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-Project Gutenberg's The Principles of Stratigraphical Geology, by J. E. Marr
-
-This eBook is for the use of anyone anywhere at no cost and with
-almost no restrictions whatsoever.  You may copy it, give it away or
-re-use it under the terms of the Project Gutenberg License included
-with this eBook or online at www.gutenberg.org
-
-
-Title: The Principles of Stratigraphical Geology
-
-Author: J. E. Marr
-
-Release Date: October 16, 2013 [EBook #43963]
-
-Language: English
-
-Character set encoding: ISO-8859-1
-
-*** START OF THIS PROJECT GUTENBERG EBOOK STRATIGRAPHICAL GEOLOGY ***
-
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-Produced by Chris Curnow, Tom Cosmas and the Online
-Distributed Proofreading Team at http://www.pgdp.net (This
-file was produced from images generously made available
-by The Internet Archive)
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-
-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 _AŽ_, _BŽ_,
-and _CŽ_ is discovered, of which _AŽ_ has the fauna of _A_ in the
-former area, and similarly _BŽ_ the fauna of _B_, and _CŽ_ that of
-_C_.
-
-It cannot be assumed that the stratum _A_ is therefore contemporaneous
-with _AŽ_, _B_ with _BŽ_, and _C_ with _CŽ_, 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 _AŽ_ 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 _AŽ_
-contain the same fauna as _A_, and _CŽ_ the same fauna as _C_ whilst
-the fauna of _BŽ_ is different from that of _B_, we can nevertheless
-correlate the strata _B_ and _BŽ_ (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 _BŽ_.
-
-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 _AAŽ_, _BBŽ_ 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 _AŽ_
-and adding these thicknesses together, whereas the actual thickness of
-the middle bed is the distance between _A_ and _B_ or _AŽ_ and _BŽ_.
-
-[Illustration: Fig. 4.]
-
-When rocks have been affected by thrust-planes, the simulation of
-bedding may be carried out to a very full extent. Not only do the
-major thrust-planes resemble bedding-planes but the minor thrusts
-produce an appearance of divisional planes separating stratula or
-laminæ, and a close approximation to false-bedding is the result. To
-this structure Prof. Bonney has given the name
-'pseudo-stromatism[17].' It may be developed in rocks of all kinds,
-whether possessing original planes of stratification or not, and as a
-result of its existence the geologist may be seriously misled, not
-merely by mistaking the direction of the strata, but also the nature
-of the rock, for we may find it produced in an unstratified glacial
-till, and in a massive igneous rock, and in each case the resulting
-rock will resemble a sedimentary deposit, and of course the observer
-may be confirmed in his erroneous opinion by the formation of apparent
-fossils, ripple-marks or other objects which he might expect to
-discover in sediments. As illustrative examples, reference may be made
-to a number of schistose rocks, in which the planes of discontinuity
-(which are in truth planes of foliation) have been taken for
-bedding-planes and the rocks claimed as sedimentary though they are in
-reality igneous; for instance many of the rocks of the Laurentian of
-Canada, of the Hebridean of the North West Highlands, and some of the
-ancient rocks of Anglesey.
-
-[Footnote 17: Bonney, T. G., _Quart. Journ. Geol. Soc._, vol. XLII.
-_Proc._ p. 65.]
-
-A foliated structure may, as is now well known, be simulated by a
-structure developed in a rock prior to its consolidation. The
-similarity of flow structure of some lavas to the foliated structure
-of a schist was long ago pointed out by Darwin and Scrope, and recent
-work has proved that parallel structure due to differential movement
-prior to consolidation may be developed in plutonic rocks, as shown
-by Lieut.-General McMahon in the Himalayan granites, and by Lawson
-amongst the plutonic rocks of the Rainy Lake Region; and as the
-foliated structure may be mistaken for original stratification the
-same may occur, and has occurred, when dealing with this
-flow-structure.
-
-This is not the place to discuss the truth of the old theory of
-progressive metamorphism, in which it was maintained that a gradual
-passage could be traced between ordinary sediments and plutonic rocks,
-but it may be pointed out that much of the evidence which was relied
-upon to prove the theory was fallacious and due to the confusion of
-the parallel structure set up in plutonic rocks prior to, or
-subsequent to, consolidation, with original stratification. Recent
-study of metamorphic rocks has proved that the parallel structures
-developed in the rocks of an area which has undergone metamorphism may
-be produced by three distinct processes; they may be original planes
-of deposition, or formed in a solid rock subsequently to its
-formation, or in an igneous rock before its consolidation, and
-although it is sometimes possible to separate the structures produced
-by these processes, this is not always the case[18]. When a plutonic
-rock contains large phenocrysts and an eye-structure is developed in
-it, it may simulate a conglomerate, the rounded phenocrysts being
-taken for pebbles[19]. Still closer simulation of an epiclastic
-conglomerate may be produced in other ways and will be referred to
-immediately.
-
-[Footnote 18: It must be noticed that the rock in which parallel
-structure is produced before consolidation, if it undergoes no further
-change, though often associated with metamorphic rocks, is not itself
-metamorphic. The term _gneiss_ applied to these rocks is a misnomer,
-unless the term be used even more vaguely than it is at present.]
-
-[Footnote 19: See Lehmann, _Untersuchungen über die Entstehung der
-Altkrystallinischen Schiefergesteine mit besonderer Bezugnahme auf das
-Sächsische Granulitgebirge_, Plate XI. fig. 1.]
-
-We have already seen that the existence of unconformities has been
-utilised in the demarcation of large divisions of strata in various
-regions, and whether they be utilised in this manner or not, their
-detection is a matter of importance to the stratigraphical geologist,
-as they afford information concerning the occurrence of great physical
-changes during their production. These unconformities may also be
-closely simulated by structures produced in very different manner.
-
-The occurrence of an unconformity implies the denudation of one set of
-beds before the deposition of another set upon them, and accordingly
-the denuded edges of the lower set will somewhere abut against the
-lower surface of the lowest deposit or deposits of the overlying
-set[20]. The existence of an unconformity may often be detected in
-section, but when the unconformity is upon a large scale this may not
-be possible, but it will be discovered by mapping the strata and will
-be apparent on a map owing to the deposits of the lower set of beds
-abutting against the others. This is well seen where the Permian rocks
-of Durham, Yorkshire, and Nottinghamshire rest upon different members
-of the underlying Carboniferous series, and will be noticed on any
-good geological map of England. But a similar effect may be caused by
-a fault, so that mere inspection of a map or even of the strata in the
-field and discovery of one set of beds ending off against another does
-not prove unconformity. When the fault is a normal one, with low hade
-(that is, having a fissure approaching the vertical position), the
-outcrop of the fault-fissure will approximate to a straight line if
-the fault has a straight course, even if the ground be very uneven,
-whereas, if the plane of unconformity has not been tilted to a high
-angle from its original horizontal position, it will crop out in a
-sinuous manner across uneven ground, in a way similar to that of beds
-which are nearly horizontal, so that though the general trend of the
-outcrop of the plane of unconformity may be fairly straight, its
-deviation from a straight line will be frequent and marked, as seen in
-the case of the Permian unconformity above referred to. But if the
-unconformable junction has been highly inclined its outcrop will
-resemble that of a normal fault, or if the fault be a thrust-plane
-with high hade, the outcrop of this will resemble that of an
-unconformable junction which has not been greatly tilted from its
-original horizontal position. In these cases we require more evidence
-before we can decide whether we are dealing with an unconformable
-junction or a faulted one.
-
-[Footnote 20: An unconformity may be simulated or an actual
-unconformity rendered apparently more important, as the result of
-underground solution of the underlying strata subsequently to the
-deposition of the upper set upon them, and any insoluble materials in
-the underlying strata may be left as an apparent pebble-bed at the
-base of the upper beds. This is seen at the junction of the Tertiary
-beds with the chalk near London. Subterranean water has dissolved the
-upper part of the chalk, increasing the unconformity which naturally
-exists between chalk and Tertiary beds, and the insoluble flint of the
-dissolved chalk is left as a layer of 'green-coated flint' at the base
-of the Tertiary deposits.]
-
-The lowest deposits of the newer set of strata lying above an
-unconformity have probably been laid down in water near the
-shore-line. As the unconformity, if large, implies elevation above the
-sea-level, the deposits first formed after this elevation has ceased,
-and depression commenced, will necessarily be littoral in character
-and possibly of beach-formation, and accordingly we often find that an
-unconformity is marked by the existence of an epiclastic conglomerate
-immediately above the plane of unconformity and, although this need
-not be continuous, it is usually found somewhere along the line of
-junction. The conglomeratic base of the Lowest Carboniferous strata
-when they repose upon the upturned edges of the Lower Palæozoic rocks
-of the dales of West Yorkshire is well known, and may be cited as an
-example. The association of conglomerates with unconformities is
-indeed so frequent that its possible occurrence will always be
-suspected and sought by the geologist. Unfortunately the result of
-recent observation is to show that along thrust-planes of which the
-outcrop simulates those of unconformable junctions, the difficulty of
-discrimination may be increased by the existence of cataclastic rocks
-which bear a close resemblance to epiclastic conglomerates, and which
-may be and have been styled conglomerates. It is well known that
-fragments of the adjoining rocks are knocked into a fault-fissure
-during the occurrence of the movements which cause the fault, to
-constitute a _fault-breccia_, and as the result of the abrasion of
-these fragments by chemical or mechanical agency, the angular
-fragments may become rounded and converted into rounded pebble-like
-bodies, when the rock is changed into a _fault-conglomerate_. Fig. 5,
-from a photograph kindly supplied by Prof. W. W. Watts, shows a stage
-in the formation of a conglomerate of this nature from a
-fault-breccia; the fragment on the right remains angular, whilst those
-on the left have become much more rounded. The illustration is from a
-case described by Mr Lamplugh occurring in the slaty rocks of the Isle
-of Man, and Mr Lamplugh's paper[21] furnishes the reader with
-references to other examples of the production of similar rocks. No
-general rule can be laid down for distinguishing the true from the
-apparent unconformity, for the attendant phenomena will differ in each
-case; but if a fault-conglomerate should be suspected, the observer
-should try to ascertain whether fragments of a newer rock are imbedded
-in an older one, which sometimes occurs; he should note the existence
-of extensive slickensiding along the plane of junction and along
-planes of faulting, though the existence of these, implying as it does
-the occurrence of differential movement along the plane, does not
-prove that the movement was necessarily great, or that it did not take
-place along a plane of original unconformity; above all, he should
-look for structures such as mylonitic structure, pseudo-stromatism,
-development of new minerals, crushing out and stretching of fossils
-and fragments and, in short, for any structure which is familiar to
-him as a result of orogenic movements.
-
-[Footnote 21: Lamplugh, G. W., "On the Crush-Conglomerates of the Isle
-of Man," _Quart. Journ. Geol. Soc._, vol. LI. p. 563.]
-
-[Illustration: Fig. 5.]
-
-The effects of thrusting not only give rise to appearances suggestive
-of unconformity, but naturally also to a simulation of overlap. The
-thrust-planes are often parallel to original bedding-planes for some
-distance, but must cut across them sooner or later, producing
-lenticular masses which might be supposed to be due to the thinning
-out of beds as the result of cessation of deposition in a lateral
-direction.
-
-Attention has already been directed to the deceptive appearance of
-great thickness of strata which is due to repetition of one stratum or
-set of strata by a series of thrust-planes, so that there is no actual
-inversion of any part of a bed. When masses of limestone are affected
-in this way, the thrust-planes may become sealed up, as the result of
-chemical change, and a compact irregular mass of limestone devoid of
-any definite divisional planes may be the consequence, and beds of
-grit sometimes exhibit the same feature to some extent.
-
-Enough has been said to show that simulation of one structure by
-another has frequently occurred in rocks in so marked a degree as to
-render mistakes easy; and that these examples of 'mimicry' in the
-inorganic world are particularly frequent in rocks which have been
-subjected to great orogenic movements. The student will do well to
-acquaint himself with the macroscopic and microscopic structures
-which may be taken as characteristic of the rocks which have been thus
-affected, some of which can usually be detected with ease, and when he
-discovers them he may suspect that many phenomena which appear
-explicable in one way were in reality produced in a different one, for
-it is frequently very true of a region in which the rocks have been
-violently squeezed, stretched and broken that 'things are not what
-they seem.'
-
-
-
-
-CHAPTER VIII.
-
-GEOLOGICAL MAPS AND SECTIONS.
-
-
-The writer does not propose to give an account of the intricacies of
-geological mapping, for their right consideration requires a separate
-treatise[22]; all he desires is to call attention to some of the uses
-of geological maps as a means of conveying information. A geological
-map may be looked upon as an attempt to express as far as possible in
-two dimensions phenomena which possess three dimensions; this can be
-done to some extent on the actual surface of the map, by conventional
-signs, still more fully, by supplementing the map with sections; but
-best of all by a geological model, which is cut across in various
-directions in order to show the underground structure as well as that
-of the surface.
-
-[Footnote 22: The student is recommended to consult in particular,
-Appendix I. "On Geological Surveying" in _The Student's Manual of
-Geology_, by J. B. Jukes (Third Edition, Edited by A. Geikie), p. 747,
-and _Outlines of Field Geology_, by Sir A. Geikie (Macmillan and
-Co.).]
-
-The ordinary geological map is one which shows the outcrop of the
-strata, subdivided according to age, as they would be seen upon the
-surface of the earth after stripping off the superficial
-accumulations, and it is to be feared that the term 'geological map'
-is associated in the minds of most students with a map of this
-character and of no other. Nevertheless, a great many most important
-observations other than those connected with the order of succession
-of the strata are capable of representation upon a geological map, and
-the possession of a large number of maps of any area upon the geology
-of which a person is engaged--each map to be used for recording
-observations of a particular kind--will save much writing in
-note-books and, what is of more importance, will allow him to compare
-observations which have been made at different times at a glance,
-instead of causing him to search through a series of note-books.
-Still, however well furnished with maps, the geologist will find a
-note-book essential[23].
-
-[Footnote 23: As a result of some experience, the writer recommends
-every student to acquire some skill in the use of the pencil, and if
-to such a degree that he can combine artistic effect with accuracy, so
-much the better. An acquaintance with photography is invaluable: often
-the possession of a camera would enable a section to be recorded,
-which is otherwise lost to science.]
-
-The earliest geological maps represented the variations in the surface
-soils, or at most the general lithological characters of the rocks
-which by their decay furnished the materials for the soils. We have
-seen that the first chronological map was due to William Smith, and
-most subsequent English geological maps have been based upon his map
-of the strata of England and Wales. The order of succession of the
-strata is represented in these maps to some extent by the use of
-arrows to indicate the direction of dip of the strata, though this is
-not an unerring guide where strata are reversed, and accordingly the
-addition of a legend at the side of the map may be looked upon as
-essential to the correct understanding of the map itself. The legend
-is usually in the form of a section of a column, the strata being
-arranged in right order, the oldest at the base and the newest at the
-summit, the colours by which the strata are indicated being similar to
-those placed upon the map. Other information besides the mere order of
-succession of the strata may appear in the legend; thus their relative
-and actual thicknesses can be indicated if the column is drawn to some
-definite scale, and a brief description of the lithological characters
-of the rocks may well be appended to the side of the column. On the
-actual maps it is customary to exhibit the outcrop of the junctions of
-all igneous rocks as well as of the sedimentary ones: the nature of
-the metamorphism which sedimentary rocks have undergone at the contact
-with igneous ones may be and often is indicated by suitable signs; the
-position of faults is shown, and often also that of metalliferous
-veins, the nature of the ore in the latter being further indicated in
-some suitable manner, as by giving the recognised symbol for the
-metal; and in many maps an attempt is made to show the variations in
-dip and strike of the cleavage-planes.
-
-The Geological Survey of the United Kingdom publishes two sets of
-maps, one showing the 'solid geology' and the other the 'superficial
-geology.' It is easier to understand these terms than to define them,
-for in Britain there is a sharp line between the two everywhere except
-near Cromer. The maps showing the superficial geology represent
-gravels, glacial drifts and other incoherent accumulations of
-geologically recent origin, which to a greater or less extent mask the
-strata below which are usually composed of more or less solidified
-material. The maps showing the solid geology display the outcrops of
-these strata, though it is usual to insert alluvium upon these maps,
-as it is often impossible to trace the junction-lines of the strata
-below it. Attention has already been directed to the fact that these
-maps of solid geology, though chronological, that is, having the
-strata represented according to age, are founded largely upon
-lithological differences, rather than upon included organisms; and it
-has been stated that for theoretical purposes two sets of
-chronological maps, one founded upon lithological differences, the
-other upon difference of fossil organisms, would be extremely
-valuable.
-
-Other phenomena are often best represented upon separate maps, for if
-all observations are crowded upon one map the result will be very
-confusing. Special glacial maps showing the contour of the country,
-with the portions between the contour lines coloured differently
-according to altitude, say the country between sea-level and 500 feet
-light green, that between 500 and 1000 dark green, that between 1000
-and 1500 light brown and so on, exhibiting the direction of all
-observed glacial striae, the distribution of boulders so far as it is
-possible, and any other glacial phenomena which can be noted upon the
-map, will be valuable to the student of glaciation[24].
-
-[Footnote 24: For examples see Tiddeman, R. H., "Evidence for the
-Ice-Sheet in North Lancashire and the adjacent parts of Yorkshire and
-Westmorland," _Quart. Journ. Geol. Soc._, vol. XXVIII. pl. XXX., and
-Goodchild, J. G., "Glacial Phenomena of the Eden Valley" &c., _Quart.
-Journ. Geol. Soc._, vol. XXXI. pl. II.; and for a map of distribution
-of boulders, Ward, J. C., "Geology of the Northern Part of the English
-Lake District" (_Mem. Geol. Survey_), pl. IV.]
-
-Various structural features may be well displayed on separate maps.
-The trend of the axes of folds will be useful, and may be accompanied
-by other information of cognate character[25]; maps of the
-distribution of joint planes may be given in combination with those
-showing the folding of the strata if it be desired to exhibit the
-relationship between these; or with the physical features of the
-country, if the dependence of physical features upon joint structure
-be under consideration[26]. Much information concerning cleavage may
-be acquired from a map showing anticlinal and synclinal axes of
-cleavage[27], or the actual strike of the cleavage over different
-parts of a map may be represented, and its relationship to the
-geological structure of the district exhibited[28].
-
-[Footnote 25: See Bertrand, M., "Sur le Raccordement des Bassins
-Houillers du nord de la France et du sud de l'Angleterre," _Annales
-des Mines_, Jan. 1893, Plate 1.]
-
-[Footnote 26: See Daubrée, A., _Études Synthétiques de Géologie
-Expérimentale_, 1^{ère} Partie, Plates III.-VI., for an example of the
-latter, which is also interesting as showing the utility of a map on
-transparent paper super-posed on another, when illustrating the
-connexion between two sets of structures.]
-
-[Footnote 27: Ward, J. C., _Geology of the Northern Part of the
-English Lake District_, Plate IX.]
-
-[Footnote 28: Harker, Alfred, "The Bala Volcanic Series of
-Caernarvonshire" (_Sedgwick Essay_ for 1888), Fig. 5.]
-
-Maps exhibiting changes in physical geography appertain to the
-geologist as well as to the geographer. The position of ancient
-beaches, former lakes, representation of the changes in the courses of
-rivers and kindred phenomena may be shown upon maps, and will prove
-useful[29].
-
-[Footnote 29: For examples of maps of this kind, see Kjerulf, Th.,
-_Die Geologie des südlichen und mittleren Norwegen_.]
-
-A perusal of the maps to which reference has been made above will give
-the student some notion of the extent to which maps may be utilised to
-represent geological structures, and may suggest other methods by
-which they may be utilised.
-
-A geological section is usually drawn in order to exhibit the lie of
-the rocks, as it would be seen if a vertical cutting were made in that
-part of the earth's crust which is under consideration. The character
-of the section will depend upon circumstances. The Geological Survey
-of Great Britain issues two kinds of sections which are usually spoken
-of as vertical sections and horizontal sections, though each is in
-truth a vertical section; but whereas in the former the horizontal
-distance represented is small as compared with the thickness of the
-strata, in the latter the rocks of a considerable horizontal extent of
-country are exhibited in the section, and the section is not carried
-down to a great depth below the earth's surface. There is no essential
-difference between the two kinds of section, and often sections are
-drawn which cannot be definitely classed as belonging to either kind,
-but in extreme cases the vertical section is a representation of the
-order of succession as it would appear if the rocks were horizontal,
-no matter how disturbed they may be in reality; whereas the horizontal
-section represents the strata as they actually occur, with all the
-folds and faults by which they are affected. The accompanying figure
-(Fig. 6) represents a horizontal section on the left side of the
-figure with a vertical section of the same rocks on the right side.
-
-[Illustration: Fig. 6.]
-
-Vertical sections are extremely useful when it is desirable to compare
-variations in the strata over wide extents of country: this can be
-done by drawing a series of columns of the strata, each showing in
-vertical section the lithological characters and thicknesses of the
-strata in one place, whilst the relationship between the strata of
-two different places may be indicated by joining the beds of the same
-age by dotted lines as shown in Fig. 7[30].
-
-[Illustration: Fig. 7.]
-
-[Footnote 30: It is useful to adopt conventional symbols for the
-representation of strata of different lithological characters, and so
-far as possible to adhere to the same kind of symbol for any one kind
-of deposit. Those which are generally in use, are rough pictorial
-representations of the characters of the deposits, as shown in Fig. 7.
-The conglomerate is indicated by circular marks representing
-cross-sections of the pebbles, a breccia by triangular marks
-signifying that the fragments are angular and not rounded; a sandstone
-is indicated by dots to represent the grains of sand; a mud, clay or
-shale by continuous or broken horizontal lines, which reproduce the
-appearance of the planes of lamination so frequent in beds of this
-composition; a limestone is usually marked by the use of regular
-horizontal lines illustrating the pronounced bedding, with vertical
-lines at intervals to represent the regular jointing which occurs in
-so many limestones: the nature of the bedding may be further shown by
-drawing the lines comparatively far apart when the limestone is a
-thick-bedded one, nearer together when it is thin-bedded. Igneous
-rocks are represented by crosses or irregular V-shaped marks,
-illustrating the absence of stratification and presence of joints.
-
-Volcanic ashes are sometimes represented by dots, at other times by
-signs somewhat similar to those which are used for true igneous rocks.
-Sedimentary rocks which are composed of more than one kind of material
-may be further shown by a combination of two symbols, thus the
-existence of a sandy clay may be shown by means of a combination of
-horizontal lines and dots, and so with other combinations. The
-practical geologist should become accustomed to the use of these
-symbols in his note-book; if used, they will save much writing.
-
-These symbols are used in some of the later illustrations to this
-book.]
-
-The horizontal section is one which is in constant use by the
-practical geologist: the results of the first traverse of a district
-may be jotted down in his note-book in the form of a horizontal
-section (with accompanying notes), and the written memoir on the
-geology of any district composed largely of stratified rocks will
-almost certainly require illustration by means of these sections.
-Perhaps nothing more clearly marks the careful observer than the
-nature of the sections which he makes, and geological literature is
-too frequently marred by the publication of slovenly sections. A badly
-drawn section not only offends the eye, it may and frequently does
-convey inaccurate information.
-
-[Illustration: Fig. 8.]
-
-In the above figure (Fig. 8) taken from Sir Henry de la Beche's
-"Sections and Views Illustrative of Geological Phænomena," Plate II.,
-the lower drawing represents a section drawn to true scale, while that
-above shows one which is exaggerated. The student who saw this would
-infer that the uppermost beds on the left side of the upper section
-rested unconformably upon the dotted beds beneath, and once abutted
-against them in that portion of the figure where the beds have been
-removed by denudation in the deep valley, whereas an examination of
-the section drawn to true scale shows that the unconformity does not
-exist (although there is one at the base of the deposits marked by
-dots), and that there is room for the higher deposits to pass above
-those marked by dots at the place where the former have been removed
-by denudation. Whenever possible, horizontal sections should be drawn
-to true scale, the vertical heights being on the same scale as the
-horizontal distances. Sections which are so drawn represent the nature
-of the surface of the country as well as the relationship of the
-strata, and often illustrate in a marked degree the influence which
-the character of the strata has exerted upon the nature of the
-superficial features of a country. If it be impossible to draw a
-section in which the elevations and horizontal distances are
-represented upon a true scale, the former ought to be drawn on a scale
-which is a multiple of the latter; thus the vertical heights may be
-shown on 2, 3, or 4 or more times the scale chosen for the horizontal
-distances; when this is done, it will often be necessary to show the
-strata with an exaggerated dip, and accordingly the exaggerated
-section loses some of its value, though if vertical and horizontal
-scales bear some definite proportion it will still be more valuable
-than a rough diagram which is not drawn to any scale.
-
-Section-drawing cannot be satisfactorily accomplished without some
-practice, and the student is strongly advised to acquire the art of
-drawing good sections; the writer can assert as the result of
-considerable experience in the conduct of examinations of all kinds,
-that slovenly sections are the rule in candidates' papers, and good
-sections very rarely appear. Study of the six-inch maps and horizontal
-sections (drawn on the same scale) of the Geological Survey of the
-United Kingdom will enable the student to familiarise himself with
-admirable sections, and it should be his aim to produce sections like
-these. He is recommended to take some of these six-inch maps which
-show contour-lines as well as the disposition of the strata, and to
-draw sections on the scale of six inches to the mile, vertical and
-horizontal, exhibiting the proper outline of the ground and the
-arrangement of the strata, and afterwards to compare them with the
-published sections. The sections should be drawn as far as possible at
-right angles to the general strike of the strata. Some datum-line is
-taken for the base of the section (say sea-level) and offsets drawn
-vertically from this where the section crosses a contour-line or
-recorded height. The height is marked on these offsets; thus if a
-recorded height of 2700 feet (just over half a mile) occurred on the
-line of section a height of somewhat over three inches is marked on
-the offset, and so with the other points where the section crosses
-contours or recorded heights. By joining these points on the offsets,
-giving the connecting lines curves similar to those which are likely
-to occur in nature, the general character of the surface of the ground
-is represented. The geology of the district is next shown. Wherever a
-dip is marked on the map, the direction and amount of dip is shown by
-a short line on the section, and where dips are not actually seen
-along the line of section, the dips which are nearest to that line on
-the map must be considered, and marked on the section. The lines of
-junction between the various deposits shown by different colours upon
-the map are inserted on the section as short lines, the inclination
-being judged by study of the nearest dips; faults and igneous rocks
-must be marked off, and any indication of the hade of the fault or the
-slope of the edges of the igneous rock which the map affords will be
-taken into account. The section will then appear somewhat as shown in
-the following figure:
-
-[Illustration: Fig. 9.]
-
-and sufficient indication of the trend of the rocks will be obtained
-to shew that they form portions of curves which may then be filled in
-as shown in Fig. 10 and the section will be complete.
-
-[Illustration: Fig. 10.]
-
-It will be noticed that the small dyke of igneous rock on the right of
-the main dyke is joined to it lower down, though no indication of this
-is given along the line of section; but the requisite information for
-this and evidence of the existence of the small dyke proceeding from
-the left-hand side of the main one may be obtained by the study of
-the rocks in a valley on one side or other of the line of section.
-
-After the student has become conversant with the nature of geological
-maps and sections, and has read Sir A. Geikie's _Outlines of Field
-Geology_, he should on no account omit to learn something of the art
-of making geological maps, by going into the field and attempting to
-produce a map, for the art of geological surveying does not come
-naturally to any one, and some acquaintance with the methods of
-surveying is a necessity to everyone who wishes to make original
-geological observations, though all cannot expect to afford the time
-and acquire the skill necessary for the production of maps vying with
-the detailed maps of the Government Survey. Before actually attempting
-to draw lines on a map on his own account, he will do well to tramp
-over a portion of a district with the published geological map in his
-hands, selecting a country which is not characterised by great
-intricacy of geological structure, and he can then attempt to
-represent the geology of another portion of the same district without
-consulting the published map. Of all the districts of Britain with
-which he is acquainted the writer believes that the basin of the river
-Ribble, in the neighbourhood of the town of Settle in the West Riding
-of Yorkshire, is best adapted for studying field geology in the way
-suggested above, for the main geological features are marked by
-extreme simplicity, and the exposures are good, whilst the presence of
-an important fault-system and of a great unconformity relieve the area
-from monotony. Anyone who stands on the summit of Ingleborough or
-Penyghent will grasp the main features of a portion of the district
-without any difficulty, for it lies beneath his feet like a geological
-model, and when the student has mastered and mapped in the leading
-features, he can find bits of country with geology of varying degrees
-of complexity amongst the Lower Palæozoic rocks of the valleys which
-run down to Ingleton, Clapham, Austwick and Settle.
-
-The biologist is supplied with laboratories at home and abroad, where
-he may study his science under the best conditions. Would that some
-munificent person would found, in a district like that referred to
-above, a geological station where Cambridge students would have the
-means of acquiring a knowledge of field-geology under conditions more
-favourable than those presented by the flats around the sluggish Cam!
-
-
-
-
-CHAPTER IX.
-
-EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED.
-
-
-The establishment of the order of succession of the strata, and the
-correlation of strata of different areas merely pave the way for the
-geologist. To write the history of the earth during various geological
-ages, he has to ascertain the physical and climatic conditions which
-prevailed during the successive geological periods, and to study the
-various problems connected with the life of each period. In the
-present chapter an attempt will be made to illustrate the methods
-which have been pursued in order to write to the fullest degree which
-is compatible with our present knowledge, the earth-history of various
-ages of the past. In making this attempt, the physical and climatic
-conditions may be first considered, and their consideration followed
-by that of the changes in the faunas, though it will frequently be
-necessary to refer to one set of conditions as illustrative of the
-other.
-
-It will be assumed here that the great principle of geology, that the
-modern changes of the earth and its inhabitants are illustrative of
-past changes, is rigidly true. Reference will be made to this
-principle in a later chapter, but it is sufficient to state here that
-the study of the sediments which have been deposited from the
-commencement of Lower Palæozoic times to the times in which we now
-live bear the marks of having been formed under physical conditions,
-which, in the main, are similar in kind to those which prevail upon
-some part of the surface of the lithosphere at the present day.
-
-One of the most important inferences of the stratigrapher relates to
-the existence of marine or terrestrial conditions over an area at any
-particular time, and we may, in the first place, consider the evidence
-which supplies us with a clue to this subject.
-
-It has been previously stated that the ocean is essentially the
-theatre of deposition, the land that of destruction, and accordingly,
-the presence of deposit as a general rule indicates the evidence of
-marine conditions during the formation of those deposits, though this
-is not universally the case. Again, as denudation is practically
-confined to the land areas, and the shallow-waters at their margins,
-unconformity on a large scale gives evidence of the existence of
-terrestrial conditions in the area in which it is developed, during
-its production. Accordingly a mass of deposit separated from deposits
-above and below by marked unconformities shows the alternation of
-terrestrial conditions (during which the unconformity was produced)
-and marine conditions (during which the deposits were laid down). The
-deposits formed after an unconformity has been developed will
-naturally be of shallow-water character, as will also be those of the
-period immediately preceding the incoming of conditions which will
-cause the occurrence of another unconformity, and between these two
-shallow-water periods will occur a period when deeper-water conditions
-probably prevailed. We can therefore not only divide the history of
-any particular area into a series of chapters, of which every two
-successive ones will describe a continental period and a marine one,
-but each marine period may be divided into three phases--a
-shallow-water phase at the commencement, an intermediate deeper-water
-phase, and a shallow-water phase at the end. These phases are
-frequently complicated by the occurrence of a host of minor changes,
-but on eliminating these, the effects of the three great phases are
-shown by study of the nature of the strata, and their recognition does
-much to simplify the detailed study of the stratigraphical geology of
-various parts of the earth's surface.
-
-In discriminating between terrestrial conditions and marine ones, the
-existence of unconformities is of great importance in marking
-terrestrial conditions and is often the only available evidence, for
-no accumulations or deposits formed on the land may be preserved to
-testify to the terrestrial conditions[31]. When terrestrial deposits
-and accumulations do occur, they are extremely important, and it is
-necessary to allude to the points wherein they differ from marine
-deposits.
-
-[Footnote 31: The term terrestrial is used above in opposition to
-marine, to include the conditions prevalent above sea-level. The term
-continental would be better if it did not exclude insular conditions.
-Accordingly deposits formed in rivers, and fresh-water and salt-water
-lakes are spoken of as terrestrial.]
-
-Apart from organic contents, the mechanically formed deposits of
-rivers and lakes resemble in general characters the shallow-water
-deposits of the ocean, though they are usually less widely
-distributed. It is the accumulations which have actually been formed
-as æolian rocks, or those which have been laid down as chemical
-precipitates in salt-lakes which, by study of lithological characters,
-furnish the most convincing evidence of their terrestrial origin.
-
-Many æolian accumulations may be looked upon as soils, if the term
-soil be used in a special sense to refer to the accumulations which
-are produced as the result of the excess of disintegration over
-transportation in an area, whilst others are due to transport which
-has not been sufficiently effective to carry the material to the sea.
-When the weathered material accumulates above the weathered rock, it
-depends chiefly upon climate whether the disintegrated rock becomes
-mingled with much decayed organic matter forming humus. If this
-organic matter exists in quantity, the probability is that the
-accumulation is a terrestrial one, though this is by no means
-necessarily the case, for under exceptional circumstances a good deal
-of humus may be deposited in the sea, as beneath the mangrove-swamps
-which line the coasts of some regions, and to go further back, in the
-case of the Cromer Forest series of Pliocene times, or some coals,
-such as the Wigan Cannel Coal of the Carboniferous strata.
-
-In addition to the work of water, which affects both land and
-sea-deposits, the land is especially characterised by the operations
-of wind and frost upon it, for these produce results which may
-frequently serve to differentiate a land-accumulation from a deposit
-laid down beneath sea-level. The effect of wind in rounding the grains
-of sand which are blown by it is well-known, and samples of the
-'millet-seed' sands of desert regions are preserved in most museums.
-The greater rounding which characterises wind-borne as compared with
-water-borne sand grains is due, in great measure, to the greater
-friction between the grains when carried by the air than when swept
-along by the water. Under favourable circumstances water-worn grains
-may become rounded, especially when agitated by gentle currents
-sweeping over a shoal[32]; but a large mass of sand, in which most of
-the grains have undergone much rounding so as to give rise to
-'millet-seed' sand, will nevertheless be probably formed by
-wind-action except where a marine deposit is formed of material
-largely derived from an earlier æolian one. The effect of frost is to
-split rocks into fragments which are more or less angular before they
-are subjected to water-action. The broken fragments are prone to
-collect on slopes as screes, and as any scree-material falling into
-the sea is likely to become rounded except under conditions which
-rarely prevail, the existence of much scree-material in a rock
-suggests its terrestrial origin. Glaciers gave rise to terrestrial
-moraines, which may occasionally be identified as land-accumulations
-by mere inspection of their physical characters, but all geologists
-are aware of the difficulties with which they are confronted when they
-attempt to discriminate between terrestrial and marine glacial
-deposits.
-
-[Footnote 32: Cf. Hunt, A. R., "The Evidence of the Skerries Shoal on
-the wearing of Fine Sands by Waves," _Trans. Devon. Assoc._, 1887,
-vol. XIX. p. 498.]
-
-The existence of much material amongst the stratified rocks which has
-been precipitated from a state of solution is an indication of the
-terrestrial origin of the rocks, which were laid down on the floors of
-the inland seas, separated more or less completely from the open
-ocean; for the waters of the ocean are capable of retaining in
-solution all of the material which is brought down to them, and
-accordingly precipitates of carbonate of lime, rock-salt, gypsum and
-other compounds formed from solution, are only formed on a large scale
-in inland lakes, though they may be formed to some extent when the
-water of a lagoon is only slightly connected with that of the open
-ocean, and the evaporation is great, for instance in the lagoons of
-coral reefs. Certain physical features often mark the deposits of
-chemical origin, cubical or hopper-crystals of rock-salt may be
-dissolved, and the hollow afterwards filled with mud, so that the rock
-surfaces are sometimes marked with pseudomorphs of mud after
-rock-salt. Sun-cracks and rain-prints impressed on the rock are not
-actual indications of terrestrial origin of the rocks on which they
-are found, for the shallow-water muds of an estuary may be deposited
-in the sea and yet exposed to the action of the air at low tide, but
-they mark very shallow-water deposits which have been exposed to the
-atmosphere immediately after their formation if not during the time
-they were formed, and they frequently occur amongst the deposits of
-inland lakes.
-
-It will be observed that the characters of the terrestrial
-accumulations serve to distinguish them to some extent from the marine
-ones, but they also enable one to detect to some degree the actual
-conditions under which the accumulation was produced, whether on the
-mountain-slope, or in the plain, the desert or the fen, the river-bank
-or the lake-floor.
-
-The conditions of formation of the marine deposits may be
-distinguished within certain limits with ease, by examination of their
-physical characters, for the near-shore deposits will generally be
-coarser and contain more mechanically-transported material than the
-sediments which accumulate at a greater distance from the shore,
-though it is not safe to infer that deposits are formed away from the
-shore on account of the absence of mechanically-transported sediments.
-In districts where the mechanically-transported material is rapidly
-deposited, organic deposits of great purity may form close to the
-coast-line; for instance, when the rivers of a country end in fjords,
-the mechanical sediments are deposited in the fjords, and the sea
-around the coast is free from this sediment, and there the organisms
-can build up deposits of great purity; and a similar thing may happen
-when the rivers on one side of a country have short courses, and do
-not carry down much sediment, which occurs when the watershed is near
-the coast. On the one hand, clay may be formed in considerable purity
-near the coast, where the supply of mud is so great that the organisms
-existing there can do little in the way of contribution to the mass of
-the deposit, or it may be formed on the other hand in great depths of
-the ocean, where the supply of sediment is extremely small, but where
-all the organic tests become dissolved; as the characters of the deep
-sea clays are mainly negative, a geologist examining the rocks of the
-geological column would have much difficulty in distinguishing a
-deep-water clay from a shallow-water one by its lithological
-characters only. In cases of difficulty, information of importance is
-likely to be furnished by examination of the relative thickness of
-equivalent deposits in adjoining areas, for if we find a mass of clay
-a few feet thick in one region represented by hundreds of feet of clay
-and limestone in another, the former mass probably accumulated slowly
-and at some distance from the land; again, the uniformity of
-lithological characters of a deposit over a very wide area is a
-possible indication of its formation away from land, but this is not a
-safe guide, for reasons which will eventually appear, unless it can be
-shown that the deposit is everywhere of the same age.
-
-A clue to climatic conditions is frequently furnished by the physical
-characters of accumulations, especially terrestrial ones. The
-accumulations containing a large percentage of hydrocarbons have
-probably been formed under fairly temperate and moist climatic
-conditions, whilst the existence of millet-seed sandstones associated
-with chemical deposits points to desert conditions and inland lakes,
-requiring a dry climate and probably a warm one. Glaciated surfaces
-and glacial deposits of course indicate a low temperature. Some
-geologists profess that occasionally they can even determine the
-direction of the prevailing winds during past periods, by examination
-of the character of ripple-marks, rain-pits and other features, though
-it is doubtful whether much reliance can be placed upon these obscure
-indications.
-
-Useful as is the physical evidence supplied by deposits, as an index
-to the conditions under which they were formed, it is usually only
-supplementary to the evidence derived from a study of the fossils.
-Fossils when present in the rocks, usually supply considerable
-information concerning the prevalent conditions during the deposition
-of the rocks. By them we can not only separate marine from terrestrial
-deposits, but also freshwater deposits from æolian accumulations; each
-kind of deposit will generally contain the remains of organisms which
-existed under the conditions prevalent in the area of formation of the
-rock, though it is of course a frequent thing for a terrestrial
-creature or plant to be washed into a freshwater area or into the sea.
-In an æolian deposit, the invertebrate remains may be those of any
-air-breathing forms, as insects, galley-worms, spiders, scorpions and
-molluscs. The land-molluscs are all univalve. Of vertebrates, we may
-find the bones and teeth of amphibians, reptiles, birds and mammals.
-Occasionally freshwater or even marine forms may be found in an æolian
-deposit, but they will be exceptional. Marine shells are often blown
-amongst the sand-grains of the coastal dunes, and seagulls and other
-birds frequently carry marine organisms far inland.
-
-The creatures frequenting fresh water differ from those of the land
-and of the sea. The most abundant vertebrate remains will be those of
-fishes, and of the invertebrates we find mollusca preponderate. The
-variety of molluscs is not so great as in the case of marine faunas.
-The bivalves always possess two muscular scars on each valve (except
-adult _Mulleria_); whilst many marine shells as the oyster have only
-one muscular scar on each valve. (See Fig. 11.)
-
-[Illustration: Fig. 11.
-
-_A._ Monomyary shell with one scar.
-
-_B._ Dimyary shell with two scars.]
-
-These scars mark the attachment of the adductor muscles, for drawing
-the valves together, and the shells with only one impression on each
-valve are called _monomyary_, those with two impressions _dimyary_.
-The discovery of monomyary shells indicates with tolerable certainty
-the marine character of the deposit in which they are found, though
-their absence cannot be taken as proof of freshwater origin. The
-beaks or umbones of the bivalves are often corroded in freshwater
-deposits, as may be seen by examining shells of the common freshwater
-mussel. "All univalve shells of land and freshwater species, with the
-exception of _Melanopsis_ and _Achatina_, which has a slight
-indentation, have entire mouths; and this circumstance may often serve
-as a convenient rule for distinguishing freshwater from marine strata;
-since if any univalves occur of which the mouths are not entire, we
-may presume that the formation is marine[33]."
-
-[Footnote 33: Lyell's _Students' Elements of Geology_, Second Edition
-(1874), Chap. III. A good account of the differences between
-freshwater and marine organisms, from which some of the facts here
-cited are extracted, will be there found.]
-
-[Illustration: Fig. 12.
-
-_A._ Holostomatous shell.
-
-_B._ Siphonostomatous shell.]
