in
Bornu a depth of more than 300 feet differs considerably in composition
from the characteristic drift of Hausaland. Upon the open plains of
Bornu the drift is composed for the most part of a mixture of white or
yellow sand and light or dark grey clay. A red ferruginous staining of
the drift is comparatively rare to the east and north-east of Gujba. The
clays in places yield irregular concretions of kankar and small nodules
of concretionary ironstone are dug from the surface sands in the
neighbourhood of Kukawa, but no evidence has been anywhere obtained of
the formation or earlier existence of any continuous sheet of vesicular
ironstone. Well sections exhibit below the surface alternations of sand
and clay in beds of very varying thickness.[135] The sand varies much in
coarseness of grain and is in part angular and of drifted fluviatile
origin and in part well rounded and of æolian origin, the latter
apparently predominating largely towards the north-east. The clays are
finely divided and flocculent in water and mixed with minute grains of
blown sand. In places they become darker coloured from the inclusion of
organic matter and pass into the “firki” (p. 52) or so-called black
cotton soil of Chad and the lower Shari. Where exposed at the surface,
the clay becomes plastic and very tenacious in the rains, but hard and
brittle and much cracked and fissured in the dry season. These clays are
rich in azote which contributes much to their fertility.[136] This
advantage, however, is counterbalanced by the almost complete absence of
lime, potash, magnesia, and phosphoric acid, and by the presence of
large quantities of silicate of alumina and of salts of soda. Fragments
of broken shells of land snails and lacustrine molluscs are common in
the clays of eastern Bornu, while thin beds of calcareous marl[137] with
abundant remains of recent molluscs are intercalated among the sands and
clays of Mongonu and Kukawa. It is evident that the conditions of
accumulation of the drift in Bornu were different from those under which
the drift of Hausaland was accumulated. The latter was preeminently a
terrestrial or sub-aerial accumulation while the former in the finer-
grained character of the sand, the abundance of clay and the absence of
pebbles and of any intense ferruginous staining affords indications of
having been deposited to a great extent under water in a shallow
lacustrine or swampy region. It is probable, however, that the
accumulation of the drift of Bornu was, at least in the earlier stages,
contemporaneous with the accumulation of the drift of the plains of
Hausaland and may therefore be referred to a considerable extent to the
second period of depression. No traces are left, however, upon the
plains of Bornu of the earlier periods of depression and elevation, and
it may therefore be presumed that the surface upon which the drift was
deposited was entirely cleared of earlier accumulations during the first
period of erosion and that its minor irregularities have been completely
obscured by the later drift. The second period of elevation which
brought about the erosion of the drift in Hausaland appears to have
resulted in comparatively little erosive action upon the drift of Bornu.
It is highly probable indeed that Bornu retained its swampy and
lacustrine and therefore receptive character to a comparatively recent
period and that the oscillations of less and more humid conditions which
probably prevailed during the accumulation and erosion of the drift of
Hausaland are reflected in Bornu in the intermingling of æolian and
fluviatile and lacustrine material. The local abundance of alkaline
salts in the drift of Bornu may also be taken as indicating an
occasional and local desiccation.
The presence of swampy and lacustrine conditions in Bornu dates,
therefore, from the second period of depression and there is no reason
to believe that there has been any marked break in continuity between
these later Tertiary swamps and the present Lake Chad. There is,
however, no evidence that the present lake or the former more extensive
swamps in any way represent the shrunken relics of the early Tertiary
seas. The later swamps indeed, with which the lake is more or less
continuous in point of time, appear never to have reached any great
depth and to have been liable to repeated desiccation. Desert periods
may have changed them partially or wholly into sand fields, but the
return of humid conditions constantly re-established their former
character.[138] It may be presumed also that during periods of
exceptional rainfall the swamps became united into a shallow lake of
considerable extent which may even at times have covered the greater
part of central and Eastern Bornu. There is no reason to believe,
however, that this was to any extent a normal condition of things, or
that the drift was primarily accumulated in a large permanent lake. So
far as known, marly beds occur in Bornu only in the neighbourhood of the
present lake, and whatever the extent of the marginal swamps may have
been, the deepest and clearest water seems always to have been confined
to eastern Bornu. The thinness and local character of the marls would
seem to imply, however, that the requisite conditions for their
formation were by no means common or universal. Captain Freydenberg has
described in sufficient detail the former extension of the swamps to the
north, east, and south of Chad.
The character and thickness of the drift deposits appear to indicate
that Bornu has in the past formed part of an area of predominant
depression. The shelly marl of Kukawa which lies at least fifty feet
above the present level of Chad, undoubtedly indicates that the floor of
the lake was at one time at a higher level than it is at present. The
long-continued accumulation was probably conditioned at first by the
second period of depression and later by the gradual uprise of the Congo
watershed during the latest period of elevation. In the neighbourhood of
Kukawa, however, the low hillocks of drift show evident signs of
erosion, and it may therefore be presumed that the present floor of the
lake has been excavated out of the earlier lacustrine and drift
deposits. This erosion which M. Garde[139] would presumably ascribe
entirely to æolian activity, was probably in great part conditioned by
the continued relative subsidence of the Bodele depression, the
consequent eastward draining of the lacustrine region and the
establishment of the Bahr el Ghazal as a waterway from west to east. The
lower relative level of Bodele has now been definitely established, and
M. Chudeau’s[140] arguments in favour of the eastward flow of the Bahr
el Ghazal may be provisionally accepted until fuller information has
confirmed or contradicted them. Desert conditions, however, followed
close upon erosion. The worn and rounded hillocks of drift around Kukawa
were partially buried in blown sand and the Bahr el Ghazal was blocked
by the action of the wind. On the establishment of the present
conditions the lake as we now know it was formed and limited towards the
west by the line of sand dunes, now covered with abundant vegetation,
which stretches from Kukawa to the river Yo along the margin of the
lake. There is no reason, however, to postulate the presence of an
earlier lake actually on the site of the present Chad at a period
immediately anterior to the coming of the desert.
It may be presumed that during the period of erosion which preceded the
advent of the desert, shallow watercourses were cut in the drift of
Bornu and that they were in part dismembered and filled up again during
the succeeding period of desiccation. One such watercourse may be
clearly traced from Murguba to Maigumeri in central Bornu. The present
channels of the Yo, the Maidugari river, the Yedseram, and the Shari
were probably also established at the same period. The cuvettes of Manga
probably originated at the same epoch and represent portions of earlier
watercourses detached and isolated by æolian accumulations. The sites of
the cuvettes may possibly have been determined in part by the presence
of springs in the earlier river beds. While, therefore, there is little
reason to believe with Captain Freydenberg[141] that the bottoms of the
cuvettes represent detached portions of the original floor of an
extensive lake almost entirely filled up with æolian accumulations,
there is equally little reason to believe with M. Chudeau[142] that the
cuvettes have arisen through the local subsidence of the underlying
rocks brought about by the dissolution of beds of soluble salts,
hypothetically contained within the sandstones of the Tegama below. M.
Chudeau’s objection that an approximately level surface such as that of
the plains of Bornu cannot be produced under the influence of the wind
is clearly untenable in view of the known occurrence of extensive level
sandfields in modern deserts. Moreover, in Katagum solitary dunes or
isolated hummocks of blown sand formed during the recent arid period may
be found alone in the midst of an open undulating plain. There is some
reason also to believe that the winds which never attained any great
intensity varied in direction in different parts of the Protectorate. In
Bornu the dunes are elongated in a N.–S. direction while in Katagum they
are more generally E.–W. and it is possible that a successive variation
in the direction of the winds over the same area may have been
responsible for much of the levelling of the present surface of the
plain.
The lacustrine conditions which prevailed over central and eastern Bornu
during the deposition of the drift extended also for a considerable
distance westward through Katagum. The drift of Katagum is very similar
in character to that of Bornu and the abundance in it of wind-blown
material, as in the drift of Bornu, bears witness to the constant
struggle between humid and desert conditions during the period of its
accumulation. The predominance of the latter at a period immediately
preceding the present is indicated as in Bornu by worn and rounded dunes
and banks of sand. There is every reason to believe that Bornu and
Katagum have been for long on the margin of the desert and that the
history of the repeated advance and retreat of the desert can be clearly
read in the varied character and composition of the drift.
3. _The Gongola Valley._
To the west of Gujba, thick accumulations of red sandy loam effectually
obscure the irregularities of the topography of eastern Kerri Kerri. The
drift has evidently been largely derived from the disintegration of the
red sandstones and ironstones of Kerri Kerri, but the surface of the
original deposits has been in great part rearranged by subsequent æolian
action. Upon the plateau itself the æolian accumulations reach in places
a very considerable thickness. In the valley of the middle Gongola to
the south of Kerri Kerri there is as in Bornu and Katagum, a remarkable
absence of surface ironstone. Around Nafada and southward upon the
narrow plain to the west of the Gongola the drift is predominantly a red
sandy loam. To the east of Ashaka, however, the drift consists very
largely of a stiff tenacious black or dark grey clay, very similar in
appearance to the “firki” of eastern Bornu. This material also covers
large areas upon the central plain of Bauchi to the south and south-east
of Deba Habe. It is clear from the manner in which the red loam and the
dark coloured clay obscure the irregularities of the surface and creep
up the flanks of the sandstone hills that the superficial accumulations
are here also, as on the plains of Hausaland, of drifted or travelled
origin and not primarily of the nature of material weathered and
disintegrated in situ. The local origin of the drift may however be
presumed. The red sandy loam of the northern and western plains has been
derived very largely from the disintegration of the Kerri Kerri and
Gombe sandstones, while the clays of the east and south have probably
arisen in great part from the decomposition of the lavas of the Burra
plateau. The original depth of the drift is unknown, but must have been
very considerable. The Gongola in places has not yet succeeded in
cutting its way through the drift to the rock below, while the highest
portion of the plain which is still buried in drift lies about 300 feet
above the present river bed. The total thickness, however, may well have
been much greater and there is every reason to believe that what is now
the valley of the middle Gongola between Nafada and Gasi formed, along
with that portion of the central plain of Bauchi which lies between Deba
Habe and Putu, an area of special accumulation throughout the second
period of depression and that the activity of the river and its
tributaries during the most recent period of elevation has been directed
mainly towards the removal of the drift which had accumulated in the
earlier valley.
4. _The Benue Valley._
The alternation of the major periods of accumulation and of erosion as
established for the plains of Hausaland was probably to some extent of a
regional character, although the relative amount of respective
depression or elevation may have varied much in different parts of the
Protectorate. The irregular movements to which the Benue valley as a
whole was subjected during the latest period of elevation renders the
correlation of the superficial deposits with those of the northern
States a matter of considerable difficulty. There is good reason to
believe, however, that the lower Benue valley and the greater part of
the provinces of Kabba and Ilorin were exposed during the same period to
very intense erosion, in consequence of which the original drift
deposits have been almost entirely removed and the underlying Eocene
sandstones deeply trenched and worn. The middle Benue, on the other
hand, appears to have been long a region of relative depression and
accumulation, and although platforms of surface ironstone may be found
in places even on the plains of Muri,[143] the requisite conditions for
its formation appear to have been for the most part wanting. It is
possible that its absence may be due, as in Bornu and the Gongola
valley, to a prolonged water-logged condition of the subsoil. Wherever,
on the other hand, the surface of a hillock of red sandstone was exposed
above the drift, there seems to have been a rearrangement of the
ferruginous material and a concentration of the iron ores in the upper
part which has given rise upon exposure to a typical honeycombed
limonitic cap. The ferruginous crusts of the flat-topped hills and
plateaux of the lower Benue and the Niger have a somewhat similar origin
and have arisen in part through the rearrangement and deposition in the
subsoil of the original iron of the sandstones and oolites and in part,
like the ferruginous conglomerate on the summit of Mount Patti, through
the cementation of travelled material of a similar composition. Where
the summits are bare and lustrous it may be presumed, as in the case of
the ironstone platforms of Hausaland, that the surface soil, originally
overlying, has been removed in the course of denudation. At Awe, on the
middle Benue, a sheet of sintery calcareous material appears to have
been deposited upon the upturned edges of the Cretaceous before the
accumulation of the drift.
PLATE XV
[Illustration: CRETACEOUS SANDSTONES AND SHALES AT AWE.]
[Illustration: A FLAT-TOPPED HILLOCK OF DRIFT ABOVE NARAGUTA.]
Attention has been already directed (pp. 27, 77) to the occurrence in
Kabba, Nassarawa, and Southern Zaria of belts of felspathic muscovite
magnetite schists whose deeply decomposed crusts support a luxuriant
vegetation. Where partially eroded, it may be observed that the iron
ores have a tendency to accumulate in the subsoil, and it is possible
that if the overlying soil and vegetation were removed, the subsoil
might harden into a typical vesicular ironstone. It is difficult to
decide, however, to what extent the decomposed crusts may be considered
the result of weathering under present conditions. It seems probable in
view of the absence of any deep decomposition of the adjoining rocks
that they may to a large extent represent the irregular base of the
earlier sheet of decomposed rock to which the presence of Mounts Dala
and Kogon Dutsi bears witness upon the plains of Hausaland and which may
possibly have extended at the same time over the remainder of the
Protectorate. In some places, moreover, as at Kanna, there is evidence
that the crystalline floor had been locally and deeply decomposed as the
result of differential weathering underneath the Eocene sandstones. A
similar occurrence has been noted by M. Hubert[144] at Youri in the
Niger Valley.
Little consolidated accumulations of late Tertiary age, frequently
provided with a ferruginous cap like the drift of Northern Nigeria, have
been described under the designation of “Benin Sands” by Mr.
Parkinson[145] from Southern Nigeria, of “Ossa Sandstones” by Dr.
Esch[146] from the Kameruns, of “terre de barre” by M. Hubert[147] from
Dahomey, and of _reg_ from the Northern Sahara by numerous French
authorities.[148] It is yet premature, however, to attempt any
correlation of these deposits with the drift of Northern Nigeria.
[Footnote 102: Chudeau, _Sah. Soud._, 1909, p. 97.]
[Footnote 103: De Lapparent, _C. Rd. Ac. Sc._, 26 Dec., 1904.]
[Footnote 104: Chudeau, _op. cit._, p. 273.]
[Footnote 105: See Chamberlain and Salisbury’s _Geology_, 1905, p. 174.]
[Footnote 106: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 102.]
[Footnote 107: _Cf._ Lenz, _G. M._, 1879, p. 172.]
[Footnote 108: _Cf._ Passarge, Rumpffläche und Inselberge (Kordofan-
type), _Zeit. Deut. Geol. Gesell._, 56, 1904, p. 193.]
[Footnote 109: Andrews and Bailey, _Q. J. G. S._, Vol. LXVI, 1910, p.
228.]
[Footnote 110: Walther, _Das Gesetz der Wüstenbildung_, 1900.]
[Footnote 111: _Cf._ Chudeau, _Sah. Soud._, 1909, p. 285.]
[Footnote 112: Rohlfs evidently mistook the surface ironstones of Bauchi
and Zaria for rocks of sedimentary origin. _Pet. Mitt. Erg._, VII, 1872,
p. 64.]
[Footnote 113: Hubert, _Miss. Sci. au Dahomey._, p. 160, Plate VIII.]
[Footnote 114: Hubert, _Miss. Sci. au Dahomey_, 1908, pp. 109, 123.]
[Footnote 115: Passarge, _Adamaua_, 1895, p. 400.]
[Footnote 116: _Cf._ also Mennell, _G. M._, VI, 1909, p. 350.]
[Footnote 117: Chautard, _Le Fouta-Djallon_, 1905, p. 126.]
[Footnote 118: Hatch and Corstorphine, _The Geology of South Africa_,
2nd Ed., 1909, p. 330. _Cf._ also Hatch, _Mineral Resources of Natal_,
1910, p. 73; Rogers and Du Toit, _Geology of Cape Colony_, 1909, p.
390.]
[Footnote 119: Freydenberg, _Tchad et Chari_, 1908, p. 21.]
[Footnote 120: Lacoin, _Bull. Soc. Geol. Fr._, 1903, p. 484. _Cf._
Foureau, _Miss. Sah._, p. 674 _et seq._]
[Footnote 121: De Lapparent, _Traité de Géologie_, 4th Ed., p. 333.]
[Footnote 122: Walther, _Das Gesetz der Wüstenbildung_, 1900, p. 19.]
[Footnote 123: Chautard, _Le Fouta Djallon_, 1905, p. 137; _C. Rd. Ac.
Sc._, 146, p. 239.]
[Footnote 124: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 199.]
[Footnote 125: Holland, _Geol. Mag._, X, 1903, p. 59.]
[Footnote 126: Passarge, _Adamaua_, 1895, p. 396.]
[Footnote 127: Maufe, _The Geology of East Africa_, Col. Reps.
(Miscell.), No. 45, 1908, p. 53.]
[Footnote 128: Maclaren, _Geol. Mag._, 3, 1906, p. 536.]
[Footnote 129: Mennell, _Geol. Mag._, 6, 1909, p. 350.]
[Footnote 130: Campbell, _Tr. Inst. M.M._ 19, 1910, p. 432.]
[Footnote 131: Campbell, _op. cit._, p. 209.]
[Footnote 132: Maufe, _op. cit._, p. 208.]
[Footnote 133: _Cf._ also Mennell, _op. cit._, p. 209.]
[Footnote 134: Baikie, _A Journey from Bida to Kano_, 1866.]
[Footnote 135: _Cf._ Freydenberg, _Tchad et Chari_, 1908, p. 1.]
