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Title: The Principles of Stratigraphical Geology
Author: J. E. Marr
Release Date: October 16, 2013 [EBook #43963]
Language: English
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Cambridge Natural Science Manuals.
Geological Series.
THE PRINCIPLES
OF
STRATIGRAPHICAL GEOLOGY
London: C. J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE.
AND
H. K. LEWIS,
136, GOWER STREET, W.C.
[Illustration]
Leipzig: F. A. BROCKHAUS.
New York: THE MACMILLAN COMPANY.
Bombay: E. SEYMOUR HALE.
THE PRINCIPLES
OF
STRATIGRAPHICAL GEOLOGY
BY
J. E. MARR, M.A., F.R.S.
FELLOW AND LECTURER OF S. JOHN'S COLLEGE, CAMBRIDGE,
AND UNIVERSITY LECTURER IN GEOLOGY.
CAMBRIDGE:
AT THE UNIVERSITY PRESS.
1898
[_All Rights reserved._]
Cambridge:
PRINTED BY J. & C. F. CLAY,
AT THE UNIVERSITY PRESS.
PREFACE.
The present work has been written in order that students may gain by
its perusal some idea of the methods and scope of Stratigraphical
Geology. I believe that this idea can be obtained most satisfactorily,
if a large number of the details connected with the study of the
stratified rocks are omitted, and I have accordingly given very brief
accounts of the strata of the different Systems.
The work is intended for use in conjunction with any book which treats
of the strata of the Geological Column at considerable length; some of
these books are mentioned on pages 124, 125.
J. E. M.
Cambridge,
_November, 1898_.
CONTENTS.
PAGE
CHAPTER I.
Introduction 1
CHAPTER II.
Account of the growth and progress of stratigraphical geology 6
CHAPTER III.
Nature of the stratified rocks 21
CHAPTER IV.
The law of superposition 31
CHAPTER V.
The test of included organisms 40
CHAPTER VI.
Methods of classification of the strata 58
CHAPTER VII.
Simulation of structures 72
CHAPTER VIII.
Geological maps and sections 84
CHAPTER IX.
Evidences of conditions under which strata were formed 97
CHAPTER X.
Evidences of conditions under which strata were formed, continued 116
CHAPTER XI.
The classification of the stratified rocks 125
CHAPTER XII.
The Precambrian rocks 132
CHAPTER XIII.
Cycles of change in the British area 149
CHAPTER XIV.
The Cambrian system 152
CHAPTER XV.
The Ordovician system 164
CHAPTER XVI.
The Silurian system and the changes which occurred in Britain
at the close of Silurian times 174
CHAPTER XVII.
The Devonian system 183
CHAPTER XVIII.
The Carboniferous system 192
CHAPTER XIX.
The changes which occurred during the third continental period
in Britain; and the foreign Permo-Carboniferous rocks 202
CHAPTER XX.
The Permian system 209
CHAPTER XXI.
The Triassic system 218
CHAPTER XXII.
The Jurassic system 226
CHAPTER XXIII.
The Cretaceous system 236
CHAPTER XXIV.
The Eocene rocks 244
CHAPTER XXV.
The Oligocene and Miocene periods 251
CHAPTER XXVI.
The Pliocene beds 256
CHAPTER XXVII.
The Pleistocene accumulations 260
CHAPTER XXVIII.
The Steppe period 267
CHAPTER XXIX.
The Forest period 275
CHAPTER XXX.
Remarks on various questions 278
ADDENDA ET CORRIGENDA. [TN: Corrections made!]
p. 38, line 15 from bottom: for 'joining' read 'jointing'
p. 208, line 6 from bottom: for 'Dr' read 'Messrs Medlicott and'
p. 214, line 15 from bottom: after 'Permo-Carboniferous Strata'
insert 'through the Permian'
p. 217, last line of footnote: for 'Dr' read 'Messrs Medlicott and'
" insert a second footnote: 'For information concerning the
Permian volcanic rocks see Sir A. Geikie's _Ancient Volcanoes
of Great Britain_.'
p. 235, insert a footnote: 'A good account of the British Jurassic
rocks will be found in Mr H. B. Woodward's Memoir on
"The Jurassic Rocks of Britain." _Mem. Geol. Survey_,
1893--.'
p. 250, top line: for 'Gardiner' read 'Gardner'
CHAPTER I.
INTRODUCTION.
It is the aim of the Stratigraphical Geologist to record the events
which have occurred during the existence of the earth in the order in
which they have taken place. He tries to restore the physical
geography of each period of the past, and in this way to write a
connected history of the earth. His methods are in a general way
similar to those of the ethnologist, the archæologist, and the
historian, and he is confronted with difficulties resembling those
which attend the researches of the students of human history. Foremost
amongst these difficulties is that due to the imperfection of the
geological record, but similar difficulty is felt by those who pursue
the study of other uncertain sciences, and whilst this imperfection is
very patent to the geologist, it is perhaps unduly exaggerated by
those who have only a general knowledge of the principles and aims of
geology.
The history of the earth, like other histories, is a connected one, in
which one period is linked on to the next. This was not always
supposed to be the case; the catastrophic geologist of bygone times
believed that after each great geological period a convulsion of
nature left the earth's crust as a _tabula rasa_ on which a new set of
records was engraved, having no connexion with those which had been
destroyed. Careful study of the records of the rocks has proved that
the conclusions of the catastrophists were erroneous, and that the
events of one period produce their impression upon the history of the
next. Every event which occurs, however insignificant, introduces a
new complication into the conditions of the earth, and accordingly
those conditions are never quite the same. Although the changes were
no doubt very slow, so that the same general conditions may be traced
as existent during two successive periods, minor complications
occurred in the inorganic and organic worlds, and we never get an
exact recurrence of events. Vegetable deposits may now be in process
of accumulation which in ages to come may be converted into coal, but
the general conditions which were prevalent during that Carboniferous
period when most of our workable coal was deposited do not now exist,
and will never exist again. The changes which have taken place and
which are taking place show an advance from the simple to the more
complex, and the stratigraphical geologist is confronted with a
problem to which the key is development, and it is his task to trace
the development of the earth from the primitive state to the complex
condition in which we find it at the present day.
Our general ignorance of the events of the earliest periods of the
history of the earth will be emphasised in the sequel, and it will be
found that the complexity which marks the inorganic and organic
conditions which existed during the deposition of the earliest rocks
of which we have detailed knowledge points to the lapse of enormous
periods of time subsequent to the formation of the earth, and previous
to the deposition of those rocks. The imperfection of the record is
most pronounced for that long period of time, but in this respect the
geologist is in the same condition as the student of human history,
for the relics of the early stone age prove that man in that age had
attained a fairly high state of civilisation, and the gap which
separates palæolithic man from the first of our race is relatively
speaking as great as that which divides the Cambrian period from the
commencement of earth-history. Nevertheless, human history is a
science which has made gigantic strides towards the solution of many
problems connected with the development of man and civilisation, and
similarly geology has advanced some way in its task of elucidating the
history of our globe.
The task of the stratigraphical geologist is two-fold. In the first
place, he must establish the order of succession of the strata, for a
correct chronology is of paramount importance to the student of
earth-lore. The precautions which must be taken in making out the
order of deposition of the rocks of any area, and correlating those of
one area with those of another will be considered in the body of the
work. When this task is completed, there yet remains the careful
examination of all the information supplied by a study of the rocks of
the crust, in order to ascertain the actual conditions which existed
during the deposition of any stratum or group of strata. In practice,
it is generally very difficult to separate these two departments of
the labour of the stratigraphical geologist, and the two kinds of work
are often done to a large extent simultaneously, or sometimes
alternately. Frequently the general succession of the deposits
comprising an important group is ascertained, and at the same time
observations made concerning the physical characters of the deposits
and the nature of their included organisms, which are sufficient to
afford some insight into the general history of the period when these
deposits were laid down; a more detailed classification of the same
set of deposits may be subsequently made, and as the result of this,
more minute observations as to the variations in the physical and
biological conditions of the period are possible, which permit us to
write a much more concise history of the period. So great has been the
tendency to carry on work in a more and more detailed manner, that it
is very difficult if not impossible to tell when any approach to
finality is reached in the study of a group of strata in any area.
Roughly speaking, we may state that our knowledge of a group of strata
is obtained by three processes, or rather modifications of one
process. The general order of succession is established by the
pioneer, frequently as the result of work carried on through one or
two seasons. Subsequently to this, a more minute subdivision of the
rocks is possible as the result of labours conducted by one or more
workers who are enabled to avail themselves of the work of the
pioneer, and our knowledge of the rocks is largely increased thereby.
But the minutiæ, often of prime importance, are supplied by workers
who must spend a large portion of their time in the area where the
work lies, and it is only in districts where work of this character
has been performed, that our knowledge of the strata approaches
completion. The strata of the Arctic regions, for example, have in
many places been examined by pioneers, but a great deal remains to be
done in those regions; the main subdivisions only have been defined in
many cases, and our information concerning the physical history of
Arctic regions in past times is comparatively meagre. To come nearer
home--a few miles north of Cambridge lies the little patch of
Corallian rock at Upware; it has been frequently visited, and a large
suite of organic remains extracted from it, but no one has devoted the
time to the collection of remains from this deposit which has been
devoted to that of some other formations presently to be mentioned,
and accordingly our knowledge of the fauna of that deposit is far from
complete. Contrast with this the information we possess of the little
seam known as the Cambridge Greensand, from which organic remains have
been sedulously collected during the extensive operations which have
been carried on for the extraction of the phosphatic nodules which
occur in the seam. The suite of relics of the organisms of that period
is accordingly far more perfect than in the case of many other beds,
and indeed the large and varied collection of relics of the vertebrata
of the period which furnish much information of value to the
palæontologist would not have been gathered together, had not this
seam been so carefully worked, and an important paragraph in the
chapter bearing on the history of this period would have remained
unknown to us. Again, two little patches of limestone of the same age,
one in central England and the other in the island of Gothland, have
been the objects of sedulous inquiry by local observers, and we find
again that our knowledge of the physical history of the period, as
regards these two regions, is exceptionally perfect. Special stress is
laid upon this point, for in these days, when every county possesses
its learned societies whose members are desirous of advancing in every
possible way the progress of science, it is well to insist upon the
importance of this detailed work which can only be done by those who
have a large amount of time to devote to the rigorous examination of
the rocks of a limited area.
CHAPTER II.
ACCOUNT OF THE GROWTH AND PROGRESS OF STRATIGRAPHICAL GEOLOGY.
The history of the growth of a science is not always treated as an
essential part of our knowledge of that science, and many text-books
barely allude to the past progress of the science with which they
deal. The importance of a review of past progress has, however,
attracted the attention of many geologists, and Sir Charles Lyell, in
his _Principles of Geology_, gave prominence to an historical sketch
of the rise and progress of the science. Historical studies of this
nature have more than an academic value; the very errors made by men
in past times are useful as warnings to prevent those of the present
day from going astray; the lines along which a science has progressed
in the past may often be used as guides to indicate how work is to be
conducted in the future; but perhaps the greatest lesson which is
taught by a careful consideration of the rise and progress of a study
is one which has a moral value, for he who pays attention to the
growth of his science in past times, gains a reverence for the old
masters, and at the same time learns that a slavish regard for
authority is a dangerous thing. This is a lesson which is of the
utmost importance to the student who wishes to advance his science,
and will prevent him from paying too little attention to the work of
those who have gone before him, whilst it will enable him to perceive
that as great men have fallen into error through not having sufficient
data at their disposal, he need not be unduly troubled should he find
that conclusions which he has lawfully attained after consideration of
evidence unknown to his predecessors clash with those which they
adopted. Want of this historic knowledge has no doubt caused many
workers to waste their time on work which has already been performed,
but it has also led others to withhold important conclusions from
their fellow-workers because they were supposed to be heterodox. In an
uncertain science like geology one of the great difficulties is to
keep an even balance between contempt and undue respect for authority,
and assuredly a scientific study of the past history of a science will
do much to enable a student to attain this end. It will be useful,
therefore, at this point to give a brief account of the rise and
progress of the study of stratigraphical geology, so far as that can
be done without entering into technical details, at the same time
recommending the student to survey the progress of this branch of our
science for himself, after he has mastered the principles of the
subject, and such details as are the property of all who have studied
the science from the various text-books written for advanced students.
William Smith, the 'Father of English Geology,' is rightly regarded as
the founder of stratigraphical geology on a true scientific basis, but
like all great discoverers, his work was foreshadowed by others,
though so dimly, that this does not and cannot detract from his fame.
It is desirable, however, to begin our historical review at a time
somewhat further back than that at which Smith gave to the world his
epoch-making map and memoirs.
Before the eighteenth century, stratigraphical geology cannot be said
to have existed as a branch of science--the way had not been prepared
for it. Data had been accumulated which would have been invaluable if
at the disposal of open-minded philosophers, but with few exceptions
prejudice prevented the truth from becoming known. There were two
great stumbling-blocks to the establishment of a definite system of
stratigraphical geology by the writers of the Middle Ages, firstly,
the contention that fossils were not the relics of organisms, and,
secondly, when it was conceded that they represented portions of
organisms which had once existed, the assertion that they had reached
their present positions out of reach of the sea during the Noachian
Deluge. For full details concerning the mischievous effects of these
tenets upon the science the reader is referred to the luminous sketch
of the growth of geology in the first four chapters of Sir Charles
Lyell's _Principles of Geology_.
The disposition of rocks in strata, and the occurrence of different
fossils in different strata, was known to Woodward when he published
his _Essay toward a Natural History of the Earth_ in 1695, and the
valuable collections made by Woodward and now deposited in the
Woodwardian Museum at Cambridge, show how fully he appreciated the
importance of these facts, though he formed very erroneous conclusions
from them, owing to the manner in which he drew upon his imagination
when facts failed him, maintaining that fossils were deposited in the
strata according to their gravity, the heaviest sinking first, and the
lightest last, during the time of the universal deluge. The following
extracts from Part II. of Woodward's book, show the position in which
our knowledge of the strata stood at the end of the seventeenth
century: "The Matter, subsiding ..., formed the _Strata_ of Stone, of
Marble, of Cole, of Earth, and the rest; of which Strata, lying one
upon another, the Terrestrial Globe, or at least as much of it as is
ever displayed to view, doth mainly consist.... The Shells of those
Cockles, Escalops, Perewinkles, and the rest, which have a greater
degree of Gravity, were enclosed and lodged in the _Strata_ of Stone,
Marble, and the heavier kinds of Terrestrial Matter: the lighter
Shells not sinking down till afterwards, and so falling amongst the
lighter Matter, such as Chalk, and the like ... accordingly we now
find the lighter kinds of Shells, such as those of the _Echini_, and
the like, very plentifully in Chalk.... Humane Bodies, the Bodies of
Quadrupeds, and other Land-Animals, of Birds, of Fishes, both of the
Cartilaginous, the Squamose, and Crustaceous kinds; the Bones, Teeth,
Horns, and other parts of Beasts, and of Fishes: the Shells of
Land-Snails: and the Shells of those River and Sea Shell-Fish that
were lighter than Chalk &c. Trees, Shrubs, and all other Vegetables,
and the Seeds of them: and that peculiar Terrestrial Matter whereof
these consist, and out of which they are all formed, ... were not
precipitated till the last, and so lay above all the former,
constituting the supreme or outmost _Stratum_ of the Globe.... The
said _Strata_, whether of Stone, of Chalk, of Cole, of Earth, or
whatever other Matter they consisted of, lying thus each upon other,
were all originally parallel: ... they were plain, eaven, and
regular.... After some time the _Strata_ were broken, on all sides of
the Globe: ... they were dislocated, and their Situation varied, being
elevated in some places, and depressed in others ... the Agent, or
force, which effected this Disruption and Dislocation of the
_Strata_, was seated _within_ the Earth."
Woodward's writings no doubt exercised a direct influence on the
growth of our subject, but the indirect effects of his munificent
bequest to the University of Cambridge and his foundation of the Chair
of Geology in that University were even greater, for as will be
pointed out in its proper place, two of the occupants of that chair
played a considerable part in raising stratigraphical geology to the
position which it now occupies.
The discoveries which were made after the publication of Woodward's
book and before the appearance of the map and writings of William
Smith are given in the memoir of the latter author, written by his
nephew, who formerly occupied the Chair of Geology at Oxford[1]. It
would appear that the fact that "the strata, considered as definitely
extended masses, were arranged one upon another in a certain _settled
order_ or _series_" was first published by John Strachey in the
_Philosophical Transactions_ for 1719 and 1725. "In a section he
represents, in their true order, chalk, oolites, lias, red marls and
coal, and the metalliferous rocks" of Somersetshire, but confines his
attention to the rocks of a limited district.
[Footnote 1: _Memoirs of William Smith, LL.D._ By J. Phillips, F.R.S.,
F.G.S. 1844.]
The Rev. John Michell published in the _Philosophical Transactions_
for 1760 an "Essay on the Cause and Phænomena of Earthquakes," but
Prof. Phillips gives proofs that Michell, who in 1762 became
Woodwardian Professor, had before 1788 discovered (what he never
published) the first approximate succession of the Mesozoic rocks, in
the district extending from Yorkshire to the country about Cambridge.
Michell's account was discovered written by Smeaton on the back of a
letter dated 1788. The following is the succession as quoted in
Phillips' memoir (p. 136):
Yards of thickness.
"Chalk 120
Golt 50
Sand of Bedfordshire 10 to 20
Northamptonshire lime and Portland
lime, lying in several strata 100
Lyas strata 78 to 100
Sand of Newark about 30
Red Clay of Tuxford, and several 100
Sherwood Forest pebbles and gravel 50 unequal
Very fine white sand uncertain
Roche Abbey and Brotherton limes 100
Coal strata of Yorkshire --"
The order of succession of the Cretaceous, Jurassic, Triassic and
Permian beds will be readily recognised as indicated in this section,
though the discovery of the detailed succession of the Jurassic rocks
was reserved for Smith.
In the year 1778, John Whitehurst published _An Inquiry into the
Original State and Formation of the Earth_, containing an Appendix in
which the general succession of the strata of Derbyshire is noted. The
main points of interest are that the author clearly recognised the
'toad-stones' of Derbyshire as igneous rocks, "as much a _lava_ as
that which flows from Hecla, Vesuvius, or Ætna," though he believed
that they were intrusive and not contemporaneous, and he also
foreshadows the distinction between the solid strata and the
superficial deposits,--"we may conclude," he says, "that all beds of
sand and gravel are assemblages of adventitious bodies and not
original _strata_: therefore wherever sand or gravel form the surface
of the earth, they conceal the original _strata_ from our observation,
and deprive us of the advantages of judging, whether coal or limestone
are contained in the lower regions of the earth, and more especially
in flat countries where the _strata_ do not basset."
Werner, who was born in 1750, exercised more influence by his teaching
than by his writings. His ideas of stratigraphical geology were
somewhat vitiated by his theoretical views concerning the deposition
of sediment from a universal ocean, in a definite order, beginning
with granite, followed by gneiss, schists, serpentines, porphyries and
traps, and lastly ordinary sediments. He recognised and taught that
these rocks had a definite order "in which the remains of living
bodies are successively accumulated, in an order not less determinate
than that of the rocks which contain them[2]." The limited value of
Werner's stratigraphical teaching is accounted for by Lyell, who
remarks that "Werner had not travelled to distant countries; he had
merely explored a small portion of Germany, and conceived and
persuaded others to believe that the whole surface of our planet, and
all the mountain-chains in the world, were made after the model of his
own province," and the author of the _Principles_ justly calls
attention to the great importance of travel to the geologist. Those
who cannot travel extensively should at any rate pay special attention
to the works published upon districts other than their own, and even
at the present time, the writings of some British workers are apt to
be marked by some of that 'insularity' which our neighbours regard as
a national characteristic.
[Footnote 2: Cuvier's _Eloge_.]
It is now time to turn directly to the work of William Smith, who, of
all men, exercised the most profound influence upon the study of
stratigraphical geology and may indeed be regarded as the true founder
of that branch of the science. The memoir of his life which was
before mentioned is all too short to illustrate the methods of work
which he followed, but in it we can trace his success to three
things:--firstly, his 'eye for a country,' to use a phrase which is
thoroughly understood by practical geologists, though it is hard
to explain to others, inasmuch as it epitomises a number of
qualifications of which the most important are, a ready recognition of
the main geological features from some coign of vantage, an intuitive
perception of what to note and what to neglect, and the power of
storing up acquired information in the mind rather than the note-book,
so that one may use it almost unconsciously for future work; secondly,
ability to draw conclusions from his observations, and thirdly, and
perhaps most important of all in its ultimate results, a facility for
checking these conclusions by means of further observations, and
dropping those which were clearly erroneous, whilst extracting the
truth from those which contained a germ of truth mixed with error.
Besides writers referred to above "some foreign writers, in particular
Scilla and Rouelle, appear to have made very just comparisons of the
natural associations of fossil shells, corals, &c. in the earth, with
the groups of similar objects as they are found in the sea, and thus
to have produced new proofs of the organic origin of these fossil
bodies; but they give no sign of any knowledge of the _limitation of
particular tribes of organic remains to particular strata_, of the
_successive existence of different groups of organization_, on
_successive beds of the antient sea_. Mr Smith's claim to this happy
and fertile induction is clear and unquestionable[3]." We get a clue
to the manner in which he arrived at his view in the following
passage[4]:--"Accustomed to view the surfaces of the several strata
which are met with near Bath uncovered in large breadths at once, Mr
Smith saw with the distinctness of certainty, that 'each stratum had
been in succession the bed of the sea'; finding in several of these
strata abundance of the exuviae of marine animals, he concluded that
these animals had lived and died during the period of time which
elapsed between the formation of the stratum below and the stratum
above, at or near the places where now they are imbedded; and
observing that in the successively-deposited strata the organic
remains were of different forms and structures--Gryphites in the lias,
Trigoniæ in the inferior oolite, hooked oysters in the fuller's
earth,--and finding these facts repeated in other districts, he
inferred that each of the separate periods occupied in the formation
of the strata was accompanied by a peculiar series of the forms of
organic life, that these forms characterized those periods, and that
the different strata could be identified in different localities and
otherwise doubtful cases by peculiar imbedded organic remains[5]."
[Footnote 3: _Memoir of William Smith_, p. 142.]
[Footnote 4: _Ibid._ p. 141.]
[Footnote 5: The work of Smith which directly bears upon the
establishment of the law of identification of strata by included
organisms is published in two treatises, entitled:--
(i) _Strata identified by Organized Fossils_, 4to. (intended to
comprise seven parts, of which four only were published), commenced in
1816.
(ii) _A Stratigraphical System of Organized Fossils_, compiled from
the original Geological Collection deposited in the British Museum.
4to. 1817.]
William Smith seems to have recognised intuitively the truth of a law
which was but dimly understood before his time,--the law of
superposition, which may be thus stated: "of any two strata, the one
which was originally the lower, is the older." This may appear
self-evident but it was certainly not so. As the result of this
recognition he established the second great stratigraphical law, with
which his name will ever be linked, that strata are identifiable by
their included organisms.
Before Smith's time, geological maps were lithological rather than
stratigraphical, they represented the different kinds of rocks seen
upon the surface without regard to their age; since Smith
revolutionised geology, the maps of a country composed largely of
stratified rocks are essentially stratigraphical, but partly no doubt
on account of adherence to old custom, partly on economic grounds, the
majority of our stratigraphical maps are lithological rather than
palæontological, that is the subdivisions of the strata represented
upon the map are chosen rather on account of lithological
peculiarities than because of the variations in their enclosed
organisms. It is hardly likely that Government surveys will be allowed
to publish palæontological maps, which will be almost exclusively of
theoretical interest, and it remains for zealous private individuals
to accomplish the production of such maps. When they are produced, a
comparison of stratigraphical maps founded on lithological and
palæontological considerations will furnish results of extreme
scientific interest.
Turning now from Smith's contributions to the science as a whole, we
may now consider what he did for British geology. His geological map
was published in 1815 and was described as follows:--"A Geological Map
of England and Wales, with part of Scotland; exhibiting the
Collieries, Mines, and Canals, the Marshes and Fen Lands originally
overflowed by the Sea, and the varieties of Soil, according to the
variations of the Substrata; illustrated by the most descriptive Names
of Places and of Local Districts; showing also the Rivers, Sites of
Parks, and principal Seats of the Nobility and Gentry, and the
opposite Coast of France. By William Smith, Mineral Surveyor." The map
was originally on the scale of five miles to an inch. In 1819 a
reduced map was published, and in later years a series of county maps.
He also published several geological sections, including one (in 1819)
showing the strata from London to Snowdon.
The student should compare Smith's map of the strata with one
published in modern times in order to see how accurate was Smith's
delineation of the outcrop of the later deposits of our island.
The following table, taken from Phillips' memoir, p. 146, is also of
interest as showing the development of Smith's work and the
completeness of his classification in his later years, and as
illustrating how much we are indebted to Smith for our present
nomenclature, so much so that as Prof. Sedgwick remarked when
presenting the first Wollaston Medal of the Geological Society to
Smith, "If in the pride of our present strength, we were disposed to
forget our origin, our very speech would bewray us: for we use the
language which he taught us in the infancy of our science. If we, by
our united efforts, are chiselling the ornaments and slowly raising up
the pinnacles of one of the temples of nature, it was he who gave the
plan, and laid the foundations, and erected a portion of the solid
walls by the unassisted labour of his hands."[6]
[Footnote 6: The reader may consult an interesting paper by Professor
Judd, on "William Smith's Manuscript Maps," _Geological Magazine_,
Decade IV. vol. IV. (1897) p. 439.]
Comparative View of the Names and Succession of the Strata.
--------------------+-------------------------+--------------------------
| | Improved table drawn up
Table drawn up | Table accompanying the | in 1815 and 1816 after
in 1799. | map, drawn up in 1812. | the first copies of the
| | map had been issued.
--------------------+-------------------------+--------------------------
| London Clay | 1 London Clay
| Clay or Brick-earth | 2 Sand
| | 3 Crag
| Sand or light loam | 4 Sand
1 Chalk | Chalk | 5 Chalk { Upper
| | { Lower
2 Sand | Green Sand | 6 Green Sand
| Blue Marl | 7 Brick Earth
| Purbeck Stone, Kentish {| 8 Sand
| Rag and Limestone {| 9 Portland Rock
| of the vales {| 10 Sand
| of Pickering and {| 11 Oaktree Clay
| Aylesbury, {| 12 Coral Rag and Pisolite
| Iron Sand and Carstone {| 13 Sand
3 Clay | Dark Blue Shale | 14 Clunch Clay and Shale
| | 15 Kelloway's Stone
| Cornbrash | 16 Cornbrash
4 Sand and Stone | | 17 Sand and Sandstone
5 Clay | |
6 Forest Marble | Forest Marble Rock | 18 Forest Marble
| | 19 Clay over Upper
| | Oolite
7 Freestone | Great Oolite Rock | 20 Upper Oolite
8 Blue Clay }| |
9 Yellow Clay }| |
10 Fuller's Earth }| | 21 Fuller's Earth and
}| | Rock
11 Bastard ditto }| |
and Sundries }| |
12 Freestone | Under Oolite | 22 Under Oolite
13 Sand | | 23 Sand
| | 24 Marlstone
14 Marl Blue | Blue Marl | 25 Blue Marl
15 Blue Lias | Blue Lias | 26 Blue Lias
16 White Lias | White Lias | 27 White Lias
17 Marlstone, Indigo| |
and Black Marls | |
18 Red Ground | Red Marl and Gypsum | 28 Red Marl
19 Millstone | Magnesian Limestone | 29 Redland Limestone
| Soft Sandstone |
20 Pennant Street }| |
21 Grays }| Coal Districts | 30 Coal Measures
22 Cliff }| |
23 Coal }| |
| Derbyshire Limestone | 31 Mountain Limestone
| Red and Dunstone | 32 Red Rhab and Dunstone
| Killas or Slate | 33 Killas
| Granite, Sienite and | 34 Granite, Sienite and
| Gneiss | Gneiss
--------------------+-------------------------+--------------------------
The above table contains a very complete classification of the British
Mesozoic rocks, one of the Tertiary strata which is less complete, and
a preliminary division of the Palæozoic rocks into Permian (Redland
Limestone), Carboniferous (Coal Measures and Mountain Limestone),
Devonian (Red Rhab and Dunstone) and Lower Palæozoic (Killas).
Since Smith's time the main work which has been done in classification
is a fuller elucidation of the sequence of the Tertiary and Palæozoic
Rocks, and this we may now consider.
The Mesozoic rocks are developed in Britain under circumstances which
render the application of the test of superposition comparatively
simple, for the various subdivisions crop out on the surface over long
distances, and the stratification is not greatly disturbed. With the
Tertiary and Palæozoic Rocks it is otherwise, for some members of the
former are found in isolated patches, whilst the latter have usually
been much disturbed after their formation.
Commencing with the Tertiary deposits we may note that "the first
deposits of this class, of which the characters were accurately
determined, were those occurring in the neighbourhood of Paris,
described in 1810 by MM. Cuvier and Brongniart.... Strata were soon
afterwards brought to light in the vicinity of London, and in
Hampshire, which although dissimilar in mineral composition were
justly inferred by Mr T. Webster to be of the same age as those of
Paris, because the greater number of fossil shells were specifically
identical[7]." It is to Lyell that we owe the establishment of a
satisfactory classification of the Tertiary deposits which is the
basis of later classifications. Recognising the difficulty of
applying the ordinary test of superposition to deposits so scattered
as are those of Tertiary age in north-west Europe, he in 1830,
assisted by G. P. Deshayes, proposed a classification based on the
percentage of recent mollusca in the various deposits. It may be
noted, that although this method was sufficient for the purpose, it
has been practically superseded, as the result of increase of our
knowledge of the Tertiary faunas, by the more general method of
identifying the various divisions by their actual fossils without
reference to the number of living forms contained amongst them. The
further study of the British Tertiary rocks was largely carried on by
Joseph Prestwich, formerly Professor of Geology in the University of
Oxford.
[Footnote 7: Lyell, _Students' Elements of Geology_. 2nd Edition, p.
118.]
Amongst the Palæozoic rocks, it has been seen that the Permian,
Carboniferous and some of the Devonian beds were recognised as
distinct by Smith, though a large number of deposits now known to
belong to the last named were thrown in with other rocks as 'killas.'
The Devonian system was established and the name given to it in 1838
by Sedgwick and Murchison, largely owing to the palæontological
researches of Lonsdale. An attempt was subsequently made to abolish
the system, but the detailed palæontological studies of R. Etheridge
finally placed it upon a secure basis. The establishment of the
Devonian system cleared the way for the right understanding of the
Lower Palæozoic rocks, which Sedgwick and Murchison had commenced to
study before the actual establishment of the Devonian system, and to
these workers belongs the credit of practically completing what was
begun by William Smith, namely, the establishment of the Geological
Sequence of the British strata. The controversy which unfortunately
marked the early years of the study of the British Lower Palæozoic
Rocks is well-nigh forgotten, and in the future the names of Sedgwick
and Murchison will be handed down together, in the manner which is
most fitting.
Our account of the growth of British Stratigraphical Geology is not
yet complete. In 1854, Sir William Logan applied the term Laurentian
to a group of rocks discovered in Canada, which occurred beneath the
Lower Palæozoic Rocks. Murchison shortly afterwards claimed certain
rocks in N.W. Scotland as being of generally similar age, and since
then a number of geologists, most of whom are still living, have
proved the occurrence of several large subdivisions of rocks in
Britain, each of which is of pre-Palæozoic age.
The above is a brief description of the growth of our knowledge of the
order of succession of the strata which is the foundation of
Stratigraphical Geology. A sketch of the manner in which the knowledge
which has been obtained has been applied to the elucidation of the
earth's history of different times would require far more space than
can be devoted to it in a work like the present, but some idea of it
may be gained from a study of the later chapters of the book. It will
suffice here to remark, that to Godwin-Austen we owe the foundation of
what may be termed the physical branch of Palæo-physiography, which is
concerned with the restoration of the physical conditions of past
ages, while Cuvier and Darwin have exerted the most influence on the
study of Stratigraphical Palæontology.
CHAPTER III.
NATURE OF THE STRATIFIED ROCKS.
The present constituents of the earth which are accessible for direct
study are divisible into three parts. The inner portion, consisting of
_rocks_, is known as the _lithosphere_; outside this, with portions of
the lithosphere projecting through into the outermost part, is the
_hydrosphere_, comprising the ocean, lakes, rivers, and all masses of
water which rest upon the lithosphere in a liquid condition. The
outermost envelope, which is continuous and unbroken is the
_atmosphere_, in a gaseous condition. It is well known that some of
the constituents of any one of these parts may be abstracted from it,
and become a component of either of the others; thus the atmosphere
abstracts aqueous vapour from the hydrosphere, and the lithosphere
takes up water from the hydrosphere, and carbonic anhydride from the
atmosphere.
The nebular hypothesis of Kant and Laplace necessitates the former
existence of the present solid portions of the lithosphere in a molten
condition, and accordingly the first formed solid covering of the
lithosphere, if this hypothesis be true, must have been formed from
molten material, or in the language of Geology, it was an _igneous
rock_. Consequently, the earliest _sedimentary rock_ was necessarily
derived directly from an igneous rock, with possible addition of
material from the early hydrosphere and atmosphere, and all
subsequently formed sedimentary rocks have therefore been derived from
igneous rocks (with the additions above stated) either directly, or
indirectly through the breaking up of other sedimentary rocks which
were themselves derived directly or indirectly from igneous rocks. The
observations of geologists show that this supposition that the
materials of sediments have been directly or indirectly obtained for
the most part from once-molten rocks is in accordance with the
observed facts, and so far their observations testify to the truth of
the nebular hypothesis. This being the case, the study of the
petrology of the igneous rocks is necessary, in order to arrive at a
true understanding of the composition of the sedimentary ones. The
igneous rocks are largely composed of four groups of minerals,
viz.--quartz, felspars, ferro-magnesian minerals, and ores. Of these
the quartz (composed of silica) yields particles of silica for the
formation of sedimentary rocks; the felspars, which are double
silicates of alumina and an alkali or alkaline earth, being prone to
decomposition furnish silicate of alumina and compounds of soda,
potash, lime, &c. The ferro-magnesian minerals (such as augite,
hornblende and mica) may undergo a certain amount of decomposition,
and yield compounds of iron, lime, &c. We may also have fragments of
any of these minerals, and of the ore group in an unaltered condition.
The composition of a sedimentary rock which has undergone no
alteration after its formation will therefore depend upon the
character of the rock from which it was derived, the chemical changes
which take place in the materials which compose it, before they enter
into its mass, and the mechanical sorting which they undergo prior to
their deposition.
In the above passage the terms igneous rock and sedimentary rock have
been used, and it is necessary to give some account of the sense in
which they were used.
An _igneous_ rock is one which has been _consolidated_ from a state of
_fusion_. It is not necessary to discuss here the exact significance
of the word fusion, and whether certain rocks which are included in
the igneous division were formed rather from solution at high
temperature than from actual fusion. This point is of importance to
the petrologist, but to the student of stratigraphical geology the
term igneous rock may be used in its most comprehensive sense. These
igneous rocks were consolidated either upon the surface of the
lithosphere or in its interior.
The other great group of rocks is one to which it is difficult to
apply a satisfactory name. They have been termed by different writers,
_sedimentary_, _stratified_, _derivative_, _aqueous_, and _clastic_,
but no one of these terms is strictly accurate. The term _sedimentary_
implies that they have settled down, at the bottom of a sheet of water
for instance. It can hardly be maintained that limestones formed by
organic agency, like the limestones of coral reefs, are sedimentary in
the strict sense of the term, and an accumulation like surface-soil
can only be called a sediment by straining the term. _Stratified_
rocks are those which are formed in strata or layers, but many of the
rocks which we are considering do not show layers on a small scale,
and igneous rocks (such as lava-flows) are also found in layers,
though such layers are not true strata in the sense in which the term
is used by geologists; the term _stratified_ is perhaps the least open
to objection of any of those named above. _Derivative_ implies that
the fragments have been derived from some pre-existing rock, but as
there are many ways in which fragments of one rock may be derived from
another, the term is too comprehensive. _Aqueous_ rocks should be
formed in water, and most of the class of rocks which we are
considering have been so formed, but others such as sand-dunes and
surface-soil have not. (The term Aerial or Æolian has been suggested
to include these rocks which are thus separated from the Aqueous rocks
proper; the objection to this is that the origin of these rocks is
closely connected with that of the true Aqueous rocks, and moreover
the group is too small to be raised to the dignity of a separate
subdivision.) Lastly, the name _clastic_ has been given, because the
rocks so called are formed by the _breaking up_ of pre-existing rocks.
There are two objections to this name. In the first place, some rocks
included under the head clastic are formed by solution of material and
its consolidation from a state of solution by chemical or organic
agency, though we may perhaps speak of rocks being broken up by
chemical as well as by mechanical action. The most important objection
is that many clastic rocks are formed by the breaking up of rocks
subsequently to their formation, and it has been proposed that rocks
of this nature should be termed _cataclastic_, while those which are
formed by the breaking up of pre-existing rocks upon the earth's
surface should be termed _epiclastic_; another group formed of
materials broken up within the earth, and accumulated upon its surface
as the result of ejection of fragmental material from volcanic vents
being termed _pyroclastic_. This classification is scientific, and
under special circumstances is extremely useful, but the older terms
have been used so generally, and with this explanation their use is so
unobjectionable, that they may be retained, and the term _stratified_
will be generally used to indicate all rocks which are not of igneous
origin or formed as mineral veins in the earth's interior.
The division of rocks into _three_ great groups, the Igneous,
Stratified and Metamorphic (the latter name being applied to those
rocks which have undergone considerable alteration since their
formation), is objectionable, since we have metamorphic igneous rocks
as well as metamorphic stratified ones. The most convenient
classification is as follows:--
A. Igneous 1. { Unaltered.
2. { Metamorphic.
B. Stratified 1. { Unaltered.
2. { Metamorphic.
It must be distinctly understood that all geological phenomena must be
taken into account by the stratigraphical geologist. The upheaval of
strata, the production of jointing and cleavage in them, their
intrusion by igneous material, their metamorphism, give indications of
former physical conditions equally with the lithological characters of
the strata, and their fossil contents. Nevertheless it is not proposed
to give a full account of the various phenomena displayed by rocks;
the student is referred to Text-books of General Geology for this
information. It will be as well here, however, to point out in a few
words the exact significance of the existence of strata in the
lithosphere.
The formation of strata and their subsequent destruction to supply
material for fresh strata are due to three great classes of changes.
Beginning with a portion of lithosphere composed of rock, it is found
that rock is broken up by agents of denudation, as wind, rain, frost,
rivers and sea. These agents perform their function mainly upon the
portion of the lithosphere which projects through the hydrosphere to
form _land_, and the land is the main area of denudation. The
materials furnished by denudation are carried away, and owing to
gravitation, naturally proceed from a higher to a lower level, often
resting on the way, but if nothing else occurs, ultimately finding
their way to the _sea_, where they are deposited as strata. The sea is
the principal area for the reception of this material, and it is there
accordingly that the bulk of stratified rock is formed. If nothing
else occurred, in time the whole of the land would be destroyed, and
the wreckage of the land deposited beneath the sea as stratified rock.
As it is there is a third class of change, underground change, causing
movements of the earth's crust (to use a term which can hardly be
defined in few words but which is generally understood), and as the
result of the relative uplift of portions of the earth's crust, the
stratified rocks formed beneath the oceans are raised above its level,
giving rise to new masses of land, which are once more ready for
destruction by the agents of denudation. This cycle of change (all
parts of which are ever proceeding simultaneously) is one of the
utmost importance to the stratigraphical geologist.
_Stratification_ is the rock-structure of prime importance in
stratigraphical geology, and a few words must here be devoted to its
consideration, leaving further details to be dealt with hereafter. The
surface of the ocean-floor is, when viewed on a large scale, so level,
that it may be considered practically horizontal, and accordingly in
most places the materials which are laid down on the ocean-floor give
rise to accumulations which at all times have a general horizontal
surface (when the ocean-slopes depart markedly from horizontality the
deposits tend to abut against these slopes rather than to lie with
their upper surfaces parallel to their original angle). A practically
horizontal surface of this character may give rise to a _plane of
stratification_ (or _bedding-plane_) in more than one way. A pause may
occur during which there is a cessation of the supply of material, so
that the material which has already been accumulated has sufficient
time to become partially consolidated before the deposition of fresh
material upon it. In this way a want of coherence between the two
masses is produced, along the plane of junction, which after
consolidation of the deposits causes an actual divisional plane along
which the two deposits may be separated. This is a plane of
stratification. The pause may be produced in various ways, sometimes
between successive high tides, at others as the result of physical
changes which may have taken ages to happen. Again, after material of
one kind has been deposited, say sand, some other substance such as
clay may be accumulated on its upper surface, giving rise to a plane
of stratification between two deposits of different lithological
characters. If this occurs alone, there may be actual coherence
between the two strata, so that it is erroneous to speak of a plane of
stratification as if it were always one along which one deposit could
be readily split from the other, though as a general though by no
means universal rule, change from one kind of deposit to another is
also marked by want of coherence between the two. The material between
two planes of stratification forms a _stratum_ or _bed_, though if the
deposit be very thin it is known as a _lamina_, and the planes are
spoken of as _planes of lamination_ (no hard and fast line can be
drawn between strata and laminæ; several of the latter usually occur
in the space of an inch).
A _stratum_ will have its upper and lower surface apparently parallel,
though not really so, for no stratum extends universally round the
earth, and many of them disappear at no great distance when traced in
any direction. Parts of one stratum may be composed of different
materials from other parts when traced laterally, thus one stratum may
be found composed essentially of sand in one place, of mud in another,
and of a mixture of the two in an intervening locality. Whatever be
the composition of a stratum it dies out eventually, owing to the
coming together of the upper and lower bounding planes of
stratification. The stratum is thickest at some spot, from that spot
it becomes thinner in all directions, until it disappears at last by
the coalescence of the bounding-planes. This is spoken of as
_thinning-out_. Strata, then, consist of lenticular masses of rock,
separated from the underlying and overlying strata by planes of
stratification. The shape of the lenticle may vary immensely, the
thickness bearing no definite relationship to the horizontal extent.
Some strata, many feet in thickness, may thin out and disappear
completely in the course of a few yards, whilst others an inch or two
in thickness may be traced horizontally for many miles. We often find
thin strata of coal and limestone, extending for great distances,
strata of mud thinning out more rapidly, and sandstones still more
rapidly, but no universal rule connecting rapidity of thinning-out
with composition of the strata can be laid down.
Having seen what a stratum is, it now remains to speak of the
composition of the stratified rocks. They have been classified
according to their composition, and according to their origin.
According to composition they have been divided into:
_Arenaceous_ rocks, composed essentially of grains of sand.
_Argillaceous_ rocks, composed essentially of particles of
mud.
_Calcareous_ rocks, composed essentially of particles of
carbonate of lime.
_Carbonaceous_ rocks, composed largely of hydrocarbon
compounds.
_Siliceous rocks_, composed essentially of silica not in the
form of grains;
whilst according to their origin they have been separated into:--
_Mechanically-formed_ rocks, composed of fragments derived
from other rocks by mechanical fracture.
_Chemically-formed_ rocks, composed of particles which have
been chemically deposited from a state of solution.
_Organically-formed_ rocks, composed of materials which have
been derived from a state of solution or from the gaseous
condition by the agency of organisms.
Whichever classification be adopted (and each is useful for special
purposes), it must be noted that no hard and fast line can be drawn
between one division and another. A rock may be partly arenaceous and
partly calcareous, composed of a mixture of sand and lime, and the
same rock may similarly be partly mechanically and partly organically
formed, the sand being due to mechanical fracture, and the lime to the
agency of organisms, and so with the other divisions.
As many of the changes which have occurred in past times have been
concerned in destruction and obliteration, whilst deposition is the
cause of preservation, the study of deposits is peculiarly adapted for
testing the truth of the grand principle of geology that the changes
which have taken place in past times are generally speaking similar in
kind and in intensity of action to those which are in progress at the
present day, and a study of the modern deposits is specially important
as throwing light upon the characters of those which have been formed
in past times. It will be abundantly shown in the sequel that the
deposits of the strata are in general comparable in all essential
respects with those which are being formed at present, and accordingly
they give most valuable indications as to the nature of the physical
and other conditions under which they were laid down. The desert sand,
the precipitate of the inland sea, the reef-limestone and many another
deposit can thus be detected by an examination of their lithological
characters, combined with consideration of other kinds of evidence.
The petrology of the sedimentary rocks is still in its infancy, though
much has already been done, but it offers a wide field of inquiry to
the field-geologist and worker with the microscope[8].
[Footnote 8: The student will do well to consult _The Challenger
Report_ by Messrs Murray and Renard (1891), for information concerning
many modern sediments, and Harker's _Petrology for Students_ Section
D, for general information on the Petrology of the Stratified Rocks.]
CHAPTER IV.
THE LAW OF SUPERPOSITION.
In a previous chapter this law was given as follows: "Of any two
strata, the one which was originally the lower is the older;" the
general truth of the law depends upon the fact that except under very
exceptional circumstances the strata are deposited upon the surface of
the lithosphere, and not beneath it. There are occasions where strata
may be deposited beneath the lithosphere, but as a general rule the
geologist will not be misled by such occurrences. In caverns,
accumulations often occur which are newer than the strata over the
cavern roof, and so long as caverns are formed in ordinary sedimentary
rocks, no great difficulty will result from this exception to the law
of superposition. When caverns occur beneath masses of land ice, the
order of superposition may be misleading. A deposit may be formed on
the surface of the ice, and subsequently to this a newer deposit may
be laid down in a sub-glacial or englacial cavern; upon the melting of
the ice the newer deposit would be found with the older one resting
upon its surface.
Apart from these exceptional cases, the law as stated holds good, but
the reader will notice the insertion of the word 'originally' which
requires some comment.
A geologist speaks of one bed lying _upon_ another not only when the
beds are horizontal, but when they are inclined at any angle, until
they become vertical, so that until beds have been turned through an
angle of 90° by earth-movement the test of superposition is
applicable, but when they have been turned more than 90°, the stratum
which was originally lower rests upon that which was originally above
it, and in the case of these _inverted_ strata, the test of
superposition is no longer applicable. It was formerly supposed that
cases of inversion were comparatively rare and local, and that the
test of superposition could therefore be generally applied with
confidence, but it is now known that though this is generally true of
such strata as have been subjected only to those widespread, fairly
uniform movements which are spoken of as _epeirogenic_ or
continent-forming, where the radius of each curve is very long,
inversion is a frequent accompaniment of the more local _orogenic_ or
mountain-forming movements, where the radius of a curve is short.
Though orogenic movements are limited as compared with those of
epeirogenic character, they often affect large tracts of country, in
which case the apparent order of succession of the strata need not be
the true one, and examples of inversion may be frequent[9].
[Footnote 9: For a discussion of the principles of mountain-building
see Heim, A., _Untersuchungen über den Mechanismus der
Gebirgsbildung_, and Lapworth, C. "The Secret of the Highlands,"
_Geological Magazine_, Decade II. vol. x. pp. 120, 193, 337.]
It is not easy to lay down any definite rules for detecting inverted
strata, where the top of an inverted arch is swept off by denudation
or the bottom of an inverted trough concealed beneath the surface,
beyond stating that if an easily recognised set of beds is obviously
repeated in inverse order, inversion must have occurred, though even
then it may not be clear which side of the fold shows the beds in
original and which in inverted sequence. Suggestions are frequently
made that ripple-marks and worm-tracks may be utilised in order to
discover inversion, for the well-formed ripple-marks will appear
convex on the upper surface of a bed which is not inverted, and we may
note concave casts of these ripple-marks on the under surface of the
overlying bed, whilst worm-tracks will appear concave on the upper
surface, and their casts convex on the lower surface of the succeeding
bed under similar conditions. In the case of inversion the occurrences
will be the exact opposite to these. Unfortunately ripple-marks and
worm-tracks may, as will appear in the sequel, be simulated by
structures produced in quite a different way, and unless the observer
is certain that he is confronted with true ripple-marks and
worm-tracks he may be seriously misled. The geologist must take into
account all the evidence at his disposal, when he is dealing with
cases of possible inversion, but oftentimes he will after due
consideration of all the phenomena be left in doubt unless he is able
to supplement his observations on the succession of the strata by
evidence derived from the included fossils.
The test of superposition is most apt to be misleading when the strata
have been affected by the faults known as reversed faults or
thrust-planes.
Reference to text-books will show that a fold consists of two parts,
the arch and the trough, and that the two are connected by a common-,
middle-, or partition-limb. In the case of an inverted fold, an
=S=-shaped or sigmoidal structure is the result (Fig. 1 A).
[Illustration: Fig. 1.
A. A sigmoidal fold, showing a bed _xx_ in an overfold with arch
(_a_), trough (_t_) and common limb _c_.
B. A similar bed _xx_ affected by a thrust-plane _tt_ which replaces
the common limb.]
Here the portions of any bed (_xx_) which occur in the arch or trough
are in normal position, and have not been moved round through an angle
of 90°, whilst the portion which occurs in the common limb c has been
moved round through an angle greater than 90° and is inverted, so that
its former upper surface now faces downwards. In Fig. 1 B the common
limb is replaced by a reversed fault, or thrust-plane, and the
inverted portion of the bed seen in the common limb is therefore
absent. An observer, applying the test of superposition, might suppose
that the position of the bed _x_ on the left-hand side of the figure
was a different bed from the portion which is seen on the right-hand
side, instead of belonging to the same bed, and in this way, if a
number of parallel thrust-planes affected one bed or a set of beds, he
might be led to infer the occurrence of a great thickness of strata
where there was in reality a slight thickness, or even one bed only
repeated again and again by faulting. It is quite certain that
exaggerated estimates of the thickness of deposits have frequently
been made owing to the non-recognition of the occurrence of repetition
as a consequence of the existence of thrust-planes.
Where thrust-planes are suspected, it is well to look for some of the
following features:
(_a_) The strata of a country affected by thrust-planes often crop out
as lenticular masses, thinning out rapidly in the direction of the
strike[10]. This is true of beds thrown into sharp folds whether or
not inverted, but the lenticles will be wider in a direction at right
angles to that of the strike as compared with their length when
inversion has not occurred. It is also true of beds which were
originally deposited as lenticles, such as many massive sandstones,
and as almost any kind of deposit may be formed originally as a
lenticle, the test by itself is by no means sufficient as a proof of
thrusting, though it is suggestive.
[Footnote 10: For definitions of the terms dip, strike, outcrop and
allied expressions, the reader is referred to a _Text-Book of
Geology_.]
(_b_) The _surfaces_ of the strata are often affected by the
striations known as slickensides, and the joint-faces of gently
inclined beds are also frequently marked by slickensides which often
run in a nearly horizontal direction.
(_c_) A parallel structure presenting the appearances characteristic
of the mechanically-formed features of a foliated rock is often
developed, and one or more of certain accompanying phenomena will
probably be found, which will be noticed more fully in a later
chapter.
(_d_) Extension or stretching of the rocks will have been frequently
produced, causing rupture, and the resulting fissures are usually
filled with mineral-veins, though this occurrence is by no means
characteristic of rocks which have been affected by thrust-planes.
(_e_) Chemical changes may have occurred which have resulted in the
reconstitution of some of the rock-constituents, which may crystallise
where pressure is least, thus we often find rocks which have undergone
movements of the type we are considering marked by the existence of
sericitic films upon the surfaces.
Another reservation must be made when considering the law of
superposition. The test is only applicable for limited areas. Suppose
we find a deposit of clay _a_ resting upon another deposit of
limestone _b_ in the south of England, and can prove that the apparent
succession is the true one, that is, that there has been no inversion;
it is clear that the test of superposition is applicable in that area.