-
-In Fig. 12 _A_ shows a freshwater shell (_Vivipara_) with entire
-mouth, whilst _B_ exhibits the shell of a marine gastropod
-(_Pleurotoma_) with a notched mouth. The entire-mouthed shells are
-called _holostomatous_ whilst those which are notched, the notch being
-often prolonged into a canal, are termed _siphonostomatous_.
-
-Many groups of invertebrates are seldom or never found in fresh water.
-Of exclusively or nearly exclusively marine creatures we may name the
-foraminifera, radiolaria, sponges with a hard framework, most hydrozoa
-which secrete hard parts, corals, echinoderms, cirripedes, king-crabs,
-locust-shrimps, most polyzoa, brachiopods, pteropods, heteropods, and
-cephalopods. Of extinct groups, the graptolites and trilobites seem to
-have been entirely confined to the sea.
-
-In the modern and comparatively modern deposits, the forms frequently
-belong to existing genera, and we get fairly conclusive evidence of
-the conditions of deposit by determination of the genera. The
-terrestrial (including freshwater) molluscs have mostly a long range
-in time. We find pulmoniferous gastropods of living genera in the
-Carboniferous period, one (_Dendropupa_) belongs to a subgenus of the
-modern land-shell _Pupa_, the other (_Zonites_) to a subgenus of the
-snail group _Helix_. Many freshwater molluscs as _Unio_, _Cyclas_, and
-_Physa_ are found amongst the secondary rocks, and give a clue to the
-origin of the deposits which contain them. Many extinct genera are
-closely allied to modern genera, and their mode of existence may be
-assumed with fair certainty. With all these guides, we may sometimes
-be left in doubt as to the conditions of deposit when organisms are
-few in number; thus, it is yet a matter for discussion whether the Old
-Red Sandstone and many of the deposits of the Coal Measures of Britain
-were of freshwater or marine origin.
-
-In considering the possibility of fossils having been carried from
-land to water or _vice versa_, it will be remembered that generally
-speaking they are more readily transferred from a higher to a lower
-level, so we are more likely to find remains of land-animals and
-plants in fresh water or the sea, and relics of freshwater animals and
-plants in the sea, than of marine or freshwater animals and plants in
-land, or marine organisms in fresh water. River-gravels and lacustrine
-deposits are especially prone to contain a considerable intermixture
-of land-forms with those proper to the station.
-
-Fossils supply much information concerning the depth and distance from
-land at which the deposits were laid down. When portions of the
-ocean-water have been separated to form inland lakes, the water
-becomes saltier than that of the open ocean, if the evaporation is
-greater than the supply of fresh water, and the life of the inland sea
-undergoes change under the unfavourable conditions set up. Many forms
-disappear altogether, and those which survive tend to become stunted,
-and the shells of many of the mollusca are abnormally thin; the fauna
-of an inland sea though it may have abundance of individuals is apt to
-be characterised by paucity of species.
-
-Turning now to the faunas of the open oceans, it is found that in
-addition to latitude, the distribution of organisms is affected by
-depth, and by the nature of the sea-floor, and accordingly we find
-different organisms in different areas; and in examining the same area
-the organisms inhabiting different depths are not all the same, and at
-the same depth some kinds of animals have different _stations_ from
-those of others, one creature being confined to a sandy floor, another
-to a muddy one, and so on[34]. The oceans have been divided into 18
-_provinces_, each of which is more or less characterised by the
-possession of peculiar forms which are termed _endemic_, in contrast
-to the _sporadic_ forms which are widely distributed. In any area
-which is margined by a coast line, the molluscs are distributed in
-zones which were formerly classed as follows:--the _littoral_ zone
-between tide marks, the _laminarian_ zone from low water to fifteen
-fathoms, the _coralline_ zone between fifteen and fifty fathoms, and
-the _deep-sea coral_ zone from fifty fathoms to one hundred fathoms or
-more; this last depth was once supposed to mark the limit of the
-downward extension of marine life, but as the result of modern
-deep-sea soundings we know that organisms extend to a much greater
-depth, and the deep-sea fauna, owing to uniformity of conditions over
-wide areas, contains fewer endemic forms in proportion to the sporadic
-ones than the shallow-water[35]. The deep-sea deposits entomb the
-remains of these deep-sea organisms and also of numerous _pelagic_
-organisms which live upon the surface of the ocean, whose remains sink
-to the ocean-floor after death. Amongst the deposits of the deeper
-parts of the ocean, we find many which are almost exclusively composed
-of the tests of foraminifera, radiolaria and pteropods, the spicules
-of sponges, and the frustules of diatoms; and accordingly the
-existence of foraminiferal, pteropodan, radiolarian, and diatomaceous
-oozes, amongst the strata of the geological column, has been taken by
-some as indicating the prevalence of deep-sea conditions during the
-formation of those deposits: as the purity of a calcareous ooze
-depends upon the absence of mechanical sediment, or volcanic dust, and
-as the component organisms of these oozes are pelagic forms which live
-near the continents as well as in the open oceans, the presence of
-calcareous oozes implies the existence of a _clear_ sea during their
-deposition but not necessarily of a deep one, for if the sea-area be
-far away from land masses, or if the sediment be strained off in
-fjords, calcareous oozes may be formed in shallow water. The existence
-of pure radiolarian or diatomaceous deposits is better evidence of
-deep water, for if they were formed in shallow water we should expect
-an intermixture of calcareous tests, whereas these are dissolved
-whilst sinking into the extreme depths of the ocean. As the deep-sea
-creatures are under very different conditions from those of shallower
-waters, we might expect marked structural differences between the deep
-and shallow-water creatures: one such difference has been emphasized,
-namely the occurrence of animals which are blind or have enormously
-developed eyes in the great depths of the sea, where the only light is
-due to phosphorescent organisms. This is well seen in the case of many
-recent crustacea, and has been noted by Suess in the case of the
-trilobites of some beds which he accordingly infers to be of
-deep-water origin, and it is interesting to find that these creatures
-are found in deposits which give independent evidence of an open-water
-origin. The _Æglinæ_ of the Ordovician strata are frequently furnished
-with enormous eyes, and they are often accompanied by blind
-trilobites, and in Bohemia the blind and large-eyed forms are
-sometimes different species of the same genus, for instance
-_Illænus_[36].
-
-[Footnote 34: For an account of the distribution of one group of
-organisms see Woodward, S. P., _A Manual of the Mollusca_, from which
-many of the following observations are taken.]
-
-[Footnote 35: For an account of the deep-sea fauna, see Hickson, S.
-J., _The Fauna of the Deep Sea_, 1894.]
-
-[Footnote 36: Suess, E., _Das Antlitz der Erde_, 2^{er}. Bd., p. 266.]
-
-As one would naturally expect, the actual depth at which deposits were
-formed can generally be calculated with a greater degree of certainty
-amongst the newer rocks than amongst the older ones. In the case of
-the Pliocene Crags, the depth in fathoms may be confidently given. In
-the Cretaceous rocks attempts have been made to give numerical
-estimates of the depths at which different accumulations were formed,
-but some differences of opinion have arisen in the case of these
-rocks. In the Palæozoic rocks, only a rough idea of the general depth
-can usually be obtained, and no attempt to calculate the depth in
-fathoms is likely to be even approximately correct in the present
-state of our knowledge.
-
-The comminution of fossils has sometimes been taken as an indication
-of shallower water origin of the deposits which contain them, but
-although the hard parts of organisms in a broken condition have
-frequently been shattered by the action of the waves, they may also be
-broken at great depths by predaceous creatures, and in many instances
-the fracture is the result of earth-movements occurring subsequently
-to the formation of the deposits.
-
-Turning now to the difference in organisms which results from
-difference of station, it will be sufficient to give a quotation from
-Woodward's _Manual of the Mollusca_ as an illustration:--"In Europe
-the characteristic genera of _rocky_ shores are _Littorina_,
-_Patella_, and _Purpura_; of sandy beaches, _Cardium_, _Tellina_,
-_Solen_; gravelly shores, _Mytilus_; and on muddy shores, _Lutraria_
-and _Pullastra_. On rocky coasts are also found many species of
-_Haliotis_, _Siphonaria_, _Fissurella_, and _Trochus_; they occur at
-various levels, some only at the high-water line, others in a middle
-zone, or at the verge of low-water. _Cypræa_ and _Conus_ shelter under
-coral-blocks, and _Cerithium_, _Terebra_, _Natica_ and _Pyramidella_
-bury in sand at low-water, but may be found by tracing the marks of
-their long burrows (Macgillivray)[37]."
-
-[Footnote 37: Woodward, S. P., _A Manual of the Mollusca_, p. 151.]
-
-The geologist will naturally select sporadic forms rather than endemic
-ones in comparing the strata of different areas, but how far
-differences in faunas are the result of existence at different times,
-and how far they are due to difference of conditions affecting
-contemporaneous organisms can only be discovered as the result of
-accurate observation. The main points to be regarded when comparing
-the successive faunas of different regions have been noticed in this
-and the preceding chapters, and it has been shown that as the evidence
-is cumulative, it requires the collection of a large number of facts
-obtained by observation of the strata before accurate inferences can
-be drawn.
-
-The indications of climatic conditions furnished by organisms require
-some consideration. In the comparatively recent deposits it is not
-difficult to get some notion of the prevalent climatic conditions when
-the fossils belong to forms closely related to modern genera. The
-existence of the arctic birch and arctic willow, and of shells
-belonging to species now living north of the British Isles, in
-deposits of comparatively recent date in Britain would afford
-convincing evidence of the occurrence of colder climatic conditions
-than those which are now prevalent in the area, even if the evidence
-were not confirmed as it is, by physical proof of glaciation in
-deposits of the same age. Nevertheless, even in these recent beds, we
-have a useful warning, by finding species of elephant and rhinoceros
-associated with northern forms like the lemming, glutton, and musk-ox.
-We know that the species of elephant and rhinoceros (the mammoth and
-woolly rhinoceros) were provided with thick coverings which would
-enable them to resist the severity of an arctic climate, but had not
-these coverings been found, we might have been puzzled by the
-association of forms whose nearest allies are sub-tropical with others
-of arctic character. As we go back in time and deal with earlier
-deposits, the ascertainment of the climatic conditions becomes more
-difficult, as the fossils mostly belong to extinct species, genera or
-even families.
-
-In these circumstances, it is very dangerous to draw conclusions as to
-climatic conditions from examination of a few forms, but when we find
-that plants and animals, terrestrial and marine forms, vertebrates and
-invertebrates alike point to the same conclusion, as in the London
-Clay, where all the fossils belong to forms allied to those now living
-under sub-tropical conditions, the state of the climate may be
-inferred with considerable certainty[38]. The character of the fossils
-must be taken into account rather than their size. There was a
-tendency amongst geologists to believe that large organisms probably
-indicate warm conditions. Recent researches in arctic seas have
-dispelled this belief. Marine algæ of enormous size are found in the
-cold seas, and the size of creatures, abundance of individuals and
-variety of forms in the arctic faunas of some regions is very
-noteworthy. In the Kara Sea, for instance, a variety of creatures were
-dredged up during the voyage of the Vega, and Baron Nordenskjöld makes
-the following pertinent remarks about them: "For the science of our
-time, which so often places the origin of a northern form in the
-south, and _vice versa_, as the foundation of very wide theoretical
-conclusions, a knowledge of the types which can live by turns in
-nearly fresh water of a temperature of +10°, and in water cooled down
-to -2·7° and of nearly the same salinity as that of the Mediterranean,
-must have a certain interest. The most remarkable were, according to
-Dr Stuxberg, the following: a species of Mysis, _Diastylis Rathkei_
-Kr., _Idothea entomon_ Lin., _Idothea Sabinei_ Kr., two species of
-Lysianassida, _Pontoporeia setosa_ Stbrg., _Halimedon brevicalcar_
-Goës, an Annelid, a Molgula, _Yoldia intermedia_ M. Sars, _Yoldia_ (?)
-_arctica_ Gray, and a Solecurtus[39]. "The temperatures were taken by
-a centigrade thermometer. Again we read of the results of dredging off
-Cape Chelyuskin. "The yield of the trawling was extraordinarily
-abundant; large asterids, crinoids, sponges, holothuria, a gigantic
-sea-spider (Pycnogonid), masses of worms, crustacea, etc. _It was the
-most abundant yield that the trawl-net at any one time brought up
-during the whole of our voyage round the coast of Asia_, and this from
-the sea off the northern extremity of that continent[40]."
-
-[Footnote 38: For a discussion as to the value of plants as indices of
-climate see Seward, A. C., Sedgwick Essay for 1892.]
-
-[Footnote 39: Nordenskjöld, A. E., _The Voyage of the Vega_, Vol. I.
-Chap. IV.]
-
-[Footnote 40: _Ibid._ Chap. VII.]
-
-Amongst the marine invertebrates reef-building corals and mollusca
-perhaps furnish the best evidence of climatic conditions. The
-coral-reefs of the Jurassic rocks with large gastropods and
-lamellibranchs clustered around them have been appealed to in proof of
-the existence of sub-tropical conditions during their formation;
-further back in time we find evidence of climate furnished by the
-fossils of the Silurian rocks of the Isle of Gothland in the Baltic
-Sea. Of these, Lindström writes "_The fauna had a tropical character_.
-In consideration of the great numbers of Pleurotomariae, Trochi,
-Turbinidae and the large Pteropods the assumption of a tropical
-character of the fauna may seem justifiable[41]."
-
-[Footnote 41: Lindström, G., _On the Silurian Gastropoda and Pteropoda
-of Gotland_, Stockholm, 1884, p. 33.]
-
-Structure may give some indication of climate even though the organism
-is not allied to living species. The bark of trees in arctic regions
-is often thicker than in more temperate regions, and the leaves of
-arctic plants often have special characters to enable them to resist
-the long periods during which they are deprived of water, though the
-fact that desert-plants frequently shew similar modifications deprives
-this test of any particular value except as a means of corroborating
-conclusions reached from other evidence[42]. The shells of arctic
-mollusca may become stunted, but this is not by any means universal,
-and the same result may be brought about by other abnormal conditions,
-as for instance the increase of salt in a water area by evaporation.
-
-[Footnote 42: For an account of the modifications of the leaves of
-arctic plants, see Warming, Eug., _Om Grønlands Vegetation_,
-Meddelelser om Grønland, 12th part, p. 105.]
-
-On the whole, an examination of the evidence available for
-ascertaining the character of climate by reference to included
-organisms, shews that inferences may be drawn within certain limits,
-but that the task is a difficult one not unaccompanied by danger, and
-every kind of available evidence derived from a study of physical
-phenomena and the included organisms should be utilised before any
-conclusion is drawn.
-
-The likelihood of accurate inference is increased by comparing the
-faunas of various areas; should they seem to indicate a progressive
-lowering of climate when passing from lower to higher latitudes, it is
-probable that the indication is correct. The student is referred to a
-paper by the late Professor Neumayr for an account of the existence of
-climatic zones during the Mesozoic Period[43].
-
-[Footnote 43: Neumayr, M., "Ueber klimatische Zonen während der Jura-
-und Kreidezeit," _Denkschrift. der Math.-Naturwissensch. Classe der k.
-Akad. der Wissenschaften_, Bd. XLVII. Vienna, 1883.]
-
-
-
-
-CHAPTER X.
-
-EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED, CONTINUED.
-
-
-In the preceding chapter, attention was drawn to the indications as to
-conditions of deposition furnished by the sediments of any one
-locality, and only passing reference was made to variation in the
-nature of the sediments and their organic contents, when the deposits
-are traced laterally from place to place; some attention must now be
-paid to this matter.
-
-It is sometimes inferred that, whereas similarity of organisms is a
-dangerous guide in correlating the strata of two areas, accurate
-correlations may be made, if the deposits can be traced continuously
-through the intervening interval; no doubt the task is simplified when
-this can be done, but the continuity of deposit of one particular
-composition is no more proof of contemporaneity than the occurrence of
-the same fossils continuously through the interval, imbedded in strata
-of different character, indeed probably not so much so. The existence
-of widespread masses of conglomerate, which are not found as linear
-strips, but which extend in all directions, is in itself an indication
-of this; the Oldhaven pebble bed for instance, in the Tertiary rocks
-of the London basin, is very widely distributed. We cannot suppose
-that coastal conditions prevailed far away from the shore-line, and
-accordingly when a conglomerate occurs in a widespread sheet, and not
-in a linear strip, this is indicative that the deposit has not been
-formed continuously but that strip has been added to strip along an
-advancing or receding shore line, and if this happens with
-conglomerates, it must occur also in the case of other deposits.
-
-[Illustration: Fig. 13.]
-
-In fig. 13[44] let _A_ represent a shore line of a continent which is
-undergoing gradual elevation. A deposit of pebbles _a_ will be formed
-against the coast, one of sand _b_ further away, then one of mud _c_
-and lastly limestone _d_, may be formed in the open sea away from
-land. Naturally there may be intermingling of two kinds of deposit at
-the junctions, but for the sake of simplicity this may be disregarded.
-During the accumulation of the deposits _a_, _b_, _c_, _d_, certain
-sporadic forms may be distributed throughout all the deposits, and
-some of them may become extinct before the deposition of these beds is
-completed, if the process is carried out on a large scale; we may
-speak of the characteristic fossils of this period as fauna I. As the
-result of elevation or of mere silting up of the sea-margin, or of
-both combined, the next mass of pebble-deposit will be laid down
-further away from the original shore, for the shore line will now be
-at _AŽ_ and not at _A_, and it will partly overlap the mass of sand
-_b_; the sand _b_^1 will also be deposited somewhat further out and
-partly overlap the mud _c_, and similarly the mud _c_^{1} will partly
-overlie the limestone _d_. During the formation of _a_^{1}, _b_^{1},
-_c_^{1}, _d_^{1}, other sporadic forms belonging to a fauna II may
-replace those of the first fauna. In the same way _a_^{2}, _b_^{2},
-_c_^{2}, _d_^{2} will be deposited, and in the meantime a new fauna
-III may arise and replace II. So the process will go on until we
-finally have a group of deposits lying one over the other, consisting
-of a basal accumulation of limestone, succeeded by mud, sandstone and
-pebble-beds in succession. Each of these will be continuous, though
-the inner part of the pebble-deposit was formed long before the outer
-part of the limestone, which is nevertheless beneath a mass of
-pebble-deposit continuous with that formed first, and the various
-deposits will be separated by fairly horizontal planes _x_, _y_, _z_,
-which might be regarded as bedding planes, but which are not so,
-strictly speaking. The true bedding planes will occur at a slight
-angle to these planes of separation, for the structure resembles false
-bedding on a gigantic scale, but of course, the lines separating two
-masses of similar deposit will be practically horizontal and parallel
-to the planes of demarcation of two distinct kinds of material. The
-lines separating two faunas would, under the conditions postulated,
-run approximately parallel to the planes of separation of adjoining
-deposits of the same lithological character but would pass from
-conglomerate, through sandstone, mud and limestone, as indicated by
-the lines 1, 2, 3, ... and the deposits between adjoining lines would
-be contemporaneous[45]. In nature, complications will arise, owing to
-the gradual appearance and disappearance of forms, and the existence
-of endemic species in contemporaneous deposits formed in different
-stations and having different lithological characters.
-
-[Footnote 44: The writer gratefully acknowledges his indebtedness to
-Prof. Lapworth for some of his views concerning deposition of strata.]
-
-[Footnote 45: The lines 1, 2, 3 ... are incorrectly drawn in the
-figure. Line 1 should be drawn so as to separate _a_, _b_, _c_, _d_
-from _a_^{1}, _b_^{1}, _c_^{1}, _d_^{1}, line 2 to separate _a_^{1},
-_b_^{1}, _c_^{1}, _d_^{1} from _a_^{2}, _b_^{2}, _c_^{2}, _d_^{2}, and
-so with the others.]
-
-If elevation ceased and were succeeded by depression, the exact
-opposite would occur, and the pebble beds would be overlain by
-sandstones, these by muds, and lastly limestones would appear. It
-follows that during a marine phase occurring between two
-unconformities we should have a =V=-shaped accumulation of deposits
-with the apex pointing to the part of the shore line which was last
-submerged before the commencement of elevation, as shewn in fig. 14,
-though the beds of the apex will in most cases be denuded during the
-re-emergence.
-
-[Illustration: Fig. 14.]
-
-Indications of the non-coincidence of the planes separating faunas and
-those which separate deposits of one lithological character from those
-of another have already been detected, for instance the 'greensand'
-condition of the Cretaceous period occurs in some places during the
-existence of one fauna, and in others during that of another, though
-the planes have not been traced continuously. Mr Lamplugh has
-furnished another example amongst the Cretaceous rocks of Yorkshire
-and Lincolnshire, but as has already been observed, a great deal
-remains to be done in this direction, and geologists are much in want
-of two sets of stratigraphical maps, in one of which the lines are
-drawn with reference to the differences of lithological character,
-whilst in the other they separate different faunas.
-
-The student will notice the normal recurrence of deposits in definite
-order; conglomerate succeeded by sandstone, mud and limestone, in a
-sinking area, and limestone succeeded by mud, sandstone and
-conglomerate in a rising area. Naturally many instances of departure
-from this rule are seen, owing to local conditions, but on a large
-scale, it is very frequently noted, and recognition of this will
-enable the student to remember the variations in the lithological
-characters of the deposits more easily, than if he simply acquired
-them from a text-book without taking heed as to their significance.
-
-Upon the variations in the lithological characters of deposits and of
-their faunas, when the beds are traced laterally depends very largely
-the successful ascertainment of the existence of former coast-lines,
-the restoration of which constitutes an important part, of
-Palæo-physiography, concerning which some observations may here be
-made[46]. If a set of deposits having different lithological
-characters can be proved to be contemporaneous, the coarser detrital
-accumulations will point to the approach to a coast-line, and the
-actual position of the coast during the period of accumulation of the
-deposits may be very accurately fixed. The pebble-beds at the base of
-the Cambrian rocks of Llanberis indicate the existence of a
-coast-line in that position during the accumulation of those
-pebble-beds. Similar pebble-beds occur at St David's, at the base of
-the Cambrian, but it is impossible in the case of these rapidly
-accumulated sediments to say that two deposited so far away from one
-another were actually contemporaneous, and therefore although we
-might draw a line through Llanberis and St David's to indicate the
-old coast-line of the period, it does not follow that the actual
-beach existed simultaneously at the positions indicated. The
-palæo-physiographer, however, attempts to restore the physical
-conditions of greater thicknesses of deposit; for instance, the
-distribution of land and sea during Lower Carboniferous times over the
-area now occupied by the British Isles is often taken to illustrate
-the methods of restoration of ancient features, and all admit that the
-lithological and palæontological characters of the rocks indicate a
-shallowing of the Carboniferous sea when passing northwards towards
-Scotland. For conveying an idea of the restorations to the student, it
-is almost imperative to portray the distribution of land and sea upon
-a map, and this can only be done by drawing definite lines. It must be
-distinctly understood that these lines are necessarily only an
-approximation to the actual position of the ancient shore-lines, which
-must have shifted again and again during the long period occupied by
-the accumulation of the Lower Carboniferous strata, so that a true
-idea of the positions of the Lower Carboniferous shore-lines could
-only be obtained by placing on a series of maps the successive
-shore-lines of different parts of the Lower Carboniferous period, and
-taking a composite photograph of these, which would appear as a wide
-belt of shaded portion of the map with no definite boundaries. The
-utmost that the maker of palæo-physiographical maps can expect to
-indicate, when dealing with considerable thicknesses of strata, is an
-approximation to the mean position of the shore-lines of the period
-when these strata were deposited. This is extremely valuable in
-enabling the student to understand the significance of the variations
-in the characters of the strata and their organic contents, if he
-distinctly recognises the generalised nature of the map. Examination
-of any two palæo-physiographical maps of the same period by different
-authors will shew wide divergences in the details, but a general
-resemblance of the main features. The reader will do well to consult
-Prof. Hull's restoration of the physical features of Old Red Sandstone
-and Lower Carboniferous Times on Plate VI. of his _Contributions to
-the Physical History of the British Isles_, and compare it with the
-map drawn by Prof. Green (_Coal: its History and Uses_, by Profs.
-Green, Miall, Thorpe, Rücker, and Marshall, Fig. 3, p. 38), which will
-be found to bear out this statement.
-
-[Footnote 46: On this subject, the student may consult Prof. E. Hull's
-_Contributions to the Physical History of the British Isles_.]
-
-Valuable as the published maps of palæo-physiography are as an aid to
-the student in understanding the significance of the variations of
-characters amongst the sediments, he will do well to supplement them
-by maps which he fills in for himself. He is recommended to procure a
-number of outline maps of England, or of the British Isles, and when
-studying in detail the characters of the British sedimentary rocks
-formed during the various periods, to place a blank map by his side
-when beginning the study of each period or important portion of a
-period. On this map he should jot down the geographical distribution
-of the different kinds of sediments, using the conventional signs
-indicated at p. 90: thus, in the case of the Lower Carboniferous
-rocks he would place the conventional sign for limestone in
-Derbyshire, a combination of those for limestone and shale in
-Yorkshire, and would add to these the sandstone sign in
-Northumberland. He should also note the general character of the
-fossils, using abbreviations for such terms as fresh-water fossils,
-shallow-sea fossils, deep-water fossils. After reading the account of
-the group of rocks in a comprehensive text-book, and inserting his
-notes on the map, he should proceed to insert the probable position of
-the coast-lines. He should also take notes of any indications of
-contemporaneous volcanic action, though these might well be inserted
-on a separate map. If this course be pursued, the student will not
-only have the significance of the variations amongst the strata
-impressed upon his mind, but he will have a means of obtaining at a
-glance the distribution of sediments and faunas of different kinds in
-the British area during the principal geological periods. On another
-set of maps he may indicate the axes of the orogenic movements which
-have occurred at different times, and when his various maps are
-completed, he will have the materials for the construction of a
-general account of the various geological processes which have been
-concerned with the building of the British area.
-
-When an area like Britain has been studied, the student may proceed to
-construction of maps of wider regions, and he will find that in doing
-this, new sets of facts must be taken into consideration, as for
-instance the occurrence of different faunas on opposite sides of
-once-existing continental masses, and the problems connected with the
-present distribution of the faunas and floras. For an instance of the
-importance of the former distribution of life the reader may consult
-the twelfth section of the first part of Professor Suess' _Das
-Antlitz der Erde_, whilst a good account of the value of recent
-geographical distribution of organisms in supplying a clue to former
-distribution of land and sea will be found in Mr A. R. Wallace's
-_Island Life_, Chapter xxii.
-
-Should the method suggested above be adopted, the student is likely to
-acquire a much more coherent idea of the significance of the facts of
-stratigraphical geology than can be obtained by a mere perusal of the
-accounts of the strata given in those portions of the various
-text-books which are devoted to a consideration of the stratigraphical
-branch of the science.
-
-
-
-
-CHAPTER XI.
-
-THE CLASSIFICATION OF THE STRATIFIED ROCKS.
-
-
-In the succeeding chapters, a general account of the characters of the
-Geological Deposits of different periods will be given, for the
-purposes of illustrating the principles to the consideration of which
-the earlier chapters have been devoted. It is not proposed to enter
-into a description of numberless details, which would only confuse the
-student who wished to grasp the main principles, for many facts have
-been recorded which it is necessary to notice in a comprehensive
-text-book treating of stratigraphical geology, though their full
-significance is not yet grasped. The writer, while noting the main
-characters of the various subdivisions of the different
-stratigraphical systems, will assume that this work is used in
-conjunction with some recognised text-book. The stratigraphical
-portion of Sir A. Geikie's _Class Book of Geology_ gives an admirable
-general account of the British Strata, while the larger text-book by
-the same author has a condensed though very full account of the rocks
-of the stratigraphical column in all parts of the world, and this is
-supplemented by numerous references to the original works wherein
-further descriptions may be found. The English edition of Prof. E.
-Kayser's _Text-Book of Comparative Geology_, edited by P. Lake, is
-also well adapted to the wants of the student, and an excellent
-account of the strata is given in Mr A. J. Jukes-Browne's _Handbook of
-Historical Geology_, which may be read with the same author's
-_Building of the British Isles_.
-
-The reader who refers to different text-books will be struck with the
-variations of nomenclature even amongst the larger stratigraphical
-divisions, for two authors seldom subdivide the geological column into
-the same number of rock-systems. The following classification will be
-here adopted:--
-
-  Groups.              Systems.
-
-                  { Recent
-                  { Pleistocene
-  Cainozoic or    { Pliocene
-  Tertiary        { Miocene
-                  { Oligocene
-                  { Eocene
-
-                  { Cretaceous
-  Mesozoic or     { Jurassic
-  Secondary       { Triassic
-
-                  { Permian
-                  { Permo-Carboniferous
-                  { Carboniferous
-  Palæozoic       { Devonian
-                  { Silurian
-                  { Ordovician
-                  { Cambrian.
-
-  Precambrian.
-
-A few remarks may be given as to the reason for adopting this
-classification.
-
-It is not for a moment suggested that the Systems have the same value,
-if the time taken for their accumulation be alone considered. The beds
-classified as Recent, for example, were probably accumulated during a
-lapse of time far shorter than that occupied for the deposit of some
-of the series or even stages of a system like the Silurian, but the
-recent rocks acquire a special significance from the fact that we are
-living in the period, and the Cainozoic rocks as a whole are capable
-of greater subdivision than the earlier groups, on account of the
-greater ease with which they can be studied, owing to the small amount
-of disturbance which they have usually undergone when compared with
-that which has affected older rocks, and the closer resemblance of
-their faunas and floras to those of existing times.
-
-With reference to the groups, the writer has already commented upon
-the use of the terms Palæozoic, Mesozoic and Cainozoic; below the
-lowest Palæozoic rocks (those of the Cambrian system) lie a group of
-rocks which have been variously spoken of as Azoic, Eozoic, and
-Archæan. There is an objection to the use of any one of these words in
-this sense; the objection in the case of the first two is that the
-term is theoretical and probably incorrect, whilst the word Archæan,
-otherwise suitable, has also been used in a more restricted sense. In
-these circumstances the term Precambrian will be used when referring
-to any rocks which were formed below Palæozoic times, though no doubt
-when this obscure group of rocks is more thoroughly understood a
-satisfactory classification will be applied to it.
-
-Taking the other groups into account, the lower systems of the
-Palæozoic group will be found to vary greatly according to the views
-of different writers; some make only one system, the Silurian, others
-two, the Cambrian and Silurian. The three systems are here adopted,
-not only because the one, Silurian, is too unwieldy on account of its
-size and requires subdivision (and the Cambrian and Silurian however
-defined, will be found to be of very unequal importance, whereas the
-three systems adopted are of fairly equal value), but especially
-because when the term Ordovician is used, the significance of the
-other terms Cambrian and Silurian is at once understood.
-
-An attempt has been made to shew that the Devonian system is
-non-existent, but the result of modern research is to shew that the
-rocks placed in this system are worthy of the distinction, both from
-their importance and from the distinctness of the fauna from those of
-the underlying and overlying systems.
-
-The Permo-Carboniferous system is adopted, because an important group
-of deposits has recently been brought to light which were not
-represented either in the Permian or Carboniferous system as
-originally defined.
-
-Some authors have advocated the union of the Permian and Triassic
-systems into one system placed at the base of the Mesozoic group. This
-is unnecessary, and would depart from the classification originally
-proposed, which is to be deprecated, unless there is any strong reason
-for it.
-
-The Mesozoic systems are classified according to the method generally
-adopted. Were a fresh classification to be proposed, a portion of the
-Cretaceous system might be included with the Jurassic rocks, but it is
-better to adhere to the old classification.
-
-The divisions of the Cainozoic rocks are hardly systems in the sense
-in which the term is used in the case of the older rocks, but the
-reason for using these smaller subdivisions has already been
-mentioned. The addition of the Oligocene to the original divisions
-suggested by Lyell has been found useful, and the term will be used
-in this work.
-
-The reasons for the adoption of the particular minor subdivisions
-(series and stages) in the following chapters will frequently appear
-when the rocks of the various systems are described, and need not be
-further alluded to in this place.
-
-Although most geologists describe the stratified rocks in ascending
-sequence beginning with the oldest, and proceeding towards the newest,
-others, and notably Lyell, adopted the opposite method and commenced
-with an account of the newest beds. The argument generally used for
-the latter method is that it is easier to work from the study of the
-known to that of the less known, and as the faunas of the newest rocks
-are most like the existing faunas, the student would more readily
-follow a description of the rocks in the order which is opposite to
-that in which they were deposited.
-
-In practice, the study of the sediments in their proper order, that
-is, in the order of deposit, will not be found to task the student to
-any great extent, especially if, as is very desirable, he has studied
-the main facts and principles of Palæontology before commencing the
-study of the rock-systems in detail. There is one reason for beginning
-with the study of the older sediments which outweighs any reasons
-which can be advanced against it, namely that the events of any period
-produce their effect not only upon the strata of that period, but also
-on those of succeeding periods.
-
-The task of the stratigraphical geologist is really to learn the
-evolution of the earth, in its changes from the simple to the more
-complex conditions, and it is quite obvious that it is unnatural to
-attempt any study of evolution by working backward. For this reason
-the study of the sediments will be here made in the order which is
-usually adopted, by passing from the older to the newer, and from the
-simple to the more complex.
-
-The British strata will be mainly considered, though references will
-frequently be made to their foreign equivalents, and a fuller account
-of the latter will be added when the British strata are abnormal, as
-are those of Triassic times, and also when a period is not represented
-amongst the strata of the British Isles, as for instance, the
-Permo-Carboniferous and Miocene periods.
-
-The student is recommended to refer constantly to good geological maps
-of the British Isles, of Europe, and of the world. Of maps of the
-British Isles, mention may be made of Sir A. Ramsay's geological map
-of England, Sir A. Geikie's map of Scotland, and his map of the
-British Isles, J. G. Goodchild's map of England and Wales, a map of
-Europe by W. Topley and one of the world reduced from that by J.
-Marcou, accompanying the first and second volumes of the late Sir J.
-Prestwich's _Geology_. For special purposes more detailed maps will be
-studied, including the one-inch maps of H. M. Geological Survey, and
-the index map on a smaller scale. Lastly, for an account of British
-Geology, reference must be made to H. B. Woodward's _Geology of
-England and Wales_, where the British formations are described in
-order, and to W. J. Harrison's _Geology of the Counties of England and
-Wales_, where the stratigraphical geology of the country is given
-under the head of the different counties, which are taken in
-alphabetical order.
-
-In concluding this chapter, it is hardly necessary to say that every
-opportunity of studying the characters of the deposits and their
-fossils in the field should be eagerly seized, and that much
-information may be acquired even on a railway journey, especially as
-to the influence which the deposits exert upon the scenery of a
-region[47].
-
-[Footnote 47: In the first edition of H. B. Woodward's _Geology of
-England and Wales_, an account of the geology of the main lines of
-English railways is given, which is omitted in the later edition. It
-is well worth consulting by those who take a long journey, and it will
-be found useful to take a geological map with one on the journey so as
-to discover when one is passing from one formation to another.]
-
-
-
-
-CHAPTER XII.
-
-THE PRECAMBRIAN ROCKS.
-
-
-Study of a geological map of the world will shew that extensive
-regions, such as parts of Scandinavia, many tracts of Central Europe,
-a large area in Canada, and a considerable portion of Brazil and the
-adjoining countries are occupied by crystalline schists, which
-underlie the oldest known sedimentary strata in those places. These
-crystalline schists form the floor upon which the sediments
-constituting the bulk of the geological column rest, and it is
-necessary that we should know something of the character of this
-floor. Other rocks which can be definitely proved to be of Precambrian
-age are often found associated with the crystalline schists, and these
-associated rocks have often undergone more or less alteration
-subsequently to their formation. The difference between the coarser
-types of crystalline schists and these associated rocks is sometimes
-so marked that geologists have necessarily paid attention to it, and
-separated the two groups of rocks; the term Archæan has been used by
-some geologists to include the crystalline schists, and Eparchæan for
-the associated rocks of known Precambrian age, but though this
-separation may sometimes be effected, there are cases when it is
-impossible to draw any sharp line of demarcation between 'Archæan'
-and 'Eparchæan' types.
-
-In the present state of our knowledge, a chronological classification
-of the Precambrian rocks when applied to wide and distant regions is
-destined to break down, and it will be convenient if we consider at
-some length the features of the Precambrian rocks of a particular
-region, and apply the knowledge thus gained to a study of Precambrian
-rocks of other areas, and to a consideration of our knowledge of the
-Precambrian rocks as a whole. In doing so, the term 'crystalline
-schists' will be used somewhat vaguely with reference to a complex of
-schistose rocks of which the mode of origin cannot be fully
-determined. We may take our own country as a region where a good
-development of the Precambrian rocks occurs.
-
-A few explanatory remarks concerning the mode of detection of
-Precambrian rocks may not be amiss. If any true organisms have been
-hitherto discovered amongst the rocks formed before Cambrian times
-they are valueless as a means of correlating rocks, and accordingly
-lithological characters only are available in attempting to correlate
-the rocks of one area with those of another. Those who have read the
-preceding chapters will have gathered that comparisons founded on
-similarity of lithological character are not so valuable as those made
-after careful scrutiny of the fossils of strata, but they are by no
-means valueless, and when the rocks of two areas which are not far
-distant from one another present close lithological resemblances,
-their general contemporaneity may be inferred with some degree of
-certainty.
-
-It is only when we get the lowest Cambrian strata overlying earlier
-rocks that we have absolute proof of the Precambrian age of the
-latter, and it is necessary, therefore, that we should have some
-definite lower limit to the rocks of the Cambrian system. It is now
-generally agreed that that limit shall be drawn at the base of a group
-of rocks containing what is known as the _Olenellus_-fauna, which will
-be considered at greater length in the next chapter, and it will be
-well, if the term Cambrian be not in future applied to any rocks
-beneath the ones containing the relics of this fauna, for otherwise
-there is danger of the indefinite downward extension of the Cambrian
-system. We need not be surprised to find great thicknesses of rock
-below the rocks containing the _Olenellus_-fauna, and passing upwards
-with complete conformity into those rocks; nevertheless, if it can be
-shewn that the _Olenellus_-fauna had not appeared during the
-deposition of the underlying group, the rocks of that group should be
-termed Precambrian. A case of this nature has not yet been detected in
-our area, and all the rocks which have been proved to be Precambrian
-in Britain are separated from the overlying Cambrian rocks by a
-physical break, though that break is not necessarily very large, and
-in some districts is probably of little importance. Hitherto the
-_Olenellus_-fauna has been detected in Ross, Warwickshire, Shropshire,
-Worcestershire and probably in Pembrokeshire, and the rocks underlying
-the _Olenellus_-beds in those counties can be proved to be Precambrian
-(i.e. if the _Olenellus_-age of the Pembrokeshire rocks be ultimately
-established, and the researches of Dr Hicks tend to prove that it will
-almost certainly be done). It will be convenient if we take the
-instances where the age of the rocks can be proved with certainty or
-with a considerable degree of probability first, and then consider the
-examples of rocks which are found below Cambrian strata, though these
-have not hitherto yielded the _Olenellus_-fauna, concluding with a
-notice of rocks which have been claimed to be of Precambrian age on
-account of their lithological characters, though they are not now seen
-to be immediately succeeded by strata appertaining to the Cambrian
-system.
-
-Commencing with the region where we have the greatest development of
-the known Precambrian rocks, namely Ross, Sutherland and the Hebrides,
-we may explain the general relationship of the rocks by means of a
-generalised section (fig. 15).
-
-[Illustration: Fig. 15.]
-
-The lowest rocks _a_ are crystalline schists, they are succeeded by a
-set of arenaceous rocks _b_ known as the Torridonian beds, which rest
-unconformably upon the upturned edges of the crystalline schists,
-whilst the Cambrian rocks, _c_, rest with another unconformity
-sometimes upon the partly denuded Torridonian beds, or where the
-latter have been completely removed, as on the right side of the
-figure, directly upon the crystalline schists, thus presenting an
-example of unconformable overlap. The occurrence of the
-_Olenellus_-fauna in the basement beds of the Cambrian system near
-Loch Maree, proves the Precambrian age of the Torridonian strata,
-whilst the unconformable junction between the latter and the
-crystalline schists indicates that we are here dealing with two
-distinct sets of Precambrian rocks, one of Eparchæan and the other of
-Archæan type.
-
-The crystalline schists consist of rocks of very varied lithological
-characters, some with gneissose, and others with schistose structure,
-and they vary in degree of acidity from ultrabasic rocks to those of
-acid composition. Most of them exhibit parallel structures, which in
-many cases can be shewn to have been impressed on the rocks
-subsequently to their consolidation, though this need not have
-occurred and probably did not occur with some of them, especially the
-granitoid gneisses. The researches of the members of H. M. Geological
-Survey have shewn that many of these rocks were originally intrusive
-igneous rocks, though it is not yet known into what rocks those which
-were first consolidated were injected, and the origin of the bulk of
-the schists still remains to be elucidated. Subsequently to their
-consolidation and before the deposition of the earliest Torridonian
-rocks they were subjected to more than one set of earth-movements,
-which folded them and impressed a series of parallel structures upon
-many of them; and accordingly we find that the pebbles of the
-crystalline schists which are found amongst the basal conglomerates of
-the Torridonian rocks consist of fragments which had undergone the
-alteration caused by these earth-movements before they were denuded
-from their parent-rocks[48].
-
-[Footnote 48: For an account of these rocks, their characters, and the
-effects of earth movement upon them, the reader should consult a
-"Report on the Recent Work of the Geological Survey in the North-West
-Highlands of Scotland": _Quart. Journ. Geol. Soc._, vol. XLIV. p.
-378.]