[Footnote 136: Courtet, _C. Rd. Ac. Sc._, 140, p. 163.]
[Footnote 137: _Cf._ Courtet, _C. Rd. Ac. Sc._, 140, p. 160; Garde, _C.
Rd. Ac. Sc._, 148, pp. 1616, 1698.]
[Footnote 138: Freydenberg, _Tchad et Chari_, 1908, p. 63.]
[Footnote 139: Garde, _C. Rd. Ac. Sc._, 148, pp. 1698, 1616.]
[Footnote 140: Chudeau, _Sah. Soud._, 1909, p. 232.]
[Footnote 141: Freydenberg, _Tchad et Chari_, 1908, p. 61.]
[Footnote 142: Chudeau, _Sah. Soud._, p. 84.]
[Footnote 143: Passarge, _Adamaua_, 1895, p. 400.]
[Footnote 144: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 368.]
[Footnote 145: Parkinson, _Q. J. G. S._, 63, 1907, p. 309.]
[Footnote 146: Esch, _Geol. von Kamerun_, 1904, p. 14.]
[Footnote 147: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 219.]
[Footnote 148: See Gautier, _Sahara Algerien_, 1908.]
CHAPTER VII
TERTIARY CRUSTAL MOVEMENTS AND DRAINAGE CHANGES
Post-Cretaceous erosion in the Sudan; the Middle Eocene depression;
western and eastern basins of sedimentation; crustal oscillations of the
Middle Tertiary period; later Tertiary oscillations; the first period of
depression; the first period of elevation; early drainage system of the
Protectorate; the second period of depression; lacustrine conditions in
Bornu; the second period of elevation; flexuring of the crust; α- and
β-flexures; differential elevation of the Nassarawa tableland and the
Bauchi plateau; deflection of the Gongola; depression of the Chad area.
Peculiarities of the watersheds and river valleys of the Protectorate:
explanation of the characteristic features of the valleys of the Niger,
Benue, and Gongola; the origin of inselberge; M. Hubert’s explanation of
the hydrography of Dahomey; the period of formation of the “dallols” of
Sokoto; correlation of the Tertiary crustal movements in the Sudan with
those of the Mediterranean area.
The close of the Cretaceous was marked by regional upheaval, accompanied
by fracturing and folding of the crust and by subsequent intense
erosion. The Cretaceous sandstones, shales, and limestones, which may be
presumed to have at one time covered the greater part of the
Protectorate, were largely removed from the underlying hummocky and
irregular surface of crystalline rocks. The plateau of southern Yola, as
described by Mr. Longbottom (p. 32), and the eastern cliffs of the
central tableland of Kabba (p. 16), would appear to have originated at
this period. The final character of the post-Cretaceous drainage system
and of the early Eocene land surface is, however, very imperfectly
known. Mr. Longbottom believes that there were earlier river valleys in
the place of the Upper Benue and Lower Gongola of the present day (p.
190), and there is some reason to believe that there was also an earlier
river in the present valley of the Lower Niger. At all events it is in
these localities that the greatest thickness of Eocene sediments is now
preserved, although there is little doubt that the later crustal
movements have also, to some extent, affected their present
distribution.
The movement of depression in the Middle Eocene which resulted in the
establishment of open sea in the north-west and in the deposition of the
marine beds of Sokoto province, brought about also the gradual
submergence of the land surface and the predominance of shallow water
conditions over the greater part of the Protectorate. In the earlier
river valleys the greatest thickness of sediments would naturally be
deposited, while only the ferruginous sandstones and ironstones of the
latest lacustrine phase would be deposited over the higher lying
portions of the early Eocene land surface. The formation of the lava
fields of Bauchi and Burra was probably contemporaneous with the latest
phase of sedimentation. There is some reason to believe, moreover, that
during the greater part of the period occupied by the movements of
depression, there were two major regions or basins of sedimentation, a
western and an eastern, separated by a stretch of higher ground which
extended from Zinder and the northern frontier of Hausaland to the
southern border of the Benue valley, and of which the Murchison Range
formed the only striking physical feature. The thinning out of the
Eocene sandstones to the east of the Niger valley, on the middle Benue,
and to the west of the Gongola, lends some support to this hypothesis.
The accumulations within the western basin, which was probably
continuous westward with the Atlantic, would then include the lacustrine
series of Southern Nigeria, the shallow water deposits of the middle
Niger, the marine limestones of Sokoto and the sandstones of Dahomey and
the Futa Djallon, while the Duguri, Gombe, and Kerri Kerri sandstones,
the Upper Benue sandstones and their prolongation into the northern
Kameruns, together with the ferruginous sandstones of the Upper Shari
and the Congo, would represent the accumulations within the eastern
basin. The outliers of Eocene upon the Kanna hills and the relics of
ferruginous sandstones upon the plains of Muri indicate, however, that
the western and eastern basins became finally united, and it seems
probable that towards the close of Eocene sedimentation, shallow water
and lacustrine conditions prevailed over the greater part of the
southern Sudan. Free communication may even have been established at
this time over Central Africa,[149] between the western seas of the
Upper Niger and the eastern seas of Lybia and Egypt. There is no reason
to believe, however, that the lower marine facies of Sokoto ever
extended eastward much beyond its present limits.
There can be little doubt that the close of the Eocene was marked by the
cessation of the predominant movement of depression and the beginning of
those crustal oscillations which culminated in late Pliocene or early
Pleistocene times in the formation of the Bauchi plateau and the
establishment of the present river system. No deposits of later
recognisable age than Eocene have been discovered within the
Protectorate and it may therefore be presumed that until the close of
the Miocene, when the sea is believed to have finally retired from the
Sahara (p. 194), the surface of the Protectorate remained near or at the
level of the sea. The predominant movement, however, was probably that
of elevation, with the result that the uppermost members of the Eocene
suffered some considerable erosion and were entirely removed from the
greater part of the surface of the central provinces. The later Tertiary
oscillations, which may be conveniently grouped as Pliocene, began with
a negative or downward movement, which has already been termed the
“first period of depression” (p. 213), and during which the crystalline
rocks of the central provinces were deeply decomposed and probably also
covered with a layer of drift. The “first period of elevation” which
followed was marked by a somewhat irregular upheaval of the surface of
the Protectorate. The maximum movement appears to have been attained in
the east, where the Eocene and underlying rocks were elevated in a low
extended arch whose axis formed the primary watershed and ran
approximately from Eastern Yola to Zinder and Air. As a result of the
erosion which followed the movement of elevation the greater part of the
sheet of decomposed rock formed during the first period of depression
was removed from the surface of the central plain. The crystalline areas
assumed very largely their present physical features and characteristic
scenery. The plains of Hausaland, however, were continuous with the
plains of Nassarawa and Muri across the site of the Bauchi plateau and
the Nassarawa tableland. The Murchison Range, the Wadai Hills, and the
Ningi and Kagoro massifs remained then as now relics of a former
erosion, and beyond acting as secondary watersheds exerted comparatively
little influence upon the general system of drainage. The relics of
higher beds occasionally found on the sandstone plateaux of the Niger
may, however, be referred to this period of erosion. The larger rivers
probably drained south-westward into Southern Nigeria and Dahomey across
the present valley of the lower Benue and the Niger. The gorges of the
Niger above Lokoja and between Jebba and Yelwa clearly indicate the much
more recent origin of the present river valley. In the east, however,
along the watershed the sedimentary rocks were deeply trenched and worn
on either side by the headwaters of the rivers. The valleys of the
middle Gongola and of the Upper Benue present when compared with the
Niger Valley all the characteristics of a more ancient topography. It
seems probable indeed that towards the close of erosion the rivers which
drained westward over the plains of Bauchi had already established the
escarpments of Kerri Kerri, the narrow plains of the middle Gongola, and
the cliffs of Waja and Tangale. An earlier river, moreover, flowed
westward from the hills of Yola with its valley limited as now by the
highlands of Wurkum and Waja, which for a time interposed a continuous
barrier between the rivers of Bauchi and the headwaters of the Benue.
The establishment of the gap at Kombo was probably accomplished by a
tributary of the Maio Haul towards the close of erosion. To the east of
the watershed the plains of eastern and central Bornu were likewise
established at this period by rivers which probably drained eastward and
southward over the site of Chad and the Shari valley into the Congo
basin.
There followed, however, the “second period of depression,” during which
the drift was accumulated upon the irregular surface of the plain. The
maximum movement would appear to have taken place in the east in the
region of the former elevation. It seems probable that not only was the
earlier watershed depressed to such an extent that the lateral valleys
in Bauchi and Bornu were filled with drift to a depth of 400 to 500
feet, but that lacustrine conditions prevailed farther east over the
plains of Bornu and Chad and the present valleys of the Logone and the
Shari. Another area of special depression was apparently situated in
southern Zaria and western Bauchi, where the remains of a covering of
drift 200 to 300 feet in thickness forms a series of flat-topped hills
on the highest portion of the plateau. The older Benue gravels, as
described by Passarge[150] and Edlinger,[151] were probably also
accumulated at this period. The drainage in the earlier stages may be
presumed to have been, in general, towards the areas of maximum
depression, while the close of the period was marked by the prevalence
of conditions of accumulation and of uncertain drainage over the whole
surface. The inland seas of the Upper Niger, as described by MM.
Chevalier,[152] Gautier and Chudeau,[153] should probably also be
referred to this period, but it should be remembered that, although they
may have been at times connected with the open sea to the west, there
is, as in the case of Lake Chad (p. 217), no reason to postulate any
kind of continuity between them and the earlier Miocene or Eocene
seas.[154]
Radical changes ensued upon the cessation of the movement of depression
and the return of a positive movement of elevation in late Pliocene or
early Pleistocene times. It should be remembered, however, that the
irregular upheaval of the surface during the “second period of
elevation” and the consequent changes of drainage were in no sense
catastrophic. On the contrary, the movements and changes were gradual,
and possibly to some extent successive and spread over a considerable
interval of time. The regional elevation of the Protectorate, with the
exception of the middle Benue valley, to an average height of 1,000 feet
was accompanied and complicated by a gentle flexuring of the crust into
low arches and shallow troughs along axes which ran approximately
W.S.W.–E.N.E. and N.N.W.–S.S.E. (see map, p. 236). The two sets of
flexures may be termed respectively the α-flexures and the β-flexures,
and it may be presumed that they were produced for the most part
simultaneously and only to a small extent successively by crustal
movements, which were themselves contemporaneously functional. The
irregular elevation and depression of the surface which gave rise to the
present system of drainage may therefore be looked upon as the result of
the interference of the two sets of flexures. Two axial arches of the
α-series, a northern (α₁) and a southern (α₂), are readily
distinguishable within the Protectorate, the southern being the more
important and of the greater elevation. The northern (α₁) runs from
Borgu over the high plains of Kontagora, Sokoto, and Katsina towards
Machina and Zinder, and forms the watershed between the Gulbin Kebbi and
its tributaries on the one hand, and the Kaduna and the Yo on the other.
These rivers may be presumed to occupy the shallow troughs on either
side of the axial ridge. The axis of the southern arch (α₂) runs from
central Kabba through Nassarawa and southern Bauchi into northern Yola,
and the Benue may be presumed to occupy the trough to the south. The two
α-arches are not strictly parallel, but diverge somewhat towards the
east. Of the β-flexures two axial arches are also distinguishable within
the Protectorate, a central (β₁) and a western (β₂), the latter being
the more important of the two. The central arch (β₁) is of a very
partial character, and is recognisable only in the sinuous watershed
which runs northward between the two α-arches from the summit of the
plateau towards the Sokoto-Kano frontier and separates the headwaters of
the Kaduna from those of the Yo. The western arch (β₂) forms, in part,
the western watershed of the Niger, and runs northward from the high
plains to the south of Ilorin approximately along the frontier of the
Protectorate to the neighbourhood of Nikki in Dahomey.
The arches throughout are asymmetrical in character, and vary somewhat
in elevation from point to point. The sinuous character of the crests is
probably due to the mutual influence of the complementary forces which
produced the flexuring at right angles. The intensity of the flexuring
appears also to have varied, and in the case of two intersecting
flexures one or other usually predominates. In the central region, for
example, the α-arches clearly predominate at their intersection with the
β-arch (β₁), while in the west the β-arch (β₂) has evidently been the
determining factor in the establishment of the present course of the
Niger. The interference of the β-arch (β₂) with the western extension of
the α-flexures is clearly reflected in the repeated bending of the
river. Where the α-arches were crossed by the β-arch, dome-shaped
elevations of a regional character were formed, whose effect may still
be traced in the westwardly concave curves which the Niger describes at
Yelwa and above Lokoja. Where the intervening α-trough, however, was
crossed by the β-arch, the resulting curve was concave eastward as at
Jebba. The asymmetrical position of the Niger valley indicates a shorter
radius for the western portion of the β-arch, while the fact that the
river was able to pursue a continuous course across the α-arches and
troughs makes it sufficiently obvious that the β-arch (β₂) must have
reached originally a somewhat greater elevation than the western portion
of the α-arches. The influence of the formation of the western β-trough,
now occupied by the Niger valley, may also be traced in the southward
deflection of the lower courses of the Kebbi, Kaduna and Gurara. It is
obvious also from the height of the sandstone plateaux above Lokoja that
the greatest and perhaps most prolonged movement of elevation in the
Niger valley took place in the region of the confluence of the Niger and
the Benue. This movement, moreover, was evidently sufficiently slow to
allow of the Niger cutting a continuous trench across the rising arch, a
result, however, which was rendered possible only by the presence of
sedimentary rocks in the river bed. There is no reason to believe with
Gurich[155] that, during the formation of their trench-like valleys in
the region of the confluence, the middle courses of the Niger and the
Benue were ever converted into a series of detached basins or inland
seas. It should be remembered also that the relative elevation of the
arches was for the most part insufficient to produce any marked
divergence from the apparent horizontality of the sandstones in the
Niger valley.
[Illustration: NORTHERN NIGERIA
Axes of Elevation.
MAP No. 3.
London. Macmillan & Co., Ltd.
_Face p._ 236.]
The symmetrical elevation of the southern α-arch and the southward
prolongation of the central β-arch were modified and interrupted by the
formation of lines of weakness longitudinally along the former and in
the region of the intersection of the two arches. The relative influence
of the forces which produced the flexures can be traced in the varying
direction and sinuous character of the lines of weakness. Along these
lines differential movements took place which resulted in the
disproportionate elevation of the Nassarawa tableland and of the Bauchi
plateau with reference to the middle Benue valley and the northern
plains of Bauchi. On the north the fracturing of the crust, as already
described (p. 39), was confined only to the central region, while to the
west and east the tableland and the plateau merged respectively into the
general slope of the arch. On the south, however, a continuous line of
fracture extended from western Nassarawa by way of Darroro and Assab to
Sura and Angass in southern Bauchi. There is no reason to believe that
the southern α-arch as a whole ever reached the elevation of the Bauchi
plateau or that the plateau originated in any sense as a “horst,”[156]
separated from the adjoining tableland and the plains of Nassarawa and
Muri by the later differential subsidence of the latter. On the
contrary, the movement appears to have been in one direction only, and
characterised by the differential elevation of the plateau and the
tableland and the lagging behind of the lower plains of Nassarawa, Muri
and northern Bauchi. The trough of the Benue above the confluence of the
Niger affords clear evidence indeed both of the comparatively stationary
character of the middle Benue valley and of the gradual relative
elevation of the Lokoja sandstones in the region of the confluence.
In the east the prolongation of the northern α-arch through Manga and
the region to the north of Chad extends beyond the limits of the
Protectorate. The southern arch, on the other hand, is prolonged as a
simple flexure of varying elevation through eastern Bauchi into Yola
province. In the trough between the northern and southern arches, the
river Yo occupies the same relative position to the east of the central
β-arch as the Kaduna does to the west. The presence in the east,
however, of the former watershed of the first period of elevation has
introduced complications into the otherwise simple relations of the
α-axes and troughs. It may be presumed that on the formation of the
southern α-axis and the differential elevation of the Bauchi plateau,
the river Gongola, like the Shidya and the Delime, would have followed a
general north-easterly course and ultimately joined the Yo but for the
intervention of the higher ground along the earlier watershed which
diverted the river to the south. It should be remembered that the whole
of this region had been deeply trenched and eroded during the first
period of elevation (p. 231) and that the earlier valleys had been
largely filled with drift during the second period of depression. It
seems probable, therefore, that on the subsequent elevation of the
southern α-arch the Gongola on leaving the high plains of Bauchi was
able to make its way for long distances through the earlier river
valleys and stream courses and ultimately to escape into the Benue
valley through the gap at Kombo. The trench of the Gongola above Gombe
may possibly be to some extent the product of recent erosion, but there
is little doubt that the plain of the Gongola below Nafada was excavated
at an earlier period and is in no way to be regarded as the result of
the activity of the present river, whose energy has been almost entirely
directed to the removal of the drift, which formerly obscured the
hummocky surface of the plain.
The prolongation of the southern α-axis into eastern Bauchi appears to
have been characterised by the formation of a flexure of a flattened
monoclinal type, which resulted in the elevation of the plains of
eastern Bauchi and the middle Gongola to a height of 500 feet above the
valley of the Benue. The actual summit of the flexure is probably
situated in the vicinity of the rapids of Kombo. The hydrography of
northern Yola is very imperfectly known and complicated by the presence
of the earlier watershed. It is probable, however, that the present
watershed between the Benue and Lake Chad on the eastern frontier
corresponds approximately with the prolongation of the α-axis, which
here resumes its normal character of an asymmetrical arch. The last
trace of the flexure may be recognised to the east of the Mandara hills
in the swampy watershed between the Benue and the Logone. The
configuration of southern Yola suggests an ancient topography, and its
physical character is probably very similar to that which prevailed at
the close of the first period of elevation. The Chad area to the north
remained a region of relative depression during all the later flexures,
and it seems probable that accumulation continued and the escape
southward of the surplus waters remained possible until the final
elevation of the Congo watershed along a parallel flexure to the south.
