The Principles of Stratigraphical Geology
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Title: The Principles of Stratigraphical Geology
Author: J. E. Marr
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*** START OF THE PROJECT GUTENBERG EBOOK THE PRINCIPLES OF STRATIGRAPHICAL GEOLOGY ***
Transcriber's Note
Emphasized text denoted as _Italic_ and =Bold= respectively.
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
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