-
-The Torridonian system is composed of rocks which are largely of
-arenaceous character, the most prominent beds being formed of red
-sandstones, and the bulk of the fragments in them have clearly been
-derived by denudation from the crystalline schists, many of the beds
-being composed of arkose, where the quartz is mixed with a large
-proportion of felspar and often of ferro-magnesian minerals. The
-deposits are clearly sedimentary, and are as little altered as many
-strata of much more recent origin, only possessing structures produced
-by metamorphic action under exceptional circumstances. The detailed
-researches of the geological surveyors prove that the rocks of this
-system have a much greater thickness and are of more varied
-lithological characters than was previously supposed. The total
-thickness of the strata is over 10,000 feet, and the sandstones are
-associated with deposits of a muddy character, and with occasional
-bands of limestone; in these circumstances the discovery of fossils
-would excite no surprise, and in 1891 Sir A. Geikie announced the
-detection of "traces of annelids and some more obscure remains of
-other organisms in these strata," which have not yet been
-described[49]. These Torridonian strata furnish us with the most
-satisfactory group of Precambrian sediments yet detected in
-Britain[50].
-
-[Footnote 49: An account of the subdivisions and lithological
-characters of the rocks of the Torridonian System will be found in the
-_Annual Report of the Geological Survey of the United Kingdom_ for
-1893.]
-
-[Footnote 50: It has been recently maintained that some of the
-Torridonian rocks are of Æolian origin.]
-
-In the south-east Highlands is a great mass of crystalline schists of
-a less gneissose character than that of the north-west, to which Sir
-A. Geikie has applied the name Dalradian. Many of these schists will
-be found by examination of the geological map of Scotland to be
-separable into divisions, which by means of their lithological
-characters can be traced long distances across the country, and they
-present all the characters of sedimentary rocks, though they are
-associated with intrusive igneous rocks, and have undergone great
-metamorphic changes since their formation. Cambrian rocks have not yet
-been discovered immediately above them, though they are clearly older
-than Ordovician times, but the existence of rocks associated with them
-along their north-west borders, which in lithological characters
-closely resemble some of the rocks of the crystalline schists of the
-north-west Highlands, indicates the probability of their general
-Precambrian age. In some instances, the extreme types of metamorphism
-which they exhibit are the result of the kind of action usually termed
-pyrometamorphic as has been shewn by Mr G. Barrow[51].
-
-[Footnote 51: Barrow, G. "On an Intrusion of Muscovite-biotite gneiss
-in the S.E. Highlands of Scotland, and its accompanying metamorphism."
-_Quart. Journ. Geol. Soc._, vol. XLIX. p. 330.]
-
-In England and Wales the rocks which have been shewn or inferred to be
-Precambrian, when not intrusive, are largely of volcanic origin. The
-most satisfactory example of the occurrence of the _Olenellus_-fauna
-is that of the Cambrian Comley sandstone of Shropshire, which rests
-unconformably upon a set of rocks termed by Dr Callaway the Uriconian
-rocks; the latter are essentially volcanic, and strongly resemble
-Precambrian rocks of other British areas. There is also strong reason
-to suppose that the sediments to which the name Longmyndian has been
-applied, which have been described by the Rev. J. F. Blake, are of
-Precambrian age, for, as Professor Lapworth has pointed out, the three
-great subdivisions of the Cambrian system are present in the area
-under consideration, and the rocks of each are entirely different from
-those of the adjoining Longmynd area. In Shropshire therefore we meet
-with one set of volcanic rocks, and another set consisting of
-sedimentary rocks, of which the former is certainly, the latter
-almost certainly of Precambrian age, and as the Longmyndian rocks are
-in a comparatively unaltered condition, consisting of normal
-sediments, we may well expect the discovery of fossils in them
-also[52]. The _Olenellus_-fauna has been found near Nuneaton in
-Warwickshire in beds which unconformably succeed volcanic rocks, the
-Caldecote series of Prof. Lapworth, and the latter are therefore of
-Precambrian age[53]. A few fossils belonging to the _Olenellus_-fauna
-have occurred in the oldest Cambrian rocks of the Malvern district,
-and these rocks rest unconformably upon those of an old ridge which is
-therefore composed of Precambrian rocks. The rocks of this ridge are
-largely of intrusive igneous origin, though parallel structures have
-been impressed upon them as the result of subsequent deformation, but
-some of the rocks are almost certainly of contemporaneous volcanic
-origin[54]. In the Wrekin ridge, igneous and pyroclastic rocks are
-found succeeded unconformably by Cambrian rocks which resemble those
-of the Malvern and Nuneaton districts, and probably belong to the
-period of existence of the _Olenellus_-fauna, and these igneous and
-pyroclastic rocks are presumably of Precambrian age, and the
-contemporaneous rocks constitute Dr Callaway's typical Uriconian
-group. Volcanic ashes and breccias are accompanied by devitrified
-pitchstones and intruded granitic rocks, which may or may not be all
-of the same general age[55]. The rocks which have been claimed as
-Precambrian in Pembrokeshire and in Caernarvonshire have the same
-general characters as those of the Wrekin ridge. Pyroclastic rocks
-underlie the oldest Cambrian rocks, with discordance between the two,
-and associated with these pyroclastic rocks are quartz felsites which
-according to some are of contemporaneous nature whilst others maintain
-their intrusive origin. In each county granites are found which are
-now generally recognised to be intrusive, though there seems to be no
-doubt as to their being of the same general age as the rocks with
-which they are associated, and therefore presumably Precambrian. The
-Pembrokeshire rocks are marked by the occurrence of a certain amount
-of metamorphism, probably of more than one kind, which has converted
-pyroclastic volcanic rocks into sericitic-schists and quartz-felsites
-into hälleflintas[56]. The term Pebidian given by Dr Hicks to the
-contemporaneous volcanic fragmental rocks should be retained, and if
-these rocks be eventually shewn to be contemporaneous with similar
-volcanic rocks of other districts, may be applied generally, as it has
-priority over other terms as Uriconian and Caldecote series. The term
-Dimetian was applied to rocks known to be intrusive, and must be
-dropped as a chronological term, whilst the existence of an Arvonian
-system separate from the Pebidian system is not fully proved.
-
-[Footnote 52: The reader may consult a paper by Prof. Lapworth "On
-_Olenellus Callavei_ and its geological relationships," _Geol. Mag._
-Dec III. vol. VIII. p. 529, for information concerning the
-relationship of the _Olenellus_ beds of Shropshire to the more ancient
-rocks; the Uriconian rocks are described by Dr Callaway in a series of
-papers, especially in the _Quarterly Journal of the Geological
-Society_, vol. XXXV. p. 643, vol. XXXVIII. p. 119, vol. XLII. p. 481
-and vol. XLVII. p. 109, whilst the lithological characters of the
-Longmyndian rocks are described by the Rev. J. F. Blake (_Quart.
-Journ. Geol. Soc._, vol. XLVI. p. 386).]
-
-[Footnote 53: See Lapworth, C., "On the sequence and systematic
-position of the Cambrian rocks of Nuneaton," _Geol. Mag._ Dec III.
-vol. III. p. 319; and Waller, T. H., "Preliminary Note on the Volcanic
-and Associated Rocks of the neighbourhood of Nuneaton," _ibid._ p.
-322.]
-
-[Footnote 54: For details concerning the rocks of the Malvern Hills
-see papers by Callaway in the _Quarterly Journal of the Geological
-Society_, vol. XXXVI. p. 536, XLIII. p. 525, XLV. p. 475, and XLIX. p.
-398, and a paper by Prof. A. H. Green, _ibid._ vol. LVI. p. 1.]
-
-[Footnote 55: Callaway, C., _Quart. Journ. Geol. Soc._, vol. XXXV. p.
-643.]
-
-[Footnote 56: The Pembrokeshire area is of interest as the probable
-existence of Precambrian rocks in Britain was first indicated on good
-evidence in this county. The general structure of the district is
-fairly simple, consisting of Cambrian rocks beneath which Precambrian
-rocks are exposed in at least two ridges of which the northerly and
-more important one runs through St Davids. The rocks of the St Davids
-ridge consist of a binary granite (granitoidite), felsites, and
-volcanic ashes and breccias of intermediate composition. Much
-diversity of opinion has existed, and to some extent still exists as
-to questions of detail, and a very extensive literature has been
-devoted to these rocks. Amongst the numerous papers which treat of
-them, the student may consult the following:--Hicks, H., _Quart.
-Journ. Geol. Soc._, vol. XXXIII. p. 229, XXXIV. p. 147, XXXV. p. 285,
-XL. p. 507, XLII. p. 351, Geikie, A., _ibid._ vol. XXXIV. p. 261,
-Blake, J. F., _ibid._ vol. XL. p. 294, and Morgan, C. Ll., _ibid._
-vol. XLVI. p. 241. Much of the matter contained in these papers is
-controversial, and need not be fully read by those who merely wish to
-obtain a general account of the rocks of the district.]
-
-In Caernarvonshire two ridges are found, the one running from Bangor
-to Caernarvon, and the other through Llanberis lake. The rocks of
-these are generally similar to those of St Davids, and as the lowest
-Cambrian rocks of the area closely resemble those of St Davids, the
-Precambrian age of the rocks of these ridges is rendered highly
-probable, though until the discovery of the _Olenellus_-fauna in the
-area, it cannot be regarded as proved[57].
-
-[Footnote 57: These rocks are described by T. M^{c}K. Hughes, _Quart.
-Journ. Geol. Soc._, vol. XXXIV. p. 137, and XXXV. p. 682; by Prof. T.
-G. Bonney, _ibid._ vol. XXXIV. p. 144; and by Dr Hicks, _ibid._ vol.
-XXXV. p. 295.]
-
-The actual position of the similar rocks of Anglesey has not been so
-clearly fixed, as the rocks associated with them are of Ordovician
-age, but their resemblance to the rocks of the adjoining regions
-renders their Precambrian age highly probable. It is interesting to
-find in association with the rocks which resemble those of
-Caernarvonshire, others which Sir A. Geikie recognises as quite
-similar to some existing amongst the crystalline schists of the
-north-west Highlands of Scotland, and when these ancient rocks of
-Anglesey have been mapped in detail, they will probably be found to
-present greater variety than is afforded by any Precambrian rocks of
-Great Britain occurring S. of the Scotch border[58].
-
-[Footnote 58: Papers upon the old rocks of Anglesey will be found in
-many volumes of the _Quarterly Journal of the Geological Society_; see
-especially Hicks, vol. XXXV. p. 295, Callaway, vol. XXXVI. p. 536,
-XXXVII. p. 210, and Blake, XLIV. p. 463.]
-
-Of rocks whose age is more uncertain, but which are probably of
-Precambrian age, those of Charnwood Forest in Leicestershire may first
-be noticed. They are largely of pyroclastic origin, and from their
-likeness to similar rocks of proved Precambrian age, they are very
-probably of this age, as suggested by Messrs Hill and Bonney[59]. A
-group of crystalline schists is found in the south of Cornwall,
-especially near the Lizard, and similar rocks are found in the Channel
-Isles. As their relationship to newer rocks is not clear, little can
-be said about them, which has not already been noticed in mentioning
-the crystalline schists of other regions[60].
-
-[Footnote 59: Hill and Bonney, _Quart. Journ. Geol. Soc._, vol.
-XXXIII. p. 754, XXXIV. p. 199 and XLVII. p. 78; see also Watts, W. W.,
-_Rep. Brit. Assoc._ for 1896, p. 795.]
-
-[Footnote 60: For an account of the Volcanic History of Britain in
-Precambrian times, see Sir A. Geikie, Presidential Address to the
-Geological Society, _Quart. Journ. Geol. Soc._, vol. XLVII. p. 63.]
-
-The Precambrian rocks of the European continent consist largely of
-crystalline schists which in their general aspects recall those of the
-north-west Highlands of Scotland. Important masses are found in
-Bavaria, Bohemia, France, Spain, Scandinavia and Russia. The
-Scandinavian and Russian rocks of Archæan type are in places succeeded
-by the _Olenellus_-bearing beds of the Cambrian rocks, and rocks of
-Eparchæan character are not extensively developed, though certain
-Norwegian rocks may be the equivalents of the Torridonian rocks of
-Scotland, and other rocks of this type are found in places in Sweden.
-In Bohemia and in Brittany Precambrian strata of Eparchæan type have
-been discovered, and this type probably occurs elsewhere in Europe.
-
-The North American rocks require some notice, for it was in Canada
-that the existence of Precambrian rocks was first recognised, and the
-term Laurentian, originally applied to an Archæan type of Precambrian
-rocks in Canada, was subsequently adopted in speaking of many
-Precambrian rocks elsewhere, though it is now wisely restricted to the
-type of rock in the original area to which the name was first given.
-These Laurentian rocks acquired a special, interest on account of the
-occurrence in their limestones of a supposed reef-building
-foraminifer, _Eozoon canadense_, but detailed study of its structure
-and mode of occurrence has convinced most geologists that the
-structure is inorganic.
-
-The Laurentian rocks of the typical Laurentide region are largely
-crystalline schists associated with massive crystalline rocks. The
-attempt to separate them chronologically into a Lower and Upper
-division was premature, as shewn by the fact that many of them, upon
-detailed study, prove to be intrusive igneous rocks. In the
-neighbourhood of Lake Huron, a set of sedimentary rocks overlying the
-Archæan rocks is of Eparchæan type, consisting to a great extent of
-volcanic rocks, clay-slates and schists with intrusive igneous rocks;
-it has been termed the Huronian System, and this term has also been
-extensively applied to other Eparchæan types found elsewhere, but
-should be restricted to the rocks of the Huron district. A number of
-other rocks of Eparchæan type have been discovered in various parts of
-North America, and have been grouped together under the title of
-Algonkian, a name proposed for them by Dr C. D. Walcott, and an
-attempt has been made to arrange them in chronological order, though
-in the absence of fossils, the rocks of different districts can only
-be so arranged by reference to lithological characters; nevertheless a
-detailed study of the Eparchæan and some of the more finely
-crystalline schistose rocks points to the existence of a number of
-divisions of sedimentary rocks of Precambrian age, some of which may
-attain to the dignity of forming separate systems[61]. By far the most
-instructive development of American Precambrian rocks has been found
-in the Rainy Lake region of Canada, and it is the subject of a special
-memoir by Dr A. C. Lawson[62]. The Archæan rocks of the region are
-divided into a lower Laurentian and an upper division, which is
-further subdivided into the Coutchiching series below and the Keewatin
-series above, though the rocks of the Keewatin series are largely of
-Eparchæan character. The Laurentian rocks of this region resemble
-those of the Laurentide area, and consist of highly crystalline
-schistose and gneissose rocks associated with compact rocks. The
-Coutchiching series consists of mica schists and grey laminated
-gneisses, which appear to have been of sedimentary origin, altered by
-subsequent metamorphic action, while the Keewatin series, which
-reposes sometimes upon the rocks of the Coutchiching series (when the
-junction is an unconformable one), sometimes upon the Laurentian
-rocks, is formed of pyroclastic rocks and lava flows with intercalated
-sedimentary rocks; some of the Keewatin rocks are highly metamorphosed
-but others have undergone little or no metamorphic change. The most
-important point in connexion with these rocks of the Rainy Lake Region
-has reference to the relationship between the Laurentian rocks and
-those of the Coutchiching and Keewatin series. Lawson demonstrates the
-igneous nature of the Laurentian rocks, and brings forward evidence of
-various kinds that they were formed "by the fusion of the basement or
-floor upon which the formations of the upper division of the Archæan
-were originally deposited. With the fusion of this floor it seems
-probable that portions of the superincumbent strata, which once formed
-integral parts of either the Coutchiching series or the Keewatin, have
-also been absorbed into the general magma, and reappeared on
-crystallization as Laurentian gneiss. This fusion, however, only
-extended up to a certain uneven surface, which surface constitutes the
-demarcation between the present upper and lower Archæan. Above this
-surface, or upper limit of fusion, the formation of the Coutchiching
-and Keewatin series retained their stratiform or bedded disposition,
-and rested as a crust of hard and brittle rocks upon the magma,
-subject to its metamorphosing influences[63]."
-
-[Footnote 61: A large number of classifications have been proposed for
-the Archæan rocks of America; the most plausible one is given in Sir
-A. Geikie's _Text Book of Geology_, Third Edition, p. 716.]
-
-[Footnote 62: Lawson, A. C., _Report on the Geology of the Rainy Lake
-Region_. Montreal, 1888.]
-
-[Footnote 63: Lawson, _op. cit._ p. 139.]
-
-We may now pass briefly in review the evidence which has been so far
-obtained as to the mode of formation of the various Precambrian rocks.
-The existence of a very varied fauna amongst the earliest Cambrian
-strata has been commented upon by many geologists, and according to
-accepted explanations of the origin of that fauna, an enormous period
-of time elapsed before the deposition of the earliest Cambrian strata.
-During portions of that long period, the undoubtedly clastic rocks of
-Eparchæan type were deposited, and probably many others which are now
-so altered by metamorphism, like some of the Coutchiching rocks of
-Canada, that their original clastic origin can only be inferred and
-not directly proved. Volcanic activity was very rife during the
-deposition of some of these Eparchæan rocks, though perhaps not more
-so than during the formation of some of the Lower Palæozoic Rocks. All
-attempts to prove the occurrence of organisms in Precambrian strata
-have hitherto failed, for no undoubted fossil has been described which
-is unhesitatingly accepted as of Precambrian age, notwithstanding the
-many asserted occurrences of such fossils. That fossils will
-eventually be discovered is more than probable, and their
-non-detection at the present time is in no way very surprising, when
-we remember the long time that elapsed after the existence of
-stratified rocks below the Upper Palæozoic rocks had been recognised,
-before definite faunas were discovered in them. The determination of
-the Precambrian age of stratified rocks is recent, and now that this
-determination has been made, the search for fossils will be more
-eager, and is likely to be rewarded by their discovery. Furthermore,
-experience shows that when fossils are discovered in rocks of unknown
-age, there is a tendency to refer those rocks to some known period,
-and consequently we may actually possess Precambrian fossils, out of
-beds which have been erroneously referred to the Cambrian or a later
-period.
-
-Another important question is that of the metamorphism of a large
-number of Precambrian rocks, and here again recent research tends to
-show that the metamorphism is not of a kind different from that which
-occurred after the end of Precambrian times; the discovery of
-crystalline schists in Norway, Kirkcudbrightshire and Westmorland
-amongst Lower Palæozoic rocks, which resemble those of Archæan masses
-in all respects except in the extent of area which they cover, shows
-that similar processes to those which occurred in Precambrian times
-went on during later periods, though perhaps not on so large a scale.
-The great extent of these metamorphic rocks of Precambrian age can
-hardly be due in any great degree to the longer time during which they
-have been subjected to metamorphic influence, for there is evidence
-that much of the change took place in Precambrian times, far more than
-has occurred since, and it is a significant fact that these old rocks
-are more extensively penetrated by intrusive igneous masses than those
-of later periods; here again we find that much of the intrusion
-actually occurred in Precambrian times. The greater extent of
-intrusion and metamorphism amongst these Precambrian rocks than
-amongst later sediments indicates some differences of conditions in
-the case of Precambrian and later times. If besides intrusion, actual
-fusion of floors of Precambrian rocks occurred, we may well suppose
-that the earlier records of the rocks are for ever lost to us, the
-earliest sediments having been fused, but that the history of life
-upon our earth is to be revealed to us first in so late a stage as
-that of Cambrian times is highly improbable, and we may look forward
-with confidence to laying bare the records of the rocks composing the
-geological column some way below the Cambrian portion of the column.
-
-Upon this foundation of igneous rock, sediment and volcanic material,
-formed in Precambrian times, whose history we have only begun to
-study, was laid down the great mass of sediment which the geologist
-has more completely studied, where abundant traces of life are
-preserved, and concerning whose history we can gain a greater insight
-than is permitted us in the case of the old Foundation Stones.
-
-
-
-
-CHAPTER XIII.
-
-CYCLES OF CHANGE IN THE BRITISH AREA.
-
-
-Before studying in further detail the strata of the geological column,
-it will be convenient to deal with the great physical changes which
-have occurred in the British area from Precambrian times to the
-present day, as this will clear the way for a right appreciation of
-the main variations in the characters and distribution of the strata.
-
-At the end of Precambrian times there was a general upheaval of the
-British area, and this we may speak of as the First Continental
-Period. It was followed by depression and extensive sedimentation,
-proceeding more or less continuously though with local interruptions
-through Lower Palæozoic times, so that so far as Britain is concerned
-we may speak of Lower Palæozoic times as constituting the First Marine
-Period. Extensive upheaval gave rise to continental tracts and
-mountain chains, and deposits of abnormal character (as compared with
-ordinary marine deposits) at the end of Lower Palæozoic times;--the
-Devonian period was one of elevation and denudation, and we may
-therefore refer to it as the Second Continental Period. This was
-followed by depression and sedimentation in Carboniferous times, and
-these Carboniferous times constitute the Second Marine Period.
-Elevation gave rise to continental tracts and mountain chains at the
-end of Carboniferous times, and here again we find proofs of extensive
-denudation and the formation of abnormal deposits:--the Permo-Triassic
-period is the Third Continental Period. Depression set in during early
-Jurassic times and continued throughout the Mesozoic and the early
-part of Tertiary times, which form the Third Marine Period.
-Disturbances culminating in Miocene times once more produced
-terrestrial conditions. In this, the Fourth Continental Period, we are
-still living.
-
-From what has been previously written it will be seen that each of the
-marine periods should be marked by an early and late shallow-water
-phase, separated by an intervening marine phase, and the importance of
-the phases will depend upon the length of time during which they
-existed, and will differ markedly in different cases, whilst the
-distinctness of the middle phase from the upper and lower, will depend
-upon the magnitude of the maximum submergence.
-
-During the first marine period submergence was comparatively rapid,
-and the shallow-water phase only lasted through very early Cambrian
-times in most regions, whilst the deep-water phase, complicated by
-many minor upheavals, extended through the main part of Cambrian,
-Ordovician and Silurian times, and was replaced by the later
-shallow-water phase at the end of Silurian times.
-
-The second marine period again was ushered in by rapid submergence, so
-that the shallow-water phase was brief, and the main mass of the Lower
-Carboniferous strata was deposited in deep water; but, unlike the
-first marine period, the second was characterised by the occurrence of
-a long interval of time marking the later shallow-water phase, during
-which the whole of the Upper Carboniferous strata were deposited. The
-Carboniferous Marine Period is the simplest of the three with which we
-have to deal, as the local oscillations occurring on a fairly large
-scale for such movements were less frequent than was the case during
-the first and third marine periods.
-
-The third marine period had a long shallow-water phase at the
-commencement, with many minor oscillations, causing great variation in
-the character of the deposits and frequent minor unconformities. This
-shallow-water phase existed throughout Jurassic and Lower Cretaceous
-times. The deep-water phase existed during the deposition of the Upper
-Cretaceous deposits, and was succeeded by the second shallow-water
-phase, when the early Tertiary strata were accumulated.
-
-The difference between the elevations which accompanied the
-Continental Periods and those which have been alluded to as minor
-elevations is no doubt one of degree, but in considering the British
-strata only no confusion is likely to arise on this account, as the
-difference was here very great.
-
-The events which occurred during the continental periods are of
-extreme importance to the geologist. Every great upheaval was
-accompanied by crumpling and stiffening of portions of the earth's
-crust, and a definite trend was given to the strata as the result of
-these movements. It is to the earth-movements of the four great
-continental periods that the present structure of the British Isles is
-largely due, and in any attempt to restore the physical history of our
-islands considerable attention must be paid to the changes which were
-produced in the stratified rocks during these periods of
-earth-movement.
-
-
-
-
-CHAPTER XIV.
-
-THE CAMBRIAN SYSTEM.
-
-
-_Classification._ The rocks of the Cambrian system when found reposing
-on Precambrian rocks in Britain are always separated from the latter
-by an unconformity. The typical development of the rocks of the
-system, as the name implies, is in the hilly region of Caernarvonshire
-and Merionethshire in North Wales, and they are also well represented
-in South Wales, the border counties between England and Wales, and the
-North-West Highlands of Scotland. Two distinct classifications of the
-Cambrian rocks of Britain are in use, the original one founded on
-variations of lithological character, whilst the second depends upon
-faunistic differences, but the original lithological classification
-has been to some extent modified to make it locally correspond with
-the classification based upon palæontological grounds. The following
-table will shew the differences:--
-
-  Lithological Classification.          Palæontological Classification.
-
-  Tremadoc Slate Series[64]              Beds with Intermediate Fauna
-
-  Lingula Flags Series                  Beds with _Olenus_ Fauna
-
-  Menevian beds (formerly included   }
-     in Lingula Flags)               }  Beds with _Paradoxides_ Fauna
-               } Formerly grouped    }
-  Solva beds   } together as Harlech
-  Caerfai beds } or Llanberis beds      Beds with _Olenellus_ Fauna
-
-[Footnote 64: In accordance with the custom usually observed in
-Britain, the Tremadoc slates are placed in the Cambrian system; most
-continental geologists place them in the succeeding Ordovician system.
-The matter is not an important one, as the fauna is an intermediate
-one between that of the Lingula Flags and that of the Arenig series of
-the Ordovician system, and the beds are true beds of passage. As the
-lithological classification is essentially British, it will be as well
-to retain the Tremadoc Slates in the Cambrian system.]
-
-The original lithological classification was essentially the result of
-Prof. Sedgwick's work in North Wales, while the classification
-according to faunas is the outcome of the researches of Dr Hicks in
-South Wales.
-
-_Description of the Strata._ The Cambrian rocks of North Wales occur
-in two complex anticlines, separated by an intermediate syncline of
-Ordovician strata occupying the Snowdonian hills. The southerly or
-Harlech anticline forms a part of Merionethshire to the east of
-Harlech, whilst the northern one is developed around Bangor and
-Llanberis. The South Welsh Cambrian rocks are chiefly found on either
-side of the Pembrokeshire axis of Precambrian rocks which runs through
-St David's. As the corresponding rocks of the two regions were
-deposited in bathymetrical zones of much the same depth, it will be
-convenient to give a general account of the rocks of the two regions
-at the same time, leaving the student to acquire information of the
-detailed variations in the larger text-books and in special
-memoirs[65].
-
-[Footnote 65: A general account of the Cambrian, Ordovician and
-Silurian rocks will be found in the Sedgwick Essay for 1883, _A
-Classification of the Cambrian and Silurian Rocks_, though the use of
-a cumbrous nomenclature therein will tend to confuse the reader. For a
-detailed account of the Cambrian rocks of North Wales the reader is
-referred to the Geological Survey Memoir, _The Geology of North
-Wales_, by Sir A. Ramsay (2nd edition), he may also consult Belt, T.,
-"On the Lingula Flags or Festiniog Group of the Dolgelly district,"
-_Geol. Mag._, Dec I. vol. IV. pp. 493, 536, vol. V. p. 5. The geology
-of the Cambrian rocks is described in a series of Memoirs in the
-_Quarterly Journal of the Geological Society_ by Dr H. Hicks; the
-following should be consulted: Harkness, R. and Hicks, H., "On the
-Ancient Rocks of the St David's Promontory, South Wales, and their
-Fossil Contents," vol. XXVII. p. 384; Hicks, H., "On some Undescribed
-Fossils from the Menevian Group," vol. XXVIII. p. 173; and "On the
-Tremadoc Rocks in the neighbourhood of St David's, South Wales, and
-their Fossil Contents," vol. XXIX. p. 39. See also Hicks, "The
-Classification of the Eozoic and Lower Palæozoic Rocks of the British
-Isles," _Popular Science Review_, New Series, vol. V., and Hicks,
-"Life-zones in the Lower Palæozoic Rocks," _Geol. Mag._ Dec IV. vol.
-I. pp. 368, 399 and 441.]
-
-The strata of the Caerfai and Solva groups show the prevalence of the
-shallow-water phase almost uninterruptedly through the whole of the
-time occupied by their accumulation in the Welsh areas. They consist
-chiefly of basal conglomerates, succeeded by alternations of grits and
-shales, though the latter are often converted into slates, owing to
-the subsequent production of cleavage. The basal conglomerates of the
-Caerfai beds are frequently marked by the existence of enormous
-pebbles, composed of fragments of the rocks of the underlying
-Precambrian groups, and the possibility of the occurrence of glacial
-action during their accumulation as advocated by Dr Hicks must be
-taken into account. Above these beds are various coloured grits, with
-alternations of muddy sediments often coloured red[66]. The Solva
-group consists of massive grits, of various colours, also with
-alternations of mud, which have prevalent purple and green hues. The
-great thickness of the strata of the Caerfai and Solva Series, which
-sometimes exceeds 10,000 feet, must also be noted.
-
-[Footnote 66: In giving this description the red (Glyn) slates of
-North Wales are treated as belonging to the Caerfai series, though
-this correlation depends on lithological characters only at present.]
-
-The Menevian beds consist essentially of very fine, well laminated
-black and grey muds, which are of a texture favourable for the
-production of a somewhat regular jointing, causing the rock to break
-into small rectangular blocks. They are thin, not exceeding 600 feet
-in thickness, and indicate the incoming of the general deep-water
-phase of the Lower Palæozoic epoch. The Lingula Flags mark a local
-return to shallower water conditions, especially in the central
-portion. The total thickness is over 3,000 feet, of which the lower
-stage (locally the Maentwrog series) is over 500 feet, and consists of
-blackish muds, the middle (Festiniog stage[67]) is about 2,000 feet
-thick, and is composed chiefly of shallower water gritty flags, whilst
-the upper (Dolgelly) stage is of about the same thickness as the lower
-stage and has similar lithological characters.
-
-[Footnote 67: The term Festiniog has been used for the whole Lingula
-Flag series as well as for the middle stage. It will be well to use it
-with reference to the stage only.]
-
-The Tremadoc Slates are about 1,000 feet thick. They are divided into
-a lower and upper stage, of about equal thickness, and are essentially
-composed of iron-stained slates, with a considerable admixture of
-calcareous matter in some parts of South Wales, when they furnish the
-nearest approach to a limestone which has been found amongst the Welsh
-Cambrian strata. They were probably formed in a fairly deep sea.
-
-Much pyroclastic rock and some lava flows are intercalated amongst the
-Welsh Cambrian sediments. Tuffs are formed in the lower beds of St
-David's, and lavas and ashes have been found amongst the Lingula Flags
-and Tremadoc Slates of North Wales, while the Lingula Flags of South
-Wales have furnished several bands of ash to the north of
-Haverfordwest. Much of the material of the grits and muds may be
-derived from volcanic rocks, though how far this is so cannot be
-stated in the absence of information obtained by detailed petrological
-examination of the rocks.
-
-The various isolated outcrops of Cambrian strata amongst the counties
-of the Welsh borders and adjoining Midland counties indicate a great
-thinning of the Cambrian rocks in this direction.
-
-The probable equivalents of the Caerfai rocks occur at Nuneaton,
-Comley, and on the flanks of the Wrekin and Malvern hills. The thin
-basal conglomerates are succeeded by quartzites, and sometimes red
-calcareous sandstones (Comley sandstone). These rocks are succeeded by
-thin arenaceous and calcareous beds which represent either the Solva
-or Menevian beds of Wales. The Lingula Flags are represented by the
-Malvern Shales of the Malvern area and the Stockingford Shales of
-Nuneaton, whilst the Tremadoc Slates have as their equivalents the
-Shineton Shales. The exact thicknesses of these deposits do not seem
-to have been recorded, but Prof. Lapworth observes that in central
-Shropshire "the Comley and Shineton groups which ... have a collective
-thickness of perhaps less than 3,000 feet, we have apparently a
-condensed epitome of the entire Cambrian system as at present
-generally defined."
-
-The Cambrian rocks of the North-west Highlands consist of a thin
-conglomerate succeeded by grits and flags with shaley beds, and above
-these a mass of limestone, which may represent some of the Ordovician
-deposits as well as those of Cambrian age. Pending a complete
-description of the faunas of these rocks, it is sufficient to state
-that the only fauna which has hitherto been described in detail
-indicates the existence of Lowest Cambrian rocks. Further remarks will
-be made on this head when describing the character of the Cambrian
-faunas. The Cambrian rocks of the North-west Highlands are also very
-thin as compared with those of Wales, so that the Highland and Welsh
-borderland regions appear to have existed as a deeper sea area than
-that which is indicated by the Cambrian rocks of Wales, an inference
-which is to some extent borne out by study of the Cambrian rocks of
-extra-British areas, to which we may now turn.
-
-The principal European developments of Cambrian rock are found in
-Scandinavia, Russia, Bohemia and Spain, and of these the Scandinavian
-one is by far the most fully developed, as there is a complete
-sequence in the rocks of that peninsula. They occur both in Norway and
-Sweden, but the Swedish exposures are the most interesting in most
-respects, especially those of Westrogothia and Scania. The rocks are
-of no great thickness, and consist essentially of black carbonaceous
-shales, with inconstant bands of impure black limestone composed
-almost entirely of the remains of trilobites or more rarely of
-brachiopods. These Alum Shales, as they are termed, rest unconformably
-upon Precambrian rocks, and have arenaceous and conglomeratic deposits
-at the base. In Russia the rocks are still further attenuated, and
-have not yielded the relics of so many faunas as have been found in
-the Scandinavian Cambrian rocks.
-
-The Bohemian development is incomplete, owing apparently to an
-unconformity at the base of the overlying Ordovician rocks, while the
-Spanish deposits which seem fairly thick and composed largely of
-mechanical sediments have not been worked out in very great detail.
-
-The American development of Cambrian rocks resembles the European one
-in many striking particulars, and as in the case of Europe, there are
-lateral variations in the lithological characters of the rocks, though
-in the opposite direction, the shallow-water deposits occurring on the
-east coast, and the deep-water deposits further west.
-
-The general distribution of the different types of Cambrian strata in
-Europe and North America has been accounted for on the supposition
-that in Cambrian times a tract of land lay over much of the present
-site of the North Atlantic Ocean, and that the detritus of that land
-formed the shallow-water accumulations of Wales and the east of
-Canada, whilst further away from it were deposited the open-sea
-accumulations of Scandinavia and Russia on one side and of the more
-westerly regions of North America on the other, as indicated in Fig.
-16.
-
-[Illustration: Fig. 16.
-
-  P. Precambrian Rocks.
-  A. Land.
-  X, XŽ. Sea level.
-  BBŽ. Shore deposits.
-  CCŽ. Deep-water deposits.
-  DDŽ. Abyssal deposits.
-]
-
-_The Cambrian Faunas._ The Cambrian Period has been termed the age of
-trilobites, for they are the dominant forms of the time, but they are
-associated with many other forms of invertebrata; indeed all the great
-groups of this division are represented in the earliest Cambrian
-fauna. Dr C. D. Walcott records representatives of Spongiae, Hydrozoa,
-Echinodermata, Annelida, Brachiopoda, Lamellibranchiata, Gastropoda,
-Pteropoda, Crustacea and Trilobita as occurring in the _Olenellus_
-beds of North America and other groups are represented in the rocks of
-this age in the Old World. The Cambrian trilobites as a whole are of
-more generalised types than those of the later systems which furnish
-their remains, as indicated especially by the looseness of the body,
-and the large number of body rings in many of the genera, while the
-tail or pygidium was small and formed of only a few coalesced
-segments, as pointed out by Barrande. In the later trilobites the test
-is more compact, there are on the whole fewer body rings, as more of
-these have become fused into a tail which is therefore larger than
-that of the average tail of the Cambrian trilobite.
-
-Taking the faunas in order, the oldest or _Olenellus_ fauna has
-furnished a great variety of forms in the North-west Highlands of
-Scotland, Shropshire, Scandinavia, Esthonia, Sardinia, Canada, and
-Newfoundland, whilst representative species of the fauna have been
-recorded also from Worcestershire, Warwickshire, Pembrokeshire, India,
-China, and Australia.
-
-The dominant form is the trilobite of the genus or group _Olenellus_,
-which contains a great variety of species referable to three or four
-divisions which have been ranked as separate genera by some writers.
-Associated with _Olenellus_ are trilobites belonging to other genera,
-which are found in higher deposits, though there represented by
-different species.
-
-Brachiopods are fairly abundant, especially those provided with a
-horny shell; of these, the genus _Kutorgina_ is widely distributed.
-
-The zoological relationships of several of the fossils of this horizon
-are as yet doubtful. The Archæocyathinæ show affinities with certain
-corals; a number of tests, included in the genus _Hyolithes_ and its
-allies are doubtfully referred to the Pteropods, and the position of
-the genus _Volborthella_ is uncertain. Special attention is directed
-to these doubtful relationships, as it is possible that a number of
-'generalised forms' of organisms occur in these strata[68].
-
-[Footnote 68: For an account of the _Olenellus_ fauna see Walcott, C.
-D., "The Fauna of the Lower Cambrian or Olenellus Zone," _Tenth Annual
-Report of the Director of the United States Geological Survey_,
-Washington, 1890. It is possible that some of the fossils mentioned in
-that report belong to strata above that containing _Olenellus_.]
-
-It should be noticed here that faunas have been discovered which are
-possibly of earlier date than the _Olenellus_ fauna, as they do not
-correspond with it, or with those of newer strata. One, the _Neobolus_
-fauna of the Salt Range of India, occurs in beds below those with
-_Olenellus_, though it is not yet clear that _Olenellus_ will not be
-eventually discovered associated with it, whilst the other, the
-_Protolenus_ fauna of Canada, is of unknown age[69].
-
-[Footnote 69: For an account of the _Neobolus_ beds see Noetling, F.,
-"On the Cambrian Formation of the Eastern Salt Range," _Records Geol.
-Survey, India_, vol. XXVII. p. 71, and for the Protolenus fauna
-consult a paper by Matthew, G. F., "The _Protolenus_ Fauna," _Trans.
-New York Acad. of Science_, 1895, vol. XIV. p. 101.]
-
-The _Olenellus_ beds are succeeded by beds containing the
-_Paradoxides_ fauna, which have been found in North and South Wales,
-Shropshire, Scandinavia, Bohemia, Spain, and North and South America.
-_Olenellus_ and its allies became extinct (or else so scarce that no
-relics of them have been discovered in the _Paradoxides_ beds) before
-the commencement of the deposition of the strata containing the
-_Paradoxides_ fauna, and few genera pass from the beds with the one
-fauna to that containing the other. The _Paradoxides_ fauna existed
-for a considerable period, and the beds have been divided into a
-series of zones characterised by different species of _Paradoxides_,
-thus
-
-Dr Hicks records the following zones in Pembrokeshire[70]:--
-
-  Zone of _Paradoxides_  _Davidis_   }  Menevian.
-    "             "      _Hicksii_   }
-
-    "             "      _Aurora_    }
-    "             "      _Solvensis_ }  Solva.
-    "             "      _Harknessi_ }
-
-[Footnote 70: The order here as elsewhere is _ascending_, i.e. the
-newest deposit is placed at the top.]
-
-Dr Tullberg divides the _Paradoxides_ beds of Scania into thirteen
-zones, though only a few of these are characterised by definite
-species of _Paradoxides_. The _Olenellus_ beds have not yet been
-divided into zones, though this will probably be the outcome of
-further study[71].
-
-[Footnote 71: The _Paradoxides_ fauna is described in the following
-works: Britain, Hicks, H. and Salter J. W., _Quart. Journ. Geol.
-Soc._, vol. XXIV. p. 510, XXV. p. 51, XXVII. p. 173, and Hicks, H. and
-Harkness, R., _ibid._ vol. XXVII. p. 384; Scandinavia, Angelin, N. P.,
-_Palæontologia Scandinavica_; Brögger, W. C., _Nyt Magazin for
-Naturvidenskaberne_, vol. XXIV., Linnarsson, G., _Sveriges Geologiska
-Undersökning_, Ser. C. No. 35; Bohemia, Barrande, J., _Système
-Silurien du centre de la Bohême_; Spain, Prado, C. de, "Sur
-l'existence de la faune Primordiale dans la chaîne Cantabrique suivie
-de la description des Fossiles par MM. de Verneuil et Barrande,"
-_Bull. Soc. Geol. France_, 2 Series, vol. XVII. p. 516; America,
-Walcott, C. D., _Bull. U. S. Geol. Survey_: "The Cambrian Faunas of
-North America," and Matthew, G. F., _Trans. Roy. Soc. Canada_, 1882
-and succeeding years.]
-
-The strata with _Paradoxides_ are succeeded by those with the _Olenus_
-fauna, characterised by the genus _Olenus_ and a large number of
-allied genera or sub-genera as some prefer to term them. The genus
-_Olenus_ (_sensu stricto_) is very abundant in the lower part of the
-series, whilst the allied forms are more abundant in the upper beds.
-The genus _Paradoxides_ and its associates disappeared before the
-deposition of these strata containing _Olenus_ and its allies, and
-indeed the complete change in the character of the faunas in Europe is
-very remarkable. The _Olenus_ fauna has been found in North Wales,
-Pembrokeshire, Warwickshire, Worcestershire, and abroad in Scandinavia
-and Canada. It is interesting to note among the fossils of the
-_Olenus_ beds the occurrence of a graptolite which is associated with
-_Olenus_ in Scandinavia; this is the earliest recorded appearance of a
-group which is destined to play so important a role amongst the
-fossils of the succeeding system[72]. The following zones have been
-detected by Dr S. A. Tullberg amongst the _Olenus_ beds of Scania:--
-
-  Zone of _Acerocare ecorne_.
-     "    _Dictyograptus flabelliformis_.
-     "    _Cyclognathus micropygus_.
-     "    _Peltura scarabæoides_.
-     "    _Eurycare camuricorne_.
-     "    _Parabolina spinulosa_.
-     "    _Ceratopyge_ sp.
-     "    _Olenus_ (proper).
-     "    _Leperditia_.
-     "    _Agnostus pisiformis_.
-
-[Footnote 72: For descriptions of the _Olenus_ fauna consult the
-following:--Wales, Belt, T., _Geol. Mag._ Dec. I. vol. V. p. 5, and
-Salter, J. W., _Decades Geol. Survey_, Decade II. Pl. IX. and Decade
-XI. Pl. VIII.; Scandinavia, Angelin, N. P., _Palæontologia
-Scandinavica_, and Brögger, W. C., _Die Silurischen Etagen 2 und 3 im
-Kristianiagebiet und auf Eker_; Canada, Matthew, G. F., "Illustrations
-of the Fauna of the St John Group, No. VI.," _Trans. Roy. Soc.