Now, we may be able to trace the two deposits continuously across the
country, one as a clay, the other as a limestone; so that when we
reach the north of England we find the clay _a_ still reposing upon
the limestone _b_. The test of superposition is applicable in that
area also, the clay of the northern area being newer than the
limestone of the same region. But, for reasons which will ultimately
appear, it by no means follows that the clay of the north is newer
than the limestone of the south, although the two deposits are
continuously traceable with the same lithological characters; it may
have been formed simultaneously with the limestone of the south, or
even before it. Something more, therefore, than the test of
superposition is necessary in order to make out the relative ages of
continuous deposits in a wide region, and this is still truer in the
case of deposits which are discontinuous, whether separated from one
another by the sea, or by outcrops of older or newer rocks.
A few words of warning may be added with reference to the detection of
bedding-planes. A bedding-plane is one which separates two beds, and
its existence is determined during the deposition of the beds. Many
other planes are formed in rocks subsequently to their deposition, and
it is not always easy to distinguish these from true bedding-planes.
That even experienced observers may be led astray is shown by the fact
that, of recent years, it has been proved that great masses of rock
have been claimed as of sedimentary origin, and their apparent order
of succession noted, which are in truth naught but irregular masses of
intrusive igneous rocks affected by divisional planes which simulate
bedding, produced in the rocks subsequently to their consolidation.
Joints, faults, and cleavage-planes may all at times simulate planes
of bedding, and it is frequently very difficult to distinguish them in
the limited exposures with which a geologist has oftentimes to deal.
It is easier to make suggestions for distinguishing bedding-planes
from other planes which simulate them, than to apply the suggestions
in practice, and the student of field geology will find that
experience is the only guide, though after years of experience he may
be confronted with cases where the evidence is insufficient to
convince him that he is dealing with planes of stratification and not
with some other structure.
From what has been remarked, it will be inferred that the test of
superposition though of prime importance to the geologist is
frequently insufficient to enable him to ascertain the true order of
succession of the strata, and he is compelled to supplement this test
by some other. There are several useful physical tests which may
frequently be applied. Thus, if a rock _a_ contains fragments of
another rock _b_, _under such circumstances as to show that the
fragments of_ b _were included in a during its deposition_, it is
clear that _b_ is older than _a_. Here again, it will be found from
what appears in a later chapter that the student is confronted with
difficulties when actually examining rocks, for fragmental rocks of
cataclastic origin, where the fragments have been formed as the result
of fracture produced by earth-movements subsequently to the deposition
of the rock, simulate epiclastic rocks in which the fragments were
introduced during the accumulation of the deposits to so surprising a
degree as sometimes to baffle the most experienced observer. Not only
are the fragments of these cataclastic rocks broken up, but they may
be further rounded so as to imitate in a remarkable manner the
water-worn pebbles of an epiclastic conglomerate. Again, an older
series of rocks may have had structures impressed upon them as the
result of changes subsequent to their formation, and before the
formation of a newer set which the latter therefore do not exhibit.
Jointing, cleavage, and various metamorphic phenomena may thus be
exhibited by the older rocks, but great care is required in applying
this test, especially with a limited thickness of rocks, as one set
may not exhibit the structures not because they were not in existence
when the structures were developed, but because their nature is such
that they were incapable of receiving or retaining the structures. For
instance a mass of grit which is older than a mass of clay-slate may
not be cleaved, because, although subjected to the pressure which
produced the cleavage, it was of a nature not adapted to the
development of cleavage structure.
On the whole, application of tests dependent upon physical features of
rocks, does not often supplement to any great extent the information
supplied by ascertaining the order of superposition, and in all
cases, where possible, every other kind of information should be
supplemented, by that which is acquired after examination of the
included organisms of the strata.
CHAPTER V.
THE TEST OF INCLUDED ORGANISMS.
The second great law of the Stratigraphical Geologist is that
fossiliferous strata are identifiable by their included organisms, in
other words, that we can tell the geological age of deposits by
examination of the fossils contained in them, though the determination
of age must be given in more general terms in some cases than in
others. Considerable misconception has arisen concerning the value of
fossils as indices of age, and it is necessary therefore to discuss
the significance of the law of identification of strata by their
included organisms at some length.
The comparison between fossils and medals has frequently been made and
fossils have well been styled the "Medals of Creation"; and the
significance of fossils as guides to the age of deposits may perhaps
be made clearer if we pursue this comparison some way. In the first
place there is clear indication of a gradual increase in the
complexity of organisation of the fossils as one passes from the
earlier to the later rocks, and accordingly the general facies of a
fauna is likely to furnish a clue to the age of the rocks in which it
is found, even though every species or even genus represented in the
fauna was previously unknown to science. So an antiquary versed in the
evolution of art or metallurgy, might detect the general age of a
medal with whose image and superscription he was not acquainted. He
would know that a medal struck in iron was formed subsequently to the
bronze age, or that one formed of palladium appertained to the present
century. But quite apart from any theoretical knowledge, an antiquary
would find as the result of accumulated experience that certain medals
are characteristic of certain periods; he would learn that the
denarius is characteristic of a different period from that indicated
by the coin of the Victorian era, even though he had no knowledge of
the technicalities of numismatics. The same is the case with the
geologist. He may not be influenced by any knowledge of the evolution
of faunas and floras, but actual work amongst the rocks will show him
that the trilobite is not found with the belemnite or the ichthyosaur
with the elephant, save under exceptional circumstances, which only
prove the rule, as for instance when worn bones of ichthyosaurs are
washed from their proper strata into gravels with elephant-bones.
It must be distinctly understood that the determination of fossils as
characteristic of different periods is solely made as the result of
experience. No _à priori_ reasoning may give one indication of the
actual range in time of a species or genus; no one can say why
_Discina_ has a long range in time, whilst that of the closely related
_Trematis_ is very limited. This being the case, the greater the mass
of evidence which is accumulated as to the range of a fossil, the
greater will be the value of that fossil as a clue to the age of the
deposit in which it is found. This is so important, that it requires
more than mere notice. If a fossil is found in abundance in a group of
strata _B_ in any one area, and is not found in an underlying group
_A_ or overlying group _C_ in that area after prolonged search, we
may confidently speak of the fossil as characteristic of the strata
_B_ in that area, though without further work, the value of the fossil
as a clue to age in other areas would be unproved. It may nevertheless
happen, that after more prolonged search in _A_ or _C_, in the
original area a few specimens of the fossil which has been spoken of
as characteristic of _B_ may be found in one or other of them, in
small quantity. The value of the fossil as one characteristic of _B_
will be slightly diminished, though only slightly, as it is not likely
to turn up in numbers in the strata _A_ or _C_ after the prolonged
search. Should the fossil be found also to be characteristic of the
strata _B_ in areas other than the original one, it becomes of more
than local value, and if, after much study it is found to characterise
the same strata over wide areas, the cumulative evidence now obtained
will render the fossil peculiarly important to the stratigraphical
geologist. The detection of characteristic fossils is not quite so
simple as might be supposed from the above remarks, for examination of
the position of one fossil will not prove the contemporaneity of beds
in different places, to prove this, all the evidence at our disposal
must be considered, for reasons which will be presently pointed out.
As the result of accumulated knowledge, we can now compile lists of
characteristic fossils of the major subdivisions of the strata, which
are of world-wide utility and as our knowledge increases, we are
enabled to subdivide the strata into minor divisions of more than
local value.
_What is a fossil?_ Before discussing the value of fossils as aids to
the stratigraphical geologist, it may be well to make a few
observations as to what constitutes a fossil. It is difficult to give
any concise definition, and as is often the case in geology, an
explanatory paragraph is of more value than a mere definition. The
term fossil was originally applied to anything dug up from the rocks
of the earth's crust, and was used with reference to inorganic objects
as well as organic remains, for instance minerals were spoken of as
fossils. It is now applied essentially though not exclusively to
relics of former organisms, though one still reads of fossil
rain-drops, fossil sun-cracks, and so on. Furthermore, the relics need
not necessarily be parts of the organism, the track of a worm or a
bird's nest if embedded in the strata would be termed a fossil. It is
generally agreed that no sharp line can be drawn between recent and
fossil organic remains which is based upon the degree of
mineralisation (or as it was sometimes termed petrifaction) of the
relics, for many true fossils have not undergone mineralisation,
subsequent to their entombment.
It has been suggested that the name fossil should be applied to
organic remains which have been entombed by some process other than
human agency, but this restriction is undesirable. The stone-implement
of the river gravels is as genuine a fossil as the ammonite extracted
from the chalk, and the human relics of very recent date may give
information of a character quite similar to that supplied by other
remains, for instance, the occurrence of moa-bones in New Zealand in
accumulations below those containing biscuit-tins and jam-pots has
been used as a geological argument pointing to the extinction of the
moa before the arrival of Europeans in New Zealand. The biscuit-tin
here serves all the purposes of a fossil, and there is no valid reason
why it should not be spoken of as such.
This statement brings one to consider another method which has been
adopted in order to separate fossil organisms from recent ones,
namely the time-test. This again is inapplicable, for no line can be
drawn between the shell which was buried in yesterday's tidal deposit
and that which has lain in the strata through geological ages, and
each may be equally useful to the geologist.
Whilst, then, we can give no definition of fossil which is likely to
meet with general acceptance, the term can be so used, as not to give
rise to any doubts as to its meaning, and it is generally applicable
to any organic relics which have been embedded in any deposit or
accumulation by any agent human or otherwise.
_Mode of occurrence of fossils._ It will not be out of place to say a
few words as to the way in which fossils are found in strata, as beds
are often inferred to be unfossiliferous, because of ignorance of
methods which should be pursued in searching for organic relics. It is
unnecessary to dilate upon the actual modes of preservation of
organisms, which is treated of fully in other works. In the first
place, it is rash to assert that any deposit is unfossiliferous
because no fossils have been found in it, even after prolonged search.
The Llanberis slates had been eagerly searched for fossils for many
years without result, but that the search was not exhaustive was
proved by the discovery of trilobites in them some years ago. Seekers
after fossils are rather prone to confine their attention to strata
which are already known to be fossiliferous than to pay much attention
to those which have hitherto yielded no organic remains.
Some kinds of deposits are more often fossiliferous than others.
Limestones which are frequently largely of organic origin, are often
rich in remains, and muddy deposits more frequently furnish fossils
than those of a purely sandy nature. The difference in the yield is
not necessarily due to the original inclusion of more remains in one
rock than in another, but is often caused by the obliteration of
former relics owing to changes which have taken place in the rocks
subsequently to their deposition. No sedimentary rock must be regarded
as unfossiliferous, however unfitted it appears for the preservation
of fossils. The writer has seen fossils, not only in coarse
conglomerates, rocks which frequently contain no traces of organisms,
but in deposits composed largely of specular iron ore, and even in
intrusive igneous rocks, though in the latter case, the inclusion of
fossils was due to circumstances which cannot have occurred with
frequency.
In sandy strata, the substance of the fossils has often been
completely removed, leaving hollow casts, which may be almost or quite
unrecognisable. In these circumstances, much information may be
obtained by taking impressions of the casts in modelling wax or some
other material. The importance of this process may be judged from the
results it yielded to Mr Clement Reid in the case of the fossils of
the Pliocene deposits occurring in pipe-like hollows in the Cretaceous
rocks of Kent and the discovery of the remarkable reptiles described
by Mr E. T. Newton from the Triassic sandstones of Elgin.
In argillaceous rocks which have been affected by the processes
producing cleavage, the fossils may be distorted beyond recognition or
owing to the difficulty of breaking the rocks along the original
planes of deposition, may remain invisible. Under such circumstances,
small nodules of sandy or calcareous nature may sometimes be found
included in the argillaceous deposits and may perhaps yield fossils.
Oftentimes, also, where the argillaceous rock is in close proximity
to a harder rock, such as massive grit, the argillaceous rock in
close contiguity to the hard rock may escape the impress of
cleavage-structure, and fossils may be readily extracted from rocks in
this position when not obtainable from other parts of the deposit. It
was under these circumstances that the trilobites alluded to above
were obtained from the Llanberis slates.
The fossils of calcareous rocks are often very obvious, but difficult
to extract, as they break across when the rock is fractured. They are
frequently obtainable in a perfect condition when the rock is
weathered. Occasionally they may be extracted from certain
argillaceous limestones if the limestone be heated to redness, and
suddenly plunged into cold water. Fossils are often found in a state
which enables them to be readily extracted when a limestone is
coarsely crystalline, though they cannot be extracted in a perfect
condition when the same limestone is in a different state.
Many microzoa, which are invisible in rocks, even when viewed through
a lens, may be found in microscopic sections of calcareous and
silicious rocks, and plant structures may be detected under similar
circumstances in the case of carbonaceous rocks.
Various special methods of extracting fossils from rocks have been
described by different writers, many of which are very complex, and
require much time. The mechanical action of the sand-blast and the
solvent action of various acids as hydrochloric and hydrofluosilicic
have been found of use upon different occasions[11]. The various
processes which have been utilised in order to extract and develop
fossils can, however, be best learned by information obtainable from
curators of palæontological collections, and by actual experience,
and there is yet much information to be acquired as to the manner of
extracting fossils from various kinds of rocks.
[Footnote 11: For information concerning use of acids see especially
Wiman, C. "Ueber die Graptoliten," _Bull. Geol. Inst._, Upsala, No. 4,
vol. II. Part II.]
_Relative value of fossils to the Stratigraphical Geologist._ It has
been hinted above that no general rule as to the relative value of
fossils as guides to the age of strata can be laid down, and that the
ascertainment of their relative value is largely the result of actual
experience. It may be noted, however, that organisms which possess
hard parts are naturally more important to the geologist than those
which do not, as few traces of the latter are preserved in the fossil
state, and even when preserved are usually too obscure to be of much
practical use. Of the organisms which do possess hard parts, different
groups have been utilised to a different degree, and one group will be
more or less important than another, according to the use to which it
is applied. Groups of organisms which have a long range in time are
naturally useful for the identification of large subdivisions of the
strata, whilst those which have had a shorter range are valuable when
separating minor subdivisions.
Again, as the bulk of the sedimentary deposits has been formed beneath
the waters of the ocean, relics of marine organisms are naturally more
useful than those of freshwater ones. Other things being equal, the
more easily the organism is recognisable, and the more abundant are
its remains, the greater its value to the stratigraphical geologist,
and as the remains of invertebrates are usually found in greater
quantities and in more readily recognisable condition than those of
the vertebrates, they have been used more extensively as indices of
age. Of the invertebrates, the mollusca are often very abundant, their
remains are adapted for preservation, and their characteristics have
been extensively studied, and accordingly they have been and are of
great use to the geologist. Of other groups, the graptolites, corals,
echinids, brachiopods, and trilobites have been very largely utilised.
The Lower Palæozoic strata have been divided into numerous groups,
each characterised by definite forms of graptolites, and a similar use
has been made of the ammonites in the case of the Mesozoic rocks. It
is not to be inferred that these groups of organisms are naturally
more useful than other groups, on account of the extent to which they
have been used; we can merely state that they have been proved to be
useful as the result of prolonged study; when other groups have
received equal attention, they may well be found to be equally useful
for the purposes which we have in view.
_Contemporaneity and Homotaxis._ From what has been already stated, it
will be recognised that the ages of the various fossiliferous rocks of
the geological column[12] in any one area can be identified with
greater or less degree of certainty by reference to their included
organisms, the various subdivisions being marked by the possession of
characteristic fossils, and it will be naturally and rightly inferred
that the greater the number of characteristic fossils of any one
deposit, the more certain is the identification of that deposit. In
practice, geologists are wont to ascertain the age of the strata after
consideration of all the fossils found therein, some of which may be
actually characteristic whilst many may come up from the strata
below, or pass into those above. Having ascertained the order of
succession and fossil contents of the strata in various regions, it is
the task of the geologist to compare the strata of these two regions,
and this task is fraught with considerable difficulty. Much
controversy has arisen as to the degree of accuracy with which strata
of remote regions can be correlated, and the subject is one which
requires full consideration.
[Footnote 12: Although the rocks do not always lie on one another in
regular succession, it is often convenient to speak of them as though
they did, and as though a column of strata could be carved out in any
region consisting of horizontal bands of deposit one above another. We
speak of such an ideal arrangement as constituting a 'geological
column.']
Suppose that a series of strata which we will call _A_, _B_, and _C_
is found in any one area, each member of which contains characteristic
fossils which enable it to be recognised in that area, and we will
further suppose that in another area a series of strata _A´_, _B´_,
and _C´_ is discovered, of which _A´_ has the fauna of _A_ in the
former area, and similarly _B´_ the fauna of _B_, and _C´_ that of
_C_.
It cannot be assumed that the stratum _A_ is therefore contemporaneous
with _A´_, _B_ with _B´_, and _C_ with _C´_, but on the other hand, it
must not be assumed that they are not contemporaneous. This is a
statement which requires some comment. It has been urged that if the
deposits _A_ and _A´_ in different localities contain the same fauna,
this is a proof that the two are not contemporaneous, for some time
must have elapsed in order to allow of the migration of the organisms
from one area to another, it being justifiably assumed that they did
not originate simultaneously in the two areas. But everything depends
on the time taken for migration as compared with the period of
existence of the fauna. If the former was extremely short as compared
with the latter it may be practically ignored, for we might then speak
of the strata as contemporaneous, just as a historian would rightly
speak of events in the same way which occurred upon the same
afternoon, though one might have happened an hour before the other.
Let us then glance at the evidence which we have at our disposal,
which bears upon this matter.
The objection to identification of strata with similar faunas as
contemporaneous was urged by Whewell, Herbert Spencer, and Huxley, and
the latter suggested the term Homotaxis or similarity of arrangement
as applicable to groups of strata in different areas, in which a
similar succession of faunas was traceable, maintaining that though
not contemporaneous the strata might be spoken of as homotaxial.
Huxley went so far as to assert that "for anything that geology or
palæontology are able to show to the contrary, a Devonian fauna and
flora in the British Islands may have been contemporaneous with
Silurian life in North America, and with a Carboniferous fauna and
flora in Africa[13]," a statement which few if any living geologists
will endorse. If the statement be true, and the fauna which we speak
of as Devonian, when present be always found (as it is) above that
which we in Britain know as Silurian and below that which we term
Carboniferous, the faunas must have originated independently in the
three centres, and disappeared before the appearance of the next
fauna, or having originated at the same centre, each must have
migrated in the same direction, spread over the world, and become
extinct as it reached the point or line from which it started. Suppose
for instance a fauna _A_ originates at the meridian of Greenwich,
migrates eastward, and dies out again when it once more reaches
Greenwich, that _B_ and _C_ do the same, at a later period, then the
fauna _B_ will always be found above _A_ and _C_ above _B_, but if
_B_ did not become extinct when it reached the Greenwich meridian, it
would continue its eastward course, and _C_ having in the meantime
started on its first round, the fossils of the fauna _B_ would be
found both above and below those of _C_. It will be shown below that
cases of recurrence do occur, but nowhere do we find a Silurian fauna
above a Devonian one, or a Devonian one above one belonging to the
Carboniferous, nor is the fauna of a great group of rocks found in one
region above the fauna of another group, and in another region below
the same. And this is true not only of the faunas of one major
division, such as those of the Silurian and Carboniferous periods, but
also of the faunas of many minor subdivisions into which the large
ones are separated, for instance we do not find the Llandovery fauna
of the Silurian period which in Britain is found below the Wenlock
fauna embedded elsewhere in strata above the Wenlock. I have
simplified the statement by assuming that the faunas are identical in
the different localities, and exactly similar throughout the whole
thickness of the containing strata, which is naturally not the case,
but the additional complexity does not conceal the truth of what has
been stated. In the absence of actual inversion of well-marked faunas,
only one explanation is possible, namely, that the time for migration
of forms is so short as compared with the entire period during which
the forms existed, that it may be practically ignored, and the strata
containing similar faunas may be therefore spoken of truthfully as
contemporaneous and not merely homotaxial[14].
[Footnote 13: Huxley, T. H. "Geological Contemporaneity and Persistent
Types of Life," being the Anniversary Address to the Geological
Society for 1862; reprinted in _Lay Sermons, Addresses and Reviews_.]
[Footnote 14: For fuller discussion of this matter see a paper by the
Author 'On Homotaxis,' _Proc. Camb. Phil. Soc._, vol. VI. Part II. p.
74.]
_Apparent anomalies in the distribution of fossils._ There are several
occurrences which have tended to augment the distrust frequently felt
concerning the value of fossils as indices of the age of the beds in
which they occur, which may be here considered.
Though the greater number of fossil remains belonged to organisms
which lived during the time of accumulation of the deposits in which
they are now embedded, this is by no means universally the case, and
the occurrence of _remanié_ fossils, which have been derived from
deposits more ancient than the ones in which they are now found is far
from being a rare event. The existence of remains of this nature in
the superficial drifts and river-gravels of our own country has long
been recognised, and no one would suppose that the _Gryphæa_ and other
shells furnished by these gravels had lived contemporaneously with the
species of _Corbicula_, _Unio_ and other molluscs which are part of
the true fauna of the gravels. In this case the water-worn nature of
the remains is a good index to their origin, but in other cases, it is
by no means an infallible guide, for we sometimes find on the one hand
that remains of organisms proper to the deposits in which they occur
are water-worn, whilst on the other the relics of _remanié_ fossils
are not. The now well-known gault fossils of the Cambridge Greensand
at the base of the chalk were not always recognised as having been
derived from older beds, and there are certain fossils found in
nodules in the Cretaceous rocks of Lincolnshire, which still form a
subject for difference of opinion, for while some writers maintain
that they belong to the deposits in which they are now found, others
suppose that the nodules have been washed out of earlier beds.
Occasionally we find forms which occurring in a set of beds _A_ in an
area, are absent from the overlying beds _B_, and appear again in the
succeeding deposits _C_. Such cases of _recurrence_ are by no means
rare, though many supposed instances of recurrence have been recorded
as the result of stratigraphical or palæontological errors. The best
examples have been noted by Barrande among the Lower Palæozoic
deposits of Bohemia. The stage _D_ of Bohemia consists of five
'bandes' or subdivisions, the lowest (_d_ 1), central (_d_ 3) and
uppermost (_d_ 5) divisions are mainly argillaceous, whilst the second
(_d_ 2) and fourth (_d_ 4) are essentially arenaceous. Some of the
forms found in _d_ 1, _d_ 3 and _d_ 5 have not been found in _d_ 2 and
_d_ 4. The best-known example is the trilobite _Æglina rediviva_. It
is clear that this and other forms did not become extinct during the
deposition of the strata of _d_ 2 and _d_ 4, though they may have
disappeared temporarily from the Bohemian area, or else lingered on in
such diminished numbers that their remains have not been discovered.
The range of the organism is in fact right through the deposits of the
stage _D_, and the discontinuity of distribution is not a real
anomaly; it may be compared to some extent with cases of discontinuous
distribution in space. It is needless to remark that the whole fauna
does not disappear for a time and then reappear, but only a few out of
the many forms which compose it. The comparative rarity of examples of
recurrence after long intervals is an indication that the
palæontological record as it is termed is not so imperfect as some
suppose, for if our knowledge of fossils were very imperfect, we
should expect cases of apparent recurrence to be common, as the result
of the non-detection of fossils in the intermediate beds. One of the
most marked cases of apparent recurrence known some years ago was the
reappearance of a genus of trilobite _Ampyx_ in Ludlow rocks, found in
the Bala rocks, but not in the Llandovery or Wenlock strata. It has
since been discovered in Llandovery beds, and its eventual discovery
in beds of Wenlock age may be regarded as certain. A supposed case of
recurrence which would have been remarkable, that of the disappearance
of _Phillipsia_ in Ordovician rocks, its entire absence in those of
Silurian age, and its reappearance in the Devonian, has broken down,
for the supposed Ordovician form has been shown to belong to an
entirely different group of trilobites from that containing the genus
_Phillipsia_, and it has been therefore renamed _Phillipsinella_.
Many apparent anomalies of distribution have been explained as due to
migration, but it is doubtful whether any one of these supposed
anomalies is actual and not due to errors in determining the position
of the beds or the nature of their included fossils. Some of the
supposed anomalies have already been shown to be due to error, and the
others will almost certainly be cleared up. In speaking of anomalies
of distribution, the geologist can only be guided by experience as to
what constitutes an anomaly. For instance the existence of a complete
fauna in any one place in the beds of a system above that to which it
is elsewhere confined would be regarded as anomalous and as probably
due to error, whilst the reappearance of several forms in beds of a
system higher than that in which they had hitherto been found, could
hardly be considered as an anomaly. A geologist would suspect the
statement that after the disappearance of an Ordovician fauna in an
area and its replacement by a Silurian fauna, the Ordovician fauna
reappeared for a time, but would not regard the statement that a
Cenomanian fauna partly reappeared in the Chalk Rock with surprise.
The existence of a Silurian fauna in Ordovician times was maintained
by Barrande in the case of the Bohemian basin. Lenticular patches of
Silurian rocks having the lithological characters of the Silurian
strata are found in the Ordovician beds of that region, and they
contain fossils specifically identical with those of the Silurian
rocks. Barrande explained this appearance as due to the existence of a
fauna in other regions resembling the Silurian fauna of Bohemia,
during the Ordovician period, when the normal Ordovician fauna of
Bohemia inhabited that area. He supposed that in parts of the basin,
when favourable conditions arose, _colonies_ of the foreign fauna
settled for a time, but did not get a permanent footing in the basin
until the commencement of Silurian times. The theory of colonies has
now been rejected for the Bohemian area, and the phenomena shown to be
due to repetition of strata by folding and faulting, but it is a
theory which is again and again advocated in order to explain
apparently anomalous phenomena in other areas, and these apparent
anomalies which are so explained, must be regarded with grave
suspicion.
The various complexities alluded to in the foregoing pages increase
the difficulty experienced by the geologist in correlating strata in
different areas by their included organisms, but no one of them
disproves the possibility of making these correlations, which can be
carried on to a greater or less extent according to the nature of the
faunas.
A good deal of misconception has arisen concerning the geographical
distribution of former faunas, owing to the tendency to compare them
exclusively with the littoral faunas of the present day. These
littoral faunas have a comparatively limited geographical
distribution, the forms of one marine province often differing
considerably from those of an adjoining one, and still more widely
from one which is remote, so that anyone confronted with the relics
of faunas from the existing Australian and European seas, would find
no indications furnished by identity of species that the faunas were
contemporaneous. Recent researches have shown, however, that the
creatures whose remains are deposited at some distance from the
coast-line have a much stronger resemblance to one another than the
littoral organisms have, if the fauna of two distant areas be
compared. It is still a moot point which will be discussed in a later
chapter, how far the deep-sea deposits of modern times are represented
amongst the strata of the geological column by deposits of similar
origin. But it is certain that many of the ancient strata are not
littoral deposits, and it will be found that it is by comparison of
the faunas of the deeper-water deposits that the geologist correlates
the strata of remote regions: where shallow water deposits are formed,
the faunas differ markedly in different regions, and these
shallow-water forms can only be correlated owing to their occurrence
between deeper-water strata. Thus if strata _A_, _B_ and _C_ be found
in one area, and the fauna of _A_ and _C_ are deep-water forms, those
of _B_ being shallow-water forms, and in another area beds _A´_
contain the same fauna as _A_, and _C´_ the same fauna as _C_ whilst
the fauna of _B´_ is different from that of _B_, we can nevertheless
correlate the strata _B_ and _B´_ (if they be conformable with the
underlying and overlying beds), because of the identity of age of the
associated beds in the two areas. It will possibly be found that the
strata _A_ and _C_ can be further subdivided into _A_{1}_, _A_{2}_,
... &c. _C_{1}_, _C_{2}_, ... by the existence of minor faunas, which
are comparable in the two cases, but such subdivisions may not be
established in the case of the beds _B_ and _B´_.
To take actual examples:--The Llandovery beds of Dumfriesshire can be
subdivided into several minor divisions each of which can be
recognised in the Lake District of England, and to a large extent in
Scandinavia and elsewhere, for the deposits in these areas are of
deep-water character, and the sub-faunas of the subdivisions are
similar in the different areas, but the Llandovery rocks of the Welsh
borderland are shallow-water deposits, with a different fauna from
that of the deep-water deposits of this age, and can only be stated to
be contemporaneous with the Llandovery rocks elsewhere, because the
deeper-water faunas of the underlying Bala rocks and overlying Wenlock
rocks of the Welsh borders are respectively similar to those of the
Bala and Wenlock rocks of the other regions. The shallow-water
Llandoveries of the Welsh borders have only been separated into two
divisions, upper and lower, and have not been split up into a number
of subdivisions, each characterised by a sub-fauna, and each
comparable with one of the subdivisions of Dumfriesshire, Lakeland and
the other regions where the deep-water facies is found.
It will be seen that though the principle of William Smith that strata
can be recognised by their included organisms has been extended since
his time, and shown to apply to far smaller subdivisions of the strata
than was suspected, the method of application is the same, and is more
or less successful according to the amount of evidence which is
accumulated in support of it.
CHAPTER VI.
METHODS OF CLASSIFICATION OF THE STRATA.
Earth-history like human history is the record of an unbroken chain of
events. The agents which have produced geological phenomena have been
in operation since the earth came into existence. Accordingly a
perfect earth-history would be written as a continuous narrative, just
as would a complete history of the human race. The historian of man
finds it not only convenient but necessary to divide the epoch of
which he is writing into periods of time, and so does the geologist,
and in each case the division is necessarily more or less arbitrary.
It is true that in writing the history or geology of a country, marked
events stand out which form a convenient means of making divisions,
but the marked events occurring in one country are not likely to take
place simultaneously with those of another country, and consequently a
classification of this character is only locally applicable.
The classification which is at present used by geologists was
originally founded upon definite principles, and although our
principles of classification have, as will appear, been somewhat
altered subsequently, it has been found more convenient to modify the
original classification than to adopt a new one in its entirety.
The largest divisions into which the strata of the geological column
were separated were instituted because of the supposed extinction of
faunas, and sudden or rapid replacement by other faunas of an entirely
different character. This supposed rapid extinction and replacement is
now known to have been only apparent and due to observation in
restricted areas, and it is doubtful whether the three great divisions
founded upon them are not rather mischievous than useful, as tending
to disseminate wrong notions.
Moreover there is considerable diversity of opinion as to the terms to
be adopted. The rocks were formerly divided into Primary, Secondary,
and Tertiary. Owing chiefly to the use of the term Primary in another
sense, the alternative titles Palæozoic, Mesozoic and Cainozoic (or
Cænozoic) were suggested, and though the term Primary has been
definitely abandoned in favour of Palæozoic, the words Secondary and
Tertiary are used extensively as synonyms of Mesozoic and Cainozoic.
It was soon perceived that the period of time included in the
Palæozoic age was much longer than the combined periods of Secondary
and Tertiary ages, and it was proposed to group the latter under one
title Neozoic, whilst another suggestion was to split the Palæozoic
age into an earlier Proterozoic and later Deuterozoic division. The
interest excited by the advent of man is probably the cause of the
attempt to establish a Quaternary division, which some hold to be a
minor subdivision of the Tertiary, whilst others would separate it
altogether. The terms Palæozoic, Mesozoic (or Secondary) and Cainozoic
(or Tertiary) are now used so generally that any attempt to abolish
them would be doomed to failure, but it must be remembered that they
are purely arbitrary expressions, and the other terms which are not in
general use, might be dropped with advantage.
The other subdivisions have been used somewhat loosely, and although
an attempt has been made by the International Geological Congress to
restrict certain names to subdivisions of varying degrees of value, it
will probably be found best to allow of a certain elasticity in the
use of terms, merely agreeing that they shall be used as nearly as
possible with the signification assigned to them by the Congress.
According to this classification, and apart from the division into
Palæozoic, Mesozoic and Cainozoic, the strata of the geological column
are grouped into _Systems_, which are subdivided into _Series_, and
the series are further split up into _Stages_. A number of
chronological terms were also suggested, of equivalent importance,
thus the beds of a _system_ would be deposited during a _Period_,
those of a _series_ during an _Epoch_, and those of a _stage_ during
an _Age_[15].
[Footnote 15: The chronological words have been used so loosely that
it is doubtful whether any good will come of trying to restrict their
use, and Sir A. Geikie has pointed out the confusion which would arise
if the term _group_ be employed for the largest divisions (Palæozoic,
&c.). The terms _System_, _Series_ and _Stage_ may well be employed in
the senses suggested by the Congress.]
The rocks of the Geological Column were originally divided into
systems, owing to the occurrence of marked physical and
palæontological breaks between the rocks of two adjacent systems,
except in cases where a complete change occurred locally in the
lithological characters of the rocks of two systems which were in
juxtaposition: it is necessary to consider for awhile the nature of
these breaks.
The most apparent physical break is where the rocks of one set of
deposits rest unconformably upon the rocks of another one, indicating
that the older set has been uplifted and to some extent eroded before
the deposition of the strata of the newer set. This uplift and erosion
signifies a change from oceanic to continental conditions in the area
in which unconformity is found on a large scale, and accordingly a
long period of time would elapse during which the continental surface
would not receive deposits, so that the highest rocks of the
underlying system would be considerably older than the lowest rocks of
the one which succeeds it. Such a break may be obviously utilised for
purposes of classification, but as some areas of the earth's surface
must have been occupied by the waters of the ocean when other regions
formed land, deposit in some areas must constantly have occurred
simultaneously with denudation in others, and any classification
founded upon the existence of unconformities will therefore have a
purely local value.
Another, and less apparent physical break, which will also be locally
applicable, may be due to the depression of an area to so great a
depth that little or no deposit was formed upon the ocean floor there
during the period of great depression; but as a break of this
character is difficult to detect, the existence of unconformities has
alone been practically utilised as a means of separating strata into
systems owing to marked physical change, except in the cases where the
lithological character of the strata completely changes, as between
the Triassic and Jurassic rocks of England.
[Illustration: Fig. 2.]
Palæontological breaks or breaks in the succession of organisms are in
many cases, the result of physical breaks, and accordingly it is often
possible to separate one set of strata from another by the existence
of a combined physical and palæontological break between them. It is
by no means necessary however that a physical break should be
accompanied by a break in succession of the organisms, and the latter
may also occur without the former. It was once maintained that a
palæontological break was due to the complete and sudden extinction of
a fauna and its entire replacement by a new one, but this is far from
true, and accordingly the breaks differ in degree. Study of the strata
shows that when the succession is not to any extent interrupted, the
species do not appear simultaneously, but come in at different
horizons, and they disappear in the same way. In Figure 2 let _A_
represent a set of conformable strata _ab ... k_, and suppose the
vertical lines represent the ranges of the various species found in
these strata. It will be seen that of 27 species whose range is shown
only 2 pass through the whole thickness, so that the fauna of _k_ is
very different from the fauna of _a_, nevertheless the fauna of each
stratum is closely similar to that of the underlying as well as to
that of the overlying stratum, and though most of the species of _k_
are different from those of _a_, this need not be the case with the
genera. The fauna of the set of strata would contain every species
whose range is represented, and for convenience' sake it might be said
to be composed of sub-faunas, one of which occurs in each division
_ab_ ..., but the separation into sub-faunas would be artificial and
merely for convenience' sake, for there is no break between any two
sub-faunas. Turning now to _B_ (Fig. 2), an attempt is made there to
show what happens when there has been a physical break, resulting in
the denudation of the strata _ghik_, and the deposition of another set
_op_ ... unconformably upon those deposits of the earlier set which
have not been denuded. As the result of this we note, first, that the
relics of organisms which existed in the area during the deposition of
_ghik_, and were entombed in those strata, are destroyed by the
processes of denudation, and a large number of organisms which lived
long after the deposition of _f_, and disappeared not simultaneously
but at different times during the period when denudation was in
operation, seem to become extinct simultaneously at the top of _f_,
though, if we could visit an area which was receiving sediment during
the period of denudation, we should find them dying out in the rocks
of that region at different levels. Furthermore, whilst denudation is
going on, a longer or shorter period of time elapses, during which the
upheaved area receives no deposit, and accordingly no organisms which
lived during that period are preserved in the upheaved area. During
this time a set of deposits _lmn_ may have been laid down elsewhere,
and besides the gradual disappearance of some of the organisms of _ab
... k_, there will have been a gradual appearance of new species.
When the upheaved area is once more submerged, a new set of deposits
_op_ ... is accumulated in it, and the species which gradually
appeared in adjoining regions will now migrate to it, and will seem to
come in simultaneously at the bottom of _o_; accordingly we may find
that there is not a single species which passes through from _f_ to
_o_ and the palæontological break in this area is complete, though it
is clear that it only implies local change, and that we may and indeed
must find intermediate forms in other regions which fill up the gap.
As an illustration of the local character of a palæontological break
we may cite the case of the Carboniferous and Permian systems of
Britain. These rocks are separated from one another in our area by a
physical and palæontological break, but in parts of India, and other
places, we find a group of rocks now known as the Permo-Carboniferous
rocks which contain a fauna intermediate in character between those of
the Permian and Carboniferous systems, and a study of this fauna shows
that the hiatus which exists locally is filled by the species
contained in the Permo-Carboniferous rocks.
A palæontological break may, like a physical one, result from
depression of the ocean-floor to so great a depth, that no organisms
are preserved there during the period of great depression, and the
remarks made concerning a depression of this nature when speaking of
physical breaks will apply here also.
A local palæontological break may result owing to physical changes
without the production of an unconformity in the area, or its
submergence to a great depth, or if an unconformity is found, the
break may be more marked owing to other physical changes. The
difference between the Upper and Lower Carboniferous faunas is very
marked in England, where the Upper Carboniferous beds were deposited
under physical conditions different from those of the Lower
Carboniferous, and accordingly the corals, crinoids and other
open-water animals which flourished in Lower Carboniferous times are
rare or altogether absent in the higher rocks. Where the change of
conditions did not occur to a great extent as in parts of Spain and
North America, the similarity between the two faunas is much more
pronounced. Again, there is an unconformity between the Cretaceous and
Eocene beds of England, which is accompanied by a palæontological
break, but this break is more pronounced owing to difference of
physical conditions, for we find abundance of gastropods in the lower
Tertiary beds, and a rarity of these shells at the top of the chalk of
England, though where physical conditions were favourable for the
growth of gastropods, their shells are found in the higher strata of
chalk age, and the palæontological break is not so apparent.
A palæontological break may occur also as the result of climatic
change, though actual instances of this occurrence are much more
difficult to detect owing to the general absence of any evidence of
climatic change other than that supplied by the organisms themselves.
Still, when no physical break exists, and the lithological characters
of a group of sediments remain constant throughout, indicating the
prevalence of similar physical conditions through the period of
deposition of the sediments, if the fauna suddenly changes, there must
have been cause for the change, and in the absence of any other cause
which is likely to produce the change, alteration of the character of
the climate may be suspected.
It follows from the observations which have been made, that although
the rocks of the Geological Column may be divided into systems owing
to the existence of physical and palæontological breaks, and this
classification may be and has been applied generally, the line of
demarcation between the rocks of two systems will be a purely
conventional one, where there is no break, and, to avoid confusion,
that line when once drawn should be adopted by everyone, unless good
cause can be shown for its abandonment.
The subdivision of systems into series has been conducted in a manner
generally similar to that in which large masses of strata have been
grouped into systems, with the exception that actual breaks need not
occur. The subdivision was usually made on account of marked
differences in the lithological characters or fossil contents of the
rocks of the various series, and frequently the lithological
characters as well as the fossil contents are dissimilar; taking the
rocks of the Silurian system of the typical Silurian area as an
example, we find the Llandovery rocks largely arenaceous, the Wenlock
rocks largely calcareo-argillaceous, and the Ludlow rocks
argillaceo-arenaceous, whilst the fauna of the Wenlock rocks differs
from that of the Llandovery rocks below and also from that of the
Ludlow rocks above. The Llandovery, Wenlock and Ludlow therefore
constitute three series of the Silurian system, but the lines of
demarcation between these series are nevertheless conventional, for it
has been suggested that a more natural division, as far as the British
rocks are concerned, could be made by drawing a line, not as at
present at the base of the Ludlow, but in the middle of that series as
now defined, and uniting the Lower Ludlow beds with the Wenlock strata
to form a single series.
The same process as that adopted in the case of series has been
essentially pursued in subdividing these into stages. Each stage is
usually different from that above and below in its lithological
characters, fossil contents, or both, though the difference is usually
less in degree than that which has been utilised for the demarcation
of series. A stage is often, though not always, composed of deposits
of one kind of sediment, and is furthermore frequently characterised
by the possession of one or, it may be, two, three or more
characteristic fossils. Thus the Wenlock series is divided in the
typical area into Woolhope limestone, Wenlock shale, and Wenlock
limestone, and the very names given to these stages indicate that each
is largely composed of one kind of material. Their fossils are also to
some extent different, though the difference between them is not
likely to be of so marked a nature as that which exists between the
faunas of separate series.
It will be seen that the system differs from the series and the series
from the stage in degree rather than in kind, and no hard line can be
drawn between divisions of different degrees of magnitude. It follows
therefore that frequently a mass of sediment which one author will
consider sufficiently important to constitute a system will be defined
by another as a series, and similarly a series of one writer may
become a stage of another.
The student of Stratigraphical Geology will find the expression
'fossil zone' occurring over and over again in geological literature,
and as the term has been used somewhat vaguely by many writers and is
apt to be misunderstood, it will be useful to notice the expression at
some length.
Strictly speaking the term zone (a belt or girdle), when applied to
distribution of fossils, should refer to the belt of strata through
which a fossil or group of fossils ranges. Generally speaking, the
expression is used in connexion with one fossil; thus we speak of the
zone of _Coenograptus gracilis_, the zone of _Cidaris florigemma_ and
the zone of _Belemnites jaculum_, though sometimes it is used with
reference to more than one species, as the zone of Micrasters and the
_Olenellus_ zone. The term has been used not of a belt of strata but
of a group of organisms[16], and zones defined as "assemblages of
organic remains of which one abundant and characteristic form is
chosen as an index," but if it be agreed that the term should be
applied to strata and not to organisms this might be modified and the
definition run:--'Zones are belts of strata, each of which is
characterised by an assemblage of organic remains of which one
abundant and characteristic form is chosen as an index.'
[Footnote 16: See H. B. Woodward, "On Geological Zones," _Proc. Geol.
Assoc._, vol. XII. Part 7, p. 295, and vol. XII. Part 8, p. 313.]
It has been objected that the subdivision of strata into zones has
been pushed too far, but this is merely because in the establishment
of zones, workers find it easier to work out the successive zones
where the strata are thin and presumably deposited with extreme
slowness, than where they are much thicker and have been rapidly
accumulated, and accordingly, as the subdivision of strata into zones
is a recent event, geological literature contains many more references
to thin zones than to those of great thickness. Where an abundant and
characteristic form (which is chosen as an index) of an assemblage of
organic remains ranges through a great thickness of deposit, there is
no objection to speaking of the whole as a zone, and it cannot be
divided. To give some idea of the variations in the thickness of
strata through which these abundant and characteristic forms will
range, I append a list of the zones of graptolites which have been
established amongst the Silurian rocks of English Lakeland and the
thickness of each (which in the case of the thicker deposits is
naturally only approximate):--
Thickness.
Zone of Feet. Inches.
_Monograptus leintwardinensis_ 5000 0
_Monograptus bohemicus_ 5000 0
_Monograptus Nilssoni_ 1000 0
_Cyrtograptus Murchisoni_ 1000 0
_Monograptus crispus_ 22 0
_Monograptus turriculatus_ 60 0
_Rastutes maximus_ 25 0
_Monograptus spinigerus_ 3 0
_Monograptus Clingani_ 3 0
_Monograptus convolutus_ 7 6
_Monograptus argenteus_ 0 8
_Monograptus fimbriatus_ 7 6
_Dimorphograptus confertus_ 25 0
_Diplograptus acuminatus_ 2 6
It must not be supposed that each of the subdivisions in the above
list is of equal importance, and has occupied approximately the same
length of time for its formation, but a study of the strata proves by
various kinds of evidence that the deposits in which the
characteristic forms range through a small thickness of rock were on
the whole deposited much more slowly than where the range is
continuous through a great thickness of deposit.
The geological systems, as originally founded, were not very
accurately separated from one another except locally. A comprehensive
view of the characters of a system was taken, and accordingly the
lines of demarcation between the same systems adopted by workers in
different countries were by no means necessarily at or near the same
geological horizon. As the result of more recent work, the
establishment of fossil zones has been growing apace, and though many
of these are seen to have only local significance, it is found as the
result of experience that many of them are widely spread and occur in
the same order in different localities; accordingly the remarks that
have been made concerning the contemporaneity of strata apply to these
zones also. After a study of this kind, a much more accurate
comparison of strata is possible, and correlation of strata can be
carried on to a much greater extent than when the systems were only
roughly subdivided by reference to breaks, differences of lithological
character, and general comparison of the faunas; accordingly whilst
largely retaining the old names, the old method of classification is
being partly superseded, and the included faunas alone are utilised to
establish accurate correlations of the strata in various parts of the
world. How far this correlation can be carried on remains to be seen,
for the work though well advanced has by no means reached completion,
and predictions as to the ultimate issue are useless without the
experience by means of which only the work can be done. The difference
between the methods of classification is well shown by an examination
of the old and new divisions of the chalk. It was formerly roughly
divided mainly by lithological characters into Chalk Marl, Lower Chalk
without flints, Middle Chalk with few flints and Upper Chalk with many
flints, but no two observers would probably agree as to where the
deposit with few flints ceased and that with many commenced. The chalk
is now separated on palæontological grounds into Cenomanian, Turonian,
Senonian and Danian, and the superiority of the new method to the old
is practically shown by the abandonment of the old classification
except for very rough purposes, and the general acceptance of the new
one. Many other examples might be given, but this one will suffice. In
the case of some of the systems, the Carboniferous for example, the
old classification founded upon lithological characters is largely
extant, and it has been inferred therefore that no accurate
subdivisions of the Carboniferous rocks can be made by reference to
the faunas, owing to the rapidity with which the deposits were
accumulated. It is by no means certain because the work has not been
done that it cannot be done, and the experience obtained from a study
of other strata in which subdivisions have been established by
reference to the fauna would lead one to suppose that the
non-establishment of subdivisions of the Carboniferous strata is due
to our want of knowledge rather than to their non-existence.
The establishment of a classification on palæontological lines by no
means does away with the necessity for local classifications on a
lithological basis, and it has already been remarked that important
results will follow from a comparison of the classifications of
sediments founded on the two lines, results which have hitherto
largely escaped our attention owing to the existence of a cumbrous
classification attained by the application sometimes of one method, at
other times of the alternative one.
CHAPTER VII.
SIMULATION OF STRUCTURES.
Although it is easy to give an account of the structures which are of
importance to the student of the stratified rocks, actual observation
of these structures is frequently attended with difficulties owing to
the close imitation of one structure by another, and the past history
of the science shows that erroneous conclusions have been reached
again and again on account of the incorrect interpretation of
structures.
Simulation of organisms has frequently been the cause of error.
Inorganic substances take on the form of organisms with various
degrees of closeness. The dendritic markings produced by
efflorescences of oxide of manganese are familiar to all, and as the
name implies, they simulate, to some extent, plant remains. More
complex chemical changes have resulted in the production of
rock-masses in which, not the outward form alone but, the internal
structure of organisms is reproduced with more or less approach to
fidelity, as the rocks which contain the supposed organisms described
as _Eozoon bohemicum_, _E. bavaricum_, and, we may add, _E.
canadense_. Mechanical changes in rocks subsequent to their formation
may also cause the simulation of organisms by inorganic substances.
Prof. Sollas has given reasons for considering the structure
described as _Oldhamia_ to be inorganic, and in the Carboniferous
Sandstones of Little Haven, Pembrokeshire, every stage in the
formation of tubular bodies resembling worm-tubes, as the result of
complex folding of the strata, may be observed, whilst in other cases
we find imitation of worm-tracks, as has been observed before.
It is when one inorganic structure is simulated by another that the
stratigraphical geologist is most likely to be led astray, and
accordingly it is worth noting some cases where this has occurred, as
a warning, for it must not be supposed that the cases here noted are
the only ones which are likely to occur.
It has been seen that the existence of bedding-planes is of prime
importance to the geologist, and their detection is a matter of
supreme moment. Under ordinary circumstances there is no great
difficulty in distinguishing bedding-planes from other planes, but the
importance of discovering them is often greatest when the difficulty
is most pronounced. In rocks which have undergone no great amount of
disturbance the planes of stratification are often marked by their
regular parallelism, the separation of layers having different
lithological characters by these planes, the arrangement of the longer
axes of pebbles parallel to them, and the occurrence of fossils and
also of rain-prints, ripple-marks and other structures produced during
deposition, upon the surfaces of the strata, but none of these
appearances is necessarily conclusive, especially in areas where the
rocks have been subjected to orogenic movements. In regularly-jointed
rocks, jointing may well be mistaken for bedding, and there is often
great difficulty in discriminating between bedding and cleavage,
especially when the exposures of rock are of small extent. Fossils may
be dragged out along planes at an angle to the true bedding, pebbles
will be compressed by cleavage so that their longer axes do not remain
parallel to the bedding-planes but now lie parallel to the
superinduced planes of cleavage, and a structure closely resembling
'ripple-mark' may be produced on planes other than those of original
bedding, as the result of puckering. The alternation of rocks having
different lithological characters may also be misleading. Intrusion of
dykes along cleavage-planes, followed by decomposition of the
dyke-rock causing it to resemble a sediment, and formation of mineral
veins along the same planes, may give rise to an apparent succession
of rocks of different lithological characters which could easily
mislead an observer and cause him to mistake the cleavage-planes for
planes of stratification. In rocks which have undergone great lateral
pressure, the beds of different lithological character may be folded
in such a way as to give very erroneous ideas of the true dip of the
rock on a large scale. In Fig. 3 the dip of the rocks in a small
exposure might appear to be in the direction indicated by the
unfeathered arrow, whilst the true dip of the strata as a whole,
leaving the minor foldings out of account, is in the direction of the
feathered arrow, at the inclination represented by the dotted line.
The minor folds in a case like that represented may extend upwards for
scores or even hundreds of feet, so that an error as to the direction
and amount of dip may be made, even if the observer faces a cliff of
considerable height.
[Illustration: Fig. 3.]
False-bedding on a large scale may be a cause of error. In the Penrith
Sandstone of Cumberland, the planes of deposition are often found
dipping in one direction in a large quarry, but inspection of a wider
area shows that this is not the true dip of the beds as a whole, but
merely a local dip due to deposition on a slope, and any one
attempting to calculate the total thickness of the beds by reference
to these divisional planes might be seriously led astray. A reference
to Fig. 4 will explain this. The lines _AA´_, _BB´_ are the true
bedding-planes cut across in the section, whilst the lines sloping to
the right from _xx_ are only lines of false-bedding on a large scale.
An exaggerated estimate of the thickness of the deposit would be made
by measuring the thickness of each of these stratula from _A_ to _A´_
and adding these thicknesses together, whereas the actual thickness of
the middle bed is the distance between _A_ and _B_ or _A´_ and _B´_.
[Illustration: Fig. 4.]
When rocks have been affected by thrust-planes, the simulation of
bedding may be carried out to a very full extent. Not only do the
major thrust-planes resemble bedding-planes but the minor thrusts
produce an appearance of divisional planes separating stratula or
laminæ, and a close approximation to false-bedding is the result. To
this structure Prof. Bonney has given the name
'pseudo-stromatism[17].' It may be developed in rocks of all kinds,
whether possessing original planes of stratification or not, and as a
result of its existence the geologist may be seriously misled, not
merely by mistaking the direction of the strata, but also the nature
of the rock, for we may find it produced in an unstratified glacial
till, and in a massive igneous rock, and in each case the resulting
rock will resemble a sedimentary deposit, and of course the observer
may be confirmed in his erroneous opinion by the formation of apparent
fossils, ripple-marks or other objects which he might expect to
discover in sediments. As illustrative examples, reference may be made
to a number of schistose rocks, in which the planes of discontinuity
(which are in truth planes of foliation) have been taken for
bedding-planes and the rocks claimed as sedimentary though they are in
reality igneous; for instance many of the rocks of the Laurentian of
Canada, of the Hebridean of the North West Highlands, and some of the
ancient rocks of Anglesey.
