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.