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+*** START OF THE PROJECT GUTENBERG EBOOK 43963 ***
+
+Transcriber's Note
+
+Emphasized text denoted as _Italic_ and =Bold= respectively.
+
+
+
+
+  Cambridge Natural Science Manuals.
+  Geological Series.
+
+  THE PRINCIPLES
+  OF
+  STRATIGRAPHICAL GEOLOGY
+
+
+  London: C. J. CLAY AND SONS,
+  CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
+  AVE MARIA LANE.
+  AND
+  H. K. LEWIS,
+  136, GOWER STREET, W.C.
+
+
+  [Illustration]
+
+
+  Leipzig: F. A. BROCKHAUS.
+  New York: THE MACMILLAN COMPANY.
+  Bombay: E. SEYMOUR HALE.
+
+
+
+
+  THE PRINCIPLES
+  OF
+  STRATIGRAPHICAL GEOLOGY
+
+
+  BY
+  J. E. MARR, M.A., F.R.S.
+  FELLOW AND LECTURER OF S. JOHN'S COLLEGE, CAMBRIDGE,
+  AND UNIVERSITY LECTURER IN GEOLOGY.
+
+
+  CAMBRIDGE:
+  AT THE UNIVERSITY PRESS.
+  1898
+
+  [_All Rights reserved._]
+
+
+  Cambridge:
+  PRINTED BY J. & C. F. CLAY,
+  AT THE UNIVERSITY PRESS.
+
+
+
+
+PREFACE.
+
+
+The present work has been written in order that students may gain by
+its perusal some idea of the methods and scope of Stratigraphical
+Geology. I believe that this idea can be obtained most satisfactorily,
+if a large number of the details connected with the study of the
+stratified rocks are omitted, and I have accordingly given very brief
+accounts of the strata of the different Systems.
+
+The work is intended for use in conjunction with any book which treats
+of the strata of the Geological Column at considerable length; some of
+these books are mentioned on pages 124, 125.
+
+    J. E. M.
+
+    Cambridge,
+    _November, 1898_.
+
+
+
+
+CONTENTS.
+
+
+                                                                     PAGE
+  CHAPTER I.
+    Introduction                                                        1
+
+  CHAPTER II.
+    Account of the growth and progress of stratigraphical geology       6
+
+  CHAPTER III.
+    Nature of the stratified rocks                                     21
+
+  CHAPTER IV.
+    The law of superposition                                           31
+
+  CHAPTER V.
+    The test of included organisms                                     40
+
+  CHAPTER VI.
+    Methods of classification of the strata                            58
+
+  CHAPTER VII.
+    Simulation of structures                                           72
+
+  CHAPTER VIII.
+    Geological maps and sections                                       84
+
+  CHAPTER IX.
+    Evidences of conditions under which strata were formed             97
+
+  CHAPTER X.
+    Evidences of conditions under which strata were formed, continued 116
+
+  CHAPTER XI.
+    The classification of the stratified rocks                        125
+
+  CHAPTER XII.
+    The Precambrian rocks                                             132
+
+  CHAPTER XIII.
+    Cycles of change in the British area                              149
+
+  CHAPTER XIV.
+    The Cambrian system                                               152
+
+  CHAPTER XV.
+    The Ordovician system                                             164
+
+  CHAPTER XVI.
+    The Silurian system and the changes which occurred in Britain
+      at the close of Silurian times                                  174
+
+  CHAPTER XVII.
+    The Devonian system                                               183
+
+  CHAPTER XVIII.
+    The Carboniferous system                                          192
+
+  CHAPTER XIX.
+    The changes which occurred during the third continental period
+      in Britain; and the foreign Permo-Carboniferous rocks           202
+
+  CHAPTER XX.
+    The Permian system                                                209
+
+  CHAPTER XXI.
+    The Triassic system                                               218
+
+  CHAPTER XXII.
+    The Jurassic system                                               226
+
+  CHAPTER XXIII.
+    The Cretaceous system                                             236
+
+  CHAPTER XXIV.