[Footnote 17: Bonney, T. G., _Quart. Journ. Geol. Soc._, vol. XLII.
_Proc._ p. 65.]
A foliated structure may, as is now well known, be simulated by a
structure developed in a rock prior to its consolidation. The
similarity of flow structure of some lavas to the foliated structure
of a schist was long ago pointed out by Darwin and Scrope, and recent
work has proved that parallel structure due to differential movement
prior to consolidation may be developed in plutonic rocks, as shown
by Lieut.-General McMahon in the Himalayan granites, and by Lawson
amongst the plutonic rocks of the Rainy Lake Region; and as the
foliated structure may be mistaken for original stratification the
same may occur, and has occurred, when dealing with this
flow-structure.
This is not the place to discuss the truth of the old theory of
progressive metamorphism, in which it was maintained that a gradual
passage could be traced between ordinary sediments and plutonic rocks,
but it may be pointed out that much of the evidence which was relied
upon to prove the theory was fallacious and due to the confusion of
the parallel structure set up in plutonic rocks prior to, or
subsequent to, consolidation, with original stratification. Recent
study of metamorphic rocks has proved that the parallel structures
developed in the rocks of an area which has undergone metamorphism may
be produced by three distinct processes; they may be original planes
of deposition, or formed in a solid rock subsequently to its
formation, or in an igneous rock before its consolidation, and
although it is sometimes possible to separate the structures produced
by these processes, this is not always the case[18]. When a plutonic
rock contains large phenocrysts and an eye-structure is developed in
it, it may simulate a conglomerate, the rounded phenocrysts being
taken for pebbles[19]. Still closer simulation of an epiclastic
conglomerate may be produced in other ways and will be referred to
immediately.
[Footnote 18: It must be noticed that the rock in which parallel
structure is produced before consolidation, if it undergoes no further
change, though often associated with metamorphic rocks, is not itself
metamorphic. The term _gneiss_ applied to these rocks is a misnomer,
unless the term be used even more vaguely than it is at present.]
[Footnote 19: See Lehmann, _Untersuchungen über die Entstehung der
Altkrystallinischen Schiefergesteine mit besonderer Bezugnahme auf das
Sächsische Granulitgebirge_, Plate XI. fig. 1.]
We have already seen that the existence of unconformities has been
utilised in the demarcation of large divisions of strata in various
regions, and whether they be utilised in this manner or not, their
detection is a matter of importance to the stratigraphical geologist,
as they afford information concerning the occurrence of great physical
changes during their production. These unconformities may also be
closely simulated by structures produced in very different manner.
The occurrence of an unconformity implies the denudation of one set of
beds before the deposition of another set upon them, and accordingly
the denuded edges of the lower set will somewhere abut against the
lower surface of the lowest deposit or deposits of the overlying
set[20]. The existence of an unconformity may often be detected in
section, but when the unconformity is upon a large scale this may not
be possible, but it will be discovered by mapping the strata and will
be apparent on a map owing to the deposits of the lower set of beds
abutting against the others. This is well seen where the Permian rocks
of Durham, Yorkshire, and Nottinghamshire rest upon different members
of the underlying Carboniferous series, and will be noticed on any
good geological map of England. But a similar effect may be caused by
a fault, so that mere inspection of a map or even of the strata in the
field and discovery of one set of beds ending off against another does
not prove unconformity. When the fault is a normal one, with low hade
(that is, having a fissure approaching the vertical position), the
outcrop of the fault-fissure will approximate to a straight line if
the fault has a straight course, even if the ground be very uneven,
whereas, if the plane of unconformity has not been tilted to a high
angle from its original horizontal position, it will crop out in a
sinuous manner across uneven ground, in a way similar to that of beds
which are nearly horizontal, so that though the general trend of the
outcrop of the plane of unconformity may be fairly straight, its
deviation from a straight line will be frequent and marked, as seen in
the case of the Permian unconformity above referred to. But if the
unconformable junction has been highly inclined its outcrop will
resemble that of a normal fault, or if the fault be a thrust-plane
with high hade, the outcrop of this will resemble that of an
unconformable junction which has not been greatly tilted from its
original horizontal position. In these cases we require more evidence
before we can decide whether we are dealing with an unconformable
junction or a faulted one.
[Footnote 20: An unconformity may be simulated or an actual
unconformity rendered apparently more important, as the result of
underground solution of the underlying strata subsequently to the
deposition of the upper set upon them, and any insoluble materials in
the underlying strata may be left as an apparent pebble-bed at the
base of the upper beds. This is seen at the junction of the Tertiary
beds with the chalk near London. Subterranean water has dissolved the
upper part of the chalk, increasing the unconformity which naturally
exists between chalk and Tertiary beds, and the insoluble flint of the
dissolved chalk is left as a layer of 'green-coated flint' at the base
of the Tertiary deposits.]
The lowest deposits of the newer set of strata lying above an
unconformity have probably been laid down in water near the
shore-line. As the unconformity, if large, implies elevation above the
sea-level, the deposits first formed after this elevation has ceased,
and depression commenced, will necessarily be littoral in character
and possibly of beach-formation, and accordingly we often find that an
unconformity is marked by the existence of an epiclastic conglomerate
immediately above the plane of unconformity and, although this need
not be continuous, it is usually found somewhere along the line of
junction. The conglomeratic base of the Lowest Carboniferous strata
when they repose upon the upturned edges of the Lower Palæozoic rocks
of the dales of West Yorkshire is well known, and may be cited as an
example. The association of conglomerates with unconformities is
indeed so frequent that its possible occurrence will always be
suspected and sought by the geologist. Unfortunately the result of
recent observation is to show that along thrust-planes of which the
outcrop simulates those of unconformable junctions, the difficulty of
discrimination may be increased by the existence of cataclastic rocks
which bear a close resemblance to epiclastic conglomerates, and which
may be and have been styled conglomerates. It is well known that
fragments of the adjoining rocks are knocked into a fault-fissure
during the occurrence of the movements which cause the fault, to
constitute a _fault-breccia_, and as the result of the abrasion of
these fragments by chemical or mechanical agency, the angular
fragments may become rounded and converted into rounded pebble-like
bodies, when the rock is changed into a _fault-conglomerate_. Fig. 5,
from a photograph kindly supplied by Prof. W. W. Watts, shows a stage
in the formation of a conglomerate of this nature from a
fault-breccia; the fragment on the right remains angular, whilst those
on the left have become much more rounded. The illustration is from a
case described by Mr Lamplugh occurring in the slaty rocks of the Isle
of Man, and Mr Lamplugh's paper[21] furnishes the reader with
references to other examples of the production of similar rocks. No
general rule can be laid down for distinguishing the true from the
apparent unconformity, for the attendant phenomena will differ in each
case; but if a fault-conglomerate should be suspected, the observer
should try to ascertain whether fragments of a newer rock are imbedded
in an older one, which sometimes occurs; he should note the existence
of extensive slickensiding along the plane of junction and along
planes of faulting, though the existence of these, implying as it does
the occurrence of differential movement along the plane, does not
prove that the movement was necessarily great, or that it did not take
place along a plane of original unconformity; above all, he should
look for structures such as mylonitic structure, pseudo-stromatism,
development of new minerals, crushing out and stretching of fossils
and fragments and, in short, for any structure which is familiar to
him as a result of orogenic movements.
[Footnote 21: Lamplugh, G. W., "On the Crush-Conglomerates of the Isle
of Man," _Quart. Journ. Geol. Soc._, vol. LI. p. 563.]
[Illustration: Fig. 5.]
The effects of thrusting not only give rise to appearances suggestive
of unconformity, but naturally also to a simulation of overlap. The
thrust-planes are often parallel to original bedding-planes for some
distance, but must cut across them sooner or later, producing
lenticular masses which might be supposed to be due to the thinning
out of beds as the result of cessation of deposition in a lateral
direction.
Attention has already been directed to the deceptive appearance of
great thickness of strata which is due to repetition of one stratum or
set of strata by a series of thrust-planes, so that there is no actual
inversion of any part of a bed. When masses of limestone are affected
in this way, the thrust-planes may become sealed up, as the result of
chemical change, and a compact irregular mass of limestone devoid of
any definite divisional planes may be the consequence, and beds of
grit sometimes exhibit the same feature to some extent.
Enough has been said to show that simulation of one structure by
another has frequently occurred in rocks in so marked a degree as to
render mistakes easy; and that these examples of 'mimicry' in the
inorganic world are particularly frequent in rocks which have been
subjected to great orogenic movements. The student will do well to
acquaint himself with the macroscopic and microscopic structures
which may be taken as characteristic of the rocks which have been thus
affected, some of which can usually be detected with ease, and when he
discovers them he may suspect that many phenomena which appear
explicable in one way were in reality produced in a different one, for
it is frequently very true of a region in which the rocks have been
violently squeezed, stretched and broken that 'things are not what
they seem.'
CHAPTER VIII.
GEOLOGICAL MAPS AND SECTIONS.
The writer does not propose to give an account of the intricacies of
geological mapping, for their right consideration requires a separate
treatise[22]; all he desires is to call attention to some of the uses
of geological maps as a means of conveying information. A geological
map may be looked upon as an attempt to express as far as possible in
two dimensions phenomena which possess three dimensions; this can be
done to some extent on the actual surface of the map, by conventional
signs, still more fully, by supplementing the map with sections; but
best of all by a geological model, which is cut across in various
directions in order to show the underground structure as well as that
of the surface.
[Footnote 22: The student is recommended to consult in particular,
Appendix I. "On Geological Surveying" in _The Student's Manual of
Geology_, by J. B. Jukes (Third Edition, Edited by A. Geikie), p. 747,
and _Outlines of Field Geology_, by Sir A. Geikie (Macmillan and
Co.).]
The ordinary geological map is one which shows the outcrop of the
strata, subdivided according to age, as they would be seen upon the
surface of the earth after stripping off the superficial
accumulations, and it is to be feared that the term 'geological map'
is associated in the minds of most students with a map of this
character and of no other. Nevertheless, a great many most important
observations other than those connected with the order of succession
of the strata are capable of representation upon a geological map, and
the possession of a large number of maps of any area upon the geology
of which a person is engaged--each map to be used for recording
observations of a particular kind--will save much writing in
note-books and, what is of more importance, will allow him to compare
observations which have been made at different times at a glance,
instead of causing him to search through a series of note-books.
Still, however well furnished with maps, the geologist will find a
note-book essential[23].
[Footnote 23: As a result of some experience, the writer recommends
every student to acquire some skill in the use of the pencil, and if
to such a degree that he can combine artistic effect with accuracy, so
much the better. An acquaintance with photography is invaluable: often
the possession of a camera would enable a section to be recorded,
which is otherwise lost to science.]
The earliest geological maps represented the variations in the surface
soils, or at most the general lithological characters of the rocks
which by their decay furnished the materials for the soils. We have
seen that the first chronological map was due to William Smith, and
most subsequent English geological maps have been based upon his map
of the strata of England and Wales. The order of succession of the
strata is represented in these maps to some extent by the use of
arrows to indicate the direction of dip of the strata, though this is
not an unerring guide where strata are reversed, and accordingly the
addition of a legend at the side of the map may be looked upon as
essential to the correct understanding of the map itself. The legend
is usually in the form of a section of a column, the strata being
arranged in right order, the oldest at the base and the newest at the
summit, the colours by which the strata are indicated being similar to
those placed upon the map. Other information besides the mere order of
succession of the strata may appear in the legend; thus their relative
and actual thicknesses can be indicated if the column is drawn to some
definite scale, and a brief description of the lithological characters
of the rocks may well be appended to the side of the column. On the
actual maps it is customary to exhibit the outcrop of the junctions of
all igneous rocks as well as of the sedimentary ones: the nature of
the metamorphism which sedimentary rocks have undergone at the contact
with igneous ones may be and often is indicated by suitable signs; the
position of faults is shown, and often also that of metalliferous
veins, the nature of the ore in the latter being further indicated in
some suitable manner, as by giving the recognised symbol for the
metal; and in many maps an attempt is made to show the variations in
dip and strike of the cleavage-planes.
The Geological Survey of the United Kingdom publishes two sets of
maps, one showing the 'solid geology' and the other the 'superficial
geology.' It is easier to understand these terms than to define them,
for in Britain there is a sharp line between the two everywhere except
near Cromer. The maps showing the superficial geology represent
gravels, glacial drifts and other incoherent accumulations of
geologically recent origin, which to a greater or less extent mask the
strata below which are usually composed of more or less solidified
material. The maps showing the solid geology display the outcrops of
these strata, though it is usual to insert alluvium upon these maps,
as it is often impossible to trace the junction-lines of the strata
below it. Attention has already been directed to the fact that these
maps of solid geology, though chronological, that is, having the
strata represented according to age, are founded largely upon
lithological differences, rather than upon included organisms; and it
has been stated that for theoretical purposes two sets of
chronological maps, one founded upon lithological differences, the
other upon difference of fossil organisms, would be extremely
valuable.
Other phenomena are often best represented upon separate maps, for if
all observations are crowded upon one map the result will be very
confusing. Special glacial maps showing the contour of the country,
with the portions between the contour lines coloured differently
according to altitude, say the country between sea-level and 500 feet
light green, that between 500 and 1000 dark green, that between 1000
and 1500 light brown and so on, exhibiting the direction of all
observed glacial striae, the distribution of boulders so far as it is
possible, and any other glacial phenomena which can be noted upon the
map, will be valuable to the student of glaciation[24].
[Footnote 24: For examples see Tiddeman, R. H., "Evidence for the
Ice-Sheet in North Lancashire and the adjacent parts of Yorkshire and
Westmorland," _Quart. Journ. Geol. Soc._, vol. XXVIII. pl. XXX., and
Goodchild, J. G., "Glacial Phenomena of the Eden Valley" &c., _Quart.
Journ. Geol. Soc._, vol. XXXI. pl. II.; and for a map of distribution
of boulders, Ward, J. C., "Geology of the Northern Part of the English
Lake District" (_Mem. Geol. Survey_), pl. IV.]
Various structural features may be well displayed on separate maps.
The trend of the axes of folds will be useful, and may be accompanied
by other information of cognate character[25]; maps of the
distribution of joint planes may be given in combination with those
showing the folding of the strata if it be desired to exhibit the
relationship between these; or with the physical features of the
country, if the dependence of physical features upon joint structure
be under consideration[26]. Much information concerning cleavage may
be acquired from a map showing anticlinal and synclinal axes of
cleavage[27], or the actual strike of the cleavage over different
parts of a map may be represented, and its relationship to the
geological structure of the district exhibited[28].
[Footnote 25: See Bertrand, M., "Sur le Raccordement des Bassins
Houillers du nord de la France et du sud de l'Angleterre," _Annales
des Mines_, Jan. 1893, Plate 1.]
[Footnote 26: See Daubrée, A., _Études Synthétiques de Géologie
Expérimentale_, 1^{ère} Partie, Plates III.-VI., for an example of the
latter, which is also interesting as showing the utility of a map on
transparent paper super-posed on another, when illustrating the
connexion between two sets of structures.]
[Footnote 27: Ward, J. C., _Geology of the Northern Part of the
English Lake District_, Plate IX.]
[Footnote 28: Harker, Alfred, "The Bala Volcanic Series of
Caernarvonshire" (_Sedgwick Essay_ for 1888), Fig. 5.]
Maps exhibiting changes in physical geography appertain to the
geologist as well as to the geographer. The position of ancient
beaches, former lakes, representation of the changes in the courses of
rivers and kindred phenomena may be shown upon maps, and will prove
useful[29].
[Footnote 29: For examples of maps of this kind, see Kjerulf, Th.,
_Die Geologie des südlichen und mittleren Norwegen_.]
A perusal of the maps to which reference has been made above will give
the student some notion of the extent to which maps may be utilised to
represent geological structures, and may suggest other methods by
which they may be utilised.
A geological section is usually drawn in order to exhibit the lie of
the rocks, as it would be seen if a vertical cutting were made in that
part of the earth's crust which is under consideration. The character
of the section will depend upon circumstances. The Geological Survey
of Great Britain issues two kinds of sections which are usually spoken
of as vertical sections and horizontal sections, though each is in
truth a vertical section; but whereas in the former the horizontal
distance represented is small as compared with the thickness of the
strata, in the latter the rocks of a considerable horizontal extent of
country are exhibited in the section, and the section is not carried
down to a great depth below the earth's surface. There is no essential
difference between the two kinds of section, and often sections are
drawn which cannot be definitely classed as belonging to either kind,
but in extreme cases the vertical section is a representation of the
order of succession as it would appear if the rocks were horizontal,
no matter how disturbed they may be in reality; whereas the horizontal
section represents the strata as they actually occur, with all the
folds and faults by which they are affected. The accompanying figure
(Fig. 6) represents a horizontal section on the left side of the
figure with a vertical section of the same rocks on the right side.
[Illustration: Fig. 6.]
Vertical sections are extremely useful when it is desirable to compare
variations in the strata over wide extents of country: this can be
done by drawing a series of columns of the strata, each showing in
vertical section the lithological characters and thicknesses of the
strata in one place, whilst the relationship between the strata of
two different places may be indicated by joining the beds of the same
age by dotted lines as shown in Fig. 7[30].
[Illustration: Fig. 7.]
[Footnote 30: It is useful to adopt conventional symbols for the
representation of strata of different lithological characters, and so
far as possible to adhere to the same kind of symbol for any one kind
of deposit. Those which are generally in use, are rough pictorial
representations of the characters of the deposits, as shown in Fig. 7.
The conglomerate is indicated by circular marks representing
cross-sections of the pebbles, a breccia by triangular marks
signifying that the fragments are angular and not rounded; a sandstone
is indicated by dots to represent the grains of sand; a mud, clay or
shale by continuous or broken horizontal lines, which reproduce the
appearance of the planes of lamination so frequent in beds of this
composition; a limestone is usually marked by the use of regular
horizontal lines illustrating the pronounced bedding, with vertical
lines at intervals to represent the regular jointing which occurs in
so many limestones: the nature of the bedding may be further shown by
drawing the lines comparatively far apart when the limestone is a
thick-bedded one, nearer together when it is thin-bedded. Igneous
rocks are represented by crosses or irregular V-shaped marks,
illustrating the absence of stratification and presence of joints.
Volcanic ashes are sometimes represented by dots, at other times by
signs somewhat similar to those which are used for true igneous rocks.
Sedimentary rocks which are composed of more than one kind of material
may be further shown by a combination of two symbols, thus the
existence of a sandy clay may be shown by means of a combination of
horizontal lines and dots, and so with other combinations. The
practical geologist should become accustomed to the use of these
symbols in his note-book; if used, they will save much writing.
These symbols are used in some of the later illustrations to this
book.]
The horizontal section is one which is in constant use by the
practical geologist: the results of the first traverse of a district
may be jotted down in his note-book in the form of a horizontal
section (with accompanying notes), and the written memoir on the
geology of any district composed largely of stratified rocks will
almost certainly require illustration by means of these sections.
Perhaps nothing more clearly marks the careful observer than the
nature of the sections which he makes, and geological literature is
too frequently marred by the publication of slovenly sections. A badly
drawn section not only offends the eye, it may and frequently does
convey inaccurate information.
[Illustration: Fig. 8.]
In the above figure (Fig. 8) taken from Sir Henry de la Beche's
"Sections and Views Illustrative of Geological Phænomena," Plate II.,
the lower drawing represents a section drawn to true scale, while that
above shows one which is exaggerated. The student who saw this would
infer that the uppermost beds on the left side of the upper section
rested unconformably upon the dotted beds beneath, and once abutted
against them in that portion of the figure where the beds have been
removed by denudation in the deep valley, whereas an examination of
the section drawn to true scale shows that the unconformity does not
exist (although there is one at the base of the deposits marked by
dots), and that there is room for the higher deposits to pass above
those marked by dots at the place where the former have been removed
by denudation. Whenever possible, horizontal sections should be drawn
to true scale, the vertical heights being on the same scale as the
horizontal distances. Sections which are so drawn represent the nature
of the surface of the country as well as the relationship of the
strata, and often illustrate in a marked degree the influence which
the character of the strata has exerted upon the nature of the
superficial features of a country. If it be impossible to draw a
section in which the elevations and horizontal distances are
represented upon a true scale, the former ought to be drawn on a scale
which is a multiple of the latter; thus the vertical heights may be
shown on 2, 3, or 4 or more times the scale chosen for the horizontal
distances; when this is done, it will often be necessary to show the
strata with an exaggerated dip, and accordingly the exaggerated
section loses some of its value, though if vertical and horizontal
scales bear some definite proportion it will still be more valuable
than a rough diagram which is not drawn to any scale.
Section-drawing cannot be satisfactorily accomplished without some
practice, and the student is strongly advised to acquire the art of
drawing good sections; the writer can assert as the result of
considerable experience in the conduct of examinations of all kinds,
that slovenly sections are the rule in candidates' papers, and good
sections very rarely appear. Study of the six-inch maps and horizontal
sections (drawn on the same scale) of the Geological Survey of the
United Kingdom will enable the student to familiarise himself with
admirable sections, and it should be his aim to produce sections like
these. He is recommended to take some of these six-inch maps which
show contour-lines as well as the disposition of the strata, and to
draw sections on the scale of six inches to the mile, vertical and
horizontal, exhibiting the proper outline of the ground and the
arrangement of the strata, and afterwards to compare them with the
published sections. The sections should be drawn as far as possible at
right angles to the general strike of the strata. Some datum-line is
taken for the base of the section (say sea-level) and offsets drawn
vertically from this where the section crosses a contour-line or
recorded height. The height is marked on these offsets; thus if a
recorded height of 2700 feet (just over half a mile) occurred on the
line of section a height of somewhat over three inches is marked on
the offset, and so with the other points where the section crosses
contours or recorded heights. By joining these points on the offsets,
giving the connecting lines curves similar to those which are likely
to occur in nature, the general character of the surface of the ground
is represented. The geology of the district is next shown. Wherever a
dip is marked on the map, the direction and amount of dip is shown by
a short line on the section, and where dips are not actually seen
along the line of section, the dips which are nearest to that line on
the map must be considered, and marked on the section. The lines of
junction between the various deposits shown by different colours upon
the map are inserted on the section as short lines, the inclination
being judged by study of the nearest dips; faults and igneous rocks
must be marked off, and any indication of the hade of the fault or the
slope of the edges of the igneous rock which the map affords will be
taken into account. The section will then appear somewhat as shown in
the following figure:
[Illustration: Fig. 9.]
and sufficient indication of the trend of the rocks will be obtained
to shew that they form portions of curves which may then be filled in
as shown in Fig. 10 and the section will be complete.
[Illustration: Fig. 10.]
It will be noticed that the small dyke of igneous rock on the right of
the main dyke is joined to it lower down, though no indication of this
is given along the line of section; but the requisite information for
this and evidence of the existence of the small dyke proceeding from
the left-hand side of the main one may be obtained by the study of
the rocks in a valley on one side or other of the line of section.
After the student has become conversant with the nature of geological
maps and sections, and has read Sir A. Geikie's _Outlines of Field
Geology_, he should on no account omit to learn something of the art
of making geological maps, by going into the field and attempting to
produce a map, for the art of geological surveying does not come
naturally to any one, and some acquaintance with the methods of
surveying is a necessity to everyone who wishes to make original
geological observations, though all cannot expect to afford the time
and acquire the skill necessary for the production of maps vying with
the detailed maps of the Government Survey. Before actually attempting
to draw lines on a map on his own account, he will do well to tramp
over a portion of a district with the published geological map in his
hands, selecting a country which is not characterised by great
intricacy of geological structure, and he can then attempt to
represent the geology of another portion of the same district without
consulting the published map. Of all the districts of Britain with
which he is acquainted the writer believes that the basin of the river
Ribble, in the neighbourhood of the town of Settle in the West Riding
of Yorkshire, is best adapted for studying field geology in the way
suggested above, for the main geological features are marked by
extreme simplicity, and the exposures are good, whilst the presence of
an important fault-system and of a great unconformity relieve the area
from monotony. Anyone who stands on the summit of Ingleborough or
Penyghent will grasp the main features of a portion of the district
without any difficulty, for it lies beneath his feet like a geological
model, and when the student has mastered and mapped in the leading
features, he can find bits of country with geology of varying degrees
of complexity amongst the Lower Palæozoic rocks of the valleys which
run down to Ingleton, Clapham, Austwick and Settle.
The biologist is supplied with laboratories at home and abroad, where
he may study his science under the best conditions. Would that some
munificent person would found, in a district like that referred to
above, a geological station where Cambridge students would have the
means of acquiring a knowledge of field-geology under conditions more
favourable than those presented by the flats around the sluggish Cam!
CHAPTER IX.
EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED.
The establishment of the order of succession of the strata, and the
correlation of strata of different areas merely pave the way for the
geologist. To write the history of the earth during various geological
ages, he has to ascertain the physical and climatic conditions which
prevailed during the successive geological periods, and to study the
various problems connected with the life of each period. In the
present chapter an attempt will be made to illustrate the methods
which have been pursued in order to write to the fullest degree which
is compatible with our present knowledge, the earth-history of various
ages of the past. In making this attempt, the physical and climatic
conditions may be first considered, and their consideration followed
by that of the changes in the faunas, though it will frequently be
necessary to refer to one set of conditions as illustrative of the
other.
It will be assumed here that the great principle of geology, that the
modern changes of the earth and its inhabitants are illustrative of
past changes, is rigidly true. Reference will be made to this
principle in a later chapter, but it is sufficient to state here that
the study of the sediments which have been deposited from the
commencement of Lower Palæozoic times to the times in which we now
live bear the marks of having been formed under physical conditions,
which, in the main, are similar in kind to those which prevail upon
some part of the surface of the lithosphere at the present day.
One of the most important inferences of the stratigrapher relates to
the existence of marine or terrestrial conditions over an area at any
particular time, and we may, in the first place, consider the evidence
which supplies us with a clue to this subject.
It has been previously stated that the ocean is essentially the
theatre of deposition, the land that of destruction, and accordingly,
the presence of deposit as a general rule indicates the evidence of
marine conditions during the formation of those deposits, though this
is not universally the case. Again, as denudation is practically
confined to the land areas, and the shallow-waters at their margins,
unconformity on a large scale gives evidence of the existence of
terrestrial conditions in the area in which it is developed, during
its production. Accordingly a mass of deposit separated from deposits
above and below by marked unconformities shows the alternation of
terrestrial conditions (during which the unconformity was produced)
and marine conditions (during which the deposits were laid down). The
deposits formed after an unconformity has been developed will
naturally be of shallow-water character, as will also be those of the
period immediately preceding the incoming of conditions which will
cause the occurrence of another unconformity, and between these two
shallow-water periods will occur a period when deeper-water conditions
probably prevailed. We can therefore not only divide the history of
any particular area into a series of chapters, of which every two
successive ones will describe a continental period and a marine one,
but each marine period may be divided into three phases--a
shallow-water phase at the commencement, an intermediate deeper-water
phase, and a shallow-water phase at the end. These phases are
frequently complicated by the occurrence of a host of minor changes,
but on eliminating these, the effects of the three great phases are
shown by study of the nature of the strata, and their recognition does
much to simplify the detailed study of the stratigraphical geology of
various parts of the earth's surface.
In discriminating between terrestrial conditions and marine ones, the
existence of unconformities is of great importance in marking
terrestrial conditions and is often the only available evidence, for
no accumulations or deposits formed on the land may be preserved to
testify to the terrestrial conditions[31]. When terrestrial deposits
and accumulations do occur, they are extremely important, and it is
necessary to allude to the points wherein they differ from marine
deposits.
[Footnote 31: The term terrestrial is used above in opposition to
marine, to include the conditions prevalent above sea-level. The term
continental would be better if it did not exclude insular conditions.
Accordingly deposits formed in rivers, and fresh-water and salt-water
lakes are spoken of as terrestrial.]
Apart from organic contents, the mechanically formed deposits of
rivers and lakes resemble in general characters the shallow-water
deposits of the ocean, though they are usually less widely
distributed. It is the accumulations which have actually been formed
as æolian rocks, or those which have been laid down as chemical
precipitates in salt-lakes which, by study of lithological characters,
furnish the most convincing evidence of their terrestrial origin.
Many æolian accumulations may be looked upon as soils, if the term
soil be used in a special sense to refer to the accumulations which
are produced as the result of the excess of disintegration over
transportation in an area, whilst others are due to transport which
has not been sufficiently effective to carry the material to the sea.
When the weathered material accumulates above the weathered rock, it
depends chiefly upon climate whether the disintegrated rock becomes
mingled with much decayed organic matter forming humus. If this
organic matter exists in quantity, the probability is that the
accumulation is a terrestrial one, though this is by no means
necessarily the case, for under exceptional circumstances a good deal
of humus may be deposited in the sea, as beneath the mangrove-swamps
which line the coasts of some regions, and to go further back, in the
case of the Cromer Forest series of Pliocene times, or some coals,
such as the Wigan Cannel Coal of the Carboniferous strata.
In addition to the work of water, which affects both land and
sea-deposits, the land is especially characterised by the operations
of wind and frost upon it, for these produce results which may
frequently serve to differentiate a land-accumulation from a deposit
laid down beneath sea-level. The effect of wind in rounding the grains
of sand which are blown by it is well-known, and samples of the
'millet-seed' sands of desert regions are preserved in most museums.
The greater rounding which characterises wind-borne as compared with
water-borne sand grains is due, in great measure, to the greater
friction between the grains when carried by the air than when swept
along by the water. Under favourable circumstances water-worn grains
may become rounded, especially when agitated by gentle currents
sweeping over a shoal[32]; but a large mass of sand, in which most of
the grains have undergone much rounding so as to give rise to
'millet-seed' sand, will nevertheless be probably formed by
wind-action except where a marine deposit is formed of material
largely derived from an earlier æolian one. The effect of frost is to
split rocks into fragments which are more or less angular before they
are subjected to water-action. The broken fragments are prone to
collect on slopes as screes, and as any scree-material falling into
the sea is likely to become rounded except under conditions which
rarely prevail, the existence of much scree-material in a rock
suggests its terrestrial origin. Glaciers gave rise to terrestrial
moraines, which may occasionally be identified as land-accumulations
by mere inspection of their physical characters, but all geologists
are aware of the difficulties with which they are confronted when they
attempt to discriminate between terrestrial and marine glacial
deposits.
[Footnote 32: Cf. Hunt, A. R., "The Evidence of the Skerries Shoal on
the wearing of Fine Sands by Waves," _Trans. Devon. Assoc._, 1887,
vol. XIX. p. 498.]
The existence of much material amongst the stratified rocks which has
been precipitated from a state of solution is an indication of the
terrestrial origin of the rocks, which were laid down on the floors of
the inland seas, separated more or less completely from the open
ocean; for the waters of the ocean are capable of retaining in
solution all of the material which is brought down to them, and
accordingly precipitates of carbonate of lime, rock-salt, gypsum and
other compounds formed from solution, are only formed on a large scale
in inland lakes, though they may be formed to some extent when the
water of a lagoon is only slightly connected with that of the open
ocean, and the evaporation is great, for instance in the lagoons of
coral reefs. Certain physical features often mark the deposits of
chemical origin, cubical or hopper-crystals of rock-salt may be
dissolved, and the hollow afterwards filled with mud, so that the rock
surfaces are sometimes marked with pseudomorphs of mud after
rock-salt. Sun-cracks and rain-prints impressed on the rock are not
actual indications of terrestrial origin of the rocks on which they
are found, for the shallow-water muds of an estuary may be deposited
in the sea and yet exposed to the action of the air at low tide, but
they mark very shallow-water deposits which have been exposed to the
atmosphere immediately after their formation if not during the time
they were formed, and they frequently occur amongst the deposits of
inland lakes.
It will be observed that the characters of the terrestrial
accumulations serve to distinguish them to some extent from the marine
ones, but they also enable one to detect to some degree the actual
conditions under which the accumulation was produced, whether on the
mountain-slope, or in the plain, the desert or the fen, the river-bank
or the lake-floor.
The conditions of formation of the marine deposits may be
distinguished within certain limits with ease, by examination of their
physical characters, for the near-shore deposits will generally be
coarser and contain more mechanically-transported material than the
sediments which accumulate at a greater distance from the shore,
though it is not safe to infer that deposits are formed away from the
shore on account of the absence of mechanically-transported sediments.
In districts where the mechanically-transported material is rapidly
deposited, organic deposits of great purity may form close to the
coast-line; for instance, when the rivers of a country end in fjords,
the mechanical sediments are deposited in the fjords, and the sea
around the coast is free from this sediment, and there the organisms
can build up deposits of great purity; and a similar thing may happen
when the rivers on one side of a country have short courses, and do
not carry down much sediment, which occurs when the watershed is near
the coast. On the one hand, clay may be formed in considerable purity
near the coast, where the supply of mud is so great that the organisms
existing there can do little in the way of contribution to the mass of
the deposit, or it may be formed on the other hand in great depths of
the ocean, where the supply of sediment is extremely small, but where
all the organic tests become dissolved; as the characters of the deep
sea clays are mainly negative, a geologist examining the rocks of the
geological column would have much difficulty in distinguishing a
deep-water clay from a shallow-water one by its lithological
characters only. In cases of difficulty, information of importance is
likely to be furnished by examination of the relative thickness of
equivalent deposits in adjoining areas, for if we find a mass of clay
a few feet thick in one region represented by hundreds of feet of clay
and limestone in another, the former mass probably accumulated slowly
and at some distance from the land; again, the uniformity of
lithological characters of a deposit over a very wide area is a
possible indication of its formation away from land, but this is not a
safe guide, for reasons which will eventually appear, unless it can be
shown that the deposit is everywhere of the same age.
A clue to climatic conditions is frequently furnished by the physical
characters of accumulations, especially terrestrial ones. The
accumulations containing a large percentage of hydrocarbons have
probably been formed under fairly temperate and moist climatic
conditions, whilst the existence of millet-seed sandstones associated
with chemical deposits points to desert conditions and inland lakes,
requiring a dry climate and probably a warm one. Glaciated surfaces
and glacial deposits of course indicate a low temperature. Some
geologists profess that occasionally they can even determine the
direction of the prevailing winds during past periods, by examination
of the character of ripple-marks, rain-pits and other features, though
it is doubtful whether much reliance can be placed upon these obscure
indications.
Useful as is the physical evidence supplied by deposits, as an index
to the conditions under which they were formed, it is usually only
supplementary to the evidence derived from a study of the fossils.
Fossils when present in the rocks, usually supply considerable
information concerning the prevalent conditions during the deposition
of the rocks. By them we can not only separate marine from terrestrial
deposits, but also freshwater deposits from æolian accumulations; each
kind of deposit will generally contain the remains of organisms which
existed under the conditions prevalent in the area of formation of the
rock, though it is of course a frequent thing for a terrestrial
creature or plant to be washed into a freshwater area or into the sea.
In an æolian deposit, the invertebrate remains may be those of any
air-breathing forms, as insects, galley-worms, spiders, scorpions and
molluscs. The land-molluscs are all univalve. Of vertebrates, we may
find the bones and teeth of amphibians, reptiles, birds and mammals.
Occasionally freshwater or even marine forms may be found in an æolian
deposit, but they will be exceptional. Marine shells are often blown
amongst the sand-grains of the coastal dunes, and seagulls and other
birds frequently carry marine organisms far inland.
The creatures frequenting fresh water differ from those of the land
and of the sea. The most abundant vertebrate remains will be those of
fishes, and of the invertebrates we find mollusca preponderate. The
variety of molluscs is not so great as in the case of marine faunas.
The bivalves always possess two muscular scars on each valve (except
adult _Mulleria_); whilst many marine shells as the oyster have only
one muscular scar on each valve. (See Fig. 11.)
[Illustration: Fig. 11.
_A._ Monomyary shell with one scar.
_B._ Dimyary shell with two scars.]
These scars mark the attachment of the adductor muscles, for drawing
the valves together, and the shells with only one impression on each
valve are called _monomyary_, those with two impressions _dimyary_.
The discovery of monomyary shells indicates with tolerable certainty
the marine character of the deposit in which they are found, though
their absence cannot be taken as proof of freshwater origin. The
beaks or umbones of the bivalves are often corroded in freshwater
deposits, as may be seen by examining shells of the common freshwater
mussel. "All univalve shells of land and freshwater species, with the
exception of _Melanopsis_ and _Achatina_, which has a slight
indentation, have entire mouths; and this circumstance may often serve
as a convenient rule for distinguishing freshwater from marine strata;
since if any univalves occur of which the mouths are not entire, we
may presume that the formation is marine[33]."
[Footnote 33: Lyell's _Students' Elements of Geology_, Second Edition
(1874), Chap. III. A good account of the differences between
freshwater and marine organisms, from which some of the facts here
cited are extracted, will be there found.]
[Illustration: Fig. 12.
_A._ Holostomatous shell.
_B._ Siphonostomatous shell.]
In Fig. 12 _A_ shows a freshwater shell (_Vivipara_) with entire
mouth, whilst _B_ exhibits the shell of a marine gastropod
(_Pleurotoma_) with a notched mouth. The entire-mouthed shells are
called _holostomatous_ whilst those which are notched, the notch being
often prolonged into a canal, are termed _siphonostomatous_.
Many groups of invertebrates are seldom or never found in fresh water.
Of exclusively or nearly exclusively marine creatures we may name the
foraminifera, radiolaria, sponges with a hard framework, most hydrozoa
which secrete hard parts, corals, echinoderms, cirripedes, king-crabs,
locust-shrimps, most polyzoa, brachiopods, pteropods, heteropods, and
cephalopods. Of extinct groups, the graptolites and trilobites seem to
have been entirely confined to the sea.
In the modern and comparatively modern deposits, the forms frequently
belong to existing genera, and we get fairly conclusive evidence of
the conditions of deposit by determination of the genera. The
terrestrial (including freshwater) molluscs have mostly a long range
in time. We find pulmoniferous gastropods of living genera in the
Carboniferous period, one (_Dendropupa_) belongs to a subgenus of the
modern land-shell _Pupa_, the other (_Zonites_) to a subgenus of the
snail group _Helix_. Many freshwater molluscs as _Unio_, _Cyclas_, and
_Physa_ are found amongst the secondary rocks, and give a clue to the
origin of the deposits which contain them. Many extinct genera are
closely allied to modern genera, and their mode of existence may be
assumed with fair certainty. With all these guides, we may sometimes
be left in doubt as to the conditions of deposit when organisms are
few in number; thus, it is yet a matter for discussion whether the Old
Red Sandstone and many of the deposits of the Coal Measures of Britain
were of freshwater or marine origin.
In considering the possibility of fossils having been carried from
land to water or _vice versa_, it will be remembered that generally
speaking they are more readily transferred from a higher to a lower
level, so we are more likely to find remains of land-animals and
plants in fresh water or the sea, and relics of freshwater animals and
plants in the sea, than of marine or freshwater animals and plants in
land, or marine organisms in fresh water. River-gravels and lacustrine
deposits are especially prone to contain a considerable intermixture
of land-forms with those proper to the station.
Fossils supply much information concerning the depth and distance from
land at which the deposits were laid down. When portions of the
ocean-water have been separated to form inland lakes, the water
becomes saltier than that of the open ocean, if the evaporation is
greater than the supply of fresh water, and the life of the inland sea
undergoes change under the unfavourable conditions set up. Many forms
disappear altogether, and those which survive tend to become stunted,
and the shells of many of the mollusca are abnormally thin; the fauna
of an inland sea though it may have abundance of individuals is apt to
be characterised by paucity of species.
Turning now to the faunas of the open oceans, it is found that in
addition to latitude, the distribution of organisms is affected by
depth, and by the nature of the sea-floor, and accordingly we find
different organisms in different areas; and in examining the same area
the organisms inhabiting different depths are not all the same, and at
the same depth some kinds of animals have different _stations_ from
those of others, one creature being confined to a sandy floor, another
to a muddy one, and so on[34]. The oceans have been divided into 18
_provinces_, each of which is more or less characterised by the
possession of peculiar forms which are termed _endemic_, in contrast
to the _sporadic_ forms which are widely distributed. In any area
which is margined by a coast line, the molluscs are distributed in
zones which were formerly classed as follows:--the _littoral_ zone
between tide marks, the _laminarian_ zone from low water to fifteen
fathoms, the _coralline_ zone between fifteen and fifty fathoms, and
the _deep-sea coral_ zone from fifty fathoms to one hundred fathoms or
more; this last depth was once supposed to mark the limit of the
downward extension of marine life, but as the result of modern
deep-sea soundings we know that organisms extend to a much greater
depth, and the deep-sea fauna, owing to uniformity of conditions over
wide areas, contains fewer endemic forms in proportion to the sporadic
ones than the shallow-water[35]. The deep-sea deposits entomb the
remains of these deep-sea organisms and also of numerous _pelagic_
organisms which live upon the surface of the ocean, whose remains sink
to the ocean-floor after death. Amongst the deposits of the deeper
parts of the ocean, we find many which are almost exclusively composed
of the tests of foraminifera, radiolaria and pteropods, the spicules
of sponges, and the frustules of diatoms; and accordingly the
existence of foraminiferal, pteropodan, radiolarian, and diatomaceous
oozes, amongst the strata of the geological column, has been taken by
some as indicating the prevalence of deep-sea conditions during the
formation of those deposits: as the purity of a calcareous ooze
depends upon the absence of mechanical sediment, or volcanic dust, and
as the component organisms of these oozes are pelagic forms which live
near the continents as well as in the open oceans, the presence of
calcareous oozes implies the existence of a _clear_ sea during their
deposition but not necessarily of a deep one, for if the sea-area be
far away from land masses, or if the sediment be strained off in
fjords, calcareous oozes may be formed in shallow water. The existence
of pure radiolarian or diatomaceous deposits is better evidence of
deep water, for if they were formed in shallow water we should expect
an intermixture of calcareous tests, whereas these are dissolved
whilst sinking into the extreme depths of the ocean. As the deep-sea
creatures are under very different conditions from those of shallower
waters, we might expect marked structural differences between the deep
and shallow-water creatures: one such difference has been emphasized,
namely the occurrence of animals which are blind or have enormously
developed eyes in the great depths of the sea, where the only light is
due to phosphorescent organisms. This is well seen in the case of many
recent crustacea, and has been noted by Suess in the case of the
trilobites of some beds which he accordingly infers to be of
deep-water origin, and it is interesting to find that these creatures
are found in deposits which give independent evidence of an open-water
origin. The _Æglinæ_ of the Ordovician strata are frequently furnished
with enormous eyes, and they are often accompanied by blind
trilobites, and in Bohemia the blind and large-eyed forms are
sometimes different species of the same genus, for instance
_Illænus_[36].
[Footnote 34: For an account of the distribution of one group of
organisms see Woodward, S. P., _A Manual of the Mollusca_, from which
many of the following observations are taken.]
[Footnote 35: For an account of the deep-sea fauna, see Hickson, S.
J., _The Fauna of the Deep Sea_, 1894.]
[Footnote 36: Suess, E., _Das Antlitz der Erde_, 2^{er}. Bd., p. 266.]
As one would naturally expect, the actual depth at which deposits were
formed can generally be calculated with a greater degree of certainty
amongst the newer rocks than amongst the older ones. In the case of
the Pliocene Crags, the depth in fathoms may be confidently given. In
the Cretaceous rocks attempts have been made to give numerical
estimates of the depths at which different accumulations were formed,
but some differences of opinion have arisen in the case of these
rocks. In the Palæozoic rocks, only a rough idea of the general depth
can usually be obtained, and no attempt to calculate the depth in
fathoms is likely to be even approximately correct in the present
state of our knowledge.
The comminution of fossils has sometimes been taken as an indication
of shallower water origin of the deposits which contain them, but
although the hard parts of organisms in a broken condition have
frequently been shattered by the action of the waves, they may also be
broken at great depths by predaceous creatures, and in many instances
the fracture is the result of earth-movements occurring subsequently
to the formation of the deposits.
Turning now to the difference in organisms which results from
difference of station, it will be sufficient to give a quotation from
Woodward's _Manual of the Mollusca_ as an illustration:--"In Europe
the characteristic genera of _rocky_ shores are _Littorina_,
_Patella_, and _Purpura_; of sandy beaches, _Cardium_, _Tellina_,
_Solen_; gravelly shores, _Mytilus_; and on muddy shores, _Lutraria_
and _Pullastra_. On rocky coasts are also found many species of
_Haliotis_, _Siphonaria_, _Fissurella_, and _Trochus_; they occur at
various levels, some only at the high-water line, others in a middle
zone, or at the verge of low-water. _Cypræa_ and _Conus_ shelter under
coral-blocks, and _Cerithium_, _Terebra_, _Natica_ and _Pyramidella_
bury in sand at low-water, but may be found by tracing the marks of
their long burrows (Macgillivray)[37]."
[Footnote 37: Woodward, S. P., _A Manual of the Mollusca_, p. 151.]
The geologist will naturally select sporadic forms rather than endemic
ones in comparing the strata of different areas, but how far
differences in faunas are the result of existence at different times,
and how far they are due to difference of conditions affecting
contemporaneous organisms can only be discovered as the result of
accurate observation. The main points to be regarded when comparing
the successive faunas of different regions have been noticed in this
and the preceding chapters, and it has been shown that as the evidence
is cumulative, it requires the collection of a large number of facts
obtained by observation of the strata before accurate inferences can
be drawn.
The indications of climatic conditions furnished by organisms require
some consideration. In the comparatively recent deposits it is not
difficult to get some notion of the prevalent climatic conditions when
the fossils belong to forms closely related to modern genera. The
existence of the arctic birch and arctic willow, and of shells
belonging to species now living north of the British Isles, in
deposits of comparatively recent date in Britain would afford
convincing evidence of the occurrence of colder climatic conditions
than those which are now prevalent in the area, even if the evidence
were not confirmed as it is, by physical proof of glaciation in
deposits of the same age. Nevertheless, even in these recent beds, we
have a useful warning, by finding species of elephant and rhinoceros
associated with northern forms like the lemming, glutton, and musk-ox.
We know that the species of elephant and rhinoceros (the mammoth and
woolly rhinoceros) were provided with thick coverings which would
enable them to resist the severity of an arctic climate, but had not
these coverings been found, we might have been puzzled by the
association of forms whose nearest allies are sub-tropical with others
of arctic character. As we go back in time and deal with earlier
deposits, the ascertainment of the climatic conditions becomes more
difficult, as the fossils mostly belong to extinct species, genera or
even families.
In these circumstances, it is very dangerous to draw conclusions as to
climatic conditions from examination of a few forms, but when we find
that plants and animals, terrestrial and marine forms, vertebrates and
invertebrates alike point to the same conclusion, as in the London
Clay, where all the fossils belong to forms allied to those now living
under sub-tropical conditions, the state of the climate may be
inferred with considerable certainty[38]. The character of the fossils
must be taken into account rather than their size. There was a
tendency amongst geologists to believe that large organisms probably
indicate warm conditions. Recent researches in arctic seas have
dispelled this belief. Marine algæ of enormous size are found in the
cold seas, and the size of creatures, abundance of individuals and
variety of forms in the arctic faunas of some regions is very
noteworthy. In the Kara Sea, for instance, a variety of creatures were
dredged up during the voyage of the Vega, and Baron Nordenskjöld makes
the following pertinent remarks about them: "For the science of our
time, which so often places the origin of a northern form in the
south, and _vice versa_, as the foundation of very wide theoretical
conclusions, a knowledge of the types which can live by turns in
nearly fresh water of a temperature of +10°, and in water cooled down
to -2·7° and of nearly the same salinity as that of the Mediterranean,
must have a certain interest. The most remarkable were, according to
Dr Stuxberg, the following: a species of Mysis, _Diastylis Rathkei_
Kr., _Idothea entomon_ Lin., _Idothea Sabinei_ Kr., two species of
Lysianassida, _Pontoporeia setosa_ Stbrg., _Halimedon brevicalcar_
Goës, an Annelid, a Molgula, _Yoldia intermedia_ M. Sars, _Yoldia_ (?)
_arctica_ Gray, and a Solecurtus[39]. "The temperatures were taken by
a centigrade thermometer. Again we read of the results of dredging off
Cape Chelyuskin. "The yield of the trawling was extraordinarily
abundant; large asterids, crinoids, sponges, holothuria, a gigantic
sea-spider (Pycnogonid), masses of worms, crustacea, etc. _It was the
most abundant yield that the trawl-net at any one time brought up
during the whole of our voyage round the coast of Asia_, and this from
the sea off the northern extremity of that continent[40]."
[Footnote 38: For a discussion as to the value of plants as indices of
climate see Seward, A. C., Sedgwick Essay for 1892.]
[Footnote 39: Nordenskjöld, A. E., _The Voyage of the Vega_, Vol. I.
Chap. IV.]
[Footnote 40: _Ibid._ Chap. VII.]
Amongst the marine invertebrates reef-building corals and mollusca
perhaps furnish the best evidence of climatic conditions. The
coral-reefs of the Jurassic rocks with large gastropods and
lamellibranchs clustered around them have been appealed to in proof of
the existence of sub-tropical conditions during their formation;
further back in time we find evidence of climate furnished by the
fossils of the Silurian rocks of the Isle of Gothland in the Baltic
Sea. Of these, Lindström writes "_The fauna had a tropical character_.
In consideration of the great numbers of Pleurotomariae, Trochi,
Turbinidae and the large Pteropods the assumption of a tropical
character of the fauna may seem justifiable[41]."
[Footnote 41: Lindström, G., _On the Silurian Gastropoda and Pteropoda
of Gotland_, Stockholm, 1884, p. 33.]
Structure may give some indication of climate even though the organism
is not allied to living species. The bark of trees in arctic regions
is often thicker than in more temperate regions, and the leaves of
arctic plants often have special characters to enable them to resist
the long periods during which they are deprived of water, though the
fact that desert-plants frequently shew similar modifications deprives
this test of any particular value except as a means of corroborating
conclusions reached from other evidence[42]. The shells of arctic
mollusca may become stunted, but this is not by any means universal,
and the same result may be brought about by other abnormal conditions,
as for instance the increase of salt in a water area by evaporation.
[Footnote 42: For an account of the modifications of the leaves of
arctic plants, see Warming, Eug., _Om Grønlands Vegetation_,
Meddelelser om Grønland, 12th part, p. 105.]
On the whole, an examination of the evidence available for
ascertaining the character of climate by reference to included
organisms, shews that inferences may be drawn within certain limits,
but that the task is a difficult one not unaccompanied by danger, and
every kind of available evidence derived from a study of physical
phenomena and the included organisms should be utilised before any
conclusion is drawn.
The likelihood of accurate inference is increased by comparing the
faunas of various areas; should they seem to indicate a progressive
lowering of climate when passing from lower to higher latitudes, it is
probable that the indication is correct. The student is referred to a
paper by the late Professor Neumayr for an account of the existence of
climatic zones during the Mesozoic Period[43].
[Footnote 43: Neumayr, M., "Ueber klimatische Zonen während der Jura-
und Kreidezeit," _Denkschrift. der Math.-Naturwissensch. Classe der k.
Akad. der Wissenschaften_, Bd. XLVII. Vienna, 1883.]
CHAPTER X.
EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED, CONTINUED.
In the preceding chapter, attention was drawn to the indications as to
conditions of deposition furnished by the sediments of any one
locality, and only passing reference was made to variation in the
nature of the sediments and their organic contents, when the deposits
are traced laterally from place to place; some attention must now be
paid to this matter.