The contemporaneous formation of the Bodele depression, however, brought
about the establishment of the effluent of Chad through the Bahr el
Ghazal and the partial erosion of the drift and lake deposits of Bornu.
The subsequent advent of arid conditions during the period immediately
preceding the present resulted, as already indicated (p. 219), in the
diminution of the water supply, the closure of the Bahr el Ghazal, and
the restriction of the lake to its present limits.
This attempted reconstruction of the more recent hydrographical changes
affords an explanation of many of the more striking peculiarities in the
physical geography of the Protectorate. Attention has already been
directed to the fact that the larger tributaries of the Niger and the
Benue offer throughout their courses a reversal of the normal sequence
of mountain tract, valley tract and plain tract. The Kebbi, the Kaduna,
and the Gongola rise upon lofty plains and flow for long distances in
broad sandy channels bounded by imperceptible slopes until they enter,
in their lower courses, well-defined trench-like valleys bounded on
either hand by steep walls of rock. The watersheds are marked by no
ranges of jagged hills. On the contrary, the lofty plains on which the
rivers rise are frequently more deeply buried in drift than the flood
plains of their lower courses. The rivers flow peacefully around and
between the lofty granite domes which rise abruptly from the upper
plains, and even such larger groups of hills as those of Ningi form
merely local centres of dispersion for the smaller streams. Moreover,
the hydrographical centre of the Protectorate is not a rugged mountain
mass, but an open treeless plain, a plateau 4,000 feet in height,
bounded by steep cliffs on the south and west and provided with worn and
flattened hillocks of drifted alluvium perched in places upon the very
margin of the cliffs. A complete reorganisation and rejuvenation of the
drainage system is clearly indicated, and it is possible only in some
such manner as that already described. The crust has been flexured and
fractured during a recent regional elevation, and from the crests of the
arches the consequent streams have been thrown off to pick a course for
themselves over the drift-covered plains and between the solitary hills
and finally to unite to form the major rivers in the bottoms of the
shallow troughs. Dr. Passarge[157] has frequently called attention to
the youthful age of the rivers of Africa, and his suggestion would
appear to be amply confirmed by the whole character of the present
river-system of Nigeria.
The sinuous course of the Niger within the Protectorate still represents
in a general way the original adaptation of the drainage system to the
tectonic conditions. The diversity of scenery in the present valley is,
however, largely due to the influence of the lithological character of
the underlying rocks. There is little doubt that at first the Niger
flowed throughout upon a sandstone surface, relics of which now lie from
500–1,000 feet above its present level, and that it eroded its channel
originally as a continuous, deep and narrow trench. Complications first
ensued when the river succeeded in cutting through the overlying
sandstones between Yelwa and Jebba, and in reaching the crystalline
rocks below. The greater thickness of sandstones below Jebba conditioned
the cutting back of the gorge between Jebba and Yelwa with which the
river is still occupied. The resistant character of the crystalline
rocks brought about the formation of falls and rapids, while the
consequent lateral erosion at the head of the gorge gave rise to the
broad sandy plains between Yelwa and Ilo. Below Jebba the prolonged
elevation of the southern α-arch brought about the continued trenching
of the sandstones above Lokoja and the lateral erosion of the banks in
Nupe and Ilorin, where the level of the river remained stationary for
considerable intervals of time. The irregular degradation of the channel
of the Niger conditioned also the formation of the broad, flat-bottomed
valleys (dallols) of Sokoto and the narrow canyon valleys of southern
Kontagora, the dissection of the sandstone plateaux of Nupe, the removal
of the Eocene rocks from northern Kabba and the cutting of the lower
gorge of the Gurara. Everything, indeed, points to the recent erosion of
the Niger valley, while the rapids and falls of the Kaduna and the
Gurara beyond the limit of the sandstones and the ill-defined valleys of
their upper and middle courses clearly indicate that they have only just
begun to cut their way backwards and downwards into the central
crystalline plains.
Compared with the Niger valley, the plains of the middle and upper Benue
present all the characteristics of a more ancient topography, and it is
probable that they retain to a great extent the configuration which they
possessed at the close of the first period of erosion. Only in its lower
course has the Benue cut a trench in the Eocene sandstones at all
comparable with the gorge of the Niger above Lokoja, and this has
probably been accomplished, as already explained (p. 238), during the
slow differential elevation of the region of the confluence. There seems
reason to believe, also, that during the earlier stages of the more
recent erosion the Benue was provided with a greater volume of water
than the Niger, and that to this influence is to be ascribed the greater
imperfection of the trench of the lower Benue and the removal of the
sandstones from the hummocky crystalline plain of eastern Kabba. This
increase of volume, however, was probably only relative and
contemporaneous with the diminution of the Niger during the recent arid
period in the Sudan. The return of more humid conditions, however,
together with the capture of the upper Niger by a tributary of the
Taffassasset, as described by M. Chudeau,[158] restored the activity of
the Niger and enabled it to cut its present channel in the surface of
the crystalline plains above and below Lokoja.
The northern tributaries of the Benue, with the exception of the
Gongola, take their rise on the southern border of the Nassarawa
tableland and the Bauchi plateau. Their headwaters alone, however,
exhibit evidence of any recent intense erosive activity. Behind Jagindi
and in the neighbourhood of Assab the headwaters of the Modu river have
trenched and worn the margin of the plateau. The Wase river has laid
bare in Yergum and Angass the hummocky and irregular slopes which
separate the upper from the lower plains. The headwaters of the Kudu
river have cut picturesque steep-sided valleys in the rocks of Duguri,
and narrow canyon-like trenches in the central plains of Bauchi. In the
Gongola valley, on the other hand, the only traces of recent erosion are
to be found in the rapids of Baddera and Kombo and, in part, in the
imperfect trench which the river has cut in the sandstones between Gombe
and Deu. From its source on the plateau until it enters the plains to
the east of Gombe, the topography of the river valley recalls in every
detail the characteristic features of the valleys of the Kaduna and the
Gurara. Nevertheless, although of similar age and origin, the Gongola
flows from Nafada southward through a valley which presents all the
features of a more intense erosion than the Kaduna or the Gurara or even
the Niger itself has been able to accomplish in its lower course. It is
difficult to escape the conclusion that upon the rearrangement of the
drainage system and the establishment of the present upper Gongola, the
river entered to the east of Gombe, an older river valley which had been
excavated during some former period of erosion. The interlacing valleys
of the margin of the Kerri Kerri plateau may possibly have arisen during
the latest period of erosion, but if so, the streams which excavated
them have never regained the activity which they possessed before the
last appearance of the desert in the Sudan.
PLATE XVI
[Illustration: UPPER BENUE SANDSTONES AND CONGLOMERATES UPON A HUMMOCKY
GRANITIC FLOOR.]
The isolated domes and kopjes (Plates III., XII.) and detached groups of
hills which form the most striking feature of the higher plains are
characteristic also of all the older gneissic areas of Africa, and the
question of their origin has given rise to considerable difference of
opinion. The particular type of scenery to which they give rise has been
termed by German writers the “Inselberglandschaft” (Plate XVII., Figs.
1, 2). Passarge[159] sees in them the product of arid conditions, and
concludes that the surfaces which they characterise assumed their
present character for the most part in early Mesozoic times when the
greater part of Africa was a barren desert. Bornhardt,[160] on the other
hand, believes that the inselberge have arisen as the result of the
superposition of several cycles of erosion. There is, however, no reason
to believe that this peculiar topography cannot arise simply through an
alternation of periods of weathering and periods of erosion, brought
about either by a gentle oscillation of the crust or by a repeated base
levelling of the plains and rejuvenation of the drainage system. A plane
surface of granite and gneiss subjected to long-continued weathering at
base level would be decomposed to unequal depths, mainly according to
the composition and texture of the various rocks. When elevation and
erosion ensued, the weathered crust would be removed, and an irregular
surface would be produced from which the more resistant rocks would
project. Those rocks which had offered the greatest resistance to
chemical weathering beneath the surface would upon exposure naturally
assume that configuration of surface which afforded the least scope for
the activity of the agents of denudation. In this way would arise the
characteristic domes and turtle-backs which suffer further denudation
only through insolation and exfoliation.[161] Their general elliptical
outlines, which Merrill[162] would ascribe very largely to the influence
of crustal stress and strain, are probably in great part due simply to
the modification by weathering of original phacolitic intrusions. A
slight subsequent movement of depression would suffice to cover with
drift large areas of the irregular surface between the domes and to
produce the characteristic topography of isolated hills rising abruptly
from the level surface of the plain (Passarge’s Kordofan Type). That
such indeed has been the mode of formation of the kopjes of Hausaland
can be demonstrated within the walls of Kano. Kogon Dutsi, the larger of
the two flat-topped hills of diorite, although deeply decomposed, still
preserves in its lower part detached boulders or cores of unweathered
rock. If the subsequent erosion had continued until the weathered
material had been entirely removed, the flattened hill would have been
replaced by a typical kopje of loose boulders resting upon a smooth and
rounded surface of rock below. On the plains around ample evidence can
be found of the levelling of the surface by means of the accumulation of
drift. A repetition of these conditions of formation would give rise to
an accentuation of the earlier domes and kopjes and the formation of
others of lower level. A similar origin to that of the domes may be
ascribed also to the detached groups of hills and even to the larger
granite massifs like those of the Ningi, Ruruma, Kagoro, and Vere Hills.
There is no reason to believe with Passarge[163] that the latter partake
in any way of the nature of “horsts,” which have assumed their present
position through the differential subsidence of the surrounding plains.
Their flattened though hummocky summits evidently bear witness to an
earlier regional level of erosion, but their characteristic mode of
occurrence is to be ascribed not to marginal faulting, but to a
succession of periods of decomposition and accumulation and periods of
erosion which has brought about the relative lowering of the surrounding
and less resistant gneisses and schists.
The pre-Cretaceous land surface, as exposed around Tangalto, was marked
like the present with domes and inselberge. The pre-Eocene surface was
somewhat broken and irregular, but probably also characterised, as in
the Niger valley below Lokoja, by detached and solitary hills. There is
no reason, however, to suppose that the actual pre-Cretaceous
configuration of the plains of Hausaland is still retained. It is much
more probable that domes and solitary hills have been formed in the
manner indicated at many different periods, and are a characteristic
product of sub-aerial tropical weathering of a crystalline surface when
exposed to oscillations of level.
Dr. Passarge,[164] in explanation of the structure of Adamawa, has
assumed the existence of two tectonic directions, an E.–W. or Benue
line, and a S.S.W.–N.N.E. or Kamerun line, along both of which extensive
faulting has taken place at various periods and in such a manner as to
leave a numerous series of isolated hills and ranges of hills standing
out as horsts upon the lower plains. According to him, also, the Bagele
and other hills of Yola have assumed their present position through the
subsidence of the surrounding plains, while the Benue valley between
Yola and Garua is of tectonic origin, and bounded by faulted margins to
the north and south. While, however, there is some reason to believe
that the Cretaceous rocks of Yola have been faulted down against the
crystallines to the north and south, and that in pre-Eocene times an
earlier river occupied a tectonic valley in the region of the upper
Benue (p. 190), there is no evidence to show that there has been a
recurrence of faulting on the upper Benue in post-Eocene times, or that
the slopes of any of the hills of “Upper Benue Sandstone” have been
defined by later Tertiary or recent movements. Passarge’s hypothesis is
applicable only to pre-Eocene topography, and cannot be extended in
explanation of those features of the upper Benue valley which are of
more recent origin.
M. Hubert[165] has suggested in explanation of the hydrography of
Dahomey that the courses of the present streams bear a direct
relationship to the strike of the gneisses and schists, and that the
surface of the colony has remained stationary since the close of the
Eocene. His hypothesis, however, is admittedly inadequate to explain the
transverse character of the Niger valley. In Nigeria, moreover, all the
principal rivers without exception flow across the strike of the
gneisses, while the drift-covered character of the lofty plains and of
the watersheds and the nature of the whole hydrographical system clearly
indicate the recent elevation of the present surface of the
Protectorate. It is very probable indeed that Dahomey has participated
in all the recent crustal movements which have affected Nigeria, and
that the central watershed of the colony corresponds with the westward
prolongation of the northern α-arch and the western β-arch of the
Protectorate. A recent rejuvenation of the drainage system is
postulated, moreover, by M. Chudeau[166] in the French Sudan, by Mr.
Parkinson[167] in Southern Nigeria, and by Dr. Esch[168] in the
Kameruns, and it is improbable that Dahomey should have escaped the
general movement of elevation.
M. Chudeau[169] has recognised in the Sudan the recent period of arid
conditions of which traces are still to be found within Northern
Nigeria. He believes, however, that the dallols were excavated during
the period of desert conditions by rivers which drew their supplies from
the mountains of Air. It is possible that there may have been repeated
alternations of desert and humid conditions within the latest period of
erosion; but it is clear in Nigeria that the dallols of Sokoto were
already excavated before the advent of the latest arid period, and that
the rivers which formed them have not regained the activity which
formerly characterised them. Capt. Meynier’s suggestion, quoted by M.
Chudeau, that the rivers of Hausaland turned their backs to the desert,
though picturesque, is little in harmony with the facts. The courses
they follow within the Protectorate were defined long before the advent
of the desert, and not by any adaptation to climatic conditions, but by
the direct influence of the early Pleistocene crustal movements which
brought about the rejuvenation of the drainage system of Nigeria and the
formation of the Bauchi plateau. These movements were probably also
responsible for the establishment of the dallols of Tahoua and the
quaternary wadis of the northern Sahara, as well as for the elevation of
the plateaux of Air and Adr’ar’ des Ifor’as. These movements, moreover,
were probably contemporaneous with the later Tertiary movements in the
Mediterranean basin and in East Africa.
M. Chudeau[170] has pointed out the analogy between the pre-Devonian and
the post-Carboniferous movements to the south of the Atlas and the
Caledonian and hercynian movements to the north of the Alps. In Nigeria
and the Sudan some analogy may perhaps be permitted between the general
meridional strike and vertical folding of the gneisses and schists and
the Caledonian folding of Scotland and Scandinavia. Of hercynian
movements, however, there is little or no trace in the Sudan, while the
presence of an unconformability at the base of the Eocene and the
absence of any later folding makes the post-Cretaceous and early
Tertiary movements in the Sudan more nearly parallel with the sequence
of events in northern Europe than with the sequence in the Alps and the
Atlas. The period of maximum movement in the Alps would appear to be
represented in Nigeria and the Sudan only by minor post-Eocene
oscillations of the crust, while the major movements in the Sudan which
gave birth to the present drainage system appear to be contemporaneous
with the later Mediterranean stages of the Alpine movements.[171] At the
same time, however, it should be emphasised that, while the movements
may have been contemporaneous, there was probably little or no genetic
connection between them. The Tertiary period was everywhere a period of
great instability of the crust, and the movements would appear to have
varied both in intensity and in kind, and to have been localised now in
one place and now in another, at different periods of their history.
[Footnote 149: _Cf._ De Lapparent, _C. Rd. Ac. Sc._, 136, p. 1118.]
[Footnote 150: Passarge, _Adamaua_, 1895, p. 385.]
[Footnote 151: Edlinger, _Niger-Tsad Expedition_, 1904, p. 157.]
[Footnote 152: Chevalier, _C. Rd. Ac. Sc._, 15 April, 1901, p. 926.]
[Footnote 153: Gautier and Chudeau, _Sah. Soud._, 1909.]
[Footnote 154: _Cf._ Suess, _The Face of the Earth_, Vol. IV., Oxf.
Trans., p. 92.]
[Footnote 155: Gurich, _Zeit. Deut. Geol. Gesell._, xxxix, 1887, p.
126.]
[Footnote 156: _Cf._ Passarge, _Adamaua_, 1895, p. 377.]
[Footnote 157: Passarge, _Die Kalahari_, 1904, p. 637.]
[Footnote 158: Chudeau, _Sah. Soud._, 1909, p. 222.]
[Footnote 159: Passarge, _Rumpffläche und Inselberge_, _Z. D. G. G._,
56, 1904, p. 193.]
[Footnote 160: Bornhardt, _Deutsch Ost Afrika_, Bd. VII, 1900, p. 37.]
[Footnote 161: _Cf._ Harrison, J. B., _Handbook of British Guiana_, p.
87.]
[Footnote 162: Merrill, _Rocks and Soils_, 1897, p. 245.]
[Footnote 163: Passarge, _Adamaua_, 1895, p. 377. _Cf._ also Suess, _The
Face of the Earth_, Vol. IV, Oxford Trans., 1909, p. 283.]
[Footnote 164: Passarge, _Adamaua_, 1895, p. 387. See also Suess, _The
Face of the Earth_, Vol. IV, 1909, p. 280.]
[Footnote 165: Hubert, _Miss. Sc. au Dahomey_, 1908, p. 443.]
[Footnote 166: Chudeau, _Sah. Soud._, 1909, p. 220.]
[Footnote 167: Parkinson, _G. J._, XXIX., 1907, p. 56.]
[Footnote 168: Esch, _Geologie von Kamerun_, 1904.]