-Canada_, 1891.]
-
-The beds with _Dictyograptus flabelliformis_ form a wonderfully
-constant horizon at or near the top of the _Olenus_ beds. They are
-found in North Wales, the Border Counties between Wales and England,
-France, Scandinavia, Russia and Canada.
-
-The passage fauna of the beds which are the equivalents of the
-Tremadoc Slates may be spoken of as the _Ceratopyge_ fauna, for
-_Ceratopyge forficula_, a remarkable species of trilobite,
-characterises it in Scandinavia, and will probably be found
-elsewhere. _Ceratopyge_ beds have been found in North and South Wales,
-Shropshire, Scandinavia, Bavaria and North America, and in each case
-the fauna is intermediate in character between that of the Cambrian
-and that of the Ordovician system, containing the loosely-formed
-trilobites of the former with the more compact ones of the latter. The
-genus _Bryograptus_, a many-branched graptolite, also appears to
-characterise this fauna[73].
-
-[Footnote 73: For accounts of the Tremadoc Slates Fauna in England and
-Wales see Ramsay, A. C., _Geology of North Wales_, Appendix; Hicks,
-H., _Quart. Journ. Geol. Soc._, vol. XXIX. p. 39; Callaway, C.,
-_ibid._ vol. XXXIII. p. 652, whilst many of the foreign fossils are
-noticed in Brögger's _Die Silurischen Etagen 2 und 3_ and Barrande's
-_Faune silurienne des Environs de Hof en Bavière_.]
-
-The faunas of the Cambrian rocks have not been studied in sufficient
-detail, with reference to the physical surroundings of the organisms,
-to throw much light upon the conditions under which the strata were
-deposited, though the evidence obtained from an examination of the
-lithological characters of the deposits is generally corroborated by
-study of the organic contents.
-
-
-
-
-CHAPTER XV.
-
-THE ORDOVICIAN SYSTEM.
-
-
-_Classification._ The Ordovician strata were originally divided into
-series by Sedgwick as follows:--
-
-  Upper Bala,
-  Middle Bala,
-  Lower Bala,
-  Arenig.
-
-The Arenig series was at one time included by some writers with the
-Lower Bala under the name Llandeilo, but the word Llandeilo is now
-used in the sense of Sedgwick's Lower Bala. The Middle Bala is often
-spoken of as Caradoc, but the terms Bala and Caradoc are sometimes
-used interchangeably. As much confusion attaches to the use of the
-name Bala without explanation, the alternative titles have been
-largely adopted, and as the series are well defined there is no
-objection to their use, save that some expression is wanted equivalent
-to Upper Bala. The local term Ashgill shales was originally applied by
-Mr W. Talbot Aveline to beds of this age in Lakeland, and I have
-elsewhere suggested the use of this name for the whole series in that
-region; its use may well be extended to the series which is developed
-in many parts of Britain and the continent. The terms which will be
-used here, therefore, for the different series of the Ordovician
-system are the following:--
-
-  Ashgill Series (= Upper Bala)
-  Caradoc    "   (= Middle " )
-  Llandeilo  "   (= Lower  " )
-  Arenig     "
-
-Adopting a palæontological classification, we may speak of the Arenig
-and Llandeilo beds as those containing the _Asaphus_ fauna, whilst the
-Caradoc and Ashgill beds possess the _Trinucleus_ fauna; this is the
-terminology employed by Angelin for the equivalent strata of Sweden.
-It must be noted that here the names applied are not those of
-absolutely characteristic genera, as was the case with those adopted
-for naming the Cambrian faunas, for both _Asaphus_ and _Trinucleus_
-range through the beds of the system; but whereas _Asaphus_ is most
-abundant in the beds of the two lower series, _Trinucleus_ occurs most
-frequently in those of the two upper series.
-
-_Description of the strata._ The Ordovician rocks are found over large
-tracts in North and South Wales, in the counties on the Welsh border,
-in Lakeland and the outlying districts in the Southern Uplands of
-Scotland, and in detached areas in Ireland. There are three main types
-of deposit:--(i) the volcanic type, in which the ordinary sediments
-are associated with a large amount of contemporaneous volcanic matter,
-(ii) the black shale type, with a fauna consisting largely of
-graptolites, and (iii) the ordinary sedimentary type, in which we find
-alternations of grits, shales, and more or less impure limestones. We
-also find developments which are intermediate between any two or even
-all three of these types. The first type is characteristically
-developed in Caernarvonshire and Merionethshire, the second in the
-Dumfriesshire Uplands, and the third in the Girvan district of
-Ayrshire. The variation in the thickness of these three types of
-deposit is shown in the accompanying sections of the Caernarvon,
-Merioneth, Moffat and Girvan regions (see Fig. 17).
-
-[Illustration: Fig. 17.
-
-Showing the variations in the characters of the Ordovician deposits of
-the three principal types.
-
-  Scale 1 in. = 1000 feet.
-
-  A = Arenig.  L = Llandeilo.  C = Caradoc.
-
-The thickness of the Arenig rocks of the Scotch areas is unknown.]
-
-The North Welsh area gives two different developments of the
-Ordovician strata, one of which is much less volcanic than the other.
-In the Merioneth-Caernarvon area, two great masses of volcanic rock
-form the Aran and Arenig hills of Merioneth and the Snowdonian group
-of Caernarvon. The former are of Arenig, the latter of Caradoc age.
-The Merionethshire volcanic rocks consist of a great thickness of
-lavas and ashes of intermediate composition (anderites), associated
-with sandy and muddy sediments of no great vertical depth. The
-Llandeilo beds of this area are chiefly of the nature of black shales,
-while the Caradoc series is represented by volcanic lavas and ashes of
-acid composition (felsites) with a few thin interbedded sediments. A
-calcareous ash forming the summit of Snowdon is of importance as being
-on the same horizon as a limestone (the Bala limestone) found in the
-other North Welsh area. The Ashgill series is not represented in
-Snowdonia.
-
-In the other North Welsh tract, around Bala Lake, the volcanic matter
-is much less conspicuous. The Arenig rocks are not seen nearer than
-the Arenig mountains which form the western boundary of this second
-tract. The Llandeilo beds consist of shaley deposits with a
-well-marked limestone, the Llandeilo limestone, in the centre, whilst
-the Caradoc beds consist chiefly of muddy sediments with some thin
-ashes and a limestone, the Bala limestone, at the top. The Ashgill
-series contains a basal limestone, the Rhiwlas limestone, succeeded by
-shales, and another thin limestone called the Hirnant limestone at the
-summit.
-
-In South Wales the Arenig beds[74] are chiefly composed of slates, and
-are divisible into an upper and lower group. The total thickness is
-about 2000 feet. The Llandeilo beds contain three series:--
-
-  Upper Llandeilo Slates 1000
-  Llandeilo Limestone     200
-  Lower Llandeilo Slates  800.
-
-[Footnote 74: A remarkable fauna, fairly well represented in Britain
-and exceedingly well developed on the continent, exists in the
-Uppermost Arenig and Lower Llandeilo beds, and it is well separated
-from the dominant Arenig fauna below and Llandeilo fauna above. To the
-beds which contain it Dr Hicks has given the name Llanvirn series.]
-
-The Caradoc beds consist of black graptolitic shales of no great
-thickness, succeeded by an impure limestone on the horizon of the Bala
-limestone, while the Ashgill series like that of North Wales is
-separated into upper and lower limestone stages with an intervening
-stage composed of shales.
-
-The deposits of the Welsh borderland are well developed in Shropshire,
-where there is practically a repetition of the Caernarvon-Merioneth
-development, with variations in detail. The Arenig and Caradoc
-volcanic rocks are not so thick as those of the Welsh district, but
-are nevertheless of considerable importance[75].
-
-[Footnote 75: For information concerning these beds see Lapworth, C.
-and Watts, W. W., "The Geology of South Shropshire," _Proc. Geol.
-Assoc._, vol. XIII. p. 297.]
-
-In the hilly region of Cumberland, Westmorland, and the adjoining
-parts of Yorkshire the succession differs from that of any of the
-Welsh regions, for the great period of volcanicity was during the
-formation of the Llandeilo rocks, and there were merely sporadic
-outbursts in Arenig and Caradoc times. The Arenig rocks consist of
-black shales with interstratified beds of coarser sediment, and some
-thin lavas and ashes of intermediate type. The Llandeilo series is
-represented by a very great thickness of volcanic rocks, varying in
-composition from basic to acid lavas, with associated pyroclastic
-rocks. The rocks of the Caradoc period largely consist of impure
-limestone with associated argillaceous rocks, and contemporaneous
-volcanic rocks of acid character. A marked unconformity is found
-locally in the centre of these. The Ashgill series consists of a basal
-limestone with shales above, and there is evidence that volcanic
-activity had not become extinct during the deposition of the rocks of
-this series.
-
-Passing on to Scotland, the graptolitic type is admirably shown in the
-southern Uplands of the neighbourhood of Moffat, Dumfriesshire. The
-base of the Ordovician system has not been found, but the lowest
-series seems to be represented by shales with a graptolite possibly of
-Arenig age. Above this are volcanic beds succeeded by a group of black
-shales known as the Moffat shales. They are only about six hundred
-feet in thickness, and yet represent much of the Ordovician and part
-of the Silurian strata as developed elsewhere. The beds belonging to
-the Ordovician system are divided into two series, the Glenkiln shales
-below and the Hartfell shales above. The former consist of intensely
-black muds with few fossils save graptolites, and a deposit of chert
-at the base which is composed of radiolaria. The graptolites of the
-black shales are Upper Llandeilo forms, but the thin deposit of
-radiolarian chert may represent the rest of the Llandeilo period and
-part of the Arenig period also. The Hartfell shales are also usually
-black graptolite shales with lighter deposits nearly barren of organic
-remains; they represent the Caradoc and Ashgill series and pass
-conformably into the deposits of Silurian age[76]. The ordinary
-sedimentary type of Ordovician rocks is found in Ayrshire, though a
-few thin graptolitic seams are intercalated with the conglomerates
-and shelly sands, clays and limestones of the region, which is
-therefore peculiarly valuable as affording a means of comparison of
-the shelly type with the graptolitic type of Ordovician deposits. The
-Arenig series consists of black shales with graptolites, and these
-rocks are succeeded by a volcanic group which is probably of Llandeilo
-age. Above these volcanic beds, as in Dumfriesshire, we find three
-great divisions, two of which are of Ordovician, the third of Silurian
-age. The Ordovician divisions are respectively termed the Barr series,
-which is the equivalent of the Glenkiln shales, and the Ardmillan
-series above, equivalent to the Hartfell shales[77].
-
-[Footnote 76: The Moffat beds are described in a paper by Prof.
-Lapworth entitled "The Moffat Series" in the _Quarterly Journal of the
-Geological Society_, vol. XXXIV. p. 239. This paper, which is a
-masterpiece of detailed work, has furnished a clue to many problems.
-Few students will be able to follow the numerous details, and for
-general information concerning the beds they are recommended to read
-another paper by the same author "On the Ballantrae Rocks of South
-Scotland," _Geol. Mag._ Dec. III. vol. VI. p. 20. An account of the
-radiolarian cherts by Dr G. J. Hinde will be found in the _Annals and
-Magazine of Natural History_ for July, 1890, p. 40.]
-
-[Footnote 77: See Lapworth, C., "The Girvan Succession," _Quart.
-Journ. Geol. Soc._, vol. XXXVIII. p. 537, and also the paper on the
-Ballantrae Rocks referred to in the preceding footnote. The latter
-paper should be carefully read by all students of the stratigraphy of
-the Lower Palæozoic Rocks.]
-
-It is interesting to find that in the north of Ireland the rocks
-generally coincide in characters with those which are found along the
-same line of strike in Great Britain; thus, the Girvan type appears in
-Londonderry, Tyrone and Fermanagh, the Moffat type in County Down, and
-the Lake District type in the counties of Dublin and Kildare.
-
-On the continent the volcanic material which plays so important a part
-in the constitution of the Ordovician accumulations of Britain is
-practically absent, and the strata are largely composed of
-accumulations of shale and limestone with occasional coarser deposits.
-In Scandinavia, the Arenig beds consist of limestones with a few
-shales, the Llandeilo deposits are largely calcareous, those of
-Caradoc age are partly calcareous and towards the top usually
-argillaceous, while the equivalents of the British Ashgill series are
-calcareous at the base and argillaceous at the summit. In Russia the
-calcareous matter preponderates over the argillaceous material.
-
-Ordovician strata are also found in Belgium, France, Bohemia, and
-other places, and are largely composed of mechanical sediments of
-varying degrees of fineness mixed occasionally with some calcareous
-matter.
-
-The variation in the characters of the Ordovician strata of Britain
-points to accumulation in a fairly deep sea, usually at some distance
-from the land, but dotted over with volcanoes which often rose above
-the water, causing the addition of much volcanic material to the
-ordinary sediments, and the existence of minor unconformities at
-different horizons along their flanks. As these unconformities are not
-always associated with volcanic material it is obvious that uplifts
-must have occurred occasionally during the deposition of the rocks;
-one important uplift is indicated by the occurrence of an unconformity
-in the Arenig rocks of Wales, while another is seen amongst the
-Caradoc rocks of the Welsh borders. On the whole, however, the period
-was one of slow subsidence, the deposition of material generally
-keeping pace with this subsidence, and accordingly there is a great
-uniformity of characters amongst the strata over wide areas. The
-probable continuation through the Ordovician period of the tract of
-land over the present site of the N. Atlantic ocean which as we have
-reason to suppose existed during Cambrian times, is indicated by
-similar changes of lithological character amongst the strata when
-traced from Britain eastward to Russia in both Cambrian and Ordovician
-times, and the continuance of these conditions over the American area
-is also indicated by study of the variations amongst the American
-Ordovician deposits.
-
-_The Ordovician Faunas._ The Ordovician period has justly been termed
-the Period of Graptolites, which are the dominant forms of the time,
-and continue in abundance throughout the period. The abundance of
-graptolites in black shales associated with few other organisms has
-often been noted. It appears to be due to a large extent to the slow
-accumulation of the graptolitic deposits, allowing an abundance of
-these creatures to be showered upon the ocean floor, after death, for
-the evidence derived from detailed examination of their structure
-points to their existence as floating organisms. The tests of other
-creatures largely calcareous may well have been dissolved before
-reaching the sea-floor. In support of the view that these black shales
-are abysmal deposits may be noted the singular persistence of their
-lithological characters over wide areas, their replacement by much
-greater thicknesses of normal sediments along the ancient coast-lines,
-the frequent occurrence together of blind trilobites with those having
-abnormally large eyes when these creatures are associated with
-graptolites in the black shales, and lastly the interstratification of
-the black shales with radiolarian cherts similar to the modern abysmal
-radiolarian oozes. If this be so, we ought to find graptolites in
-marine deposits of all kinds, and indeed they are found there, though
-largely masked by the mass of sediment and the hosts of other included
-fossils, so that their discovery is rendered much more difficult than
-when they occur in the black shales,--a state of things which is
-familiar in the case of other pelagic organisms as _Globigerinæ_,
-radiolaria, and pteropods, whose tests abound in the abysmal deposits
-and are comparatively rare in those of terrigenous origin[78].
-
-[Footnote 78: The importance of the graptolites as indices of the
-geological age will be seen by perusal of Prof. Lapworth's paper "On
-the Geological Distribution of the Rhabdophora," _Ann. and Mag. Nat.
-Hist._, Ser. 5, vol. III. (1897).]
-
-The characters of the Ordovician trilobites have already been noticed.
-These organisms are abundant, and occur in sediments of all kinds. Of
-other groups, the significance of the radiolaria has been referred to
-above. Corals occasionally form reef-like masses of limestone as in
-the limestones of the Caradoc epoch; the echinoderms are well
-represented, cystids being locally abundant; of the crustacea, many
-remains of tests of phyllocarida have been recorded; the brachiopods
-are very abundant, and of the mollusca, lamellibranchs, gastropods and
-cephalopods all occur with frequency though none of these groups is
-very prevalent. Certain forms have been referred to pteropods though
-with doubt, and other shells seem to be referable to the heteropods.
-The existence of vertebrates during Ordovician times is not, in the
-opinion of many geologists, proved, though remains of fishes have been
-recorded from the Ordovician strata of North America; but it is
-desirable that more evidence of this occurrence should be given[79].
-
-[Footnote 79: Walcott, C. D., "Preliminary Notes on the Discovery of a
-Vertebrate Fauna in Silurian (Ordovician) Strata," _Bulletin Geol.
-Soc. America_, vol. III. p. 153.]
-
-The distribution of the Ordovician faunas like that of the sediments
-points to the prevalence of open ocean conditions over wide areas
-during the period, with occasional approaches to land, which was often
-of a volcanic nature. Around this land clustered the ordinary
-invertebrates, building up coral-reefs and shell-banks, whilst away in
-the open oceans the graptolites floated, almost alone, and sank to the
-ocean floor after death.
-
-
-
-
-CHAPTER XVI.
-
-THE SILURIAN SYSTEM AND THE CHANGES WHICH OCCURRED IN BRITAIN AT THE
-CLOSE OF SILURIAN TIMES.
-
-
-_Classification._ The Silurian system was originally divided by its
-founder, Sir R. I. Murchison, into three series, as follows:--
-
-  Ludlow Series
-  Wenlock    "
-  Llandovery "
-
-The term May Hill, proposed by Sedgwick, is sometimes used as
-synonymous with Llandovery. This classification omits a somewhat
-important set of beds intercalated between those of the Llandovery and
-Wenlock series known as the Tarannon shales, and in Britain if we were
-to classify afresh, it would be more convenient to include some of the
-beds formerly referred to the Ludlow in the Wenlock. I shall, however,
-adopt the old and well-established classification, adding the term
-Tarannon to Llandovery, and speaking of the Llandovery-Tarannon
-series. The nature of the two classifications is shown in the
-following table:
-
-                             Old             New           Palæontological
-         Stages.         Classification.  Classification.  Classification.
-
-  1 Upper Ludlow       }                }                }
-  2 Aymestry Limestone }  Ludlow        }  Downtonian    }
-  3 Lower Ludlow       }                                 }  Fauna
-                                        }                }  with
-  4 Wenlock Limestone  }                }                }  _Encrinurus_
-  5 Wenlock Shale      }  Wenlock       }  Salopian      }
-  6 Woolhope Limestone }                }                }
-
-  7 Tarannon Shales                     }                }  Fauna
-  8 Upper Llandovery   }  Llandovery    }  Valentian     }  with
-  9 Lower Llandovery   }                }                }  _Harpes_
-
-[Illustration: Fig. 18.
-
-L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.]
-
-_Description of the strata._ Lithologically the Silurian deposits of
-Britain form a continuation of those of the Ordovician period, with a
-local interruption due to the elevation of portions of Wales and the
-Welsh borders at the close of Ordovician times. Elsewhere we find a
-predominance of shales passing into grits at the top of the system,
-the change indicating the incoming of the shallow-water phase before
-the commencement of the second continental period. Particular stress
-is laid upon the predominant shaley character of the beds, for, on
-account of the richness and variety of the faunas of the calcareous
-rocks, greater attention is naturally paid to them in geological
-works, and the student may get a false idea of their relative
-importance. An attempt is made below (Fig. 18) to give a general idea
-of the variations in lithological characters of the Silurian rocks in
-different parts of Britain.
-
-The Silurian strata are mostly found in the same localities as those
-which furnish exposures of the rocks of Ordovician age.
-
-The development in the typical Silurian region of the Welsh borders is
-characterised by the abundance of calcareous matter which is found
-there as compared with that which exists in the other British
-localities.
-
-The Llandovery strata are sandy, often conglomeratic, with a fair
-amount of calcareous matter in places. The arenaceous nature is
-undoubtedly due to the proximity of land caused by local upheaval at
-the end of Ordovician times, and the Upper Llandovery rocks sometimes
-rest unconformably on the Lower ones, at other times on Ordovician,
-Cambrian, or even Precambrian rocks. The Tarannon shales are light
-green shales with intercalated grits. The Wenlock series consists of a
-group of shales separating a lower, very inconstant, earthy limestone
-from an upper, more constant, thicker and purer limestone. The latter,
-the Wenlock limestone, is composed of fragments and perfect specimens
-of various fossils, and the fragmentary nature of many of the shells
-indicates the occurrence of wave-action and probable formation in
-shallow water, in some places against coral-reefs.
-
-The Lower Ludlow beds consist of sandy shales; they are separated from
-the Upper Ludlow beds by an impure limestone, the Aymestry limestone.
-The Upper Ludlow beds consist mainly of grits and flags, often
-coloured red towards the summit.
-
-In North Wales the Llandovery beds occasionally present the shelly
-arenaceous types of deposit as near Llangollen, at other times as near
-Conway, Corwen, and in Anglesey, the graptolitic shale type. They
-also rest unconformably upon the Ordovician rocks in this area. The
-Tarannon shales resemble those of the border county. The Wenlock
-series consists essentially of shales, while the Ludlow development
-differs from that of the borders in its greater thickness and the
-absence of any calcareous band in the centre. In Central Wales the
-graptolitic type of the Llandovery-Tarannon series is found, but the
-graptolite-bearing shales of the Llandovery epoch are thin beds
-occurring between grits and flags no doubt deposited in shallow water,
-and this division of the series is of very great thickness.
-
-In South Wales the Silurian rocks are very similar to those of the
-Welsh borders, save that the calcareous deposits are fewer and
-thinner.
-
-The Lake District Silurian strata generally resemble those of North
-Wales. The Llandovery-Tarannon rocks are of the graptolite-shale type,
-intercalated with fine grits in the case of the beds of Tarannon age.
-The Wenlock beds consist of shales, and the Ludlow beds of gritty
-shales beneath, and massive flags and grits at the summit. These
-Ludlow beds are here of great thickness (certainly not less than 7000
-feet) and were obviously accumulated for the most part in shallow
-water.
-
-The Llandovery-Tarannon rocks of Southern Scotland show the two types
-which prevailed in the Moffat and Girvan areas in later Ordovician
-times. The Llandovery beds of Moffat are known as the Birkhill shales,
-and are very thin. The representatives of the Tarannon shales,
-however, the Gala beds, consist mainly of grits, and attain a great
-thickness. In the Girvan area, the Llandovery beds are of the shelly
-type. Here as at Moffat and in the Lake District there is perfect
-conformity between the beds of Ordovician and those of Silurian age,
-and accordingly it is instructive to note the completeness of the
-palæontological break, especially in the Moffat district. The higher
-Silurian beds of Southern Scotland present a general resemblance to
-those of North Wales and the Lake District[80].
-
-[Footnote 80: For descriptions of the Silurian beds of the typical
-region see Lapworth and Watts, _Proc. Geol. Assoc._, vol. XIII. p.
-297, those of Wales are described by Lake and Groom, _Quart. Journ.
-Geol. Soc._, vol. XLIX. p. 426, and Lake, _ibid._ vol. LI. p. 9. A
-description of those of Lakeland will be found in the Memoir of the
-Geological Survey "The Geology of the Country around Kendal, etc."
-while the Scotch Rocks are described in Lapworth's papers on Moffat
-and Girvan.]
-
-On the European continent we find indications of conditions similar to
-those which prevailed during the Ordovician period; the strata become
-much thinner and more calcareous in Scandinavia, and still thinner in
-the Baltic provinces of Russia, where they consist very largely of
-calcareous matter. In central Europe the greater abundance of
-calcareous matter, compared with that which is found in the Ordovician
-strata of that region, points to a change in physical conditions which
-became still more marked after Silurian times.
-
-In North America, the succession is very similar to that of Britain,
-the calcareous development of the Silurian rocks being found around
-Niagara, but towards the close of Silurian times the shallow-water
-phase became marked in places by the deposition of chemical
-precipitates which indicate the separation of a portion of the late
-Silurian ocean from the main mass during the period of formation of
-these abnormal deposits.
-
-The conditions of Silurian times, until the advent of the
-shallow-water phase, recall those of Ordovician times and point to a
-wide expanse of ocean at some distance from the land, though the
-earliest deposits become arenaceous where they were deposited against
-an old land surface formed by the elevation of the Welsh Ordovician
-rocks, which were denuded to supply this material. One marked
-difference existed between the physical conditions of our area during
-Ordovician and Silurian times, for the volcanic activity which was
-rife during Ordovician times almost ceased during Silurian times,
-except in the region now occupied by the extreme south-west of
-Ireland, and accordingly volcanic material does not appreciably
-contribute to the formation of the Silurian deposits. The shallowness
-of the sea-floor at times is marked by the occurrence of masses of
-reef-building corals in the limestones, and these probably indicate
-the prevalence of a fairly warm climate, an inference supported by the
-nature of the Gastropod fauna of Gothland, as noticed in Chap. IX.
-
-The shallow-water phase commences fairly simultaneously over the whole
-area at the beginning of the deposition of the Lower Ludlow rocks, and
-becomes more marked in the Upper Ludlow rocks, being most noticeable
-at their extreme summit, when a change occurred which will be
-considered at the conclusion of this chapter.
-
-_The Silurian Faunas[81]._ The Silurian period has been termed the
-period of Crinoids, and this group of creatures certainly contained a
-great variety of very remarkable forms, which are specially numerous
-in the Wenlock Limestone of the Welsh borders, Gothland, and North
-America, but many of the rocks of the system display few traces of
-these organisms. The trilobites and graptolites still contribute
-largely to the fauna, the latter becoming very scarce at the summit of
-the system, though a few specimens have been detected in the rocks of
-the succeeding system. The trilobites belong to few genera though
-these are mostly more highly organised than those of the Ordovician
-period. The genus _Harpes_ has been taken as fairly characteristic of
-the lower part of the system in Sweden, and it occurs there abundantly
-in places in Britain, whilst _Encrinurus_ is more abundant in the
-upper series, but both of these genera range from higher Ordovician
-beds into the Devonian. Mention has already been made of the corals.
-Brachiopods are very abundant, and Mollusca appear with considerable
-frequency. The appearance of true insects is of importance,
-cockroaches have been recorded from Silurian rocks and a number of
-other insects have lately been recorded from Canada[82]. Eurypterids
-occur in considerable abundance in the higher parts of the system, as
-do also the remains of fish.
-
-[Footnote 81: For an account of the Silurian faunas the student may
-consult Sir R. I. Murchison's _Silurian System_ or the shorter
-_Siluria_ and Lapworth's paper on the Geological Distribution of the
-Rhabdophora.]
-
-[Footnote 82: See an article by Dr G. F. Matthew, "Description of an
-extinct Palæozoic Insect and a review of the Fauna with which it
-occurs," _Bulletin_ XV. _of the Natural History Society of New
-Brunswick_. The Silurian Rocks of the Little River Group of St John,
-New Brunswick, have yielded species of land snails, two doubtful
-saw-bugs, several arachnids, and myriopods, two insects of the order
-Thysanura (Spring-tails), and eight Palæodictyoptera.]
-
-The close of Silurian times ushered in the second continental period
-in Britain when a large part of our area and the adjoining areas to
-the north and north-east were uplifted to form land, which in the case
-of our area was interpenetrated by watery tracts, whose exact nature
-is still a subject of dispute. Accordingly the deposits which were
-formed during this period are local and in some cases abnormal, but
-they will be considered in the next chapter. Simultaneously with the
-formation of these deposits, uplift of the sea-floor converted wider
-and wider areas into land, and this land underwent considerable
-denudation, so that the tops of the anticlines were worn away. The
-general trend of the anticlines was east-north-east and
-west-south-west, and accordingly a series of mountain chains possessed
-that direction, for the epeirogenic movements were accompanied by
-orogenic ones. Between the regions of uplifts were depressions in
-which sediments accumulated. The principal axes of uplift in our area
-range through the North of Scotland towards Scandinavia, across the
-Southern Scotch Uplands to the North of Ireland, through the Lake
-District and through Wales. As the result of lateral pressure, a
-cleavage structure was impressed on many of the Lower Palæozoic rocks,
-the strike of the rocks extended in the direction of the ridges and
-depressions, and the rocks as a whole became considerably compacted
-and hardened, thus producing one of the most important portions of the
-framework of our island, for although the ancient mountain chains were
-largely denuded during their elevation, and their stumps were
-afterwards covered by later deposits, upon the removal of these, the
-ancient stumps were once more exposed as fairly rigid masses which do
-not yield greatly to denuding influences, and accordingly stand out as
-the most important upland regions of Britain at the present day.
-
-It is interesting to notice, as an illustration of the now well
-established fact that successive earth movements often occur in the
-same direction, that the axes of the folds produced during this second
-continental (Devonian) period, run parallel with the lines separating
-tracts of different lithological characters. It has been seen that
-the Ordovician and Silurian rocks of the Southern Uplands continue
-into Ireland, and that the beds of similar characters run in belts
-having a general east-north-east and west-south-west trend, which
-accordingly must have been the direction of the coast-line parallel to
-which they were deposited, and as that coast-line was due to uplift,
-the movement which produced it would naturally produce foldings with
-east-north-east and west-south-west trend. This is one of many cases
-where the lines separating belts of rock having different lithological
-characters run parallel to axial lines of folds which have been
-produced in the rocks at a later period.
-
-As the result of the existence of land over parts of north-west Europe
-in Devonian times, it is comparatively rare to find a passage from
-normal Silurian rocks into normal Devonian ones; there is often an
-unconformity above the Silurian strata. As we proceed southwards
-towards central Europe, where the epeirogenic and orogenic movements
-died out, this is not the case, and we get complete conformity between
-marine sediments of the Silurian and Devonian periods.
-
-
-
-
-CHAPTER XVII.
-
-THE DEVONIAN SYSTEM.
-
-
-_Classification._ As a result of the movements which were briefly
-described in the last chapter, two types of Devonian deposit are found
-in the British Isles, and are called respectively the Devon type and
-the Old Red Sandstone type. The latter rocks, formerly divided into
-three divisions, are now separated into two only, the upper and lower
-Old Red Sandstone, and the exact relation of these to the different
-subdivisions of the rocks of Devon type remains to be settled. The
-Devon type itself has given rise to much difference of opinion, two
-local classifications have been applied, one for the rocks of North
-Devon and another for those of South Devon. The classification which
-has been most generally adopted is as follows:--
-
-                          N. Devon.               S. Devon[83].
-
-                   { Pilton Beds              { Entomis Slates
-  Upper Devonian   { Cucullæa (Marwood)       { Goniatite Limestones
-  (Clymenian)      { Beds                     { and Slates
-                   { Pickwell Down Sandstone  { Massive Limestones
-
-  Middle Devonian  { Morte Slates             { Middle Devonian
-     (Eifelian)    { Ilfracombe Beds          {  Limestones
-                                              { Ashprington Volcanic
-                                              {  Series
-                                              { Eifelian Slates and
-                                              {  Shaly Limestones
-
-                                              { Lower Devonian
-  Lower Devonian   { Hangman Grits            {  Slates
-    (Coblenzian)   { Lynton Slates            { Lincombe and Warberry
-                   { Foreland Grits           {  Grits and
-                                              {  Meadfoot Sands
-
-[Footnote 83: An account of the South Devon rocks by Mr Ussher will be
-found in the _Quart. Journ. Geol. Soc._, vol. XLVI. p. 487; from it
-the above classification of the rocks of S. Devon is taken.]
-
-The division into Lower Middle and Upper Devonian is generally
-adopted, though the alternative titles given to these divisions are
-not always used with the same signification, and the distribution of
-the different local stages given in the above classifications is
-usually adopted in the main, though a detailed comparison of the
-Devonian beds of North and South Devon is still attended with
-difficulty.
-
-More than once an attempt has been made to prove that the apparent
-succession of the North Devon rocks, which is that given in the above
-table, is not the true one, and of recent years Dr Hicks has obtained
-a number of fossils from the Morte Slates which had hitherto yielded
-none, and he believes that these fossils indicate that the Morte
-Slates are on a lower horizon than the beds on which they rest.
-Whatever be the ultimate verdict, we can, at any rate, say that the
-"Devonian Question," as it is termed, is not settled[84].
-
-[Footnote 84: See Hicks, H., "On the Morte Slates and Associated Beds
-in North Devon and West Somerset," _Quart. Journ. Geol. Soc._, vols.
-LII. p. 254, LIII. p. 438.]
-
-_Description of the Strata._ The general variations in the
-lithological characters of the deposits of Devonian age will be seen
-from the accompanying figure which represents the deposits of Britain
-as they occurred from north to south before they had been affected by
-subsequent earth-movements (Fig. 19). The conventional signs which are
-used are similar to those which have been used in other parts of this
-work, and will save description of the section.
-
-[Illustration: Fig. 19.
-
-  A.   Lower Palæozoic and Precambrian Rocks.
-  N.S. North of Scotland       }
-  C.V. Central valley of ditto } Old Red Sandstone Type.
-  W.   Wales                   }
-  N.D. North Devon } Devon Type.
-  S.D. South Devon }
-]
-
-The ridges separate different deposits of Devonian rocks, which were
-possibly deposited in isolated areas, though there was probably
-connexion between them at any rate at times.
-
-The Old Red Sandstone type consists to a large extent, as the name
-implies, of sandstones which are coloured red by a deposit of peroxide
-of iron around the sand grains. They are separable into a lower and
-upper division with an unconformity often occurring between them. The
-lower Old Red passes down in places into the Silurian rocks with
-perfect conformity, and the upper Old Red similarly passes up into the
-Carboniferous strata. The existence of pebble beds at different
-horizons is a noteworthy feature. They are frequently found at or near
-the base of the two divisions. The sandstones of the lower division
-are often accompanied by flagstones, while the red sandstones of the
-upper division usually have deposits of yellow and brown sandstone
-intercalated between them. Inconstant beds of limestone, known as
-cornstones, are found in both divisions, and Prof. Sollas has shown
-that some of these, at any rate, are true mechanical deposits, formed
-by the destruction of pre-existing strata of limestone and the
-deposition of the resulting fragments from a state of suspension. In
-Scotland a great thickness of volcanic material of various kinds is
-associated with the two divisions. For the sake of simplicity this is
-omitted from Fig. 19[85]. It is not known how far normal sediments are
-associated with the Old Red Sandstone type of deposit. The existence
-of some in South Wales is suggested by evidence supplied by the late
-Mr J. W. Salter.
-
-[Footnote 85: For an account of these and all other British volcanic
-rocks the reader is referred to Sir A. Geikie's work on _The Ancient
-Volcanoes of Great Britain_. Macmillan and Co., 1897.]
-
-The Devon type, as will be seen in the figure, consists of rocks which
-are to a great extent of normal character. We find in Devonshire
-alternations of sandstones, shales and limestones, but even here, red
-sandstones, which are comparable with those of the Old Red type occur
-in diminished amount: the Foreland Grits and Pickwell Down Sandstones
-are both coloured red, and are like the sandstones formed further
-north. The recognition of this fact induces one to believe that the
-contrast between the two types of rock which are found at a short
-distance from one another on opposite sides of the Bristol Channel is
-not so marked as one is sometimes led to suppose.
-
-The rocks of North Devon differ from those of South Devon chiefly
-owing to the amount of calcareous sediment found in the two areas, for
-limestones occur in South Devon to a great extent, and in North Devon
-there is a comparative poverty of this kind of sediment. Here, again,
-the apparent difference is possibly greater than the real one. The
-North Devon limestones have in places been stretched out after their
-formation and thus rendered thinner, and the highly-cleaved limestones
-are occasionally mistaken for shales, while in South Devon there is
-evidence of thickening of the limestones by folding subsequently to
-their deposition. Allowing for these changes, however, there is still
-a marked diminution in the amount of coarse mechanical sediments and
-increase in the quantity of calcareous matter as one passes from North
-to South Devon, and this prepares one for the condition of things met
-with on parts of the continent, where the mechanical sediments become
-finer and thinner on the whole as one travels southward, until, when
-we reach the Bohemian area, the Devonian rocks are found to be largely
-composed of calcareous sediments.
-
-It is interesting to find that in North America the two types of
-Devonian strata recur, and present characters generally similar to
-those which they possess upon this side of the Atlantic.
-
-Passing now to a consideration of the conditions under which the
-Devonian rocks were deposited, we may examine the bearing of the
-character of the strata as a whole, and then proceed to more detailed
-consideration of the nature and conditions of deposits of the two
-types.
-
-The gradual increase in calcareous matter and dying out of mechanical
-sediments as one travels southward points to recession from land in
-that direction, and we have already seen that the epeirogenic and
-orogenic movements of this continental period elevated the Silurian
-sea-floor in the north, and gave rise to a Northern Continent, while
-oceanic conditions continued further South, and allowed the
-accumulation of sediments lying conformably upon those of Silurian
-age, and giving indications of the prevalence of physical conditions
-during Devonian times which were in the main similar to those of the
-preceding Silurian period.
-
-In the shallow waters adjoining the land of the Northern Continent the
-Old Red Sandstones were laid down, and the exact conditions under
-which they were accumulated is a matter of some interest. The late Sir
-Andrew Ramsay gave reasons for supposing that many red deposits were
-accumulated in the waters of inland lakes, which underwent rapid
-evaporation, and his views have been applied, with much corroborative
-evidence by Sir A. Geikie, to account for the red sandstones of
-Devonian age, which he believes to have been accumulated in a series
-of inland lakes, though others hold a different opinion, and consider
-that the Old Red Sandstone waters had a direct connexion with those of
-the open ocean; the question is too intricate to be discussed at
-length here. Besides the difference of physical characters of the two
-types of strata, the difference in the nature of their included
-organisms is significant. The ordinary invertebrates, as corals,
-crinoids, brachiopods and molluscs are extremely rare in the Old Red
-Sandstone, which contains remarkable remains of Agnatha fishes and
-eurypterids, and although these are also found associated with a true
-marine fauna in Russia, Germany and Bohemia, the rarity or apparent
-absence of the ordinary marine invertebrates, though only negative
-evidence, which is proverbially dangerous, must be regarded.
-
-The North Devon rocks are sediments which might well be accumulated on
-the shores of a continent, while those of South Devon, with their
-abundant coral reefs, and other organic limestones were no doubt
-deposited in a clearer sea, at a greater distance from the land, and
-the clear water deposits of Germany and still more of Bohemia, were
-accumulated in the open ocean. It is interesting to note in these
-Bohemian deposits abundance of shells of a Pteropod _Styliola_ which
-has been proved by Prof. H. A. Nicholson to form masses of limestone
-in the Devonian system of Canada. The modern distribution of the
-Pteropoda suggests the open ocean character of the deposits which
-contain them even so far back as Devonian times, though one cannot
-conclude that these deposits are really analogous to the so-called
-Pteropod ooze of modern seas which, as a matter of fact, is largely
-composed of foraminiferal tests with a considerable percentage of
-pteropod shells.
-
-_The Devonian flora and faunas._ The plant remains in the Lower
-Palæozoic rocks are few in number. Some undoubted terrestrial plants
-have been discovered, but the prevalent flora of lower Palæozoic
-times, so far as yet known, was one consisting of Algæ. In Devonian
-times we begin to meet with a number of Cryptogams of higher type,
-allied to those which form the dominant flora of the succeeding
-period. The fauna is in many ways remarkable. The Devonian period has
-been termed the period of ganoid fishes, and the remarkable remains,
-so graphically described by the late Hugh Miller, are indeed
-peculiarly characteristic of Devonian times, but they are largely
-though by no means exclusively entombed in rocks of the Old Red
-Sandstone type[86]. The Devon type of rock contains a great abundance
-and variety of the problematical group, the Stromatoporoids, which
-contribute extensively to the formation of many of the limestones,
-and although these organisms are not by any means confined to Devonian
-strata, their abundance and variety therein might lead one to speak of
-the period as that of Stromatoporoids. The remains of corals are very
-abundant in the limestones, and, as already stated, frequently give
-rise to true reef-masses. The graptolites, as remarked in the previous
-chapter, disappear in the rocks of the Devonian period, and as only
-one or two fragments have been found, we may assert that the group was
-practically extinct at the end of Silurian times, though species of
-one genus, _Monograptus_, lingered for a short time in greatly
-diminished quantity. The trilobites which played so important a part
-amongst the faunas of Lower Palæozoic times still occur fairly
-abundantly amongst the rocks of the Devonian system, and there is a
-very interesting point to be noticed in connexion with them. They seem
-to have become practically extinct in the succeeding Carboniferous
-period, where few genera are found, and the decadence of the group
-began in Devonian times. In these circumstances it is interesting to
-note the tendency displayed by the creatures to possess spiny
-coverings. It is true that _Acidaspis_, the most spinose of all
-trilobites, is abundant in Ordovician and Silurian strata, and that
-other spinose trilobites are found there, but the peculiarity of the
-Devonian trilobites is, that genera which were previously smooth, or
-rarely possessing one or few spines, are found represented by
-extremely spinose species in these beds,--the spines being developed
-from all parts of the test, sometimes as a fringe to head or tail,
-sometimes as prominent projections from glabella and neck segment, and
-frequently in rows down the body segments. Besides _Acidaspis_, we
-find spinose species of _Phacops_, _Homalonotus_, _Cyphaspis_,
-_Bronteus_ and _Encrinurus_ in Devonian strata, and the occurrence of
-these forms is so frequent and world-wide, that one might perhaps
-infer with confidence that an unknown fauna containing many spiny
-trilobites was of Devonian age.
-
-[Footnote 86: For an account of these see A. S. Woodward's _Vertebrate
-Palæontology_.]
-
-The abundance of Eurypterids has been previously noted. Occurring as
-they do in Silurian rocks, they are far more abundant in those of
-Devonian age, and are found indifferently in sediments of Old Red and
-Devon types. Of air breathers, several insects have been found in the
-strata of different parts of the world.
-
-The ordinary marine faunas are otherwise intermediate in character
-between those of the Silurian and Carboniferous periods, but there are
-several characteristic Devonian genera, and no one who is acquainted
-with the peculiarity of the Devonian fauna would deny to the Devonian
-strata the right to rank as a separate system, containing a fauna as
-well marked in its way as that of the Silurian system below or that of
-the Carboniferous above. Special stress is laid upon this point
-because it has been suggested that the Devonian system should be
-abolished, and its strata either divided between the Silurian and
-Carboniferous systems or referred exclusively to the latter
-system[87].