It is sometimes inferred that, whereas similarity of organisms is a
dangerous guide in correlating the strata of two areas, accurate
correlations may be made, if the deposits can be traced continuously
through the intervening interval; no doubt the task is simplified when
this can be done, but the continuity of deposit of one particular
composition is no more proof of contemporaneity than the occurrence of
the same fossils continuously through the interval, imbedded in strata
of different character, indeed probably not so much so. The existence
of widespread masses of conglomerate, which are not found as linear
strips, but which extend in all directions, is in itself an indication
of this; the Oldhaven pebble bed for instance, in the Tertiary rocks
of the London basin, is very widely distributed. We cannot suppose
that coastal conditions prevailed far away from the shore-line, and
accordingly when a conglomerate occurs in a widespread sheet, and not
in a linear strip, this is indicative that the deposit has not been
formed continuously but that strip has been added to strip along an
advancing or receding shore line, and if this happens with
conglomerates, it must occur also in the case of other deposits.
[Illustration: Fig. 13.]
In fig. 13[44] let _A_ represent a shore line of a continent which is
undergoing gradual elevation. A deposit of pebbles _a_ will be formed
against the coast, one of sand _b_ further away, then one of mud _c_
and lastly limestone _d_, may be formed in the open sea away from
land. Naturally there may be intermingling of two kinds of deposit at
the junctions, but for the sake of simplicity this may be disregarded.
During the accumulation of the deposits _a_, _b_, _c_, _d_, certain
sporadic forms may be distributed throughout all the deposits, and
some of them may become extinct before the deposition of these beds is
completed, if the process is carried out on a large scale; we may
speak of the characteristic fossils of this period as fauna I. As the
result of elevation or of mere silting up of the sea-margin, or of
both combined, the next mass of pebble-deposit will be laid down
further away from the original shore, for the shore line will now be
at _A´_ and not at _A_, and it will partly overlap the mass of sand
_b_; the sand _b_^1 will also be deposited somewhat further out and
partly overlap the mud _c_, and similarly the mud _c_^{1} will partly
overlie the limestone _d_. During the formation of _a_^{1}, _b_^{1},
_c_^{1}, _d_^{1}, other sporadic forms belonging to a fauna II may
replace those of the first fauna. In the same way _a_^{2}, _b_^{2},
_c_^{2}, _d_^{2} will be deposited, and in the meantime a new fauna
III may arise and replace II. So the process will go on until we
finally have a group of deposits lying one over the other, consisting
of a basal accumulation of limestone, succeeded by mud, sandstone and
pebble-beds in succession. Each of these will be continuous, though
the inner part of the pebble-deposit was formed long before the outer
part of the limestone, which is nevertheless beneath a mass of
pebble-deposit continuous with that formed first, and the various
deposits will be separated by fairly horizontal planes _x_, _y_, _z_,
which might be regarded as bedding planes, but which are not so,
strictly speaking. The true bedding planes will occur at a slight
angle to these planes of separation, for the structure resembles false
bedding on a gigantic scale, but of course, the lines separating two
masses of similar deposit will be practically horizontal and parallel
to the planes of demarcation of two distinct kinds of material. The
lines separating two faunas would, under the conditions postulated,
run approximately parallel to the planes of separation of adjoining
deposits of the same lithological character but would pass from
conglomerate, through sandstone, mud and limestone, as indicated by
the lines 1, 2, 3, ... and the deposits between adjoining lines would
be contemporaneous[45]. In nature, complications will arise, owing to
the gradual appearance and disappearance of forms, and the existence
of endemic species in contemporaneous deposits formed in different
stations and having different lithological characters.
[Footnote 44: The writer gratefully acknowledges his indebtedness to
Prof. Lapworth for some of his views concerning deposition of strata.]
[Footnote 45: The lines 1, 2, 3 ... are incorrectly drawn in the
figure. Line 1 should be drawn so as to separate _a_, _b_, _c_, _d_
from _a_^{1}, _b_^{1}, _c_^{1}, _d_^{1}, line 2 to separate _a_^{1},
_b_^{1}, _c_^{1}, _d_^{1} from _a_^{2}, _b_^{2}, _c_^{2}, _d_^{2}, and
so with the others.]
If elevation ceased and were succeeded by depression, the exact
opposite would occur, and the pebble beds would be overlain by
sandstones, these by muds, and lastly limestones would appear. It
follows that during a marine phase occurring between two
unconformities we should have a =V=-shaped accumulation of deposits
with the apex pointing to the part of the shore line which was last
submerged before the commencement of elevation, as shewn in fig. 14,
though the beds of the apex will in most cases be denuded during the
re-emergence.
[Illustration: Fig. 14.]
Indications of the non-coincidence of the planes separating faunas and
those which separate deposits of one lithological character from those
of another have already been detected, for instance the 'greensand'
condition of the Cretaceous period occurs in some places during the
existence of one fauna, and in others during that of another, though
the planes have not been traced continuously. Mr Lamplugh has
furnished another example amongst the Cretaceous rocks of Yorkshire
and Lincolnshire, but as has already been observed, a great deal
remains to be done in this direction, and geologists are much in want
of two sets of stratigraphical maps, in one of which the lines are
drawn with reference to the differences of lithological character,
whilst in the other they separate different faunas.
The student will notice the normal recurrence of deposits in definite
order; conglomerate succeeded by sandstone, mud and limestone, in a
sinking area, and limestone succeeded by mud, sandstone and
conglomerate in a rising area. Naturally many instances of departure
from this rule are seen, owing to local conditions, but on a large
scale, it is very frequently noted, and recognition of this will
enable the student to remember the variations in the lithological
characters of the deposits more easily, than if he simply acquired
them from a text-book without taking heed as to their significance.
Upon the variations in the lithological characters of deposits and of
their faunas, when the beds are traced laterally depends very largely
the successful ascertainment of the existence of former coast-lines,
the restoration of which constitutes an important part, of
Palæo-physiography, concerning which some observations may here be
made[46]. If a set of deposits having different lithological
characters can be proved to be contemporaneous, the coarser detrital
accumulations will point to the approach to a coast-line, and the
actual position of the coast during the period of accumulation of the
deposits may be very accurately fixed. The pebble-beds at the base of
the Cambrian rocks of Llanberis indicate the existence of a
coast-line in that position during the accumulation of those
pebble-beds. Similar pebble-beds occur at St David's, at the base of
the Cambrian, but it is impossible in the case of these rapidly
accumulated sediments to say that two deposited so far away from one
another were actually contemporaneous, and therefore although we
might draw a line through Llanberis and St David's to indicate the
old coast-line of the period, it does not follow that the actual
beach existed simultaneously at the positions indicated. The
palæo-physiographer, however, attempts to restore the physical
conditions of greater thicknesses of deposit; for instance, the
distribution of land and sea during Lower Carboniferous times over the
area now occupied by the British Isles is often taken to illustrate
the methods of restoration of ancient features, and all admit that the
lithological and palæontological characters of the rocks indicate a
shallowing of the Carboniferous sea when passing northwards towards
Scotland. For conveying an idea of the restorations to the student, it
is almost imperative to portray the distribution of land and sea upon
a map, and this can only be done by drawing definite lines. It must be
distinctly understood that these lines are necessarily only an
approximation to the actual position of the ancient shore-lines, which
must have shifted again and again during the long period occupied by
the accumulation of the Lower Carboniferous strata, so that a true
idea of the positions of the Lower Carboniferous shore-lines could
only be obtained by placing on a series of maps the successive
shore-lines of different parts of the Lower Carboniferous period, and
taking a composite photograph of these, which would appear as a wide
belt of shaded portion of the map with no definite boundaries. The
utmost that the maker of palæo-physiographical maps can expect to
indicate, when dealing with considerable thicknesses of strata, is an
approximation to the mean position of the shore-lines of the period
when these strata were deposited. This is extremely valuable in
enabling the student to understand the significance of the variations
in the characters of the strata and their organic contents, if he
distinctly recognises the generalised nature of the map. Examination
of any two palæo-physiographical maps of the same period by different
authors will shew wide divergences in the details, but a general
resemblance of the main features. The reader will do well to consult
Prof. Hull's restoration of the physical features of Old Red Sandstone
and Lower Carboniferous Times on Plate VI. of his _Contributions to
the Physical History of the British Isles_, and compare it with the
map drawn by Prof. Green (_Coal: its History and Uses_, by Profs.
Green, Miall, Thorpe, Rücker, and Marshall, Fig. 3, p. 38), which will
be found to bear out this statement.
[Footnote 46: On this subject, the student may consult Prof. E. Hull's
_Contributions to the Physical History of the British Isles_.]
Valuable as the published maps of palæo-physiography are as an aid to
the student in understanding the significance of the variations of
characters amongst the sediments, he will do well to supplement them
by maps which he fills in for himself. He is recommended to procure a
number of outline maps of England, or of the British Isles, and when
studying in detail the characters of the British sedimentary rocks
formed during the various periods, to place a blank map by his side
when beginning the study of each period or important portion of a
period. On this map he should jot down the geographical distribution
of the different kinds of sediments, using the conventional signs
indicated at p. 90: thus, in the case of the Lower Carboniferous
rocks he would place the conventional sign for limestone in
Derbyshire, a combination of those for limestone and shale in
Yorkshire, and would add to these the sandstone sign in
Northumberland. He should also note the general character of the
fossils, using abbreviations for such terms as fresh-water fossils,
shallow-sea fossils, deep-water fossils. After reading the account of
the group of rocks in a comprehensive text-book, and inserting his
notes on the map, he should proceed to insert the probable position of
the coast-lines. He should also take notes of any indications of
contemporaneous volcanic action, though these might well be inserted
on a separate map. If this course be pursued, the student will not
only have the significance of the variations amongst the strata
impressed upon his mind, but he will have a means of obtaining at a
glance the distribution of sediments and faunas of different kinds in
the British area during the principal geological periods. On another
set of maps he may indicate the axes of the orogenic movements which
have occurred at different times, and when his various maps are
completed, he will have the materials for the construction of a
general account of the various geological processes which have been
concerned with the building of the British area.
When an area like Britain has been studied, the student may proceed to
construction of maps of wider regions, and he will find that in doing
this, new sets of facts must be taken into consideration, as for
instance the occurrence of different faunas on opposite sides of
once-existing continental masses, and the problems connected with the
present distribution of the faunas and floras. For an instance of the
importance of the former distribution of life the reader may consult
the twelfth section of the first part of Professor Suess' _Das
Antlitz der Erde_, whilst a good account of the value of recent
geographical distribution of organisms in supplying a clue to former
distribution of land and sea will be found in Mr A. R. Wallace's
_Island Life_, Chapter xxii.
Should the method suggested above be adopted, the student is likely to
acquire a much more coherent idea of the significance of the facts of
stratigraphical geology than can be obtained by a mere perusal of the
accounts of the strata given in those portions of the various
text-books which are devoted to a consideration of the stratigraphical
branch of the science.
CHAPTER XI.
THE CLASSIFICATION OF THE STRATIFIED ROCKS.
In the succeeding chapters, a general account of the characters of the
Geological Deposits of different periods will be given, for the
purposes of illustrating the principles to the consideration of which
the earlier chapters have been devoted. It is not proposed to enter
into a description of numberless details, which would only confuse the
student who wished to grasp the main principles, for many facts have
been recorded which it is necessary to notice in a comprehensive
text-book treating of stratigraphical geology, though their full
significance is not yet grasped. The writer, while noting the main
characters of the various subdivisions of the different
stratigraphical systems, will assume that this work is used in
conjunction with some recognised text-book. The stratigraphical
portion of Sir A. Geikie's _Class Book of Geology_ gives an admirable
general account of the British Strata, while the larger text-book by
the same author has a condensed though very full account of the rocks
of the stratigraphical column in all parts of the world, and this is
supplemented by numerous references to the original works wherein
further descriptions may be found. The English edition of Prof. E.
Kayser's _Text-Book of Comparative Geology_, edited by P. Lake, is
also well adapted to the wants of the student, and an excellent
account of the strata is given in Mr A. J. Jukes-Browne's _Handbook of
Historical Geology_, which may be read with the same author's
_Building of the British Isles_.
The reader who refers to different text-books will be struck with the
variations of nomenclature even amongst the larger stratigraphical
divisions, for two authors seldom subdivide the geological column into
the same number of rock-systems. The following classification will be
here adopted:--
Groups. Systems.
{ Recent
{ Pleistocene
Cainozoic or { Pliocene
Tertiary { Miocene
{ Oligocene
{ Eocene
{ Cretaceous
Mesozoic or { Jurassic
Secondary { Triassic
{ Permian
{ Permo-Carboniferous
{ Carboniferous
Palæozoic { Devonian
{ Silurian
{ Ordovician
{ Cambrian.
Precambrian.
A few remarks may be given as to the reason for adopting this
classification.
It is not for a moment suggested that the Systems have the same value,
if the time taken for their accumulation be alone considered. The beds
classified as Recent, for example, were probably accumulated during a
lapse of time far shorter than that occupied for the deposit of some
of the series or even stages of a system like the Silurian, but the
recent rocks acquire a special significance from the fact that we are
living in the period, and the Cainozoic rocks as a whole are capable
of greater subdivision than the earlier groups, on account of the
greater ease with which they can be studied, owing to the small amount
of disturbance which they have usually undergone when compared with
that which has affected older rocks, and the closer resemblance of
their faunas and floras to those of existing times.
With reference to the groups, the writer has already commented upon
the use of the terms Palæozoic, Mesozoic and Cainozoic; below the
lowest Palæozoic rocks (those of the Cambrian system) lie a group of
rocks which have been variously spoken of as Azoic, Eozoic, and
Archæan. There is an objection to the use of any one of these words in
this sense; the objection in the case of the first two is that the
term is theoretical and probably incorrect, whilst the word Archæan,
otherwise suitable, has also been used in a more restricted sense. In
these circumstances the term Precambrian will be used when referring
to any rocks which were formed below Palæozoic times, though no doubt
when this obscure group of rocks is more thoroughly understood a
satisfactory classification will be applied to it.
Taking the other groups into account, the lower systems of the
Palæozoic group will be found to vary greatly according to the views
of different writers; some make only one system, the Silurian, others
two, the Cambrian and Silurian. The three systems are here adopted,
not only because the one, Silurian, is too unwieldy on account of its
size and requires subdivision (and the Cambrian and Silurian however
defined, will be found to be of very unequal importance, whereas the
three systems adopted are of fairly equal value), but especially
because when the term Ordovician is used, the significance of the
other terms Cambrian and Silurian is at once understood.
An attempt has been made to shew that the Devonian system is
non-existent, but the result of modern research is to shew that the
rocks placed in this system are worthy of the distinction, both from
their importance and from the distinctness of the fauna from those of
the underlying and overlying systems.
The Permo-Carboniferous system is adopted, because an important group
of deposits has recently been brought to light which were not
represented either in the Permian or Carboniferous system as
originally defined.
Some authors have advocated the union of the Permian and Triassic
systems into one system placed at the base of the Mesozoic group. This
is unnecessary, and would depart from the classification originally
proposed, which is to be deprecated, unless there is any strong reason
for it.
The Mesozoic systems are classified according to the method generally
adopted. Were a fresh classification to be proposed, a portion of the
Cretaceous system might be included with the Jurassic rocks, but it is
better to adhere to the old classification.
The divisions of the Cainozoic rocks are hardly systems in the sense
in which the term is used in the case of the older rocks, but the
reason for using these smaller subdivisions has already been
mentioned. The addition of the Oligocene to the original divisions
suggested by Lyell has been found useful, and the term will be used
in this work.
The reasons for the adoption of the particular minor subdivisions
(series and stages) in the following chapters will frequently appear
when the rocks of the various systems are described, and need not be
further alluded to in this place.
Although most geologists describe the stratified rocks in ascending
sequence beginning with the oldest, and proceeding towards the newest,
others, and notably Lyell, adopted the opposite method and commenced
with an account of the newest beds. The argument generally used for
the latter method is that it is easier to work from the study of the
known to that of the less known, and as the faunas of the newest rocks
are most like the existing faunas, the student would more readily
follow a description of the rocks in the order which is opposite to
that in which they were deposited.
In practice, the study of the sediments in their proper order, that
is, in the order of deposit, will not be found to task the student to
any great extent, especially if, as is very desirable, he has studied
the main facts and principles of Palæontology before commencing the
study of the rock-systems in detail. There is one reason for beginning
with the study of the older sediments which outweighs any reasons
which can be advanced against it, namely that the events of any period
produce their effect not only upon the strata of that period, but also
on those of succeeding periods.
The task of the stratigraphical geologist is really to learn the
evolution of the earth, in its changes from the simple to the more
complex conditions, and it is quite obvious that it is unnatural to
attempt any study of evolution by working backward. For this reason
the study of the sediments will be here made in the order which is
usually adopted, by passing from the older to the newer, and from the
simple to the more complex.
The British strata will be mainly considered, though references will
frequently be made to their foreign equivalents, and a fuller account
of the latter will be added when the British strata are abnormal, as
are those of Triassic times, and also when a period is not represented
amongst the strata of the British Isles, as for instance, the
Permo-Carboniferous and Miocene periods.
The student is recommended to refer constantly to good geological maps
of the British Isles, of Europe, and of the world. Of maps of the
British Isles, mention may be made of Sir A. Ramsay's geological map
of England, Sir A. Geikie's map of Scotland, and his map of the
British Isles, J. G. Goodchild's map of England and Wales, a map of
Europe by W. Topley and one of the world reduced from that by J.
Marcou, accompanying the first and second volumes of the late Sir J.
Prestwich's _Geology_. For special purposes more detailed maps will be
studied, including the one-inch maps of H. M. Geological Survey, and
the index map on a smaller scale. Lastly, for an account of British
Geology, reference must be made to H. B. Woodward's _Geology of
England and Wales_, where the British formations are described in
order, and to W. J. Harrison's _Geology of the Counties of England and
Wales_, where the stratigraphical geology of the country is given
under the head of the different counties, which are taken in
alphabetical order.
In concluding this chapter, it is hardly necessary to say that every
opportunity of studying the characters of the deposits and their
fossils in the field should be eagerly seized, and that much
information may be acquired even on a railway journey, especially as
to the influence which the deposits exert upon the scenery of a
region[47].
[Footnote 47: In the first edition of H. B. Woodward's _Geology of
England and Wales_, an account of the geology of the main lines of
English railways is given, which is omitted in the later edition. It
is well worth consulting by those who take a long journey, and it will
be found useful to take a geological map with one on the journey so as
to discover when one is passing from one formation to another.]
CHAPTER XII.
THE PRECAMBRIAN ROCKS.
Study of a geological map of the world will shew that extensive
regions, such as parts of Scandinavia, many tracts of Central Europe,
a large area in Canada, and a considerable portion of Brazil and the
adjoining countries are occupied by crystalline schists, which
underlie the oldest known sedimentary strata in those places. These
crystalline schists form the floor upon which the sediments
constituting the bulk of the geological column rest, and it is
necessary that we should know something of the character of this
floor. Other rocks which can be definitely proved to be of Precambrian
age are often found associated with the crystalline schists, and these
associated rocks have often undergone more or less alteration
subsequently to their formation. The difference between the coarser
types of crystalline schists and these associated rocks is sometimes
so marked that geologists have necessarily paid attention to it, and
separated the two groups of rocks; the term Archæan has been used by
some geologists to include the crystalline schists, and Eparchæan for
the associated rocks of known Precambrian age, but though this
separation may sometimes be effected, there are cases when it is
impossible to draw any sharp line of demarcation between 'Archæan'
and 'Eparchæan' types.
In the present state of our knowledge, a chronological classification
of the Precambrian rocks when applied to wide and distant regions is
destined to break down, and it will be convenient if we consider at
some length the features of the Precambrian rocks of a particular
region, and apply the knowledge thus gained to a study of Precambrian
rocks of other areas, and to a consideration of our knowledge of the
Precambrian rocks as a whole. In doing so, the term 'crystalline
schists' will be used somewhat vaguely with reference to a complex of
schistose rocks of which the mode of origin cannot be fully
determined. We may take our own country as a region where a good
development of the Precambrian rocks occurs.
A few explanatory remarks concerning the mode of detection of
Precambrian rocks may not be amiss. If any true organisms have been
hitherto discovered amongst the rocks formed before Cambrian times
they are valueless as a means of correlating rocks, and accordingly
lithological characters only are available in attempting to correlate
the rocks of one area with those of another. Those who have read the
preceding chapters will have gathered that comparisons founded on
similarity of lithological character are not so valuable as those made
after careful scrutiny of the fossils of strata, but they are by no
means valueless, and when the rocks of two areas which are not far
distant from one another present close lithological resemblances,
their general contemporaneity may be inferred with some degree of
certainty.
It is only when we get the lowest Cambrian strata overlying earlier
rocks that we have absolute proof of the Precambrian age of the
latter, and it is necessary, therefore, that we should have some
definite lower limit to the rocks of the Cambrian system. It is now
generally agreed that that limit shall be drawn at the base of a group
of rocks containing what is known as the _Olenellus_-fauna, which will
be considered at greater length in the next chapter, and it will be
well, if the term Cambrian be not in future applied to any rocks
beneath the ones containing the relics of this fauna, for otherwise
there is danger of the indefinite downward extension of the Cambrian
system. We need not be surprised to find great thicknesses of rock
below the rocks containing the _Olenellus_-fauna, and passing upwards
with complete conformity into those rocks; nevertheless, if it can be
shewn that the _Olenellus_-fauna had not appeared during the
deposition of the underlying group, the rocks of that group should be
termed Precambrian. A case of this nature has not yet been detected in
our area, and all the rocks which have been proved to be Precambrian
in Britain are separated from the overlying Cambrian rocks by a
physical break, though that break is not necessarily very large, and
in some districts is probably of little importance. Hitherto the
_Olenellus_-fauna has been detected in Ross, Warwickshire, Shropshire,
Worcestershire and probably in Pembrokeshire, and the rocks underlying
the _Olenellus_-beds in those counties can be proved to be Precambrian
(i.e. if the _Olenellus_-age of the Pembrokeshire rocks be ultimately
established, and the researches of Dr Hicks tend to prove that it will
almost certainly be done). It will be convenient if we take the
instances where the age of the rocks can be proved with certainty or
with a considerable degree of probability first, and then consider the
examples of rocks which are found below Cambrian strata, though these
have not hitherto yielded the _Olenellus_-fauna, concluding with a
notice of rocks which have been claimed to be of Precambrian age on
account of their lithological characters, though they are not now seen
to be immediately succeeded by strata appertaining to the Cambrian
system.
Commencing with the region where we have the greatest development of
the known Precambrian rocks, namely Ross, Sutherland and the Hebrides,
we may explain the general relationship of the rocks by means of a
generalised section (fig. 15).
[Illustration: Fig. 15.]
The lowest rocks _a_ are crystalline schists, they are succeeded by a
set of arenaceous rocks _b_ known as the Torridonian beds, which rest
unconformably upon the upturned edges of the crystalline schists,
whilst the Cambrian rocks, _c_, rest with another unconformity
sometimes upon the partly denuded Torridonian beds, or where the
latter have been completely removed, as on the right side of the
figure, directly upon the crystalline schists, thus presenting an
example of unconformable overlap. The occurrence of the
_Olenellus_-fauna in the basement beds of the Cambrian system near
Loch Maree, proves the Precambrian age of the Torridonian strata,
whilst the unconformable junction between the latter and the
crystalline schists indicates that we are here dealing with two
distinct sets of Precambrian rocks, one of Eparchæan and the other of
Archæan type.
The crystalline schists consist of rocks of very varied lithological
characters, some with gneissose, and others with schistose structure,
and they vary in degree of acidity from ultrabasic rocks to those of
acid composition. Most of them exhibit parallel structures, which in
many cases can be shewn to have been impressed on the rocks
subsequently to their consolidation, though this need not have
occurred and probably did not occur with some of them, especially the
granitoid gneisses. The researches of the members of H. M. Geological
Survey have shewn that many of these rocks were originally intrusive
igneous rocks, though it is not yet known into what rocks those which
were first consolidated were injected, and the origin of the bulk of
the schists still remains to be elucidated. Subsequently to their
consolidation and before the deposition of the earliest Torridonian
rocks they were subjected to more than one set of earth-movements,
which folded them and impressed a series of parallel structures upon
many of them; and accordingly we find that the pebbles of the
crystalline schists which are found amongst the basal conglomerates of
the Torridonian rocks consist of fragments which had undergone the
alteration caused by these earth-movements before they were denuded
from their parent-rocks[48].
[Footnote 48: For an account of these rocks, their characters, and the
effects of earth movement upon them, the reader should consult a
"Report on the Recent Work of the Geological Survey in the North-West
Highlands of Scotland": _Quart. Journ. Geol. Soc._, vol. XLIV. p.
378.]
The Torridonian system is composed of rocks which are largely of
arenaceous character, the most prominent beds being formed of red
sandstones, and the bulk of the fragments in them have clearly been
derived by denudation from the crystalline schists, many of the beds
being composed of arkose, where the quartz is mixed with a large
proportion of felspar and often of ferro-magnesian minerals. The
deposits are clearly sedimentary, and are as little altered as many
strata of much more recent origin, only possessing structures produced
by metamorphic action under exceptional circumstances. The detailed
researches of the geological surveyors prove that the rocks of this
system have a much greater thickness and are of more varied
lithological characters than was previously supposed. The total
thickness of the strata is over 10,000 feet, and the sandstones are
associated with deposits of a muddy character, and with occasional
bands of limestone; in these circumstances the discovery of fossils
would excite no surprise, and in 1891 Sir A. Geikie announced the
detection of "traces of annelids and some more obscure remains of
other organisms in these strata," which have not yet been
described[49]. These Torridonian strata furnish us with the most
satisfactory group of Precambrian sediments yet detected in
Britain[50].
[Footnote 49: An account of the subdivisions and lithological
characters of the rocks of the Torridonian System will be found in the
_Annual Report of the Geological Survey of the United Kingdom_ for
1893.]
[Footnote 50: It has been recently maintained that some of the
Torridonian rocks are of Æolian origin.]
In the south-east Highlands is a great mass of crystalline schists of
a less gneissose character than that of the north-west, to which Sir
A. Geikie has applied the name Dalradian. Many of these schists will
be found by examination of the geological map of Scotland to be
separable into divisions, which by means of their lithological
characters can be traced long distances across the country, and they
present all the characters of sedimentary rocks, though they are
associated with intrusive igneous rocks, and have undergone great
metamorphic changes since their formation. Cambrian rocks have not yet
been discovered immediately above them, though they are clearly older
than Ordovician times, but the existence of rocks associated with them
along their north-west borders, which in lithological characters
closely resemble some of the rocks of the crystalline schists of the
north-west Highlands, indicates the probability of their general
Precambrian age. In some instances, the extreme types of metamorphism
which they exhibit are the result of the kind of action usually termed
pyrometamorphic as has been shewn by Mr G. Barrow[51].
[Footnote 51: Barrow, G. "On an Intrusion of Muscovite-biotite gneiss
in the S.E. Highlands of Scotland, and its accompanying metamorphism."
_Quart. Journ. Geol. Soc._, vol. XLIX. p. 330.]
In England and Wales the rocks which have been shewn or inferred to be
Precambrian, when not intrusive, are largely of volcanic origin. The
most satisfactory example of the occurrence of the _Olenellus_-fauna
is that of the Cambrian Comley sandstone of Shropshire, which rests
unconformably upon a set of rocks termed by Dr Callaway the Uriconian
rocks; the latter are essentially volcanic, and strongly resemble
Precambrian rocks of other British areas. There is also strong reason
to suppose that the sediments to which the name Longmyndian has been
applied, which have been described by the Rev. J. F. Blake, are of
Precambrian age, for, as Professor Lapworth has pointed out, the three
great subdivisions of the Cambrian system are present in the area
under consideration, and the rocks of each are entirely different from
those of the adjoining Longmynd area. In Shropshire therefore we meet
with one set of volcanic rocks, and another set consisting of
sedimentary rocks, of which the former is certainly, the latter
almost certainly of Precambrian age, and as the Longmyndian rocks are
in a comparatively unaltered condition, consisting of normal
sediments, we may well expect the discovery of fossils in them
also[52]. The _Olenellus_-fauna has been found near Nuneaton in
Warwickshire in beds which unconformably succeed volcanic rocks, the
Caldecote series of Prof. Lapworth, and the latter are therefore of
Precambrian age[53]. A few fossils belonging to the _Olenellus_-fauna
have occurred in the oldest Cambrian rocks of the Malvern district,
and these rocks rest unconformably upon those of an old ridge which is
therefore composed of Precambrian rocks. The rocks of this ridge are
largely of intrusive igneous origin, though parallel structures have
been impressed upon them as the result of subsequent deformation, but
some of the rocks are almost certainly of contemporaneous volcanic
origin[54]. In the Wrekin ridge, igneous and pyroclastic rocks are
found succeeded unconformably by Cambrian rocks which resemble those
of the Malvern and Nuneaton districts, and probably belong to the
period of existence of the _Olenellus_-fauna, and these igneous and
pyroclastic rocks are presumably of Precambrian age, and the
contemporaneous rocks constitute Dr Callaway's typical Uriconian
group. Volcanic ashes and breccias are accompanied by devitrified
pitchstones and intruded granitic rocks, which may or may not be all
of the same general age[55]. The rocks which have been claimed as
Precambrian in Pembrokeshire and in Caernarvonshire have the same
general characters as those of the Wrekin ridge. Pyroclastic rocks
underlie the oldest Cambrian rocks, with discordance between the two,
and associated with these pyroclastic rocks are quartz felsites which
according to some are of contemporaneous nature whilst others maintain
their intrusive origin. In each county granites are found which are
now generally recognised to be intrusive, though there seems to be no
doubt as to their being of the same general age as the rocks with
which they are associated, and therefore presumably Precambrian. The
Pembrokeshire rocks are marked by the occurrence of a certain amount
of metamorphism, probably of more than one kind, which has converted
pyroclastic volcanic rocks into sericitic-schists and quartz-felsites
into hälleflintas[56]. The term Pebidian given by Dr Hicks to the
contemporaneous volcanic fragmental rocks should be retained, and if
these rocks be eventually shewn to be contemporaneous with similar
volcanic rocks of other districts, may be applied generally, as it has
priority over other terms as Uriconian and Caldecote series. The term
Dimetian was applied to rocks known to be intrusive, and must be
dropped as a chronological term, whilst the existence of an Arvonian
system separate from the Pebidian system is not fully proved.
[Footnote 52: The reader may consult a paper by Prof. Lapworth "On
_Olenellus Callavei_ and its geological relationships," _Geol. Mag._
Dec III. vol. VIII. p. 529, for information concerning the
relationship of the _Olenellus_ beds of Shropshire to the more ancient
rocks; the Uriconian rocks are described by Dr Callaway in a series of
papers, especially in the _Quarterly Journal of the Geological
Society_, vol. XXXV. p. 643, vol. XXXVIII. p. 119, vol. XLII. p. 481
and vol. XLVII. p. 109, whilst the lithological characters of the
Longmyndian rocks are described by the Rev. J. F. Blake (_Quart.
Journ. Geol. Soc._, vol. XLVI. p. 386).]
[Footnote 53: See Lapworth, C., "On the sequence and systematic
position of the Cambrian rocks of Nuneaton," _Geol. Mag._ Dec III.
vol. III. p. 319; and Waller, T. H., "Preliminary Note on the Volcanic
and Associated Rocks of the neighbourhood of Nuneaton," _ibid._ p.
322.]
[Footnote 54: For details concerning the rocks of the Malvern Hills
see papers by Callaway in the _Quarterly Journal of the Geological
Society_, vol. XXXVI. p. 536, XLIII. p. 525, XLV. p. 475, and XLIX. p.
398, and a paper by Prof. A. H. Green, _ibid._ vol. LVI. p. 1.]
[Footnote 55: Callaway, C., _Quart. Journ. Geol. Soc._, vol. XXXV. p.
643.]
[Footnote 56: The Pembrokeshire area is of interest as the probable
existence of Precambrian rocks in Britain was first indicated on good
evidence in this county. The general structure of the district is
fairly simple, consisting of Cambrian rocks beneath which Precambrian
rocks are exposed in at least two ridges of which the northerly and
more important one runs through St Davids. The rocks of the St Davids
ridge consist of a binary granite (granitoidite), felsites, and
volcanic ashes and breccias of intermediate composition. Much
diversity of opinion has existed, and to some extent still exists as
to questions of detail, and a very extensive literature has been
devoted to these rocks. Amongst the numerous papers which treat of
them, the student may consult the following:--Hicks, H., _Quart.
Journ. Geol. Soc._, vol. XXXIII. p. 229, XXXIV. p. 147, XXXV. p. 285,
XL. p. 507, XLII. p. 351, Geikie, A., _ibid._ vol. XXXIV. p. 261,
Blake, J. F., _ibid._ vol. XL. p. 294, and Morgan, C. Ll., _ibid._
vol. XLVI. p. 241. Much of the matter contained in these papers is
controversial, and need not be fully read by those who merely wish to
obtain a general account of the rocks of the district.]
In Caernarvonshire two ridges are found, the one running from Bangor
to Caernarvon, and the other through Llanberis lake. The rocks of
these are generally similar to those of St Davids, and as the lowest
Cambrian rocks of the area closely resemble those of St Davids, the
Precambrian age of the rocks of these ridges is rendered highly
probable, though until the discovery of the _Olenellus_-fauna in the
area, it cannot be regarded as proved[57].
[Footnote 57: These rocks are described by T. M^{c}K. Hughes, _Quart.
Journ. Geol. Soc._, vol. XXXIV. p. 137, and XXXV. p. 682; by Prof. T.
G. Bonney, _ibid._ vol. XXXIV. p. 144; and by Dr Hicks, _ibid._ vol.
XXXV. p. 295.]
The actual position of the similar rocks of Anglesey has not been so
clearly fixed, as the rocks associated with them are of Ordovician
age, but their resemblance to the rocks of the adjoining regions
renders their Precambrian age highly probable. It is interesting to
find in association with the rocks which resemble those of
Caernarvonshire, others which Sir A. Geikie recognises as quite
similar to some existing amongst the crystalline schists of the
north-west Highlands of Scotland, and when these ancient rocks of
Anglesey have been mapped in detail, they will probably be found to
present greater variety than is afforded by any Precambrian rocks of
Great Britain occurring S. of the Scotch border[58].
[Footnote 58: Papers upon the old rocks of Anglesey will be found in
many volumes of the _Quarterly Journal of the Geological Society_; see
especially Hicks, vol. XXXV. p. 295, Callaway, vol. XXXVI. p. 536,
XXXVII. p. 210, and Blake, XLIV. p. 463.]
Of rocks whose age is more uncertain, but which are probably of
Precambrian age, those of Charnwood Forest in Leicestershire may first
be noticed. They are largely of pyroclastic origin, and from their
likeness to similar rocks of proved Precambrian age, they are very
probably of this age, as suggested by Messrs Hill and Bonney[59]. A
group of crystalline schists is found in the south of Cornwall,
especially near the Lizard, and similar rocks are found in the Channel
Isles. As their relationship to newer rocks is not clear, little can
be said about them, which has not already been noticed in mentioning
the crystalline schists of other regions[60].
[Footnote 59: Hill and Bonney, _Quart. Journ. Geol. Soc._, vol.
XXXIII. p. 754, XXXIV. p. 199 and XLVII. p. 78; see also Watts, W. W.,
_Rep. Brit. Assoc._ for 1896, p. 795.]
[Footnote 60: For an account of the Volcanic History of Britain in
Precambrian times, see Sir A. Geikie, Presidential Address to the
Geological Society, _Quart. Journ. Geol. Soc._, vol. XLVII. p. 63.]
The Precambrian rocks of the European continent consist largely of
crystalline schists which in their general aspects recall those of the
north-west Highlands of Scotland. Important masses are found in
Bavaria, Bohemia, France, Spain, Scandinavia and Russia. The
Scandinavian and Russian rocks of Archæan type are in places succeeded
by the _Olenellus_-bearing beds of the Cambrian rocks, and rocks of
Eparchæan character are not extensively developed, though certain
Norwegian rocks may be the equivalents of the Torridonian rocks of
Scotland, and other rocks of this type are found in places in Sweden.
In Bohemia and in Brittany Precambrian strata of Eparchæan type have
been discovered, and this type probably occurs elsewhere in Europe.
The North American rocks require some notice, for it was in Canada
that the existence of Precambrian rocks was first recognised, and the
term Laurentian, originally applied to an Archæan type of Precambrian
rocks in Canada, was subsequently adopted in speaking of many
Precambrian rocks elsewhere, though it is now wisely restricted to the
type of rock in the original area to which the name was first given.
These Laurentian rocks acquired a special, interest on account of the
occurrence in their limestones of a supposed reef-building
foraminifer, _Eozoon canadense_, but detailed study of its structure
and mode of occurrence has convinced most geologists that the
structure is inorganic.
The Laurentian rocks of the typical Laurentide region are largely
crystalline schists associated with massive crystalline rocks. The
attempt to separate them chronologically into a Lower and Upper
division was premature, as shewn by the fact that many of them, upon
detailed study, prove to be intrusive igneous rocks. In the
neighbourhood of Lake Huron, a set of sedimentary rocks overlying the
Archæan rocks is of Eparchæan type, consisting to a great extent of
volcanic rocks, clay-slates and schists with intrusive igneous rocks;
it has been termed the Huronian System, and this term has also been
extensively applied to other Eparchæan types found elsewhere, but
should be restricted to the rocks of the Huron district. A number of
other rocks of Eparchæan type have been discovered in various parts of
North America, and have been grouped together under the title of
Algonkian, a name proposed for them by Dr C. D. Walcott, and an
attempt has been made to arrange them in chronological order, though
in the absence of fossils, the rocks of different districts can only
be so arranged by reference to lithological characters; nevertheless a
detailed study of the Eparchæan and some of the more finely
crystalline schistose rocks points to the existence of a number of
divisions of sedimentary rocks of Precambrian age, some of which may
attain to the dignity of forming separate systems[61]. By far the most
instructive development of American Precambrian rocks has been found
in the Rainy Lake region of Canada, and it is the subject of a special
memoir by Dr A. C. Lawson[62]. The Archæan rocks of the region are
divided into a lower Laurentian and an upper division, which is
further subdivided into the Coutchiching series below and the Keewatin
series above, though the rocks of the Keewatin series are largely of
Eparchæan character. The Laurentian rocks of this region resemble
those of the Laurentide area, and consist of highly crystalline
schistose and gneissose rocks associated with compact rocks. The
Coutchiching series consists of mica schists and grey laminated
gneisses, which appear to have been of sedimentary origin, altered by
subsequent metamorphic action, while the Keewatin series, which
reposes sometimes upon the rocks of the Coutchiching series (when the
junction is an unconformable one), sometimes upon the Laurentian
rocks, is formed of pyroclastic rocks and lava flows with intercalated
sedimentary rocks; some of the Keewatin rocks are highly metamorphosed
but others have undergone little or no metamorphic change. The most
important point in connexion with these rocks of the Rainy Lake Region
has reference to the relationship between the Laurentian rocks and
those of the Coutchiching and Keewatin series. Lawson demonstrates the
igneous nature of the Laurentian rocks, and brings forward evidence of
various kinds that they were formed "by the fusion of the basement or
floor upon which the formations of the upper division of the Archæan
were originally deposited. With the fusion of this floor it seems
probable that portions of the superincumbent strata, which once formed
integral parts of either the Coutchiching series or the Keewatin, have
also been absorbed into the general magma, and reappeared on
crystallization as Laurentian gneiss. This fusion, however, only
extended up to a certain uneven surface, which surface constitutes the
demarcation between the present upper and lower Archæan. Above this
surface, or upper limit of fusion, the formation of the Coutchiching
and Keewatin series retained their stratiform or bedded disposition,
and rested as a crust of hard and brittle rocks upon the magma,
subject to its metamorphosing influences[63]."
[Footnote 61: A large number of classifications have been proposed for
the Archæan rocks of America; the most plausible one is given in Sir
A. Geikie's _Text Book of Geology_, Third Edition, p. 716.]
[Footnote 62: Lawson, A. C., _Report on the Geology of the Rainy Lake
Region_. Montreal, 1888.]
[Footnote 63: Lawson, _op. cit._ p. 139.]
We may now pass briefly in review the evidence which has been so far
obtained as to the mode of formation of the various Precambrian rocks.
The existence of a very varied fauna amongst the earliest Cambrian
strata has been commented upon by many geologists, and according to
accepted explanations of the origin of that fauna, an enormous period
of time elapsed before the deposition of the earliest Cambrian strata.
During portions of that long period, the undoubtedly clastic rocks of
Eparchæan type were deposited, and probably many others which are now
so altered by metamorphism, like some of the Coutchiching rocks of
Canada, that their original clastic origin can only be inferred and
not directly proved. Volcanic activity was very rife during the
deposition of some of these Eparchæan rocks, though perhaps not more
so than during the formation of some of the Lower Palæozoic Rocks. All
attempts to prove the occurrence of organisms in Precambrian strata
have hitherto failed, for no undoubted fossil has been described which
is unhesitatingly accepted as of Precambrian age, notwithstanding the
many asserted occurrences of such fossils. That fossils will
eventually be discovered is more than probable, and their
non-detection at the present time is in no way very surprising, when
we remember the long time that elapsed after the existence of
stratified rocks below the Upper Palæozoic rocks had been recognised,
before definite faunas were discovered in them. The determination of
the Precambrian age of stratified rocks is recent, and now that this
determination has been made, the search for fossils will be more
eager, and is likely to be rewarded by their discovery. Furthermore,
experience shows that when fossils are discovered in rocks of unknown
age, there is a tendency to refer those rocks to some known period,
and consequently we may actually possess Precambrian fossils, out of
beds which have been erroneously referred to the Cambrian or a later
period.
Another important question is that of the metamorphism of a large
number of Precambrian rocks, and here again recent research tends to
show that the metamorphism is not of a kind different from that which
occurred after the end of Precambrian times; the discovery of
crystalline schists in Norway, Kirkcudbrightshire and Westmorland
amongst Lower Palæozoic rocks, which resemble those of Archæan masses
in all respects except in the extent of area which they cover, shows
that similar processes to those which occurred in Precambrian times
went on during later periods, though perhaps not on so large a scale.
The great extent of these metamorphic rocks of Precambrian age can
hardly be due in any great degree to the longer time during which they
have been subjected to metamorphic influence, for there is evidence
that much of the change took place in Precambrian times, far more than
has occurred since, and it is a significant fact that these old rocks
are more extensively penetrated by intrusive igneous masses than those
of later periods; here again we find that much of the intrusion
actually occurred in Precambrian times. The greater extent of
intrusion and metamorphism amongst these Precambrian rocks than
amongst later sediments indicates some differences of conditions in
the case of Precambrian and later times. If besides intrusion, actual
fusion of floors of Precambrian rocks occurred, we may well suppose
that the earlier records of the rocks are for ever lost to us, the
earliest sediments having been fused, but that the history of life
upon our earth is to be revealed to us first in so late a stage as
that of Cambrian times is highly improbable, and we may look forward
with confidence to laying bare the records of the rocks composing the
geological column some way below the Cambrian portion of the column.
Upon this foundation of igneous rock, sediment and volcanic material,
formed in Precambrian times, whose history we have only begun to
study, was laid down the great mass of sediment which the geologist
has more completely studied, where abundant traces of life are
preserved, and concerning whose history we can gain a greater insight
than is permitted us in the case of the old Foundation Stones.
CHAPTER XIII.
CYCLES OF CHANGE IN THE BRITISH AREA.
Before studying in further detail the strata of the geological column,
it will be convenient to deal with the great physical changes which
have occurred in the British area from Precambrian times to the
present day, as this will clear the way for a right appreciation of
the main variations in the characters and distribution of the strata.
At the end of Precambrian times there was a general upheaval of the
British area, and this we may speak of as the First Continental
Period. It was followed by depression and extensive sedimentation,
proceeding more or less continuously though with local interruptions
through Lower Palæozoic times, so that so far as Britain is concerned
we may speak of Lower Palæozoic times as constituting the First Marine
Period. Extensive upheaval gave rise to continental tracts and
mountain chains, and deposits of abnormal character (as compared with
ordinary marine deposits) at the end of Lower Palæozoic times;--the
Devonian period was one of elevation and denudation, and we may
therefore refer to it as the Second Continental Period. This was
followed by depression and sedimentation in Carboniferous times, and
these Carboniferous times constitute the Second Marine Period.
Elevation gave rise to continental tracts and mountain chains at the
end of Carboniferous times, and here again we find proofs of extensive
denudation and the formation of abnormal deposits:--the Permo-Triassic
period is the Third Continental Period. Depression set in during early
Jurassic times and continued throughout the Mesozoic and the early
part of Tertiary times, which form the Third Marine Period.
Disturbances culminating in Miocene times once more produced
terrestrial conditions. In this, the Fourth Continental Period, we are
still living.
From what has been previously written it will be seen that each of the
marine periods should be marked by an early and late shallow-water
phase, separated by an intervening marine phase, and the importance of
the phases will depend upon the length of time during which they
existed, and will differ markedly in different cases, whilst the
distinctness of the middle phase from the upper and lower, will depend
upon the magnitude of the maximum submergence.
During the first marine period submergence was comparatively rapid,
and the shallow-water phase only lasted through very early Cambrian
times in most regions, whilst the deep-water phase, complicated by
many minor upheavals, extended through the main part of Cambrian,
Ordovician and Silurian times, and was replaced by the later
shallow-water phase at the end of Silurian times.
The second marine period again was ushered in by rapid submergence, so
that the shallow-water phase was brief, and the main mass of the Lower
Carboniferous strata was deposited in deep water; but, unlike the
first marine period, the second was characterised by the occurrence of
a long interval of time marking the later shallow-water phase, during
which the whole of the Upper Carboniferous strata were deposited. The
Carboniferous Marine Period is the simplest of the three with which we
have to deal, as the local oscillations occurring on a fairly large
scale for such movements were less frequent than was the case during
the first and third marine periods.
The third marine period had a long shallow-water phase at the
commencement, with many minor oscillations, causing great variation in
the character of the deposits and frequent minor unconformities. This
shallow-water phase existed throughout Jurassic and Lower Cretaceous
times. The deep-water phase existed during the deposition of the Upper
Cretaceous deposits, and was succeeded by the second shallow-water
phase, when the early Tertiary strata were accumulated.
The difference between the elevations which accompanied the
Continental Periods and those which have been alluded to as minor
elevations is no doubt one of degree, but in considering the British
strata only no confusion is likely to arise on this account, as the
difference was here very great.
The events which occurred during the continental periods are of
extreme importance to the geologist. Every great upheaval was
accompanied by crumpling and stiffening of portions of the earth's
crust, and a definite trend was given to the strata as the result of
these movements. It is to the earth-movements of the four great
continental periods that the present structure of the British Isles is
largely due, and in any attempt to restore the physical history of our
islands considerable attention must be paid to the changes which were
produced in the stratified rocks during these periods of
earth-movement.
CHAPTER XIV.
THE CAMBRIAN SYSTEM.
_Classification._ The rocks of the Cambrian system when found reposing
on Precambrian rocks in Britain are always separated from the latter
by an unconformity. The typical development of the rocks of the
system, as the name implies, is in the hilly region of Caernarvonshire
and Merionethshire in North Wales, and they are also well represented
in South Wales, the border counties between England and Wales, and the
North-West Highlands of Scotland. Two distinct classifications of the
Cambrian rocks of Britain are in use, the original one founded on
variations of lithological character, whilst the second depends upon
faunistic differences, but the original lithological classification
has been to some extent modified to make it locally correspond with
the classification based upon palæontological grounds. The following
table will shew the differences:--
Lithological Classification. Palæontological Classification.
Tremadoc Slate Series[64] Beds with Intermediate Fauna
Lingula Flags Series Beds with _Olenus_ Fauna
Menevian beds (formerly included }
in Lingula Flags) } Beds with _Paradoxides_ Fauna
} Formerly grouped }
Solva beds } together as Harlech
Caerfai beds } or Llanberis beds Beds with _Olenellus_ Fauna
[Footnote 64: In accordance with the custom usually observed in
Britain, the Tremadoc slates are placed in the Cambrian system; most
continental geologists place them in the succeeding Ordovician system.
The matter is not an important one, as the fauna is an intermediate
one between that of the Lingula Flags and that of the Arenig series of
the Ordovician system, and the beds are true beds of passage. As the
lithological classification is essentially British, it will be as well
to retain the Tremadoc Slates in the Cambrian system.]
The original lithological classification was essentially the result of
Prof. Sedgwick's work in North Wales, while the classification
according to faunas is the outcome of the researches of Dr Hicks in
South Wales.
_Description of the Strata._ The Cambrian rocks of North Wales occur
in two complex anticlines, separated by an intermediate syncline of
Ordovician strata occupying the Snowdonian hills. The southerly or
Harlech anticline forms a part of Merionethshire to the east of
Harlech, whilst the northern one is developed around Bangor and
Llanberis. The South Welsh Cambrian rocks are chiefly found on either
side of the Pembrokeshire axis of Precambrian rocks which runs through
St David's. As the corresponding rocks of the two regions were
deposited in bathymetrical zones of much the same depth, it will be
convenient to give a general account of the rocks of the two regions
at the same time, leaving the student to acquire information of the
detailed variations in the larger text-books and in special
memoirs[65].
[Footnote 65: A general account of the Cambrian, Ordovician and
Silurian rocks will be found in the Sedgwick Essay for 1883, _A
Classification of the Cambrian and Silurian Rocks_, though the use of
a cumbrous nomenclature therein will tend to confuse the reader. For a
detailed account of the Cambrian rocks of North Wales the reader is
referred to the Geological Survey Memoir, _The Geology of North
Wales_, by Sir A. Ramsay (2nd edition), he may also consult Belt, T.,
"On the Lingula Flags or Festiniog Group of the Dolgelly district,"
_Geol. Mag._, Dec I. vol. IV. pp. 493, 536, vol. V. p. 5. The geology
of the Cambrian rocks is described in a series of Memoirs in the
_Quarterly Journal of the Geological Society_ by Dr H. Hicks; the
following should be consulted: Harkness, R. and Hicks, H., "On the
Ancient Rocks of the St David's Promontory, South Wales, and their
Fossil Contents," vol. XXVII. p. 384; Hicks, H., "On some Undescribed
Fossils from the Menevian Group," vol. XXVIII. p. 173; and "On the
Tremadoc Rocks in the neighbourhood of St David's, South Wales, and
their Fossil Contents," vol. XXIX. p. 39. See also Hicks, "The
Classification of the Eozoic and Lower Palæozoic Rocks of the British
Isles," _Popular Science Review_, New Series, vol. V., and Hicks,
"Life-zones in the Lower Palæozoic Rocks," _Geol. Mag._ Dec IV. vol.
I. pp. 368, 399 and 441.]
The strata of the Caerfai and Solva groups show the prevalence of the
shallow-water phase almost uninterruptedly through the whole of the
time occupied by their accumulation in the Welsh areas. They consist
chiefly of basal conglomerates, succeeded by alternations of grits and
shales, though the latter are often converted into slates, owing to
the subsequent production of cleavage. The basal conglomerates of the
Caerfai beds are frequently marked by the existence of enormous
pebbles, composed of fragments of the rocks of the underlying
Precambrian groups, and the possibility of the occurrence of glacial
action during their accumulation as advocated by Dr Hicks must be
taken into account. Above these beds are various coloured grits, with
alternations of muddy sediments often coloured red[66]. The Solva
group consists of massive grits, of various colours, also with
alternations of mud, which have prevalent purple and green hues. The
great thickness of the strata of the Caerfai and Solva Series, which
sometimes exceeds 10,000 feet, must also be noted.
[Footnote 66: In giving this description the red (Glyn) slates of
North Wales are treated as belonging to the Caerfai series, though
this correlation depends on lithological characters only at present.]
The Menevian beds consist essentially of very fine, well laminated
black and grey muds, which are of a texture favourable for the
production of a somewhat regular jointing, causing the rock to break
into small rectangular blocks. They are thin, not exceeding 600 feet
in thickness, and indicate the incoming of the general deep-water
phase of the Lower Palæozoic epoch. The Lingula Flags mark a local
return to shallower water conditions, especially in the central
portion. The total thickness is over 3,000 feet, of which the lower
stage (locally the Maentwrog series) is over 500 feet, and consists of
blackish muds, the middle (Festiniog stage[67]) is about 2,000 feet
thick, and is composed chiefly of shallower water gritty flags, whilst
the upper (Dolgelly) stage is of about the same thickness as the lower
stage and has similar lithological characters.