[Footnote 169: Chudeau, _Sah. Soud._, 1909, p. 220.]
[Footnote 170: Chudeau, _Sah. Soud._, 1909, p. 1.]
[Footnote 171: Suess, _The Face of the Earth_, Vol. I., Oxf. Trans.,
1904, p. 279.]
CHAPTER VIII
TERTIARY VOLCANIC ACTION
Absence of pre-Tertiary volcanic rocks within the Protectorate: two
periods of Tertiary volcanic activity. The early Tertiary period: the
Maradu vent; the nepheline basalts of Wase and Tangalto; the phonolites
of Wase Rock, Tangale Peak and Bantaji; the plateau lavas of southern
Bornu, Bauchi and Nassarawa; petrographical character of the lavas;
early Tertiary lavas of the Sahara, Sudan and Adamawa. The later
Tertiary volcanic period: the puys of northern Yola, the middle Benue
valley and the Bauchi plateau; petrographical character of the lavas;
petrographical provinces.
It may be presumed that the intrusion of the alkali granites and dyke
rocks throughout the Protectorate was accompanied in pre-Cretaceous
times by considerable superficial volcanic activity (p. 142). M.
Chautard[172] has, moreover, described the intercalation of volcanic
rocks in the Upper Cretaceous of Senegal. All trace of pre-Tertiary
effusive rocks, however, within the Protectorate appears to have been
removed in the course of denudation. The fracturing and folding of the
crust at the close of the Cretaceous was apparently accompanied in
places by the formation of mineral veins within the fissures, but no
contemporaneous igneous activity has been definitely identified within
the Protectorate (p. 140). Within the Tertiary era itself two separate
periods of volcanic activity may be recognised, one in the middle Eocene
and probably to some extent contemporaneous with the accumulation of the
Eocene sandstones within the Protectorate, and one in late Pliocene or
early Pleistocene times and contemporaneous with the later Tertiary
oscillations of the crust. For the most part the eruptions were of an
isolated character and gave rise to the formation of detached pipes,
cones and puys of igneous material. More rarely, as in southern Bornu
and on the borders of Bauchi and Nassarawa, the extruded lavas united to
form a continuous sheet of volcanic rock, which covered a considerable
area of the underlying sedimentary and crystalline rocks.
The earliest outburst of volcanic activity appears to have taken place
towards the close of the post-Cretaceous erosion or possibly in the
middle Eocene at a period contemporaneous with the accumulation of the
lower marine beds of Sokoto, and is represented now by the extensive
neck or vent of fragmental material near Maradu in Sokoto province whose
levelling was accomplished during the later extension of the middle
Eocene sea. The greater number of the boulders in the agglomerate are
composed of a microgranitic quartz porphyry with phenocrysts of
orthoclase, biotite and brown hornblende in addition to quartz (1323).
To the same period is probably also to be referred the line of well-worn
stumps of basalt, phonolite and trachyte which stretches from the
Gongola valley westward through Tangale and Angass to Namu in northern
Muri. The Tangale Peak (Pl. XIII, Fig. 2) and the Wase Rock (Pl. V, Fig.
2), which still retain a portion of the original flow attached to the
rock in the vent, were probably among the latest eruptions of the
series. The intrusive dolerites of Bashar (41) and Awe (1014) may be
ascribed to the same phase of igneous activity, while the Rock of
Bantaji in Southern Muri may be taken to represent contemporaneous
volcanic action to the south of the Benue.
In petrographical character the basaltic pipe rocks are always olivine-
bearing and usually porphyritic with olivine and augite. Felspar is
found only in the ground-mass which at times contains a little
intersertal glassy base. The basalt of Wase Topa is remarkable for the
number and size of the nodules of olivine which it encloses. A worn vent
immediately south of the town of Wase is filled with a fine-grained
nepheline basalt (32) containing glomeroporphyritic aggregates of augite
crystals and well-formed crystals of olivine. Felspar is entirely
wanting, the ground-mass being composed of small rods of augite and
granules of magnetite set in a brown glassy base studded with minute
crystals of nepheline. A somewhat similar but coarser-grained rock from
Tangalto (1661) contains large crystals of decomposed olivine and purple
and well-zoned augite in a holocrystalline ground-mass of rods and
granules of augite, magnetite grains, flakes of biotite and abundant
nepheline in well-formed and irregular crystals. The nepheline was
evidently the last to crystallise, and to some extent takes the place of
matrix, being in part moulded upon the other constituents of the ground-
mass.
The phonolite of the Wase Rock (31) is of a trachytoid type, containing
columnar phenocrysts of sanidine, ragged crystals of ægirine augite and
scattered pseudomorphs after nepheline in a ground-mass consisting
predominantly of minute felspar prisms with a well-marked fluxional
arrangement, mingled with magnetite grains and rods and granules of
green pyroxene. The phonolite of Tangale Peak (1665–6) contains
phenocrysts of sanidine and soda pyroxene in a ground-mass of a more
orthophyric type, composed of minute idiomorphic crystals of nepheline,
imperfect prisms of ægirine and ægirine augite and elongated microlites
of felspar which pass into an ill-defined felspathic matrix. Traces of
brown hornblende in irregular porous growths may also be observed in the
ground-mass. A similar rock from the immediate neighbourhood of Tangalto
(1663) contains soda pyroxene, but no evident nepheline in the ground-
mass, and may be grouped provisionally as a trachyte. The phonolite of
Bantaji Rock is of a more nephelinitoid type, with the ground-mass
enclosed in a somewhat intersertal manner between the abundant but much
decomposed phenocrysts of felspar. With the felspar are associated as
phenocrysts hypidiomorphic crystals of ægirine and ægirine augite
frequently in zonal growths and smaller idiomorphic and abundant
crystals of somewhat weathered nepheline. The ground-mass is composed of
lath-shaped felspars, granular pyroxene and irregular patches of
analcime probably of secondary origin. The association of phonolite and
nepheline basalt, observed in the neighbourhood of Wase and the Peak,
has not been recognised around Bantaji.
The period of formation of the lava fields in southern Bornu and on the
borders of Bauchi and Nassarawa may be provisionally referred to the
close of the middle Eocene, on the strength of the apparent
intercalation of a friable red sandstone amongst the lavas in the
escarpment of the Barbur plateau behind Dukshi. The extensive erosion
which the plateau lavas themselves have undergone indicates, moreover,
that they are of no very recent age. The eastern lava field is the more
extensive of the two and perhaps somewhat younger, so far as one can
judge from the less denuded character of the cones. The rich content in
iron of the upper part of the Eocene accumulations may possibly have
been conditioned to a considerable extent by the distribution of much
fragmental basic igneous material upon the surface of the waters.
In petrographical character the lavas are basaltic throughout and rich
in olivine, and vary from felspathic doleritic types to limburgitic
varieties in which felspar is greatly reduced in amount.[173] Nepheline-
bearing types have nowhere been recognised, nor any effusive rocks more
acid than basalt. MM. Chudeau and Gentil[174] have, however, indicated
the occurrence of ægirine rhyolites and phonolites, as well as of
basalts, tephrites and limburgites, amongst the older lavas of Ahaggar
and Air, which are probably of similar age to those of Nigeria. Dr.
Passarge[175] has described the occurrence of basalt, augite andesite
and phonolite on the summit of the Shebshi hills on the borders of
Nigeria and the Kameruns. MM. Gentil and Freydenberg[176] believe that
the ægirine porphyries of Mounio and of Hadj el Hamis on the southern
shore of Chad are of volcanic origin. M. Garde,[177] however, is of
opinion that they possess an intrusive character and belong to the
hypabyssal phase of the alkaline magma which gave rise to the Mounio and
Melfi granites.
The plutonic rocks of this early Tertiary volcanic period are nowhere
exposed within the Protectorate, and hypabyssal types are almost
unknown. The doleritic sills of Awe and Bashar were probably injected
during one of the earlier phases of igneous activity. Dr. Passarge[178]
has indicated the occurrence of basaltic dykes in the sandstones of
Adamawa and a boss of nepheline syenite between Yola and Garua on the
upper Benue. The latter, however, more probably belongs to the
underlying crystalline floor. M. Chudeau[179] believes that the alkali
granites of Zinder and Mounio have been intruded since the close of the
Cretaceous; but while there is every reason to believe that the
nepheline basalts and ægirine rhyolites of Tertiary age drew their
origin from a magma rich in soda, there is, as already stated (p. 140),
no evidence within the Protectorate upon which any immediate connection
between the younger soda granites and the Tertiary volcanic rocks can be
established. On the contrary, there would appear to have been in the
middle Eocene within the Protectorate merely a renewal of volcanic
activity from the same alkaline source from which the pre-Cretaceous
granites and dyke rocks originated. If, however, further investigation
should confirm M. Chudeau’s suggestion of the Tertiary age of the
granites and porphyries of Zinder, Mounio and Goure, these rocks may
then be taken to represent the plutonic and hypabyssal phases of this
middle Eocene period of igneous activity.
To the later Tertiary period of volcanic activity, which may be referred
to the close of the Pliocene and which perhaps also extended into the
early Pleistocene, are to be assigned the puys of Kereng (Plate X, Fig.
2) on the southern margin of the Bauchi plateau, the puys of Song and
Mboi (Plates VI, Fig. 2; VII, Figs. 1, 2) in northern Yola, and somewhat
doubtfully also those of the Awe district on the middle Benue (Plate V,
Fig. 1). The puys of the Kereng group are remarkably perfect and exhibit
steep crater walls of fragmental material breached by a single short
flow of olivine basalt. Elsewhere on the plateau also, as between Vom
and Bukuru, there are small puys with breached craters which are
probably of a similar recent age. From the cliffs of Assab, moreover,
later flows of lava may in places be seen on the worn slopes of the
plateau edge, and there is every reason to believe that the erosion of
the drift and of the margin of the plateau was considerably advanced
before the final close of volcanic activity. The presence of craters and
the local character of the associated flows serve to distinguish these
later products of igneous activity from the earlier lavas of the
plateau. A similar superposition of later volcanic material upon earlier
lavas has been observed by M. Chudeau[180] in the Ahaggar and in Air.
Unlike the central Sahara, however, there is in Nigeria no reason to
believe that there has been any actual continuity in volcanic action
between the earlier and the later periods.
In Yola province the Mboi Hills are crossed by a line of small craters
four or five miles long, running roughly in a N.W.–S.E. direction. To
the south-east between the Mboi Hills and Song the line is continued by
a number of hillocks of basalt for a few more miles, but how far it
extends to the N.W. is not known. Only a relatively small amount of
basalt was erupted from these vents. In the case of the largest crater
with a crater floor of 250 yards in diameter the volcanic material was
not sufficient to conceal the granite through which it came. The granite
was broken away during the eruption so as to form a cliff extending one-
third round the crater, while the other two-thirds is formed of
agglomerate and tuff with very many fragments of granite. The crater
thus resembles a swallow’s nest attached to its support. With one
exception, where the wall is entire, all the craters were breached by a
lava flow. They are in excellent preservation, and the cliff-like walls,
with insignificant talus heaps at their bases, indicate a very recent
origin. (A. L.)
PLATE XVIII
[Illustration: THE SALT-FIELDS OF AWE; VOLCANIC HILLS IN THE
BACKGROUND.]
[Illustration: AKIRI SALTINGS.]
The vents on the plain to the south-east are not marked by craters or by
fragmental materials, but by mounds of basalt less than fifty feet high
spreading out into lava flows, which cover most of the surface near by.
Here the eruptions seem to have consisted in a much quieter extrusion of
lava alone. The basalts are all vesicular and very commonly quite light
and cellular. (A. L.)
The basalts of the Benue plain likewise form low hillocks or mounds with
no tuff or fragmental material to be seen. Generally there is no lava
flow proceeding from them, and the extrusion of quite a small quantity
of lava seems to have sufficed to relieve the pressure. Kaddemi Hill is
an exception; an old basalt flow leads from it for two to three miles to
the south-east. (A. L.)
All the basalt flows around Awe are raised considerably above the level
of the plain, and protect the limestone shales below from denudation,
though elsewhere the shales are very badly exposed. In consequence the
flows rest on “pedestals” from thirty to fifty feet in height. Assuming
that the lava chose the level surface of the plain rather than ridges
upon it, it follows that since the eruptions around Awe there has been a
general lowering of the surface by that amount. From this it appears
that the Awe eruptions were not quite contemporaneous with those of
Mboi, for at the latter place only a small amount of talus has been
broken off from the crater cliffs. (A. L.)
The ash cones and puys of the middle Benue are much more imperfectly
preserved than those of Kereng or Mboi and are certainly considerably
older. Their comparatively recent age is presumed, however, as in the
case of the later volcanic period in Air and Tibesti,[181] from the
presence of thermal springs in the immediate neighbourhood. It seems
probable that they were established as the result of the crustal
movements anterior to the accumulation of the drift, while the puys of
Kereng and Mboi owe their origin to the differential movements of the
latest period of elevation.
In petrographical character these later rocks are very similar to the
earlier Tertiary lavas. All are basalts rich in olivine and augite and
varying in the same locality from types with abundant felspar in the
ground-mass to thoroughly limburgitic[182] varieties (12) from which
felspar is entirely wanting. The ash of the Kereng puy is remarkable for
the number and size of the rounded augite crystals and olivine nodules
which it contains. Recent flows of olivine basalt and limburgite are
known also from Ahaggar and Air[183] and from the Kameruns.[184] M.
Chudeau[185] has described rhyolites from Inzize to which he assigns a
later Tertiary age, Dr. Passarge[186] has noted a nepheline basalt from
an isolated cone of recent origin in the Benue valley between Yola and
Garua, and Dr. Esch[187] has described nephelinites and leucitites from
the Kamerun Mt. which is still periodically in active eruption. No rocks
of similar composition, however, have been recognised amongst the
products of the most recent volcanic period within the Protectorate.
MM. Chudeau[188] and Chautard,[189] following M. Lacroix,[190] have
suggested that the alkaline rocks of North Africa may be grouped into
three petrographical provinces, a western province including the Azores,
Canaries, Cape Verde and Senegal, a central province including Ahaggar,
Inzize, Air, Zinder and Mounio, Chad and the Shari valley, the Kameruns,
and Dahomey, and an eastern province including Abyssinia, Arabia and
Somaliland. Upon this hypothesis the alkaline rocks of Northern Nigeria
belong to and complete the central province of Lacroix, as extended by
M. Chudeau. It is probable, however, that this central province includes
intrusive and volcanic rocks of many different ages, and that alkaline
types have been a characteristic product of differentiation from the
earliest times to the present day, (p. 142). There is, moreover, every
reason to believe that the progress of investigation will reveal many
connecting links between the various provinces, which are themselves
somewhat too extensive and comprehensive to warrant their recognition as
such. A petrographical province in the stricter sense of the term may
perhaps be set up to include the alkaline plutonic rocks of Bauchi
province and their associated hypabyssal types, but there can be little
doubt that the general alkaline character of the igneous rocks of
northern Africa, to which Lacroix has given expression, may be quite
sufficiently indicated by emphasising their natural classification
within the Atlantic branch of igneous rocks.[191]
[Footnote 172: Chautard, _Bull. Soc. Geol. Fr._, 4th Series, VII., 1907,
p. 427.]
[Footnote 173: These rocks present in microscopical structure a decided
resemblance to the Carboniferous lavas of the Bathgate Hills (Falconer,
_Trans. Roy. Soc. Edin._, Vol. XLV., 1906, p. 133) in Scotland. With the
exception that the doleritic types are somewhat less weathered, the
description and classification of the Bathgate rocks may be applied
almost word for word to these Tertiary lavas of Nigeria. Doleritic rocks
(Falconer, _op. cit._, Plate I., Fig. 2) with phenocrysts of augite and
olivine, and more rarely of felspar are common on the Morrua plateau in
eastern Nassarawa and along the base of the escarpment between Darroro
and Assab (928, 1573, 1630). Felspar is abundant in the ground-mass and
the augite occurs in granules, granulitic aggregates, small ophitic
plates, or minute idiomorphic crystals between the felspars. The basalts
of Zammagan (1578) and Buratai (1640) show a typical fluxional
arrangement of the felspars (Falconer, _op. cit._, Pl. I., Fig. 3). The
basalt of Kwoia (1654) exhibits an intimate mixture of felspar and
augite in the ground-mass (Falconer, _op. cit._, Pl. I., Fig. 4). The
basalt of Assab (934) contains scattered felspars with abundant minute
granular aggregates of augite, while a similar rock from Buratai and
Dukshi (1641) contains some residual brown glassy material and passes
into limburgitic varieties with phenocrysts of augite and olivine in an
ill-defined microlitic base.]
[Footnote 174: Chudeau, _Sah. Soud._, 1909, p. 258; Gentil, _Doc. Sc.
Miss. Sah._, 1905, p. 726.]
[Footnote 175: Passarge, _Adamaua_, 1895, p. 374. See also Downes,
_Geol. Rep. Yola-Cross River Boundary Commission_, 1910, p. 35.]
[Footnote 176: Gentil and Freydenberg, _Bull. Soc. Geol. Fr._, 4th Ser.,
VIII., 1908, p. 35; Lacoin, _C. Rd. Ac. Sc._, 22 June, 1903.]
[Footnote 177: Garde, _C. Rd. Ac. Sc._, 149, 5 July, 1909, p. 43.]
[Footnote 178: Passarge, _Adamaua_, 1895, pp. 363, 384.]
[Footnote 179: Chudeau, _Sah. Soud._, 1909, p. 266.]
[Footnote 180: Chudeau, _Sah. Soud._, 1909, p. 361.]