-
-[Footnote 87: The literature of the fauna of the Devonian rocks is a
-rich one. For an account of the Devonian rocks of Britain, the reader
-may consult the Monograph of the Devonian Fossils of the South of
-England by Rev. G. F. Whidbourne, which is now appearing in the series
-of Monographs of the Palæontographical Society, and in the
-publications of the same Society he will find a Monograph of the
-Eurypterids from the pen of Dr Henry Woodward. The richest Devonian
-fauna is undoubtedly that of the Bohemian area, for the work of Dr E.
-Kayser has conclusively proved that the stages _F_, _G_ and _H_ of
-that basin, formerly referred to the Silurian, are of Devonian age,
-and an excellent idea of the richness of the Devonian fauna may be
-obtained by studying the descriptions of the fossils from those stages
-which have appeared and are appearing in Barrande's classic work.]
-
-
-
-
-CHAPTER XVIII.
-
-THE CARBONIFEROUS SYSTEM.
-
-
-_The Classification._ The British rocks of the Carboniferous system
-have been classified according to their lithological characters, but
-as the classification has been altered from time to time, we may use
-that which seems most acceptable to the majority of British geologists
-at the present day. According to this, the beds are grouped as
-below:--
-
-                      {                  { Ardwick Stage
-  Upper Carboniferous { Coal Measures    { Pennant Stage
-                      {                  { Gannister Stage
-                      { Millstone Grit
-  Lower Carboniferous { Carboniferous (Mountain) Limestone
-                      { Series.
-
-The Lower Carboniferous beds have been further subdivided into:--
-
-    Yoredale Series or Upper Limestone Shales,
-    Mountain Limestone,
-    Lower Limestone Shales, with Sandstones and Conglomerates,
-
-but as these lithological types are found to be very variable when
-traced laterally for comparatively short distances, it is found more
-satisfactory to use the terms in a purely lithological sense rather
-than with chronological significance.
-
-The somewhat abnormal development of the higher portions of the
-Carboniferous rocks of Britain renders the local classification only
-partially applicable in other regions, and as our knowledge
-progresses, a palæontological classification will probably be adopted.
-This has already been done with the more purely open-water sediments
-of Russia and Eastern Asia, where the development of the beds is more
-normal. There the rocks are classified as under:--
-
-    Upper Carboniferous or Gshellian,
-    Middle Carboniferous or Moscovian,
-    Lower Carboniferous,
-
-and as this classification has already been found to be applicable
-over rather wide areas, it is almost certain that, as in the case of
-the rocks of other systems, it will prove more serviceable than one
-which is mainly (though not quite exclusively) based upon vertical
-variation of lithological characters, especially as the Carboniferous
-rocks over large tracts in North America possess faunas which are
-similar to those which have been discovered in Russia, Eastern Asia
-and North Africa.
-
-_Description of the strata._ The variations in the lithological
-characters and fossil contents of the British Carboniferous strata
-when traced from north to south have been so frequently described, and
-utilised as a means of illustrating the indications as to local
-variations in physical conditions which are supplied by those strata,
-that little need be said upon the subject. The restoration of the
-physical geography of Carboniferous times over the British area will
-be found in a chapter by the late Professor Green in the work upon
-_Coal_ by various professors at the Yorkshire College of Science and
-also in Prof. Hull's _Physical History of the British Isles_. Some
-modifications must be made in these restorations as the result of
-recent research, the principal being caused by discoveries amongst the
-Carboniferous rocks of Devonshire.
-
-Taking the strata in vertical succession, we find evidence of the
-occurrence of a complete marine period (the second great marine
-period) between the second and third continental periods. The first
-shallow-water phase over a great portion of the British Isles is
-marked by thin terrigenous sediments, indicating that the period was a
-brief one; it was followed by the deep-water phase, probably of some
-length, lasting through the greater part of the remainder of Lower
-Carboniferous times; while the concluding shallow-water phase was
-lengthy as compared with that of the beginning of the period, and is
-marked by the accumulation of the great thickness of deposits
-belonging to the Millstone Grit and Coal Measures. There is no doubt,
-however, that in some parts of the British area minor changes produced
-local terrestrial conditions during the period, and accordingly we
-find that the deepest water deposits of the system in Britain are
-succeeded by an unconformable junction with the sediments of the upper
-portion of the system.
-
-The general change in the lithological characters of the beds of the
-Lower Carboniferous division when traced from south to north is shewn
-in the following diagram (Fig. 20).
-
-It will be seen that the land and open sea areas were in the
-respective positions which they occupied during Devonian times, but
-that as the result of greater submergence, with which the accumulation
-of sediment did not keep pace, the shallow-water marine deposits of
-Devonian age are in Devon replaced by open-sea deposits[88], while
-shallow-water marine deposits further north replace the anomalous
-deposits which were found there during the Devonian period.
-
-[Footnote 88: The Radiolarian Cherts of the Lower Carboniferous rocks
-of Devon, and the associated sediments, together with the unconformity
-between these and the Upper Carboniferous beds are described by Messrs
-Hinde and Fox, Quart. _Journ. Geol. Soc._, vol. LI. p. 609.]
-
-[Illustration: Fig. 20.
-
-    _a._ Radiolarian cherts of Devon.
-    _b._ Mountain limestone of Central England.
-    _c._ Mechanical sediments of Northern England.
-    _d._ Freshwater deposits of Southern Scotland.
-    O.R. Older rocks.
-]
-
-Owing to the accumulation of thick masses of sediment, the Lower
-Carboniferous sea of the north of England appears to have been largely
-silted up, and although the organic deposits of the south are so thin
-that they did not render the sea shallow in that region, the general
-level of the Lower Carboniferous floor of the south was also uplifted,
-and actually converted into land, as the result of the upward movement
-which took place in Devonshire and tracts of France; and owing to
-silting up in the north, and elevation in the south, a general plane
-surface was produced over very extensive areas, not only in Britain
-but upon the Continent, upon which the peculiar deposits and
-accumulations of Upper Carboniferous times were laid down, sometimes
-in shallow water, sometimes upon the land, and often under conditions
-which cannot at present be determined with accuracy. That the deposits
-of the Millstone Grit and Coal Measure epochs were to a large extent
-laid down in water is admitted by all, and in the case of many of the
-deposits of the Millstone Grit, and some thin deposits of the Coal
-Measures, it is equally clear that the water area was part of an
-expanse of ocean, for we find marine fossils, as corals, crinoids, and
-cephalopods, in these beds. Associated with them in the Coal Measures
-are other beds in which the ordinary Carboniferous genera of marine
-invertebrates are absent, and their place is taken by shells which
-bear much resemblance to the modern fresh-water mussel, and it has
-been maintained with good reason that as the ordinary marine forms are
-rarely or never mixed with those resembling recent fresh-water shells,
-the latter are truly fresh-water[89]. If this be so, many of the
-mechanically formed sediments of the Coal Measures were of fresh-water
-origin, laid down in shallow lagoon-like expanses, probably shut off
-from the main ocean by a narrow portion of intervening land, which was
-occasionally destroyed, thus permitting incursions of salt-water when
-some of the ordinary marine invertebrates of the period obtained a
-temporary footing in the area.
-
-[Footnote 89: For further information upon this subject the student
-should consult the Introduction to a Monograph on _Carbonicola_,
-_Anthracomya_ and _Naiadites_ (the shells in question) by Dr Wheelton
-Hind, being one of the Monographs of the Palæontographical Society.]
-
-There is not only a difference of opinion as to the mode of
-accumulation of many of the mechanical sediments of the Coal Measures,
-but also as to that of the coal-seams which accompanied them. Two
-different theories have been put forward to account for these
-coal-seams, which are usually spoken of as the drift theory and the
-growth-in-place theory. According to the former, in its extreme
-application, coal is an aqueous deposit formed by the settlement of
-drifted masses of vegetation upon the floor of a water-tract, while
-those who push the growth-in-place theory to its extreme limits
-maintain that coal is the result of growth of vegetation upon the
-actual site where the coal is now found. Much apparently conflicting
-evidence has been advanced by the advocates of the two hypotheses, and
-special cases of coal-formation have been appealed to by each in
-support of their views; thus the existence of coal composed largely of
-bodies which resemble the spores of modern lycopods,--objects of so
-resinous a nature that they float on the surface until they are
-decomposed,--is cited by the upholders of the growth-in-place theory,
-while the supporters of the other hypothesis can point with equal
-force to the occurrence of the finely divided carbonaceous mud
-containing remains of fishes which gives rise to cannel coal in some
-places. One of the main assertions in support of the growth-in-place
-theory was that of the supposed universality of 'underclays' or old
-surface soils beneath all coal-seams, but though these are common,
-they are far from universal. It is impossible to do justice in small
-compass to this question of coal-formation, but it may be pointed out
-that much of the difference of opinion can be understood if it be
-remembered that the term 'coal' is rather a popular term which has
-been admitted into scientific terminology, and therefore used somewhat
-loosely, than a strictly scientific term applied to a definite
-substance, and accordingly, just as at the present day we find
-carbonaceous substances growing in one place on land to form peat, in
-other places on a tract sometimes dry and sometimes submerged, to form
-the carbonaceous deposits of the cypress-swamps, and once more
-accumulated beneath the shallows of a sea as a sediment to form the
-carbonaceous muds of the ocean margins where the mangroves grow, so
-the diverse substances which are included under the general term coal
-may have accumulated in one place on land, in another beneath water,
-and in a third on an area alternately dry and submerged. This is not a
-question of great importance; the important point is that
-accumulations of vegetation on a fairly large scale are found at the
-present day on plains, for even if they grow on mountain regions, the
-deposits are readily denuded before they are covered up, and also it
-must be noted that a moist climate is necessary for the growth of much
-vegetation. The conclusion that the accumulations of coaly matter were
-formed on plains is borne out by their great horizontal extent as
-compared with their thickness, and it is now generally agreed that the
-coal vegetation which is found in the normal coal-measures was
-essentially a swamp vegetation.
-
-An attempt has been made to prove that an upland vegetation of very
-different character existed contemporaneously with it, but reasons
-will be given in the sequel for concluding that this supposed upland
-Carboniferous flora is everywhere of later date.
-
-The later shallow-water phase of Carboniferous times, as already
-stated, was unusually long, it was also very widespread, and appears
-to have been accompanied over wide areas by humid conditions during
-its continuance, and accordingly the marsh conditions which existed
-during Upper Carboniferous times were probably on a larger scale than
-that of similar conditions before or after. Special stress is laid
-upon this fact, as it is a good illustration of the view which seems
-to be gaining ground, that every period possessed peculiar conditions
-never to be repeated, which must have left their impress upon the
-character of the sediments.
-
-Though the conditions above described were widespread, they were
-naturally not universal, and accordingly in many parts of the world,
-as previously stated, we find true marine deposits of Upper
-Carboniferous times, though even these were sometimes replaced during
-part of the epoch, by conditions which were favourable for the
-formation of coal-seams in those places. Interruption in the
-continuance of a humid temperate climate over the regions of
-North-West Europe is also suggested by the discovery of deposits which
-are maintained to be of glacial origin amongst the Coal Measures of
-France[90].
-
-[Footnote 90: For an account of the numerous volcanic products see Sir
-A. Geikie's work on "The Ancient Volcanoes of Great Britain."]
-
-_The Floras and Faunas._ The flora of the Carboniferous rock is so
-noteworthy that the period has been termed the Period of Cryptogams;
-the remains of ferns, horsetails, and clubmosses predominate, and many
-of the forms reached a gigantic size. Though the floras of the various
-stages are marked by a general resemblance, there are differences
-which enable the palæobotanist to ascertain the stratigraphical
-position of the beds by reference to the included plant remains, and a
-considerable number of successive floras have been described[91]. The
-invertebrate fauna does not differ on the whole very greatly from that
-of Devonian times, though the trilobites are now becoming rare, and
-the mollusca assume a more prominent position as compared with the
-brachiopods. Corals occur in abundance in the calcareous deposits of
-the period, and frequently give rise to sheets of reef-formation, but
-the foraminifera and crinoidea certainly play the principal part as
-limestone-producers, and the influence of the latter in giving rise to
-great masses of limestone which are frequently used for ornamental
-purposes is too well known to need more than passing reference. The
-air-breathers have also been detected in greater abundance, though
-they are rare, when we consider the comparatively favourable
-conditions for their preservation presented by the Coal Measure rocks.
-Myriopods, arachnids, insects and pulmoniferous gastropods have
-however been found with tolerable frequency. The danger of arguing
-from imperfect data is well illustrated by the great addition to our
-knowledge of the insect-fauna of these times due to the exploration of
-the beds of one small coal-field, that of Commentry in France, of
-which the insects have been described by M. C. Brongniart. The
-vertebrates are represented by a considerable variety of fishes, and
-less abundant though tolerably numerous remains of Amphibia, which
-occur in the Carboniferous rocks of the North of England, Ireland,
-France, North America and elsewhere.
-
-[Footnote 91: Consult Kidston, R., "On the Various Divisions of the
-Carboniferous Rocks as determined by their Fossil Flora," _Proc. Roy.
-Phys. Soc. Edin._, vol. XII. p. 183.]
-
-The existence of definite zones of organisms in the case of the
-Carboniferous rocks has been denied, and it appears to be considered
-by some that the Carboniferous rocks were accumulated so rapidly as
-compared with rocks of some other systems that the fauna remained very
-similar throughout. It is very doubtful if this was so. In the case of
-other systems, the division into zones has only been accomplished by
-means of more detailed researches than those which have been conducted
-amongst the Carboniferous rocks of Britain: again, the occurrence of
-successive floras suggests that there may have been a similar
-succession amongst the faunas, and finally we find that zonal division
-has been carried on to some extent amongst the Carboniferous strata
-of other regions. The following classification of the Russian type of
-sediment may prove useful, as an indication of the possibility of more
-detailed separation of our own beds:--
-
-                        { Beds with _Spirifera fascigera_, _Spiriferina_
-       Gshellian        {   _Saranae_, &c.
-  (with _Fusulina_ and  { Beds with _Producta cora_, _P. uralica_,
-    _Archimedipora_)    {   _Camarophoria crumena_, &c.
-                        { Beds with _Syringopora parallela_ and
-                        {   _Spirifera striata_.
-
-     Moscovian          { Stage of _Spirifera mosquensis_.
-
-                        { Stage of _Spirifera Kleini_.
-                        { Coals, Sandstones and Shales with _Noeggerathia_
-  Lower Carboniferous   {   _tenuistriata_ and _Producta_
-                        {   _gigantea_.
-                        { Stage of _Producta mesoloba_.
-
-The marine fauna of the Upper Carboniferous beds, which is so poorly
-represented in Britain, but is well developed in Spain, Russia, Asia
-and North America, is largely characterised by the abundance of
-foraminifers of the genus _Fusulina_ and _Fusulinella_ and of bryozoa
-of the genus _Archimedipora_. It is very desirable that the truly
-marine fauna of the _Spirorbis_ limestone and other marine bands of
-the British Coal Measures should be carefully studied to see if they
-present any close relationship with that of the Gshellian beds[92].
-
-[Footnote 92: A good idea of the general characters of the
-Carboniferous fauna of Britain will be obtained from an examination of
-Professor Phillips' _Geology of Yorkshire_, Part I., and Mr (now Sir
-F.) M^{c}Coy's _Carboniferous Fossils of Ireland_, while the nature of
-the European fauna is well illustrated in Prof. de Koninck's
-well-known work _Description des animaux fossiles qui se trouvent dans
-le terrain carbonifère de Belgique_. For an account of the characters
-of the marine fauna of the Upper Carboniferous rocks the reader should
-consult the work on Geology and Palæontology published by the
-Geological Survey of the State of Illinois in 1866.]
-
-
-
-
-CHAPTER XIX.
-
-THE CHANGES WHICH OCCURRED DURING THE THIRD CONTINENTAL PERIOD IN
-BRITAIN; AND THE FOREIGN PERMO-CARBONIFEROUS ROCKS.
-
-
-At the close of Carboniferous times a marked change took place in the
-nature of the earth-movements. The prevalent depression which occurred
-over the British and adjoining regions during Carboniferous times was
-replaced by upward movement, accompanied by orogenic folds, which once
-more brought on continental conditions and developed a series of
-mountain ranges. The change is marked even at the close of
-Carboniferous times by the abnormal red sandstones of the uppermost
-part of the Carboniferous system which are found around Whitehaven in
-Cumberland and Rotherham in Yorkshire, as the Whitehaven Sandstone and
-Rotherham Red Rock. These movements continued through Permian and
-Triassic times, and it is to them and to the climatic conditions of
-the periods, that the anomalous nature of the Permo-Triassic deposits
-is largely due, as will be shewn in the succeeding chapters. At
-present it is our purpose to call attention to the effect of these
-movements upon the sediments which had been deposited previously to
-their occurrence.
-
-Over the British area, two different systems of orogenic movement can
-be detected, producing folds of which the axes run approximately at
-right angles to one another. One of these, of which the Pennine system
-is the best representative in Britain, caused the production of
-elevations having axes in a general north and south direction, and we
-may therefore speak of it as the Pennine system of movement, while the
-other, which gave rise to folds running in an east and west direction,
-is well represented in the Mendip Hills, and may be therefore termed
-the Mendip system, though it is more widely known as the Hercynian
-system, as, on the Continent, the rocks which are greatly affected by
-it form the foundations of the region occupied by the ancient
-Hercynian forest.
-
-The effects of these systems were in the main similar; they resulted
-in the uplift of parallel belts of country to form hill-ranges with
-intervening lowlands, but when studied in detail the movements are
-seen to be of a different character. The Pennine system of movements
-was of a type which is familiar to the geologists as developed in the
-Great Basin Region of the western territories of North America, and
-produced what is spoken of as Basin-Range structure. The movements
-were of the nature of direct uplift, causing fracture, only
-accompanied by folding in a minor degree, and accordingly the hills
-are composed of terraced scarps, with one gently sloping side, and one
-steep scarp-side, the latter on the upthrow side of the fault, as seen
-in fig. 21.
-
-In the Mendip system, the folds were of the Alpine type, which is a
-familiar product of lateral pressure, consisting essentially of
-overfolds, though these are often complicated by reversed faults.
-
-Of the Pennine system, the Pennine Chain itself furnishes the most
-noteworthy example in Britain, but we have indications of other folds
-of this system, such as that which runs from the Lake District to the
-Ayrshire coast, which is partly concealed as the result of other
-movements, and a still more marked one, in the rocks of the Malvern
-Hills.
-
-[Illustration: Fig. 21.
-
-_a aŽ_. One stratum displaced by faults _f f_. _h._ Hills.]
-
-The Mendip system is well shewn in the Mendip Hills, but the remains
-of a still more important anticline are seen in South Devon and
-Cornwall, separated from the Mendip Hills by the great syncline of
-Devon. Another parallel anticline runs from Lancashire to Yorkshire at
-right angles to the Pennine Chain and separates the coal-field of
-Cumberland and that of Northumberland and Durham, from those of South
-Lancashire, and Yorkshire, Notts, and Derbyshire.
-
-On the European continent the Ural Chain is the most important uplift
-of the system of which the Pennine Chain forms a minor representative,
-while the Hercynian system has caused the compression and stiffening
-of many of the Carboniferous and earlier rocks which now rise to the
-surface in many parts of central Europe.
-
-The extensive continental area which was the result of these uplifts
-not only determined the formation of abnormal deposits, but allowed
-the occurrence of a long period of time subsequently to the close of
-the Carboniferous period, of which few deposits now exposed in Europe
-are representative, and we must accordingly seek other regions in
-order to find typical representatives of this _Permo-Carboniferous_
-period, of which the strata developed in the Salt Range of India have
-been most carefully worked, especially by Dr Waagen, though marine
-sediments of the period are known elsewhere, as in Spitsbergen, the
-Ural Mountains, China and Australasia; and a group of somewhat
-anomalous sediments of this age in parts of India, Australia and South
-America is of peculiar interest, on account of the insight as to the
-climatic conditions of the times which it affords.
-
-_The Permo-Carboniferous Rocks._ In the Salt Range of the North-West
-of India an interesting series of sandstones alternating with
-limestones rests unconformably upon lower rocks. The sandstones are
-known as the Speckled Sandstones, while the limestones are termed the
-_Productus_ Limestones. The Lower and Middle Speckled Sandstones are
-succeeded by the Lower _Productus_ Limestone which is separated from
-the Lower division of the Middle _Productus_ Limestone by the Upper
-Speckled Sandstone; these are all of the Permo-Carboniferous period,
-while the upper part of the Middle _Productus_ Limestone and the Upper
-_Productus_ Limestone belongs to the Permian period. The fossils,
-largely invertebrates, are intermediate in character between those of
-Carboniferous and Permian ages. Similar fossils are found in the
-marine Permo-Carboniferous beds of the other areas which have been
-named above. The Lower Speckled Sandstone is of interest on account of
-the occurrence of boulder-beds within it, and this division of the
-sandstone has been correlated with the lowest (Talchir) stage of the
-Permo-Carboniferous strata of other parts of India, while the other
-Speckled Sandstones and those divisions of _Productus_ Limestone which
-are referred to the Permo-Carboniferous are correlated with the higher
-divisions of other parts.
-
-Special mention is made of the Talchir division, on account of the
-occurrence therein of boulder beds which have long been known, and
-whose glacial origin was inferred by Dr W. T. Blanford forty years
-ago. The accumulations shew signs of having been deposited in water,
-but the existence of large subangular, sometimes striated boulders
-therein, which must have come from distant sources, and the occasional
-occurrence of striated rock surfaces on the strata upon which the
-Talchir beds repose unconformably points to ice-action; this would not
-be so very remarkable if it were an isolated case, though sufficiently
-so, from the comparative nearness of the region to the equator; but
-researches conducted in different parts of the southern hemisphere
-have brought to light similar, and sometimes even more striking
-evidences of glacial action in widely distinct regions[93]. In
-Australia they have been found in New South Wales, Victoria,
-South Australia, East Australia and Tasmania; the Dwyka
-boulder-conglomerates of South Africa and certain deposits of similar
-character discovered by Prof. Derby in Southern Brazil have been
-referred to the same period, and their glacial origin has also been
-inferred. This widespread distribution of deposits which are generally
-contemporaneous, of which the glacial origin may now be taken as
-established, is extremely remarkable, and must be taken into careful
-consideration by those who put forward theories framed to account for
-former climatic changes.
-
-[Footnote 93: The reader will find an excellent account of the
-Permo-Carboniferous glacial deposits in a paper by Prof. Edgworth
-David, entitled "Evidences of Glacial Action in Australia in
-Permo-Carboniferous Time" (_Quart. Journ. Geol. Soc._ Vol. LII. p.
-289). In this paper other glacial beds besides those of Australia are
-noticed.]
-
-_The Flora and Fauna._ The flora of the Permo-Carboniferous beds has
-caused as much discussion as the question concerning the origin of the
-boulder-deposits. In the southern hemisphere, the Permo-Carboniferous
-rocks of those countries which have yielded boulder-beds also contain
-remains of a flora which is now known as the _Glossopteris_ flora,
-from the prevailing genus, which is associated with other genera, such
-as _Gangamopteris_. These fossils appear to be ferns, though their
-modern allies have not been indicated with certainty; associated with
-them are rare cycads and conifers. The _Glossopteris_ flora is
-markedly contrasted with the Coal-Measure flora of the northern
-hemisphere with its giant lycopods. Moreover _Glossopteris_ appears in
-the northern hemisphere in rocks of later date than the
-Permo-Carboniferous period. It has been suggested that the
-_Glossopteris_ flora originated in a continent in the southern
-hemisphere, on which the boulder beds were also formed in isolated
-water areas, and that some of the forms migrated northwards. To this
-continent the name Gondwanaland has been applied by Prof. Suess, from
-the _Gondwana_ series of the Permo-Carboniferous rocks of India, in
-which the _Glossopteris_ flora is found, and it has also been
-maintained that the southern _Glossopteris_ flora was contemporaneous
-with the northern flora of ordinary Coal-measure type, though whether
-this was so to any extent remains to be proved, for the beds
-containing the _Glossopteris_ flora are distinctly newer than any
-which have furnished a typical northern Coal-measure flora. In
-any case, the change of floras between Coal Measure and
-Permo-Carboniferous times is very marked, and when taken in connexion
-with the widespread glacial deposits, is one of the most striking
-phenomena displayed by the rocks of the stratified column[94].
-
-[Footnote 94: For an account of the Glossopteris flora and its
-geological relations, consult Seward, A. C., _Science Progress_,
-January, 1897, p. 178.]
-
-The fauna has already been noticed. It consists of brachiopods, some
-of which are of peculiar genera. The general similarity of the faunas
-in regions so remote as Spitsbergen, the Ural Mountains, India, and
-New South Wales, indicates an extensive sea during the period. It can
-hardly be supposed that the fauna of Permo-Carboniferous times has
-been completely described, for the fossils of one or two areas only
-have been made known to us with any degree of fulness, and when the
-Permo-Carboniferous and marine Permian faunas are as well known as
-those of Triassic times (and the latter have only been fully described
-very recently) there is no doubt that the important break which was at
-one time supposed to exist between Palæozoic and Mesozoic faunas will
-be filled in satisfactorily[95].
-
-[Footnote 95: The Permo-Carboniferous beds are described in Messrs
-Medlicott and Blanford's _Geology of India_, second edition (edited by
-Mr R. D. Oldham), and figures of some of the important fossils given
-therein. For fuller information the reader should refer to Waagen's
-account of the Salt Range Fossils and Feistmantel's description of the
-plants in the _Memoirs of the Geological Survey of India_.]
-
-
-
-
-CHAPTER XX.
-
-THE PERMIAN SYSTEM.
-
-
-_Classification._ It has already been observed that as the result of
-the Pennine and Mendip systems of earth-movement, the Carboniferous
-rocks of Britain are succeeded by a marked unconformity, and that the
-rocks of the succeeding Permian and Triassic systems of Britain shew
-an abnormal development. The principal areas where Permian rocks are
-found are on either side of the Pennine Chain in the North of England,
-but sporadic exposures of rocks of this age are found in some of the
-Midland and Southern counties. The Permian rocks have been well
-studied in Germany, and the German names are sometimes adopted in
-Britain, and the following comparison will prove useful:--
-
-           Britain.               Germany.
-    Magnesian Limestone       Magnesian Limestone } Zechstein.
-    Marl Slate                Kupferschiefer      }
-    Lower Permian Sandstones  Rothliegende.
-
-The term Zechstein has been applied in a somewhat different sense by
-different writers, but the one given in the table appears to find most
-favour.
-
-In a region which was essentially continental, considerable variations
-in the lithological characters of the rocks may be expected, when the
-strata are traced laterally, but we nevertheless find that the
-differences are not so great as was formerly supposed to be the case
-when certain red sandstones lying above recognised Permian strata in
-the district on the west side of the Pennine Chain towards its
-northern extremity were also referred to the Permian; these sandstones
-(the St Bees Sandstones) are now generally admitted to be of Triassic
-age, and comparison between the rocks on opposite sides of the Pennine
-Chain is much simplified, as seen below.
-
-        West side.                           East side.
-  Thin Magnesian Limestones and Marls    Magnesian Limestone
-  Hilton Shales                          Marl Slate
-  Penrith Sandstone and Brockrams        Lower Permian Sandstones.
-
-_Description of the Strata._ On the east side of the Pennine Chain,
-the Lower Permian sandstone is an inconstant deposit often consisting
-of yellow false-bedded arenaceous strata. The Marl Slate is an
-argillaceous shale, often containing bituminous matter, and yielding
-several fish-remains and some plants; it is usually only a few feet in
-thickness. The Magnesian Limestone is typically developed in Durham as
-a yellow or greyish limestone containing a variable percentage of
-carbonate of magnesia; when traced southward, it alters its
-characters, becoming mixed with mechanical deposits, and some chemical
-precipitates in places, so that at Mansfield it appears as a red
-sandstone with grains cemented by a mixture of carbonates of lime and
-magnesia; and, like the rest of the Permian strata, it has disappeared
-when we reach Nottingham. In addition to the southward thinning of the
-Permian beds of this area, there is some evidence of their
-disappearance in a westerly direction, though, as the present strike
-of the beds is nearly north and south, the indications of this are
-less convincing.
-
-On the east side of the Pennine Chain, the main difference observable
-is the relative thickness of the major divisions. The Lower Permian
-sandstones have thickened out considerably, while the reputed
-representatives of the Magnesian Limestone are thin. The Penrith
-sandstone is of considerable interest. It contains in places, as near
-Appleby, thick deposits of breccia consisting of angular fragments
-chiefly composed of Carboniferous Limestone, which in many cases have
-undergone subsequent dolomitisation, embedded in a matrix of red
-sandstone. This breccia is known as brockram. Many beds of the Penrith
-sandstone are composed of crystalline grains of sand, due to
-deposition of silica in crystalline continuity with the quartz of the
-original grain after the formation of the deposit; of more
-significance, for our present purpose, is the presence of other
-accumulations of the sand, in which the individual grains often
-approach the form of spheres, thus resembling the 'millet-seed' sands
-of modern desert regions. The Hilton shales are grey sandy shales,
-with plant remains, and above them are variable deposits including
-thin magnesian limestones which have yielded no fossils.
-
-The isolated Permian deposits of the midland and southern counties of
-England consist of red marls and sandstones with occasional breccias,
-and in the absence of fossils, their exact position in the Permian
-series is still unknown.
-
-The German Permian rocks resemble those of Britain, especially as seen
-in Durham, in many particulars, and give indications of formation
-under physical and climatic conditions generally similar to those
-which were then prevalent in the British area. At Stassfurt, in
-Germany, the less soluble constituents of ocean water are accompanied
-by a great variety of salts:--chlorides, sulphates and borates; and
-the very soluble salts of potassium and magnesium known as the Abraum
-salts are found in abundance as well as the less soluble salts of
-sodium and calcium. The occurrence of these very soluble salts is so
-infrequent on a large scale among the rocks of the Geological Column,
-and the matter is one of so great theoretical import, that it is
-necessary to take special note of their presence in the Permian
-strata.
-
-The frequent existence of chemical deposits in the Permian Rocks of
-N.W. Europe, the formation of red sandstones, and the dolomitisation
-of limestone beds and fragments of pre-existing limestones point to
-inland seas of a Caspian character, while the evaporation necessary
-for the formation of the precipitates also indicates a fairly warm
-temperature. The presence of millet-seed sands, in very lenticular
-patches, suggesting former sand-dunes, and the occurrence in places of
-breccias (like some parts of the brockram) almost devoid of matrix,
-piled up against pre-existing cliffs, recalling screes of modern
-times, give almost certain evidence of the occurrence of land tracts
-most probably of desert character, during part of the period of
-accumulation of the materials of the Permian rocks. The fossil
-evidence supports this view, and geologists are mostly agreed that the
-Permian rocks of north-west Europe were accumulated in an area of
-desert character, occupied in part by inland seas, though there is
-much difference of opinion as to the extent of these seas, some
-geologists holding that a number of isolated sheets of water were
-necessary to produce the distribution and character of the
-accumulations. It is still a vexed question with British geologists
-how far the Pennine ridge stood up as land during the period, but
-leaving this and other minor considerations out of account, it may be
-noted that the similarity of deposits in the different areas, whether
-we examine the order of succession, the lithological characters or the
-included fossils, suggests communication between the water tracts of
-different regions, though this communication need not have been more
-than a series of straits, or comparatively narrow belts of water[96].
-
-[Footnote 96: It should be mentioned that some writers have inferred
-the evidence of glacial conditions over parts of the British area, on
-account of the resemblance of some of the Permian breccias to recent
-glacial deposits. The question is still _sub judice_. It is not
-necessarily opposed to the existence of desert conditions, if the
-mountains were sufficiently high, for the Wahsatch regions adjoining
-the Basin Region of N. America have been glaciated.]
-
-The extensive development of Permian and Triassic rocks with
-terrestrial characters in the southern hemisphere also, and the
-absence of newer deposits in many places, suggests that the land areas
-of these times in that hemisphere have largely remained such ever
-since, in which case, the Permo-Triassic series of movements produced
-a marked direct effect upon our present continental areas, and at any
-rate produced an indirect one upon the British land tracts.
-
-The presence of anomalous deposits of Permian age over wide areas need
-not be surprising, but it would be indeed remarkable if no ordinary
-marine type of Permian rocks was known, and the researches of recent
-years have proved that this type is extensively developed, in Eastern
-Europe, Asia, and North America, where Permian rocks consisting of
-limestones, with a greater or less admixture of mechanical deposits,
-occur in some abundance. The studies of Waagen and others in India
-have given us the farthest insight into the nature of these beds.
-Below is a general classification taken from Waagen's work:--
-
-       Salt Range.                       Germany.
-
-  Base of Trias                 }
-  Unfossiliferous Shale and     }
-    Sandstone                   } Passage Beds into Trias
-  Top Beds of Upper _Productus_ }
-    Limestone                   }
-
-  Cephalopoda Beds of Upper     } Gypsum Beds
-    _Productus_ Limestone       }
-
-  Middle Division of Upper      }
-    _Productus_ Limestone       } Zechstein (in restricted sense)
-  Lower Division of Upper       }
-    _Productus_ Limestone       }
-
-  Upper Division of Middle      } Weissliegende and Kupferschiefer
-    _Productus_ Limestone       }
-
-  Middle Division of Middle     } Rothliegende.
-    _Productus_ Limestone       }
-
-It will be seen that in the Salt Range there is a complete passage
-from the Permo-Carboniferous strata through the Permian into the
-Trias, and the detailed work which has been carried out by Waagen and
-others amongst the rocks of the Salt Range must make this, for the
-present at all events, the type area for the marine development of the
-strata of Permo-Carboniferous and Permian ages.
-
-_The Permian flora and fauna._ The Permian flora presents some
-difficulties. The flora of the Zechstein consists largely of ferns and
-conifers, but that of the Rothliegende of Germany has been compared
-with that of the Carboniferous, and if a true Permian flora of the
-northern hemisphere has many forms of Carboniferous affinities, the
-presence of the Glossopteris flora in Permo-Carboniferous rocks of
-more southerly regions seems to imply its origin there and _slow_
-migration northwards. It must be noted, however, that the Rothliegende
-has been divided by some geologists into an upper and lower division,
-of which the lower is actually referred to the Carboniferous system.
-All that can be now said is, that our knowledge of the floras of
-Permo-Carboniferous and Permian times is still incomplete, and that
-the difficulties will no doubt be cleared up as the result of further
-work.
-
-The invertebrate fauna of the north-west European Permian deposits is
-chiefly noticeable on account of the paucity of species, though
-individuals are often abundant. The shells are also sometimes stunted
-and occasionally distorted. These characters bear out the supposition
-that the aqueous deposits were laid down in inland seas of Caspian
-character and not in the open ocean. Polyzoa, brachiopods, and
-lamellibranchs predominate, but other groups are found. The
-vertebrates consist of forms of fish, amphibia and reptiles, and the
-Permian rocks are the earliest strata in which the remains of true
-Reptilia are known to occur with certainty. The Reptiles belong to the
-orders Anomodontia (Theromora) and Rhynchocephalia, of which the
-former is exclusively Permian and Triassic, while the latter is
-abundant in the strata of those periods, but is represented at the
-present day by the genus _Sphenodon_ of New Zealand. The Amphibia
-belong to the order Labyrinthodontia which ranges from Carboniferous
-to Lower Jurassic, but the members of the order are most abundant in
-Permian and Triassic strata, and these periods may be spoken of as the
-Periods of Labyrinthodonts.
-
-A few words must be said of the fauna of the truly marine Permian
-beds. It is much richer than that of the abnormal deposits of
-north-western Europe, and its study is important as furnishing another
-link between Palæozoic and Mesozoic life. Many Palæozoic genera pass
-up into the Permian rocks, and, as will be ultimately seen, several
-occur in those of the Triassic system, and one or two even in the
-basal Jurassic strata, though Mesozoic forms predominate in the Lower
-Jurassic Rocks, and there is a fairly equal admixture of forms usually
-considered as Palæozoic and of those generally regarded as Mesozoic in
-Triassic rocks, while the Palæozoic forms still predominate over the
-Mesozoic in the Permian strata. Along with these characteristic
-Palæozoic genera, it is interesting to find representatives of more
-than one genus of the tribe of Ammonites, which is to take so
-prominent a place in the fauna of the Mesozoic rocks, amongst the true
-marine Permian sediments of India and other areas. The announcement of
-the contemporaneity of ammonites with fossils regarded as exclusively
-palæozoic was received with considerable doubt, but this
-contemporaneity is now clearly established, and need not be regarded
-as in any way anomalous.
-
-With the deposition of the Permian rocks, Palæozoic time comes to an
-end, but as already remarked there is no marked and sudden change to
-characterise it. Had our classification been originally founded on
-study of the Indian Rocks instead of those of Britain, and similar
-terms adopted, the line of demarcation between Palæozoic and Mesozoic
-rocks would probably have been drawn below the Permo-Carboniferous
-deposits, and if it had been based on study of other areas, perhaps
-elsewhere. The palæontological break is purely local, and it is of the
-utmost importance that it should be recognised as such, and that it
-should not be considered that division into Palæozoic and Mesozoic
-implies some great and widespread change which occurred between the
-times covered by the deposits of each of these great divisions[97].
-
-[Footnote 97: The Permian Fossils of Britain are described by
-Professor King in the Monographs of the Palæontographical Society (the
-Brachiopods by Dr Davidson in the Monographs of the same Society). For
-a general account of the marine type the student may consult the
-second edition of Messrs Medlicott and Blanford's _Geology of India_.
-For information concerning the Permian volcanic rocks see Sir A.
-Geikie's _Ancient Volcanoes of Great Britain_.]
-
-
-
-
-CHAPTER XXI.
-
-THE TRIASSIC SYSTEM.
-
-
-_Classification._ The term Triassic has been applied to these rocks on
-account of the threefold division into which those of Germany
-naturally fall. These three divisions are:--
-
-    Keuper,
-    Muschelkalk,
-    Bunter;
-
-but above the Keuper beds we find a group of deposits of some
-importance, which shew affinities with both Triassic and Jurassic
-rocks, which may be looked upon as true passage beds, though they are
-generally placed in the Triassic System. They are known as Rhætic or
-locally in Britain as Penarth Beds. The Muschelkalk is usually
-considered to be unrepresented in Britain, and accordingly the British
-deposits may be, and are usually grouped as under:--
-
-  Rhætic or Penarth beds
-  Keuper                  { Keuper Marls
-                          { Keuper Sandstones
-  [Muschelkalk] absent
-                          { Upper Red and Mottled Sandstones
-  Bunter                  { Bunter Pebble Beds
-                          { Lower Red and Mottled Sandstones.
-
-The threefold grouping has been applied more or less universally, but
-when used outside the north-west European area, it loses its
-significance, as the conditions which enable one to differentiate the
-rocks of the three divisions were naturally only prevalent over a
-limited area.
-
-_Description of the strata._ The British Triassic rocks possess a
-certain sameness as regards their general characters, consisting
-mainly of mechanical sediments coloured red by peroxide of iron, with
-occasional chemical precipitates of rock-salt and gypsum. They have a
-wider distribution over Britain than have the Permian rocks, and the
-lithological characters of the different subdivisions do not as a rule
-vary to a remarkable degree when traced laterally. The differences in
-detail in the characters of the various deposits are noteworthy, and
-an explanation of the exact origin of some of these abnormal deposits
-which will satisfy everyone is not yet forthcoming. Leaving the
-details out of consideration for the moment, and looking at the
-general aspect of the deposits, the prevalence of conditions generally
-similar to those which existed over the British Isles in the preceding
-Permian period is decidedly indicated by the nature of the strata,
-though the continental conditions appear to have been more widely
-established over our area, as shewn by the general absence of any
-calcareous deposits resembling the Magnesian Limestone. We find
-chemical precipitates, millet-seed sandstones, and scree-like breccias
-in the British Triassic rocks as well as in those of Permian age, and
-the paucity of a marine invertebrate fauna in the Triassic rocks of
-Britain is even more apparent than in the Permian strata. It is only
-at the extreme close of the Triassic period, during the deposition of
-the rocks which are admitted on all hands to be of Rhætic age, that
-we note the incoming of those marine conditions over our area, which
-prevailed so extensively, with few local exceptions, during the
-remainder of the Mesozoic and the early part of Tertiary times; the
-Rhætic beds, in fact, mark the commencement of the third marine
-period. Referring to the strata in further detail, we may proceed to
-consider the character of the different subdivisions in the order of
-their formation, commencing as usual with the oldest. The Bunter
-deposits rest in places upon those of Permian age with an unconformity
-at the junction, but as these unconformities occur frequently among
-the British Triassic rocks, it is doubtful whether this unconformity
-marks more than very local change of physical conditions. The lower
-and upper divisions of the Bunter sandstone consist of false-bedded
-red and variegated sandstones, and there is no great difficulty in
-explaining their formation in desert areas with tracts of water, but
-the great change which marks the appearance and disappearance of the
-middle division, the Bunter pebble beds, requires some explanation,
-for the contrast between the lithological characters of the rocks of
-this division and those of the rocks appertaining to the preceding and
-succeeding division is very marked. The matrix differs, but the main
-difference is the abundance of pebbles, mostly of fairly uniform size,
-well rounded, and largely consisting of liver-coloured quartzite. Much
-difference of opinion exists as to the exact origin of these pebble
-beds, and the source of the pebbles, but without entering into this
-vexed question, it may be remarked that the agency of rivers has been
-somewhat generally invoked to account for their transport, and the
-conditions during their accumulation need not have been very different
-from those which are now found in northern India where the torrential
-rivers of the south side of the Himalayan chains debouch upon the
-plain, and spread an abundant deposit of well-worn pebbles over the
-finer silts which were previously laid down thereon.