[Footnote 67: The term Festiniog has been used for the whole Lingula
Flag series as well as for the middle stage. It will be well to use it
with reference to the stage only.]
The Tremadoc Slates are about 1,000 feet thick. They are divided into
a lower and upper stage, of about equal thickness, and are essentially
composed of iron-stained slates, with a considerable admixture of
calcareous matter in some parts of South Wales, when they furnish the
nearest approach to a limestone which has been found amongst the Welsh
Cambrian strata. They were probably formed in a fairly deep sea.
Much pyroclastic rock and some lava flows are intercalated amongst the
Welsh Cambrian sediments. Tuffs are formed in the lower beds of St
David's, and lavas and ashes have been found amongst the Lingula Flags
and Tremadoc Slates of North Wales, while the Lingula Flags of South
Wales have furnished several bands of ash to the north of
Haverfordwest. Much of the material of the grits and muds may be
derived from volcanic rocks, though how far this is so cannot be
stated in the absence of information obtained by detailed petrological
examination of the rocks.
The various isolated outcrops of Cambrian strata amongst the counties
of the Welsh borders and adjoining Midland counties indicate a great
thinning of the Cambrian rocks in this direction.
The probable equivalents of the Caerfai rocks occur at Nuneaton,
Comley, and on the flanks of the Wrekin and Malvern hills. The thin
basal conglomerates are succeeded by quartzites, and sometimes red
calcareous sandstones (Comley sandstone). These rocks are succeeded by
thin arenaceous and calcareous beds which represent either the Solva
or Menevian beds of Wales. The Lingula Flags are represented by the
Malvern Shales of the Malvern area and the Stockingford Shales of
Nuneaton, whilst the Tremadoc Slates have as their equivalents the
Shineton Shales. The exact thicknesses of these deposits do not seem
to have been recorded, but Prof. Lapworth observes that in central
Shropshire "the Comley and Shineton groups which ... have a collective
thickness of perhaps less than 3,000 feet, we have apparently a
condensed epitome of the entire Cambrian system as at present
generally defined."
The Cambrian rocks of the North-west Highlands consist of a thin
conglomerate succeeded by grits and flags with shaley beds, and above
these a mass of limestone, which may represent some of the Ordovician
deposits as well as those of Cambrian age. Pending a complete
description of the faunas of these rocks, it is sufficient to state
that the only fauna which has hitherto been described in detail
indicates the existence of Lowest Cambrian rocks. Further remarks will
be made on this head when describing the character of the Cambrian
faunas. The Cambrian rocks of the North-west Highlands are also very
thin as compared with those of Wales, so that the Highland and Welsh
borderland regions appear to have existed as a deeper sea area than
that which is indicated by the Cambrian rocks of Wales, an inference
which is to some extent borne out by study of the Cambrian rocks of
extra-British areas, to which we may now turn.
The principal European developments of Cambrian rock are found in
Scandinavia, Russia, Bohemia and Spain, and of these the Scandinavian
one is by far the most fully developed, as there is a complete
sequence in the rocks of that peninsula. They occur both in Norway and
Sweden, but the Swedish exposures are the most interesting in most
respects, especially those of Westrogothia and Scania. The rocks are
of no great thickness, and consist essentially of black carbonaceous
shales, with inconstant bands of impure black limestone composed
almost entirely of the remains of trilobites or more rarely of
brachiopods. These Alum Shales, as they are termed, rest unconformably
upon Precambrian rocks, and have arenaceous and conglomeratic deposits
at the base. In Russia the rocks are still further attenuated, and
have not yielded the relics of so many faunas as have been found in
the Scandinavian Cambrian rocks.
The Bohemian development is incomplete, owing apparently to an
unconformity at the base of the overlying Ordovician rocks, while the
Spanish deposits which seem fairly thick and composed largely of
mechanical sediments have not been worked out in very great detail.
The American development of Cambrian rocks resembles the European one
in many striking particulars, and as in the case of Europe, there are
lateral variations in the lithological characters of the rocks, though
in the opposite direction, the shallow-water deposits occurring on the
east coast, and the deep-water deposits further west.
The general distribution of the different types of Cambrian strata in
Europe and North America has been accounted for on the supposition
that in Cambrian times a tract of land lay over much of the present
site of the North Atlantic Ocean, and that the detritus of that land
formed the shallow-water accumulations of Wales and the east of
Canada, whilst further away from it were deposited the open-sea
accumulations of Scandinavia and Russia on one side and of the more
westerly regions of North America on the other, as indicated in Fig.
16.
[Illustration: Fig. 16.
P. Precambrian Rocks.
A. Land.
X, X´. Sea level.
BB´. Shore deposits.
CC´. Deep-water deposits.
DD´. Abyssal deposits.
]
_The Cambrian Faunas._ The Cambrian Period has been termed the age of
trilobites, for they are the dominant forms of the time, but they are
associated with many other forms of invertebrata; indeed all the great
groups of this division are represented in the earliest Cambrian
fauna. Dr C. D. Walcott records representatives of Spongiae, Hydrozoa,
Echinodermata, Annelida, Brachiopoda, Lamellibranchiata, Gastropoda,
Pteropoda, Crustacea and Trilobita as occurring in the _Olenellus_
beds of North America and other groups are represented in the rocks of
this age in the Old World. The Cambrian trilobites as a whole are of
more generalised types than those of the later systems which furnish
their remains, as indicated especially by the looseness of the body,
and the large number of body rings in many of the genera, while the
tail or pygidium was small and formed of only a few coalesced
segments, as pointed out by Barrande. In the later trilobites the test
is more compact, there are on the whole fewer body rings, as more of
these have become fused into a tail which is therefore larger than
that of the average tail of the Cambrian trilobite.
Taking the faunas in order, the oldest or _Olenellus_ fauna has
furnished a great variety of forms in the North-west Highlands of
Scotland, Shropshire, Scandinavia, Esthonia, Sardinia, Canada, and
Newfoundland, whilst representative species of the fauna have been
recorded also from Worcestershire, Warwickshire, Pembrokeshire, India,
China, and Australia.
The dominant form is the trilobite of the genus or group _Olenellus_,
which contains a great variety of species referable to three or four
divisions which have been ranked as separate genera by some writers.
Associated with _Olenellus_ are trilobites belonging to other genera,
which are found in higher deposits, though there represented by
different species.
Brachiopods are fairly abundant, especially those provided with a
horny shell; of these, the genus _Kutorgina_ is widely distributed.
The zoological relationships of several of the fossils of this horizon
are as yet doubtful. The Archæocyathinæ show affinities with certain
corals; a number of tests, included in the genus _Hyolithes_ and its
allies are doubtfully referred to the Pteropods, and the position of
the genus _Volborthella_ is uncertain. Special attention is directed
to these doubtful relationships, as it is possible that a number of
'generalised forms' of organisms occur in these strata[68].
[Footnote 68: For an account of the _Olenellus_ fauna see Walcott, C.
D., "The Fauna of the Lower Cambrian or Olenellus Zone," _Tenth Annual
Report of the Director of the United States Geological Survey_,
Washington, 1890. It is possible that some of the fossils mentioned in
that report belong to strata above that containing _Olenellus_.]
It should be noticed here that faunas have been discovered which are
possibly of earlier date than the _Olenellus_ fauna, as they do not
correspond with it, or with those of newer strata. One, the _Neobolus_
fauna of the Salt Range of India, occurs in beds below those with
_Olenellus_, though it is not yet clear that _Olenellus_ will not be
eventually discovered associated with it, whilst the other, the
_Protolenus_ fauna of Canada, is of unknown age[69].
[Footnote 69: For an account of the _Neobolus_ beds see Noetling, F.,
"On the Cambrian Formation of the Eastern Salt Range," _Records Geol.
Survey, India_, vol. XXVII. p. 71, and for the Protolenus fauna
consult a paper by Matthew, G. F., "The _Protolenus_ Fauna," _Trans.
New York Acad. of Science_, 1895, vol. XIV. p. 101.]
The _Olenellus_ beds are succeeded by beds containing the
_Paradoxides_ fauna, which have been found in North and South Wales,
Shropshire, Scandinavia, Bohemia, Spain, and North and South America.
_Olenellus_ and its allies became extinct (or else so scarce that no
relics of them have been discovered in the _Paradoxides_ beds) before
the commencement of the deposition of the strata containing the
_Paradoxides_ fauna, and few genera pass from the beds with the one
fauna to that containing the other. The _Paradoxides_ fauna existed
for a considerable period, and the beds have been divided into a
series of zones characterised by different species of _Paradoxides_,
thus
Dr Hicks records the following zones in Pembrokeshire[70]:--
Zone of _Paradoxides_ _Davidis_ } Menevian.
" " _Hicksii_ }
" " _Aurora_ }
" " _Solvensis_ } Solva.
" " _Harknessi_ }
[Footnote 70: The order here as elsewhere is _ascending_, i.e. the
newest deposit is placed at the top.]
Dr Tullberg divides the _Paradoxides_ beds of Scania into thirteen
zones, though only a few of these are characterised by definite
species of _Paradoxides_. The _Olenellus_ beds have not yet been
divided into zones, though this will probably be the outcome of
further study[71].
[Footnote 71: The _Paradoxides_ fauna is described in the following
works: Britain, Hicks, H. and Salter J. W., _Quart. Journ. Geol.
Soc._, vol. XXIV. p. 510, XXV. p. 51, XXVII. p. 173, and Hicks, H. and
Harkness, R., _ibid._ vol. XXVII. p. 384; Scandinavia, Angelin, N. P.,
_Palæontologia Scandinavica_; Brögger, W. C., _Nyt Magazin for
Naturvidenskaberne_, vol. XXIV., Linnarsson, G., _Sveriges Geologiska
Undersökning_, Ser. C. No. 35; Bohemia, Barrande, J., _Système
Silurien du centre de la Bohême_; Spain, Prado, C. de, "Sur
l'existence de la faune Primordiale dans la chaîne Cantabrique suivie
de la description des Fossiles par MM. de Verneuil et Barrande,"
_Bull. Soc. Geol. France_, 2 Series, vol. XVII. p. 516; America,
Walcott, C. D., _Bull. U. S. Geol. Survey_: "The Cambrian Faunas of
North America," and Matthew, G. F., _Trans. Roy. Soc. Canada_, 1882
and succeeding years.]
The strata with _Paradoxides_ are succeeded by those with the _Olenus_
fauna, characterised by the genus _Olenus_ and a large number of
allied genera or sub-genera as some prefer to term them. The genus
_Olenus_ (_sensu stricto_) is very abundant in the lower part of the
series, whilst the allied forms are more abundant in the upper beds.
The genus _Paradoxides_ and its associates disappeared before the
deposition of these strata containing _Olenus_ and its allies, and
indeed the complete change in the character of the faunas in Europe is
very remarkable. The _Olenus_ fauna has been found in North Wales,
Pembrokeshire, Warwickshire, Worcestershire, and abroad in Scandinavia
and Canada. It is interesting to note among the fossils of the
_Olenus_ beds the occurrence of a graptolite which is associated with
_Olenus_ in Scandinavia; this is the earliest recorded appearance of a
group which is destined to play so important a role amongst the
fossils of the succeeding system[72]. The following zones have been
detected by Dr S. A. Tullberg amongst the _Olenus_ beds of Scania:--
Zone of _Acerocare ecorne_.
" _Dictyograptus flabelliformis_.
" _Cyclognathus micropygus_.
" _Peltura scarabæoides_.
" _Eurycare camuricorne_.
" _Parabolina spinulosa_.
" _Ceratopyge_ sp.
" _Olenus_ (proper).
" _Leperditia_.
" _Agnostus pisiformis_.
[Footnote 72: For descriptions of the _Olenus_ fauna consult the
following:--Wales, Belt, T., _Geol. Mag._ Dec. I. vol. V. p. 5, and
Salter, J. W., _Decades Geol. Survey_, Decade II. Pl. IX. and Decade
XI. Pl. VIII.; Scandinavia, Angelin, N. P., _Palæontologia
Scandinavica_, and Brögger, W. C., _Die Silurischen Etagen 2 und 3 im
Kristianiagebiet und auf Eker_; Canada, Matthew, G. F., "Illustrations
of the Fauna of the St John Group, No. VI.," _Trans. Roy. Soc.
Canada_, 1891.]
The beds with _Dictyograptus flabelliformis_ form a wonderfully
constant horizon at or near the top of the _Olenus_ beds. They are
found in North Wales, the Border Counties between Wales and England,
France, Scandinavia, Russia and Canada.
The passage fauna of the beds which are the equivalents of the
Tremadoc Slates may be spoken of as the _Ceratopyge_ fauna, for
_Ceratopyge forficula_, a remarkable species of trilobite,
characterises it in Scandinavia, and will probably be found
elsewhere. _Ceratopyge_ beds have been found in North and South Wales,
Shropshire, Scandinavia, Bavaria and North America, and in each case
the fauna is intermediate in character between that of the Cambrian
and that of the Ordovician system, containing the loosely-formed
trilobites of the former with the more compact ones of the latter. The
genus _Bryograptus_, a many-branched graptolite, also appears to
characterise this fauna[73].
[Footnote 73: For accounts of the Tremadoc Slates Fauna in England and
Wales see Ramsay, A. C., _Geology of North Wales_, Appendix; Hicks,
H., _Quart. Journ. Geol. Soc._, vol. XXIX. p. 39; Callaway, C.,
_ibid._ vol. XXXIII. p. 652, whilst many of the foreign fossils are
noticed in Brögger's _Die Silurischen Etagen 2 und 3_ and Barrande's
_Faune silurienne des Environs de Hof en Bavière_.]
The faunas of the Cambrian rocks have not been studied in sufficient
detail, with reference to the physical surroundings of the organisms,
to throw much light upon the conditions under which the strata were
deposited, though the evidence obtained from an examination of the
lithological characters of the deposits is generally corroborated by
study of the organic contents.
CHAPTER XV.
THE ORDOVICIAN SYSTEM.
_Classification._ The Ordovician strata were originally divided into
series by Sedgwick as follows:--
Upper Bala,
Middle Bala,
Lower Bala,
Arenig.
The Arenig series was at one time included by some writers with the
Lower Bala under the name Llandeilo, but the word Llandeilo is now
used in the sense of Sedgwick's Lower Bala. The Middle Bala is often
spoken of as Caradoc, but the terms Bala and Caradoc are sometimes
used interchangeably. As much confusion attaches to the use of the
name Bala without explanation, the alternative titles have been
largely adopted, and as the series are well defined there is no
objection to their use, save that some expression is wanted equivalent
to Upper Bala. The local term Ashgill shales was originally applied by
Mr W. Talbot Aveline to beds of this age in Lakeland, and I have
elsewhere suggested the use of this name for the whole series in that
region; its use may well be extended to the series which is developed
in many parts of Britain and the continent. The terms which will be
used here, therefore, for the different series of the Ordovician
system are the following:--
Ashgill Series (= Upper Bala)
Caradoc " (= Middle " )
Llandeilo " (= Lower " )
Arenig "
Adopting a palæontological classification, we may speak of the Arenig
and Llandeilo beds as those containing the _Asaphus_ fauna, whilst the
Caradoc and Ashgill beds possess the _Trinucleus_ fauna; this is the
terminology employed by Angelin for the equivalent strata of Sweden.
It must be noted that here the names applied are not those of
absolutely characteristic genera, as was the case with those adopted
for naming the Cambrian faunas, for both _Asaphus_ and _Trinucleus_
range through the beds of the system; but whereas _Asaphus_ is most
abundant in the beds of the two lower series, _Trinucleus_ occurs most
frequently in those of the two upper series.
_Description of the strata._ The Ordovician rocks are found over large
tracts in North and South Wales, in the counties on the Welsh border,
in Lakeland and the outlying districts in the Southern Uplands of
Scotland, and in detached areas in Ireland. There are three main types
of deposit:--(i) the volcanic type, in which the ordinary sediments
are associated with a large amount of contemporaneous volcanic matter,
(ii) the black shale type, with a fauna consisting largely of
graptolites, and (iii) the ordinary sedimentary type, in which we find
alternations of grits, shales, and more or less impure limestones. We
also find developments which are intermediate between any two or even
all three of these types. The first type is characteristically
developed in Caernarvonshire and Merionethshire, the second in the
Dumfriesshire Uplands, and the third in the Girvan district of
Ayrshire. The variation in the thickness of these three types of
deposit is shown in the accompanying sections of the Caernarvon,
Merioneth, Moffat and Girvan regions (see Fig. 17).
[Illustration: Fig. 17.
Showing the variations in the characters of the Ordovician deposits of
the three principal types.
Scale 1 in. = 1000 feet.
A = Arenig. L = Llandeilo. C = Caradoc.
The thickness of the Arenig rocks of the Scotch areas is unknown.]
The North Welsh area gives two different developments of the
Ordovician strata, one of which is much less volcanic than the other.
In the Merioneth-Caernarvon area, two great masses of volcanic rock
form the Aran and Arenig hills of Merioneth and the Snowdonian group
of Caernarvon. The former are of Arenig, the latter of Caradoc age.
The Merionethshire volcanic rocks consist of a great thickness of
lavas and ashes of intermediate composition (anderites), associated
with sandy and muddy sediments of no great vertical depth. The
Llandeilo beds of this area are chiefly of the nature of black shales,
while the Caradoc series is represented by volcanic lavas and ashes of
acid composition (felsites) with a few thin interbedded sediments. A
calcareous ash forming the summit of Snowdon is of importance as being
on the same horizon as a limestone (the Bala limestone) found in the
other North Welsh area. The Ashgill series is not represented in
Snowdonia.
In the other North Welsh tract, around Bala Lake, the volcanic matter
is much less conspicuous. The Arenig rocks are not seen nearer than
the Arenig mountains which form the western boundary of this second
tract. The Llandeilo beds consist of shaley deposits with a
well-marked limestone, the Llandeilo limestone, in the centre, whilst
the Caradoc beds consist chiefly of muddy sediments with some thin
ashes and a limestone, the Bala limestone, at the top. The Ashgill
series contains a basal limestone, the Rhiwlas limestone, succeeded by
shales, and another thin limestone called the Hirnant limestone at the
summit.
In South Wales the Arenig beds[74] are chiefly composed of slates, and
are divisible into an upper and lower group. The total thickness is
about 2000 feet. The Llandeilo beds contain three series:--
Upper Llandeilo Slates 1000
Llandeilo Limestone 200
Lower Llandeilo Slates 800.
[Footnote 74: A remarkable fauna, fairly well represented in Britain
and exceedingly well developed on the continent, exists in the
Uppermost Arenig and Lower Llandeilo beds, and it is well separated
from the dominant Arenig fauna below and Llandeilo fauna above. To the
beds which contain it Dr Hicks has given the name Llanvirn series.]
The Caradoc beds consist of black graptolitic shales of no great
thickness, succeeded by an impure limestone on the horizon of the Bala
limestone, while the Ashgill series like that of North Wales is
separated into upper and lower limestone stages with an intervening
stage composed of shales.
The deposits of the Welsh borderland are well developed in Shropshire,
where there is practically a repetition of the Caernarvon-Merioneth
development, with variations in detail. The Arenig and Caradoc
volcanic rocks are not so thick as those of the Welsh district, but
are nevertheless of considerable importance[75].
[Footnote 75: For information concerning these beds see Lapworth, C.
and Watts, W. W., "The Geology of South Shropshire," _Proc. Geol.
Assoc._, vol. XIII. p. 297.]
In the hilly region of Cumberland, Westmorland, and the adjoining
parts of Yorkshire the succession differs from that of any of the
Welsh regions, for the great period of volcanicity was during the
formation of the Llandeilo rocks, and there were merely sporadic
outbursts in Arenig and Caradoc times. The Arenig rocks consist of
black shales with interstratified beds of coarser sediment, and some
thin lavas and ashes of intermediate type. The Llandeilo series is
represented by a very great thickness of volcanic rocks, varying in
composition from basic to acid lavas, with associated pyroclastic
rocks. The rocks of the Caradoc period largely consist of impure
limestone with associated argillaceous rocks, and contemporaneous
volcanic rocks of acid character. A marked unconformity is found
locally in the centre of these. The Ashgill series consists of a basal
limestone with shales above, and there is evidence that volcanic
activity had not become extinct during the deposition of the rocks of
this series.
Passing on to Scotland, the graptolitic type is admirably shown in the
southern Uplands of the neighbourhood of Moffat, Dumfriesshire. The
base of the Ordovician system has not been found, but the lowest
series seems to be represented by shales with a graptolite possibly of
Arenig age. Above this are volcanic beds succeeded by a group of black
shales known as the Moffat shales. They are only about six hundred
feet in thickness, and yet represent much of the Ordovician and part
of the Silurian strata as developed elsewhere. The beds belonging to
the Ordovician system are divided into two series, the Glenkiln shales
below and the Hartfell shales above. The former consist of intensely
black muds with few fossils save graptolites, and a deposit of chert
at the base which is composed of radiolaria. The graptolites of the
black shales are Upper Llandeilo forms, but the thin deposit of
radiolarian chert may represent the rest of the Llandeilo period and
part of the Arenig period also. The Hartfell shales are also usually
black graptolite shales with lighter deposits nearly barren of organic
remains; they represent the Caradoc and Ashgill series and pass
conformably into the deposits of Silurian age[76]. The ordinary
sedimentary type of Ordovician rocks is found in Ayrshire, though a
few thin graptolitic seams are intercalated with the conglomerates
and shelly sands, clays and limestones of the region, which is
therefore peculiarly valuable as affording a means of comparison of
the shelly type with the graptolitic type of Ordovician deposits. The
Arenig series consists of black shales with graptolites, and these
rocks are succeeded by a volcanic group which is probably of Llandeilo
age. Above these volcanic beds, as in Dumfriesshire, we find three
great divisions, two of which are of Ordovician, the third of Silurian
age. The Ordovician divisions are respectively termed the Barr series,
which is the equivalent of the Glenkiln shales, and the Ardmillan
series above, equivalent to the Hartfell shales[77].
[Footnote 76: The Moffat beds are described in a paper by Prof.
Lapworth entitled "The Moffat Series" in the _Quarterly Journal of the
Geological Society_, vol. XXXIV. p. 239. This paper, which is a
masterpiece of detailed work, has furnished a clue to many problems.
Few students will be able to follow the numerous details, and for
general information concerning the beds they are recommended to read
another paper by the same author "On the Ballantrae Rocks of South
Scotland," _Geol. Mag._ Dec. III. vol. VI. p. 20. An account of the
radiolarian cherts by Dr G. J. Hinde will be found in the _Annals and
Magazine of Natural History_ for July, 1890, p. 40.]
[Footnote 77: See Lapworth, C., "The Girvan Succession," _Quart.
Journ. Geol. Soc._, vol. XXXVIII. p. 537, and also the paper on the
Ballantrae Rocks referred to in the preceding footnote. The latter
paper should be carefully read by all students of the stratigraphy of
the Lower Palæozoic Rocks.]
It is interesting to find that in the north of Ireland the rocks
generally coincide in characters with those which are found along the
same line of strike in Great Britain; thus, the Girvan type appears in
Londonderry, Tyrone and Fermanagh, the Moffat type in County Down, and
the Lake District type in the counties of Dublin and Kildare.
On the continent the volcanic material which plays so important a part
in the constitution of the Ordovician accumulations of Britain is
practically absent, and the strata are largely composed of
accumulations of shale and limestone with occasional coarser deposits.
In Scandinavia, the Arenig beds consist of limestones with a few
shales, the Llandeilo deposits are largely calcareous, those of
Caradoc age are partly calcareous and towards the top usually
argillaceous, while the equivalents of the British Ashgill series are
calcareous at the base and argillaceous at the summit. In Russia the
calcareous matter preponderates over the argillaceous material.
Ordovician strata are also found in Belgium, France, Bohemia, and
other places, and are largely composed of mechanical sediments of
varying degrees of fineness mixed occasionally with some calcareous
matter.
The variation in the characters of the Ordovician strata of Britain
points to accumulation in a fairly deep sea, usually at some distance
from the land, but dotted over with volcanoes which often rose above
the water, causing the addition of much volcanic material to the
ordinary sediments, and the existence of minor unconformities at
different horizons along their flanks. As these unconformities are not
always associated with volcanic material it is obvious that uplifts
must have occurred occasionally during the deposition of the rocks;
one important uplift is indicated by the occurrence of an unconformity
in the Arenig rocks of Wales, while another is seen amongst the
Caradoc rocks of the Welsh borders. On the whole, however, the period
was one of slow subsidence, the deposition of material generally
keeping pace with this subsidence, and accordingly there is a great
uniformity of characters amongst the strata over wide areas. The
probable continuation through the Ordovician period of the tract of
land over the present site of the N. Atlantic ocean which as we have
reason to suppose existed during Cambrian times, is indicated by
similar changes of lithological character amongst the strata when
traced from Britain eastward to Russia in both Cambrian and Ordovician
times, and the continuance of these conditions over the American area
is also indicated by study of the variations amongst the American
Ordovician deposits.
_The Ordovician Faunas._ The Ordovician period has justly been termed
the Period of Graptolites, which are the dominant forms of the time,
and continue in abundance throughout the period. The abundance of
graptolites in black shales associated with few other organisms has
often been noted. It appears to be due to a large extent to the slow
accumulation of the graptolitic deposits, allowing an abundance of
these creatures to be showered upon the ocean floor, after death, for
the evidence derived from detailed examination of their structure
points to their existence as floating organisms. The tests of other
creatures largely calcareous may well have been dissolved before
reaching the sea-floor. In support of the view that these black shales
are abysmal deposits may be noted the singular persistence of their
lithological characters over wide areas, their replacement by much
greater thicknesses of normal sediments along the ancient coast-lines,
the frequent occurrence together of blind trilobites with those having
abnormally large eyes when these creatures are associated with
graptolites in the black shales, and lastly the interstratification of
the black shales with radiolarian cherts similar to the modern abysmal
radiolarian oozes. If this be so, we ought to find graptolites in
marine deposits of all kinds, and indeed they are found there, though
largely masked by the mass of sediment and the hosts of other included
fossils, so that their discovery is rendered much more difficult than
when they occur in the black shales,--a state of things which is
familiar in the case of other pelagic organisms as _Globigerinæ_,
radiolaria, and pteropods, whose tests abound in the abysmal deposits
and are comparatively rare in those of terrigenous origin[78].
[Footnote 78: The importance of the graptolites as indices of the
geological age will be seen by perusal of Prof. Lapworth's paper "On
the Geological Distribution of the Rhabdophora," _Ann. and Mag. Nat.
Hist._, Ser. 5, vol. III. (1897).]
The characters of the Ordovician trilobites have already been noticed.
These organisms are abundant, and occur in sediments of all kinds. Of
other groups, the significance of the radiolaria has been referred to
above. Corals occasionally form reef-like masses of limestone as in
the limestones of the Caradoc epoch; the echinoderms are well
represented, cystids being locally abundant; of the crustacea, many
remains of tests of phyllocarida have been recorded; the brachiopods
are very abundant, and of the mollusca, lamellibranchs, gastropods and
cephalopods all occur with frequency though none of these groups is
very prevalent. Certain forms have been referred to pteropods though
with doubt, and other shells seem to be referable to the heteropods.
The existence of vertebrates during Ordovician times is not, in the
opinion of many geologists, proved, though remains of fishes have been
recorded from the Ordovician strata of North America; but it is
desirable that more evidence of this occurrence should be given[79].
[Footnote 79: Walcott, C. D., "Preliminary Notes on the Discovery of a
Vertebrate Fauna in Silurian (Ordovician) Strata," _Bulletin Geol.
Soc. America_, vol. III. p. 153.]
The distribution of the Ordovician faunas like that of the sediments
points to the prevalence of open ocean conditions over wide areas
during the period, with occasional approaches to land, which was often
of a volcanic nature. Around this land clustered the ordinary
invertebrates, building up coral-reefs and shell-banks, whilst away in
the open oceans the graptolites floated, almost alone, and sank to the
ocean floor after death.
CHAPTER XVI.
THE SILURIAN SYSTEM AND THE CHANGES WHICH OCCURRED IN BRITAIN AT THE
CLOSE OF SILURIAN TIMES.
_Classification._ The Silurian system was originally divided by its
founder, Sir R. I. Murchison, into three series, as follows:--
Ludlow Series
Wenlock "
Llandovery "
The term May Hill, proposed by Sedgwick, is sometimes used as
synonymous with Llandovery. This classification omits a somewhat
important set of beds intercalated between those of the Llandovery and
Wenlock series known as the Tarannon shales, and in Britain if we were
to classify afresh, it would be more convenient to include some of the
beds formerly referred to the Ludlow in the Wenlock. I shall, however,
adopt the old and well-established classification, adding the term
Tarannon to Llandovery, and speaking of the Llandovery-Tarannon
series. The nature of the two classifications is shown in the
following table:
Old New Palæontological
Stages. Classification. Classification. Classification.
1 Upper Ludlow } } }
2 Aymestry Limestone } Ludlow } Downtonian }
3 Lower Ludlow } } Fauna
} } with
4 Wenlock Limestone } } } _Encrinurus_
5 Wenlock Shale } Wenlock } Salopian }
6 Woolhope Limestone } } }
7 Tarannon Shales } } Fauna
8 Upper Llandovery } Llandovery } Valentian } with
9 Lower Llandovery } } } _Harpes_
[Illustration: Fig. 18.
L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.]
_Description of the strata._ Lithologically the Silurian deposits of
Britain form a continuation of those of the Ordovician period, with a
local interruption due to the elevation of portions of Wales and the
Welsh borders at the close of Ordovician times. Elsewhere we find a
predominance of shales passing into grits at the top of the system,
the change indicating the incoming of the shallow-water phase before
the commencement of the second continental period. Particular stress
is laid upon the predominant shaley character of the beds, for, on
account of the richness and variety of the faunas of the calcareous
rocks, greater attention is naturally paid to them in geological
works, and the student may get a false idea of their relative
importance. An attempt is made below (Fig. 18) to give a general idea
of the variations in lithological characters of the Silurian rocks in
different parts of Britain.
The Silurian strata are mostly found in the same localities as those
which furnish exposures of the rocks of Ordovician age.
The development in the typical Silurian region of the Welsh borders is
characterised by the abundance of calcareous matter which is found
there as compared with that which exists in the other British
localities.
The Llandovery strata are sandy, often conglomeratic, with a fair
amount of calcareous matter in places. The arenaceous nature is
undoubtedly due to the proximity of land caused by local upheaval at
the end of Ordovician times, and the Upper Llandovery rocks sometimes
rest unconformably on the Lower ones, at other times on Ordovician,
Cambrian, or even Precambrian rocks. The Tarannon shales are light
green shales with intercalated grits. The Wenlock series consists of a
group of shales separating a lower, very inconstant, earthy limestone
from an upper, more constant, thicker and purer limestone. The latter,
the Wenlock limestone, is composed of fragments and perfect specimens
of various fossils, and the fragmentary nature of many of the shells
indicates the occurrence of wave-action and probable formation in
shallow water, in some places against coral-reefs.
The Lower Ludlow beds consist of sandy shales; they are separated from
the Upper Ludlow beds by an impure limestone, the Aymestry limestone.
The Upper Ludlow beds consist mainly of grits and flags, often
coloured red towards the summit.
In North Wales the Llandovery beds occasionally present the shelly
arenaceous types of deposit as near Llangollen, at other times as near
Conway, Corwen, and in Anglesey, the graptolitic shale type. They
also rest unconformably upon the Ordovician rocks in this area. The
Tarannon shales resemble those of the border county. The Wenlock
series consists essentially of shales, while the Ludlow development
differs from that of the borders in its greater thickness and the
absence of any calcareous band in the centre. In Central Wales the
graptolitic type of the Llandovery-Tarannon series is found, but the
graptolite-bearing shales of the Llandovery epoch are thin beds
occurring between grits and flags no doubt deposited in shallow water,
and this division of the series is of very great thickness.
In South Wales the Silurian rocks are very similar to those of the
Welsh borders, save that the calcareous deposits are fewer and
thinner.
The Lake District Silurian strata generally resemble those of North
Wales. The Llandovery-Tarannon rocks are of the graptolite-shale type,
intercalated with fine grits in the case of the beds of Tarannon age.
The Wenlock beds consist of shales, and the Ludlow beds of gritty
shales beneath, and massive flags and grits at the summit. These
Ludlow beds are here of great thickness (certainly not less than 7000
feet) and were obviously accumulated for the most part in shallow
water.
The Llandovery-Tarannon rocks of Southern Scotland show the two types
which prevailed in the Moffat and Girvan areas in later Ordovician
times. The Llandovery beds of Moffat are known as the Birkhill shales,
and are very thin. The representatives of the Tarannon shales,
however, the Gala beds, consist mainly of grits, and attain a great
thickness. In the Girvan area, the Llandovery beds are of the shelly
type. Here as at Moffat and in the Lake District there is perfect
conformity between the beds of Ordovician and those of Silurian age,
and accordingly it is instructive to note the completeness of the
palæontological break, especially in the Moffat district. The higher
Silurian beds of Southern Scotland present a general resemblance to
those of North Wales and the Lake District[80].
[Footnote 80: For descriptions of the Silurian beds of the typical
region see Lapworth and Watts, _Proc. Geol. Assoc._, vol. XIII. p.
297, those of Wales are described by Lake and Groom, _Quart. Journ.
Geol. Soc._, vol. XLIX. p. 426, and Lake, _ibid._ vol. LI. p. 9. A
description of those of Lakeland will be found in the Memoir of the
Geological Survey "The Geology of the Country around Kendal, etc."
while the Scotch Rocks are described in Lapworth's papers on Moffat
and Girvan.]
On the European continent we find indications of conditions similar to
those which prevailed during the Ordovician period; the strata become
much thinner and more calcareous in Scandinavia, and still thinner in
the Baltic provinces of Russia, where they consist very largely of
calcareous matter. In central Europe the greater abundance of
calcareous matter, compared with that which is found in the Ordovician
strata of that region, points to a change in physical conditions which
became still more marked after Silurian times.
In North America, the succession is very similar to that of Britain,
the calcareous development of the Silurian rocks being found around
Niagara, but towards the close of Silurian times the shallow-water
phase became marked in places by the deposition of chemical
precipitates which indicate the separation of a portion of the late
Silurian ocean from the main mass during the period of formation of
these abnormal deposits.
The conditions of Silurian times, until the advent of the
shallow-water phase, recall those of Ordovician times and point to a
wide expanse of ocean at some distance from the land, though the
earliest deposits become arenaceous where they were deposited against
an old land surface formed by the elevation of the Welsh Ordovician
rocks, which were denuded to supply this material. One marked
difference existed between the physical conditions of our area during
Ordovician and Silurian times, for the volcanic activity which was
rife during Ordovician times almost ceased during Silurian times,
except in the region now occupied by the extreme south-west of
Ireland, and accordingly volcanic material does not appreciably
contribute to the formation of the Silurian deposits. The shallowness
of the sea-floor at times is marked by the occurrence of masses of
reef-building corals in the limestones, and these probably indicate
the prevalence of a fairly warm climate, an inference supported by the
nature of the Gastropod fauna of Gothland, as noticed in Chap. IX.
The shallow-water phase commences fairly simultaneously over the whole
area at the beginning of the deposition of the Lower Ludlow rocks, and
becomes more marked in the Upper Ludlow rocks, being most noticeable
at their extreme summit, when a change occurred which will be
considered at the conclusion of this chapter.
_The Silurian Faunas[81]._ The Silurian period has been termed the
period of Crinoids, and this group of creatures certainly contained a
great variety of very remarkable forms, which are specially numerous
in the Wenlock Limestone of the Welsh borders, Gothland, and North
America, but many of the rocks of the system display few traces of
these organisms. The trilobites and graptolites still contribute
largely to the fauna, the latter becoming very scarce at the summit of
the system, though a few specimens have been detected in the rocks of
the succeeding system. The trilobites belong to few genera though
these are mostly more highly organised than those of the Ordovician
period. The genus _Harpes_ has been taken as fairly characteristic of
the lower part of the system in Sweden, and it occurs there abundantly
in places in Britain, whilst _Encrinurus_ is more abundant in the
upper series, but both of these genera range from higher Ordovician
beds into the Devonian. Mention has already been made of the corals.
Brachiopods are very abundant, and Mollusca appear with considerable
frequency. The appearance of true insects is of importance,
cockroaches have been recorded from Silurian rocks and a number of
other insects have lately been recorded from Canada[82]. Eurypterids
occur in considerable abundance in the higher parts of the system, as
do also the remains of fish.
[Footnote 81: For an account of the Silurian faunas the student may
consult Sir R. I. Murchison's _Silurian System_ or the shorter
_Siluria_ and Lapworth's paper on the Geological Distribution of the
Rhabdophora.]
[Footnote 82: See an article by Dr G. F. Matthew, "Description of an
extinct Palæozoic Insect and a review of the Fauna with which it
occurs," _Bulletin_ XV. _of the Natural History Society of New
Brunswick_. The Silurian Rocks of the Little River Group of St John,
New Brunswick, have yielded species of land snails, two doubtful
saw-bugs, several arachnids, and myriopods, two insects of the order
Thysanura (Spring-tails), and eight Palæodictyoptera.]
The close of Silurian times ushered in the second continental period
in Britain when a large part of our area and the adjoining areas to
the north and north-east were uplifted to form land, which in the case
of our area was interpenetrated by watery tracts, whose exact nature
is still a subject of dispute. Accordingly the deposits which were
formed during this period are local and in some cases abnormal, but
they will be considered in the next chapter. Simultaneously with the
formation of these deposits, uplift of the sea-floor converted wider
and wider areas into land, and this land underwent considerable
denudation, so that the tops of the anticlines were worn away. The
general trend of the anticlines was east-north-east and
west-south-west, and accordingly a series of mountain chains possessed
that direction, for the epeirogenic movements were accompanied by
orogenic ones. Between the regions of uplifts were depressions in
which sediments accumulated. The principal axes of uplift in our area
range through the North of Scotland towards Scandinavia, across the
Southern Scotch Uplands to the North of Ireland, through the Lake
District and through Wales. As the result of lateral pressure, a
cleavage structure was impressed on many of the Lower Palæozoic rocks,
the strike of the rocks extended in the direction of the ridges and
depressions, and the rocks as a whole became considerably compacted
and hardened, thus producing one of the most important portions of the
framework of our island, for although the ancient mountain chains were
largely denuded during their elevation, and their stumps were
afterwards covered by later deposits, upon the removal of these, the
ancient stumps were once more exposed as fairly rigid masses which do
not yield greatly to denuding influences, and accordingly stand out as
the most important upland regions of Britain at the present day.
It is interesting to notice, as an illustration of the now well
established fact that successive earth movements often occur in the
same direction, that the axes of the folds produced during this second
continental (Devonian) period, run parallel with the lines separating
tracts of different lithological characters. It has been seen that
the Ordovician and Silurian rocks of the Southern Uplands continue
into Ireland, and that the beds of similar characters run in belts
having a general east-north-east and west-south-west trend, which
accordingly must have been the direction of the coast-line parallel to
which they were deposited, and as that coast-line was due to uplift,
the movement which produced it would naturally produce foldings with
east-north-east and west-south-west trend. This is one of many cases
where the lines separating belts of rock having different lithological
characters run parallel to axial lines of folds which have been
produced in the rocks at a later period.
As the result of the existence of land over parts of north-west Europe
in Devonian times, it is comparatively rare to find a passage from
normal Silurian rocks into normal Devonian ones; there is often an
unconformity above the Silurian strata. As we proceed southwards
towards central Europe, where the epeirogenic and orogenic movements
died out, this is not the case, and we get complete conformity between
marine sediments of the Silurian and Devonian periods.
CHAPTER XVII.
THE DEVONIAN SYSTEM.
_Classification._ As a result of the movements which were briefly
described in the last chapter, two types of Devonian deposit are found
in the British Isles, and are called respectively the Devon type and
the Old Red Sandstone type. The latter rocks, formerly divided into
three divisions, are now separated into two only, the upper and lower
Old Red Sandstone, and the exact relation of these to the different
subdivisions of the rocks of Devon type remains to be settled. The
Devon type itself has given rise to much difference of opinion, two
local classifications have been applied, one for the rocks of North
Devon and another for those of South Devon. The classification which
has been most generally adopted is as follows:--
N. Devon. S. Devon[83].
{ Pilton Beds { Entomis Slates
Upper Devonian { Cucullæa (Marwood) { Goniatite Limestones
(Clymenian) { Beds { and Slates
{ Pickwell Down Sandstone { Massive Limestones
Middle Devonian { Morte Slates { Middle Devonian
(Eifelian) { Ilfracombe Beds { Limestones
{ Ashprington Volcanic
{ Series
{ Eifelian Slates and
{ Shaly Limestones
{ Lower Devonian
Lower Devonian { Hangman Grits { Slates
(Coblenzian) { Lynton Slates { Lincombe and Warberry
{ Foreland Grits { Grits and
{ Meadfoot Sands
[Footnote 83: An account of the South Devon rocks by Mr Ussher will be
found in the _Quart. Journ. Geol. Soc._, vol. XLVI. p. 487; from it
the above classification of the rocks of S. Devon is taken.]
The division into Lower Middle and Upper Devonian is generally
adopted, though the alternative titles given to these divisions are
not always used with the same signification, and the distribution of
the different local stages given in the above classifications is
usually adopted in the main, though a detailed comparison of the
Devonian beds of North and South Devon is still attended with
difficulty.
More than once an attempt has been made to prove that the apparent
succession of the North Devon rocks, which is that given in the above
table, is not the true one, and of recent years Dr Hicks has obtained
a number of fossils from the Morte Slates which had hitherto yielded
none, and he believes that these fossils indicate that the Morte
Slates are on a lower horizon than the beds on which they rest.
Whatever be the ultimate verdict, we can, at any rate, say that the
"Devonian Question," as it is termed, is not settled[84].
[Footnote 84: See Hicks, H., "On the Morte Slates and Associated Beds
in North Devon and West Somerset," _Quart. Journ. Geol. Soc._, vols.
LII. p. 254, LIII. p. 438.]
_Description of the Strata._ The general variations in the
lithological characters of the deposits of Devonian age will be seen
from the accompanying figure which represents the deposits of Britain
as they occurred from north to south before they had been affected by
subsequent earth-movements (Fig. 19). The conventional signs which are
used are similar to those which have been used in other parts of this
work, and will save description of the section.
[Illustration: Fig. 19.
A. Lower Palæozoic and Precambrian Rocks.
N.S. North of Scotland }
C.V. Central valley of ditto } Old Red Sandstone Type.
W. Wales }
N.D. North Devon } Devon Type.
S.D. South Devon }
]
The ridges separate different deposits of Devonian rocks, which were
possibly deposited in isolated areas, though there was probably
connexion between them at any rate at times.
The Old Red Sandstone type consists to a large extent, as the name
implies, of sandstones which are coloured red by a deposit of peroxide
of iron around the sand grains. They are separable into a lower and
upper division with an unconformity often occurring between them. The
lower Old Red passes down in places into the Silurian rocks with
perfect conformity, and the upper Old Red similarly passes up into the
Carboniferous strata. The existence of pebble beds at different
horizons is a noteworthy feature. They are frequently found at or near
the base of the two divisions. The sandstones of the lower division
are often accompanied by flagstones, while the red sandstones of the
upper division usually have deposits of yellow and brown sandstone
intercalated between them. Inconstant beds of limestone, known as
cornstones, are found in both divisions, and Prof. Sollas has shown
that some of these, at any rate, are true mechanical deposits, formed
by the destruction of pre-existing strata of limestone and the
deposition of the resulting fragments from a state of suspension. In
Scotland a great thickness of volcanic material of various kinds is
associated with the two divisions. For the sake of simplicity this is
omitted from Fig. 19[85]. It is not known how far normal sediments are
associated with the Old Red Sandstone type of deposit. The existence
of some in South Wales is suggested by evidence supplied by the late
Mr J. W. Salter.
[Footnote 85: For an account of these and all other British volcanic
rocks the reader is referred to Sir A. Geikie's work on _The Ancient
Volcanoes of Great Britain_. Macmillan and Co., 1897.]
The Devon type, as will be seen in the figure, consists of rocks which
are to a great extent of normal character. We find in Devonshire
alternations of sandstones, shales and limestones, but even here, red
sandstones, which are comparable with those of the Old Red type occur
in diminished amount: the Foreland Grits and Pickwell Down Sandstones
are both coloured red, and are like the sandstones formed further
north. The recognition of this fact induces one to believe that the
contrast between the two types of rock which are found at a short
distance from one another on opposite sides of the Bristol Channel is
not so marked as one is sometimes led to suppose.
The rocks of North Devon differ from those of South Devon chiefly
owing to the amount of calcareous sediment found in the two areas, for
limestones occur in South Devon to a great extent, and in North Devon
there is a comparative poverty of this kind of sediment. Here, again,
the apparent difference is possibly greater than the real one. The
North Devon limestones have in places been stretched out after their
formation and thus rendered thinner, and the highly-cleaved limestones
are occasionally mistaken for shales, while in South Devon there is
evidence of thickening of the limestones by folding subsequently to
their deposition. Allowing for these changes, however, there is still
a marked diminution in the amount of coarse mechanical sediments and
increase in the quantity of calcareous matter as one passes from North
to South Devon, and this prepares one for the condition of things met
with on parts of the continent, where the mechanical sediments become
finer and thinner on the whole as one travels southward, until, when
we reach the Bohemian area, the Devonian rocks are found to be largely
composed of calcareous sediments.
It is interesting to find that in North America the two types of
Devonian strata recur, and present characters generally similar to
those which they possess upon this side of the Atlantic.
Passing now to a consideration of the conditions under which the
Devonian rocks were deposited, we may examine the bearing of the
character of the strata as a whole, and then proceed to more detailed
consideration of the nature and conditions of deposits of the two
types.
The gradual increase in calcareous matter and dying out of mechanical
sediments as one travels southward points to recession from land in
that direction, and we have already seen that the epeirogenic and
orogenic movements of this continental period elevated the Silurian
sea-floor in the north, and gave rise to a Northern Continent, while
oceanic conditions continued further South, and allowed the
accumulation of sediments lying conformably upon those of Silurian
age, and giving indications of the prevalence of physical conditions
during Devonian times which were in the main similar to those of the
preceding Silurian period.
In the shallow waters adjoining the land of the Northern Continent the
Old Red Sandstones were laid down, and the exact conditions under
which they were accumulated is a matter of some interest. The late Sir
Andrew Ramsay gave reasons for supposing that many red deposits were
accumulated in the waters of inland lakes, which underwent rapid
evaporation, and his views have been applied, with much corroborative
evidence by Sir A. Geikie, to account for the red sandstones of
Devonian age, which he believes to have been accumulated in a series
of inland lakes, though others hold a different opinion, and consider
that the Old Red Sandstone waters had a direct connexion with those of
the open ocean; the question is too intricate to be discussed at
length here. Besides the difference of physical characters of the two
types of strata, the difference in the nature of their included
organisms is significant. The ordinary invertebrates, as corals,
crinoids, brachiopods and molluscs are extremely rare in the Old Red
Sandstone, which contains remarkable remains of Agnatha fishes and
eurypterids, and although these are also found associated with a true
marine fauna in Russia, Germany and Bohemia, the rarity or apparent
absence of the ordinary marine invertebrates, though only negative
evidence, which is proverbially dangerous, must be regarded.
The North Devon rocks are sediments which might well be accumulated on
the shores of a continent, while those of South Devon, with their
abundant coral reefs, and other organic limestones were no doubt
deposited in a clearer sea, at a greater distance from the land, and
the clear water deposits of Germany and still more of Bohemia, were
accumulated in the open ocean. It is interesting to note in these
Bohemian deposits abundance of shells of a Pteropod _Styliola_ which
has been proved by Prof. H. A. Nicholson to form masses of limestone
in the Devonian system of Canada. The modern distribution of the
Pteropoda suggests the open ocean character of the deposits which
contain them even so far back as Devonian times, though one cannot
conclude that these deposits are really analogous to the so-called
Pteropod ooze of modern seas which, as a matter of fact, is largely
composed of foraminiferal tests with a considerable percentage of
pteropod shells.
_The Devonian flora and faunas._ The plant remains in the Lower
Palæozoic rocks are few in number. Some undoubted terrestrial plants
have been discovered, but the prevalent flora of lower Palæozoic
times, so far as yet known, was one consisting of Algæ. In Devonian
times we begin to meet with a number of Cryptogams of higher type,
allied to those which form the dominant flora of the succeeding
period. The fauna is in many ways remarkable. The Devonian period has
been termed the period of ganoid fishes, and the remarkable remains,
so graphically described by the late Hugh Miller, are indeed
peculiarly characteristic of Devonian times, but they are largely
though by no means exclusively entombed in rocks of the Old Red
Sandstone type[86]. The Devon type of rock contains a great abundance
and variety of the problematical group, the Stromatoporoids, which
contribute extensively to the formation of many of the limestones,
and although these organisms are not by any means confined to Devonian
strata, their abundance and variety therein might lead one to speak of
the period as that of Stromatoporoids. The remains of corals are very
abundant in the limestones, and, as already stated, frequently give
rise to true reef-masses. The graptolites, as remarked in the previous
chapter, disappear in the rocks of the Devonian period, and as only
one or two fragments have been found, we may assert that the group was
practically extinct at the end of Silurian times, though species of
one genus, _Monograptus_, lingered for a short time in greatly
diminished quantity. The trilobites which played so important a part
amongst the faunas of Lower Palæozoic times still occur fairly
abundantly amongst the rocks of the Devonian system, and there is a
very interesting point to be noticed in connexion with them. They seem
to have become practically extinct in the succeeding Carboniferous
period, where few genera are found, and the decadence of the group
began in Devonian times. In these circumstances it is interesting to
note the tendency displayed by the creatures to possess spiny
coverings. It is true that _Acidaspis_, the most spinose of all
trilobites, is abundant in Ordovician and Silurian strata, and that
other spinose trilobites are found there, but the peculiarity of the
Devonian trilobites is, that genera which were previously smooth, or
rarely possessing one or few spines, are found represented by
extremely spinose species in these beds,--the spines being developed
from all parts of the test, sometimes as a fringe to head or tail,
sometimes as prominent projections from glabella and neck segment, and
frequently in rows down the body segments. Besides _Acidaspis_, we
find spinose species of _Phacops_, _Homalonotus_, _Cyphaspis_,
_Bronteus_ and _Encrinurus_ in Devonian strata, and the occurrence of
these forms is so frequent and world-wide, that one might perhaps
infer with confidence that an unknown fauna containing many spiny
trilobites was of Devonian age.
[Footnote 86: For an account of these see A. S. Woodward's _Vertebrate
Palæontology_.]
The abundance of Eurypterids has been previously noted. Occurring as
they do in Silurian rocks, they are far more abundant in those of
Devonian age, and are found indifferently in sediments of Old Red and
Devon types. Of air breathers, several insects have been found in the
strata of different parts of the world.
The ordinary marine faunas are otherwise intermediate in character
between those of the Silurian and Carboniferous periods, but there are
several characteristic Devonian genera, and no one who is acquainted
with the peculiarity of the Devonian fauna would deny to the Devonian
strata the right to rank as a separate system, containing a fauna as
well marked in its way as that of the Silurian system below or that of
the Carboniferous above. Special stress is laid upon this point
because it has been suggested that the Devonian system should be
abolished, and its strata either divided between the Silurian and
Carboniferous systems or referred exclusively to the latter
system[87].
[Footnote 87: The literature of the fauna of the Devonian rocks is a
rich one. For an account of the Devonian rocks of Britain, the reader
may consult the Monograph of the Devonian Fossils of the South of
England by Rev. G. F. Whidbourne, which is now appearing in the series
of Monographs of the Palæontographical Society, and in the
publications of the same Society he will find a Monograph of the
Eurypterids from the pen of Dr Henry Woodward. The richest Devonian
fauna is undoubtedly that of the Bohemian area, for the work of Dr E.