[Footnote 181: Chudeau, _Sah. Soud._, 1909, p. 264.]
[Footnote 182: _Cf._ Falconer, _Trans. Roy. Soc. Ed._, XLV., 1906, Pl.
I, Fig. 6.]
[Footnote 183: Chudeau, _Sah. Soud._, 1909, p. 361.]
[Footnote 184: Esch, _Geologie von Kamerun_, p. 40.]
[Footnote 185: Chudeau, _op. cit._, p. 259.]
[Footnote 186: Passarge, _Adamaua_, 1895, p. 384.]
[Footnote 187: Esch, _Sitz. Ber. Ak. Wiss. Berlin_, 1901, p. 277.]
[Footnote 188: Chudeau, _op. cit._, p. 268.]
[Footnote 189: Chautard, _Bull. Soc. Geol. Fr._, 4th Ser., VII., 1907,
p. 427.]
[Footnote 190: Lacroix, _C. Rd. Ac. Sc._, 24 May, 1899, p. 1353; 1905,
140, p. 22. _Nouv. Arch. Mus._, 4th Ser., IV., p. 156; V., p. 246.]
[Footnote 191: _Cf._ Prior, _Min. Mag._, 1903, XIII., p. 228; Harker,
_Nat. Hist. of Igneous Rocks_, 1910, p. 91.]
APPENDIX I
MINERAL RESOURCES
Salt — Iron — Gold — Monazite — Galena — Tinstone.
Detailed information respecting the principal mineral resources of
Northern Nigeria will be found in Professor Dunstan’s Reports on the
Results of the Mineral Survey.[192] The following is a brief summary of
the more important facts.
(1). _Salt._
Salt has long been a staple article of inter-tribal commerce on the
middle Benue, and its manufacture has in the past been extensively
carried on in the province of Muri. The introduction of European salt
has, however, adversely affected the native industry, and salt-making is
now confined only to those localities in which the natural conditions
are most favourable. In Muri province the salt is prepared by the
concentration and evaporation of the waters of weak brine springs which
are distributed over a western and an eastern area. In the western area
the town of Awe is the centre of the industry, while Akiri and Azara are
subordinate in both production and population. (Plate XVIII, Figs. 1,
2.) Keana on the Nassarawa border forms the western limit of the salt-
producing area, while Akwana to the south of the Benue marks its
southernmost extension. In addition to these towns which still
manufacture a surplus for export, salt-making for local purposes is
carried on at Kanji, Doya, Abuni, Ribi and Arofu within the same area.
At a former period also salt was made at Lafin Gisheri, between Tunga
and Ibi, and this locality may be taken to mark the eastern limit of the
salt area. In eastern Muri the salt-producing district is of less
importance than that in the west. The centres of the industry are
Bomanda and Jenoe, but small quantities of salt are also prepared at
Jebjeb, Jebero and Langa.
With regard to geological occurrence it is believed, as already
indicated, that the weak brine from which the salt is made in Muri
province is located within the upper part of the “Muri sandstones” and
immediately underneath the passage beds and the Turonian limestone. Mr.
Longbottom found that at Awe the springs supplied from the sandstones of
the passage beds are entirely free from salt and cease flowing before
the end of the dry season, while the neighbouring brine springs which
rise from rocks lying below the surface drainage maintain a constant
flow throughout the year. Moreover, wherever the shaly bands within the
passage beds were pierced by boreholes, the brine rose with a steady
flow under considerable pressure. The sandstones and grits underneath
the passage beds appear to be saturated with weak brine which is
retained, as in a reservoir, by the overlying shales. It is believed
that the western and eastern salt-bearing districts form detached and
irregular synclinal areas affected by a subordinate folding. The brine
springs at Awe, Azara and Bomanda are especially associated with the
subordinate anticlinal axes along which the salt-bearing beds within the
synclinals approach the surface. Some of the minor salt towns, such as
Ribi, Keana and Jenoe, are probably located on or near the outcrop of
the salt-bearing beds. Wherever the brine springs are found, bubbles of
a colourless gas, probably carbon dioxide, rise with the water.
Calcareous incrustations round the orifices of the springs are found at
Awe and Jebero. Some of the springs of Awe are, moreover, slightly
thermal and possess a temperature a few degrees above sun heat. This is
probably to be correlated with the recent volcanic action in the
neighbourhood.
[Illustration: NORTHERN NIGERIA
Distribution of Salt, Tinstone, Gold and Galena.
MAP No. 4.
London. Macmillan & Co., Ltd.
_Face p._ 266.]
The present springs rise in stream courses and are unavailable during
the rainy season. Boring, however, within the synclinal areas would
afford a copious and continuous supply. Professor Dunstan has analysed
the salt of various localities and discussed the possibilities of
increasing the native output. The general weakness of the brine,
however, would appear to be unfavourable to European exploitation.
Salt is won from brine pools in south-eastern Bauchi over an extensive
belt of country which stretches northward from the Pero Hills and passes
to the east of Chongwom and Tangalto. The pools are presumably fed by
springs and the salt is said to be abundant and easily prepared. The
whole area, however, is very imperfectly known and largely unexplored.
Scanty incrustations of natron, utilised by the local herdsmen, are
known from the Gongola valley in the neighbourhood of Nafada.
In Bornu salt is made from the ash of the siwak tree (_Salvadora
persica_) which grows abundantly on the ancient dunes which fringe the
western shore of Chad. The slaves of Mongonu and Musara prepare salt
also from the ash of three varieties of grass, “pagam, kalaslim and
kanido,” which are found in the immediate neighbourhood of the lake.
Over the whole of central Bornu the waters of the wells are charged with
salts of soda in very varying amount. The ancient alluvium of Bornu is
evidently impregnated with soda, and incrustations are found in places
upon the surface of desiccated swamps. Natron pools, similar to those of
Manga and presumably fed by springs, are unknown in central Bornu.
In Sokoto province incrustations of salt are found in the dry season at
Zauro in the valley of the Gulbin Kebbi, and natron (kungwa) occurs in a
similar manner between Gulma and Argungu. Knots and streaks of gypsum
are common in the shales and clays around Sokoto, while efflorescences
of gypsum and alum are found on the cliffs of Wurnu to the north-east.
Scanty incrustations of natron are found upon desiccated swamps in many
localities in the provinces of Sokoto, Kano, Zaria and Ilorin, but never
in sufficient quantity to give rise to even a native industry. Hausaland
has been in the past supplied with salt and natron from Asben, Bilma and
Manga in the French Sudan. European salt, however, is rapidly replacing
the native article, and many caravans now bring only natron from the
north to the markets of Kano.
(2). _Iron._
As in the case of salt, so in the case of iron, the native manufacture
has been very largely superseded by the importation of European bar-
iron. Only in the more inaccessible districts or in those places where
the ore is abundant do the natives still smelt the local ironstones in
their primitive fashion. The surface ironstones were at one time
generally utilised throughout the Protectorate as a source of iron. The
magnetite sands are still washed and smelted by the pagans of the Mumuye
hills in eastern Muri, while in northern Nassarawa the Koro tribe around
Kao is still noted for its proficiency in the manufacture of iron from
the local limonitic ores which occur within the schists in veins of
varying thickness.[193] Similar ores occur in veins in the sandstones
around Akiri in western Muri, but are no longer utilised by the natives.
The oolitic and earthy ironstones of sedimentary origin and of Eocene
age give rise to an extensive native industry around Akwi and Ojerami in
central Kabba, around Mahorro in eastern Kontagora, around Aliero in
southern Sokoto, and around Kantana in southern Bauchi, as well as in
the Kerri Kerri country in western Bornu and in the Tagum hills in
southern Bornu. So far as European exploitation is concerned, the
sedimentary ores of Mt. Patti and the lower Niger are most favourably
situated for transport. The ores, however, while abundant, are of a
somewhat low grade character and relatively rich in phosphoric acid.
Magnetite and hæmatite schists are extensively developed in eastern
Kabba, but their composition would render necessary costly crushing and
separating processes before the ores could be available for export.[194]
(3). _Gold._
Traces of gold have been found in some of the quartz veins which are so
widely distributed throughout the Protectorate, but no payable reefs
have yet been discovered. Small quantities of free gold occur in the
streams between Koriga and Birnin Gwari in northern Zaria and around
Tagum in southern Muri. In Zaria the gold is evidently located within
the northern prolongation of the Zungeru belt of slates and schists, and
further exploration may possibly lead to valuable discoveries. All the
slaty belts already described within the Protectorate are, indeed,
worthy of careful prospecting for gold. Small quantities of gold are
also found associated with cassiterite in some parts of Bauchi province.
(4). _Monazite._
Monazite is widely distributed throughout the Protectorate, and appears
to be a fairly constant constituent of the gneisses and schist. Small
quantities are contained in almost every river and stream within the
crystalline areas. It has been found most abundantly in central Kabba
and in northern Nassarawa, but, so far as known, it occurs nowhere in
sufficient quantity to afford any hope of profitable European
exploitation.
(5). _Galena._
A slightly argentiferous galena occurs in veins within the Cretaceous
rocks around Arofu and Akwana in western Muri, and in the Kudu valley to
the west of Jebjeb in northern Muri. In the Arofu district the galena is
associated with fluorspar and calcite, and the formation of the veins
has been accompanied as already described by much silicification of the
adjoining limestones. The Arofu galena is excavated by the natives for
use as a cosmetic, and from time to time small pockets of silver are
discovered within the veinstuff. To the west of Jebjeb the galena is no
longer dug by the natives, and the old workings are now levelled and
grassed over. The galena appears to have been associated with quartz,
hæmatite and blende, but there is no evidence of silicification of the
country rock.
(6). _Cassiterite (Tinstone)._
Tinstone is the most valuable asset of the Protectorate from the point
of view of mineral resources. Its occurrence in western Bauchi was known
to the natives long before the advent of Europeans. Recent
investigation, however, by the Mineral Survey has proved its presence
over considerable areas in widely separated localities within the
Protectorate. The most extensive area over which tinstone is found in
the surface alluvium occurs in western Bauchi, and includes the summit
of the plateau and the greater part of its northern margin. The alluvial
deposits of Naraguta, which lie within this area, were originally
developed by the Niger Co., but have now been acquired by the
Consolidated Gold Fields, Ltd. Elsewhere, however, tinstone becomes
equally abundant, and there is little doubt that further investigation
will reveal many localities within this extensive area in which alluvial
workings may be profitably carried on. Several syndicates indeed are now
engaged in exploiting certain portions of the tinfield. In the
neighbourhood of Ngell the tinstone, accompanied by sulphide ores of
copper, zinc and lead, occurs, as already described, in part as a
pneumatolytic modification of the Bukuru granite, and in part as a true
lode or pegmatitic formation. This particular occurrence is being
developed by the Niger Co., but there is every reason to believe that
similar occurrences will be found throughout the area as investigation
proceeds.
Apart from this extensive tin-field in the centre of the Protectorate,
cassiterite is also found in Bauchi province in many places throughout
the Ningi and Burra hills, and detailed prospecting will probably here
also reveal several payable localities. The completion of the Baro-Kano
railway[195] within the next two years will afford an easy means of
access to the tin-producing areas of Bauchi, and will at the same time
largely overcome the difficulty of transport which is at present the
greatest obstacle in the way of the development of the tin fields.
Beyond the limits of Bauchi province small quantities of tinstone have
been found in the neighbourhood of Gadama and Fagam on the borders of
Bauchi and Kano, around Gantam and Aribi on the Nassarawa tableland as
well as on the eastern slopes of the Vere hills in Yola, the western
slopes of the Shebshi hills in southern Muri, and in the neighbourhood
of Eri in eastern Ilorin. In the latter locality the tinstone was found
to be derived from a richly stanniferous pegmatite vein, but the actual
extent and possibilities of the occurrence are as yet unknown. The same
may be said also of the other localities indicated, and there can be
little doubt that all of them are worthy of further investigation.
Tinstone has been reported to occur abundantly in the Mada country in
eastern Nassarawa, but this discovery likewise awaits confirmation.
[Footnote 192: Dunstan, _Col. Reps._ (_Miscell._), Nos. 32 (1906), 46
(1908), 47 (1908). A fourth report on the results of the Mineral Survey
will shortly be issued.]
[Footnote 193: Sir William Wallace has reported an extensive native iron
industry at Fawa in Katsina, where the ore appears to occur in a similar
manner to that in the neighbourhood of Kao.—_G. J._, VIII., 1896.]
[Footnote 194: In _The Iron Ore Resources of the World_, Vol. II., 1910,
p. 1029, Dr. J. W. Evans has given a _résumé_ of the results of the
investigations of the Mineral Survey into the extent and value of the
ferruginous deposits of Northern Nigeria.]
[Footnote 195: And the branch line to Leri in Zaria province.]
APPENDIX II
THE PALÆONTOLOGY OF THE UPPER CRETACEOUS DEPOSITS OF NORTHERN NIGERIA
By HENRY WOODS, M.A., _University Lecturer in Palæozoology, Cambridge_
The fossils collected by Dr. J. D. Falconer and Mr. A. Longbottom from
the Upper Cretaceous deposits of Northern Nigeria include Fishes,
Ammonites, Gasteropods, and Lamellibranchs. The localities from which
these fossils were obtained are: Awe, Deba Habe, Jibaru, Gongila,[196]
Gulani, Kumberi, Kunini, and Reme. At present the number of species
which can be identified is comparatively small, but the collection is
sufficient to show that under more favourable conditions a large and
important fauna could probably be obtained from the deposits of this
region.
Teeth, vertebræ, &c., of fishes were collected at Kumberi and Jibaru,
and have been examined by Dr. A. S. Woodward who identifies them as
_Gigantichthys_, a genus which is known from the Chalk of Egypt and the
Sahara.[197] Ammonites have been found at Gongila, Reme, Kunini, and
Kumberi, and they show that the deposits are of Turonian age—probably
Lower Turonian. The Ammonites belong to the genera _Vascoceras_,
_Mammites_ (_Pseudaspidoceras_), and _Hoplitoides_. _Vascoceras_ is
confined to the Turonian. _Pseudaspidoceras_ is mainly of Turonian age.
The species of _Hoplitoides_ belongs to the section of that genus which
occurs in Turonian deposits. The species of _Vascoceras_ and _Mammites_
(_Pseudaspidoceras_) found in Nigeria are closely allied to forms which
occur in the Lower Turonian of Northern Africa and the Iberian
peninsula.
The larger number of the Gasteropods and Lamellibranchs were found at
Awe, but no Ammonites were obtained at that locality; these fossils, so
far as they can be identified, do not give any definite evidence that
the deposit at Awe is of earlier age than the Turonian.
At Deba Habe _Ostrea (Exogyra) olisiponensis_, Sharpe, is abundant and
is the only fossil obtained at that place. In Portugal that species
occurs in both Turonian and Cenomanian deposits. In Nigeria it has also
been found at Gongila, where the Turonian _Hoplitoides_ occurs. So that
it is probable that the deposit at Deba Habe is of the same age as the
one at Gongila.
At Gulani _Ostrea prælonga_, Sharpe, is abundant, but no other fossil
was found at that locality. In Portugal _O. prælonga_ occurs in the
Bellasian—an horizon which is believed to come between the Aptian and
the Cenomanian. The same species is found at a similar level in Tunis
and Algeria. The occurrence of this one species can, I think, hardly be
taken as sufficient evidence that the deposit at Gulani is of Bellasian
age, since it is not unlikely that, as is the case with _O. (Exogyra)
olisiponensis_, this form may recur at later horizons when a favourable
facies of deposit again appears.
At present, then, we can state that the deposits in which Ammonites
occur are of Turonian age, and that there is not sufficient evidence to
show that beds of earlier age are represented.
The fauna of the Turonian deposits of Northern Nigeria resembles closely
that found in beds of similar age in Portugal, Tunis, and Algeria. This
is shown especially by the Ammonites and oysters. At present the genus
_Vascoceras_ is known only from Portugal, Spain, Algeria, Tunis, Egypt,
Nigeria and Brazil. _Hoplitoides_ occurs in Algeria, Tunis, Nigeria, the
Kameruns, and probably in Egypt. The species of _Pseudaspidoceras_ found
in Nigeria is allied to forms found in Portugal, Tunis and Brazil. The
oysters likewise belong to species characteristic of Northern Africa and
South-western Europe, and one of them has also been found as far south
as Angola. The Ammonites also show, as Pervinquière has already pointed
out is the case in Tunis, that the fauna is related to that found in the
Upper Cretaceous of Brazil.
Amongst other places in West Africa, Cretaceous deposits have been found
in the Kameruns, and are regarded by Solger as of Upper Turonian and
Lower Senonian age, but by Harbort[198] they are referred to the
Emscherian (Lower Senonian). They certainly appear to be of later date
than the Nigerian deposits with which they are connected chiefly by the
presence of _Hoplitoides_.
Until the fauna of the Nigerian Cretaceous is more completely known any
further discussion of its relationship to the faunas of other areas must
be postponed.
The list on the following page shows the distribution of the species
found in Nigeria, and is followed by palæontological notes and
descriptions of some of the species.