-
-The junction of the Bunter and Keuper beds requires a short notice. It
-is usually if not always an unconformable one in Britain, and it is
-generally assumed that the absence of the Muschelkalk of the Continent
-is due to the presence of land undergoing denudation in Britain during
-the time when the Muschelkalk was elsewhere deposited, though it is
-quite possible that the Muschelkalk epoch is represented in Britain
-not only by the time which elapsed when the unconformity was being
-impressed on the rocks, but also during the true deposition of the
-upper part of the Bunter beds, or the lower part of the Keuper, or
-both.
-
-The Keuper sandstones and marls contain a great development of
-chemical deposits, of millet-seed sands, and of many other features
-pointing to desert conditions, such as sun-cracks, tracks of animals
-impressed upon a rapidly drying surface, and pseudomorphs of mud after
-rock salt in the form of cubes and hopper-crystals; furthermore we
-find the scree-like breccias at different horizons of the Keuper beds
-where they abut against the old Mendip ridge composed largely of
-mountain-limestone which furnished the fragments, as was the case with
-the brockrams abutting against the Pennine ridge. It must be noted
-that the chemical precipitates of Triassic age consist of the less
-soluble substances dissolved in ocean water, namely, gypsum and rock
-salt, whilst the more deliquescent potash and magnesia salts are not
-represented in Britain.
-
-Turning to these continental beds, we get evidence of a general
-approach to open sea conditions as we pass away from Britain in a
-south-easterly direction as roughly shewn in the following diagram
-(fig. 22), where _B_ represents the Bunter beds, _M_ the Muschelkalk,
-and _K_ the Keuper.
-
-[Illustration: Fig. 22.]
-
-It will be seen that the mechanical sediments gradually die out and
-become replaced by calcareous material as one passes from Britain
-towards Switzerland; the Muschelkalk is very thin in the east of
-France and thickens out in Germany, while in Switzerland Keuper,
-Muschelkalk and Bunter are alike largely represented by calcareous
-deposits, and the mechanical deposits are chiefly argillaceous, the
-only important sandstone being situated at the extreme base of the
-Bunter series.
-
-The marine development of the Triassic system is naturally the one
-which is most widely spread, though full appreciation of its
-importance has only taken place as the result of researches in distant
-climes of recent years. It is found in southern Europe, in
-Spitsbergen, in considerable tracts of Asia, including India, and
-along the Pacific coast region of North America, and everywhere
-possesses much the same characters.
-
-It will be seen from the above remarks that the physical conditions
-which prevailed in the continental area of Triassic times which is now
-partly occupied by the British Isles are most closely represented by
-those of the desert regions of central Asia, hemmed in by the
-mountain ranges which intercept the vapour-laden winds of the oceans,
-and cause them to precipitate the great bulk of their vapour on the
-seaward slopes of the mountains, so that they blow over the deserts as
-dry winds, causing the fall of any large amount of rain to be a rare
-though by no means unknown event in the desert regions.
-
-_Flora and Fauna of the Period._ The Triassic flora is essentially
-similar to that of the higher Permian strata, though many of the
-genera are different.
-
-The invertebrate fauna of the British deposits is, as might be
-expected, very poor until the beds of the Rhætic series are reached.
-In the beds below the Rhætics, the principal invertebrate remains are
-the tests of the crustacean genus _Estheria_, though a few obscure
-lamellibranch shells have been recorded. The vertebrate fauna is of
-great interest. A number of fishes have been found, the most
-remarkable of which is the genus _Ceratodus_, occurring in the Rhætic
-beds of Britain and lower Triassic strata of foreign countries. It is
-closely related to the Barramunda of the Queensland rivers belonging
-to the order Dipnoi. As in the Permian strata, abundance of
-Labyrinthodont amphibians have been discovered, and the reptiles
-belong to the orders Anomodontia and Rhynchocephalia. In the Rhætic
-beds of Britain and in still lower Triassic beds abroad the orders
-Ichthyopterygia and Sauropterygia (represented by _Ichthyosaurus_ and
-_Plesiosaurus_) are found.
-
-The Triassic rocks also yield the earliest known mammals, the best
-known, _Microlestes_, occurring in the Triassic rocks of Britain and
-the Continent. These mammals are now placed in a subclass Metatheria
-of the order Monotremata.
-
-The marine invertebrate fauna of the normal Triassic rocks presents
-some points of considerable interest. As already remarked, the fauna
-may be looked upon as a passage fauna between that of Palæozoic and
-that of Mesozoic times, the number of Palæozoic forms which pass into
-the Trias being approximately comparable with those which appear here
-and range upwards into higher Mesozoic strata. This may be well seen
-by examining the table given in Chapter XXI. of the Second Edition of
-Sir Charles Lyell's _Student's Elements of Geology_, in which three
-columns shew the genera of Mollusca common to older rocks, those
-characteristic of the Trias, and those common to newer rocks. Amongst
-the first are _Orthoceras_, _Bactrites_, _Loxonema_, _Murchisonia_,
-and _Euomphalus_, in the second column are _Ceratites_, _Halobia_
-(_Daonella_), _Koninckina_, and _Myophoria_, and in the third,
-Ammonites, _Cerithium_, _Opis_, _Plicatula_ and _Thecidium_[98].
-
-[Footnote 98: It has been seen that some of the Ammonites appear
-earlier, namely, in Permian strata. _Myophoria_ is extremely abundant
-in the Trias, but ranges into newer strata.]
-
-The Ammonites are largely utilised in the case of the Mesozoic strata
-for separation of these strata into zones, each zone being
-characterised by some species of Ammonite, and the researches of
-Mojsisovics have proved that this zonal subdivision, long adopted for
-Jurassic rocks, is also applicable to those of Triassic age[99]. He
-gives the following table of the classification of the Triassic rocks
-of the Mediterranean Province, which is reproduced, as it is founded
-upon Palæontological evidence, and will probably be widely adopted.
-
-[Footnote 99: von Mojsisovics, Dr E., "Faunistische Ergebnisse aus der
-Untersuchung der Ammoneen-faunen der Mediterranen Trias." _Abhandl.
-der k. k. Geologisch. Reichsanstalt_, VI. Band 2 Abtheilung. Vienna,
-1893.]
-
-     Series                                    Zonal Divisions
-  --------------+-------------------+--------------------------------------
-  Rhætic        |                   |  1. Zone of _Avicula Contorta_
-  --------------+-------------------+--------------------------------------
-                |                   |  2. Zone of _Sirenites Argonautae_
-                | Upper Juvavic     |  3. Zone of _Pinnacoceras
-                |                   |       Metternichi_
-  Juvavic       | Middle Juvavic    |  4. Zone of _Cyrtopleurites
-                |                   |       bicrenatus_
-                |                   |  5. Zone of _Cladiscites ruber_
-                | Lower Juvavic     |  6. Zone of _Sagenites Giebeli_
-  --------------+-------------------+--------------------------------------
-                | Upper Carnic      |  7. Zone of _Tropites subbullatus_
-  Carnic        | Middle Carnic     |  8. Zone of _Trachyceras Aonoides_
-                | Lower Carnic      |  9. Zone of _Trachyceras Aon_
-  --------------+-------------------+--------------------------------------
-                | Upper Noric       | 10. Zone of _Protrachyceras
-  Noric         |                   |       Archelaus_
-                | Lower Noric       | 11. Zone of _Protrachyceras Curionii_
-  --------------+-------------------+--------------------------------------
-                | Upper Muschelkalk | 12. Zone of _Ceratiles trinodosus_
-  Muschelkalk   |                   |
-                | Lower Muschelkalk | 13. Zone of _Ceratiles binodosus_
-  --------------+-------------------+--------------------------------------
-  Buntsandstein | Werfener Schichten| 14. Zone of _Tirolites Cassianus_
-  --------------+-------------------+--------------------------------------
-
-
-
-
-CHAPTER XXII.
-
-THE JURASSIC SYSTEM.
-
-
-The Jurassic rocks were formerly separated on account of differences
-of lithological character into Oolites and Lias, but it was apparent
-that the Oolites were more important than the Lias, and a fourfold
-division was made into:--
-
-  Upper or Portland Oolites }
-  Middle or Oxford Oolites  } = Malm
-  Lower or Bath Oolites       = Dogger
-    Lias.
-
-The Lias strata have also been spoken of as the Black Jura, the Lower
-Oolites and part of the Oxford Oolites as Brown Jura, and the rest of
-the Oxford Oolites with the Portland Oolites as White Jura.
-
-As the outcome of a detailed study of the faunas of the Jurassic
-rocks, a further subdivision has been made, partly based upon the
-original British series, but the divisions are defined with greater
-accuracy, so that they are applicable over wider areas. They are as
-follows:--
-
-                 { Purbeckian
-  Upper Oolites  { Portlandian
-                 { Kimmeridgian
-
-                 { Corallian
-  Middle Oolites { Oxfordian
-                 { Callovian
-
-  Lower Oolites  { Bathonian
-                 { Bajocian
-
-                 { Toarcian
-  Lias           { Liassian
-                 { Sinemurian.
-
-Many of these series have been still farther subdivided into smaller
-stages, and the whole differentiated into a number of zones
-characterised by different forms of Ammonites. Dr E. von Mojsisovics
-gives thirty-two Ammonite zones, of which fourteen occur in the Lias,
-eight in the Lower Oolites, six in the Middle Oolites, and four in the
-Upper Oolites.
-
-_Characters of the strata._ The whole of the Jurassic rocks and also
-those of Lower Cretaceous age may be regarded as having been deposited
-during the first shallow water phase of the third marine period, but
-this shallow water phase is represented by strata which are varied
-owing to numerous marine changes resulting in the production of land
-at times, and estuarine conditions, shallow water, marine conditions,
-and somewhat deeper sea conditions respectively at other times, and
-accordingly the strata of the British Isles vary greatly when traced
-laterally. That the uplifts of the Permo-Triassic periods produced
-some effect on the nature and distribution of the Jurassic rocks is
-certain, but it is not quite clear how far the ridges produced by
-these uplifts were submerged and denuded during the deposition of the
-main portion of the Jurassic strata.
-
-Viewed broadly, the Jurassic rocks of Britain may be regarded as
-consisting of three great clay deposits, the Lias, Oxford and
-Kimmeridge Clays, alternating with the deposits of variable
-lithological characters, which compose the Bajocian, Bathonian,
-Corallian, Portlandian and Purbeckian subdivisions. This essentially
-argillaceous character of a large part of the deposits of Jurassic age
-is often overlooked, as, owing to their sameness and the comparative
-paucity of organisms constituting the faunas in the clays, their
-description in text-books can be given at much shorter length than
-that of the more variable and highly fossiliferous deposits which
-separate the clays. The following figure (Fig. 23) roughly represents
-the nature of the different divisions of the rocks of this system when
-traced across England from south-west to north-east.
-
-[Illustration: Fig. 23.
-
-Vertical scale: 1 in. = about 1000 feet.]
-
-It will be seen that the greatest variations in lithological character
-occur in the Bathonian and Bajocian beds, and it will be of interest
-to give some account of the principal variations and to attempt to
-account for them. In so doing it will be convenient to consider the
-four major divisions of the Jurassic rocks separately, and to enter
-into particulars concerning the local classification applied to the
-rocks of these divisions.
-
-_The Lias._ The British Lias deposits are divided into the Lower Lias,
-the Marlstone, and the Upper Lias corresponding in general terms only
-with the Sinemurian, Liassian, and Toarcian. The Marlstone is
-separated from the Upper and Lower Lias on account of the greater
-percentage of carbonate of lime which it contains, so that the bands
-of argillaceous limestone are much more marked in the Marlstone than
-in the upper and lower divisions, which consist chiefly of clay. The
-three divisions possess very much the same characters throughout the
-country, though the presence of the Mendip ridge and its continuation
-beneath London is marked by the attenuation of this and succeeding
-strata, and by the conglomeratic character of some of the Liassic
-strata where they abut against it. The British Lias, as a whole, seems
-to have been deposited in a fairly shallow sea at no great distance
-from the land. It passes down conformably into the Rhætic beds, indeed
-the zone of Ammonites (_Aegoceras_) _planorbis_, referred by British
-geologists to the Lower Lias is included by some continental writers
-with the Rhætic beds, and the plane of demarcation here as in other
-cases is conventional.
-
-_The Lower Oolites._ Of all the British strata, these perhaps cause
-most trouble to the learner, on account of the different nomenclature
-applied to the rocks in different parts of England, and the rapid
-variations in lithological character, when the beds are traced
-laterally. The following divisions are usually adopted for the beds of
-the south-western counties where the most marked marine development
-occurs:--
-
-    Cornbrash,
-    Forest Marble,
-    Great Oolite (with Bradford Clay),
-    Fuller's Earth,
-    Inferior Oolite.
-
-Of these divisions, the uppermost one, the Cornbrash, though thin,
-retains its characters with great constancy across the island. Of the
-others the Forest Marble may be looked upon as a local development of
-the upper portion of the Great Oolite, and the Fuller's Earth is a
-local deposit, so that the Inferior Oolite and Great Oolite constitute
-the important divisions of the Lower Oolites. The variations in the
-characters of the rocks may be best shown in tabular form.
-
- -----------------+------------------+-------------------+-----------------
- Gloucestershire, |      South       |        N.         |
-       &c.        | Northamptonshire | Northamptonshire  |    Yorkshire
-                  |                  |    and Lincoln    |
- -----------------+------------------+-------------------+-----------------
-     Cornbrash    |    Cornbrash     |     Cornbrash     |    Cornbrash
- -----------------+------------------+-------------------+-----------------
- Great Oolite     | Great Oolite     | Great Oolite Clay |
-                  | (Upper part)     | Great Oolite      | Upper
-                  |                  |   Limestone       |
-                  |                  | Upper             | Estuarine
-                  | Northamptonshire | Estuarine         |
-  ............... | ................ | ................. | ................
-                  |                  | Series            |   Series
-                  |                  | Lincolnshire      | Scarbro'
-                  |                  |   Limestone       |   Limestone
-                  | Sands            |                   | Middle Estuarine
-                  |                  |                   |   Series
- Inferior Oolite  |                  | Lower Estuarine   | Millepore Oolite
-                  |                  |   Series          |
-                  |                  |                   | Lower Estuarine
-                  |                  |                   |   Series
- -----------------+------------------+-------------------+-----------------
-    Upper Lias    |    Upper Lias    |     Upper Lias    |    Upper Lias
- -----------------+------------------+-------------------+-----------------
-
-     The dotted line shows roughly the division between Bathonian
-     and Bajocian.
-
-The changes may be explained very simply if we leave out of account
-for the moment the development of Lincolnshire Limestone, with its
-equivalent the Scarbro' Limestone, and the Millepore series. The beds
-in Gloucestershire and other south-western counties are essentially
-marine; whilst in Northamptonshire and Lincolnshire estuarine
-conditions set in after the deposition of the Upper Lias, and
-continued throughout the deposition of the Bajocian and Lower
-Bathonian beds, being replaced by marine conditions during the
-formation of the Upper Bathonian strata, and still further north in
-Yorkshire the estuarine conditions generally prevailed throughout
-Bajocian and Bathonian times. These changes point to the existence of
-land towards the north. The general simplicity is modified by
-temporary prevalence of marine conditions twice over (during the
-deposition of the Millepore Oolite and the Scarbro' Limestone) in
-Yorkshire, and once (during the deposition of the Lincolnshire
-Limestone) in Lincolnshire.
-
-Certain local deposits have not been noticed, but two of them merit
-brief reference. At the base of the Great Oolite of Oxfordshire is an
-estuarine deposit of finely laminated mechanical sediment mixed with
-calcareous matter known as the Stonesfield Slate, especially
-interesting on account of its fossils, while a bed with similar
-lithological characters but with a different fauna occurring at the
-base of the Lincolnshire Limestone (of Bajocian age) is termed the
-Collyweston Slate. Neither of these deposits is a slate in the true
-sense of the word, as they have not been affected by cleavage
-subsequently to their accumulation, but each has been somewhat
-extensively used for roofing purposes.
-
-The Middle Oolites are much less complicated though considerable
-variations arise with respect to the Corallian Rocks. The Oxfordian
-with Callovian consist chiefly of clay, though the Callovian of the
-south of England is represented by calcareous sandstone, with a
-peculiar fauna which seems to be represented in the lower part of the
-Oxford Clay further north, though this Callovian fauna has not been
-everywhere recognised.
-
-The Corallian of the southern counties consists of limestones with
-calcareous grits, the limestones being often largely composed of the
-remains of reef-building corals, and a similar development of the
-rocks of this series is found in Yorkshire, while a local development
-of the same character is found at Upware in Cambridgeshire, though in
-the other parts of the Fenland counties the Corallian is represented
-by an argillaceous deposit with Corallian fossils known as the
-Ampthill Clay.
-
-The Upper Oolites have a tolerably constant base, the Kimmeridge Clay,
-usually consisting of laminated bituminous argillaceous material, but
-the Portlandian and Purbeckian divisions vary greatly, and are only
-locally developed, though their absence in some parts of central
-England is no doubt due to unconformity.
-
-The Portlandian rocks of the south of England consist of limestones
-and sandstones which pass further northward into shallower water
-mechanical deposits often charged with iron hydrate, and the beds
-disappear in Oxfordshire. The Purbeckian rocks of the south are also
-limited as regards area of exposure: they consist of estuarine
-deposits with some terrestrial accumulations of the nature of old
-surface soils. Representations of the Portlandian and Purbeckian beds
-are found in Lincolnshire and Yorkshire, as arenaceous deposits in the
-former county and argillaceous ones in the latter. Both are marine
-deposits of a northern type, developed elsewhere in northern European
-and circumpolar regions, and in these counties we find a complete
-passage from the Jurassic rocks through the Cretaceous rocks, but the
-exact lines of demarcation between the different series of the passage
-beds are difficult to define.
-
-The foreign Jurassic rocks of Europe and of some parts of Asia
-strongly resemble in general characters those which have been
-described above as occurring in Britain. One of the most remarkable
-features of the Jurassic rocks as a whole, is the absence of the Lias
-over wide areas, the continental period which in Britain existed in
-Permo-Triassic times is elsewhere frequently replaced by one of
-Liassic age.
-
-The Jurassic and Cretaceous rocks are of interest on account of the
-evidence which they supply as to the existence of climatic zones in
-these periods, which run fairly parallel with those at present
-existing. The late Dr Neumayr in a paper already cited divides the
-world during later Mesozoic times into four distinct climatic zones,
-equatorial, north and south temperate and boreal zones (the
-corresponding austral zone is not known owing no doubt to the
-extensive sea of South Polar regions and our general ignorance of its
-lands). In Europe the Mediterranean Province belongs to the equatorial
-zone, the Middle European to the North temperate zone, and the Russian
-or Boreal to the Boreal zone. The last-named is marked partly by
-negative characters, the absence of certain Ammonite-genera and of
-coral reefs being noticeable, whilst the lamellibranch _Aucella_ is
-very frequent. In the North temperate zone, certain Ammonite genera as
-_Aspidoceras_ and _Oppelia_ are abundant and there are also extensive
-coral-reefs. The Equatorial zone is marked by the Ammonite-genera
-_Phylloceras_ and _Lytoceras_ and by the _Diphya_ group of
-_Terebratulæ_. It is of special interest to note that the fauna of the
-South temperate bears closer relationship to that of the North
-temperate than to that of the intermediate Equatorial zone.
-
-_Jurassic floras and faunas._ The Jurassic flora is very similar in
-its characters to that of the Lower Cretaceous rocks, and the two
-taken together afford a decided contrast with that of later Palæozoic
-times, and also with that which succeeds them in the Upper Cretaceous
-rocks, which bears a marked resemblance to the existing flora. Cycads
-predominate, accompanied by conifers, and a fair number of ferns and
-Equisetaceæ.
-
-The Jurassic fauna is specially noteworthy on account of the character
-of the vertebrata, but some notice of the invertebrates must also be
-taken. The abundance of corals in the Temperate zones has already been
-pointed out, but the mollusca form the bulk of the invertebrate fauna,
-lamellibranchs, gastropods and cephalopods being all abundant; of the
-last-named the ammonites and belemnites contribute most largely. The
-vertebrates include remains of fishes, amphibia, reptiles, birds and
-mammals. The Jurassic reptilia furnish representatives of some modern
-orders as the Chelonia and Crocodilia, but the most important orders
-are essentially characteristic of later Mesozoic times and their
-representatives abound in the Jurassic strata. These are the
-Sauropterygia (including the Plesiosaurs), the Ichthyopterygia
-(including the Ichthyosaurs), the Dinosauria, and the Pterosauria
-commonly known as Pterodactyls. No birds have hitherto been discovered
-in the British Jurassic rocks, but the Solenhofen Slate of Bavaria (of
-Kimmeridgian age) has furnished the celebrated _Archæopteryx
-macrura_, which is not only placed in a family but also in an order by
-itself, the order Saururæ. Many remains of mammals have been extracted
-from the estuarine deposits of Stonesfield, and the old surface soils
-of the Purbeckian beds; representatives of the Monotremata are
-furnished by the _Plagiaulacidæ_ and _Tritylodontidæ_, the former
-family containing the genus _Plagiaulax_ of the Purbeck Beds and the
-latter, _Stereognathus_ of the Stonesfield slate. The Marsupialia are
-represented by the _Amphitheridæ_, _Spalacotheridæ_ and
-_Triconodontidæ_. Some forms have been referred to the Insectivora,
-but there is still disagreement concerning the correctness of this
-reference.
-
-Before dismissing the subject of the Jurassic fossils, attention may
-be called to a feature which has been frequently commented upon,
-namely, the general resemblance of the flora and fauna of Jurassic
-times to the modern Australian fauna and flora. The explanation which
-has been offered to account for this resemblance has been given in a
-preceding chapter, where it was stated that Mr A. R. Wallace
-considers, after review of the geological and biological evidence,
-that Australia was severed from the adjoining continental lands in
-Mesozoic times, and that the higher forms of life which on the larger
-continents have replaced the earlier and lower forms have not
-succeeded in obtaining a footing in Australia, which therefore
-furnishes us with a local survival of a once widespread fauna. In
-connection with this matter the actual existence of the genus
-_Trigonia_ (a form peculiarly abundant in Jurassic strata and
-characteristic of Mesozoic strata in Britain) in the Australian sea is
-of considerable interest.[100]
-
-[Footnote 100: A good account of the British Jurassic rocks will be
-found in Mr H. B. Woodward's Memoir on "The Jurassic Rocks of
-Britain." _Mem. Geol. Survey_, 1893--.]
-
-
-
-
-CHAPTER XXIII.
-
-THE CRETACEOUS SYSTEM.
-
-
-_Classification._ The rocks of the Cretaceous system are conveniently
-divided into Upper and Lower Cretaceous. The following classification
-has been widely used for the British deposits, and is founded on
-lithological characters:
-
-             { Upper Chalk with flints      }
-             { Middle Chalk with few flints } Chalk
-  Upper      { Lower Chalk without flints   }
-  Cretaceous { Chalk Marl                   }
-             { Upper Greensand
-             { Gault
-
-             { Lower Greensand
-  Lower      { Wealden
-  Cretaceous { Hastings sands
-
-As the result of examination of the faunas, a more generally
-applicable classification has been established and is now largely
-adopted. It is as follows:
-
-  Danian     }
-  Senonian   } Upper Cretaceous
-  Turonian   }
-  Cenomanian }
-
-  Albian     }
-  Aptian     }Lower Cretaceous.
-  Neocomian  }
-
-In this classification the Neocomian practically represents the
-Wealden and Hastings beds, the Aptian the Lower Greensand and the
-Albian the Gault, placed according to this classification in the Lower
-Cretaceous, while the Upper divisions represent the strata above the
-Gault, consisting essentially of Chalk in England.
-
-
-_Description of the Strata._
-
-(i) _The Neocomian and Aptian Beds._ In the south of England the Lower
-Cretaceous beds succeed the Jurassic rocks with little or no break,
-and the type of the lower beds is similar to that of the beds
-deposited during the Purbeck age, consisting of estuarine deposits of
-variable characters, chiefly arenaceous below (the Hastings sands) and
-argillaceous above (the Wealden series), though impure limestones are
-found, largely composed of the shells of the freshwater _Paludina_,
-and much ironstone is developed in places. At the close of Neocomian
-times, the freshwater conditions in southern England were replaced by
-marine conditions and the Lower Greensand strata with their marine
-fauna were deposited in the Aptian sea. The Neocomian and Aptian beds
-thin out westward, and much more rapidly to the northward, so that
-both divisions disappear against the now buried ridge which forms a
-continuation of the Mendip axis. North of this they appear in another
-form. At first the highest Aptian beds alone are developed as shore
-deposits. Passing into Norfolk lower beds come in until in
-Lincolnshire we get a complete development of the Neocomian and Aptian
-beds with a marine facies, though of fairly shallow water character,
-whilst in Yorkshire the two divisions are represented by a deeper
-water clay, forming the Upper portion of the Speeton series. There is
-a consensus of opinion in favour of the Neocomian beds of southern
-Britain having been laid down in an estuary of a river flowing from
-the west over a continent now destroyed. To the north of this river
-stood the London ridge of the Palæozoic rocks, the northern borders of
-which formed the coast line off which were deposited the sediments of
-Neocomian and Aptian ages which occur in northern England. Before the
-deposition of the Albian beds a considerable upheaval of some parts of
-Britain occurred, and an unconformity separates the higher Cretaceous
-beds from older strata of Cretaceous and Jurassic ages, thus
-complicating the major phases by local changes in the characters of
-the strata.
-
-(ii) _The Albian and higher Cretaceous Beds._ The commencement of the
-deep-water phase of the third marine period may be said to occur in
-Albian times in Britain, reaching its maximum during the deposition of
-the chalk. The existence of a deeper sea towards the north of England
-is indicated by the characters of the Albian and newer strata. The
-Albian beds of gault consist of a stiff clay in southern England,
-replaced by coarser mechanical sediments towards the west. As one
-passes north from the London ridge (which exerted its influence in
-Albian times, after which it was finally buried in sediment) the gault
-thins out, and becomes gradually replaced by calcareous deposit when
-it is known as the red chalk which replaces the gault in northern
-Norfolk, Lincolnshire and Yorkshire.
-
-A local unconformity separating the chalk and gault in parts of East
-Anglia points to another local uplift with its accompanying
-complications in the characters of the strata. After the uplift had
-ceased, general depression must have occurred, and the various
-divisions of the chalk were accumulated in a fairly open sea, though,
-for reasons to be given presently, this was probably of no great
-lateral extent, save when united with the open ocean, probably in a
-manner similar to the connexion between the Gulf of Mexico and the
-Atlantic.
-
-The general variations in the lithological characters of the various
-members of the Cretaceous system will probably be rendered clearer by
-reference to the accompanying diagram (fig. 24) representing the
-variations when traced across England from south to north[101].
-
-[Footnote 101: For information concerning the British Cretaceous beds,
-see Topley and Foster, "Geology of the Weald," _Mem. Geol. Survey_,
-1875; Bristow and Strahan, "Geology of the Isle of Wight," _Mem. Geol.
-Survey_, 1889; Lamplugh, "On the Speeton Clay," _Q. J. G. S._, vol.
-XLV. p. 575, and "The Speeton Series in Yorkshire and Lincolnshire,"
-_ibid._, vol. LII. p. 179; Barrois "Recherches sur le Terrain Crétacé
-supérieur de l'Angleterre et d'Irlande," Lille, 1876; and various
-papers by Messrs Hill and Jukes-Browne, in the _Quarterly Journal of
-the Geological Society_ and _Geological Magazine_ of recent years. For
-the Scotch deposits consult a paper by Prof. Judd, _Q. J. G. S._, vol.
-XXXIV. p. 736, and for those of Ireland, see Hume, _Q. J. G. S._, vol.
-LII. p. 540.]
-
-[Illustration: Fig. 24.
-
-  Ch. Chalk.
-  Al. Albian.
-
-  Ap. Aptian.
-  N. Neocomian.
-
-  J. Jurassic.
-]
-
-The clue to the physical geography of Britain during Cretaceous times
-is furnished to a considerable extent by study of the foreign
-deposits. In Northern Europe the Cretaceous beds of England are met
-with in Northern France, and there the characters are generally
-speaking similar to those of our British deposits. In Germany
-shallower water conditions prevailed, the lower beds gradually
-disappear, and the upper beds are replaced by mechanical sediments of
-various degrees of coarseness, becoming on the whole coarser, as one
-travels eastward, so that in Saxony the chalk is partly replaced by
-arenaceous deposits (the 'Quader' sandstones) which are responsible
-for the remarkable scenery of the Elbe district above Dresden. In
-passing northwards, indications of similar change are noted in the
-deposits of Denmark and Scania, whilst to the south, we get a complete
-change in the character of the rocks, after crossing the Loire in
-France, and a similar change is observable in districts lying further
-east. Furthermore, as will be noted more fully in a subsequent
-paragraph, the character of the Upper Cretaceous flora indicates the
-existence of a large tract of land lying to the north and north-west
-of Europe, so that it would appear that the Cretaceous rocks of
-Northern Europe were deposited in a gulf-like expansion of a western
-ocean, bounded on the north by Scandinavia, on the west by eastern
-Germany, and on the south by a ridge running eastward from the mouth
-of the Loire[102]. We may speak of this gulf as the Chalk gulf. To the
-south of the presumed ridge the character of the strata alters, and
-also that of the included organisms. This southern type of Cretaceous
-rocks is one which is very widely spread, being found in Europe south
-of the Loire, and of the Alps, and in Greece and Turkey, while it
-also occurs in the northern parts of Africa. The beds of this type are
-traceable through Asia Minor into India and to the shores of the
-Indian Ocean, indicating the existence of a widespread Cretaceous
-ocean, which is sometimes spoken of as the Hippurite-limestone sea,
-for reasons which will eventually appear. The deposits are largely
-formed of hard limestone which is very different in its character from
-the soft chalk of the northern gulf.
-
-[Footnote 102: The reader will find the existence of this gulf
-maintained and supported by a considerable mass of detail in Mr A. R.
-Wallace's _Island Life_.]
-
-The climatic conditions which prevailed during Cretaceous times were
-apparently similar in most respects to those of the preceding Jurassic
-period, and as already stated the climatic zones which Neumayr defined
-for Jurassic times are also maintained by him to have existed during
-the Cretaceous period. The existence of cold has sometimes been
-inferred from the presence of large foreign blocks in the chalk,
-especially at its base, but if these are due to the transport, they
-might well be caused by masses of floating ice, which are often found
-at considerable distances from the coast in temperate regions after
-the break-up of the frost which succeeds an unusually hard winter. The
-flora and fauna are not suggestive of severe conditions.
-
-_The Cretaceous flora and fauna._ It has been noted in the last
-chapter that the gymnospermous flora of the Jurassic period, in which
-cycads form a considerable percentage of the whole flora, was
-prevalent in Lower Cretaceous times. In the Upper Cretaceous rocks
-this flora is replaced by one which consists to a large extent of
-dicotyledonous angiosperms. These are found in the Upper Cretaceous
-rocks of Europe and North America, and as the researches of botanists
-indicate their origin in circumpolar regions, their arrival in Europe
-is an additional argument in favour of the existence of an extensive
-northern continent, sending a prolongation to the southward in eastern
-Europe.
-
-The invertebrate fauna bears considerable resemblance to that of
-Jurassic times, and many of the dominant Jurassic genera are also
-found in Cretaceous rocks. A most interesting feature is connected
-with the character and geographical distribution of the Ammonites. In
-Europe they are almost exclusively confined to the deposits of the
-northern gulf, and before their final disappearance they undergo many
-changes of form. We find the discoid spiral shells of earlier times,
-but these are accompanied by shells which are straight, curved,
-boat-shaped, and coiled into various helicoid spirals, sometimes
-having the whorls in contact, while at other times they are separate.
-
-In the chalk of Britain gastropods are on the whole rare, and this
-fact serves to emphasize the palæontological break which occurs
-between the Cretaceous and Tertiary rocks; but when conditions were
-favourable, as during the deposition of some of the strata of the
-Middle Chalk, gastropods are abundant, and some are related to
-Tertiary genera, so that we may assume that the palæontological break
-alluded to is exaggerated by the difference of conditions which
-prevailed during the deposition of the earliest Tertiary and latest
-Cretaceous sediments.
-
-In the Cretaceous deposits of the southern sea, where the Ammonite
-tribe is almost unknown, the remarkable family of the lamellibranchs
-known as the Hippuritidæ furnish the dominant invertebrates of the
-period, and the representatives of this family are exceedingly scarce
-amongst the Cretaceous strata of the northern gulf, though they are
-found on two or three horizons.
-
-Of vertebrates, the most interesting are the reptiles. The families
-which predominate in Jurassic times have many representatives amongst
-the Cretaceous strata also, but the order Squamata is represented by
-the sub-order Pythonomorpha, which is characteristic of the Cretaceous
-rocks. The best known representative is the gigantic _Mosasaurus_.
-Lastly, we have the remarkable toothed birds or Odontornithes, now
-placed in different orders, the genus _Hesperornis_ being the only
-representative of the sub-order Odontolcæ of the Ratitæ, whilst
-_Ichthyornis_ and allied forms are placed in the sub-order Odontormæ
-of the Carinatæ.
-
-
-
-
-CHAPTER XXIV.
-
-THE EOCENE ROCKS.
-
-
-_Classification._ The Eocene Beds of the south of England have been
-subdivided according to the variations in their lithological
-characters, and the subdivisions have received local names. The
-following classification is generally adopted, though the different
-subdivisions are by no means of equal value:
-
-  Upper Eocene  { Upper Bagshot Beds
-                { Barton Beds
-
-  Middle Eocene   Bracklesham Beds
-
-                { Lower Bagshot Beds
-                { London Clay[103]
-  Lower Eocene  { Oldhaven Beds             } Lower London
-                { Woolwich and Reading Beds } Tertiary Strata
-                { Thanet Sands              }
-
-[Footnote 103: Some writers place the London Clay in the Middle
-Eocene.]
-
-The deposits vary greatly when traced abroad, and the exact
-equivalents of the minor subdivisions of the British rocks can seldom
-be ascertained at any distance from England, though the division into
-Upper, Middle, and Lower Eocene can be made over wide areas.
-
-_Description of the strata._ The character of the strata of Europe and
-Asia indicates the persistence of the northern gulf and southern
-ocean of Cretaceous times in Eocene times also, though the area of
-each had shrunk in the meantime, owing to the physiographical changes
-which occurred at the end of Cretaceous times, giving rise to more
-extended land areas, and producing a shallow water phase over wide
-extents of ocean,--the final shallow water phase of the third and last
-great marine period of the British area. It is difficult to ascertain
-the exact importance of the physical break between Cretaceous and
-Eocene rocks in the south-east of England, owing to the subterranean
-solution of the upper part of the chalk, subsequently to the
-deposition of the Eocene strata, but the contraction of the Cretaceous
-gulf is shown in several ways, one of the most significant being the
-distribution of Cretaceous and Eocene rocks in the south-west of
-England. The existence of an outlier of Cretaceous rock at Buckland
-Brewer in North Devon, only three miles from the Atlantic Ocean,
-indicates the former extension westward of the Upper Cretaceous beds,
-while the occurrence of an outlier of Eocene rocks at Bovey Tracey in
-South Devon, resting not on Cretaceous but on Palæozoic rocks, shows
-that there was an uplift after the deposition of the Cretaceous rocks
-and before the Eocene rocks were deposited there, and that during the
-period of uplift the Cretaceous rocks were removed.
-
-Owing to these physical changes, the Eocene rocks of Britain are
-mainly mechanical sediments, some, as the Oldhaven beds, being
-composed of coarse pebbles over a fairly wide district, while some of
-the earlier Eocene rocks are estuarine or fluvio-marine.
-
-The Eocene rocks of Britain occur in four areas, namely, the London
-Basin, the Hampshire Basin, the Bovey Tracey outlier, and the
-north-east of Ireland and western Isles of Scotland. The deposits of
-the three southern areas may be considered together, and give general
-indications of an approach to land when passing westward. The Lower
-London Tertiary strata are fluvio-marine at the east end of the London
-Basin; they become shallower water deposits when traced westward, and
-begin to disappear. The London Clay is an estuarine deposit, which is
-generally supposed to have been laid down at the mouth of a large
-river flowing from the west. It is absent in the Bovey Tracey outlier.
-
-Local disturbances caused the existence of a shallow water region in
-the east during the deposition of the Middle and Upper Eocene
-deposits, and accordingly the well-marked marine deposits which form
-the representatives of these divisions in Hampshire are replaced by
-the Bagshot beds of the London Basin, consisting chiefly of coarse
-mechanical sediments with a poor marine fauna, but even in the west
-shallow water prevailed at times during the accumulation of various
-plant-bearing strata. The Middle Eocene beds only are found in the
-Bovey Tracey outlier, though the Upper Eocene beds may originally have
-been laid down in that area, and subsequently denuded.
-
-The fourth area displays a very different succession of Eocene strata,
-and one of extreme interest. Mechanical sediments and plant-bearing
-clays and lignites alternate with a vast accumulation of basaltic
-lavas, indicating the outbreak of the volcanic forces in the British
-area, after a period of quiescence which lasted through the greater
-part of Mesozoic times. The region in which these lavas were poured
-out was probably a land area during the greater part of the period of
-volcanic activity, but the horizontal lie of the lava flows and their
-wide extent indicate the existence of a flat tract of country,
-gradually raised into a plateau by the accumulation of sheet over
-sheet of basalt. How far this plateau extended it is impossible to
-say. The distribution of the lavas at the present day is somewhat
-limited in our isles, but there is no sign of dying out at the present
-margins of the accumulations, and they have probably escaped
-denudation in these regions, as maintained by Professor Judd, on
-account of the faults which have depressed them, while the portions
-which were not depressed have been removed by denudation. Two views as
-to the origin of the lavas have been put forward: according to Prof.
-Judd, they were poured forth from gigantic volcanoes, while Sir A.
-Geikie maintains that they represent portions of massive or fissure
-eruptions, the molten rock having welled out from great cracks in the
-earth, which are now filled by once molten rock in the form of dykes.
-As these dykes extend far away from the present volcanic plateau, one
-actually extending to the Yorkshire coast, we may well believe,
-whatever was the origin of the sheets of lava, that they were formerly
-spread far away from their present terminations[104]. Without entering
-here into a discussion of the exact nature of extrusion of these
-igneous sheets, it will suffice to say that all the evidence points to
-the formation of extensive plateaux, which must have presented a
-fairly uniform surface, similar to that which is still found
-characterising the volcanic districts of the western territories of
-North America.
-
-[Footnote 104: Prof. Judd's views will be found in a series of papers
-by him on the "Secondary Rocks of Scotland," _Quart. Journ. Geol.
-Soc._, vol. XXIX. p. 95, XXX. p. 220, XXXIV. p. 660, while Sir A.
-Geikie's explanation is advanced in a paper in the _Transactions of
-the Royal Society of Edinburgh_, vol. XXXV.; see also the same
-author's _Ancient Volcanoes of Great Britain_.]
-
-The Eocene rocks of the north-west of Europe possess characters very
-similar to those of the south of England, and there are indications
-that the northern gulf had diminished in extent towards the east as
-well as towards the west.
-
-Passing to southern Europe, Central Asia and northern Africa, we find
-the conditions of Cretaceous times reproduced, and an extensive series
-of marine deposits extends very widely over these regions, the most
-persistent deposit being a mass of limestone of Middle Eocene age,
-which is almost entirely composed of the tests of Nummulites, whence
-the development is known as the Nummulitic Limestone facies, and we
-may speak of the ocean as the Nummulitic Limestone Sea. The incoming
-of shallow water conditions marked by accumulation of coarse
-mechanical sediments towards the end of the Eocene period in some
-parts of the southern European area indicates the setting in, even
-then, of those continental conditions which culminated during the
-Miocene period.
-
-In North America we get similar evidence of the contractions of the
-oceans which in Mesozoic times occupied large expanses of our present
-continents.
-
-The climatic conditions of Eocene times have been noticed in passing
-in chapter IX., and evidence was given to prove the prevalence of a
-warmer climate over the British area than that which now exists. A
-study of the floras of various parts of the northern hemisphere
-suggests that climatic zones, whose lines of demarcation ran
-practically parallel with the Equator, existed in Eocene times also,
-though further information upon this subject is desirable.
-
-_The Eocene flora and fauna._ The flora of prevalent dicotyledonous
-angiosperms, which appeared in Upper Cretaceous times, also marks the
-Eocene and later deposits, but a study of the floras indicates that
-the differentiation which now marks off the floras of different areas
-from one another had not occurred to so great an extent as at the
-present time. The existence of a rich flora in the Eocene beds of
-circumpolar regions in the northern hemisphere should be noted, though
-perhaps its importance has been somewhat exaggerated.
-
-The invertebrate fauna shows an approximation to that of the present
-day. The remarkable ammonite fauna of Mesozoic times has disappeared,
-and gastropods and lamellibranchs predominate, many of the forms
-belonging to existing genera, though very rarely to existing species.
-The Nummulites are the most characteristic Eocene fossils, and the
-period may be spoken of as the Nummulitic Period, though it is now
-known that Nummulites are not confined to the Eocene strata.
-
-The vertebrate fauna is very noteworthy. The fishes and reptiles are
-closely related to existing forms, and the orders of reptiles which
-predominated in Mesozoic times have completely disappeared. But the
-mammals are the most interesting vertebrates of the Eocene period.
-Instead of the lowly organised forms of Mesozoic times, we find
-representatives of many orders, including the highest, the Primates.
-The generalised forms which serve as links between groups which are
-now separated to a considerable extent are of particular importance.
-They have been detected in Eocene rocks of various regions, though the
-most complete series have been obtained from the Eocene rocks of North
-America and made known to us through the numerous memoirs of
-Professors Cope and Marsh[105].
-
-[Footnote 105: The Eocene floras of Britain are described by Mr J.