Kayser has conclusively proved that the stages _F_, _G_ and _H_ of
that basin, formerly referred to the Silurian, are of Devonian age,
and an excellent idea of the richness of the Devonian fauna may be
obtained by studying the descriptions of the fossils from those stages
which have appeared and are appearing in Barrande's classic work.]
CHAPTER XVIII.
THE CARBONIFEROUS SYSTEM.
_The Classification._ The British rocks of the Carboniferous system
have been classified according to their lithological characters, but
as the classification has been altered from time to time, we may use
that which seems most acceptable to the majority of British geologists
at the present day. According to this, the beds are grouped as
below:--
{ { Ardwick Stage
Upper Carboniferous { Coal Measures { Pennant Stage
{ { Gannister Stage
{ Millstone Grit
Lower Carboniferous { Carboniferous (Mountain) Limestone
{ Series.
The Lower Carboniferous beds have been further subdivided into:--
Yoredale Series or Upper Limestone Shales,
Mountain Limestone,
Lower Limestone Shales, with Sandstones and Conglomerates,
but as these lithological types are found to be very variable when
traced laterally for comparatively short distances, it is found more
satisfactory to use the terms in a purely lithological sense rather
than with chronological significance.
The somewhat abnormal development of the higher portions of the
Carboniferous rocks of Britain renders the local classification only
partially applicable in other regions, and as our knowledge
progresses, a palæontological classification will probably be adopted.
This has already been done with the more purely open-water sediments
of Russia and Eastern Asia, where the development of the beds is more
normal. There the rocks are classified as under:--
Upper Carboniferous or Gshellian,
Middle Carboniferous or Moscovian,
Lower Carboniferous,
and as this classification has already been found to be applicable
over rather wide areas, it is almost certain that, as in the case of
the rocks of other systems, it will prove more serviceable than one
which is mainly (though not quite exclusively) based upon vertical
variation of lithological characters, especially as the Carboniferous
rocks over large tracts in North America possess faunas which are
similar to those which have been discovered in Russia, Eastern Asia
and North Africa.
_Description of the strata._ The variations in the lithological
characters and fossil contents of the British Carboniferous strata
when traced from north to south have been so frequently described, and
utilised as a means of illustrating the indications as to local
variations in physical conditions which are supplied by those strata,
that little need be said upon the subject. The restoration of the
physical geography of Carboniferous times over the British area will
be found in a chapter by the late Professor Green in the work upon
_Coal_ by various professors at the Yorkshire College of Science and
also in Prof. Hull's _Physical History of the British Isles_. Some
modifications must be made in these restorations as the result of
recent research, the principal being caused by discoveries amongst the
Carboniferous rocks of Devonshire.
Taking the strata in vertical succession, we find evidence of the
occurrence of a complete marine period (the second great marine
period) between the second and third continental periods. The first
shallow-water phase over a great portion of the British Isles is
marked by thin terrigenous sediments, indicating that the period was a
brief one; it was followed by the deep-water phase, probably of some
length, lasting through the greater part of the remainder of Lower
Carboniferous times; while the concluding shallow-water phase was
lengthy as compared with that of the beginning of the period, and is
marked by the accumulation of the great thickness of deposits
belonging to the Millstone Grit and Coal Measures. There is no doubt,
however, that in some parts of the British area minor changes produced
local terrestrial conditions during the period, and accordingly we
find that the deepest water deposits of the system in Britain are
succeeded by an unconformable junction with the sediments of the upper
portion of the system.
The general change in the lithological characters of the beds of the
Lower Carboniferous division when traced from south to north is shewn
in the following diagram (Fig. 20).
It will be seen that the land and open sea areas were in the
respective positions which they occupied during Devonian times, but
that as the result of greater submergence, with which the accumulation
of sediment did not keep pace, the shallow-water marine deposits of
Devonian age are in Devon replaced by open-sea deposits[88], while
shallow-water marine deposits further north replace the anomalous
deposits which were found there during the Devonian period.
[Footnote 88: The Radiolarian Cherts of the Lower Carboniferous rocks
of Devon, and the associated sediments, together with the unconformity
between these and the Upper Carboniferous beds are described by Messrs
Hinde and Fox, Quart. _Journ. Geol. Soc._, vol. LI. p. 609.]
[Illustration: Fig. 20.
_a._ Radiolarian cherts of Devon.
_b._ Mountain limestone of Central England.
_c._ Mechanical sediments of Northern England.
_d._ Freshwater deposits of Southern Scotland.
O.R. Older rocks.
]
Owing to the accumulation of thick masses of sediment, the Lower
Carboniferous sea of the north of England appears to have been largely
silted up, and although the organic deposits of the south are so thin
that they did not render the sea shallow in that region, the general
level of the Lower Carboniferous floor of the south was also uplifted,
and actually converted into land, as the result of the upward movement
which took place in Devonshire and tracts of France; and owing to
silting up in the north, and elevation in the south, a general plane
surface was produced over very extensive areas, not only in Britain
but upon the Continent, upon which the peculiar deposits and
accumulations of Upper Carboniferous times were laid down, sometimes
in shallow water, sometimes upon the land, and often under conditions
which cannot at present be determined with accuracy. That the deposits
of the Millstone Grit and Coal Measure epochs were to a large extent
laid down in water is admitted by all, and in the case of many of the
deposits of the Millstone Grit, and some thin deposits of the Coal
Measures, it is equally clear that the water area was part of an
expanse of ocean, for we find marine fossils, as corals, crinoids, and
cephalopods, in these beds. Associated with them in the Coal Measures
are other beds in which the ordinary Carboniferous genera of marine
invertebrates are absent, and their place is taken by shells which
bear much resemblance to the modern fresh-water mussel, and it has
been maintained with good reason that as the ordinary marine forms are
rarely or never mixed with those resembling recent fresh-water shells,
the latter are truly fresh-water[89]. If this be so, many of the
mechanically formed sediments of the Coal Measures were of fresh-water
origin, laid down in shallow lagoon-like expanses, probably shut off
from the main ocean by a narrow portion of intervening land, which was
occasionally destroyed, thus permitting incursions of salt-water when
some of the ordinary marine invertebrates of the period obtained a
temporary footing in the area.
[Footnote 89: For further information upon this subject the student
should consult the Introduction to a Monograph on _Carbonicola_,
_Anthracomya_ and _Naiadites_ (the shells in question) by Dr Wheelton
Hind, being one of the Monographs of the Palæontographical Society.]
There is not only a difference of opinion as to the mode of
accumulation of many of the mechanical sediments of the Coal Measures,
but also as to that of the coal-seams which accompanied them. Two
different theories have been put forward to account for these
coal-seams, which are usually spoken of as the drift theory and the
growth-in-place theory. According to the former, in its extreme
application, coal is an aqueous deposit formed by the settlement of
drifted masses of vegetation upon the floor of a water-tract, while
those who push the growth-in-place theory to its extreme limits
maintain that coal is the result of growth of vegetation upon the
actual site where the coal is now found. Much apparently conflicting
evidence has been advanced by the advocates of the two hypotheses, and
special cases of coal-formation have been appealed to by each in
support of their views; thus the existence of coal composed largely of
bodies which resemble the spores of modern lycopods,--objects of so
resinous a nature that they float on the surface until they are
decomposed,--is cited by the upholders of the growth-in-place theory,
while the supporters of the other hypothesis can point with equal
force to the occurrence of the finely divided carbonaceous mud
containing remains of fishes which gives rise to cannel coal in some
places. One of the main assertions in support of the growth-in-place
theory was that of the supposed universality of 'underclays' or old
surface soils beneath all coal-seams, but though these are common,
they are far from universal. It is impossible to do justice in small
compass to this question of coal-formation, but it may be pointed out
that much of the difference of opinion can be understood if it be
remembered that the term 'coal' is rather a popular term which has
been admitted into scientific terminology, and therefore used somewhat
loosely, than a strictly scientific term applied to a definite
substance, and accordingly, just as at the present day we find
carbonaceous substances growing in one place on land to form peat, in
other places on a tract sometimes dry and sometimes submerged, to form
the carbonaceous deposits of the cypress-swamps, and once more
accumulated beneath the shallows of a sea as a sediment to form the
carbonaceous muds of the ocean margins where the mangroves grow, so
the diverse substances which are included under the general term coal
may have accumulated in one place on land, in another beneath water,
and in a third on an area alternately dry and submerged. This is not a
question of great importance; the important point is that
accumulations of vegetation on a fairly large scale are found at the
present day on plains, for even if they grow on mountain regions, the
deposits are readily denuded before they are covered up, and also it
must be noted that a moist climate is necessary for the growth of much
vegetation. The conclusion that the accumulations of coaly matter were
formed on plains is borne out by their great horizontal extent as
compared with their thickness, and it is now generally agreed that the
coal vegetation which is found in the normal coal-measures was
essentially a swamp vegetation.
An attempt has been made to prove that an upland vegetation of very
different character existed contemporaneously with it, but reasons
will be given in the sequel for concluding that this supposed upland
Carboniferous flora is everywhere of later date.
The later shallow-water phase of Carboniferous times, as already
stated, was unusually long, it was also very widespread, and appears
to have been accompanied over wide areas by humid conditions during
its continuance, and accordingly the marsh conditions which existed
during Upper Carboniferous times were probably on a larger scale than
that of similar conditions before or after. Special stress is laid
upon this fact, as it is a good illustration of the view which seems
to be gaining ground, that every period possessed peculiar conditions
never to be repeated, which must have left their impress upon the
character of the sediments.
Though the conditions above described were widespread, they were
naturally not universal, and accordingly in many parts of the world,
as previously stated, we find true marine deposits of Upper
Carboniferous times, though even these were sometimes replaced during
part of the epoch, by conditions which were favourable for the
formation of coal-seams in those places. Interruption in the
continuance of a humid temperate climate over the regions of
North-West Europe is also suggested by the discovery of deposits which
are maintained to be of glacial origin amongst the Coal Measures of
France[90].
[Footnote 90: For an account of the numerous volcanic products see Sir
A. Geikie's work on "The Ancient Volcanoes of Great Britain."]
_The Floras and Faunas._ The flora of the Carboniferous rock is so
noteworthy that the period has been termed the Period of Cryptogams;
the remains of ferns, horsetails, and clubmosses predominate, and many
of the forms reached a gigantic size. Though the floras of the various
stages are marked by a general resemblance, there are differences
which enable the palæobotanist to ascertain the stratigraphical
position of the beds by reference to the included plant remains, and a
considerable number of successive floras have been described[91]. The
invertebrate fauna does not differ on the whole very greatly from that
of Devonian times, though the trilobites are now becoming rare, and
the mollusca assume a more prominent position as compared with the
brachiopods. Corals occur in abundance in the calcareous deposits of
the period, and frequently give rise to sheets of reef-formation, but
the foraminifera and crinoidea certainly play the principal part as
limestone-producers, and the influence of the latter in giving rise to
great masses of limestone which are frequently used for ornamental
purposes is too well known to need more than passing reference. The
air-breathers have also been detected in greater abundance, though
they are rare, when we consider the comparatively favourable
conditions for their preservation presented by the Coal Measure rocks.
Myriopods, arachnids, insects and pulmoniferous gastropods have
however been found with tolerable frequency. The danger of arguing
from imperfect data is well illustrated by the great addition to our
knowledge of the insect-fauna of these times due to the exploration of
the beds of one small coal-field, that of Commentry in France, of
which the insects have been described by M. C. Brongniart. The
vertebrates are represented by a considerable variety of fishes, and
less abundant though tolerably numerous remains of Amphibia, which
occur in the Carboniferous rocks of the North of England, Ireland,
France, North America and elsewhere.
[Footnote 91: Consult Kidston, R., "On the Various Divisions of the
Carboniferous Rocks as determined by their Fossil Flora," _Proc. Roy.
Phys. Soc. Edin._, vol. XII. p. 183.]
The existence of definite zones of organisms in the case of the
Carboniferous rocks has been denied, and it appears to be considered
by some that the Carboniferous rocks were accumulated so rapidly as
compared with rocks of some other systems that the fauna remained very
similar throughout. It is very doubtful if this was so. In the case of
other systems, the division into zones has only been accomplished by
means of more detailed researches than those which have been conducted
amongst the Carboniferous rocks of Britain: again, the occurrence of
successive floras suggests that there may have been a similar
succession amongst the faunas, and finally we find that zonal division
has been carried on to some extent amongst the Carboniferous strata
of other regions. The following classification of the Russian type of
sediment may prove useful, as an indication of the possibility of more
detailed separation of our own beds:--
{ Beds with _Spirifera fascigera_, _Spiriferina_
Gshellian { _Saranae_, &c.
(with _Fusulina_ and { Beds with _Producta cora_, _P. uralica_,
_Archimedipora_) { _Camarophoria crumena_, &c.
{ Beds with _Syringopora parallela_ and
{ _Spirifera striata_.
Moscovian { Stage of _Spirifera mosquensis_.
{ Stage of _Spirifera Kleini_.
{ Coals, Sandstones and Shales with _Noeggerathia_
Lower Carboniferous { _tenuistriata_ and _Producta_
{ _gigantea_.
{ Stage of _Producta mesoloba_.
The marine fauna of the Upper Carboniferous beds, which is so poorly
represented in Britain, but is well developed in Spain, Russia, Asia
and North America, is largely characterised by the abundance of
foraminifers of the genus _Fusulina_ and _Fusulinella_ and of bryozoa
of the genus _Archimedipora_. It is very desirable that the truly
marine fauna of the _Spirorbis_ limestone and other marine bands of
the British Coal Measures should be carefully studied to see if they
present any close relationship with that of the Gshellian beds[92].
[Footnote 92: A good idea of the general characters of the
Carboniferous fauna of Britain will be obtained from an examination of
Professor Phillips' _Geology of Yorkshire_, Part I., and Mr (now Sir
F.) M^{c}Coy's _Carboniferous Fossils of Ireland_, while the nature of
the European fauna is well illustrated in Prof. de Koninck's
well-known work _Description des animaux fossiles qui se trouvent dans
le terrain carbonifère de Belgique_. For an account of the characters
of the marine fauna of the Upper Carboniferous rocks the reader should
consult the work on Geology and Palæontology published by the
Geological Survey of the State of Illinois in 1866.]
CHAPTER XIX.
THE CHANGES WHICH OCCURRED DURING THE THIRD CONTINENTAL PERIOD IN
BRITAIN; AND THE FOREIGN PERMO-CARBONIFEROUS ROCKS.
At the close of Carboniferous times a marked change took place in the
nature of the earth-movements. The prevalent depression which occurred
over the British and adjoining regions during Carboniferous times was
replaced by upward movement, accompanied by orogenic folds, which once
more brought on continental conditions and developed a series of
mountain ranges. The change is marked even at the close of
Carboniferous times by the abnormal red sandstones of the uppermost
part of the Carboniferous system which are found around Whitehaven in
Cumberland and Rotherham in Yorkshire, as the Whitehaven Sandstone and
Rotherham Red Rock. These movements continued through Permian and
Triassic times, and it is to them and to the climatic conditions of
the periods, that the anomalous nature of the Permo-Triassic deposits
is largely due, as will be shewn in the succeeding chapters. At
present it is our purpose to call attention to the effect of these
movements upon the sediments which had been deposited previously to
their occurrence.
Over the British area, two different systems of orogenic movement can
be detected, producing folds of which the axes run approximately at
right angles to one another. One of these, of which the Pennine system
is the best representative in Britain, caused the production of
elevations having axes in a general north and south direction, and we
may therefore speak of it as the Pennine system of movement, while the
other, which gave rise to folds running in an east and west direction,
is well represented in the Mendip Hills, and may be therefore termed
the Mendip system, though it is more widely known as the Hercynian
system, as, on the Continent, the rocks which are greatly affected by
it form the foundations of the region occupied by the ancient
Hercynian forest.
The effects of these systems were in the main similar; they resulted
in the uplift of parallel belts of country to form hill-ranges with
intervening lowlands, but when studied in detail the movements are
seen to be of a different character. The Pennine system of movements
was of a type which is familiar to the geologists as developed in the
Great Basin Region of the western territories of North America, and
produced what is spoken of as Basin-Range structure. The movements
were of the nature of direct uplift, causing fracture, only
accompanied by folding in a minor degree, and accordingly the hills
are composed of terraced scarps, with one gently sloping side, and one
steep scarp-side, the latter on the upthrow side of the fault, as seen
in fig. 21.
In the Mendip system, the folds were of the Alpine type, which is a
familiar product of lateral pressure, consisting essentially of
overfolds, though these are often complicated by reversed faults.
Of the Pennine system, the Pennine Chain itself furnishes the most
noteworthy example in Britain, but we have indications of other folds
of this system, such as that which runs from the Lake District to the
Ayrshire coast, which is partly concealed as the result of other
movements, and a still more marked one, in the rocks of the Malvern
Hills.
[Illustration: Fig. 21.
_a a´_. One stratum displaced by faults _f f_. _h._ Hills.]
The Mendip system is well shewn in the Mendip Hills, but the remains
of a still more important anticline are seen in South Devon and
Cornwall, separated from the Mendip Hills by the great syncline of
Devon. Another parallel anticline runs from Lancashire to Yorkshire at
right angles to the Pennine Chain and separates the coal-field of
Cumberland and that of Northumberland and Durham, from those of South
Lancashire, and Yorkshire, Notts, and Derbyshire.
On the European continent the Ural Chain is the most important uplift
of the system of which the Pennine Chain forms a minor representative,
while the Hercynian system has caused the compression and stiffening
of many of the Carboniferous and earlier rocks which now rise to the
surface in many parts of central Europe.
The extensive continental area which was the result of these uplifts
not only determined the formation of abnormal deposits, but allowed
the occurrence of a long period of time subsequently to the close of
the Carboniferous period, of which few deposits now exposed in Europe
are representative, and we must accordingly seek other regions in
order to find typical representatives of this _Permo-Carboniferous_
period, of which the strata developed in the Salt Range of India have
been most carefully worked, especially by Dr Waagen, though marine
sediments of the period are known elsewhere, as in Spitsbergen, the
Ural Mountains, China and Australasia; and a group of somewhat
anomalous sediments of this age in parts of India, Australia and South
America is of peculiar interest, on account of the insight as to the
climatic conditions of the times which it affords.
_The Permo-Carboniferous Rocks._ In the Salt Range of the North-West
of India an interesting series of sandstones alternating with
limestones rests unconformably upon lower rocks. The sandstones are
known as the Speckled Sandstones, while the limestones are termed the
_Productus_ Limestones. The Lower and Middle Speckled Sandstones are
succeeded by the Lower _Productus_ Limestone which is separated from
the Lower division of the Middle _Productus_ Limestone by the Upper
Speckled Sandstone; these are all of the Permo-Carboniferous period,
while the upper part of the Middle _Productus_ Limestone and the Upper
_Productus_ Limestone belongs to the Permian period. The fossils,
largely invertebrates, are intermediate in character between those of
Carboniferous and Permian ages. Similar fossils are found in the
marine Permo-Carboniferous beds of the other areas which have been
named above. The Lower Speckled Sandstone is of interest on account of
the occurrence of boulder-beds within it, and this division of the
sandstone has been correlated with the lowest (Talchir) stage of the
Permo-Carboniferous strata of other parts of India, while the other
Speckled Sandstones and those divisions of _Productus_ Limestone which
are referred to the Permo-Carboniferous are correlated with the higher
divisions of other parts.
Special mention is made of the Talchir division, on account of the
occurrence therein of boulder beds which have long been known, and
whose glacial origin was inferred by Dr W. T. Blanford forty years
ago. The accumulations shew signs of having been deposited in water,
but the existence of large subangular, sometimes striated boulders
therein, which must have come from distant sources, and the occasional
occurrence of striated rock surfaces on the strata upon which the
Talchir beds repose unconformably points to ice-action; this would not
be so very remarkable if it were an isolated case, though sufficiently
so, from the comparative nearness of the region to the equator; but
researches conducted in different parts of the southern hemisphere
have brought to light similar, and sometimes even more striking
evidences of glacial action in widely distinct regions[93]. In
Australia they have been found in New South Wales, Victoria,
South Australia, East Australia and Tasmania; the Dwyka
boulder-conglomerates of South Africa and certain deposits of similar
character discovered by Prof. Derby in Southern Brazil have been
referred to the same period, and their glacial origin has also been
inferred. This widespread distribution of deposits which are generally
contemporaneous, of which the glacial origin may now be taken as
established, is extremely remarkable, and must be taken into careful
consideration by those who put forward theories framed to account for
former climatic changes.
[Footnote 93: The reader will find an excellent account of the
Permo-Carboniferous glacial deposits in a paper by Prof. Edgworth
David, entitled "Evidences of Glacial Action in Australia in
Permo-Carboniferous Time" (_Quart. Journ. Geol. Soc._ Vol. LII. p.
289). In this paper other glacial beds besides those of Australia are
noticed.]
_The Flora and Fauna._ The flora of the Permo-Carboniferous beds has
caused as much discussion as the question concerning the origin of the
boulder-deposits. In the southern hemisphere, the Permo-Carboniferous
rocks of those countries which have yielded boulder-beds also contain
remains of a flora which is now known as the _Glossopteris_ flora,
from the prevailing genus, which is associated with other genera, such
as _Gangamopteris_. These fossils appear to be ferns, though their
modern allies have not been indicated with certainty; associated with
them are rare cycads and conifers. The _Glossopteris_ flora is
markedly contrasted with the Coal-Measure flora of the northern
hemisphere with its giant lycopods. Moreover _Glossopteris_ appears in
the northern hemisphere in rocks of later date than the
Permo-Carboniferous period. It has been suggested that the
_Glossopteris_ flora originated in a continent in the southern
hemisphere, on which the boulder beds were also formed in isolated
water areas, and that some of the forms migrated northwards. To this
continent the name Gondwanaland has been applied by Prof. Suess, from
the _Gondwana_ series of the Permo-Carboniferous rocks of India, in
which the _Glossopteris_ flora is found, and it has also been
maintained that the southern _Glossopteris_ flora was contemporaneous
with the northern flora of ordinary Coal-measure type, though whether
this was so to any extent remains to be proved, for the beds
containing the _Glossopteris_ flora are distinctly newer than any
which have furnished a typical northern Coal-measure flora. In
any case, the change of floras between Coal Measure and
Permo-Carboniferous times is very marked, and when taken in connexion
with the widespread glacial deposits, is one of the most striking
phenomena displayed by the rocks of the stratified column[94].
[Footnote 94: For an account of the Glossopteris flora and its
geological relations, consult Seward, A. C., _Science Progress_,
January, 1897, p. 178.]
The fauna has already been noticed. It consists of brachiopods, some
of which are of peculiar genera. The general similarity of the faunas
in regions so remote as Spitsbergen, the Ural Mountains, India, and
New South Wales, indicates an extensive sea during the period. It can
hardly be supposed that the fauna of Permo-Carboniferous times has
been completely described, for the fossils of one or two areas only
have been made known to us with any degree of fulness, and when the
Permo-Carboniferous and marine Permian faunas are as well known as
those of Triassic times (and the latter have only been fully described
very recently) there is no doubt that the important break which was at
one time supposed to exist between Palæozoic and Mesozoic faunas will
be filled in satisfactorily[95].
[Footnote 95: The Permo-Carboniferous beds are described in Messrs
Medlicott and Blanford's _Geology of India_, second edition (edited by
Mr R. D. Oldham), and figures of some of the important fossils given
therein. For fuller information the reader should refer to Waagen's
account of the Salt Range Fossils and Feistmantel's description of the
plants in the _Memoirs of the Geological Survey of India_.]
CHAPTER XX.
THE PERMIAN SYSTEM.
_Classification._ It has already been observed that as the result of
the Pennine and Mendip systems of earth-movement, the Carboniferous
rocks of Britain are succeeded by a marked unconformity, and that the
rocks of the succeeding Permian and Triassic systems of Britain shew
an abnormal development. The principal areas where Permian rocks are
found are on either side of the Pennine Chain in the North of England,
but sporadic exposures of rocks of this age are found in some of the
Midland and Southern counties. The Permian rocks have been well
studied in Germany, and the German names are sometimes adopted in
Britain, and the following comparison will prove useful:--
Britain. Germany.
Magnesian Limestone Magnesian Limestone } Zechstein.
Marl Slate Kupferschiefer }
Lower Permian Sandstones Rothliegende.
The term Zechstein has been applied in a somewhat different sense by
different writers, but the one given in the table appears to find most
favour.
In a region which was essentially continental, considerable variations
in the lithological characters of the rocks may be expected, when the
strata are traced laterally, but we nevertheless find that the
differences are not so great as was formerly supposed to be the case
when certain red sandstones lying above recognised Permian strata in
the district on the west side of the Pennine Chain towards its
northern extremity were also referred to the Permian; these sandstones
(the St Bees Sandstones) are now generally admitted to be of Triassic
age, and comparison between the rocks on opposite sides of the Pennine
Chain is much simplified, as seen below.
West side. East side.
Thin Magnesian Limestones and Marls Magnesian Limestone
Hilton Shales Marl Slate
Penrith Sandstone and Brockrams Lower Permian Sandstones.
_Description of the Strata._ On the east side of the Pennine Chain,
the Lower Permian sandstone is an inconstant deposit often consisting
of yellow false-bedded arenaceous strata. The Marl Slate is an
argillaceous shale, often containing bituminous matter, and yielding
several fish-remains and some plants; it is usually only a few feet in
thickness. The Magnesian Limestone is typically developed in Durham as
a yellow or greyish limestone containing a variable percentage of
carbonate of magnesia; when traced southward, it alters its
characters, becoming mixed with mechanical deposits, and some chemical
precipitates in places, so that at Mansfield it appears as a red
sandstone with grains cemented by a mixture of carbonates of lime and
magnesia; and, like the rest of the Permian strata, it has disappeared
when we reach Nottingham. In addition to the southward thinning of the
Permian beds of this area, there is some evidence of their
disappearance in a westerly direction, though, as the present strike
of the beds is nearly north and south, the indications of this are
less convincing.
On the east side of the Pennine Chain, the main difference observable
is the relative thickness of the major divisions. The Lower Permian
sandstones have thickened out considerably, while the reputed
representatives of the Magnesian Limestone are thin. The Penrith
sandstone is of considerable interest. It contains in places, as near
Appleby, thick deposits of breccia consisting of angular fragments
chiefly composed of Carboniferous Limestone, which in many cases have
undergone subsequent dolomitisation, embedded in a matrix of red
sandstone. This breccia is known as brockram. Many beds of the Penrith
sandstone are composed of crystalline grains of sand, due to
deposition of silica in crystalline continuity with the quartz of the
original grain after the formation of the deposit; of more
significance, for our present purpose, is the presence of other
accumulations of the sand, in which the individual grains often
approach the form of spheres, thus resembling the 'millet-seed' sands
of modern desert regions. The Hilton shales are grey sandy shales,
with plant remains, and above them are variable deposits including
thin magnesian limestones which have yielded no fossils.
The isolated Permian deposits of the midland and southern counties of
England consist of red marls and sandstones with occasional breccias,
and in the absence of fossils, their exact position in the Permian
series is still unknown.
The German Permian rocks resemble those of Britain, especially as seen
in Durham, in many particulars, and give indications of formation
under physical and climatic conditions generally similar to those
which were then prevalent in the British area. At Stassfurt, in
Germany, the less soluble constituents of ocean water are accompanied
by a great variety of salts:--chlorides, sulphates and borates; and
the very soluble salts of potassium and magnesium known as the Abraum
salts are found in abundance as well as the less soluble salts of
sodium and calcium. The occurrence of these very soluble salts is so
infrequent on a large scale among the rocks of the Geological Column,
and the matter is one of so great theoretical import, that it is
necessary to take special note of their presence in the Permian
strata.
The frequent existence of chemical deposits in the Permian Rocks of
N.W. Europe, the formation of red sandstones, and the dolomitisation
of limestone beds and fragments of pre-existing limestones point to
inland seas of a Caspian character, while the evaporation necessary
for the formation of the precipitates also indicates a fairly warm
temperature. The presence of millet-seed sands, in very lenticular
patches, suggesting former sand-dunes, and the occurrence in places of
breccias (like some parts of the brockram) almost devoid of matrix,
piled up against pre-existing cliffs, recalling screes of modern
times, give almost certain evidence of the occurrence of land tracts
most probably of desert character, during part of the period of
accumulation of the materials of the Permian rocks. The fossil
evidence supports this view, and geologists are mostly agreed that the
Permian rocks of north-west Europe were accumulated in an area of
desert character, occupied in part by inland seas, though there is
much difference of opinion as to the extent of these seas, some
geologists holding that a number of isolated sheets of water were
necessary to produce the distribution and character of the
accumulations. It is still a vexed question with British geologists
how far the Pennine ridge stood up as land during the period, but
leaving this and other minor considerations out of account, it may be
noted that the similarity of deposits in the different areas, whether
we examine the order of succession, the lithological characters or the
included fossils, suggests communication between the water tracts of
different regions, though this communication need not have been more
than a series of straits, or comparatively narrow belts of water[96].
[Footnote 96: It should be mentioned that some writers have inferred
the evidence of glacial conditions over parts of the British area, on
account of the resemblance of some of the Permian breccias to recent
glacial deposits. The question is still _sub judice_. It is not
necessarily opposed to the existence of desert conditions, if the
mountains were sufficiently high, for the Wahsatch regions adjoining
the Basin Region of N. America have been glaciated.]
The extensive development of Permian and Triassic rocks with
terrestrial characters in the southern hemisphere also, and the
absence of newer deposits in many places, suggests that the land areas
of these times in that hemisphere have largely remained such ever
since, in which case, the Permo-Triassic series of movements produced
a marked direct effect upon our present continental areas, and at any
rate produced an indirect one upon the British land tracts.
The presence of anomalous deposits of Permian age over wide areas need
not be surprising, but it would be indeed remarkable if no ordinary
marine type of Permian rocks was known, and the researches of recent
years have proved that this type is extensively developed, in Eastern
Europe, Asia, and North America, where Permian rocks consisting of
limestones, with a greater or less admixture of mechanical deposits,
occur in some abundance. The studies of Waagen and others in India
have given us the farthest insight into the nature of these beds.
Below is a general classification taken from Waagen's work:--
Salt Range. Germany.
Base of Trias }
Unfossiliferous Shale and }
Sandstone } Passage Beds into Trias
Top Beds of Upper _Productus_ }
Limestone }
Cephalopoda Beds of Upper } Gypsum Beds
_Productus_ Limestone }
Middle Division of Upper }
_Productus_ Limestone } Zechstein (in restricted sense)
Lower Division of Upper }
_Productus_ Limestone }
Upper Division of Middle } Weissliegende and Kupferschiefer
_Productus_ Limestone }
Middle Division of Middle } Rothliegende.
_Productus_ Limestone }
It will be seen that in the Salt Range there is a complete passage
from the Permo-Carboniferous strata through the Permian into the
Trias, and the detailed work which has been carried out by Waagen and
others amongst the rocks of the Salt Range must make this, for the
present at all events, the type area for the marine development of the
strata of Permo-Carboniferous and Permian ages.
_The Permian flora and fauna._ The Permian flora presents some
difficulties. The flora of the Zechstein consists largely of ferns and
conifers, but that of the Rothliegende of Germany has been compared
with that of the Carboniferous, and if a true Permian flora of the
northern hemisphere has many forms of Carboniferous affinities, the
presence of the Glossopteris flora in Permo-Carboniferous rocks of
more southerly regions seems to imply its origin there and _slow_
migration northwards. It must be noted, however, that the Rothliegende
has been divided by some geologists into an upper and lower division,
of which the lower is actually referred to the Carboniferous system.
All that can be now said is, that our knowledge of the floras of
Permo-Carboniferous and Permian times is still incomplete, and that
the difficulties will no doubt be cleared up as the result of further
work.
The invertebrate fauna of the north-west European Permian deposits is
chiefly noticeable on account of the paucity of species, though
individuals are often abundant. The shells are also sometimes stunted
and occasionally distorted. These characters bear out the supposition
that the aqueous deposits were laid down in inland seas of Caspian
character and not in the open ocean. Polyzoa, brachiopods, and
lamellibranchs predominate, but other groups are found. The
vertebrates consist of forms of fish, amphibia and reptiles, and the
Permian rocks are the earliest strata in which the remains of true
Reptilia are known to occur with certainty. The Reptiles belong to the
orders Anomodontia (Theromora) and Rhynchocephalia, of which the
former is exclusively Permian and Triassic, while the latter is
abundant in the strata of those periods, but is represented at the
present day by the genus _Sphenodon_ of New Zealand. The Amphibia
belong to the order Labyrinthodontia which ranges from Carboniferous
to Lower Jurassic, but the members of the order are most abundant in
Permian and Triassic strata, and these periods may be spoken of as the
Periods of Labyrinthodonts.
A few words must be said of the fauna of the truly marine Permian
beds. It is much richer than that of the abnormal deposits of
north-western Europe, and its study is important as furnishing another
link between Palæozoic and Mesozoic life. Many Palæozoic genera pass
up into the Permian rocks, and, as will be ultimately seen, several
occur in those of the Triassic system, and one or two even in the
basal Jurassic strata, though Mesozoic forms predominate in the Lower
Jurassic Rocks, and there is a fairly equal admixture of forms usually
considered as Palæozoic and of those generally regarded as Mesozoic in
Triassic rocks, while the Palæozoic forms still predominate over the
Mesozoic in the Permian strata. Along with these characteristic
Palæozoic genera, it is interesting to find representatives of more
than one genus of the tribe of Ammonites, which is to take so
prominent a place in the fauna of the Mesozoic rocks, amongst the true
marine Permian sediments of India and other areas. The announcement of
the contemporaneity of ammonites with fossils regarded as exclusively
palæozoic was received with considerable doubt, but this
contemporaneity is now clearly established, and need not be regarded
as in any way anomalous.
With the deposition of the Permian rocks, Palæozoic time comes to an
end, but as already remarked there is no marked and sudden change to
characterise it. Had our classification been originally founded on
study of the Indian Rocks instead of those of Britain, and similar
terms adopted, the line of demarcation between Palæozoic and Mesozoic
rocks would probably have been drawn below the Permo-Carboniferous
deposits, and if it had been based on study of other areas, perhaps
elsewhere. The palæontological break is purely local, and it is of the
utmost importance that it should be recognised as such, and that it
should not be considered that division into Palæozoic and Mesozoic
implies some great and widespread change which occurred between the
times covered by the deposits of each of these great divisions[97].
[Footnote 97: The Permian Fossils of Britain are described by
Professor King in the Monographs of the Palæontographical Society (the
Brachiopods by Dr Davidson in the Monographs of the same Society). For
a general account of the marine type the student may consult the
second edition of Messrs Medlicott and Blanford's _Geology of India_.
For information concerning the Permian volcanic rocks see Sir A.
Geikie's _Ancient Volcanoes of Great Britain_.]
CHAPTER XXI.
THE TRIASSIC SYSTEM.
_Classification._ The term Triassic has been applied to these rocks on
account of the threefold division into which those of Germany
naturally fall. These three divisions are:--
Keuper,
Muschelkalk,
Bunter;
but above the Keuper beds we find a group of deposits of some
importance, which shew affinities with both Triassic and Jurassic
rocks, which may be looked upon as true passage beds, though they are
generally placed in the Triassic System. They are known as Rhætic or
locally in Britain as Penarth Beds. The Muschelkalk is usually
considered to be unrepresented in Britain, and accordingly the British
deposits may be, and are usually grouped as under:--
Rhætic or Penarth beds
Keuper { Keuper Marls
{ Keuper Sandstones
[Muschelkalk] absent
{ Upper Red and Mottled Sandstones
Bunter { Bunter Pebble Beds
{ Lower Red and Mottled Sandstones.
The threefold grouping has been applied more or less universally, but
when used outside the north-west European area, it loses its
significance, as the conditions which enable one to differentiate the
rocks of the three divisions were naturally only prevalent over a
limited area.
_Description of the strata._ The British Triassic rocks possess a
certain sameness as regards their general characters, consisting
mainly of mechanical sediments coloured red by peroxide of iron, with
occasional chemical precipitates of rock-salt and gypsum. They have a
wider distribution over Britain than have the Permian rocks, and the
lithological characters of the different subdivisions do not as a rule
vary to a remarkable degree when traced laterally. The differences in
detail in the characters of the various deposits are noteworthy, and
an explanation of the exact origin of some of these abnormal deposits
which will satisfy everyone is not yet forthcoming. Leaving the
details out of consideration for the moment, and looking at the
general aspect of the deposits, the prevalence of conditions generally
similar to those which existed over the British Isles in the preceding
Permian period is decidedly indicated by the nature of the strata,
though the continental conditions appear to have been more widely
established over our area, as shewn by the general absence of any
calcareous deposits resembling the Magnesian Limestone. We find
chemical precipitates, millet-seed sandstones, and scree-like breccias
in the British Triassic rocks as well as in those of Permian age, and
the paucity of a marine invertebrate fauna in the Triassic rocks of
Britain is even more apparent than in the Permian strata. It is only
at the extreme close of the Triassic period, during the deposition of
the rocks which are admitted on all hands to be of Rhætic age, that
we note the incoming of those marine conditions over our area, which
prevailed so extensively, with few local exceptions, during the
remainder of the Mesozoic and the early part of Tertiary times; the
Rhætic beds, in fact, mark the commencement of the third marine
period. Referring to the strata in further detail, we may proceed to
consider the character of the different subdivisions in the order of
their formation, commencing as usual with the oldest. The Bunter
deposits rest in places upon those of Permian age with an unconformity
at the junction, but as these unconformities occur frequently among
the British Triassic rocks, it is doubtful whether this unconformity
marks more than very local change of physical conditions. The lower
and upper divisions of the Bunter sandstone consist of false-bedded
red and variegated sandstones, and there is no great difficulty in
explaining their formation in desert areas with tracts of water, but
the great change which marks the appearance and disappearance of the
middle division, the Bunter pebble beds, requires some explanation,
for the contrast between the lithological characters of the rocks of
this division and those of the rocks appertaining to the preceding and
succeeding division is very marked. The matrix differs, but the main
difference is the abundance of pebbles, mostly of fairly uniform size,
well rounded, and largely consisting of liver-coloured quartzite. Much
difference of opinion exists as to the exact origin of these pebble
beds, and the source of the pebbles, but without entering into this
vexed question, it may be remarked that the agency of rivers has been
somewhat generally invoked to account for their transport, and the
conditions during their accumulation need not have been very different
from those which are now found in northern India where the torrential
rivers of the south side of the Himalayan chains debouch upon the
plain, and spread an abundant deposit of well-worn pebbles over the
finer silts which were previously laid down thereon.
The junction of the Bunter and Keuper beds requires a short notice. It
is usually if not always an unconformable one in Britain, and it is
generally assumed that the absence of the Muschelkalk of the Continent
is due to the presence of land undergoing denudation in Britain during
the time when the Muschelkalk was elsewhere deposited, though it is
quite possible that the Muschelkalk epoch is represented in Britain
not only by the time which elapsed when the unconformity was being
impressed on the rocks, but also during the true deposition of the
upper part of the Bunter beds, or the lower part of the Keuper, or
both.
The Keuper sandstones and marls contain a great development of
chemical deposits, of millet-seed sands, and of many other features
pointing to desert conditions, such as sun-cracks, tracks of animals
impressed upon a rapidly drying surface, and pseudomorphs of mud after
rock salt in the form of cubes and hopper-crystals; furthermore we
find the scree-like breccias at different horizons of the Keuper beds
where they abut against the old Mendip ridge composed largely of
mountain-limestone which furnished the fragments, as was the case with
the brockrams abutting against the Pennine ridge. It must be noted
that the chemical precipitates of Triassic age consist of the less
soluble substances dissolved in ocean water, namely, gypsum and rock
salt, whilst the more deliquescent potash and magnesia salts are not
represented in Britain.
Turning to these continental beds, we get evidence of a general
approach to open sea conditions as we pass away from Britain in a
south-easterly direction as roughly shewn in the following diagram
(fig. 22), where _B_ represents the Bunter beds, _M_ the Muschelkalk,
and _K_ the Keuper.
[Illustration: Fig. 22.]
It will be seen that the mechanical sediments gradually die out and
become replaced by calcareous material as one passes from Britain
towards Switzerland; the Muschelkalk is very thin in the east of
France and thickens out in Germany, while in Switzerland Keuper,
Muschelkalk and Bunter are alike largely represented by calcareous
deposits, and the mechanical deposits are chiefly argillaceous, the
only important sandstone being situated at the extreme base of the
Bunter series.
The marine development of the Triassic system is naturally the one
which is most widely spread, though full appreciation of its
importance has only taken place as the result of researches in distant
climes of recent years. It is found in southern Europe, in
Spitsbergen, in considerable tracts of Asia, including India, and
along the Pacific coast region of North America, and everywhere
possesses much the same characters.
It will be seen from the above remarks that the physical conditions
which prevailed in the continental area of Triassic times which is now
partly occupied by the British Isles are most closely represented by
those of the desert regions of central Asia, hemmed in by the
mountain ranges which intercept the vapour-laden winds of the oceans,
and cause them to precipitate the great bulk of their vapour on the
seaward slopes of the mountains, so that they blow over the deserts as
dry winds, causing the fall of any large amount of rain to be a rare
though by no means unknown event in the desert regions.
_Flora and Fauna of the Period._ The Triassic flora is essentially
similar to that of the higher Permian strata, though many of the
genera are different.
The invertebrate fauna of the British deposits is, as might be
expected, very poor until the beds of the Rhætic series are reached.
In the beds below the Rhætics, the principal invertebrate remains are
the tests of the crustacean genus _Estheria_, though a few obscure
lamellibranch shells have been recorded. The vertebrate fauna is of
great interest. A number of fishes have been found, the most
remarkable of which is the genus _Ceratodus_, occurring in the Rhætic
beds of Britain and lower Triassic strata of foreign countries. It is
closely related to the Barramunda of the Queensland rivers belonging
to the order Dipnoi. As in the Permian strata, abundance of
Labyrinthodont amphibians have been discovered, and the reptiles
belong to the orders Anomodontia and Rhynchocephalia. In the Rhætic
beds of Britain and in still lower Triassic beds abroad the orders
Ichthyopterygia and Sauropterygia (represented by _Ichthyosaurus_ and
_Plesiosaurus_) are found.
The Triassic rocks also yield the earliest known mammals, the best
known, _Microlestes_, occurring in the Triassic rocks of Britain and
the Continent. These mammals are now placed in a subclass Metatheria
of the order Monotremata.
The marine invertebrate fauna of the normal Triassic rocks presents
some points of considerable interest. As already remarked, the fauna
may be looked upon as a passage fauna between that of Palæozoic and
that of Mesozoic times, the number of Palæozoic forms which pass into
the Trias being approximately comparable with those which appear here
and range upwards into higher Mesozoic strata. This may be well seen
by examining the table given in Chapter XXI. of the Second Edition of
Sir Charles Lyell's _Student's Elements of Geology_, in which three
columns shew the genera of Mollusca common to older rocks, those
characteristic of the Trias, and those common to newer rocks. Amongst
the first are _Orthoceras_, _Bactrites_, _Loxonema_, _Murchisonia_,
and _Euomphalus_, in the second column are _Ceratites_, _Halobia_
(_Daonella_), _Koninckina_, and _Myophoria_, and in the third,
Ammonites, _Cerithium_, _Opis_, _Plicatula_ and _Thecidium_[98].
[Footnote 98: It has been seen that some of the Ammonites appear
earlier, namely, in Permian strata. _Myophoria_ is extremely abundant
in the Trias, but ranges into newer strata.]
The Ammonites are largely utilised in the case of the Mesozoic strata
for separation of these strata into zones, each zone being
characterised by some species of Ammonite, and the researches of
Mojsisovics have proved that this zonal subdivision, long adopted for
Jurassic rocks, is also applicable to those of Triassic age[99]. He
gives the following table of the classification of the Triassic rocks
of the Mediterranean Province, which is reproduced, as it is founded
upon Palæontological evidence, and will probably be widely adopted.
[Footnote 99: von Mojsisovics, Dr E., "Faunistische Ergebnisse aus der
Untersuchung der Ammoneen-faunen der Mediterranen Trias." _Abhandl.
der k. k. Geologisch. Reichsanstalt_, VI. Band 2 Abtheilung. Vienna,
1893.]
Series Zonal Divisions
--------------+-------------------+--------------------------------------
Rhætic | | 1. Zone of _Avicula Contorta_
--------------+-------------------+--------------------------------------
| | 2. Zone of _Sirenites Argonautae_
| Upper Juvavic | 3. Zone of _Pinnacoceras
| | Metternichi_
Juvavic | Middle Juvavic | 4. Zone of _Cyrtopleurites
| | bicrenatus_
| | 5. Zone of _Cladiscites ruber_
| Lower Juvavic | 6. Zone of _Sagenites Giebeli_
--------------+-------------------+--------------------------------------
| Upper Carnic | 7. Zone of _Tropites subbullatus_
Carnic | Middle Carnic | 8. Zone of _Trachyceras Aonoides_
| Lower Carnic | 9. Zone of _Trachyceras Aon_
--------------+-------------------+--------------------------------------
| Upper Noric | 10. Zone of _Protrachyceras
Noric | | Archelaus_
| Lower Noric | 11. Zone of _Protrachyceras Curionii_
--------------+-------------------+--------------------------------------
| Upper Muschelkalk | 12. Zone of _Ceratiles trinodosus_
Muschelkalk | |
| Lower Muschelkalk | 13. Zone of _Ceratiles binodosus_
--------------+-------------------+--------------------------------------
Buntsandstein | Werfener Schichten| 14. Zone of _Tirolites Cassianus_
--------------+-------------------+--------------------------------------
CHAPTER XXII.
THE JURASSIC SYSTEM.
The Jurassic rocks were formerly separated on account of differences
of lithological character into Oolites and Lias, but it was apparent
that the Oolites were more important than the Lias, and a fourfold
division was made into:--
Upper or Portland Oolites }
Middle or Oxford Oolites } = Malm
Lower or Bath Oolites = Dogger
Lias.
The Lias strata have also been spoken of as the Black Jura, the Lower
Oolites and part of the Oxford Oolites as Brown Jura, and the rest of
the Oxford Oolites with the Portland Oolites as White Jura.
As the outcome of a detailed study of the faunas of the Jurassic
rocks, a further subdivision has been made, partly based upon the
original British series, but the divisions are defined with greater
accuracy, so that they are applicable over wider areas. They are as
follows:--
{ Purbeckian
Upper Oolites { Portlandian
{ Kimmeridgian
{ Corallian
Middle Oolites { Oxfordian
{ Callovian
Lower Oolites { Bathonian
{ Bajocian
{ Toarcian
Lias { Liassian
{ Sinemurian.
Many of these series have been still farther subdivided into smaller
stages, and the whole differentiated into a number of zones
characterised by different forms of Ammonites. Dr E. von Mojsisovics
gives thirty-two Ammonite zones, of which fourteen occur in the Lias,
eight in the Lower Oolites, six in the Middle Oolites, and four in the
Upper Oolites.
_Characters of the strata._ The whole of the Jurassic rocks and also
those of Lower Cretaceous age may be regarded as having been deposited
during the first shallow water phase of the third marine period, but
this shallow water phase is represented by strata which are varied
owing to numerous marine changes resulting in the production of land
at times, and estuarine conditions, shallow water, marine conditions,
and somewhat deeper sea conditions respectively at other times, and
accordingly the strata of the British Isles vary greatly when traced
laterally. That the uplifts of the Permo-Triassic periods produced
some effect on the nature and distribution of the Jurassic rocks is
certain, but it is not quite clear how far the ridges produced by
these uplifts were submerged and denuded during the deposition of the
main portion of the Jurassic strata.
Viewed broadly, the Jurassic rocks of Britain may be regarded as
consisting of three great clay deposits, the Lias, Oxford and
Kimmeridge Clays, alternating with the deposits of variable
lithological characters, which compose the Bajocian, Bathonian,
Corallian, Portlandian and Purbeckian subdivisions. This essentially
argillaceous character of a large part of the deposits of Jurassic age
is often overlooked, as, owing to their sameness and the comparative
paucity of organisms constituting the faunas in the clays, their
description in text-books can be given at much shorter length than
that of the more variable and highly fossiliferous deposits which
separate the clays. The following figure (Fig. 23) roughly represents
the nature of the different divisions of the rocks of this system when
traced across England from south-west to north-east.
[Illustration: Fig. 23.
Vertical scale: 1 in. = about 1000 feet.]
It will be seen that the greatest variations in lithological character
occur in the Bathonian and Bajocian beds, and it will be of interest
to give some account of the principal variations and to attempt to
account for them. In so doing it will be convenient to consider the
four major divisions of the Jurassic rocks separately, and to enter
into particulars concerning the local classification applied to the
rocks of these divisions.
_The Lias._ The British Lias deposits are divided into the Lower Lias,
the Marlstone, and the Upper Lias corresponding in general terms only
with the Sinemurian, Liassian, and Toarcian. The Marlstone is
separated from the Upper and Lower Lias on account of the greater
percentage of carbonate of lime which it contains, so that the bands
of argillaceous limestone are much more marked in the Marlstone than
in the upper and lower divisions, which consist chiefly of clay. The
three divisions possess very much the same characters throughout the
country, though the presence of the Mendip ridge and its continuation
beneath London is marked by the attenuation of this and succeeding
strata, and by the conglomeratic character of some of the Liassic
strata where they abut against it. The British Lias, as a whole, seems
to have been deposited in a fairly shallow sea at no great distance
from the land. It passes down conformably into the Rhætic beds, indeed
the zone of Ammonites (_Aegoceras_) _planorbis_, referred by British
geologists to the Lower Lias is included by some continental writers
with the Rhætic beds, and the plane of demarcation here as in other
cases is conventional.
_The Lower Oolites._ Of all the British strata, these perhaps cause
most trouble to the learner, on account of the different nomenclature
applied to the rocks in different parts of England, and the rapid
variations in lithological character, when the beds are traced
laterally. The following divisions are usually adopted for the beds of
the south-western counties where the most marked marine development
occurs:--
Cornbrash,
Forest Marble,
Great Oolite (with Bradford Clay),
Fuller's Earth,
Inferior Oolite.
Of these divisions, the uppermost one, the Cornbrash, though thin,
retains its characters with great constancy across the island. Of the
others the Forest Marble may be looked upon as a local development of
the upper portion of the Great Oolite, and the Fuller's Earth is a
local deposit, so that the Inferior Oolite and Great Oolite constitute
the important divisions of the Lower Oolites. The variations in the
characters of the rocks may be best shown in tabular form.
-----------------+------------------+-------------------+-----------------
Gloucestershire, | South | N. |
&c. | Northamptonshire | Northamptonshire | Yorkshire
| | and Lincoln |
-----------------+------------------+-------------------+-----------------
Cornbrash | Cornbrash | Cornbrash | Cornbrash
-----------------+------------------+-------------------+-----------------
Great Oolite | Great Oolite | Great Oolite Clay |
| (Upper part) | Great Oolite | Upper
| | Limestone |
| | Upper | Estuarine
| Northamptonshire | Estuarine |
............... | ................ | ................. | ................
| | Series | Series
| | Lincolnshire | Scarbro'
| | Limestone | Limestone
| Sands | | Middle Estuarine
| | | Series
Inferior Oolite | | Lower Estuarine | Millepore Oolite
| | Series |
| | | Lower Estuarine
| | | Series
-----------------+------------------+-------------------+-----------------
Upper Lias | Upper Lias | Upper Lias | Upper Lias
-----------------+------------------+-------------------+-----------------
The dotted line shows roughly the division between Bathonian
and Bajocian.