--------------------+----+-----+--------+-------+--------+-------+-----
|Awe.|Deba |Gongila.|Gulani.|Kumberi.|Kunini.|Reme.
| |Habe.| | | | |
--------------------+----+-----+--------+-------+--------+-------+-----
_Nuculana_, sp. | | | | | * | |
| | | | | | |
_Trigonoarca_, sp. | * | | | | | |
| | | | | | |
_Trigonia_, sp. | * | | | | | |
| | | | | | |
_Modiola_, sp. | * | | | | | |
| | | | | | |
_Pecten (Neithea) | * | | | | | |
quinquecostatus_, | | | | | | |
Sow. | | | | | | |
| | | | | | |
_Lima (Mantellum)_, | * | | | | | |
sp. | | | | | | |
| | | | | | |
_Ostrea prælonga_, | | | | * | | |
Sharpe | | | | | | |
| | | | | | |
„ _(Exogyra) | | * | * | | | |
olisiponensis_, | | | | | | |
Sharpe | | | | | | |
| | | | | | |
_Gervillia | * | | | | | |
(Pseudoptera)_, sp. | | | | | | |
| | | | | | |
_Astarte awensis_, | * | | | | | |
sp. nov. | | | | | | |
| | | | | | |
_Lucina_, sp. | | | | | * | |
| | | | | | |
_Cardium_, sp. | * | | | | | |
| | | | | | |
_Corbula_, sp. | * | | | | | |
| | | | | | |
_Pseudomelania_, sp.| * | | | | | |
| | | | | | |
_Turritella_, sp. | | | | | * | |
| | | | | | |
_Semifusus | * | | | | | |
africanus_, sp. nov.| | | | | | |
| | | | | | |
_Vascoceras | | | | | | * | *
nigeriensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
„ | | | * | | | |
_gongilensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
_Mammites | | | * | | | |
(Pseudaspidoceras)_,| | | | | | |
sp. | | | | | | |
| | | | | | |
_Hoplitoides | | | * | | | |
nigeriensis_, sp. | | | | | | |
nov. | | | | | | |
| | | | | | |
„ sp. | | | | | | | *
| | | | | | |
_Gigantichthys_, sp.| | | | | * | |
--------------------+----+-----+--------+-------+--------+-------+-----
LAMELLIBRANCHIA.
NUCULANA, sp.
Plate XIX., Fig. 4.
An internal cast resembles in form _N. Mariæ_ (d’Orbigny), but in the
absence of the shell the species cannot be identified.
_Locality._—Kumberi.
TRIGONOARCA, sp.
Plate XIX., Figs. 1–3.
Several specimens of _Trigonoarca_ were found, but are not sufficiently
well-preserved for specific identification. In general form they
resemble some examples of _T. trichinopolitensis_ (Forbes)[199] from the
Trichinopoli Beds of Southern India.
_Locality._—Awe.
TRIGONIA, sp.
Two specimens of a species belonging to the Scabræ section of _Trigonia_
are in the collection, but are not sufficiently well-preserved for
identification.
_Locality._—Awe.
MODIOLA, sp.
Examples of a small form of _Modiola_ occur at Awe.
PECTEN (NEITHEA) QUINQUECOSTATUS, Sowerby.
Three specimens with the shell preserved were found at Awe.
LIMA (MANTELLUM), sp.
An imperfect specimen from Awe resembles _Lima (Mantellum) gaultina_,
Woods.[200]
OSTREA PRÆLONGA, Sharpe.
Plate XIX., Figs. 5–8.
1850. _Ostræa prælonga_, D. Sharpe, _Quart. Journ. Geol. Soc._, VI., p.
187, Pl. XX., Fig. 4.
1869. _Ostrea prælonga_, H. Coquand, _Mon. Ostrea. Terr. Crét._, p. 169,
Pl. LXVI., Figs. 1, 2.
1890. _Ostrea prælonga_, A. Peron, _Moll. Foss. Terr. Crét. Tunisie_, p.
110, Pl. XXIII., Figs. 1–6.
_Remarks._—About twenty more or less perfect specimens of an _Ostrea_,
agreeing closely with _O. prælonga_, Sharpe, were found. _O.
pentagruelis_, Coquand,[201] is considered by Peron to be only a very
old individual of _O. prælonga_.
This species occurs in the Bellasian of Portugal. In Algeria and Tunis
it is characteristic of beds which occur below the Cenomanian, and are
referred by Peron to the Upper Albian.
_Locality._—Gulani.
OSTREA (EXOGYRA) OLISIPONENSIS, Sharpe.
Plate XX., Figs. 1–3.
1850. _Exogyra olisiponensis_, D. Sharpe, _Quart. Journ. Geol. Soc._,
VI., p. 185, Pl. XIX., Figs. 1, 2.
1862. _Ostrea Overwegi_, H. Coquand, _Géol. pal. rég. sud. prov.
Constantine_, p. 226, Pl. XIX., Figs. 1–6.
1862. _Ostrea Coquandi_, Julien, in Coquand, _ibid._, p. 324, Pl.
XXXIII., Figs. 10–12.
1869. _Ostrea olisiponensis_, H. Coquand, _Mon. Ostrea. Terr. Crét._, p.
125, Pl. XLV., Figs. 1–7.
1869. _Ostrea Overwegi_, Coquand, _ibid._, p. 140, Pl. XLIV., Figs. 1–9;
Pl. XLVI., Figs. 14, 15.
1873. _Ostrea olisiponensis_, L. Lartet, _Ann. Sci. géol._, Vol. III.,
p. 59, Pl. XI., Fig. 1.
1882. _Exogyra olisiponensis_, G. Seguenza, _Atti d. R. Accad. dei
Lincei_, Ser. 3; _Mem. Scien. Fis. Math._, Vol. XII., p. 180, Pl. XVII.,
Fig. 2.
1882. _Exogyra oxyntas_, Seguenza, _ibid._, p. 178, Pl. XVIII., Fig. 1.
1888. _Ostrea olisiponensis_, P. Choffat, _Mém. Soc. Phys. et d’hist.
nat. de Genève_, XXX., 2, p. 95.
1891. _Ostrea olisiponensis_, A. Peron, _Moll. Foss. Terr. Crét.
Tunisie_, p. 114, Pl. XXIII., Figs. 14–18.
1902. _Ostrea (Exogyra) olisiponensis_, P. Choffat, _Faune Crét.
Portugal_, Vol. I., Ser. 4, p. 166, Pl. VI., Figs. 17–19.
1903. _Ostrea olisiponensis_, R. Fortau, _Bull. Instit. Égypt._, Ser. 4,
Vol. IV., p. 283, Figs. 3–5.
1905. _Ostrea (Exogyra) olisiponensis_, P. Choffat, _Géol. Col. Portug.
d’Afrique. II, Nouvelles données sur la zone littorale d’Angola_, p. 44,
Pl. I., Figs. 4, 5.
_Remarks._—This species is represented by numerous examples, in a few of
which the surfaces of the shell are well-preserved and show a few coarse
ribs. Sharpe and other writers have noted the great variability in the
ribbing of this species. The Nigerian specimens agree with the varieties
which possess only a few ribs. Peron, Choffat, and Fortau have discussed
fully the affinities and synonymy of _O. (Exogyra) olisiponensis_. This
species is common in Northern Africa and Portugal; it occurs in the
Cenomanian of Algeria, Tunis, Egypt, and Palestine; in Portugal it is
found in both the Upper and Lower Turonian, as well as in the
Cenomanian, wherever the deposits are of the marly facies. South of
Nigeria this species has been discovered in the province of Angola.
_Localities._—Deba Habe and Gongila.
GERVILLIA (PSEUDOPTERA), sp.
Plate XX., Figs. 4–6.
There are several internal casts resembling in form _G. (Pseudoptera)
anomala_ (Sowerby),[202] but there is no evidence to show whether radial
ribs were present on the shell or not, and the species cannot be
determined at present.
Specimens from the Cenomanian of the neighbourhood of Tebessa (Algeria)
have been identified with Sowerby’s species by Coquand,[203] and
Peron[204] states that it is abundant and well-preserved at that
locality.
_Locality._—Awe.
ASTARTE AWENSIS, sp. nov.
Plate XX., Fig. 7.
_Description._—Shell rather small, moderately convex, with the postero-
dorsal part somewhat compressed, outline subquadrate or subtriangular,
very inequilateral. Anterior margin slightly concave; ventral margin
convex; posterior margin short, truncated, forming an obtuse angle with
the long, slightly convex postero-dorsal margin. Umbones pointed, rather
prominent. Lunule large, ovate, smooth, with a sharp edge. Escutcheon
large, deep, with a sharp edge. Ornamentation consists of about fourteen
broad, prominent concentric ribs, which are marked by fine concentric
lines. Length, 8·25; height, 7 mm.
_Affinities._—In the character of its ornamentation _A. awensis_
resembles several small species of _Astarte_ found in the Cretaceous
rocks. It is more inequilateral, more elongate, and has the posterior
margin more distinctly truncated than in _A. subsimilis_, J. Böhm.[205]
In similar respects it differs from _A. griesbachi_, Woods,[206] from
Pondoland.
_Remarks._—Numerous casts of this species occur in ferruginous nodules.
_Locality._—Awe.
LUCINA, sp.
Plate XX., Figs. 8, 9.
_Description._—Shell nearly orbicular, convex, slightly inequivalve;
height and length nearly equal. Anterior and ventral margins rounded;
posterior margin truncated, forming an obtuse angle with the straight or
slightly convex postero-dorsal margin. Umbones rather prominent.
Ornamentation consists of regular, concentric lamellar ribs separated by
broad, flat interspaces which are marked by fine, concentric lines.
Length, 11 mm.; height, 11·5 mm.
_Affinities._—In general form, and in the character of its
ornamentation, this species resembles one of the varieties of _Lucina
fallax_, Forbes, described by Stoliczka[207] from the Utatúr Group, but
the umbones are more prominent, and the postero-dorsal angle is more
distinct. It also resembles _L. saharica_, Quaas,[208] and some forms of
_L. subnummismalis_, d’Orbigny.[209]
_Locality._—Kumberi.
CARDIUM, sp.
Plate XX., Fig. 10.
In the possession of numerous radial ribs and in the general form of the
shell, some internal casts resemble _Cardium cenomanense_,
d’Orbigny,[210] but the valves are less convex.
_Locality._—Awe.
CORBULA, sp.
Specimens of _Corbula_ resemble _C. elegans_, Sowerby,[211] in the
character of their ornamentation, but the posterior part of the shell is
more produced.
_Locality._—Awe.
GASTEROPODA.
PSEUDOMELANIA, sp.
Plate XXI., Fig. 1.
Several specimens of a small gasteropod resemble _Pseudomelania
Pallaryi_, Peron and Fortau,[212] from the Cenomanian of Egypt, but they
are of much smaller size, being from 8 to 13 mm. long. The surface of
the shell is smooth, except for slightly curved growth-lines, and, on
the base, spiral grooves.
_Locality._—Awe.
TURRITELLA, sp.
Plate XXI., Fig. 2.
A species of _Turritella_ resembles _T. Choffati_, Thomas and
Peron,[213] from the Turonian of Tunis. The anterior part of each whorl
is bent at an angle with the large flattened posterior part, and the
ornamentation consists of four strong spiral ribs, one of which occurs
at the angle, giving the appearance of a carina. On the later whorls the
ribs become granular or tubercular. The ribs are much coarser than in
_T. Choffati_.
_Locality._—Kumberi.
SEMIFUSUS AFRICANUS, sp. nov.
Plate XXI., Figs. 3–5.
_Description._—Shell subfusiform. Spire shorter than the last whorl,
consisting of about five whorls. Whorls carinate, the carina being more
or less distinctly nodular. Posterior part of the whorls flattened, much
larger than the anterior part in the spire. Anterior part of last whorl
convex. Ornamentation consists of narrow spiral ribs with flat
interspaces crossed by growth-lines; on the later whorls the ribs become
divided up into rows of elongated tubercles.
_Remarks._—This species, which is represented by several imperfect
specimens, shows some resemblance to _Lagena nodulosa_, Stoliczka,[214]
and _L. secans_, Stoliczka,[215] from the Ariyalúr Group, and also to
_Serrifusus dakotensis_ var. _vancouverensis_, Whiteaves,[216] but
possesses one carina only.
_Locality._—Awe.
CEPHALOPODA.
_Genus_—VASCOCERAS, P. Choffat.
(_Faune Crét. Portugal_, I., 1898, p. 51.)
VASCOCERAS NIGERIENSIS, sp. nov.
Plate XXI., Fig. 6; Plate XXII., Figs. 2, 3.
_Description._—Shell inflated, rounded, subglobose. Whorls thick,
largely embracing; umbilicus deep, rather more than a third of the
diameter of the shell, with nearly vertical walls, the umbilical margin
subcarinate, without tubercles. Flanks curving gradually from the
umbilical margin and not limited from the rounded periphery. Surface of
shell with growth-rings.
Sutures: Saddles large, broad, and low; external saddle unsymmetrically
divided and with small minor divisions; first lateral smaller than the
external saddle, more rounded, less divided, with the outer margin
sloping and the inner margin nearly perpendicular; second lateral saddle
broad, only slightly divided. External lobe deeper than the first
lateral lobe; the latter is small with one main division; second lateral
lobe smaller, with two points.
(1) (2)
Diameter 88 93 mm.
Thickness 58 54
Height of last whorl 37 34
Width of umbilicus 30 33
(1) Reme. (2) Kunini.
_Affinities._—This species resembles the thicker forms of _V. durandi_
(Peron and Thomas),[217] but the walls of the umbilicus are steeper, the
umbilical margin is without tubercles, and the external and first
lateral saddles are broader and not so high. In general form, and in the
absence of tubercles at the umbilical margin, _V. nigeriensis_ resembles
_V. amierensis_, Choffat,[218] but the whorls embrace to a larger
extent, and their sides are less flattened; there are also some
differences in the suture. In the form of the umbilicus, and in the
breadth of the saddles, this species resembles _V. harttiformis_,
Choffat,[219] but the shell is much less thick. Another similar form but
with a thicker shell, is _V. hartti_, Hyatt,[220] from Brazil.
_Localities._—Reme and Kunini.
VASCOCERAS GONGILENSIS, sp. nov.
Plate XXI., Fig. 7; Plate XXII., Fig. 1.
_Description._—Shell moderately inflated. Umbilicus less than a quarter
of the diameter of the shell, with vertical walls and subcarinate
margin. Flanks somewhat compressed. Periphery flattened. Inner whorls
with strong ribs on the flanks and prominent tubercles at the umbilical
margin and at the limit of the flanks and periphery; on the last whorl
the ribs and tubercles gradually become obsolete.
Sutures: External saddle broad, subquadrate, with small divisions; first
lateral saddle broad, rounded, divided unsymmetrically by a small lobe;
second lateral saddle subquadrate; one small auxiliary saddle on the
flanks. External lobe deep and larger than the first lateral lobe, which
has only small divisions; second lateral lobe small, its lower margin
with pointed divisions.
Diameter 108 mm.
Thickness 57 „
Height of last whorl 52 „
Width of umbilicus 24 „
_Affinities._—This species resembles _V. subconciliatum_, Choffat,[221]
from the Turonian of Portugal, but the ribs and tubercles become
obsolete at an earlier stage, the umbilicus is smaller, the divisions in
the external saddle are less deep, the first and second lateral saddles
are more rounded, and the first lateral lobe is narrower. _V.
gongilensis_ also resembles some forms of _V. cauvini_, Chudeau.[222]
_Locality._—Gongila.
_Genus_—MAMMITES, Laube and Bruder.
(_Palæontographica_, XXXIII., 1887, p. 229.)
_Sub-genus_—PSEUDASPIDOCERAS, Hyatt.
(“Pseudoceratites of the Cretaceous,” _U. S. Geol. Surv. Mon._, XLIV.,
1903, p. 106.)
MAMMITES (PSEUDASPIDOCERAS), sp.
Plate XXIII., Figs. 1, 2.
In the collection there is one specimen of _Mammites
(Pseudaspidoceras)_; it resembles the form figured by Choffat[223] under
the name _Acanthoceras_ (?), cf. _Footeanus_ (Stol.), from the Turonian
of Portugal, but differs in having a smaller umbilicus and less distinct
siphonal tubercles. The sutures are not shown in the specimen from
Nigeria.
The Portuguese form differs from _A. footeanus_, Stoliczka,[224] by the
absence of secondary ribs on the siphonal region of the inner whorls,
and by the forward inclination of the ribs between the lateral tubercles
and the siphonal tubercles.
Other allied species are _M. (Pseudaspidoceras) salmuriensis_
(Courtiller) var. _byzacenica_, Pervinquière,[225] from the Lower
Turonian of Tunis, and _M. (Pseudaspidoceras) Pedroanus_ (White),[226]
from Brazil.
_Locality._—Gongila.
_Genus_—HOPLITOIDES, A. von Kœnen.
(“Nachtrag Foss. Unter. Kreide Mungo in Kamerun,” _Abhandl. d. k.
Gesellsch. d. Wiss. Göttingen, Math.-Phys. Kl., N.F._ Band 1, 1898, p.
53. Emended F. Solger, _Foss. Mungokreide in Kamerun_, 1904, p. 127; L.
Pervinquière, _Études Pal. Tunis, I. Ceph._, 1907, p. 215.)
HOPLITOIDES NIGERIENSIS, sp. nov.
Plate XXIII., Fig. 3; Plate XXIV., Figs. 1–5. Text-figure 1.
_Description._—Shell discoidal, compressed, with truncated periphery.
Whorls largely embracing, with the flanks slightly or moderately convex,
the greatest convexity being at a short distance from the umbilical
margin. Umbilicus small, or very small, about one-seventh of the
diameter of the shell, its sides nearly perpendicular. Periphery usually
rather broad, bounded on each side by a prominent rounded carina; the
prominence of the siphuncle gives rise to a median rounded ridge between
which and the carinas the shell is concave.