-Starkie Gardner and Baron von Ettingshausen in the _Monographs of the
-Palæontographical Society_; other Monographs of the same Society
-contain an account of the Eocene Mollusca by Mr F. E. Edwards and Mr
-S. V. Wood. An idea of the generalised forms of Mammalia may be
-obtained by perusal of that portion of Nicholson and Lydekker's
-_Manual of Palæontology_ in which the latter author treats of the
-Mammalia, and in this connexion the reader will do well to read Prof.
-Huxley's "Lecture on Fossil Horses," reprinted in his _American
-Addresses_.]
-
-
-
-
-CHAPTER XXV.
-
-THE OLIGOCENE AND MIOCENE PERIODS.
-
-
-(i) _The Oligocene Beds._
-
-_Classification._ The Oligocene Beds of Britain are classified as
-follows:--
-
-  Upper    Wanting
-
-  Middle   Hempstead Beds
-
-         { Bembridge Beds
-  Lower  { Osborne Beds
-         { Headon Beds
-
-_Description of the strata._ Little need be said of the deposits of
-this period, either in Britain or abroad, except to remark that they
-show the further spread of continental conditions over the regions now
-occupied by land. The British deposits are now seen in the Hampshire
-Basin only, and have been spoken of as the fluvio-marine series, as
-many of the strata were laid down in continental sheets of water,
-while the true marine sediments are thin and infrequent.
-
-The lithological characters of deposits formed under these conditions
-naturally vary greatly, consisting of different kinds of mechanical
-sediments occasionally mixed with thin freshwater marls and
-limestones. On the Continent similar conditions prevailed, though the
-occurrence of fairly wide tracts of level surface is indicated by the
-widespread distribution of beds of brown coal or lignite, and the
-coarse and thick Oligocene 'nagelfluh' of Switzerland points to the
-elevation of mountain ranges in the neighbourhood.
-
-_The flora and fauna._ The remarks made concerning the Eocene flora
-and fauna are generally applicable to those of Oligocene times, except
-that the Oligocene fossils bear a still closer resemblance to living
-forms, and the Nummulites are no longer dominant.
-
-(ii) _The Miocene Period._ Beds of Miocene age are wanting in Britain,
-and on the Continent they occur in isolated basins deposited in
-gulf-like prolongations of the ocean, never very far from land. A
-description of the strata and their fossil contents would be of little
-use for our present purposes, and the remarks made concerning the
-Oligocene beds will apply to the Miocene strata also.
-
-The period was mainly remarkable on account of the important physical
-changes which occurred, to which we must devote some consideration.
-Commencing with the British area, we find in the south evidence of the
-separation of the London and Hampshire Basins at this time, for the
-Oligocene beds of Hampshire are tilted up on the south side of an
-anticline, which separates the Hampshire Basin from that of London,
-indicating that the movement was post-Miocene, while in Kent, beds of
-Pliocene age rest on the denuded top of the chalk, showing that the
-elevation and denudation which accompanied it were pre-Pliocene; the
-great Wealden anticline is thus seen to be of Miocene age. On the
-north side of the London Basin the line of demarcation between Eocene
-and Mesozoic beds runs approximately parallel to the strike of the
-latter in that part of Britain, and this points to the elevation of
-the Mesozoic strata which gave them their present south-easterly dip
-about the same period, though in the absence of Oligocene rocks it
-cannot be definitely stated that the movement was altogether
-post-Oligocene. The present physical geography of considerable parts
-of Britain must date from Miocene times.
-
-Important as the changes were in Britain, they were slight as compared
-with those which affected Europe and many parts of Asia. The great
-mountain chains of the Old World received their maximum uplift during
-this great period of earth-movement, and orogenic structures were
-impressed upon the rocks of many regions, for the Tertiary Mountain
-Chains of the Old World have an Alpine structure impressed upon them
-as the result of intense lateral pressure, accordingly we find the
-Eocene strata lifted far above their original level to heights of
-8,000 feet in the Alps and over 12,000 feet in the Himalayas. Away
-from these marked uplifts epeirogenic movements caused the
-disappearance of the seas of earlier Eocene times, so that towards the
-close of the Miocene Period, the main features of the Eurasian
-continent were much as they are now. The present drainage-systems must
-have originated at the same time, and the sculpture of our continent
-has been carried on more or less continuously by subaerial agents from
-Miocene times to the present day. That any addition to the total area
-of land was made is doubtful. The land which appears to have existed
-to the west of Britain during Cretaceous and Eocene times finally
-disappeared beneath the waters of the Atlantic Ocean, and the movement
-probably gave rise to the prominent submarine feature which now exists
-at some distance from the coast of Ireland. A great marine period is
-now existent in our ocean areas, but so far as the existing
-continents are concerned, we are living on the fourth continental
-period which practically came into existence in Miocene times.
-
-The strike of the uplifted strata naturally coincides on the whole
-with the axes of the major uplifts, and accordingly we find the
-Mesozoic and early Tertiary strata folded around axes which have a
-prevalent east and west direction, with others which have a trend at
-right angles to this. The strike of the British Mesozoic rocks seems
-to have been determined by each of these sets of movements, so that
-although it is east and west in the south of England, it runs north
-and south in the eastern counties north of the Thames.
-
-In America, although epeirogenic movements had occurred before Miocene
-times, with the formation of wide continental tracts, these appear to
-have been of the nature of plains, diversified by extensive inland
-sheets of water, and uplift of orogenic character converted these
-plains into uneven tracts in Miocene times. Many of the movements in
-America, which like those of Europe are still progressing with
-enfeebled power, differ from those of Eurasia, giving rise to raised
-monoclinal blocks rather than to violent folds of Alpine character, as
-seen in the western territories of North America, and as proved also
-by the differential movements which are now known to affect the
-Atlantic coast of that continent.
-
-Accompanying these changes in the earth's crust were others which
-affected the climate, at any rate locally. The warm climate of Eocene
-times gradually gave way to a cooler climate in Oligocene times, and
-this lowering of temperature was still further advanced in Miocene
-times, though there is evidence that the temperature of those parts of
-Europe which have strata representative of the Miocene period was
-higher than it is at the present day.
-
-Owing to the changes which occurred in Miocene times, the area of
-sedimentation was extensively shifted to our present oceans, and
-accordingly we find that the times subsequent to those of the Miocene
-uplifts are marked by scattered accumulations of continental
-character, with a few insignificant marine strata seldom found far
-inland from the present coast-lines.
-
-
-
-
-CHAPTER XXVI.
-
-THE PLIOCENE BEDS.
-
-
-_Classification._ The Italian Pliocene Beds which have long been known
-have been divided into three stages, to which names have been applied
-which are somewhat widely used, though the division of the British
-deposits into the same three stages has not been made. The stages
-are:--
-
-  Astian.
-
-  Plaisancean.
-
-  Zanclean.
-
-The classification of the British deposits may be made as follows:--
-
-  Cromer "Forest" Series.
-
-  Weybourne Crag and Bure Valley Beds.
-
-  Chillesford Crag.
-
-  Norwich Crag and Red Crag.
-
-  Upper Coralline Crag.
-
-  Lower Coralline Crag.
-
-As the English deposits are somewhat scattered it is difficult to make
-out the exact order of succession, but the above shows the
-classification which is adopted by the best authorities, the Norwich
-Crag (or Fluvio-marine Crag as it is sometimes termed) being now
-supposed to represent the upper portion of the Red Crag.
-
-_Description of the strata._ The British deposits are chiefly found in
-the counties of Norfolk and Suffolk, but isolated patches have been
-detected in Kent and at St Erth in Cornwall; while the inclusion of
-Pliocene fossils in the glacial deposits of Aberdeenshire and on the
-west coasts and islands of Great Britain suggests the occurrence of
-Pliocene beds beneath sea-level, around the British coasts, at no
-great distance from the land.
-
-The term 'Crag' has been applied to shelly sands of which the British
-Pliocene beds are largely composed. The oldest British Pliocene strata
-are supposed to be the Lenham Beds, occurring in 'pipes' on the Chalk
-of the North Downs, which are referred to the Lower Coralline Crag,
-and some writers believe that the St Erth beds of Cornwall are of
-similar age[106]. The former are ferruginous sands, and the latter
-shelly sands and clays. The higher beds of the Coralline Crag are
-found in Suffolk, and are largely calcareous, being made of remains of
-polyzoa, molluscs, and other invertebrates. They were probably
-deposited in deeper water than the rest of the British Pliocene
-strata, and contain a far larger percentage of carbonate of lime. The
-Red Crag consists of ferruginous shelly sands, of the nature of
-sand-banks, formed near land; while the Norwich Crag is of a still
-more littoral character, and contains remains of land shells and the
-bones of mammalia mingled with the marine shells of the coast. The
-higher Pliocene deposits are also coastal accumulations, even the
-so-called Forest bed being a deposit and not a true surface soil, as
-proved by the observations of Mr Clement Reid. At the summit of the
-Cromer 'Forest' Series, however, is a true freshwater bed. These
-British deposits appear to have been laid down on a coast line which
-formed one side of the estuary of a large river, of which the present
-Rhine is the 'betrunked' portion (to use a term introduced by Prof. W.
-M. Davis)[107].
-
-[Footnote 106: See Clement Reid, _Nature_, 1886, p. 342; and Kendall
-and Bell, _Quart. Journ. Geol. Soc._, vol. XLII. p. 201.]
-
-[Footnote 107: See a paper by Mr F. W. Harmer, "On the Pliocene
-Deposits of Holland, and their relationship to the English and Belgian
-Crags," _Quart. Journ. Geol. Soc._, vol. LII. p. 748.]
-
-On the European continent, marine Pliocene beds are found in Belgium
-and Italy. The former deposits greatly resemble our Crags, whilst the
-latter are of interest on account of the mixture of volcanic beds with
-marine sediments in Sicily, showing that the formation of Etna
-commenced in Pliocene times. Various deposits formed in inland basins
-are found in France and Germany, but the most remarkable occur in the
-Vienna basin, where Caspian conditions prevailed over large areas, and
-the ordinary strata alternate with chemical deposits of which the
-best-known are the celebrated rock salt masses of Wieliczka, near
-Cracow. At the same time volcanic activity was rife to the south of
-the Carpathian mountains. Other deposits, which are partly referable
-to the Pliocene period, occur in Greece at Pikermi, and in India in
-the Siwalik hills; these are celebrated for their remarkable mammals,
-as are the Pliocene strata of the Western territories of North
-America. The occurrence of marked earth-movements since Pliocene times
-is indicated by the nature of the deposits of Barbadoes, where
-radiolarian cherts have furnished two echinids which are described by
-Dr Gregory as deep-sea forms. These beds were once referred to the
-Miocene period, but there is good reason for assigning them to a later
-date, and correlating them with the Pliocene beds of other areas, in
-which case there must have been a considerable uplift in this region
-since Pliocene times, a fact of great theoretical importance.
-
-The climatic conditions of Pliocene times show steady fall of
-temperature. The early Pliocene beds of Britain were deposited during
-the prevalence of warmer temperatures than those which now exist in
-the same area, but during later Pliocene times, the temperature was at
-first similar to that now prevailing, and afterwards distinctly
-colder, and we find in the upper Pliocene beds the remains of
-organisms of a northern type. In the uppermost deposit of the Cromer
-'Forest' Series, the arctic birch and arctic willow indicate the
-commencement of the cold which culminated in the succeeding 'Great Ice
-Age.'
-
-_The flora and fauna._ Little need be said of the Pliocene fossils:
-the flora approaches that of present times, and the invertebrates are
-in most cases specifically identical with those now living. The
-vertebrates alone differ markedly from living forms, being chiefly of
-extinct species, and in many cases belonging to extinct genera. It is
-interesting to find that the mammalian fauna of Pliocene times
-resembles the existing fauna of the area in which the beds are found,
-a fact long ago observed by Darwin. Thus the European Pliocene mammals
-are like existing European forms, whilst in Australia the mammalian
-terrestrial fauna consists of Marsupials, and in South America there
-are Edentata of Pliocene age[108].
-
-[Footnote 108: The Pliocene fauna of Britain is described by Mr
-Searles V. Wood in the _Monographs of the Palæontographical
-Society_.]
-
-
-
-
-CHAPTER XXVII.
-
-THE PLEISTOCENE ACCUMULATIONS.
-
-
-_Classification._ The term Pleistocene, as used here, is approximately
-equivalent to the expressions 'Glacial Period' and 'Great Ice Age' of
-some writers; but I have adopted it in preference to these
-expressions, because it may eventually be possible to define the
-Pleistocene period in such a manner as to give the term a strictly
-chronological meaning, whereas the other terms indicate the existence
-of climatic conditions which must have ceased in some areas sooner
-than in others. At present, climatic change gives us the best means
-for separating the accumulations formed subsequently to the Pliocene
-period over large parts of the Eurasian land-tract, and the most
-convenient division of these continental accumulations is to refer
-them to three periods, viz.:--
-
-  The Forest Period (in which we are now living).
-
-  The Steppe Period.
-
-  The Glacial Period.
-
-Some of the accumulations which were formed during the Steppe period
-are included in the Pleistocene period by many writers, but I prefer
-to treat of them as post-Pleistocene.
-
-In the present state of our knowledge of the glacial deposits any
-attempt to make a classification applicable over very wide areas is
-doomed to failure, and the very principles upon which the
-classification should be based are a subject of disagreement. The most
-promising basis for classification is founded on alternate recession
-and advance of land-ice, though the proofs that advance takes place
-simultaneously over very wide areas are not yet forthcoming. Dr J.
-Geikie in the last edition of his work _The Great Ice Age_ adopts four
-periods of glaciation, with intervening periods of recession, and this
-division accords with the observations of many foreign geologists. In
-order to understand the method of classification upon this basis, a
-few words concerning glacial deposits in general will not be out of
-place. Glacial accumulations may be divided into three classes:--(i)
-true glacial accumulations, formed on, in, and under the ice, and left
-behind upon its recession, (ii) marine glacial deposits, laid down in
-the sea, when floating ice is extensively found on its surface, and
-(iii) fluvio-glacial deposits, laid down by streams which come from
-the ice. The two former indicate glacial conditions, while the
-occurrence of fluvio-glacial deposits overlain by true glacial
-deposits indicates an advance of land-ice, for the fluvio-glacial
-deposits are accumulated in front of those which are truly glacial.
-Accordingly if we find alternations of glacial and fluvio-glacial
-deposits on a large scale, we may fairly infer the alternation of
-periods of great glaciation with others when the ice diminished, or in
-other words of glacial and interglacial periods. There is, however, in
-many cases great difficulty in distinguishing glacial deposits from
-marine glacial ones, while some of the true glacial deposits formed
-_in_ the ice (englacial deposits) cannot readily be distinguished from
-those of fluvio-glacial origin. Furthermore, as the terminal moraines
-of land-ice often rest upon other true glacial deposits, it is often
-difficult to know whether we are dealing with the products of one or
-two glaciations over limited areas. The test of superposition is often
-applicable, and one is enabled to obtain some clue as to the relative
-order of events. In England at least three periods of glaciation seem
-to be indicated by the glacial deposits. On the east coast the Cromer
-Forest Series is succeeded by the Cromer Till, and in Yorkshire the
-Basement Clay occupies a similar position with regard to the overlying
-glacial accumulations to that of the Cromer Till. Whether these
-deposits be marine or terrestrial, and the evidence is not yet
-sufficient to settle this question to the satisfaction of all
-geologists, there is no doubt that they are glacial. Above them, in
-East Anglia, lies the Contorted Drift, the origin of which is still a
-moot point, and it is overlain by the great Chalky Boulder Clay, which
-extends far and wide over East Anglia, the Midland Counties and into
-Yorkshire. Evidence has been adduced to connect this with the _till_
-or boulder clay which spreads over the upland districts of the north
-of England at the foot of the main hill-systems. This set of deposits
-indicates a second glaciation. As the upland till is often ploughed
-out by glaciers which have left their traces in the form of moraines
-in our upland regions, we seem here to have evidence of a third
-glaciation, which naturally leaves no traces in the southern
-districts, and the exact age of this cannot be ascertained in the
-absence of fossil evidence, though we may provisionally refer it to
-the Pleistocene period.
-
-Another attempt has been made to classify the glacial deposits, on the
-supposition that there have been periods of elevation and depression
-of the land during Pleistocene times. Some writers advocate one
-interglacial period when the land was depressed to an extent of 1400
-and perhaps 2000 feet, while others have advocated the occurrence of a
-number of such interglacial marine periods. The evidence for the
-supposed oscillations is furnished by the existence of shell-bearing
-sands associated with boulder clays at high levels, the best known
-being on Moel Tryfan in Caernarvonshire, near Macclesfield in
-Cheshire, and near Oswestry in Shropshire. As many geologists believe
-that these shells have been carried to their present position by ice
-in a way which it is not our province to discuss here, we may dismiss
-this method of classification as based upon events which cannot be
-proved to have occurred. In the present state of our knowledge, it is
-indeed best to avoid, as far as possible, classifications which are
-intended to be applicable over wide regions, and to devote our
-attention to local details, gradually piecing together the evidence
-which is obtained as the result of exhaustive examination of each
-separate area[109].
-
-[Footnote 109: The glacial literature of our own island only, is so
-extensive that the student may well be bewildered when he attempts to
-grapple with it. He is recommended to read the following general
-works:
-
-J. Geikie, _The Great Ice Age_. 3rd Edition, 1894.
-
-H. Carvill Lewis, _The Glacial Geology of Great Britain and Ireland_.
-1894.
-
-G. F. Wright, _Man and the Glacial Period_, 1892, and _The Ice Age in
-North America_, 1890.
-
-Sir C. Lyell, _Antiquity of Man_. 4th Edition, 1873.
-
-For the glacial geology of special regions the following papers may be
-consulted:
-
-_The Lake District and adjoining neighbourhood._ E. H. Tiddeman,
-"Evidence for the Ice Sheet in North Lancashire &c." _Quart. Journ.
-Geol. Soc._, vol. XXVIII. p. 471. J. G. Goodchild, "Glacial Phenomena
-of the Eden Valley &c." _Quart. Journ. Geol. Soc._, vol. XXXI. p. 55,
-and J. C. Ward, _Mem. Geol. Survey_, "The Geology of the Northern half
-of the Lake District."
-
-_Yorkshire._ G. W. Lamplugh, "Drift of Flamborough Head," _Quart.
-Journ. Geol. Soc._, vol. XLVII. p. 384.
-
-_Lincolnshire._ A. J. Jukes-Browne, _Quart. Journ. Geol. Soc._, vol.
-XXXV. p. 397 and XLI. p. 114.
-
-_East Anglia._ Clement Reid, _Mem. Geol. Survey_, "The Geology of the
-district around Cromer."
-
-_North Wales._ T. McK. Hughes, "Drifts of the Yale of Clwyd" &c.
-_Quart. Journ. Geol. Soc._, vol. XLIII. p. 73, and A. Strahan,
-"Glaciation of South Lancashire, Cheshire, and the Welsh Border,"
-_ibid._, vol. XLII. p. 486.
-
-_Switzerland._ C. S. du Riche Preller, "On Fluvio-glacial and
-Interglacial Deposits in Switzerland," _Quart. Journ. Geol. Soc._,
-vol. LI. p. 369 and "On Glacial Deposits, Preglacial Valleys and
-Interglacial Lake formations in Sub-Alpine Switzerland," _ibid._, vol.
-LII. p. 556.
-
-The reader will find references to other works on the Glacial Geology
-of other districts by consulting the general works referred to on the
-preceding page.]
-
-The foregoing remarks will convince the student that any attempt to
-show the distribution of land and sea during any part of the glacial
-period is not likely to meet with general acceptance, as so much
-depends upon the terrestrial or marine origin of the deposits of the
-lowlands, and the mode of formation of the shell-bearing drifts of
-high levels. The occurrence of elevation to a greater height than that
-which our country at present possesses during portions at any rate of
-the glacial period has been inferred on general grounds, but direct
-evidence in favour of it is furnished by the existence of a number of
-ancient valleys on the land around our coasts, whose floors are often
-considerably below sea-level, while the valleys are now completely
-filled up with glacial accumulations, except where they have been
-partially re-excavated by streams which for some distance run above
-the courses of the ancient streams.
-
-The climatic conditions of glacial times can only be briefly touched
-upon in this place. If the periods of advance can be proved to be
-contemporaneous over wide areas, this points to alternations of colder
-and warmer periods, or at any rate of drier and wetter periods, though
-local advance may be due to a number of causes. It must be borne in
-mind that with the temperature remaining the same, advance of ice can
-be brought about by increased precipitation of aqueous vapour in the
-form of snow.
-
-The question of the cause of the glacial period is one that only
-indirectly affects the stratigraphical geologist until he has
-accumulated sufficient evidence to indicate the cause. It must suffice
-to observe that the extremely plausible hypothesis of Croll (for which
-the student should consult Dr Croll's _Climate and Time_) does not
-explain the apparent gradual lowering of climate throughout Tertiary
-times till the cold culminated in the Pleistocene period, and the
-student will do well to remain in suspense concerning the cause of the
-Ice Age until further evidence has been brought to bear upon it.
-
-_The glacial flora and fauna._ The glacial deposits naturally yield
-few traces of life, except those which have been derived from other
-deposits, and we are dependent for our information concerning the
-fauna and flora of the glacial period upon the remains furnished by
-the interglacial deposits. Unfortunately it is very hard to ascertain
-which deposits are interglacial, and many which have been claimed as
-such are either preglacial or postglacial. The meagre evidence which
-we possess points to the existence of an arctic fauna or flora in
-Britain during the prevalence of this glacial period. A question which
-has received much attention of recent years is that of the existence
-of preglacial or interglacial man, on which much has been written. The
-existence of man in glacial times is probable, but it is the opinion
-of many of those who are most competent to form a judgment, that it
-has not been proved in the only conclusive way, namely, by the
-discovery of relics of man in deposits which are directly overlain by
-glacial deposits, or which at any rate are demonstrably older than
-glacial deposits[110].
-
-[Footnote 110: On the question of preglacial and interglacial man, see
-W. Boyd Dawkins, _Early Man in Britain_; H. Hicks, _Quart. Journ.
-Geol. Soc._, vol. XLII. p. 3, XLIV. p. 561, and XLVIII. p. 453; T.
-McK. Hughes, _ibid._, vol. XLIII. p. 73; Sir J. Evans, _Presidential
-Address to British Assoc._ 1897.]
-
-
-
-
-CHAPTER XXVIII.
-
-THE STEPPE PERIOD.
-
-
-The occurrence of a period marked by dry climate over wide areas of
-the Eurasian continent, and possibly also in North America, is
-evidenced by the widespread distribution of an accumulation known as
-_loess_, concerning the origin of which there has been much difference
-of opinion, though that it was formed subsequently to the glacial
-period seems to be generally admitted, inasmuch as it is largely
-composed of rearranged glacial mud. The formation of the loess as a
-steppe-deposit was first advocated by Baron von Richthofen, and his
-views were supported by Nehring after study of the loess-fauna.
-Richthofen's explanation of the loess as due to the spread of dust by
-wind in a dry region is becoming widely accepted, and it necessitates
-the widespread occurrence of steppe conditions, as the loess has a
-very extensive geographical range, and may be truly regarded as the
-normal continental deposit of Eurasia during the period immediately
-succeeding the glacial period. In our own country, as the sea cannot
-have been far distant during these times the normal loess is not
-found, but several accumulations occur, which on stratigraphical and
-palæontological grounds must be regarded as synchronous with the
-formation of the loess. These are certain rubble-drifts of the
-southern counties, the older river-gravels of southern England, and
-some of the older cave deposits of various parts of England. It is
-doubtful whether any classification into minute subdivisions can be
-adopted for them, though Prof. Boyd Dawkins has advocated their
-separation into an older age of River Drift Man, and a newer period of
-Cave Man, on account of the evidences of a lower state of civilisation
-afforded by examination of the River Drift implements when compared
-with those fashioned by Cave Man. Roughly speaking, the Steppe period
-corresponds with the period during which Palæolithic man existed, at
-any rate in north-west Europe, and we may speak of the Steppe period
-as the Palæolithic period, without asserting that Palæolithic man
-necessarily disappeared at the time when the climate changed and
-caused the replacement of Steppe conditions by others favourable to
-forest-growth.
-
-_Description of the accumulations._ The loess consists of unstratified
-calcareous mud or dust, with a peculiar vertical fracture, and is
-interesting rather on account of the nature of its fossils and of its
-distribution than for its lithological characters. As it is not found
-in Britain it is not necessary to say much about it, but merely to
-refer to the published descriptions[111].
-
-[Footnote 111: An account of Richthofen's views by that author will be
-found in the _Geological Magazine_, Dec. 2, vol. IX. (1882), p. 293,
-and the fauna of the loess is described by Nehring (_Ibid._, p. 570).]
-
-The British deposits require some notice, as their characters and mode
-of occurrence are of some significance. Along the south coast are
-deposits of coarse rubble which have yielded some organic remains,
-which have been described by Mr Clement Reid[112], who also discusses
-their origin. The rock, also known as the Elephant Bed, consists of
-angular fragments of flint and chalk, and seems to have been produced
-by streams which were able to flow over the surface of the chalk when
-it was frozen. Many other similar deposits in the south of England,
-which are found on the open surface, may have had a similar origin.
-
-[Footnote 112: C. Reid, "Origin of Dry Chalk Valleys and of Coombe
-Rock," _Quart. Journ. Geol. Soc._, vol. XLIII. p. 364.]
-
-The Palæolithic river-gravels are found at various distances above
-present river-levels, and are the surviving relics of alluvial
-deposits which were laid down when the rivers ran at a higher level
-than they now do. That they are newer than the main glacial drifts of
-the region in which they occur is indicated by the frequent presence
-in them of boulders derived from the drift. Their antiquity is shown
-by the physical changes which have occurred since their deposition
-(there having been sufficient time since then to allow of the
-excavation of some river-valleys to a depth of over one hundred feet
-beneath their former level), and also by the character of the included
-mammals which will presently be referred to. The deposits vary in
-coarseness, like those of modern alluvial flats, from the coarse
-gravels of the river-beds to the fine loams and marls of the
-flood-plains. They are found, in Britain, with their typical mammalian
-remains, south-east of a line drawn from the mouth of the Tees to the
-Bristol Channel.
-
-The cave-deposits have a wider distribution than those which have just
-been noticed, being also found to the north-west of the
-above-mentioned line in Yorkshire, and in North and South Wales. In
-the south of England they are found as far east as Ightham in Kent,
-and in a westerly direction to Torquay and Tenby. The Ightham deposits
-occur in fissures and consist of materials which were apparently
-introduced from above by river action[113]. The cave-deposits of
-limestone areas are sometimes found in fissures, but at other times in
-caverns with a fairly horizontal floor, on which the various
-accumulations lie in order of formation. The deposits vary in
-character and may be divided into three groups, though accumulations
-of intermediate character are found; the first group consists of
-cave-earths and cave-breccias--formed by weathering of the limestone,
-and the retention of the insoluble residue, as a more or less
-ferruginous mud, mixed with angular fragments of limestone, and with
-the remains of creatures which inhabited the caves; the second group
-consists of true deposits laid down under water, as gravels, sands,
-and laminated clays; while the third is composed of limestone
-deposited from solution in water, in the form of stalagmite[114].
-
-[Footnote 113: The Ightham fissures and their contents are described
-by Messrs Abbot and Newton, _Quart. Journ. Geol. Soc._, vol. L. pp.
-171 and 188.]
-
-[Footnote 114: The reader should consult Prof. W. Boyd Dawkins' works
-on _Cave Hunting_ and _Early Man in Britain_, for information
-concerning the Cave Deposits. See also Sir C. Lyell, _Antiquity of
-Man_; Sir J. Evans, _Ancient Stone Implements of Great Britain_, and
-Sir J. Lubbock, _Prehistoric Times_. In these works references will be
-found to papers by Messrs Pengelly, Magens Mello, Tiddeman and others
-on the Caves of Devon, Derbyshire and Yorkshire. References have
-already been made to papers upon the Caverns of North Wales.]
-
-The organic contents of the Palæolithic period are of much interest,
-and it is desirable to discuss their character before making further
-observations upon the physical conditions of the period.
-
-_The Palæolithic flora and fauna._ The plants of some of the earlier
-deposits of the age we are considering show the prevalence of cold
-conditions during their accumulation, for instance the Arctic birch
-and Arctic willow are found in the accumulations beneath the
-implement-bearing Palæolithic deposits of Hoxne in Suffolk[115]. The
-invertebrate fauna consists essentially of the remains of molluscs.
-The loess molluscs are chiefly pulmoniferous gastropods which lived
-upon the land, though swamp forms are occasionally associated with
-them. The palæolithic river-gravels have yielded numerous land- and
-freshwater-molluscs of living species, though some which are abundant
-in the British gravels are now extinct in Britain, e.g. _Cyrena
-(Cobicula) fluminalis_ and _Unio littoralis_. Marine deposits of this
-age are occasionally found, as at March, in Cambridgeshire, where the
-fauna closely resembles that of our present sea-shores.
-
-[Footnote 115: These beds are described by Messrs Reid and Ridley,
-_Geol. Mag._ Dec. III. vol. V. p. 441. See also C. Reid on the
-"History of the Recent Flora of Britain," _Annals of Botany_, vol. II.
-No. 8, Aug. 1888.]
-
-The vertebrate remains are much more remarkable, and it is not quite
-clear that the association of forms whose living allies now live under
-widely different conditions has been satisfactorily explained. The
-river-gravels and cave-deposits contain remains of temperate forms, as
-the bison, and brown bear, associated with those of northern forms, as
-the mammoth, woolly rhinoceros, glutton, reindeer, and musk ox, and
-also with those whose living allies are inhabitants of warmer regions,
-like the lion, hyæna, and hippopotamus. One of the most remarkable
-creatures is the sabre-toothed lion or _Machairodus_, remains of which
-have been discovered in Kent's Cavern, Torquay, and in the caves of
-Cresswell Crags, Derbyshire.
-
-The loess fauna consists of characteristic steppe animals, such as the
-jerboa, Saiga antelope and steppe-porcupine, and it is interesting to
-find an indication of this fauna in the Ightham fissures.
-
-The first undoubted relics of mankind are found in the Palæolithic
-deposits, which are very widely spread over the Eurasian continent.
-They consist mainly of implements of bone and stone, the latter being
-chipped, but never ground or polished, though both bone and stone
-implements are frequently ornamented with engraved figures. The
-cave-deposits have furnished implements of a higher type than those
-usually found in the river-drifts, but the latter are also found in
-caverns in deposits beneath those containing the higher type, hence
-the division of the period into two minor periods, that of river-drift
-man, and that of cave-man[116].
-
-[Footnote 116: Concerning this matter, the reader should consult Prof.
-Boyd Dawkins' _Early Man in Britain_. Sir J. Prestwich has argued in
-favour of the existence of a group of implements found on the plateau
-south of the Thames of an age antecedent to that of the ordinary
-river-drift implements. See _Quart. Journ. Geol. Soc._, vol. XLV. p.
-270.]
-
-There are several questions of interest connected with the Palæolithic
-fauna, three of which deserve some notice here. The absence of the
-relics of the Palæolithic mammalia and of the human implements in the
-river-gravels north-west of the line drawn between the Tees and
-Bristol Channel, and the presence of the mammalian remains in the
-caverns of that area requires some explanation. One such explanation
-assumes that the relics were destroyed in the open country to the
-north-west of that line, owing to glaciation, but it is not by any
-means universally accepted.
-
-Another difficulty which in the opinion of some writers has not been
-fully cleared up is the mixture of apparently southern forms like the
-Hippopotamus, with others of northern character like the Musk ox,
-under such conditions as to show that the creatures lived in the
-British area contemporaneously. Seasonal migration might account for
-it, but the wide belt of overlap of apparent northern and southern
-forms requires something more, though secular changes of climate might
-shift the belt of seasonal overlap from one place to another, causing
-the entire belt of overlap to extend over a considerable distance.
-
-The third, and perhaps most important difficulty is the abrupt change
-from the Palæolithic type of implement to the Neolithic type,
-characteristic of the next period. Some implements, as those of the
-kitchen-middens of Denmark, and those found at Brandon and Cissbury in
-this country, have been appealed to as intermediate in character, but
-evidence has been brought forward to show that each set is truly
-Neolithic, the one being the implements of the lowly fisher-folk who
-lived contemporaneously with the makers of the highly finished
-polished implements of Denmark, while the others are unfinished
-implements thrown away during the manufacture on account of flaws or
-accidental fractures. The difficulty is increased when we take into
-account the great physical and faunistic changes which occurred
-between Palæolithic and Neolithic times.
-
-The country was undoubtedly more elevated than it is at present during
-portions if not during the whole of Palæolithic times, as shown by the
-appearance of the great mammals in Britain, the discovery of their
-remains beneath sea-level, and especially the occurrence of remains in
-the caverns of rocky islands such as those of the Bristol Channel,
-where they could not possibly have existed unless the present islands
-were connected with the mainland.
-
-The fossils of the times between the Glacial period and the Neolithic
-period indicate variations of climatic conditions. Upon this point I
-cannot do better than quote the words of Sir John Evans in his
-Presidential Address to the British Association at Toronto[117]. "At
-Hoxne the interval between the deposit of the Boulder clay and of the
-implement-bearing beds is distinctly proved to have witnessed at least
-two noteworthy changes in climate. The beds immediately reposing on
-the clay are characterised by the presence of alder in abundance, of
-hazel, and yew, as well as by that of numerous flowering plants
-indicative of a temperate climate very different from that under which
-the Boulder clay itself was formed. Above these beds characterised by
-temperate plants, comes a thick and more recent series of strata, in
-which leaves of the dwarf Arctic willow and birch abound, and which
-were in all probability deposited under conditions like those of the
-cold regions of Siberia and North America.
-
-"At a higher level, and of more recent date than these--from which
-they are entirely distinct--are the beds containing the Palæolithic
-implements, formed in all probability under conditions not essentially
-different from those of the present day."
-
-[Footnote 117: _Report Brit. Assoc._ for 1897, p. 13.]
-
-
-
-
-CHAPTER XXIX.
-
-THE FOREST PERIOD.
-
-
-Subsequently to Palæolithic times, the physical conditions over
-Eurasia changed greatly, and at the commencement of Neolithic times
-the conditions were favourable for the growth of forests over wide
-regions of that continent. At the commencement of the Forest period
-the physical conditions were very much the same as they are at
-present, though minor changes have of course taken place since then,
-including probably a submergence of large parts of Britain to a depth
-of about fifty feet beneath its former level, as indicated by the
-existence of Neolithic submerged forests round many parts of our
-coast-lines.
-
-The Forest period may be best subdivided for local purposes by
-reference to the civilisation of mankind at different times, and in
-this way we obtain the following divisions:
-
-  Historic Iron age.
-  Prehistoric Iron age.
-  Bronze age.
-  Neolithic age.
-
-A classification may also be based upon changes in the flora. In
-Denmark the peat deposits of this age are divisible into five layers,
-characterised by different dominant forms of trees. These are as
-follows in descending order:
-
-  Fifth layer:   Beech ... Iron age
-  Fourth layer:  Alder
-  Third layer:   Oak ... Bronze age
-  Second layer:  Scotch Firs ... Neolithic age
-  Lowest layer:  Poplar.
-
-In our own country the forest growth has been much interfered with by
-man, but the lower fenland peat gives a good example of the material
-formed by forest growth. It is not necessary to touch on the various
-accumulations which are now being formed in different parts of our
-island, except to remark that the deposits of the Forest period give
-indications of earth-movements on a small scale, which is well seen in
-the fenland, where the forest peat is covered in places by a "buttery
-clay" with _Scrobicularia piperata_ indicating submergence, and above
-this is a marsh peat.
-
-The flora and fauna of the Forest period are practically those of the
-present day, though the larger forms of mammalia have disappeared one
-by one. The Irish elk and _Bos primogenius_ probably became extinct
-early in the period, while as far as Britain is concerned the wolf,
-bear, and beaver have disappeared within historic times.
-
-The relics of man deserve passing notice. The Neolithic period is
-characterised by the absence of metal instruments, though those made
-of stone were much more highly finished than those of Palæolithic
-times, and were often ground and polished. The first metal which was
-largely worked was bronze, which gradually replaced stone, though
-stone was extensively used in the Bronze age, as indicated by the
-imitation of bronze implements in stone. The Bronze age in turn was
-replaced by the Prehistoric iron age; at first, when iron was scarce,
-bronze implements were merely tipped with iron, but ultimately the one
-metal was practically replaced by the other.
-
-The date of the Palæolithic period is unknown; no approximate date can
-be satisfactorily assigned to it, but various calculations, founded on
-different data, have been made as to the age of the Neolithic period,
-and several of them agree in placing it at about 7000 years from the
-present time.
-
-It will be seen that no sudden and violent change marks the incoming
-of the human race, which to the geologist is but one of a large number
-of events which have followed each other in unbroken sequence, and
-accordingly the thread of the story where abandoned by the geologist
-is taken up by the antiquary, and passed on by him to the
-historian[118].
-
-[Footnote 118: The student may obtain information concerning the
-Neolithic age in Britain in Boyd Dawkins's _Early Man in Britain_; Sir
-J. Evans' _Early Stone Implements of Great Britain_, and Sir J.
-Lubbock's _Prehistoric Times_. In the latter work he will find a good
-account of the Neolithic remains of Denmark and of the Swiss Lake
-dwellings. For information concerning the Bronze age he should consult
-Evans' _Ancient Bronze Implements of Great Britain_. The varied Danish
-antiquities of Neolithic and Bronze ages are figured in H. P. Madsen's
-_Antiquités Préhistoriques du Danemark_. The Prehistoric fauna of the
-fenlands is described in Sir R. Owen's _History of British Fossil
-Mammals and Birds_.]
-
-
-
-
-CHAPTER XXX.
-
-REMARKS ON VARIOUS QUESTIONS.
-
-
-There are many problems connected with geology which can only be
-solved by detailed study of the stratified rocks, and when solved the
-principles of the science will be more fully elucidated. In the
-present state of our knowledge some of these problems are ripe for
-discussion, others can merely be indicated, while others again have
-probably remained hidden, though it will be the task of the geologist
-of the future to clear them up. Among the many questions which demand
-knowledge of stratigraphical geology for their right understanding are
-the following, which will be briefly considered in this chapter:--the
-changes in the position of land and sea in past times, and the growth
-of continents; the replacement of a school of uniformitarianism by one
-of evolutionism; and the duration of geological time.
-
-_Changes in the position of land and sea._ Certain physicists have
-arrived at the conclusion that the general position of our oceans and
-continents was determined at a very early period in the earth's
-history, and that the changes which have occurred in their position
-since then have been comparatively insignificant. The wide extent of
-land over which stratified rocks are distributed at once indicates
-that from the point of view of the geologist the changes have been
-very important, and it is worth inquiring whether they are not
-sufficiently important to prove that the primitive oceans and
-continents have undergone so much alteration as to be unrecognisable.
-Some authorities, while recognising the great changes which have
-occurred in the relative position of land and sea during those periods
-of which geologists have direct information, suppose that the changes
-took place to a large degree in certain 'critical areas' bordering the
-more stable areas of permanent ocean on the one side and permanent
-land on the other.
-
-In discussing the question of general permanence of land and ocean
-regions it will be convenient to commence with a study of the present
-land areas, and at the outset we may take into consideration the
-present distribution of marine sediment over different parts of the
-land, using the last edition of M. Jules Marcou's geological map of
-the world for the purpose[119]. A glimpse at this map indicates that
-more than half of the land areas are occupied by rocks which are as
-yet unknown (many of which _may_ be marine sediments), or by
-crystalline schists of which the mode of origin has not yet been fully
-explained, so that a large part of Central Asia, the interior of
-Africa, and of South America may have existed as land from very early
-times, and the same may be said of smaller portions of Europe and
-North America. Actual observation of a geological map therefore
-indicates the possibility that about half of the land surfaces may
-have existed as such through very long periods, but though there is a
-possibility of this, the probability is not very great. The unknown
-regions, as remarked above, may consist to a considerable extent of
-marine sediments, and the existence of isolated patches of late
-Palæozoic and of Mesozoic strata in the heart of Central Asia, points
-to the submergence of much wider regions than those in which these
-isolated patches have been found. Again, the character of the
-sediments when they abut against the crystalline schists frequently
-proves that these sediments once extended further over the crystalline
-schists, and have since been removed by denudation, so that even if we
-assume that the crystalline schists are all of very early date, and
-not necessarily formed in any case from marine sediments, we cannot
-suppose that all the area occupied by them has existed as land for
-long periods of time. On the other hand, the major part of Europe and
-North Africa, extensive tracts in Asia, the greater part of Australia,
-a very large part of North America and considerable tracts of South
-America give proofs of having been occupied by the oceans in Palæozoic
-and later times.
-
-[Footnote 119: A reduced copy of this map will be found opposite the
-title-page of the first volume of Prof. Prestwich's _Geology_.]
-
-It may be answered that most of these regions containing marine
-sediments occur in critical areas, which have undergone a certain
-amount of oscillation owing to earth-movements, and that the interior
-parts of the great continental masses have been practically
-stationary. But if these lands had been land-areas through geological
-ages they must have been acted upon by the agents of subaerial
-denudation, throughout these ages, and long ago reduced to
-peneplains[120] unless the action of these subaerial agents was
-counteracted by that of elevating forces, but if these forces were
-sufficient to counteract the action of subaerial denudation through
-countless ages, they were also sufficient to raise extensive tracts
-of land above sea-level, and materially to alter the distribution of
-land and sea, and if elevation could go on to this extent, why not
-also depression?
-
-[Footnote 120: A term proposed by Prof. W. M. Davis for a nearly level
-surface of subaerial denudation, as opposed to a plain of marine
-denudation.]