The changes may be explained very simply if we leave out of account
for the moment the development of Lincolnshire Limestone, with its
equivalent the Scarbro' Limestone, and the Millepore series. The beds
in Gloucestershire and other south-western counties are essentially
marine; whilst in Northamptonshire and Lincolnshire estuarine
conditions set in after the deposition of the Upper Lias, and
continued throughout the deposition of the Bajocian and Lower
Bathonian beds, being replaced by marine conditions during the
formation of the Upper Bathonian strata, and still further north in
Yorkshire the estuarine conditions generally prevailed throughout
Bajocian and Bathonian times. These changes point to the existence of
land towards the north. The general simplicity is modified by
temporary prevalence of marine conditions twice over (during the
deposition of the Millepore Oolite and the Scarbro' Limestone) in
Yorkshire, and once (during the deposition of the Lincolnshire
Limestone) in Lincolnshire.
Certain local deposits have not been noticed, but two of them merit
brief reference. At the base of the Great Oolite of Oxfordshire is an
estuarine deposit of finely laminated mechanical sediment mixed with
calcareous matter known as the Stonesfield Slate, especially
interesting on account of its fossils, while a bed with similar
lithological characters but with a different fauna occurring at the
base of the Lincolnshire Limestone (of Bajocian age) is termed the
Collyweston Slate. Neither of these deposits is a slate in the true
sense of the word, as they have not been affected by cleavage
subsequently to their accumulation, but each has been somewhat
extensively used for roofing purposes.
The Middle Oolites are much less complicated though considerable
variations arise with respect to the Corallian Rocks. The Oxfordian
with Callovian consist chiefly of clay, though the Callovian of the
south of England is represented by calcareous sandstone, with a
peculiar fauna which seems to be represented in the lower part of the
Oxford Clay further north, though this Callovian fauna has not been
everywhere recognised.
The Corallian of the southern counties consists of limestones with
calcareous grits, the limestones being often largely composed of the
remains of reef-building corals, and a similar development of the
rocks of this series is found in Yorkshire, while a local development
of the same character is found at Upware in Cambridgeshire, though in
the other parts of the Fenland counties the Corallian is represented
by an argillaceous deposit with Corallian fossils known as the
Ampthill Clay.
The Upper Oolites have a tolerably constant base, the Kimmeridge Clay,
usually consisting of laminated bituminous argillaceous material, but
the Portlandian and Purbeckian divisions vary greatly, and are only
locally developed, though their absence in some parts of central
England is no doubt due to unconformity.
The Portlandian rocks of the south of England consist of limestones
and sandstones which pass further northward into shallower water
mechanical deposits often charged with iron hydrate, and the beds
disappear in Oxfordshire. The Purbeckian rocks of the south are also
limited as regards area of exposure: they consist of estuarine
deposits with some terrestrial accumulations of the nature of old
surface soils. Representations of the Portlandian and Purbeckian beds
are found in Lincolnshire and Yorkshire, as arenaceous deposits in the
former county and argillaceous ones in the latter. Both are marine
deposits of a northern type, developed elsewhere in northern European
and circumpolar regions, and in these counties we find a complete
passage from the Jurassic rocks through the Cretaceous rocks, but the
exact lines of demarcation between the different series of the passage
beds are difficult to define.
The foreign Jurassic rocks of Europe and of some parts of Asia
strongly resemble in general characters those which have been
described above as occurring in Britain. One of the most remarkable
features of the Jurassic rocks as a whole, is the absence of the Lias
over wide areas, the continental period which in Britain existed in
Permo-Triassic times is elsewhere frequently replaced by one of
Liassic age.
The Jurassic and Cretaceous rocks are of interest on account of the
evidence which they supply as to the existence of climatic zones in
these periods, which run fairly parallel with those at present
existing. The late Dr Neumayr in a paper already cited divides the
world during later Mesozoic times into four distinct climatic zones,
equatorial, north and south temperate and boreal zones (the
corresponding austral zone is not known owing no doubt to the
extensive sea of South Polar regions and our general ignorance of its
lands). In Europe the Mediterranean Province belongs to the equatorial
zone, the Middle European to the North temperate zone, and the Russian
or Boreal to the Boreal zone. The last-named is marked partly by
negative characters, the absence of certain Ammonite-genera and of
coral reefs being noticeable, whilst the lamellibranch _Aucella_ is
very frequent. In the North temperate zone, certain Ammonite genera as
_Aspidoceras_ and _Oppelia_ are abundant and there are also extensive
coral-reefs. The Equatorial zone is marked by the Ammonite-genera
_Phylloceras_ and _Lytoceras_ and by the _Diphya_ group of
_Terebratulæ_. It is of special interest to note that the fauna of the
South temperate bears closer relationship to that of the North
temperate than to that of the intermediate Equatorial zone.
_Jurassic floras and faunas._ The Jurassic flora is very similar in
its characters to that of the Lower Cretaceous rocks, and the two
taken together afford a decided contrast with that of later Palæozoic
times, and also with that which succeeds them in the Upper Cretaceous
rocks, which bears a marked resemblance to the existing flora. Cycads
predominate, accompanied by conifers, and a fair number of ferns and
Equisetaceæ.
The Jurassic fauna is specially noteworthy on account of the character
of the vertebrata, but some notice of the invertebrates must also be
taken. The abundance of corals in the Temperate zones has already been
pointed out, but the mollusca form the bulk of the invertebrate fauna,
lamellibranchs, gastropods and cephalopods being all abundant; of the
last-named the ammonites and belemnites contribute most largely. The
vertebrates include remains of fishes, amphibia, reptiles, birds and
mammals. The Jurassic reptilia furnish representatives of some modern
orders as the Chelonia and Crocodilia, but the most important orders
are essentially characteristic of later Mesozoic times and their
representatives abound in the Jurassic strata. These are the
Sauropterygia (including the Plesiosaurs), the Ichthyopterygia
(including the Ichthyosaurs), the Dinosauria, and the Pterosauria
commonly known as Pterodactyls. No birds have hitherto been discovered
in the British Jurassic rocks, but the Solenhofen Slate of Bavaria (of
Kimmeridgian age) has furnished the celebrated _Archæopteryx
macrura_, which is not only placed in a family but also in an order by
itself, the order Saururæ. Many remains of mammals have been extracted
from the estuarine deposits of Stonesfield, and the old surface soils
of the Purbeckian beds; representatives of the Monotremata are
furnished by the _Plagiaulacidæ_ and _Tritylodontidæ_, the former
family containing the genus _Plagiaulax_ of the Purbeck Beds and the
latter, _Stereognathus_ of the Stonesfield slate. The Marsupialia are
represented by the _Amphitheridæ_, _Spalacotheridæ_ and
_Triconodontidæ_. Some forms have been referred to the Insectivora,
but there is still disagreement concerning the correctness of this
reference.
Before dismissing the subject of the Jurassic fossils, attention may
be called to a feature which has been frequently commented upon,
namely, the general resemblance of the flora and fauna of Jurassic
times to the modern Australian fauna and flora. The explanation which
has been offered to account for this resemblance has been given in a
preceding chapter, where it was stated that Mr A. R. Wallace
considers, after review of the geological and biological evidence,
that Australia was severed from the adjoining continental lands in
Mesozoic times, and that the higher forms of life which on the larger
continents have replaced the earlier and lower forms have not
succeeded in obtaining a footing in Australia, which therefore
furnishes us with a local survival of a once widespread fauna. In
connection with this matter the actual existence of the genus
_Trigonia_ (a form peculiarly abundant in Jurassic strata and
characteristic of Mesozoic strata in Britain) in the Australian sea is
of considerable interest.[100]
[Footnote 100: A good account of the British Jurassic rocks will be
found in Mr H. B. Woodward's Memoir on "The Jurassic Rocks of
Britain." _Mem. Geol. Survey_, 1893--.]
CHAPTER XXIII.
THE CRETACEOUS SYSTEM.
_Classification._ The rocks of the Cretaceous system are conveniently
divided into Upper and Lower Cretaceous. The following classification
has been widely used for the British deposits, and is founded on
lithological characters:
{ Upper Chalk with flints }
{ Middle Chalk with few flints } Chalk
Upper { Lower Chalk without flints }
Cretaceous { Chalk Marl }
{ Upper Greensand
{ Gault
{ Lower Greensand
Lower { Wealden
Cretaceous { Hastings sands
As the result of examination of the faunas, a more generally
applicable classification has been established and is now largely
adopted. It is as follows:
Danian }
Senonian } Upper Cretaceous
Turonian }
Cenomanian }
Albian }
Aptian }Lower Cretaceous.
Neocomian }
In this classification the Neocomian practically represents the
Wealden and Hastings beds, the Aptian the Lower Greensand and the
Albian the Gault, placed according to this classification in the Lower
Cretaceous, while the Upper divisions represent the strata above the
Gault, consisting essentially of Chalk in England.
_Description of the Strata._
(i) _The Neocomian and Aptian Beds._ In the south of England the Lower
Cretaceous beds succeed the Jurassic rocks with little or no break,
and the type of the lower beds is similar to that of the beds
deposited during the Purbeck age, consisting of estuarine deposits of
variable characters, chiefly arenaceous below (the Hastings sands) and
argillaceous above (the Wealden series), though impure limestones are
found, largely composed of the shells of the freshwater _Paludina_,
and much ironstone is developed in places. At the close of Neocomian
times, the freshwater conditions in southern England were replaced by
marine conditions and the Lower Greensand strata with their marine
fauna were deposited in the Aptian sea. The Neocomian and Aptian beds
thin out westward, and much more rapidly to the northward, so that
both divisions disappear against the now buried ridge which forms a
continuation of the Mendip axis. North of this they appear in another
form. At first the highest Aptian beds alone are developed as shore
deposits. Passing into Norfolk lower beds come in until in
Lincolnshire we get a complete development of the Neocomian and Aptian
beds with a marine facies, though of fairly shallow water character,
whilst in Yorkshire the two divisions are represented by a deeper
water clay, forming the Upper portion of the Speeton series. There is
a consensus of opinion in favour of the Neocomian beds of southern
Britain having been laid down in an estuary of a river flowing from
the west over a continent now destroyed. To the north of this river
stood the London ridge of the Palæozoic rocks, the northern borders of
which formed the coast line off which were deposited the sediments of
Neocomian and Aptian ages which occur in northern England. Before the
deposition of the Albian beds a considerable upheaval of some parts of
Britain occurred, and an unconformity separates the higher Cretaceous
beds from older strata of Cretaceous and Jurassic ages, thus
complicating the major phases by local changes in the characters of
the strata.
(ii) _The Albian and higher Cretaceous Beds._ The commencement of the
deep-water phase of the third marine period may be said to occur in
Albian times in Britain, reaching its maximum during the deposition of
the chalk. The existence of a deeper sea towards the north of England
is indicated by the characters of the Albian and newer strata. The
Albian beds of gault consist of a stiff clay in southern England,
replaced by coarser mechanical sediments towards the west. As one
passes north from the London ridge (which exerted its influence in
Albian times, after which it was finally buried in sediment) the gault
thins out, and becomes gradually replaced by calcareous deposit when
it is known as the red chalk which replaces the gault in northern
Norfolk, Lincolnshire and Yorkshire.
A local unconformity separating the chalk and gault in parts of East
Anglia points to another local uplift with its accompanying
complications in the characters of the strata. After the uplift had
ceased, general depression must have occurred, and the various
divisions of the chalk were accumulated in a fairly open sea, though,
for reasons to be given presently, this was probably of no great
lateral extent, save when united with the open ocean, probably in a
manner similar to the connexion between the Gulf of Mexico and the
Atlantic.
The general variations in the lithological characters of the various
members of the Cretaceous system will probably be rendered clearer by
reference to the accompanying diagram (fig. 24) representing the
variations when traced across England from south to north[101].
[Footnote 101: For information concerning the British Cretaceous beds,
see Topley and Foster, "Geology of the Weald," _Mem. Geol. Survey_,
1875; Bristow and Strahan, "Geology of the Isle of Wight," _Mem. Geol.
Survey_, 1889; Lamplugh, "On the Speeton Clay," _Q. J. G. S._, vol.
XLV. p. 575, and "The Speeton Series in Yorkshire and Lincolnshire,"
_ibid._, vol. LII. p. 179; Barrois "Recherches sur le Terrain Crétacé
supérieur de l'Angleterre et d'Irlande," Lille, 1876; and various
papers by Messrs Hill and Jukes-Browne, in the _Quarterly Journal of
the Geological Society_ and _Geological Magazine_ of recent years. For
the Scotch deposits consult a paper by Prof. Judd, _Q. J. G. S._, vol.
XXXIV. p. 736, and for those of Ireland, see Hume, _Q. J. G. S._, vol.
LII. p. 540.]
[Illustration: Fig. 24.
Ch. Chalk.
Al. Albian.
Ap. Aptian.
N. Neocomian.
J. Jurassic.
]
The clue to the physical geography of Britain during Cretaceous times
is furnished to a considerable extent by study of the foreign
deposits. In Northern Europe the Cretaceous beds of England are met
with in Northern France, and there the characters are generally
speaking similar to those of our British deposits. In Germany
shallower water conditions prevailed, the lower beds gradually
disappear, and the upper beds are replaced by mechanical sediments of
various degrees of coarseness, becoming on the whole coarser, as one
travels eastward, so that in Saxony the chalk is partly replaced by
arenaceous deposits (the 'Quader' sandstones) which are responsible
for the remarkable scenery of the Elbe district above Dresden. In
passing northwards, indications of similar change are noted in the
deposits of Denmark and Scania, whilst to the south, we get a complete
change in the character of the rocks, after crossing the Loire in
France, and a similar change is observable in districts lying further
east. Furthermore, as will be noted more fully in a subsequent
paragraph, the character of the Upper Cretaceous flora indicates the
existence of a large tract of land lying to the north and north-west
of Europe, so that it would appear that the Cretaceous rocks of
Northern Europe were deposited in a gulf-like expansion of a western
ocean, bounded on the north by Scandinavia, on the west by eastern
Germany, and on the south by a ridge running eastward from the mouth
of the Loire[102]. We may speak of this gulf as the Chalk gulf. To the
south of the presumed ridge the character of the strata alters, and
also that of the included organisms. This southern type of Cretaceous
rocks is one which is very widely spread, being found in Europe south
of the Loire, and of the Alps, and in Greece and Turkey, while it
also occurs in the northern parts of Africa. The beds of this type are
traceable through Asia Minor into India and to the shores of the
Indian Ocean, indicating the existence of a widespread Cretaceous
ocean, which is sometimes spoken of as the Hippurite-limestone sea,
for reasons which will eventually appear. The deposits are largely
formed of hard limestone which is very different in its character from
the soft chalk of the northern gulf.
[Footnote 102: The reader will find the existence of this gulf
maintained and supported by a considerable mass of detail in Mr A. R.
Wallace's _Island Life_.]
The climatic conditions which prevailed during Cretaceous times were
apparently similar in most respects to those of the preceding Jurassic
period, and as already stated the climatic zones which Neumayr defined
for Jurassic times are also maintained by him to have existed during
the Cretaceous period. The existence of cold has sometimes been
inferred from the presence of large foreign blocks in the chalk,
especially at its base, but if these are due to the transport, they
might well be caused by masses of floating ice, which are often found
at considerable distances from the coast in temperate regions after
the break-up of the frost which succeeds an unusually hard winter. The
flora and fauna are not suggestive of severe conditions.
_The Cretaceous flora and fauna._ It has been noted in the last
chapter that the gymnospermous flora of the Jurassic period, in which
cycads form a considerable percentage of the whole flora, was
prevalent in Lower Cretaceous times. In the Upper Cretaceous rocks
this flora is replaced by one which consists to a large extent of
dicotyledonous angiosperms. These are found in the Upper Cretaceous
rocks of Europe and North America, and as the researches of botanists
indicate their origin in circumpolar regions, their arrival in Europe
is an additional argument in favour of the existence of an extensive
northern continent, sending a prolongation to the southward in eastern
Europe.
The invertebrate fauna bears considerable resemblance to that of
Jurassic times, and many of the dominant Jurassic genera are also
found in Cretaceous rocks. A most interesting feature is connected
with the character and geographical distribution of the Ammonites. In
Europe they are almost exclusively confined to the deposits of the
northern gulf, and before their final disappearance they undergo many
changes of form. We find the discoid spiral shells of earlier times,
but these are accompanied by shells which are straight, curved,
boat-shaped, and coiled into various helicoid spirals, sometimes
having the whorls in contact, while at other times they are separate.
In the chalk of Britain gastropods are on the whole rare, and this
fact serves to emphasize the palæontological break which occurs
between the Cretaceous and Tertiary rocks; but when conditions were
favourable, as during the deposition of some of the strata of the
Middle Chalk, gastropods are abundant, and some are related to
Tertiary genera, so that we may assume that the palæontological break
alluded to is exaggerated by the difference of conditions which
prevailed during the deposition of the earliest Tertiary and latest
Cretaceous sediments.
In the Cretaceous deposits of the southern sea, where the Ammonite
tribe is almost unknown, the remarkable family of the lamellibranchs
known as the Hippuritidæ furnish the dominant invertebrates of the
period, and the representatives of this family are exceedingly scarce
amongst the Cretaceous strata of the northern gulf, though they are
found on two or three horizons.
Of vertebrates, the most interesting are the reptiles. The families
which predominate in Jurassic times have many representatives amongst
the Cretaceous strata also, but the order Squamata is represented by
the sub-order Pythonomorpha, which is characteristic of the Cretaceous
rocks. The best known representative is the gigantic _Mosasaurus_.
Lastly, we have the remarkable toothed birds or Odontornithes, now
placed in different orders, the genus _Hesperornis_ being the only
representative of the sub-order Odontolcæ of the Ratitæ, whilst
_Ichthyornis_ and allied forms are placed in the sub-order Odontormæ
of the Carinatæ.
CHAPTER XXIV.
THE EOCENE ROCKS.
_Classification._ The Eocene Beds of the south of England have been
subdivided according to the variations in their lithological
characters, and the subdivisions have received local names. The
following classification is generally adopted, though the different
subdivisions are by no means of equal value:
Upper Eocene { Upper Bagshot Beds
{ Barton Beds
Middle Eocene Bracklesham Beds
{ Lower Bagshot Beds
{ London Clay[103]
Lower Eocene { Oldhaven Beds } Lower London
{ Woolwich and Reading Beds } Tertiary Strata
{ Thanet Sands }
[Footnote 103: Some writers place the London Clay in the Middle
Eocene.]
The deposits vary greatly when traced abroad, and the exact
equivalents of the minor subdivisions of the British rocks can seldom
be ascertained at any distance from England, though the division into
Upper, Middle, and Lower Eocene can be made over wide areas.
_Description of the strata._ The character of the strata of Europe and
Asia indicates the persistence of the northern gulf and southern
ocean of Cretaceous times in Eocene times also, though the area of
each had shrunk in the meantime, owing to the physiographical changes
which occurred at the end of Cretaceous times, giving rise to more
extended land areas, and producing a shallow water phase over wide
extents of ocean,--the final shallow water phase of the third and last
great marine period of the British area. It is difficult to ascertain
the exact importance of the physical break between Cretaceous and
Eocene rocks in the south-east of England, owing to the subterranean
solution of the upper part of the chalk, subsequently to the
deposition of the Eocene strata, but the contraction of the Cretaceous
gulf is shown in several ways, one of the most significant being the
distribution of Cretaceous and Eocene rocks in the south-west of
England. The existence of an outlier of Cretaceous rock at Buckland
Brewer in North Devon, only three miles from the Atlantic Ocean,
indicates the former extension westward of the Upper Cretaceous beds,
while the occurrence of an outlier of Eocene rocks at Bovey Tracey in
South Devon, resting not on Cretaceous but on Palæozoic rocks, shows
that there was an uplift after the deposition of the Cretaceous rocks
and before the Eocene rocks were deposited there, and that during the
period of uplift the Cretaceous rocks were removed.
Owing to these physical changes, the Eocene rocks of Britain are
mainly mechanical sediments, some, as the Oldhaven beds, being
composed of coarse pebbles over a fairly wide district, while some of
the earlier Eocene rocks are estuarine or fluvio-marine.
The Eocene rocks of Britain occur in four areas, namely, the London
Basin, the Hampshire Basin, the Bovey Tracey outlier, and the
north-east of Ireland and western Isles of Scotland. The deposits of
the three southern areas may be considered together, and give general
indications of an approach to land when passing westward. The Lower
London Tertiary strata are fluvio-marine at the east end of the London
Basin; they become shallower water deposits when traced westward, and
begin to disappear. The London Clay is an estuarine deposit, which is
generally supposed to have been laid down at the mouth of a large
river flowing from the west. It is absent in the Bovey Tracey outlier.
Local disturbances caused the existence of a shallow water region in
the east during the deposition of the Middle and Upper Eocene
deposits, and accordingly the well-marked marine deposits which form
the representatives of these divisions in Hampshire are replaced by
the Bagshot beds of the London Basin, consisting chiefly of coarse
mechanical sediments with a poor marine fauna, but even in the west
shallow water prevailed at times during the accumulation of various
plant-bearing strata. The Middle Eocene beds only are found in the
Bovey Tracey outlier, though the Upper Eocene beds may originally have
been laid down in that area, and subsequently denuded.
The fourth area displays a very different succession of Eocene strata,
and one of extreme interest. Mechanical sediments and plant-bearing
clays and lignites alternate with a vast accumulation of basaltic
lavas, indicating the outbreak of the volcanic forces in the British
area, after a period of quiescence which lasted through the greater
part of Mesozoic times. The region in which these lavas were poured
out was probably a land area during the greater part of the period of
volcanic activity, but the horizontal lie of the lava flows and their
wide extent indicate the existence of a flat tract of country,
gradually raised into a plateau by the accumulation of sheet over
sheet of basalt. How far this plateau extended it is impossible to
say. The distribution of the lavas at the present day is somewhat
limited in our isles, but there is no sign of dying out at the present
margins of the accumulations, and they have probably escaped
denudation in these regions, as maintained by Professor Judd, on
account of the faults which have depressed them, while the portions
which were not depressed have been removed by denudation. Two views as
to the origin of the lavas have been put forward: according to Prof.
Judd, they were poured forth from gigantic volcanoes, while Sir A.
Geikie maintains that they represent portions of massive or fissure
eruptions, the molten rock having welled out from great cracks in the
earth, which are now filled by once molten rock in the form of dykes.
As these dykes extend far away from the present volcanic plateau, one
actually extending to the Yorkshire coast, we may well believe,
whatever was the origin of the sheets of lava, that they were formerly
spread far away from their present terminations[104]. Without entering
here into a discussion of the exact nature of extrusion of these
igneous sheets, it will suffice to say that all the evidence points to
the formation of extensive plateaux, which must have presented a
fairly uniform surface, similar to that which is still found
characterising the volcanic districts of the western territories of
North America.
[Footnote 104: Prof. Judd's views will be found in a series of papers
by him on the "Secondary Rocks of Scotland," _Quart. Journ. Geol.
Soc._, vol. XXIX. p. 95, XXX. p. 220, XXXIV. p. 660, while Sir A.
Geikie's explanation is advanced in a paper in the _Transactions of
the Royal Society of Edinburgh_, vol. XXXV.; see also the same
author's _Ancient Volcanoes of Great Britain_.]
The Eocene rocks of the north-west of Europe possess characters very
similar to those of the south of England, and there are indications
that the northern gulf had diminished in extent towards the east as
well as towards the west.
Passing to southern Europe, Central Asia and northern Africa, we find
the conditions of Cretaceous times reproduced, and an extensive series
of marine deposits extends very widely over these regions, the most
persistent deposit being a mass of limestone of Middle Eocene age,
which is almost entirely composed of the tests of Nummulites, whence
the development is known as the Nummulitic Limestone facies, and we
may speak of the ocean as the Nummulitic Limestone Sea. The incoming
of shallow water conditions marked by accumulation of coarse
mechanical sediments towards the end of the Eocene period in some
parts of the southern European area indicates the setting in, even
then, of those continental conditions which culminated during the
Miocene period.
In North America we get similar evidence of the contractions of the
oceans which in Mesozoic times occupied large expanses of our present
continents.
The climatic conditions of Eocene times have been noticed in passing
in chapter IX., and evidence was given to prove the prevalence of a
warmer climate over the British area than that which now exists. A
study of the floras of various parts of the northern hemisphere
suggests that climatic zones, whose lines of demarcation ran
practically parallel with the Equator, existed in Eocene times also,
though further information upon this subject is desirable.
_The Eocene flora and fauna._ The flora of prevalent dicotyledonous
angiosperms, which appeared in Upper Cretaceous times, also marks the
Eocene and later deposits, but a study of the floras indicates that
the differentiation which now marks off the floras of different areas
from one another had not occurred to so great an extent as at the
present time. The existence of a rich flora in the Eocene beds of
circumpolar regions in the northern hemisphere should be noted, though
perhaps its importance has been somewhat exaggerated.
The invertebrate fauna shows an approximation to that of the present
day. The remarkable ammonite fauna of Mesozoic times has disappeared,
and gastropods and lamellibranchs predominate, many of the forms
belonging to existing genera, though very rarely to existing species.
The Nummulites are the most characteristic Eocene fossils, and the
period may be spoken of as the Nummulitic Period, though it is now
known that Nummulites are not confined to the Eocene strata.
The vertebrate fauna is very noteworthy. The fishes and reptiles are
closely related to existing forms, and the orders of reptiles which
predominated in Mesozoic times have completely disappeared. But the
mammals are the most interesting vertebrates of the Eocene period.
Instead of the lowly organised forms of Mesozoic times, we find
representatives of many orders, including the highest, the Primates.
The generalised forms which serve as links between groups which are
now separated to a considerable extent are of particular importance.
They have been detected in Eocene rocks of various regions, though the
most complete series have been obtained from the Eocene rocks of North
America and made known to us through the numerous memoirs of
Professors Cope and Marsh[105].
[Footnote 105: The Eocene floras of Britain are described by Mr J.
Starkie Gardner and Baron von Ettingshausen in the _Monographs of the
Palæontographical Society_; other Monographs of the same Society
contain an account of the Eocene Mollusca by Mr F. E. Edwards and Mr
S. V. Wood. An idea of the generalised forms of Mammalia may be
obtained by perusal of that portion of Nicholson and Lydekker's
_Manual of Palæontology_ in which the latter author treats of the
Mammalia, and in this connexion the reader will do well to read Prof.
Huxley's "Lecture on Fossil Horses," reprinted in his _American
Addresses_.]
CHAPTER XXV.
THE OLIGOCENE AND MIOCENE PERIODS.
(i) _The Oligocene Beds._
_Classification._ The Oligocene Beds of Britain are classified as
follows:--
Upper Wanting
Middle Hempstead Beds
{ Bembridge Beds
Lower { Osborne Beds
{ Headon Beds
_Description of the strata._ Little need be said of the deposits of
this period, either in Britain or abroad, except to remark that they
show the further spread of continental conditions over the regions now
occupied by land. The British deposits are now seen in the Hampshire
Basin only, and have been spoken of as the fluvio-marine series, as
many of the strata were laid down in continental sheets of water,
while the true marine sediments are thin and infrequent.
The lithological characters of deposits formed under these conditions
naturally vary greatly, consisting of different kinds of mechanical
sediments occasionally mixed with thin freshwater marls and
limestones. On the Continent similar conditions prevailed, though the
occurrence of fairly wide tracts of level surface is indicated by the
widespread distribution of beds of brown coal or lignite, and the
coarse and thick Oligocene 'nagelfluh' of Switzerland points to the
elevation of mountain ranges in the neighbourhood.
_The flora and fauna._ The remarks made concerning the Eocene flora
and fauna are generally applicable to those of Oligocene times, except
that the Oligocene fossils bear a still closer resemblance to living
forms, and the Nummulites are no longer dominant.
(ii) _The Miocene Period._ Beds of Miocene age are wanting in Britain,
and on the Continent they occur in isolated basins deposited in
gulf-like prolongations of the ocean, never very far from land. A
description of the strata and their fossil contents would be of little
use for our present purposes, and the remarks made concerning the
Oligocene beds will apply to the Miocene strata also.
The period was mainly remarkable on account of the important physical
changes which occurred, to which we must devote some consideration.
Commencing with the British area, we find in the south evidence of the
separation of the London and Hampshire Basins at this time, for the
Oligocene beds of Hampshire are tilted up on the south side of an
anticline, which separates the Hampshire Basin from that of London,
indicating that the movement was post-Miocene, while in Kent, beds of
Pliocene age rest on the denuded top of the chalk, showing that the
elevation and denudation which accompanied it were pre-Pliocene; the
great Wealden anticline is thus seen to be of Miocene age. On the
north side of the London Basin the line of demarcation between Eocene
and Mesozoic beds runs approximately parallel to the strike of the
latter in that part of Britain, and this points to the elevation of
the Mesozoic strata which gave them their present south-easterly dip
about the same period, though in the absence of Oligocene rocks it
cannot be definitely stated that the movement was altogether
post-Oligocene. The present physical geography of considerable parts
of Britain must date from Miocene times.
Important as the changes were in Britain, they were slight as compared
with those which affected Europe and many parts of Asia. The great
mountain chains of the Old World received their maximum uplift during
this great period of earth-movement, and orogenic structures were
impressed upon the rocks of many regions, for the Tertiary Mountain
Chains of the Old World have an Alpine structure impressed upon them
as the result of intense lateral pressure, accordingly we find the
Eocene strata lifted far above their original level to heights of
8,000 feet in the Alps and over 12,000 feet in the Himalayas. Away
from these marked uplifts epeirogenic movements caused the
disappearance of the seas of earlier Eocene times, so that towards the
close of the Miocene Period, the main features of the Eurasian
continent were much as they are now. The present drainage-systems must
have originated at the same time, and the sculpture of our continent
has been carried on more or less continuously by subaerial agents from
Miocene times to the present day. That any addition to the total area
of land was made is doubtful. The land which appears to have existed
to the west of Britain during Cretaceous and Eocene times finally
disappeared beneath the waters of the Atlantic Ocean, and the movement
probably gave rise to the prominent submarine feature which now exists
at some distance from the coast of Ireland. A great marine period is
now existent in our ocean areas, but so far as the existing
continents are concerned, we are living on the fourth continental
period which practically came into existence in Miocene times.
The strike of the uplifted strata naturally coincides on the whole
with the axes of the major uplifts, and accordingly we find the
Mesozoic and early Tertiary strata folded around axes which have a
prevalent east and west direction, with others which have a trend at
right angles to this. The strike of the British Mesozoic rocks seems
to have been determined by each of these sets of movements, so that
although it is east and west in the south of England, it runs north
and south in the eastern counties north of the Thames.
In America, although epeirogenic movements had occurred before Miocene
times, with the formation of wide continental tracts, these appear to
have been of the nature of plains, diversified by extensive inland
sheets of water, and uplift of orogenic character converted these
plains into uneven tracts in Miocene times. Many of the movements in
America, which like those of Europe are still progressing with
enfeebled power, differ from those of Eurasia, giving rise to raised
monoclinal blocks rather than to violent folds of Alpine character, as
seen in the western territories of North America, and as proved also
by the differential movements which are now known to affect the
Atlantic coast of that continent.
Accompanying these changes in the earth's crust were others which
affected the climate, at any rate locally. The warm climate of Eocene
times gradually gave way to a cooler climate in Oligocene times, and
this lowering of temperature was still further advanced in Miocene
times, though there is evidence that the temperature of those parts of
Europe which have strata representative of the Miocene period was
higher than it is at the present day.
Owing to the changes which occurred in Miocene times, the area of
sedimentation was extensively shifted to our present oceans, and
accordingly we find that the times subsequent to those of the Miocene
uplifts are marked by scattered accumulations of continental
character, with a few insignificant marine strata seldom found far
inland from the present coast-lines.
CHAPTER XXVI.
THE PLIOCENE BEDS.
_Classification._ The Italian Pliocene Beds which have long been known
have been divided into three stages, to which names have been applied
which are somewhat widely used, though the division of the British
deposits into the same three stages has not been made. The stages
are:--
Astian.
Plaisancean.
Zanclean.
The classification of the British deposits may be made as follows:--
Cromer "Forest" Series.
Weybourne Crag and Bure Valley Beds.
Chillesford Crag.
Norwich Crag and Red Crag.
Upper Coralline Crag.
Lower Coralline Crag.
As the English deposits are somewhat scattered it is difficult to make
out the exact order of succession, but the above shows the
classification which is adopted by the best authorities, the Norwich
Crag (or Fluvio-marine Crag as it is sometimes termed) being now
supposed to represent the upper portion of the Red Crag.
_Description of the strata._ The British deposits are chiefly found in
the counties of Norfolk and Suffolk, but isolated patches have been
detected in Kent and at St Erth in Cornwall; while the inclusion of
Pliocene fossils in the glacial deposits of Aberdeenshire and on the
west coasts and islands of Great Britain suggests the occurrence of
Pliocene beds beneath sea-level, around the British coasts, at no
great distance from the land.
The term 'Crag' has been applied to shelly sands of which the British
Pliocene beds are largely composed. The oldest British Pliocene strata
are supposed to be the Lenham Beds, occurring in 'pipes' on the Chalk
of the North Downs, which are referred to the Lower Coralline Crag,
and some writers believe that the St Erth beds of Cornwall are of
similar age[106]. The former are ferruginous sands, and the latter
shelly sands and clays. The higher beds of the Coralline Crag are
found in Suffolk, and are largely calcareous, being made of remains of
polyzoa, molluscs, and other invertebrates. They were probably
deposited in deeper water than the rest of the British Pliocene
strata, and contain a far larger percentage of carbonate of lime. The
Red Crag consists of ferruginous shelly sands, of the nature of
sand-banks, formed near land; while the Norwich Crag is of a still
more littoral character, and contains remains of land shells and the
bones of mammalia mingled with the marine shells of the coast. The
higher Pliocene deposits are also coastal accumulations, even the
so-called Forest bed being a deposit and not a true surface soil, as
proved by the observations of Mr Clement Reid. At the summit of the
Cromer 'Forest' Series, however, is a true freshwater bed. These
British deposits appear to have been laid down on a coast line which
formed one side of the estuary of a large river, of which the present
Rhine is the 'betrunked' portion (to use a term introduced by Prof. W.
M. Davis)[107].
[Footnote 106: See Clement Reid, _Nature_, 1886, p. 342; and Kendall
and Bell, _Quart. Journ. Geol. Soc._, vol. XLII. p. 201.]
[Footnote 107: See a paper by Mr F. W. Harmer, "On the Pliocene
Deposits of Holland, and their relationship to the English and Belgian
Crags," _Quart. Journ. Geol. Soc._, vol. LII. p. 748.]
On the European continent, marine Pliocene beds are found in Belgium
and Italy. The former deposits greatly resemble our Crags, whilst the
latter are of interest on account of the mixture of volcanic beds with
marine sediments in Sicily, showing that the formation of Etna
commenced in Pliocene times. Various deposits formed in inland basins
are found in France and Germany, but the most remarkable occur in the
Vienna basin, where Caspian conditions prevailed over large areas, and
the ordinary strata alternate with chemical deposits of which the
best-known are the celebrated rock salt masses of Wieliczka, near
Cracow. At the same time volcanic activity was rife to the south of
the Carpathian mountains. Other deposits, which are partly referable
to the Pliocene period, occur in Greece at Pikermi, and in India in
the Siwalik hills; these are celebrated for their remarkable mammals,
as are the Pliocene strata of the Western territories of North
America. The occurrence of marked earth-movements since Pliocene times
is indicated by the nature of the deposits of Barbadoes, where
radiolarian cherts have furnished two echinids which are described by
Dr Gregory as deep-sea forms. These beds were once referred to the
Miocene period, but there is good reason for assigning them to a later
date, and correlating them with the Pliocene beds of other areas, in
which case there must have been a considerable uplift in this region
since Pliocene times, a fact of great theoretical importance.
The climatic conditions of Pliocene times show steady fall of
temperature. The early Pliocene beds of Britain were deposited during
the prevalence of warmer temperatures than those which now exist in
the same area, but during later Pliocene times, the temperature was at
first similar to that now prevailing, and afterwards distinctly
colder, and we find in the upper Pliocene beds the remains of
organisms of a northern type. In the uppermost deposit of the Cromer
'Forest' Series, the arctic birch and arctic willow indicate the
commencement of the cold which culminated in the succeeding 'Great Ice
Age.'
_The flora and fauna._ Little need be said of the Pliocene fossils:
the flora approaches that of present times, and the invertebrates are
in most cases specifically identical with those now living. The
vertebrates alone differ markedly from living forms, being chiefly of
extinct species, and in many cases belonging to extinct genera. It is
interesting to find that the mammalian fauna of Pliocene times
resembles the existing fauna of the area in which the beds are found,
a fact long ago observed by Darwin. Thus the European Pliocene mammals
are like existing European forms, whilst in Australia the mammalian
terrestrial fauna consists of Marsupials, and in South America there
are Edentata of Pliocene age[108].
[Footnote 108: The Pliocene fauna of Britain is described by Mr
Searles V. Wood in the _Monographs of the Palæontographical
Society_.]
CHAPTER XXVII.
THE PLEISTOCENE ACCUMULATIONS.
_Classification._ The term Pleistocene, as used here, is approximately
equivalent to the expressions 'Glacial Period' and 'Great Ice Age' of
some writers; but I have adopted it in preference to these
expressions, because it may eventually be possible to define the
Pleistocene period in such a manner as to give the term a strictly
chronological meaning, whereas the other terms indicate the existence
of climatic conditions which must have ceased in some areas sooner
than in others. At present, climatic change gives us the best means
for separating the accumulations formed subsequently to the Pliocene
period over large parts of the Eurasian land-tract, and the most
convenient division of these continental accumulations is to refer
them to three periods, viz.:--
The Forest Period (in which we are now living).
The Steppe Period.
The Glacial Period.
Some of the accumulations which were formed during the Steppe period
are included in the Pleistocene period by many writers, but I prefer
to treat of them as post-Pleistocene.
In the present state of our knowledge of the glacial deposits any
attempt to make a classification applicable over very wide areas is
doomed to failure, and the very principles upon which the
classification should be based are a subject of disagreement. The most
promising basis for classification is founded on alternate recession
and advance of land-ice, though the proofs that advance takes place
simultaneously over very wide areas are not yet forthcoming. Dr J.
Geikie in the last edition of his work _The Great Ice Age_ adopts four
periods of glaciation, with intervening periods of recession, and this
division accords with the observations of many foreign geologists. In
order to understand the method of classification upon this basis, a
few words concerning glacial deposits in general will not be out of
place. Glacial accumulations may be divided into three classes:--(i)
true glacial accumulations, formed on, in, and under the ice, and left
behind upon its recession, (ii) marine glacial deposits, laid down in
the sea, when floating ice is extensively found on its surface, and
(iii) fluvio-glacial deposits, laid down by streams which come from
the ice. The two former indicate glacial conditions, while the
occurrence of fluvio-glacial deposits overlain by true glacial
deposits indicates an advance of land-ice, for the fluvio-glacial
deposits are accumulated in front of those which are truly glacial.
Accordingly if we find alternations of glacial and fluvio-glacial
deposits on a large scale, we may fairly infer the alternation of
periods of great glaciation with others when the ice diminished, or in
other words of glacial and interglacial periods. There is, however, in
many cases great difficulty in distinguishing glacial deposits from
marine glacial ones, while some of the true glacial deposits formed
_in_ the ice (englacial deposits) cannot readily be distinguished from
those of fluvio-glacial origin. Furthermore, as the terminal moraines
of land-ice often rest upon other true glacial deposits, it is often
difficult to know whether we are dealing with the products of one or
two glaciations over limited areas. The test of superposition is often
applicable, and one is enabled to obtain some clue as to the relative
order of events. In England at least three periods of glaciation seem
to be indicated by the glacial deposits. On the east coast the Cromer
Forest Series is succeeded by the Cromer Till, and in Yorkshire the
Basement Clay occupies a similar position with regard to the overlying
glacial accumulations to that of the Cromer Till. Whether these
deposits be marine or terrestrial, and the evidence is not yet
sufficient to settle this question to the satisfaction of all
geologists, there is no doubt that they are glacial. Above them, in
East Anglia, lies the Contorted Drift, the origin of which is still a
moot point, and it is overlain by the great Chalky Boulder Clay, which
extends far and wide over East Anglia, the Midland Counties and into
Yorkshire. Evidence has been adduced to connect this with the _till_
or boulder clay which spreads over the upland districts of the north
of England at the foot of the main hill-systems. This set of deposits
indicates a second glaciation. As the upland till is often ploughed
out by glaciers which have left their traces in the form of moraines
in our upland regions, we seem here to have evidence of a third
glaciation, which naturally leaves no traces in the southern
districts, and the exact age of this cannot be ascertained in the
absence of fossil evidence, though we may provisionally refer it to
the Pleistocene period.
Another attempt has been made to classify the glacial deposits, on the
supposition that there have been periods of elevation and depression
of the land during Pleistocene times. Some writers advocate one
interglacial period when the land was depressed to an extent of 1400
and perhaps 2000 feet, while others have advocated the occurrence of a
number of such interglacial marine periods. The evidence for the
supposed oscillations is furnished by the existence of shell-bearing
sands associated with boulder clays at high levels, the best known
being on Moel Tryfan in Caernarvonshire, near Macclesfield in
Cheshire, and near Oswestry in Shropshire. As many geologists believe
that these shells have been carried to their present position by ice
in a way which it is not our province to discuss here, we may dismiss
this method of classification as based upon events which cannot be
proved to have occurred. In the present state of our knowledge, it is
indeed best to avoid, as far as possible, classifications which are
intended to be applicable over wide regions, and to devote our
attention to local details, gradually piecing together the evidence
which is obtained as the result of exhaustive examination of each
separate area[109].
[Footnote 109: The glacial literature of our own island only, is so
extensive that the student may well be bewildered when he attempts to
grapple with it. He is recommended to read the following general
works:
J. Geikie, _The Great Ice Age_. 3rd Edition, 1894.
H. Carvill Lewis, _The Glacial Geology of Great Britain and Ireland_.
1894.
G. F. Wright, _Man and the Glacial Period_, 1892, and _The Ice Age in
North America_, 1890.
Sir C. Lyell, _Antiquity of Man_. 4th Edition, 1873.
For the glacial geology of special regions the following papers may be
consulted:
_The Lake District and adjoining neighbourhood._ E. H. Tiddeman,
"Evidence for the Ice Sheet in North Lancashire &c." _Quart. Journ.
Geol. Soc._, vol. XXVIII. p. 471. J. G. Goodchild, "Glacial Phenomena
of the Eden Valley &c." _Quart. Journ. Geol. Soc._, vol. XXXI. p. 55,
and J. C. Ward, _Mem. Geol. Survey_, "The Geology of the Northern half
of the Lake District."
_Yorkshire._ G. W. Lamplugh, "Drift of Flamborough Head," _Quart.
Journ. Geol. Soc._, vol. XLVII. p. 384.
_Lincolnshire._ A. J. Jukes-Browne, _Quart. Journ. Geol. Soc._, vol.
XXXV. p. 397 and XLI. p. 114.
_East Anglia._ Clement Reid, _Mem. Geol. Survey_, "The Geology of the
district around Cromer."
_North Wales._ T. McK. Hughes, "Drifts of the Yale of Clwyd" &c.
_Quart. Journ. Geol. Soc._, vol. XLIII. p. 73, and A. Strahan,
"Glaciation of South Lancashire, Cheshire, and the Welsh Border,"
_ibid._, vol. XLII. p. 486.
_Switzerland._ C. S. du Riche Preller, "On Fluvio-glacial and
Interglacial Deposits in Switzerland," _Quart. Journ. Geol. Soc._,
vol. LI. p. 369 and "On Glacial Deposits, Preglacial Valleys and
Interglacial Lake formations in Sub-Alpine Switzerland," _ibid._, vol.
LII. p. 556.
The reader will find references to other works on the Glacial Geology
of other districts by consulting the general works referred to on the
preceding page.]
The foregoing remarks will convince the student that any attempt to
show the distribution of land and sea during any part of the glacial
period is not likely to meet with general acceptance, as so much
depends upon the terrestrial or marine origin of the deposits of the
lowlands, and the mode of formation of the shell-bearing drifts of
high levels. The occurrence of elevation to a greater height than that
which our country at present possesses during portions at any rate of
the glacial period has been inferred on general grounds, but direct
evidence in favour of it is furnished by the existence of a number of
ancient valleys on the land around our coasts, whose floors are often
considerably below sea-level, while the valleys are now completely
filled up with glacial accumulations, except where they have been
partially re-excavated by streams which for some distance run above
the courses of the ancient streams.
The climatic conditions of glacial times can only be briefly touched
upon in this place. If the periods of advance can be proved to be
contemporaneous over wide areas, this points to alternations of colder
and warmer periods, or at any rate of drier and wetter periods, though
local advance may be due to a number of causes. It must be borne in
mind that with the temperature remaining the same, advance of ice can
be brought about by increased precipitation of aqueous vapour in the
form of snow.
The question of the cause of the glacial period is one that only
indirectly affects the stratigraphical geologist until he has
accumulated sufficient evidence to indicate the cause. It must suffice
to observe that the extremely plausible hypothesis of Croll (for which
the student should consult Dr Croll's _Climate and Time_) does not
explain the apparent gradual lowering of climate throughout Tertiary
times till the cold culminated in the Pleistocene period, and the
student will do well to remain in suspense concerning the cause of the
Ice Age until further evidence has been brought to bear upon it.
_The glacial flora and fauna._ The glacial deposits naturally yield
few traces of life, except those which have been derived from other
deposits, and we are dependent for our information concerning the
fauna and flora of the glacial period upon the remains furnished by
the interglacial deposits. Unfortunately it is very hard to ascertain
which deposits are interglacial, and many which have been claimed as
such are either preglacial or postglacial. The meagre evidence which
we possess points to the existence of an arctic fauna or flora in
Britain during the prevalence of this glacial period. A question which
has received much attention of recent years is that of the existence
of preglacial or interglacial man, on which much has been written. The
existence of man in glacial times is probable, but it is the opinion
of many of those who are most competent to form a judgment, that it
has not been proved in the only conclusive way, namely, by the
discovery of relics of man in deposits which are directly overlain by
glacial deposits, or which at any rate are demonstrably older than
glacial deposits[110].
[Footnote 110: On the question of preglacial and interglacial man, see
W. Boyd Dawkins, _Early Man in Britain_; H. Hicks, _Quart. Journ.
Geol. Soc._, vol. XLII. p. 3, XLIV. p. 561, and XLVIII. p. 453; T.
McK. Hughes, _ibid._, vol. XLIII. p. 73; Sir J. Evans, _Presidential
Address to British Assoc._ 1897.]
CHAPTER XXVIII.
THE STEPPE PERIOD.
The occurrence of a period marked by dry climate over wide areas of
the Eurasian continent, and possibly also in North America, is
evidenced by the widespread distribution of an accumulation known as
_loess_, concerning the origin of which there has been much difference
of opinion, though that it was formed subsequently to the glacial
period seems to be generally admitted, inasmuch as it is largely
composed of rearranged glacial mud. The formation of the loess as a
steppe-deposit was first advocated by Baron von Richthofen, and his
views were supported by Nehring after study of the loess-fauna.
Richthofen's explanation of the loess as due to the spread of dust by
wind in a dry region is becoming widely accepted, and it necessitates
the widespread occurrence of steppe conditions, as the loess has a
very extensive geographical range, and may be truly regarded as the
normal continental deposit of Eurasia during the period immediately
succeeding the glacial period. In our own country, as the sea cannot
have been far distant during these times the normal loess is not
found, but several accumulations occur, which on stratigraphical and
palæontological grounds must be regarded as synchronous with the
formation of the loess. These are certain rubble-drifts of the
southern counties, the older river-gravels of southern England, and
some of the older cave deposits of various parts of England. It is
doubtful whether any classification into minute subdivisions can be
adopted for them, though Prof. Boyd Dawkins has advocated their
separation into an older age of River Drift Man, and a newer period of
Cave Man, on account of the evidences of a lower state of civilisation
afforded by examination of the River Drift implements when compared
with those fashioned by Cave Man. Roughly speaking, the Steppe period
corresponds with the period during which Palæolithic man existed, at
any rate in north-west Europe, and we may speak of the Steppe period
as the Palæolithic period, without asserting that Palæolithic man
necessarily disappeared at the time when the climate changed and
caused the replacement of Steppe conditions by others favourable to
forest-growth.
_Description of the accumulations._ The loess consists of unstratified
calcareous mud or dust, with a peculiar vertical fracture, and is
interesting rather on account of the nature of its fossils and of its
distribution than for its lithological characters. As it is not found
in Britain it is not necessary to say much about it, but merely to
refer to the published descriptions[111].
[Footnote 111: An account of Richthofen's views by that author will be
found in the _Geological Magazine_, Dec. 2, vol. IX. (1882), p. 293,
and the fauna of the loess is described by Nehring (_Ibid._, p. 570).]
The British deposits require some notice, as their characters and mode
of occurrence are of some significance. Along the south coast are
deposits of coarse rubble which have yielded some organic remains,
which have been described by Mr Clement Reid[112], who also discusses
their origin. The rock, also known as the Elephant Bed, consists of
angular fragments of flint and chalk, and seems to have been produced
by streams which were able to flow over the surface of the chalk when
it was frozen. Many other similar deposits in the south of England,
which are found on the open surface, may have had a similar origin.
[Footnote 112: C. Reid, "Origin of Dry Chalk Valleys and of Coombe
Rock," _Quart. Journ. Geol. Soc._, vol. XLIII. p. 364.]
The Palæolithic river-gravels are found at various distances above
present river-levels, and are the surviving relics of alluvial
deposits which were laid down when the rivers ran at a higher level
than they now do. That they are newer than the main glacial drifts of
the region in which they occur is indicated by the frequent presence
in them of boulders derived from the drift. Their antiquity is shown
by the physical changes which have occurred since their deposition
(there having been sufficient time since then to allow of the
excavation of some river-valleys to a depth of over one hundred feet
beneath their former level), and also by the character of the included
mammals which will presently be referred to. The deposits vary in
coarseness, like those of modern alluvial flats, from the coarse
gravels of the river-beds to the fine loams and marls of the
flood-plains. They are found, in Britain, with their typical mammalian
remains, south-east of a line drawn from the mouth of the Tees to the
Bristol Channel.
The cave-deposits have a wider distribution than those which have just
been noticed, being also found to the north-west of the
above-mentioned line in Yorkshire, and in North and South Wales. In
the south of England they are found as far east as Ightham in Kent,
and in a westerly direction to Torquay and Tenby. The Ightham deposits
occur in fissures and consist of materials which were apparently
introduced from above by river action[113]. The cave-deposits of
limestone areas are sometimes found in fissures, but at other times in
caverns with a fairly horizontal floor, on which the various
accumulations lie in order of formation. The deposits vary in
character and may be divided into three groups, though accumulations
of intermediate character are found; the first group consists of
cave-earths and cave-breccias--formed by weathering of the limestone,
and the retention of the insoluble residue, as a more or less
ferruginous mud, mixed with angular fragments of limestone, and with
the remains of creatures which inhabited the caves; the second group
consists of true deposits laid down under water, as gravels, sands,
and laminated clays; while the third is composed of limestone
deposited from solution in water, in the form of stalagmite[114].
[Footnote 113: The Ightham fissures and their contents are described
by Messrs Abbot and Newton, _Quart. Journ. Geol. Soc._, vol. L. pp.
171 and 188.]
[Footnote 114: The reader should consult Prof. W. Boyd Dawkins' works
on _Cave Hunting_ and _Early Man in Britain_, for information
concerning the Cave Deposits. See also Sir C. Lyell, _Antiquity of
Man_; Sir J. Evans, _Ancient Stone Implements of Great Britain_, and
Sir J. Lubbock, _Prehistoric Times_. In these works references will be
found to papers by Messrs Pengelly, Magens Mello, Tiddeman and others
on the Caves of Devon, Derbyshire and Yorkshire. References have
already been made to papers upon the Caverns of North Wales.]