In fully-grown individuals the shell is smooth, but in smaller examples
the flanks bear a few strong, rounded ribs, which curve slightly
forwards near the periphery; the larger ribs start from the umbilical
margin, and one or two smaller ribs may be intercalated on the flanks.
Where the ribs join the carina the latter may become more or less
distinctly tuberculate. Usually the ribs become obsolete when the shell
reaches a diameter of about 50 mm., but occasionally they persist on
larger individuals.
Sutures: External saddle broad, rather low, with a variable number of
small divisions. First and second lateral saddles similar to, but
distinctly smaller than the external saddle; each has usually some small
divisions, but in some cases these are absent, and the saddle is then
rounded. One or two auxiliary saddles may occur. External lobe deep,
sometimes nearly as wide as the space between the two carinæ. First
lateral lobe usually rather narrower and not so deep as the external
lobe, with more or less numerous small divisions. The second lateral
lobe is similar to, but smaller than the last.
(1) (2) (3) (4) (5) (6) (7) (8)
Diameter 110 95 84 67 68 62 57 44 mm.
Thickness 47 42 34 28 28 27 28 20 „
Height of last whorl 61 49 45 37 38 38 34 25 „
Width of umbilicus 15 15 10 10 7 6·5 8 6 „
(1–8) From Gongila.
[Illustration: FIG. 1.—_Hoplitoides nigeriensis_, sp. nov. Gongila.
Sutures of six specimens. Natural size.]
_Affinities._—This species appears to belong to the bicarinate group of
_Hoplitoides_. In general form it resembles _H. munieri_,
Pervinquière,[227] from the Lower Turonian of Tunis, but the whorls, are
thicker, the flattened periphery is broader and shows a distinct
siphuncular ridge. The sutures are similar to those of _H. munieri_, but
are symmetrically placed, and the first lateral lobe is relatively
narrow. The ribbing of the smaller specimens is similar to that of
_Hoplites ingens_, v. Kœnen em. Solger.[228]
In general form, _H. nigeriensis_ is similar to _Heterotissotia
neoceratoides_, Peron,[229] from the Lower Senonian of Algeria and the
Upper Turonian of Tunis. Both possess a broad, flattened periphery with
a keel on each side and a prominent siphuncular ridge, but in _H.
neoceratoides_ the ribs are retained to a later stage. The figures given
of the suture of Peron’s species are not satisfactory, and I am indebted
to Dr. L. Pervinquière for comparing figures of the sutures of
_Hoplitoides nigeriensis_ with the type of _Heterotissotia
neoceratoides_ which is now in the Sorbonne. Dr. Pervinquière states
that the differences in the suture are too great to allow of these two
species being referred to the same genus.
_Remarks._—The sutures of _Hoplitoides nigeriensis_ vary considerably in
appearance on account of the difference in the number and size of the
small divisions of the saddles. A somewhat similar variation is seen in
the sutures of _H. ingens_, v. Kœnen, em. Solger, and _H. munieri_,
Solger. Variations also occur in the thickness of the whorls, the
breadth of the periphery, the size of the umbilicus, and the period at
which the ribs become obsolete. In one specimen, which is less thick
than the other, the siphuncular ridge is absent.
This species is represented by numerous specimens, all of which are
internal casts or have only a small portion of the shell preserved, and
in nearly all cases the sutures are clearly exposed.
_Locality._—Gongila.[230]
PLATE XIX
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
(The figures are of natural size unless the amount of enlargement is
stated).
FIGS. 1–3. _Trigonoarca_, sp. Awe. 1, right valve; 2, dorsal view of the
same; 3, hinge of left valve. (Page 276.)
„ 4. _Nuculana_, sp. Kumberi. Internal cast of left valve. × 1½. (Page
276.)
„ 5–8. _Ostrea prælonga_, Sharpe. Gulani. 5, right valve; 6, 8, right
valves with umbo of left valves; 7, left valve. (Page 277.)]
PLATE XX
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
FIGS. 1–3. _Ostrea (Exogyra) olisiponensis_, Sharpe. Deba Habe. Left
valves. (Page 277.)
„ 4–6. _Gervillia (Pseudoptera)_, sp. Awe. Internal casts. 4, 5, left
valves; 6, right valve. (Page 278).
„ 7. _Astarte awensis_, sp. nov. Awe. Right valve, drawn from a wax
impression of an external cast, × 1½. (Page 279.)
„ 8, 9. _Lucina_, sp. Kumberi. 8, right valve; 9, dorsal view of the
same, × 1½. (Page 279.)
„ 10. _Cardium_, sp. Awe. Internal cast of right valve. The marginal
part of the valve is imperfect. (Page 280.)]
PLATE XXI
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
FIGS. 1. _Pseudomelania_, sp. Awe. × 1½. (Page 280.)
„ 2. _Turritella_, sp. Kumberi. × 1½. (Page 281.)
„ 3–5. _Semifusus africanus_, sp. nov. Awe. 3, drawn from a wax
impression of an external cast; 4, portion of the ornamentation of 5, ×
6. (Page 281.)
„ 6. _Vascoceras nigeriensis_, sp. nov. Reme. (Page 281.)
„ 7. _Vascoceras gongilensis_, sp. nov. Gongila. Internal cast. (Page
282.)]
PLATE XXII
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
FIGS. 1._Vascoceras gongilensis_, sp. nov. Gongila. Siphonal view of
Fig. 7, Plate XXI. (Page 282).
„ 2, 3. _Vascoceras nigeriensis_, sp. nov. Kunini. Two views of the same
specimen. (Page 281.)]
PLATE XXIII
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
FIGS. 1, 2. _Mammites (Pseudaspidoceras)_, sp. Gongila. Two views of the
same specimen. (Page 283.)
„ 3. _Hoplitoides nigeriensis_, sp. nov. Gongila. (Page 284.)]
PLATE XXIV
[Illustration: CRETACEOUS MOLLUSCA.
T. A. Brock, del.
FIGS. 1–5. _Hoplitoides nigeriensis_, sp. nov. Gongila. 1, siphonal view
of Fig. 3, Plate XXIII; 4, siphonal view of Fig. 3. (Page 284.)]
[Footnote 196: The occurrence of Turonian fossils at this locality has
been recorded by G. C. Crick, _Geogr. Journ._, Vol. XXIV., 1904, p. 522,
footnote 1.]
[Footnote 197: Dames, _Sitzungsber. Ges. nat. Freunde, Berlin_, 1887,
pp. 69, 137.]
[Footnote 198: Guillemain and Harbort, “Profil der Kreideschicht. am
Mungo,” _Abhandl. d. k. preuss. geol. Landesanst. N.F._, Heft 62, 1909,
p. 405.]
[Footnote 199: Stoliczka, _Cret. Fauna S. India_, 1871, p. 353, Pl.
XVIII., Figs. 12, 14; Pl. XIX., Figs. 2, 3; Pl. XX., Figs. 2, 8, 9, 10.]
[Footnote 200: _Mon. Cret. Lamell. England_, Vol. II., 1904, p. 31, Pl.
V., Figs. 16–20.]
[Footnote 201: _Mon. Aptien de l’Espagne_, 1865, p. 166, Pl. XXVI.,
Figs. 1, 2.]
[Footnote 202: For figures see Woods, _Mon. Cret. Lamell. England_, Vol.
II., 1905, p. 64, Pl. IX., Figs. 2–4. Compare also d’Orbigny, _Pal.
Franç. Terr. Crét._, Vol. III., 1846, p. 478, Pl. CCCXCII., Figs. 1–3.
When describing this species I overlooked Guéranger’s statement that he
had seen specimens showing ligament pits; from the presence of those
pits it is evident that this species cannot belong to _Pteria_. It seems
probable that _Pseudoptera_ should be regarded as a sub-genus of
_Gervillia_. Guéranger, _Album Paléont. de la Sarthe_, 1867, pp. 17, 20,
Pl. XXII., Figs. 9, 10; Pl. XXV., Figs. 10, 11.]
[Footnote 203: _Géol. et Pal. de Constantine_, 1862, p. 292.]
[Footnote 204: Peron, _Moll. Foss. Terr. Crét. Tunisie_, 1890, p. 238.]
[Footnote 205: “Kreidebild. des Fürbergs u. Sulzbergs,”
_Palæontographica_, XXXVIII., 1891, p. 74, Pl. III., Fig. 8.]
[Footnote 206: _Ann. S. African Mus._, IV., 1906, p. 300, Pl. XXXV.,
Figs. 18, 19.]
[Footnote 207: _Palæont. Indica, Cret. Fauna S. India_, III., 1871, p.
256, Pl. XIV., Fig. 7.]
[Footnote 208: _Palæontographica_, XXX., 2, 1902, p. 214, Pl. XXIV.,
Fig. 7.]
[Footnote 209: _Ravn, Mollusk. Danmarks Kridtafl. I. Lamellibr._, 1902,
p. 129, Pl. IV., Fig. 21.]
[Footnote 210: _Pal. Franç. Terr. Crét._, III., 1844, p. 37, Pl. CCXLIX,
Figs. 5–9.]
[Footnote 211: Woods, _Mon. Cret. Lamell. England_, Vol. II., 1908, p.
216, Pl. XXXIV., Figs. 23–28.]
[Footnote 212: _Bull. Institut Égypt._, Ser. 4, Vol. VI., 1903, p. 270,
Pl. I., Fig. 22.]
[Footnote 213: _Moll. Foss. Crét. Tunisie_, 1889, p. 47, Pl. XIX., Figs.
13, 14.]
[Footnote 214: _Cret. Fauna S. India_, Vol. II., 1867, p. 137, Pl. XI.,
Fig. 18.]
[Footnote 215: _Ibid._, p. 138, Pl. XI., Figs. 19, 20.]
[Footnote 216: _Mesozoic Fossils_, Vol. I., 1879, p. 119, Pl. XV., Fig.
5. Compare also Woods, _Ann. S. African Mus._, Vol. IV., 1906, p. 324,
Pl. XL. Figs. 5, 6. Newton, _Trans. Roy. Soc. S. Africa_, Vol. I., 1909,
p. 28, Pl. IX., Fig. 6.]
[Footnote 217: _Moll. Foss. Crét. Tunisie_, 1889, p. 27, Pl. XVIII.,
Figs. 5–8. Peron, _Amm. Crét. supér. Algérie_, 1896–7, p. 44, Pl. IV.,
Fig. 1; Pl. V., Fig. 1; Pl. XVII., Fig. 5. Pervinquière, _Paléont.
Tunisienne, Ceph. secondaires_, 1907, p. 332, Pl. XXI., Fig. 1. _V.
Douvillei_, Choffat, _Faune crét. Portugal_, Vol. I., 1898, p. 59, Pl.
X., Fig. 6; Pl. XI., Figs. 2–5; Pl. XXI., Figs. 13–16.]
[Footnote 218: Choffat, _ibid._, p. 61, Pl. XII., Figs. 1, 2; Pl. XIII.,
Figs. 1, 2; Pl. XXI., Figs. 17–21.]
[Footnote 219: _Ibid._, p. 61, Pl. XII., Fig. 3; Pl. XIII., Figs. 3–6;
Pl. XXI., Figs. 22–24.]
[Footnote 220: “Contrib. Paleont. Brazil,” _Archiv. Mus. Nac. Rio de
Janeiro_, VII., 1887, p. 226, Pl. XIX., Figs. 1, 2; Pl. XX., Fig. 3.
Hyatt, “Pseudoceratites of the Cretaceous,” _U. S. Geol. Survey, Mon._,
XLIV., 1903, p. 103, Pl. XIV., Fig. 16.]
[Footnote 221: _Faune crét. Portugal_, Vol. I., 1898, p. 64, Pl. XV.,
Figs. 1–3; Pl. XVI., Fig. 4; Pl. XXII., Figs. 28–31.]
[Footnote 222: “Ammonites du Damergou (Sahara meridional),” _Bull. Soc.
géol. de France_ (4), IX., 1909, p. 68, Pls. I., II., III.]
[Footnote 223: _Faune crét. Portugal_, Vol. I., 1898, p. 66, Pl. XVI.,
Figs. 9, 10; Pl. XXII., Figs. 3, 4.]
[Footnote 224: _Palæont. Indica, Ceph. Cret. S. India_, 1864, p. 101,
Pl. LII., Figs. 1, 2. Kossmat, “Südindische Kreideformat,” _Beitr. Pal.
Geol. Osterr-ungarns u. d. Orients_, XI., 1897, p. 20. Petrascheck,
“Amm. sächsich. Kreideformat., _ibid._, XIV., 1902, p. 144. Pl. IX.,
Fig. 1.]
[Footnote 225: _Études Paléont. Tunisienne, I. Ceph. Second._, 1907, p.
315, Pl. XIX., Fig. 1.]
[Footnote 226: “Contrib. Paleont. Brazil,” _Archiv. Mus. Nac. Rio de
Janeiro_, VII., 1887, p. 212, Pl. XXII., Figs. 1, 2.]
[Footnote 227: _Études Pal. Tunis, I, Ceph._, 1907, p. 217, Fig. 83, Pl.
X., Figs. 1, 2.]
[Footnote 228: Solger, _Foss. Mungokreide in Kamerun_, 1904, p. 137, Pl.
V., Figs. 8, 9.]
[Footnote 229: _Amm. Crét. supér. de l’Algérie_, 1897, p. 82, Pl. XVI.,
Figs. 9, 10; Pl. XVIII., Fig. 20. Pervinquière, _Études Pal. Tunis, I.,
Ceph._, 1907, p. 379, Pl. XXIII., Fig. 7.]
[Footnote 230: Specimens of _Hoplitoides_ have also been found at Reme,
but are not sufficiently well-preserved for specific identification. An
imperfect Ammonite from Kumberi resembles in the form of its shell and
in the general character of the sutures _Coilopoceras lesseli_, Brüggen,
_Neues Jahrb. für Min._, etc., Beil.-Band XXX., 1910, p. 734, Plate
XXIX.]