-
-Proceeding a step further, and examining the character of the
-sediments as well as their geographical distribution, we find
-further evidence of great crust-movements. It has been urged that
-deep-water sediments do not occur amongst the strata found on the
-continents,--that there are no representatives of the abysmal deposits
-of recent ocean floors amongst the strata of the geological
-column[121], but the researches of the last two decades have brought
-to light foraminiferal and radiolarian deposits, pteropodal deposits,
-and possibly deep-sea clays, which are comparable with those in
-process of formation at great depths in existing oceans, and though
-the proofs of their deep-sea origin are not always as full as might be
-desired in the case of the older rocks[122], we can speak with greater
-certainty when we examine those of Tertiary age, and if the deep-sea
-accumulations of this late date can be uplifted above sea-level, this
-is much more likely to have occurred with those of past times. When a
-deposit like the radiolarian rock of Barbadoes, the deep-water
-character of which has been conclusively proved, can be elevated into
-land since Miocene or possibly Pliocene times, it is evident that the
-crust-movements have been sufficient to produce the most profound
-changes in the distribution of land and sea during the long ages which
-are known to us. Another argument against the occurrence of extensive
-changes has been derived from an examination of those islands which
-are spoken of as oceanic islands. Strictly speaking an oceanic island
-is one in which the present fauna and flora give indications of their
-introduction by transport across intervening sea, and no indications
-of the existence of forms of life which inhabited it when it was once
-united to a continent; it may be inferred with a considerable degree
-of certainty that these islands have been isolated for long periods of
-time. It has been stated that these oceanic islands never contain
-marine sediments of any considerable degree of antiquity, and that
-there are therefore no traces of former continents over those wide
-tracts of ocean which are occupied by oceanic islands. The evidence is
-of a negative character. The islands would be less likely to exhibit
-ancient sediments than continents, for being near the ocean, they
-would be readily submerged, and the older deposits masked by newer
-ones, though this need not necessarily account for the entire absence
-of ancient rocks amongst them. The danger of the argument lies in the
-fact that we do not yet know how far these old rocks really are
-absent, as the geology of the oceanic isles has not been fully
-explored from this point of view, and already several cases of the
-asserted presence of ancient rocks on these islands have been
-recorded.
-
-[Footnote 121: See Mr A. R. Wallace's _Island Life_.]
-
-[Footnote 122: See chapter IX.]
-
-The argument derived from the present distribution of organisms is far
-too complex to be discussed here, and the student is recommended to
-read a masterly review of the evidence in Dr W. T. Blanford's
-Presidential Address to the Geological Society in 1890, on the
-question of the Permanence of Ocean Basins[123]. After reviewing the
-evidence furnished by a study of modern distribution he concludes that
-it "is far too contradictory to be received as proof of the permanence
-of oceans and continents."
-
-[Footnote 123: _Quart. Journ. Geol. Soc._, vol. XLVI., _Proc._, p.
-59.]
-
-The existence of former extensive land tracts over regions now
-occupied by sea is naturally more difficult to prove than that of sea
-over land, as we depend upon inference rather than actual observation
-to a much greater degree than when considering the permanence of
-continents, nevertheless a considerable amount of indirect evidence in
-favour of the existence of widespread land tracts over our present
-ocean regions has been accumulated and will be briefly noticed. We may
-take first the evidence derived from the nature of sediments, and
-afterwards that which has been acquired by studying distribution of
-organisms in past times.
-
-The indications of existence of an extensive tract of continent over
-the North Atlantic Ocean, during Palæozoic times have already been
-considered, and it was seen that the thinning out of the Palæozoic
-sediments when traced away from the present Atlantic borders in an
-easterly direction over Europe and in a westerly one over North
-America pointed to the existence of this Palæozoic 'Atlantis,' as
-maintained by Prof. Hull in his work, "Contributions to the Physical
-History of the British Isles." This writer gives some reasons for
-supposing that the continental mass began to break up towards the end
-of Palæozoic times, though it is not clear that complete replacement
-of land by sea occurred, and the nature of the Wealden deposits has
-been pointed to as evidence of the existence of an extensive tract of
-land to the west of Britain during the Cretaceous period.
-
-The Palæontological evidence in favour of destruction of ancient
-continental areas and their replacement by the sea is more
-satisfactory than that which is based on physical grounds. The
-distribution of the Glossopteris flora of the Permo-Carboniferous
-period points to the former existence of a great southern continent,
-including the sites of Australia, India, South Africa and South
-America,--the Gondwanaland of Prof. E. Suess[124].
-
-[Footnote 124: On this question and that of the other destroyed
-continental areas noted here, see W. T. Blanford's _Presidential
-Address_, _loc. cit._]
-
-Again, a study of Jurassic and Cretaceous faunas has led
-palæontologists to conclude that there was a connexion betwixt S.
-Africa and India in Mesozoic times across a portion of the area now
-occupied by the Indian Ocean, and also between S. Africa and S.
-America, and these inferences are supported by study of the
-distribution of existing forms.
-
-The sudden appearance of the Dicotyledonous Angiosperms in Upper
-Cretaceous rocks has also been used as evidence of destruction of
-considerable tracts of land subsequently to Upper Cretaceous times,
-and there is a certain amount of evidence in favour of the existence
-of this land in the north polar region, in an area now largely
-occupied by water, though relics of it are left, as the Faroe Isles,
-Spitsbergen, Novaya Zembla and Franz Josef Land.
-
-I cannot conclude the consideration of the question of permanence of
-oceans and continents more fitly than by quoting from Dr Blanford's
-address. He says, "There is no evidence whatever in favour of the
-extreme view accepted by some physicists and geologists that every
-ocean-bed now more than 1000 fathoms deep has always been ocean, and
-that no part of the continental area has ever been beneath the deep
-sea. Not only is there clear proof that some land-areas lying within
-continental limits have at a comparatively recent date been submerged
-over 1000 fathoms, whilst sea-bottoms now over 1000 fathoms deep must
-have been land in part of the Tertiary era, but there are a mass of
-facts both geological and biological in favour of land-connexion
-having formerly existed in certain cases across what are now broad and
-deep ocean[125]."
-
-[Footnote 125: _Loc. cit._, _Proc._ p. 107.]
-
-_Growth of continents._ Whatever view as to the general permanence of
-continents and oceans be ultimately established, the occurrence of
-widespread changes in the position of land and sea is indisputable,
-and it is of interest for us to consider the nature of these changes
-in the formation of continents. Prof. J. D. Dana has put forward a
-hypothesis of growth of continents by a process of accretion, causing
-diminution in the oceanic areas, which at the same time became deeper:
-such growth need not always take place in exactly the same way, and
-study of the distribution of the strata of the North American
-continent suggests that the growth there was endogenous, the older
-rocks lying to the west and north forming a horseshoe shaped continent
-enclosing a gulf-like prolongation of the Atlantic, which became
-contracted by deposition and uplift in successive geological periods,
-though it is still partly existent as the Gulf of Mexico. The Eurasian
-continent, especially its western portion, suggests more irregular
-growth around scattered nuclei of older rocks, though the process is
-not completed, and many gulf-like prolongations, as the Baltic and the
-Mediterranean, still remain as water-tracts, which have not yet been
-added to the continents.
-
-Although extensive additions to continents may be and no doubt are
-often largely due to epeirogenic movements, the influence of orogenic
-movements on continent-formation is very pronounced. As the result of
-orogenic movements, the rocks of portions of the earth's crust become
-greatly compressed, and give rise to masses which readily resist
-denudation; moreover, these comparatively rigid masses, as shown by M.
-Bertrand, tend to undergo elevation along the same lines as those
-which formed the axes of previous elevations, and accordingly after a
-continental area has undergone denudation for a considerable period,
-the uplands consist of rocks which have undergone orogenic
-disturbance, while the tracts of ground which are occupied by rocks
-which have not suffered disturbances of this character, even if
-originally uplifted far above sea-level, tend to be destroyed, and
-ultimately occupied by tracts of ocean. Stumps of former mountain
-chains may be again and again established as nuclei of continents and
-as every period of orogenic movement will add to the number of these
-nuclei, the continental areas must in course of time become more
-complex in structure. Moreover, as some areas are affected by orogenic
-movements to a greater extent than others, the complexity of different
-continental masses will vary. Thus, western Europe has been affected
-by orogenic movements during many periods since the commencement of
-Cambrian times and its structure is extremely complex, while the
-central and western parts of Russia have not been subjected to violent
-orogenic disturbances since Cambrian times, and accordingly we find
-the structure of that area comparatively simple; the greater part of
-Africa seems to have escaped these movements since remote times, and
-the structure of that continent is extremely simple when compared with
-the Eurasian continental tract. It need hardly be stated that the
-formation of extensive chains composed of volcanic material, by
-accumulation of lavas and ashes on the earth's surface, may give and
-often has given rise to more rigid tracts, which will bring about the
-same effects as those produced by orogenic disturbance as illustrated
-on a small scale by the Lower Palæozoic volcanic rocks of Cambria and
-Cumbria.
-
-_Uniformitarianism and Evolution._ According to the extreme
-uniformitarian views held by some geologists, the agents which are in
-operation at the present day are similar in kind and in intensity to
-those which were at work in past times, though no geologist will be
-found who is sufficiently bold to assert that this holds true for all
-periods of the earth's history, but only for those of which the
-geologist has direct information derived from a study of the rocks,
-and he is content to follow his master Hutton in ignoring periods of
-which he cannot find records amongst the rocks. The modern geologist,
-however, while rightly regarding the rocks as his principal source of
-information finds that he cannot afford to ignore the evidence
-furnished by the physicist, chemist, astronomer and biologist, which
-throws light upon the history of periods far earlier than those of
-which he has any records preserved amongst the outer portions of the
-earth itself, just as the modern historian is not content with written
-records, but must turn to the 'prehistoric' archæologist and geologist
-for information concerning the history of early man upon the earth.
-Interpreting the scope of geology in this general way, rigid
-uniformitarianism must be abandoned. Assuming that the tenets of the
-evolutionist school are generally true, the question is, how far does
-this affect the geologist in his study of those periods of which we
-have definite records amongst the rocks? This is a question which
-cannot readily be answered at the present day, for our study of the
-rocks is not sufficiently far advanced to enable us to point out
-effects amongst the older rocks which were clearly caused by agents
-working with greater intensity than they do at present, but as, on
-the other hand, we cannot prove that these effects are due to agents
-working with no greater intensity than that which now marks these
-operations, it is unphilosophical to assume the latter. No student of
-science at the present day would state that because there has been no
-observed case of incoming of fresh species within the time that man
-has actually observed the present faunas and floras, the hypothesis of
-evolution of organisms is disproved, for the time of observation has
-been too short, and similarly the time which has elapsed since the
-formation of, say, the Cambrian rocks may have been too short, as
-compared with the time which has elapsed since the formation of the
-earth, to allow of any important change in the operation of the
-geological agents.
-
-Leaving out of account, for the moment, the actual evidence which has
-been derived from a study of the rocks, we may briefly consider the
-theoretical grounds upon which the substitution of an evolutionist
-school of geology for one of uniformity has been suggested[126]. The
-principal sources of energy which have exerted an influence upon
-geological changes are the heat received from the sun and that given
-off from the earth itself, both of which must have diminished in
-quantity throughout geological ages. To the former source we largely
-owe climatic changes and the operations of denudation, and accordingly
-of deposition; to the latter, those of earth-movement and vulcanicity.
-It by no means follows that because the agents were once potentially
-more powerful than now, they would necessarily produce greater
-effects, for that depends to some extent upon the various conditions
-which prevailed at different times. To give an example:--if there had
-at any time been a universal ocean of considerable depth, however
-active the agents of denudation were then, they could produce no
-effect whatever, having nothing to work upon; to take a less extreme
-case, if our continents at any past time were smaller and less
-elevated than at present, agents of denudation working with greater
-intensity than that of the present agents need not necessarily have
-produced a greater amount of denudation than that which is going on at
-the present day. Again, let us consider vulcanicity: "It is as
-certain," says Lord Kelvin, "that there is less volcanic energy in the
-whole earth than there was a thousand years ago, as it is that there
-is less gunpowder in a 'Monitor' after she has been seen to discharge
-shot and shell, whether at a nearly equable rate or not, for five
-hours without receiving fresh supplies than there was at the beginning
-of the action." But it does not follow that the manifestations of
-volcanic activity were necessarily more violent in early geological
-times than now, for the degree of violence would be affected by other
-things than the volcanic energy, such as the thickness of the earth's
-crust.
-
-[Footnote 126: The student may consult an interesting article by Prof.
-Sollas bearing on this subject. See _Geol. Mag._ Dec. 2, vol. IV. p.
-1.]
-
-And now, let us consider briefly the characters of the rocks of the
-crust, to see if they throw any light upon this question. The earliest
-sediments of which we have any certain knowledge resemble in a
-striking manner those formed at the present day, and they seem to have
-been formed under very much the same conditions, though further work
-may show that there were somewhat different conditions which did
-produce definite differences in the characters of the earlier
-strata[127]. Our knowledge of earth-movement and vulcanicity which
-took place in past times is still too small to enable us to draw any
-certain conclusions connected with the subject under discussion from
-it. Perhaps the most suggestive indication of one set of conditions
-having been generally similar in those early periods of which we have
-definite records amongst the rocks is furnished by study of past
-climate. If we accept the nebular hypothesis as a starting point, we
-must admit that in the early stages of the earth's history the
-temperature of the surface, which would then be largely dependent upon
-the amount of heat given out from the earth itself as well as upon
-that received from the sun, must have been much higher than it is at
-the present day, and indeed the mere diminution of the amount of heat
-received from the sun would probably be sufficient to account for a
-very marked lowering of the temperature. Besides this change of
-temperature, resulting in gradual lowering of temperature over the
-whole earth's surface, we have other changes dependent upon different
-conditions, as proved by the fact, that there have been alternations
-of glacial and genial periods. If the general temperature had been
-very high in the early periods of which we have actual records, the
-oscillations would not be sufficient to produce a lowering of
-temperature sufficient to cause glacial periods, whereas if it had not
-been appreciably higher than now, glacial periods might be produced.
-This may be represented diagrammatically.
-
-[Footnote 127: On this matter see Teall, J. J. H., 'Presidential
-Address to Section C,' _Report of the British Association_, 1893.]
-
-Let _a_ represent the temperature at the commencement of earth-history
-and _b_ that necessary for glaciation, and _bc_ the lapse of time
-between then and now. The curved line indicates the gradual fall in
-temperature due to diminution of the amount of heat, while the zigzag
-line represents the oscillations due to secular climatic changes. If
-the Cambrian period x occurred comparatively soon after the
-commencement of earth-history as shown in fig. _A_, no glaciation
-could be produced, even during periods when secular changes caused
-colder conditions than the mean, whereas if the Cambrian period
-occurred at a time very remote from the commencement of earth-history
-as shown in _B_, glacial conditions could be produced then as now, for
-the mean temperature, as shown by the distance of the curve from the
-line _bc_, would be practically as it now is. The studies of the last
-few decades have brought into prominence the occurrence of glacial
-periods in remote times, probably in early Palæozoic times; and as far
-as the mean temperature of the earth's surface is concerned, it would
-appear, from the knowledge in our possession, that matters were not
-very different in those early times from what they now are.
-
-[Illustration: Fig. 25.]
-
-Some further remarks will be made in subsequent paragraphs concerning
-the period of the earth's history at which the geologist is first
-furnished with definite records, but in the meantime it may be
-observed that the geologist will do well, when working amongst the
-strata, to consider that the more active operation of agents, even in
-times of which he has definite knowledge, may have produced effects
-which he should be prepared to discover, as their discovery would be
-of considerable importance, and that he should not be content to infer
-that because it has been proved that agents operating with the same
-intensity as that which they have at present, _may_ have produced all
-the effects which he can actually observe, they therefore necessarily
-_did_ produce them.
-
-_Recurrences._ Absolute uniformity of conditions is impossible, even
-in a single area. Every change which takes place upon the earth
-produces conditions somewhat dissimilar from those which previously
-existed, and these will leave their effects upon the physiography of
-the area. For this reason, assuming that the conditions have gradually
-changed from simpler to more complex, every period of time will have
-been marked by conditions which never prevailed before or afterwards,
-and these will leave their impress upon the deposits of the period. It
-is doubtful for instance, as already remarked, whether the exact
-conditions which gave rise to the extensive deposits of vegetable
-matter in Carboniferous times which now form coal, ever occurred to a
-like extent in previous or subsequent periods, and accordingly, though
-we have deposits of coal of other ages, none are so extensive as those
-of the Coal Measures. Again, as the strata of one period are largely
-composed of denuded particles of pre-existing strata, which were
-derived directly or indirectly from igneous rock, the soluble material
-existing in the igneous rocks must have been gradually eliminated
-unless restored by other processes, and we might expect to find that
-early sediments have, on the whole, a larger proportion of soluble
-silicates than the later ones.
-
-Besides these changes, there are physical changes which are recurrent,
-and cause conditions generally similar to pre-existing ones to occur
-in an area after an interval of dissimilar ones. We have seen that
-deposits tend to vary according to the distance from the coast,
-limestone being succeeded by mud, this by sand and gravel, and after
-subsidence the sand and gravel are succeeded by mud, and that by
-limestone. These changes will produce some effect upon the organisms,
-and the recurrence of organisms is a well-known event, of which cases
-have been cited in a former chapter.
-
-Again we find, as already pointed out, recurrence of climatic changes,
-with alternation of glacial and warmer periods, and these may have
-been very widespread, and would influence the other physical
-conditions, as well as the distribution of the organisms. Vulcanicity
-may have been more rife at some periods than others, for instance
-there seems, in the present imperfect state of our knowledge, evidence
-of enfeebled vulcanicity in later Mesozoic times, and of its renewed
-activity in Tertiary times. Again, orogenic movements seem to have
-occurred more extensively at some times than others, as for instance
-in early upper Palæozoic times, at the end of the Palæozoic epoch, and
-in early Tertiary times, though this may also be an apparent and not
-an actual truth, due to imperfect knowledge. In any case, in limited
-areas, there seem to have been alternations of periods of uplift
-accompanied by marked orogenic movements, and of widespread
-depression, accompanied by sedimentation.
-
-The subject of rhythmic recurrence is worthy of further study. This
-recurrence in combination with evolutionary change may account for the
-apparent marked difference between Cambrian and Precambrian times, a
-difference which strikes some geologists as being too great to be
-accounted for as due to our ignorance only.
-
-_Organic evolution._ This subject is too wide for more than passing
-notice in a work of this character. The evidence of Palæontology is of
-extreme importance to the biologist, and indeed, the way in which
-evolution of organisms has occurred can only be actually demonstrated
-by reference to Palæontology, and the study of Palæontology has
-already given much information concerning the lines on which evolution
-has proceeded in different groups of organisms. It must be remembered
-that the major divisions of the invertebrata were in existence in very
-early times; indeed representatives of most of them are found in the
-rocks containing the earliest known fauna, that of the _Olenellus_
-beds of Cambrian age. If our present views as to evolution be correct,
-there is no doubt that the period which elapsed between the appearance
-of life upon the globe and the existence of the _Olenellus_ fauna must
-have been very great, possibly, as Huxley suggested, much greater than
-that which has elapsed between early Cambrian times and the present
-day. If this be so, however probable it is that we shall carry our
-knowledge of ancient faunas far back beyond Cambrian times, it is
-extremely improbable that we shall ever get traces of the very
-earliest faunas which occupied our earth.
-
-_Geological time._ Various attempts have been made to give numerical
-estimates of the lapse of time which occurred since the earth was
-formed, or since the earliest known rocks were deposited. These
-attempts may be classed under two heads, namely, those made by
-physicists, mainly on evidence obtained otherwise than by a study of
-the rocks, and those made by geologists by calculating the mean rate
-of denudation and deposition of the rocks, and estimating the average
-thickness of the rocks of the geological column.
-
-The calculations of physicists as to the age of the earth vary:--Lord
-Kelvin assigned 20,000,000 years as the minimum and 100,000,000 as the
-maximum duration of geological time. Prof. Tait has halved Lord
-Kelvin's minimum period, while Prof. G. Darwin admits the possibility
-of the lapse of 500,000,000 years.
-
-The estimates made by geologists, which will appeal more directly to
-the geological student, also vary considerably, though they bear some
-proportion to those which have been put forward by the physicists.
-Prof. S. Haughton[128] assigned a period of 200,000,000 years for the
-accumulation of the rocks of the geological column; Mr Clifton
-Ward[129] one of 62,000,000 years, after studying the rocks of the
-English Lake District, and allowing for the gaps in the succession; Mr
-A. R. Wallace[130] further lowers the time for the formation of the
-column to 28,000,000 years; Sir A. Geikie[131] gives 73,000,000 years
-as the minimum and 680,000,000 as the maximum; while Mr J. G.
-Goodchild has lately[132] estimated the period at over 700,000,000
-years.
-
-[Footnote 128: _Nature_, vol. XVIII. p. 268.]
-
-[Footnote 129: Ward, J. C., 'The Physical History of the English Lake
-District,' _Geol. Mag._ Dec 2, vol. VI. p. 110.]
-
-[Footnote 130: Wallace, A. R., _Island Life_, Chap. X.]
-
-[Footnote 131: Geikie, Sir A., 'Presidential Address to the British
-Association,' _Report Brit. Assoc._, 1892.]
-
-[Footnote 132: Goodchild, J. G., _Proc. Roy. Soc. Edinburgh_, vol.
-XIII. p. 259.]
-
-Interesting as these figures are, they probably convey little to the
-ordinary reader, and it is doubtful whether the geologist is really
-affected by them to any extent when picturing to himself the vast
-duration of geological time. One numerical estimate probably does
-impress him, namely that made by Croll as to the date of the Great Ice
-Age, for if the Ice Age be so remote as Croll imagined, the
-commencement of earth-history must be inconceivably more remote; as
-Croll's estimate is not generally accepted, it is doubtful how far
-geologists are thus influenced, and probably the fact which does
-impress them most, leaving fossils out of account, is the very little
-change which has occurred in historic or even in prehistoric times as
-compared with the vast changes which are familiar to them after
-studying the strata of the geological column.
-
-It is, after all, the succession of varied faunas which really gives
-students of the rocks the most convincing proof of the vast periods of
-geological time. If anyone doubts this assertion, let him consider
-what impression would be made upon him by observing the several
-thousand feet of strata of the column if none of them contained any
-organisms. Cognisant as he is of the slow rate of change of existing
-organisms, the fact that fauna has succeeded fauna in past times
-brings home to him in an unmistakeable manner the great antiquity of
-the earliest fossiliferous rocks, and as our detailed knowledge of
-these faunas increases the impression of great lapse of time is
-intensified. And if the earliest fossiliferous rocks be of such vast
-antiquity, and, as has been remarked, the period of their formation is
-comparatively recent with reference to the actual commencement of
-earth-history, the latter must indeed be inconceivably remote, and
-numerical estimates can do but little to familiarise us with the
-significance of the vast time which has rolled by since the world's
-birthday.
-
-
-
-
-INDEX.
-
-
-  Abraum salts, 212
-  Æolian rocks, 24, 99, 100
-  Age, definition of, 60
-  Albian series, 236, 238
-  Algonkian rocks, 144
-  Ampthill clay, 232
-  Angelin, N. P., 161, 162, 165
-  Aptian series, 236, 237
-  Aqueous rocks, 24
-  Archæan rocks, 132
-  Ardmillan series, 170
-  Ardwick stage, 192
-  Arenaceous rocks, 29
-  Arvonian rocks, 141
-  Asaphus fauna, 165
-  Ashgill series, 164, 165, 167-169
-  Ashprington series, 184
-  Astian series, 256
-  Atlantis, 283
-  Aveline, W. T., 164
-  Aymestry limestone, 175, 176
-
-  Bagshot beds, 244, 246
-  Bajocian series, 227, 231
-  Bala limestone, 167
-  Bala series, 164
-  Barr series, 170
-  Barrande, J., 53, 55, 159, 161, 163
-  Barrois, C., 239
-  Barrow, G., 138
-  Barton beds, 244
-  Bath oolites, 226
-  Bathonian series, 227, 231
-  Bed, 27
-  Bedding plane, 27
-  Bell, A., 257
-  Belt, T., 153, 162
-  Bembridge beds, 251
-  Bertrand, M., 87, 286
-  Birkhill shales, 177
-  Black Jura, 226
-  Blake, J. F., 138-140
-  Blanford, W. T., 206, 208, 217, 282, 284
-  Bonney, T. G., 76, 141, 142
-  Boulder clay, 262
-  Bracklesham beds, 244
-  Bradford clay, 230
-  Break, palæontological, 61;
-    physical, 60
-  Bristow, H., 239
-  Brockram, 211
-  Brögger, W. C., 161-163
-  Brongniart, H., 18
-  Brongniart, C., 200
-  Bronze age, 275-277
-  Brown Jura, 226
-  Bunter sandstone, 218, 220-222
-  Bure valley beds, 256
-  Buttery clay, 276
-
-
-  Caerfai beds, 152, 154, 156
-  Calcareous rocks, 29
-  Caldicote series, 139
-  Callaway, C., 138-140
-  Callovian series, 227, 232
-  Cambrian faunas, 158-163
-  Cambrian system, 152-163
-  Caradoc series, 165, 168-171
-  Carbonaceous rocks, 29
-  Carboniferous fauna and flora, 199-201
-  Carboniferous limestone, 192, 194, 195
-  Carboniferous system, 192-201
-  Carnic beds, 225
-  Cataclastic rocks, 24
-  Cave man, 268
-  Cenomanian series, 236
-  Ceratopyge fauna, 162
-  Chalk, 236, 238, 239
-  Chalk marl, 236
-  Chemically-formed rocks, 29, 101
-  Chillesford crag, 256
-  Chronological terms, 60
-  Clastic rocks, 24
-  Climatic conditions, 103, 112, 290, 291
-  Climatic zones, in Jurassic times, 233;
-    in Cretaceous times, 241
-  Clymenian beds, 183
-  Coal, 196-199
-  Coal measures, 192;
-    mode of formation of, 195-199
-  Coblenzian beds, 184
-  Collyweston slate, 231
-  Colonies, theory of, 55
-  Contemporaneity of strata, 48
-  Continents, growth of, 285-287
-  Cope, E., 249
-  Corallian series, 227, 232
-  Coralline crag, 256, 257
-  Cornbrash, 230
-  Cornstones, 186
-  Coutchiching series, 144
-  Crags, 256-259
-  Cretaceous fauna and flora, 241-243
-  Cretaceous system, 236-243
-  Croll, J., 265, 295, 296
-  Cromer Forest series, 100, 256, 259
-  Cromer till, 262
-  Cucullæa beds, 183
-  Cuvier, Baron G., 18, 20
-
-
-  Dalradian rocks, 137
-  Dana, J. D., 285
-  Danian series, 236
-  Darwin, C., 20, 76
-  Darwin, G., 295
-  Daubrée, A., 88
-  David, T. W. E., 206
-  Davis, W. M., 258, 280
-  Dawkins, W. B., 266, 268, 270, 272, 277
-  Deep-sea deposits, 109
-  De Hayes, G. P., 19
-  De la Beche, Sir H., 92
-  Deposition, order of, 37, 116
-  Derivative rocks, 23
-  Devonian flora and fauna, 189-191
-  Devonian system, 183-191
-  Dictyograptus fauna, 162
-  Dimetian rocks, 141
-  Dogger, 226
-  Downtonian beds, 175
-  Dwyka conglomerate, 206
-
-
-  Edwards, F. E., 250
-  Eifelian beds, 184
-  Encrinurus fauna, 185
-  Englacial deposits, 261
-  Entomis slates, 183
-  Eocene fauna and flora, 248, 249
-  Eocene rocks, 244-250
-  Eozoon canadense, 143
-  Eparchæan rocks, 132
-  Epeirogenic movements, 32
-  Epiclastic rocks, 24;
-    simulation by cataclastic rocks, 38, 80
-  Epoch, definition of, 60
-  Estuarine series, 230
-  Etheridge, R., 19
-  Ettingshausen, Baron von, 250
-  Evans, Sir J., 266, 270, 274, 277
-  Evolution, 287, 293
-
-
-  Feistmantel, O., 208
-  Fenland, 276
-  Fluvio-glacial deposits, 261
-  Foreland grits, 184
-  Forest marble, 230
-  Forest period, 260, 275-277
-  Fossils, 42;
-    strata identifiable by, 40;
-    mode of occurrence of, 44;
-    relative value of, 47;
-    remanié, 52;
-    geographical distribution of, 55;
-    as indicative of physical conditions, 104
-  Fossil zone, 67
-  Foster, C. Le N., 239
-  Fox, H., 195
-  Freshwater deposits, 104;
-    distinction from marine, 105
-  Fuller's earth, 230
-  Fusulina beds, 201
-
-
-  Gala beds, 177
-  Gannister stage, 192
-  Gardner, J. S., 250
-  Gault, 236, 238
-  Geikie, Sir A., 60, 84, 95, 125, 130, 137, 141, 142, 144, 186, 188,
-    199, 247, 295
-  Geikie, J., 263
-  Girvan type, 170
-  Glacial deposits, permo-carboniferous, 206;
-    Pleistocene, 260-266
-  Glacial period, 260-266
-  Glenkiln shales, 169, 170
-  Glossopteris flora, 207, 208, 214
-  Godwin-Austen, R. A. C., 20
-  Gondwana series, 207
-  Gondwanaland, 207, 284
-  Goniatite beds, 183
-  Goodchild, J. G., 87, 130, 263, 295
-  Great ice age, 295, 296
-  Great oolite, 230, 231
-  Gregory, J. G., 258
-  Green, A. H., 122, 139, 193
-  Greensand, Lower, 236;
-    Upper, 236
-  Groom, T. T., 178
-  Gshellian beds, 193, 201
-
-
-  Hampshire basin, 245
-  Hangman grits, 184
-  Harker, A., 30, 88
-  Harkness, R., 161
-  Harmer, F. W., 258
-  Harpes fauna, 175
-  Harrison, W. J., 130
-  Hartfell shales, 169, 170
-  Hastings sands, 236, 237
-  Haughton, S., 295
-  Headon beds, 251
-  Heim, A., 32
-  Hempstead beds, 251
-  Hercynian systems of folds, 203
-  Hicks, H., 134, 141, 153, 154, 160, 161, 163, 167, 184, 266
-  Hickson, S. J., 109
-  Hill, A., 239
-  Hill, E., 142
-  Hilton shales, 210, 211
-  Hind, W., 196
-  Hinde, G. J., 169, 195
-  Hippurite limestone, 241, 242
-  Hirnant limestone, 167
-  Homotaxis, 48
-  Hughes, T. McK., 141, 264, 266
-  Hull, E., 120, 122, 193, 283
-  Hume, W. F., 239
-  Hunt, A. R., 101
-  Huronian system, 143
-  Hutton, J., 287
-  Huxley, T. H., 50, 250
-
-
-  Igneous rocks, 21-23
-  Ilfracombe beds, 184
-  Inferior oolite, 230
-  Inverted strata, 32;
-    detection of, 32
-  Iron age, 275, 276
-
-
-  Judd, J. W., 239, 247
-  Jukes, J. B., 84
-  Jukes-Browne, A. J., 126, 239, 264
-  Jurassic beds, 225
-  Jurassic fauna and flora, 234, 235
-  Jurassic system, 226-235
-
-
-  Kayser, E., 125, 191
-  Keewatin series, 144
-  Kelvin, Lord, 289
-  Kendall, P., 257
-  Keuper beds, 218, 221, 222
-  Kidston, B., 199
-  Kimmeridge clay, 232
-  Kimmeridgian series, 226
-  King, W., 217
-  Kjerulf, Th., 88
-  Koninck, L. de, 201
-  Kupferschiefer, 209
-
-
-  Lake, P., 126, 178
-  Lamina, 27
-  Lamplugh, G. W., 80, 119, 264
-  Lapworth, C., 32, 138, 139, 156, 168-170, 173, 178, 179
-  Laurentian rocks, 143
-  Lawson, A. C., 144, 145
-  Lehmann, J., 77
-  Lenham beds, 257
-  Lewis, H. C., 263
-  Lias, 226, 229
-  Liassian series, 227, 229
-  Lincolnshire limestone, 230, 231
-  Lincombe and Warberry grits, 184
-  Lindström, G., 114
-  Lingula flags, 152, 155, 156
-  Linnarsson, J. G. O., 161
-  Llandeilo limestone, 167
-  Llandeilo series, 165, 167
-  Llandovery series, 174-177
-  Loess, 267
-  Logan, Sir W., 20
-  London Basin, 245
-  London clay, 113, 244, 246
-  Longmyndian rocks, 138
-  Lower London Tertiary beds, 244, 246
-  Lubbock, Sir J., 270, 277
-  Ludlow series, 174-176
-  Lydekker, R., 250
-  Lyell, Sir C., 6, 12, 19, 106, 129, 224, 263, 270
-  Lynton slates, 184
-
-
-  McCoy, Sir F., 201
-  McMahon, C. A., 77
-  Madsen, H. P., 277
-  Magnesian Limestone, 209-211
-  Malm, 226
-  Maps, geological, 84, 130;
-    use of, 86, 120, 121
-  Marcou, J., 130, 279
-  Marine deposits, 102;
-    nature of fossils in, 107
-  Marl slate, 209, 210
-  Marlstone, 229
-  Marsh, O. C., 249
-  Marwood beds, 183
-  Matthew, G. F., 160-162, 180
-  Meadfoot sands, 184
-  Mechanically formed rocks, 29, 102
-  Mello, J. M., 270
-  Mendip system of folds, 203
-  Menevian beds, 152, 154, 156, 161
-  Metamorphic rocks, 25
-  Miall, L. C., 122
-  Michell, J., 10, 11
-  Millepore oolite, 230, 231
-  Miller, H., 189
-  Millet seed sands, 100
-  Millstone grit, 192
-  Miocene period, 252-255
-  Moffat shales, 169, 177
-  Mojsisovics, E. von, 224, 227
-  Morgan, C. Ll., 141
-  Morte slates, 184
-  Moscovian beds, 193, 301
-  Mountain limestone, 192
-  Murchison, Sir R. I., 19, 20, 174, 179
-  Murray, Sir J., 30
-  Muschelkalk, 218, 221, 222
-
-
-  Nehring, A., 267, 268
-  Neobolus fauna, 160
-  Neocomian series, 236-238
-  Neolithic age, 275-277
-  Neumayr, M., 115, 233
-  Newton, E. T., 45
-  Nicholson, H. A., 189, 250
-  Noachian Deluge, 8
-  Noetling, F., 160
-  Nordenskjöld, A. E., 113, 114
-  Noric beds, 225
-  Northamptonshire sands, 230
-  Norwich crag, 256, 257
-  Nummulitic limestone, 248
-
-
-  Old red sandstone, 183, 185, 186, 188, 191
-  Oldham, R. D., 208
-  Oldhaven beds, 244, 245
-  Olenellus fauna, 134, 153, 156-160
-  Olenus fauna, 152, 161, 162
-  Oligocene beds, 251, 252
-  Oligocene fauna and flora, 252
-  Oolite, 226
-  Ordovician faunas, 172, 173
-  Ordovician system, 164-173
-  Organically formed rocks, 29, 102, 109
-  Orogenic movements, 32
-  Osborne beds, 257
-  Owen, Sir R., 277
-  Oxford clay, 232
-  Oxford oolite, 226
-  Oxfordian series, 227, 232
-
-
-  Palæolithic fauna and flora, 270-274
-  Palæolithic man, 268, 272-274
-  Palæolithic period, 267-274
-  Palæontological break, 61
-  Palæo-physiography, 120
-  Paradoxides fauna, 152, 160, 161
-  Peat deposits, 275, 276
-  Pebble beds of Bunter, 218
-  Pebidian rocks, 140
-  Pengelly, W., 270
-  Pennant stage, 192
-  Pennine system of folds, 203
-  Penrith sandstone, 75, 210, 211
-  Period, definition of, 60
-  Permanence of ocean basins, 278-285
-  Permian fauna and flora, 214-216
-  Permian system, 209-217
-  Permo-carboniferous fauna and flora, 207, 208
-  Permo-carboniferous glacial deposits, 206
-  Permo-carboniferous period, 205-208
-  Phillips, J., 10, 11, 201
-  Physical break, 60
-  Pickwell Down sandstone, 183
-  Pilton beds, 183
-  Plaisancean series, 256
-  Planes of lamination, 27
-  Planes of stratification, 27
-  Pleistocene fauna and flora, 265, 266
-  Pleistocene period, 260-266
-  Pliocene fauna and flora, 259
-  Pliocene period, 256-259
-  Portland oolites, 226
-  Portlandian series, 226, 232
-  Prado, C. de, 161
-  Precambrian rocks, 132;
-    mode of formation of, 146
-  Preller, C. S. du R., 264
-  Prestwich, Sir J., 19, 130, 279
-  Productus limestones, 205, 206, 214
-  Protolenus fauna, 160
-  Pseudo-stromatism, 76
-  Purbeckian series, 226, 232
-  Pyroclastic rocks, 24
-
-
-  Quader sandstone, 240
-
-
-  Ramsay, Sir A. C., 130, 153, 163, 188
-  Reading beds, 244
-  Recurrences, 292
-  Red crag, 256, 257
-  Reid, C., 45, 257, 264, 268, 271
-  Renard, A., 30
-  Reversed fault, 34
-  Rhætic beds, 218
-  Rhiwlas limestone, 167
-  Richthofen, Baron von, 267, 268
-  Ridley, H. N., 271
-  River drift man, 268
-  Rotherham red rock, 202
-  Rothliegende, 209
-  Rouelle, 13
-
-
-  St Bees sandstone, 210
-  St Erth beds, 257
-  Salopian beds, 175
-  Salter, J. W., 161, 162, 186
-  Scarbro' limestone, 230, 231
-  Schists, crystalline, 76, 77, 133, 147
-  Scilla, A., 13
-  Screes, 101
-  Scrope, G. P., 76
-  Sections, geological, 84;
-    use of, 88
-  Sedimentary rocks, 23
-  Sedgwick, A., 16, 19, 20, 153, 174
-  Senonian series, 236
-  Series, definition of, 60
-  Seward, A. C., 113, 208
-  Sigmoidal structure, 33
-  Siliceous rocks, 29
-  Silurian faunas, 179, 180
-  Silurian system, 174-182
-  Simulation of structures, 72
-  Sinemurian series, 227, 229
-  Smith, W., 8, 12-18, 57, 85
-  Soil, 100
-  Solenhofen slate, 234
-  Sollas, W. J., 288
-  Solva beds, 152, 154, 156, 161
-  Speckled sandstone, 205, 206
-  Speeton series, 238
-  Spencer, H., 50
-  Spirorbis limestone, 201
-  Stages, definition of, 60
-  Steppe period, 260, 267-274
-  Stonesfield slate, 231
-  Strachey, J., 10
-  Strahan, A., 239, 264
-  Strata, 27;
-    classification of, 58, 125
-  Stratification, 26
-  Stratified rocks, 23;
-    composition of, 28;
-    origin of, 29;
-    classification of, 28, 125;
-    symbols to represent, 90
-  Stratigraphical geology, aim of, 1;
-    W. Smith, founder of, 8, 12-18
-  Suess, E., 110, 123, 207, 284
-  Superposition, law of, 31
-  Surveying, geological, 84
-  Systems, definition of, 60
-
-
-  Talchir stage, 205, 206
-  Tarannon shales, 174-177
-  Teall, J. J. H., 289
-  Terrestrial rocks, 99
-  Thanet sands, 244
-  Thinning out, 28
-  Thrust plane, 34;
-    detection of, 35, 82
-  Tiddeman, B. H., 87, 263, 270
-  Till, 262
-  Time, geological, 294-296
-  Toarcian series, 227, 229
-  Topley, W., 130, 239
-  Torridonian beds, 135-137
-  Tremadoc slates, 152, 155, 162, 163
-  Triassic fauna and flora, 223-225
-  Triassic system, 218-225;
-    ammonite zones of, 225
-  Trinucleus fauna, 165
-  Tullberg, S. A., 162
-  Turonian series, 236
-
-
-  Unconformity, 60, 78, 98
-  Underclays, 197
-  Uniformitarianism, 287-292
-  Uriconian rocks, 138
-  Ussher, W. A. E., 183
-
-
-  Valentian beds, 175
-  Verneuil, E. P. de, 161
-  Volcanic rocks, Cambrian, 155;
-    Carboniferous, 199;
-    Devonian, 184, 186;
-    Eocene, 246, 247;
-    Ordovician, 165-170;
-    Precambrian, 146
-  Vulcanicity, 289
-
-
-  Waagen, W., 213, 214
-  Walcott, C. D., 144, 158, 160, 161, 173
-  Wallace, A. R., 124, 235, 240, 281, 295
-  Ward, J. C., 87, 88, 263, 295
-  Warming, E., 115
-  Watts, W. W., 142, 168, 178
-  Wealden beds, 236, 237
-  Webster, T., 18
-  Weissliegende, 214
-  Wenlock limestone, 175, 176
-  Wenlock series, 174-177
-  Wenlock shale, 175-177
-  Werfener Schichten, 225
-  Werner, A. G., 12
-  Weybourne crag, 256
-  Whewell, W., 50
-  Whidbourne, G. F., 91
-  White Jura, 226
-  Whitehaven sandstone, 202
-  Whitehurst, J., 11, 12
-  Wiman, C., 46
-  Wood, S. V., 250, 259
-  Woodward, H., 191
-
-  Woodward, H. B., 68, 130, 131
-  Woodward, J., 8-10
-  Woodward, S. P., 108, 111
-  Woolhope limestone, 175
-  Woolwich beds, 244
-  Wright, G. F., 263
-
-
-  Yoredale series, 192
-
-
-  Zanclean series, 256
-  Zechstein, 209
-  Zone, fossil, 67;
-    ammonite, 225, 237;
-    graptolite, 69
-
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-Transcriber's Note
-
-
-Any obsolete or alternate spelling and grammar was retained. All
-obvious typographical errors were corrected. Although hyphenation of
-words has been standardized to the most prevalent occurrence, the six
-occurrences of fresh-water were not converted to freshwater (30
-occurrences) due to usage. Corrected spellings: Godwin-Austen (p. 20);
-Whidbourne (p. 191); and Ichthyopterygia (p. 223).
-
-
-
-
-
-
-
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