The organic contents of the Palæolithic period are of much interest,
and it is desirable to discuss their character before making further
observations upon the physical conditions of the period.
_The Palæolithic flora and fauna._ The plants of some of the earlier
deposits of the age we are considering show the prevalence of cold
conditions during their accumulation, for instance the Arctic birch
and Arctic willow are found in the accumulations beneath the
implement-bearing Palæolithic deposits of Hoxne in Suffolk[115]. The
invertebrate fauna consists essentially of the remains of molluscs.
The loess molluscs are chiefly pulmoniferous gastropods which lived
upon the land, though swamp forms are occasionally associated with
them. The palæolithic river-gravels have yielded numerous land- and
freshwater-molluscs of living species, though some which are abundant
in the British gravels are now extinct in Britain, e.g. _Cyrena
(Cobicula) fluminalis_ and _Unio littoralis_. Marine deposits of this
age are occasionally found, as at March, in Cambridgeshire, where the
fauna closely resembles that of our present sea-shores.
[Footnote 115: These beds are described by Messrs Reid and Ridley,
_Geol. Mag._ Dec. III. vol. V. p. 441. See also C. Reid on the
"History of the Recent Flora of Britain," _Annals of Botany_, vol. II.
No. 8, Aug. 1888.]
The vertebrate remains are much more remarkable, and it is not quite
clear that the association of forms whose living allies now live under
widely different conditions has been satisfactorily explained. The
river-gravels and cave-deposits contain remains of temperate forms, as
the bison, and brown bear, associated with those of northern forms, as
the mammoth, woolly rhinoceros, glutton, reindeer, and musk ox, and
also with those whose living allies are inhabitants of warmer regions,
like the lion, hyæna, and hippopotamus. One of the most remarkable
creatures is the sabre-toothed lion or _Machairodus_, remains of which
have been discovered in Kent's Cavern, Torquay, and in the caves of
Cresswell Crags, Derbyshire.
The loess fauna consists of characteristic steppe animals, such as the
jerboa, Saiga antelope and steppe-porcupine, and it is interesting to
find an indication of this fauna in the Ightham fissures.
The first undoubted relics of mankind are found in the Palæolithic
deposits, which are very widely spread over the Eurasian continent.
They consist mainly of implements of bone and stone, the latter being
chipped, but never ground or polished, though both bone and stone
implements are frequently ornamented with engraved figures. The
cave-deposits have furnished implements of a higher type than those
usually found in the river-drifts, but the latter are also found in
caverns in deposits beneath those containing the higher type, hence
the division of the period into two minor periods, that of river-drift
man, and that of cave-man[116].
[Footnote 116: Concerning this matter, the reader should consult Prof.
Boyd Dawkins' _Early Man in Britain_. Sir J. Prestwich has argued in
favour of the existence of a group of implements found on the plateau
south of the Thames of an age antecedent to that of the ordinary
river-drift implements. See _Quart. Journ. Geol. Soc._, vol. XLV. p.
270.]
There are several questions of interest connected with the Palæolithic
fauna, three of which deserve some notice here. The absence of the
relics of the Palæolithic mammalia and of the human implements in the
river-gravels north-west of the line drawn between the Tees and
Bristol Channel, and the presence of the mammalian remains in the
caverns of that area requires some explanation. One such explanation
assumes that the relics were destroyed in the open country to the
north-west of that line, owing to glaciation, but it is not by any
means universally accepted.
Another difficulty which in the opinion of some writers has not been
fully cleared up is the mixture of apparently southern forms like the
Hippopotamus, with others of northern character like the Musk ox,
under such conditions as to show that the creatures lived in the
British area contemporaneously. Seasonal migration might account for
it, but the wide belt of overlap of apparent northern and southern
forms requires something more, though secular changes of climate might
shift the belt of seasonal overlap from one place to another, causing
the entire belt of overlap to extend over a considerable distance.
The third, and perhaps most important difficulty is the abrupt change
from the Palæolithic type of implement to the Neolithic type,
characteristic of the next period. Some implements, as those of the
kitchen-middens of Denmark, and those found at Brandon and Cissbury in
this country, have been appealed to as intermediate in character, but
evidence has been brought forward to show that each set is truly
Neolithic, the one being the implements of the lowly fisher-folk who
lived contemporaneously with the makers of the highly finished
polished implements of Denmark, while the others are unfinished
implements thrown away during the manufacture on account of flaws or
accidental fractures. The difficulty is increased when we take into
account the great physical and faunistic changes which occurred
between Palæolithic and Neolithic times.
The country was undoubtedly more elevated than it is at present during
portions if not during the whole of Palæolithic times, as shown by the
appearance of the great mammals in Britain, the discovery of their
remains beneath sea-level, and especially the occurrence of remains in
the caverns of rocky islands such as those of the Bristol Channel,
where they could not possibly have existed unless the present islands
were connected with the mainland.
The fossils of the times between the Glacial period and the Neolithic
period indicate variations of climatic conditions. Upon this point I
cannot do better than quote the words of Sir John Evans in his
Presidential Address to the British Association at Toronto[117]. "At
Hoxne the interval between the deposit of the Boulder clay and of the
implement-bearing beds is distinctly proved to have witnessed at least
two noteworthy changes in climate. The beds immediately reposing on
the clay are characterised by the presence of alder in abundance, of
hazel, and yew, as well as by that of numerous flowering plants
indicative of a temperate climate very different from that under which
the Boulder clay itself was formed. Above these beds characterised by
temperate plants, comes a thick and more recent series of strata, in
which leaves of the dwarf Arctic willow and birch abound, and which
were in all probability deposited under conditions like those of the
cold regions of Siberia and North America.
"At a higher level, and of more recent date than these--from which
they are entirely distinct--are the beds containing the Palæolithic
implements, formed in all probability under conditions not essentially
different from those of the present day."
[Footnote 117: _Report Brit. Assoc._ for 1897, p. 13.]
CHAPTER XXIX.
THE FOREST PERIOD.
Subsequently to Palæolithic times, the physical conditions over
Eurasia changed greatly, and at the commencement of Neolithic times
the conditions were favourable for the growth of forests over wide
regions of that continent. At the commencement of the Forest period
the physical conditions were very much the same as they are at
present, though minor changes have of course taken place since then,
including probably a submergence of large parts of Britain to a depth
of about fifty feet beneath its former level, as indicated by the
existence of Neolithic submerged forests round many parts of our
coast-lines.
The Forest period may be best subdivided for local purposes by
reference to the civilisation of mankind at different times, and in
this way we obtain the following divisions:
Historic Iron age.
Prehistoric Iron age.
Bronze age.
Neolithic age.
A classification may also be based upon changes in the flora. In
Denmark the peat deposits of this age are divisible into five layers,
characterised by different dominant forms of trees. These are as
follows in descending order:
Fifth layer: Beech ... Iron age
Fourth layer: Alder
Third layer: Oak ... Bronze age
Second layer: Scotch Firs ... Neolithic age
Lowest layer: Poplar.
In our own country the forest growth has been much interfered with by
man, but the lower fenland peat gives a good example of the material
formed by forest growth. It is not necessary to touch on the various
accumulations which are now being formed in different parts of our
island, except to remark that the deposits of the Forest period give
indications of earth-movements on a small scale, which is well seen in
the fenland, where the forest peat is covered in places by a "buttery
clay" with _Scrobicularia piperata_ indicating submergence, and above
this is a marsh peat.
The flora and fauna of the Forest period are practically those of the
present day, though the larger forms of mammalia have disappeared one
by one. The Irish elk and _Bos primogenius_ probably became extinct
early in the period, while as far as Britain is concerned the wolf,
bear, and beaver have disappeared within historic times.
The relics of man deserve passing notice. The Neolithic period is
characterised by the absence of metal instruments, though those made
of stone were much more highly finished than those of Palæolithic
times, and were often ground and polished. The first metal which was
largely worked was bronze, which gradually replaced stone, though
stone was extensively used in the Bronze age, as indicated by the
imitation of bronze implements in stone. The Bronze age in turn was
replaced by the Prehistoric iron age; at first, when iron was scarce,
bronze implements were merely tipped with iron, but ultimately the one
metal was practically replaced by the other.
The date of the Palæolithic period is unknown; no approximate date can
be satisfactorily assigned to it, but various calculations, founded on
different data, have been made as to the age of the Neolithic period,
and several of them agree in placing it at about 7000 years from the
present time.
It will be seen that no sudden and violent change marks the incoming
of the human race, which to the geologist is but one of a large number
of events which have followed each other in unbroken sequence, and
accordingly the thread of the story where abandoned by the geologist
is taken up by the antiquary, and passed on by him to the
historian[118].
[Footnote 118: The student may obtain information concerning the
Neolithic age in Britain in Boyd Dawkins's _Early Man in Britain_; Sir
J. Evans' _Early Stone Implements of Great Britain_, and Sir J.
Lubbock's _Prehistoric Times_. In the latter work he will find a good
account of the Neolithic remains of Denmark and of the Swiss Lake
dwellings. For information concerning the Bronze age he should consult
Evans' _Ancient Bronze Implements of Great Britain_. The varied Danish
antiquities of Neolithic and Bronze ages are figured in H. P. Madsen's
_Antiquités Préhistoriques du Danemark_. The Prehistoric fauna of the
fenlands is described in Sir R. Owen's _History of British Fossil
Mammals and Birds_.]
CHAPTER XXX.
REMARKS ON VARIOUS QUESTIONS.
There are many problems connected with geology which can only be
solved by detailed study of the stratified rocks, and when solved the
principles of the science will be more fully elucidated. In the
present state of our knowledge some of these problems are ripe for
discussion, others can merely be indicated, while others again have
probably remained hidden, though it will be the task of the geologist
of the future to clear them up. Among the many questions which demand
knowledge of stratigraphical geology for their right understanding are
the following, which will be briefly considered in this chapter:--the
changes in the position of land and sea in past times, and the growth
of continents; the replacement of a school of uniformitarianism by one
of evolutionism; and the duration of geological time.
_Changes in the position of land and sea._ Certain physicists have
arrived at the conclusion that the general position of our oceans and
continents was determined at a very early period in the earth's
history, and that the changes which have occurred in their position
since then have been comparatively insignificant. The wide extent of
land over which stratified rocks are distributed at once indicates
that from the point of view of the geologist the changes have been
very important, and it is worth inquiring whether they are not
sufficiently important to prove that the primitive oceans and
continents have undergone so much alteration as to be unrecognisable.
Some authorities, while recognising the great changes which have
occurred in the relative position of land and sea during those periods
of which geologists have direct information, suppose that the changes
took place to a large degree in certain 'critical areas' bordering the
more stable areas of permanent ocean on the one side and permanent
land on the other.
In discussing the question of general permanence of land and ocean
regions it will be convenient to commence with a study of the present
land areas, and at the outset we may take into consideration the
present distribution of marine sediment over different parts of the
land, using the last edition of M. Jules Marcou's geological map of
the world for the purpose[119]. A glimpse at this map indicates that
more than half of the land areas are occupied by rocks which are as
yet unknown (many of which _may_ be marine sediments), or by
crystalline schists of which the mode of origin has not yet been fully
explained, so that a large part of Central Asia, the interior of
Africa, and of South America may have existed as land from very early
times, and the same may be said of smaller portions of Europe and
North America. Actual observation of a geological map therefore
indicates the possibility that about half of the land surfaces may
have existed as such through very long periods, but though there is a
possibility of this, the probability is not very great. The unknown
regions, as remarked above, may consist to a considerable extent of
marine sediments, and the existence of isolated patches of late
Palæozoic and of Mesozoic strata in the heart of Central Asia, points
to the submergence of much wider regions than those in which these
isolated patches have been found. Again, the character of the
sediments when they abut against the crystalline schists frequently
proves that these sediments once extended further over the crystalline
schists, and have since been removed by denudation, so that even if we
assume that the crystalline schists are all of very early date, and
not necessarily formed in any case from marine sediments, we cannot
suppose that all the area occupied by them has existed as land for
long periods of time. On the other hand, the major part of Europe and
North Africa, extensive tracts in Asia, the greater part of Australia,
a very large part of North America and considerable tracts of South
America give proofs of having been occupied by the oceans in Palæozoic
and later times.
[Footnote 119: A reduced copy of this map will be found opposite the
title-page of the first volume of Prof. Prestwich's _Geology_.]
It may be answered that most of these regions containing marine
sediments occur in critical areas, which have undergone a certain
amount of oscillation owing to earth-movements, and that the interior
parts of the great continental masses have been practically
stationary. But if these lands had been land-areas through geological
ages they must have been acted upon by the agents of subaerial
denudation, throughout these ages, and long ago reduced to
peneplains[120] unless the action of these subaerial agents was
counteracted by that of elevating forces, but if these forces were
sufficient to counteract the action of subaerial denudation through
countless ages, they were also sufficient to raise extensive tracts
of land above sea-level, and materially to alter the distribution of
land and sea, and if elevation could go on to this extent, why not
also depression?
[Footnote 120: A term proposed by Prof. W. M. Davis for a nearly level
surface of subaerial denudation, as opposed to a plain of marine
denudation.]
Proceeding a step further, and examining the character of the
sediments as well as their geographical distribution, we find
further evidence of great crust-movements. It has been urged that
deep-water sediments do not occur amongst the strata found on the
continents,--that there are no representatives of the abysmal deposits
of recent ocean floors amongst the strata of the geological
column[121], but the researches of the last two decades have brought
to light foraminiferal and radiolarian deposits, pteropodal deposits,
and possibly deep-sea clays, which are comparable with those in
process of formation at great depths in existing oceans, and though
the proofs of their deep-sea origin are not always as full as might be
desired in the case of the older rocks[122], we can speak with greater
certainty when we examine those of Tertiary age, and if the deep-sea
accumulations of this late date can be uplifted above sea-level, this
is much more likely to have occurred with those of past times. When a
deposit like the radiolarian rock of Barbadoes, the deep-water
character of which has been conclusively proved, can be elevated into
land since Miocene or possibly Pliocene times, it is evident that the
crust-movements have been sufficient to produce the most profound
changes in the distribution of land and sea during the long ages which
are known to us. Another argument against the occurrence of extensive
changes has been derived from an examination of those islands which
are spoken of as oceanic islands. Strictly speaking an oceanic island
is one in which the present fauna and flora give indications of their
introduction by transport across intervening sea, and no indications
of the existence of forms of life which inhabited it when it was once
united to a continent; it may be inferred with a considerable degree
of certainty that these islands have been isolated for long periods of
time. It has been stated that these oceanic islands never contain
marine sediments of any considerable degree of antiquity, and that
there are therefore no traces of former continents over those wide
tracts of ocean which are occupied by oceanic islands. The evidence is
of a negative character. The islands would be less likely to exhibit
ancient sediments than continents, for being near the ocean, they
would be readily submerged, and the older deposits masked by newer
ones, though this need not necessarily account for the entire absence
of ancient rocks amongst them. The danger of the argument lies in the
fact that we do not yet know how far these old rocks really are
absent, as the geology of the oceanic isles has not been fully
explored from this point of view, and already several cases of the
asserted presence of ancient rocks on these islands have been
recorded.
[Footnote 121: See Mr A. R. Wallace's _Island Life_.]
[Footnote 122: See chapter IX.]
The argument derived from the present distribution of organisms is far
too complex to be discussed here, and the student is recommended to
read a masterly review of the evidence in Dr W. T. Blanford's
Presidential Address to the Geological Society in 1890, on the
question of the Permanence of Ocean Basins[123]. After reviewing the
evidence furnished by a study of modern distribution he concludes that
it "is far too contradictory to be received as proof of the permanence
of oceans and continents."
[Footnote 123: _Quart. Journ. Geol. Soc._, vol. XLVI., _Proc._, p.
59.]
The existence of former extensive land tracts over regions now
occupied by sea is naturally more difficult to prove than that of sea
over land, as we depend upon inference rather than actual observation
to a much greater degree than when considering the permanence of
continents, nevertheless a considerable amount of indirect evidence in
favour of the existence of widespread land tracts over our present
ocean regions has been accumulated and will be briefly noticed. We may
take first the evidence derived from the nature of sediments, and
afterwards that which has been acquired by studying distribution of
organisms in past times.
The indications of existence of an extensive tract of continent over
the North Atlantic Ocean, during Palæozoic times have already been
considered, and it was seen that the thinning out of the Palæozoic
sediments when traced away from the present Atlantic borders in an
easterly direction over Europe and in a westerly one over North
America pointed to the existence of this Palæozoic 'Atlantis,' as
maintained by Prof. Hull in his work, "Contributions to the Physical
History of the British Isles." This writer gives some reasons for
supposing that the continental mass began to break up towards the end
of Palæozoic times, though it is not clear that complete replacement
of land by sea occurred, and the nature of the Wealden deposits has
been pointed to as evidence of the existence of an extensive tract of
land to the west of Britain during the Cretaceous period.
The Palæontological evidence in favour of destruction of ancient
continental areas and their replacement by the sea is more
satisfactory than that which is based on physical grounds. The
distribution of the Glossopteris flora of the Permo-Carboniferous
period points to the former existence of a great southern continent,
including the sites of Australia, India, South Africa and South
America,--the Gondwanaland of Prof. E. Suess[124].
[Footnote 124: On this question and that of the other destroyed
continental areas noted here, see W. T. Blanford's _Presidential
Address_, _loc. cit._]
Again, a study of Jurassic and Cretaceous faunas has led
palæontologists to conclude that there was a connexion betwixt S.
Africa and India in Mesozoic times across a portion of the area now
occupied by the Indian Ocean, and also between S. Africa and S.
America, and these inferences are supported by study of the
distribution of existing forms.
The sudden appearance of the Dicotyledonous Angiosperms in Upper
Cretaceous rocks has also been used as evidence of destruction of
considerable tracts of land subsequently to Upper Cretaceous times,
and there is a certain amount of evidence in favour of the existence
of this land in the north polar region, in an area now largely
occupied by water, though relics of it are left, as the Faroe Isles,
Spitsbergen, Novaya Zembla and Franz Josef Land.
I cannot conclude the consideration of the question of permanence of
oceans and continents more fitly than by quoting from Dr Blanford's
address. He says, "There is no evidence whatever in favour of the
extreme view accepted by some physicists and geologists that every
ocean-bed now more than 1000 fathoms deep has always been ocean, and
that no part of the continental area has ever been beneath the deep
sea. Not only is there clear proof that some land-areas lying within
continental limits have at a comparatively recent date been submerged
over 1000 fathoms, whilst sea-bottoms now over 1000 fathoms deep must
have been land in part of the Tertiary era, but there are a mass of
facts both geological and biological in favour of land-connexion
having formerly existed in certain cases across what are now broad and
deep ocean[125]."
[Footnote 125: _Loc. cit._, _Proc._ p. 107.]
_Growth of continents._ Whatever view as to the general permanence of
continents and oceans be ultimately established, the occurrence of
widespread changes in the position of land and sea is indisputable,
and it is of interest for us to consider the nature of these changes
in the formation of continents. Prof. J. D. Dana has put forward a
hypothesis of growth of continents by a process of accretion, causing
diminution in the oceanic areas, which at the same time became deeper:
such growth need not always take place in exactly the same way, and
study of the distribution of the strata of the North American
continent suggests that the growth there was endogenous, the older
rocks lying to the west and north forming a horseshoe shaped continent
enclosing a gulf-like prolongation of the Atlantic, which became
contracted by deposition and uplift in successive geological periods,
though it is still partly existent as the Gulf of Mexico. The Eurasian
continent, especially its western portion, suggests more irregular
growth around scattered nuclei of older rocks, though the process is
not completed, and many gulf-like prolongations, as the Baltic and the
Mediterranean, still remain as water-tracts, which have not yet been
added to the continents.
Although extensive additions to continents may be and no doubt are
often largely due to epeirogenic movements, the influence of orogenic
movements on continent-formation is very pronounced. As the result of
orogenic movements, the rocks of portions of the earth's crust become
greatly compressed, and give rise to masses which readily resist
denudation; moreover, these comparatively rigid masses, as shown by M.
Bertrand, tend to undergo elevation along the same lines as those
which formed the axes of previous elevations, and accordingly after a
continental area has undergone denudation for a considerable period,
the uplands consist of rocks which have undergone orogenic
disturbance, while the tracts of ground which are occupied by rocks
which have not suffered disturbances of this character, even if
originally uplifted far above sea-level, tend to be destroyed, and
ultimately occupied by tracts of ocean. Stumps of former mountain
chains may be again and again established as nuclei of continents and
as every period of orogenic movement will add to the number of these
nuclei, the continental areas must in course of time become more
complex in structure. Moreover, as some areas are affected by orogenic
movements to a greater extent than others, the complexity of different
continental masses will vary. Thus, western Europe has been affected
by orogenic movements during many periods since the commencement of
Cambrian times and its structure is extremely complex, while the
central and western parts of Russia have not been subjected to violent
orogenic disturbances since Cambrian times, and accordingly we find
the structure of that area comparatively simple; the greater part of
Africa seems to have escaped these movements since remote times, and
the structure of that continent is extremely simple when compared with
the Eurasian continental tract. It need hardly be stated that the
formation of extensive chains composed of volcanic material, by
accumulation of lavas and ashes on the earth's surface, may give and
often has given rise to more rigid tracts, which will bring about the
same effects as those produced by orogenic disturbance as illustrated
on a small scale by the Lower Palæozoic volcanic rocks of Cambria and
Cumbria.
_Uniformitarianism and Evolution._ According to the extreme
uniformitarian views held by some geologists, the agents which are in
operation at the present day are similar in kind and in intensity to
those which were at work in past times, though no geologist will be
found who is sufficiently bold to assert that this holds true for all
periods of the earth's history, but only for those of which the
geologist has direct information derived from a study of the rocks,
and he is content to follow his master Hutton in ignoring periods of
which he cannot find records amongst the rocks. The modern geologist,
however, while rightly regarding the rocks as his principal source of
information finds that he cannot afford to ignore the evidence
furnished by the physicist, chemist, astronomer and biologist, which
throws light upon the history of periods far earlier than those of
which he has any records preserved amongst the outer portions of the
earth itself, just as the modern historian is not content with written
records, but must turn to the 'prehistoric' archæologist and geologist
for information concerning the history of early man upon the earth.
Interpreting the scope of geology in this general way, rigid
uniformitarianism must be abandoned. Assuming that the tenets of the
evolutionist school are generally true, the question is, how far does
this affect the geologist in his study of those periods of which we
have definite records amongst the rocks? This is a question which
cannot readily be answered at the present day, for our study of the
rocks is not sufficiently far advanced to enable us to point out
effects amongst the older rocks which were clearly caused by agents
working with greater intensity than they do at present, but as, on
the other hand, we cannot prove that these effects are due to agents
working with no greater intensity than that which now marks these
operations, it is unphilosophical to assume the latter. No student of
science at the present day would state that because there has been no
observed case of incoming of fresh species within the time that man
has actually observed the present faunas and floras, the hypothesis of
evolution of organisms is disproved, for the time of observation has
been too short, and similarly the time which has elapsed since the
formation of, say, the Cambrian rocks may have been too short, as
compared with the time which has elapsed since the formation of the
earth, to allow of any important change in the operation of the
geological agents.
Leaving out of account, for the moment, the actual evidence which has
been derived from a study of the rocks, we may briefly consider the
theoretical grounds upon which the substitution of an evolutionist
school of geology for one of uniformity has been suggested[126]. The
principal sources of energy which have exerted an influence upon
geological changes are the heat received from the sun and that given
off from the earth itself, both of which must have diminished in
quantity throughout geological ages. To the former source we largely
owe climatic changes and the operations of denudation, and accordingly
of deposition; to the latter, those of earth-movement and vulcanicity.
It by no means follows that because the agents were once potentially
more powerful than now, they would necessarily produce greater
effects, for that depends to some extent upon the various conditions
which prevailed at different times. To give an example:--if there had
at any time been a universal ocean of considerable depth, however
active the agents of denudation were then, they could produce no
effect whatever, having nothing to work upon; to take a less extreme
case, if our continents at any past time were smaller and less
elevated than at present, agents of denudation working with greater
intensity than that of the present agents need not necessarily have
produced a greater amount of denudation than that which is going on at
the present day. Again, let us consider vulcanicity: "It is as
certain," says Lord Kelvin, "that there is less volcanic energy in the
whole earth than there was a thousand years ago, as it is that there
is less gunpowder in a 'Monitor' after she has been seen to discharge
shot and shell, whether at a nearly equable rate or not, for five
hours without receiving fresh supplies than there was at the beginning
of the action." But it does not follow that the manifestations of
volcanic activity were necessarily more violent in early geological
times than now, for the degree of violence would be affected by other
things than the volcanic energy, such as the thickness of the earth's
crust.
[Footnote 126: The student may consult an interesting article by Prof.
Sollas bearing on this subject. See _Geol. Mag._ Dec. 2, vol. IV. p.
1.]
And now, let us consider briefly the characters of the rocks of the
crust, to see if they throw any light upon this question. The earliest
sediments of which we have any certain knowledge resemble in a
striking manner those formed at the present day, and they seem to have
been formed under very much the same conditions, though further work
may show that there were somewhat different conditions which did
produce definite differences in the characters of the earlier
strata[127]. Our knowledge of earth-movement and vulcanicity which
took place in past times is still too small to enable us to draw any
certain conclusions connected with the subject under discussion from
it. Perhaps the most suggestive indication of one set of conditions
having been generally similar in those early periods of which we have
definite records amongst the rocks is furnished by study of past
climate. If we accept the nebular hypothesis as a starting point, we
must admit that in the early stages of the earth's history the
temperature of the surface, which would then be largely dependent upon
the amount of heat given out from the earth itself as well as upon
that received from the sun, must have been much higher than it is at
the present day, and indeed the mere diminution of the amount of heat
received from the sun would probably be sufficient to account for a
very marked lowering of the temperature. Besides this change of
temperature, resulting in gradual lowering of temperature over the
whole earth's surface, we have other changes dependent upon different
conditions, as proved by the fact, that there have been alternations
of glacial and genial periods. If the general temperature had been
very high in the early periods of which we have actual records, the
oscillations would not be sufficient to produce a lowering of
temperature sufficient to cause glacial periods, whereas if it had not
been appreciably higher than now, glacial periods might be produced.
This may be represented diagrammatically.
[Footnote 127: On this matter see Teall, J. J. H., 'Presidential
Address to Section C,' _Report of the British Association_, 1893.]
Let _a_ represent the temperature at the commencement of earth-history
and _b_ that necessary for glaciation, and _bc_ the lapse of time
between then and now. The curved line indicates the gradual fall in
temperature due to diminution of the amount of heat, while the zigzag
line represents the oscillations due to secular climatic changes. If
the Cambrian period x occurred comparatively soon after the
commencement of earth-history as shown in fig. _A_, no glaciation
could be produced, even during periods when secular changes caused
colder conditions than the mean, whereas if the Cambrian period
occurred at a time very remote from the commencement of earth-history
as shown in _B_, glacial conditions could be produced then as now, for
the mean temperature, as shown by the distance of the curve from the
line _bc_, would be practically as it now is. The studies of the last
few decades have brought into prominence the occurrence of glacial
periods in remote times, probably in early Palæozoic times; and as far
as the mean temperature of the earth's surface is concerned, it would
appear, from the knowledge in our possession, that matters were not
very different in those early times from what they now are.
[Illustration: Fig. 25.]
Some further remarks will be made in subsequent paragraphs concerning
the period of the earth's history at which the geologist is first
furnished with definite records, but in the meantime it may be
observed that the geologist will do well, when working amongst the
strata, to consider that the more active operation of agents, even in
times of which he has definite knowledge, may have produced effects
which he should be prepared to discover, as their discovery would be
of considerable importance, and that he should not be content to infer
that because it has been proved that agents operating with the same
intensity as that which they have at present, _may_ have produced all
the effects which he can actually observe, they therefore necessarily
_did_ produce them.
_Recurrences._ Absolute uniformity of conditions is impossible, even
in a single area. Every change which takes place upon the earth
produces conditions somewhat dissimilar from those which previously
existed, and these will leave their effects upon the physiography of
the area. For this reason, assuming that the conditions have gradually
changed from simpler to more complex, every period of time will have
been marked by conditions which never prevailed before or afterwards,
and these will leave their impress upon the deposits of the period. It
is doubtful for instance, as already remarked, whether the exact
conditions which gave rise to the extensive deposits of vegetable
matter in Carboniferous times which now form coal, ever occurred to a
like extent in previous or subsequent periods, and accordingly, though
we have deposits of coal of other ages, none are so extensive as those
of the Coal Measures. Again, as the strata of one period are largely
composed of denuded particles of pre-existing strata, which were
derived directly or indirectly from igneous rock, the soluble material
existing in the igneous rocks must have been gradually eliminated
unless restored by other processes, and we might expect to find that
early sediments have, on the whole, a larger proportion of soluble
silicates than the later ones.
Besides these changes, there are physical changes which are recurrent,
and cause conditions generally similar to pre-existing ones to occur
in an area after an interval of dissimilar ones. We have seen that
deposits tend to vary according to the distance from the coast,
limestone being succeeded by mud, this by sand and gravel, and after
subsidence the sand and gravel are succeeded by mud, and that by
limestone. These changes will produce some effect upon the organisms,
and the recurrence of organisms is a well-known event, of which cases
have been cited in a former chapter.
Again we find, as already pointed out, recurrence of climatic changes,
with alternation of glacial and warmer periods, and these may have
been very widespread, and would influence the other physical
conditions, as well as the distribution of the organisms. Vulcanicity
may have been more rife at some periods than others, for instance
there seems, in the present imperfect state of our knowledge, evidence
of enfeebled vulcanicity in later Mesozoic times, and of its renewed
activity in Tertiary times. Again, orogenic movements seem to have
occurred more extensively at some times than others, as for instance
in early upper Palæozoic times, at the end of the Palæozoic epoch, and
in early Tertiary times, though this may also be an apparent and not
an actual truth, due to imperfect knowledge. In any case, in limited
areas, there seem to have been alternations of periods of uplift
accompanied by marked orogenic movements, and of widespread
depression, accompanied by sedimentation.
The subject of rhythmic recurrence is worthy of further study. This
recurrence in combination with evolutionary change may account for the
apparent marked difference between Cambrian and Precambrian times, a
difference which strikes some geologists as being too great to be
accounted for as due to our ignorance only.
_Organic evolution._ This subject is too wide for more than passing
notice in a work of this character. The evidence of Palæontology is of
extreme importance to the biologist, and indeed, the way in which
evolution of organisms has occurred can only be actually demonstrated
by reference to Palæontology, and the study of Palæontology has
already given much information concerning the lines on which evolution
has proceeded in different groups of organisms. It must be remembered
that the major divisions of the invertebrata were in existence in very
early times; indeed representatives of most of them are found in the
rocks containing the earliest known fauna, that of the _Olenellus_
beds of Cambrian age. If our present views as to evolution be correct,
there is no doubt that the period which elapsed between the appearance
of life upon the globe and the existence of the _Olenellus_ fauna must
have been very great, possibly, as Huxley suggested, much greater than
that which has elapsed between early Cambrian times and the present
day. If this be so, however probable it is that we shall carry our
knowledge of ancient faunas far back beyond Cambrian times, it is
extremely improbable that we shall ever get traces of the very
earliest faunas which occupied our earth.
_Geological time._ Various attempts have been made to give numerical
estimates of the lapse of time which occurred since the earth was
formed, or since the earliest known rocks were deposited. These
attempts may be classed under two heads, namely, those made by
physicists, mainly on evidence obtained otherwise than by a study of
the rocks, and those made by geologists by calculating the mean rate
of denudation and deposition of the rocks, and estimating the average
thickness of the rocks of the geological column.
The calculations of physicists as to the age of the earth vary:--Lord
Kelvin assigned 20,000,000 years as the minimum and 100,000,000 as the
maximum duration of geological time. Prof. Tait has halved Lord
Kelvin's minimum period, while Prof. G. Darwin admits the possibility
of the lapse of 500,000,000 years.
The estimates made by geologists, which will appeal more directly to
the geological student, also vary considerably, though they bear some
proportion to those which have been put forward by the physicists.
Prof. S. Haughton[128] assigned a period of 200,000,000 years for the
accumulation of the rocks of the geological column; Mr Clifton
Ward[129] one of 62,000,000 years, after studying the rocks of the
English Lake District, and allowing for the gaps in the succession; Mr
A. R. Wallace[130] further lowers the time for the formation of the
column to 28,000,000 years; Sir A. Geikie[131] gives 73,000,000 years
as the minimum and 680,000,000 as the maximum; while Mr J. G.
Goodchild has lately[132] estimated the period at over 700,000,000
years.
[Footnote 128: _Nature_, vol. XVIII. p. 268.]
[Footnote 129: Ward, J. C., 'The Physical History of the English Lake
District,' _Geol. Mag._ Dec 2, vol. VI. p. 110.]
[Footnote 130: Wallace, A. R., _Island Life_, Chap. X.]
[Footnote 131: Geikie, Sir A., 'Presidential Address to the British
Association,' _Report Brit. Assoc._, 1892.]
[Footnote 132: Goodchild, J. G., _Proc. Roy. Soc. Edinburgh_, vol.
XIII. p. 259.]
Interesting as these figures are, they probably convey little to the
ordinary reader, and it is doubtful whether the geologist is really
affected by them to any extent when picturing to himself the vast
duration of geological time. One numerical estimate probably does
impress him, namely that made by Croll as to the date of the Great Ice
Age, for if the Ice Age be so remote as Croll imagined, the
commencement of earth-history must be inconceivably more remote; as
Croll's estimate is not generally accepted, it is doubtful how far
geologists are thus influenced, and probably the fact which does
impress them most, leaving fossils out of account, is the very little
change which has occurred in historic or even in prehistoric times as
compared with the vast changes which are familiar to them after
studying the strata of the geological column.
It is, after all, the succession of varied faunas which really gives
students of the rocks the most convincing proof of the vast periods of
geological time. If anyone doubts this assertion, let him consider
what impression would be made upon him by observing the several
thousand feet of strata of the column if none of them contained any
organisms. Cognisant as he is of the slow rate of change of existing
organisms, the fact that fauna has succeeded fauna in past times
brings home to him in an unmistakeable manner the great antiquity of
the earliest fossiliferous rocks, and as our detailed knowledge of
these faunas increases the impression of great lapse of time is
intensified. And if the earliest fossiliferous rocks be of such vast
antiquity, and, as has been remarked, the period of their formation is
comparatively recent with reference to the actual commencement of
earth-history, the latter must indeed be inconceivably remote, and
numerical estimates can do but little to familiarise us with the
significance of the vast time which has rolled by since the world's
birthday.
INDEX.
Abraum salts, 212
Æolian rocks, 24, 99, 100
Age, definition of, 60
Albian series, 236, 238
Algonkian rocks, 144
Ampthill clay, 232
Angelin, N. P., 161, 162, 165
Aptian series, 236, 237
Aqueous rocks, 24
Archæan rocks, 132
Ardmillan series, 170
Ardwick stage, 192
Arenaceous rocks, 29
Arvonian rocks, 141
Asaphus fauna, 165
Ashgill series, 164, 165, 167-169
Ashprington series, 184
Astian series, 256
Atlantis, 283
Aveline, W. T., 164
Aymestry limestone, 175, 176
Bagshot beds, 244, 246
Bajocian series, 227, 231
Bala limestone, 167
Bala series, 164
Barr series, 170
Barrande, J., 53, 55, 159, 161, 163
Barrois, C., 239
Barrow, G., 138
Barton beds, 244
Bath oolites, 226
Bathonian series, 227, 231
Bed, 27
Bedding plane, 27
Bell, A., 257
Belt, T., 153, 162
Bembridge beds, 251
Bertrand, M., 87, 286
Birkhill shales, 177
Black Jura, 226
Blake, J. F., 138-140
Blanford, W. T., 206, 208, 217, 282, 284
Bonney, T. G., 76, 141, 142
Boulder clay, 262
Bracklesham beds, 244
Bradford clay, 230
Break, palæontological, 61;
physical, 60
Bristow, H., 239
Brockram, 211
Brögger, W. C., 161-163
Brongniart, H., 18
Brongniart, C., 200
Bronze age, 275-277
Brown Jura, 226
Bunter sandstone, 218, 220-222
Bure valley beds, 256
Buttery clay, 276
Caerfai beds, 152, 154, 156
Calcareous rocks, 29
Caldicote series, 139
Callaway, C., 138-140
Callovian series, 227, 232
Cambrian faunas, 158-163
Cambrian system, 152-163
Caradoc series, 165, 168-171
Carbonaceous rocks, 29
Carboniferous fauna and flora, 199-201
Carboniferous limestone, 192, 194, 195
Carboniferous system, 192-201
Carnic beds, 225
Cataclastic rocks, 24
Cave man, 268
Cenomanian series, 236
Ceratopyge fauna, 162
Chalk, 236, 238, 239
Chalk marl, 236
Chemically-formed rocks, 29, 101
Chillesford crag, 256
Chronological terms, 60
Clastic rocks, 24
Climatic conditions, 103, 112, 290, 291
Climatic zones, in Jurassic times, 233;
in Cretaceous times, 241
Clymenian beds, 183
Coal, 196-199
Coal measures, 192;
mode of formation of, 195-199
Coblenzian beds, 184
Collyweston slate, 231
Colonies, theory of, 55
Contemporaneity of strata, 48
Continents, growth of, 285-287
Cope, E., 249
Corallian series, 227, 232
Coralline crag, 256, 257
Cornbrash, 230
Cornstones, 186
Coutchiching series, 144
Crags, 256-259
Cretaceous fauna and flora, 241-243
Cretaceous system, 236-243
Croll, J., 265, 295, 296
Cromer Forest series, 100, 256, 259
Cromer till, 262
Cucullæa beds, 183
Cuvier, Baron G., 18, 20
Dalradian rocks, 137
Dana, J. D., 285
Danian series, 236
Darwin, C., 20, 76
Darwin, G., 295
Daubrée, A., 88
David, T. W. E., 206
Davis, W. M., 258, 280
Dawkins, W. B., 266, 268, 270, 272, 277
Deep-sea deposits, 109
De Hayes, G. P., 19
De la Beche, Sir H., 92
Deposition, order of, 37, 116
Derivative rocks, 23
Devonian flora and fauna, 189-191
Devonian system, 183-191
Dictyograptus fauna, 162
Dimetian rocks, 141
Dogger, 226
Downtonian beds, 175
Dwyka conglomerate, 206
Edwards, F. E., 250
Eifelian beds, 184
Encrinurus fauna, 185
Englacial deposits, 261
Entomis slates, 183
Eocene fauna and flora, 248, 249
Eocene rocks, 244-250
Eozoon canadense, 143
Eparchæan rocks, 132
Epeirogenic movements, 32
Epiclastic rocks, 24;
simulation by cataclastic rocks, 38, 80
Epoch, definition of, 60
Estuarine series, 230
Etheridge, R., 19
Ettingshausen, Baron von, 250
Evans, Sir J., 266, 270, 274, 277
Evolution, 287, 293
Feistmantel, O., 208
Fenland, 276
Fluvio-glacial deposits, 261
Foreland grits, 184
Forest marble, 230
Forest period, 260, 275-277
Fossils, 42;
strata identifiable by, 40;
mode of occurrence of, 44;
relative value of, 47;
remanié, 52;
geographical distribution of, 55;
as indicative of physical conditions, 104
Fossil zone, 67
Foster, C. Le N., 239
Fox, H., 195
Freshwater deposits, 104;
distinction from marine, 105
Fuller's earth, 230
Fusulina beds, 201
Gala beds, 177
Gannister stage, 192
Gardner, J. S., 250
Gault, 236, 238
Geikie, Sir A., 60, 84, 95, 125, 130, 137, 141, 142, 144, 186, 188,
199, 247, 295
Geikie, J., 263
Girvan type, 170
Glacial deposits, permo-carboniferous, 206;
Pleistocene, 260-266
Glacial period, 260-266
Glenkiln shales, 169, 170
Glossopteris flora, 207, 208, 214
Godwin-Austen, R. A. C., 20
Gondwana series, 207
Gondwanaland, 207, 284
Goniatite beds, 183
Goodchild, J. G., 87, 130, 263, 295
Great ice age, 295, 296
Great oolite, 230, 231
Gregory, J. G., 258
Green, A. H., 122, 139, 193
Greensand, Lower, 236;
Upper, 236
Groom, T. T., 178
Gshellian beds, 193, 201
Hampshire basin, 245
Hangman grits, 184
Harker, A., 30, 88
Harkness, R., 161
Harmer, F. W., 258
Harpes fauna, 175
Harrison, W. J., 130
Hartfell shales, 169, 170
Hastings sands, 236, 237
Haughton, S., 295
Headon beds, 251
Heim, A., 32
Hempstead beds, 251
Hercynian systems of folds, 203
Hicks, H., 134, 141, 153, 154, 160, 161, 163, 167, 184, 266
Hickson, S. J., 109
Hill, A., 239
Hill, E., 142
Hilton shales, 210, 211
Hind, W., 196
Hinde, G. J., 169, 195
Hippurite limestone, 241, 242
Hirnant limestone, 167
Homotaxis, 48
Hughes, T. McK., 141, 264, 266
Hull, E., 120, 122, 193, 283
Hume, W. F., 239
Hunt, A. R., 101
Huronian system, 143
Hutton, J., 287
Huxley, T. H., 50, 250
Igneous rocks, 21-23
Ilfracombe beds, 184
Inferior oolite, 230
Inverted strata, 32;
detection of, 32
Iron age, 275, 276
Judd, J. W., 239, 247
Jukes, J. B., 84
Jukes-Browne, A. J., 126, 239, 264
Jurassic beds, 225
Jurassic fauna and flora, 234, 235
Jurassic system, 226-235
Kayser, E., 125, 191
Keewatin series, 144
Kelvin, Lord, 289
Kendall, P., 257
Keuper beds, 218, 221, 222
Kidston, B., 199
Kimmeridge clay, 232
Kimmeridgian series, 226
King, W., 217
Kjerulf, Th., 88
Koninck, L. de, 201
Kupferschiefer, 209
Lake, P., 126, 178
Lamina, 27
Lamplugh, G. W., 80, 119, 264
Lapworth, C., 32, 138, 139, 156, 168-170, 173, 178, 179
Laurentian rocks, 143
Lawson, A. C., 144, 145
Lehmann, J., 77
Lenham beds, 257
Lewis, H. C., 263
Lias, 226, 229
Liassian series, 227, 229
Lincolnshire limestone, 230, 231
Lincombe and Warberry grits, 184
Lindström, G., 114
Lingula flags, 152, 155, 156
Linnarsson, J. G. O., 161
Llandeilo limestone, 167
Llandeilo series, 165, 167
Llandovery series, 174-177
Loess, 267
Logan, Sir W., 20
London Basin, 245
London clay, 113, 244, 246
Longmyndian rocks, 138
Lower London Tertiary beds, 244, 246
Lubbock, Sir J., 270, 277
Ludlow series, 174-176
Lydekker, R., 250
Lyell, Sir C., 6, 12, 19, 106, 129, 224, 263, 270
Lynton slates, 184
McCoy, Sir F., 201
McMahon, C. A., 77
Madsen, H. P., 277
Magnesian Limestone, 209-211
Malm, 226
Maps, geological, 84, 130;
use of, 86, 120, 121
Marcou, J., 130, 279
Marine deposits, 102;
nature of fossils in, 107
Marl slate, 209, 210
Marlstone, 229
Marsh, O. C., 249
Marwood beds, 183
Matthew, G. F., 160-162, 180
Meadfoot sands, 184
Mechanically formed rocks, 29, 102
Mello, J. M., 270
Mendip system of folds, 203
Menevian beds, 152, 154, 156, 161
Metamorphic rocks, 25
Miall, L. C., 122
Michell, J., 10, 11
Millepore oolite, 230, 231
Miller, H., 189
Millet seed sands, 100
Millstone grit, 192
Miocene period, 252-255
Moffat shales, 169, 177
Mojsisovics, E. von, 224, 227
Morgan, C. Ll., 141
Morte slates, 184
Moscovian beds, 193, 301
Mountain limestone, 192
Murchison, Sir R. I., 19, 20, 174, 179
Murray, Sir J., 30
Muschelkalk, 218, 221, 222
Nehring, A., 267, 268
Neobolus fauna, 160
Neocomian series, 236-238
Neolithic age, 275-277
Neumayr, M., 115, 233
Newton, E. T., 45
Nicholson, H. A., 189, 250
Noachian Deluge, 8
Noetling, F., 160
Nordenskjöld, A. E., 113, 114
Noric beds, 225
Northamptonshire sands, 230
Norwich crag, 256, 257
Nummulitic limestone, 248
Old red sandstone, 183, 185, 186, 188, 191
Oldham, R. D., 208
Oldhaven beds, 244, 245
Olenellus fauna, 134, 153, 156-160
Olenus fauna, 152, 161, 162
Oligocene beds, 251, 252
Oligocene fauna and flora, 252
Oolite, 226
Ordovician faunas, 172, 173
Ordovician system, 164-173
Organically formed rocks, 29, 102, 109
Orogenic movements, 32
Osborne beds, 257
Owen, Sir R., 277
Oxford clay, 232
Oxford oolite, 226
Oxfordian series, 227, 232
Palæolithic fauna and flora, 270-274
Palæolithic man, 268, 272-274
Palæolithic period, 267-274
Palæontological break, 61
Palæo-physiography, 120
Paradoxides fauna, 152, 160, 161
Peat deposits, 275, 276
Pebble beds of Bunter, 218
Pebidian rocks, 140
Pengelly, W., 270
Pennant stage, 192
Pennine system of folds, 203
Penrith sandstone, 75, 210, 211
Period, definition of, 60
Permanence of ocean basins, 278-285
Permian fauna and flora, 214-216
Permian system, 209-217
Permo-carboniferous fauna and flora, 207, 208
Permo-carboniferous glacial deposits, 206
Permo-carboniferous period, 205-208
Phillips, J., 10, 11, 201
Physical break, 60
Pickwell Down sandstone, 183
Pilton beds, 183
Plaisancean series, 256
Planes of lamination, 27
Planes of stratification, 27
Pleistocene fauna and flora, 265, 266
Pleistocene period, 260-266
Pliocene fauna and flora, 259
Pliocene period, 256-259
Portland oolites, 226
Portlandian series, 226, 232
Prado, C. de, 161
Precambrian rocks, 132;
mode of formation of, 146
Preller, C. S. du R., 264
Prestwich, Sir J., 19, 130, 279
Productus limestones, 205, 206, 214
Protolenus fauna, 160
Pseudo-stromatism, 76
Purbeckian series, 226, 232
Pyroclastic rocks, 24
Quader sandstone, 240
Ramsay, Sir A. C., 130, 153, 163, 188
Reading beds, 244
Recurrences, 292
Red crag, 256, 257
Reid, C., 45, 257, 264, 268, 271
Renard, A., 30
Reversed fault, 34
Rhætic beds, 218
Rhiwlas limestone, 167
Richthofen, Baron von, 267, 268
Ridley, H. N., 271
River drift man, 268
Rotherham red rock, 202
Rothliegende, 209
Rouelle, 13
St Bees sandstone, 210
St Erth beds, 257
Salopian beds, 175
Salter, J. W., 161, 162, 186
Scarbro' limestone, 230, 231
Schists, crystalline, 76, 77, 133, 147
Scilla, A., 13
Screes, 101
Scrope, G. P., 76
Sections, geological, 84;
use of, 88
Sedimentary rocks, 23
Sedgwick, A., 16, 19, 20, 153, 174
Senonian series, 236
Series, definition of, 60
Seward, A. C., 113, 208
Sigmoidal structure, 33
Siliceous rocks, 29
Silurian faunas, 179, 180
Silurian system, 174-182
Simulation of structures, 72
Sinemurian series, 227, 229
Smith, W., 8, 12-18, 57, 85
Soil, 100
Solenhofen slate, 234
Sollas, W. J., 288
Solva beds, 152, 154, 156, 161
Speckled sandstone, 205, 206
Speeton series, 238
Spencer, H., 50
Spirorbis limestone, 201
Stages, definition of, 60
Steppe period, 260, 267-274
Stonesfield slate, 231
Strachey, J., 10
Strahan, A., 239, 264
Strata, 27;
classification of, 58, 125
Stratification, 26
Stratified rocks, 23;
composition of, 28;
origin of, 29;
classification of, 28, 125;
symbols to represent, 90
Stratigraphical geology, aim of, 1;
W. Smith, founder of, 8, 12-18
Suess, E., 110, 123, 207, 284
Superposition, law of, 31
Surveying, geological, 84
Systems, definition of, 60
Talchir stage, 205, 206
Tarannon shales, 174-177
Teall, J. J. H., 289
Terrestrial rocks, 99
Thanet sands, 244
Thinning out, 28
Thrust plane, 34;
detection of, 35, 82
Tiddeman, B. H., 87, 263, 270
Till, 262
Time, geological, 294-296
Toarcian series, 227, 229
Topley, W., 130, 239
Torridonian beds, 135-137
Tremadoc slates, 152, 155, 162, 163
Triassic fauna and flora, 223-225
Triassic system, 218-225;
ammonite zones of, 225
Trinucleus fauna, 165
Tullberg, S. A., 162
Turonian series, 236
Unconformity, 60, 78, 98
Underclays, 197
Uniformitarianism, 287-292
Uriconian rocks, 138
Ussher, W. A. E., 183
Valentian beds, 175
Verneuil, E. P. de, 161
Volcanic rocks, Cambrian, 155;
Carboniferous, 199;
Devonian, 184, 186;
Eocene, 246, 247;
Ordovician, 165-170;
Precambrian, 146
Vulcanicity, 289
Waagen, W., 213, 214
Walcott, C. D., 144, 158, 160, 161, 173
Wallace, A. R., 124, 235, 240, 281, 295
Ward, J. C., 87, 88, 263, 295
Warming, E., 115
Watts, W. W., 142, 168, 178
Wealden beds, 236, 237
Webster, T., 18
Weissliegende, 214
Wenlock limestone, 175, 176
Wenlock series, 174-177
Wenlock shale, 175-177
Werfener Schichten, 225
Werner, A. G., 12
Weybourne crag, 256
Whewell, W., 50
Whidbourne, G. F., 91
White Jura, 226
Whitehaven sandstone, 202
Whitehurst, J., 11, 12
Wiman, C., 46
Wood, S. V., 250, 259
Woodward, H., 191
Woodward, H. B., 68, 130, 131
Woodward, J., 8-10
Woodward, S. P., 108, 111
Woolhope limestone, 175
Woolwich beds, 244
Wright, G. F., 263
Yoredale series, 192
Zanclean series, 256
Zechstein, 209
Zone, fossil, 67;
ammonite, 225, 237;
graptolite, 69
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London, and A. A. Kanthack, M.D., M.R.C.P., Lecturer on
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_British Medical Journal._ This manual can in every sense be most
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=The Soluble Ferments and Fermentation.= By J. Reynolds
Green, Sc.D., F.R.S., Professor of Botany to the
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_Nature._ It is not necessary to recommend the perusal of the book, to
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Also in separate parts.
Part I. =Dynamics.= 4_s._
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=Heat.= 3_s._
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=The Principles of Stratigraphical Geology.= By J. E. Marr,
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=Crystallography.= By W. J. Lewis, M.A., Professor of
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=Petrology for Students.= An Introduction to the Study of
Rocks under the Microscope. By A. Harker, M.A., F.G.S.,
Fellow of St John's College, and Demonstrator in Geology
(Petrology) in the University of Cambridge. Crown 8vo. Second
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Transcriber's Note
Any obsolete or alternate spelling and grammar was retained. All
obvious typographical errors were corrected. Although hyphenation of
words has been standardized to the most prevalent occurrence, the six
occurrences of fresh-water were not converted to freshwater (30
occurrences) due to usage. Corrected spellings: Godwin-Austen (p. 20);
Whidbourne (p. 191); and Ichthyopterygia (p. 223).
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