INDEX
INDEX OF AUTHORS
Alexander, Boyd, 2, 13, 31, 45, 55
Allen and Thomson, 2
Ammon, A. von, 122
Andrews and Bailey, 199
Baikie, W. B., 2, 214
Bailey, _see_ Andrews
Barratt, J., 122
Barth, H., 2, 52, 58, 89, 106, 181, 186, 193
Bather, F. A., 165
Böhm, J., 279
Bornhardt, W., 18, 245
Brüggen, 286
Campbell, J. M., 209, 211
Chamberlain and Salisbury, 196
Chautard, J., 122, 205, 207, 253, 263
Chevalier, A., 122, 233
Choffat, P., 282
Chudeau, R., 109, 120, 122, 135, 140, 141, 144, 162, 165, 169, 170,
191–5, 199, 219, 220, 233, 244, 250, 251, 258–60, 262, 263, 283; _see
also_ Gautier
Clapperton, _see_ Denham
Coquand, H., 277, 279
Corstorphine, _see_ Hatch
Courtet, H., 106, 122, 193, 216
Crick, G. C., 273
Crowther, S. A., 2
Dames, 273
Denham and Clapperton, 2, 90
D’Orbigny, 278, 280
Downes, W. D., 104, 258
Dunstan, W. R., 92, 96, 265
Du Toit, _see_ Rogers
Edlinger, W., 186, 233
Esch, E., 122, 163, 191, 226, 250, 263
Evans, J. W., 269
Falconer, J. D., 257–8, 263, 273
Flamand, G. B. M., 121
Flett, J. S., 131
Forteau, _see_ Peron
Foureau, F., 135, 206
Foureau and Gentil, 135
Freydenberg, H., 135, 206, 215, 218, 220; _see also_ Gentil
Garde, G., 135, 140, 141, 216, 219, 258
Gautier, E. F., 226, 233
Gentil, L., 121, 135, 140, 258
Gentil and Freydenberg, 106, 134, 135, 140, 258
Guéranger, 279
Guillemain and Harbort, 275
Gurich, G., 174, 236
Harker, A., 264
Harbort, _see_ Guillemain
Harrison, J. B., 246
Hatch, F. H., 206
Hatch and Corstorphine, 206
Haug, E., 121
Holland, Sir T. H., 207
Horwood and Wade, 122, 143
Hubert, H., 69, 109, 121, 135, 140, 168, 170, 191, 196, 202, 204, 207,
226, 249
Hyatt, 282
Knox, A., 122
Kossmat, 284
Lacoin, L., 206, 258
Lacroix, A., 135, 138, 263
Laird and Oldfield, 2, 6
Lander, R. and J., 2
Lapparent, A. de, 162, 165, 194, 207, 229
Lenfant, Capt., 2, 8
Lenz, O., 122, 197
Longbottom, A., 13–16, 20, 22, 33–5, 87, 96, 98, 103–6, 147–51, 153,
154–7, 184–91, 227–8, 261–2, 273
Lugard, Sir F. D., 2
Maclaren, J. M., 209
Maufe, H. B., 208, 211
Mennell, F. P., 204, 209, 211
Merrill, G. P., 246
Newton, R. B., 165, 281
Oldfield, _see_ Laird
Park, Mungo, 2
Parkinson, J., 81, 112, 119, 122, 163, 191, 226, 250
Passarge, S., 2, 122, 140, 186, 191, 198, 204, 207, 224, 233, 237,
241, 245, 247, 248, 258, 259, 263
Peron, A., 279, 282, 286
Peron and Fortau, 280
Peron and Thomas, 281, 282
Pervinquière, L., 282, 284–6
Petrascheck, 284
Prior, G. T., 264
Quaas, 280
Rogers and Du Toit, 206
Rohlfs, G., 200
Salisbury, _see_ Chamberlain
Solger, F., 285
Stoliczka, 276, 280, 281, 284
Suess, E., 140, 165, 233, 247, 248, 252
Thomas, _see_ Peron
Thomson, _see_ Allen
Vandeleur, S., 7
Van Hise, C. R., 118
Vogel, H., 2
Voit, F. W., 123
Wade, _see_ Horwood
Wallace, Sir W., 2, 268
Walther, J., 199, 207
Whiteaves, 281
Whitlock, G. F. A., 32
Woods, H., 166, 273, 277–81
Woodward, A. S., 273
GENERAL INDEX
Abuja: the Rock, 27
granites and gneisses of, 99
Alpine movements in Sudan, 251
Ansongo, flints of, 170
Arofu: Cretaceous rocks of, 148
saltings of, 149, 266
silicified limestone of, 149
galena veins of, 149
Augite-syenite, 135
Awe: brine springs of, 148, 266
Cretaceous rocks of, 146
volcanic hills of, 31, 260, 262
Axes of elevation: 234
Bagele hills: 14, 34, 186
Barbur basalt plateau, 146, 257
Baro, 4, 173
Basalts, 139
of Sahara, 140
of Tertiary age, 253
nepheline basalts, 255
plateau basalts, 51, 100, 257
Bassa: scenery of, 25–6
crystalline rocks of, 82
Bauchi plateau, 3, 38–43
summit of, 40
limits of, 41
scenery of, 42
volcanic hills of, 42
drift deposits of, 43, 203
former continuity with plains of Hausaland, 203
elevation of, 237
not a horst formation, 237
Bauchi province, scenery of, 50
eastern plains of, 44
crystalline rocks of, 101
intrusive rocks of, 102, 133
Benue, watersheds of, 3
tributaries of, 10
scenery of, 11
volcanic tract of, 12
plain of, 13
sandbanks of, 15
drift of Benue valley, 223
antecedent character of the upper and middle Benue valley, 231, 243
Bida, 24
Bima, hills, 50
sandstones, 161
Borgu, scenery of, 20
crystalline rocks of, 69
drift of, 196
Bornu province, scenery of, 50–56
sandstones of, 50, 179
volcanic plateau of, 51
northern plains of, 52–3
crystalline rocks of, 106
drift of, 215
area of predominant depression, 218
dry water-courses of, 220
Bussa, 8, 9, 19, 71
Calc silicate rocks, 114
Carboniferous rocks of Sahara, 144
Cassiterite, _see_ Tinstone
Chad, lake, 2, 52
crystalline rocks near, 107
stratigraphy of rocks around, 192–3
ferruginous clay on shores of, 206
alluvial deposits of, 215
shelly marls of, 216, 218
former greater extent of, 217
not continuous with Eocene seas, 217
excavation of present floor of, 219
climatic changes in Chad area, 219, 240
depression of Chad area, 216, 217, 240
Clays,
of Giro, 168
of Kurukuru, 168
of Pali, 177
of Kirifi, 178
of Deu, 178
of Gombe, 178
Climate, changes of, in Sudan, 210
in Hausaland, 214
in Chad area, 219
favourable to formation of surface ironstones, 210
Cretaceous rocks, distribution of, 144
relation to crystallines, 146
of Muri province, 147
of Yola province, 154
of Bauchi and Bornu, 157
folding of, 147, 154, 158
classification of, 161
of French Sudan, 162
of Southern Nigeria, 163
of Kameruns, 163
palæontology of, 273 _et seq._
Crustal movements, _see_ Movements
Dala, _see_ Mt. Dala
Dallols, 61, 242, 250, 251
Desert periods in Nigeria, 214, 217
recent arid period, 210, 214, 219, 240
Devonian rocks of Sahara, 144
Diabase, 139
Diorites, of Bauchi, 136
of Kano, 136
of Kontagora, 136
diorite porphyrites, 138
Drainage system, recent rearrangement of, 198, 213, 233, 240
Drift, 195
of Hausaland and Borgu, 196, 199
ferruginous crust of, 199
origin of, 201
of Bauchi plateau, 203
of Bornu, 215
of Katagum, 221
of Gongola valley, 222
of Benue valley, 223–4
of Sahara, 226
Duguri, canyon valleys of, 48
sandstones of, 175
Dunes, ancient, 53, 58, 214
Eocene rocks of Northern Nigeria, 164
unconformable upon Cretaceous, 164, 175
of Sokoto province, 165
of the Niger valley, 169
conditions of accumulation of, 171
of Kontagora, 172
of Lokoja, 174
of Nupe, 173
of Bassa, 174
of Nassarawa, 174
of Kabba, 173, 175
of Duguri, Gombe and Kerri Kerri, 175
age of, 181
unconformity upon Cretaceous, 182–3
of Yola, 184
of Muri, 189
of Upper Niger, 191
of Kameruns, 191
Eocene rocks of Southern Nigeria, 191
basins of sedimentation, 228
probable extension over Central Africa, 193, 229
Faulting, of Cretaceous rocks, 145, 147, 154, 157, 158
of margin of Bauchi plateau, 237
on Upper Benue, 190
Ferruginous crust of drift deposits, _see_ Surface ironstones
Firki of Bornu, 52, 215
Galena, of Arofu, 149, 270
of Kudu valley, 154, 270
Gateri Hills, 46, 48, 153
grits of, 153
Gneisses, softer and harder, 69, 108
replacement of softer by harder in Niger valley, 74, 120
in Kabba and Ilorin, 80, 84
near Abuja, 99
in Bauchi, 102
relations of harder and softer, 119
of Sahara, 121
of West Africa, 122
of Transvaal, 123
meridional strike of, 109
of sedimentary origin, 110
metamorphism of, 111
Gold, in Northern Zaria, 96, 269
in Southern Muri, 269
in Bauchi, 269
Gombe, grits and clays of, 175, 178
Gongola, plain of, 14, 33, 47
valley of, 46, 50
drift of, 222
antecedent character of middle Gongola valley, 231, 244
Granites, 123
petrography of older, 123
of younger, 129
soda granites, 102, 130, 135, 140
age of younger, 140
periods of intrusion of, 142
Granulites, 116
Hausaland, provinces of, 3
scenery of, 21, 35
plains of, 21, 36, 198
drift of, 196 _et seq._
surface ironstones of, 199
origin of present aspect of, 198, 246
periods of elevation and depression of, 213
Ibi, 12
Ilo, 4
Ilorin, orographical structure of, 16
great plain of, 19
crystalline rocks of, 75
Inselberge of Kabba, 17
of Borgu, 20
of Kontagora, 22
of Nassarawa, 27
of Muri, 31
of Yola, 35
of Zaria, 36
of Bauchi, 44
of Kano, 56
of Sokoto, 60
origin of, 201, 246
Inselberg landscape, 18, 245
Intrusive rocks of Bauchi province, 102
petrography of, 123
older granites, 123
younger granites, 129
pegmatites, 129
syenites and diorites, 135
porphyries, 137
basalts, 137
Iron sands, 268
Ironstones, crystalline, 78, 81, 110, 114, 269
Cretaceous, 154, 160
Eocene, 164, 269
of Sokoto, 166
of Kontagora, 172
of Baro (Mt. Gidda), 173
of Lokoja (Mt. Patti), 174, 269
of Kantana, 177
of Panguru, 178
of Tagum hills, 179
of Kerri Kerri, 180
_See also_ Surface ironstones
Jakpara hills, 18
Jakura and Wa, limestone of, 66, 78, 110, 113
Jebba, 7 _et seq._
gorge of the Niger above, 7
Ju Ju Rock of, 8, 70
quartzites of, 70
Kabba, orographical structure of, 16
central tableland of, 16
scenery of, 17, 37
crystalline rocks of, 77 _et seq._
Kaduna, scenery of, 24, 36
tributaries of, 36
gorge at Zungeru, 35–6
in axial trough, 234
Kano, province of, 56–8
Mts. Dala and Kogon Dutsi, 58, 90, 136, 201, 247
crystalline rocks of, 89
drift of, 196
Karakai quartzites, 60, 88
Kara river, 92
Katagum, 57
drift of, 221
Kato phyllites, 95
Katsina, scenery around, 59
granites of, 89
granite surface rock, 89
sandstones of, 89
Kazaure, blue quartzite of, 58, 89–90
Kerko hills, granite massifs of, 37
Kerri Kerri, plateau of, 54
trench-like valleys of, 56
rocks of, 179
ironstones of, 181
Kilba hills, 35, 105
Kogon Dutsi, _see_ Mt. Dala
Kontagora, 21
scenery of, 22
crystalline rocks of, 91, 92
sandstones of, 170
Kurefi, 89
Kwaba hill, 33
Lake Chad, _see_ Chad
Limestone, crystalline, between Jakura and Wa, 66, 78, 110
at Igbo, 82
origin of, 110
petrography of, 113
Turonian, of Muri province, 144, 147
of Awe, 147
of Kumberi, 150
of Arofu, 149
of Yola, 155
of Gongola valley, 159
of Gongila, 169
of Reme, 160
of Sudan, 162
palæontology of, 273 _et seq._
Eocene, of Sokoto, 165
nummulitic, 167
Lokoja, 2, 6, 82, 174
crystalline rocks of, 82
sandstones and ironstones of, 174
Loko, 11
Mada hills, peaks of, 30
Magnetite schists, 78, 81, 110, 114
Manga, 238
Maradu, volcanic agglomerates of, 60, 88, 254
Mahorro, ironstones of, 22, 172, 269
Maibirro, quartzite hills of, 21, 73
Malenda river, granites and quartzites of, 21
Metamorphism, absence of contact metamorphism in Kabba, 83
variation in alternate belts, 111
of granites, 123
Mineral resources of Northern Nigeria, 265
Miocene rocks of Sudan, 194
Monazite, 270
Morrua, plains of, 29
Mts. Dala and Kogon Dutsi, 58, 90, 91, 201, 205, 212, 225, 247
Mt. Patti, 6, 11, 174, 269
Mt. Purdy, 6
Movements, crustal, post-Cretaceous, 164, 227
Tertiary, 212, 227 _et seq._
alternate periods of elevation and depression, 213, 230
flexuring of crust, 234, 241
elevation of Bauchi plateau, 237
Alpine movements in Nigeria, 252
Mumuye-Manna plateau, 32, 104
Murchison hills, 2, 12, 31, 103, 231
Muri, great plain of, 12, 30
sandstones, 153
crystalline rocks of, 30, 103
Cretaceous rocks of, 145, 147
volcanic hills of, 31, 262
Muri sandstones, 153
Nassarawa, province of, 26
scenery of, 27–30
central plain of, 28
northern tableland of, 29, 39
crystalline rocks of, 100
Nepheline basalt, 255
syenite, 259
Nigeria, Northern, area of, 1
boundaries of, 1
provinces of, 2
headquarters of, 2
physical features of, 2
hydrographical systems of, 3
hydrographical centre of, 3, 40
recent origin of river system of, 240–243
crystalline rocks of, 63
Cretaceous rocks of, 144
Eocene rocks of, 164
superficial accumulations of, 194
mineral resources of, 265
Niger, the, 4 _et seq._
watersheds of, 3, 4, 19, 36
tributaries of, 5
gorge above Lokoja, 7
sandstone plateaux of, 7
gorge between Jebba and Yelwa, 8
rapids of, 9
establishment of present course of, 241
relation of scenery to character of rocks, 242
recent origin of Niger valley, 243
Ningi hills, 44
Ningi massif, 102
Nodules, phosphatic, in Turonian limestones, 156
in Eocene limestones, 166
Nupe, scenery of, 23
crystalline rocks of, 98
Odouape, granite and gneisses, 80
Olle, 114
Okoro, 77
Okuruku granite, 126
quartz magnetite rock of, 81, 115
Okwa river, 28
Orissa hills, 19, 76
Petrography, of gneisses and schists, 108
of intrusive rocks, 119
of early Tertiary lavas, 254
of later Tertiary lavas, 262
petrographical provinces, 263
Phonolites of Northern Nigeria, 256
Phyllites, of Bussa, 71
of Ngashki, 73
of Sokoto, 86–8
of Zungeru belt, 93–5
petrography of, 112
Porphyries, 137
Porphyrites, 138–9
Quartzites, of Jebba, 70
of Maibirro, 73
of Lukke, 78
of Kazaure, 89
of Zungeru, 93
petrography of, 113
River system of Northern Nigeria recent origin of, 233, 241
Salt, 265
in western Muri, 148
in eastern Muri, 151
between Putu and Tangalto, 158
vegetable salt of Bornu 267
Shappa hills, 19, 76
Shebshi hills, 30, 32
Silurian rocks of Sahara, 121
Sokoto, province of, 60
dallols of, 61, 242, 250
crystalline rocks of, 86
Eocene rocks of, 165
Superficial accumulations of Nigeria, 194
of West Africa, 226
Surface ironstones of Borgu, 20, 199
of Bauchi and Zaria, 199
platforms of, 200
occurring upon drifted alluvium, 205
mode of formation, 205–211
period of formation, 211
absence in Bornu, Katagum and Gongola valley, 215
vesicular character of, 199
lustrous surface of, 199
irregular distribution of, 209
Syenites of Bauchi, 135
Syenite porphyries, 138
Tabachi, 73
Tagum hills, 52, 179
Tangale hills, 47, 48
Peak, 49
Tinstone, in eastern Ilorin, 77, 131
in eastern Zaria, 98
in Bauchi, 103
in pegmatites, 131
in alkaline granites, 134
in plateau drift, 203
economic development of, 270
Tongolan, 176
Tourmaline, in quartzites, 70
in quartz veins, 72, 73
in pegmatites, 76, 82, 99
in granites, 85, 99, 130
in slaty schists, 88
associated with younger granites, 129
schorl rocks, 131
Trachytes of Bussa, 72, 115
Traverses, typical, 64
Ture Peak, 49
Upper Benue sandstones, 155, 184
relation to crystalline rocks, 184
unconformable upon Cretaceous, 188
cross-bedding in, 187
mode of formation, 187, 190
Valleys, canyon, of Bauchi, 46, 244
flat-bottomed valleys of Sokoto, 61, 242
shallow valleys of Borgu, 20
of Hausaland, 22, 57
canyon, of Kontagora, 23, 242
canyon of Duguri, 48, 244
trench-like, of Kerri Kerri, 56
interlacing valleys of Kerri Kerri, 56, 245
Vere hills, 14, 34, 105, 247
Volcanic hills of Muri, 12, 31, 260
of Yola, 13, 35, 260
Barbur volcanic plateau, 51, 257
volcanic agglomerate of Maradu, 60, 88, 254
Morrua volcanic plateau, 100, 203
absence of pre-Tertiary volcanic rocks, 253
character of Tertiary volcanic eruptions, 254
early Tertiary volcanic period, 254
later Tertiary volcanic period, 259
Wase river, 39, 45
Wase rock, 31
phonolite of, 256
Watersheds, primary, 3, 36, 40
character of, 36, 40, 241
Weathering: absence of weathered crust upon rocks of Sudan, 195
dry weathering not a potent factor in denudation, 199
former sheet of weathered rock in Nigeria, 201
Wuru, Great Rapid of, 8, 9
Wurkum hills, 30
Yo river, 4, 234
Yola, eastern, 230
scenery of, 32–5
southern plateau of, 32, 104, 227
volcanic hills of, 13, 35, 260
crystalline rocks of, 103
Cretaceous rocks of, 154
Eocene rocks of, 184
Zaria, province of, 35
scenery of, 37
hills of, 36
crystalline rocks of, 93–98
plains of, 38
vesicular ironstone of, 200
drift of, 197
Zungeru, 2, 35
quartzites and schists of, 93
* * * * *
ERRATUM.
p. 203, line 13, for “larvas” _read_ “lavas.”
RICHARD CLAY AND SONS, LIMITED, BREAD STREET HILL, E.C., AND BUNGAY,
SUFFOLK.
[Illustration: TOPOGRAPHICAL AND GEOLOGICAL MAP OF NORTHERN NIGERIA
Compiled by Dr. J. D. FALCONER.
London. Macmillan & Co., Ltd.]
Transcriber's note:
The change in the ERRATUM has been made.
Page 6, Changed: "down on either si de, a n from" to: "side, and"
Page 45, One ref. to footnote missing. Ref. "2" treated as ref. to
ftn. 13, and a ref. to ftn. 14 placed after "the plains of Yergum."
Page 49, Changed: "those which form the the western boundary" to:
"form the western"
Page 115, Changed: "may be recognised all the familar stages" to:
"familiar"
Page 122, Changed: "comparision which M. Chudeau" to: "comparison"
Page 123, Changed: "the epidorites, amphibolites, and granulites" to:
"epidiorites"
Page 151, Changed: "Gateri but the the associated rocks" to: "but the
associated"
Page 154, Changed: "corrugation ofthea whole series of gently" to: "of
the whole"
Page 154, Changed: "limestones of the Kudu vlley" to: "valley"
Page 167, Changed: "limestones, however, no here form thick" to:
"nowhere"
Page 207, footnote 121, Changed: "De Lapparent, _Traité des Geologie_"
to: "_Traité de Géologie_"
Page 232, Changed: "that lacustrune conditions prevailed" to:
"lacustrine"
Page 233, footnote 150, Changed: "Passarge, _Adamana_, 1895, p. 385."
to: "_Adamaua_"
Page 290, Changed: "Stroliczka" to: "Stoliczka"
Minor changes in punctuation have been done silently.
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