+    The Eocene rocks                                                  244
+
+  CHAPTER XXV.
+    The Oligocene and Miocene periods                                 251
+
+  CHAPTER XXVI.
+    The Pliocene beds                                                 256
+
+  CHAPTER XXVII.
+    The Pleistocene accumulations                                     260
+
+  CHAPTER XXVIII.
+    The Steppe period                                                 267
+
+  CHAPTER XXIX.
+    The Forest period                                                 275
+
+  CHAPTER XXX.
+    Remarks on various questions                                      278
+
+
+
+
+ADDENDA ET CORRIGENDA. [TN: Corrections made!]
+
+
+  p. 38,  line 15 from bottom: for 'joining' read 'jointing'
+
+  p. 208, line 6 from bottom: for 'Dr' read 'Messrs Medlicott and'
+
+  p. 214, line 15 from bottom: after 'Permo-Carboniferous Strata'
+          insert 'through the Permian'
+
+  p. 217, last line of footnote: for 'Dr' read 'Messrs Medlicott and'
+
+      "   insert a second footnote: 'For information concerning the
+          Permian volcanic rocks see Sir A. Geikie's _Ancient Volcanoes
+          of Great Britain_.'
+
+  p. 235, insert a footnote: 'A good account of the British Jurassic
+          rocks will be found in Mr H. B. Woodward's Memoir on
+          "The Jurassic Rocks of Britain." _Mem. Geol. Survey_,
+          1893--.'
+
+  p. 250, top line: for 'Gardiner' read 'Gardner'
+
+
+
+
+CHAPTER I.
+
+INTRODUCTION.
+
+
+It is the aim of the Stratigraphical Geologist to record the events
+which have occurred during the existence of the earth in the order in
+which they have taken place. He tries to restore the physical
+geography of each period of the past, and in this way to write a
+connected history of the earth. His methods are in a general way
+similar to those of the ethnologist, the archæologist, and the
+historian, and he is confronted with difficulties resembling those
+which attend the researches of the students of human history. Foremost
+amongst these difficulties is that due to the imperfection of the
+geological record, but similar difficulty is felt by those who pursue
+the study of other uncertain sciences, and whilst this imperfection is
+very patent to the geologist, it is perhaps unduly exaggerated by
+those who have only a general knowledge of the principles and aims of
+geology.
+
+The history of the earth, like other histories, is a connected one, in
+which one period is linked on to the next. This was not always
+supposed to be the case; the catastrophic geologist of bygone times
+believed that after each great geological period a convulsion of
+nature left the earth's crust as a _tabula rasa_ on which a new set of
+records was engraved, having no connexion with those which had been
+destroyed. Careful study of the records of the rocks has proved that
+the conclusions of the catastrophists were erroneous, and that the
+events of one period produce their impression upon the history of the
+next. Every event which occurs, however insignificant, introduces a
+new complication into the conditions of the earth, and accordingly
+those conditions are never quite the same. Although the changes were
+no doubt very slow, so that the same general conditions may be traced
+as existent during two successive periods, minor complications
+occurred in the inorganic and organic worlds, and we never get an
+exact recurrence of events. Vegetable deposits may now be in process
+of accumulation which in ages to come may be converted into coal, but
+the general conditions which were prevalent during that Carboniferous
+period when most of our workable coal was deposited do not now exist,
+and will never exist again. The changes which have taken place and
+which are taking place show an advance from the simple to the more
+complex, and the stratigraphical geologist is confronted with a
+problem to which the key is development, and it is his task to trace
+the development of the earth from the primitive state to the complex
+condition in which we find it at the present day.
+
+Our general ignorance of the events of the earliest periods of the
+history of the earth will be emphasised in the sequel, and it will be
+found that the complexity which marks the inorganic and organic
+conditions which existed during the deposition of the earliest rocks
+of which we have detailed knowledge points to the lapse of enormous
+periods of time subsequent to the formation of the earth, and previous
+to the deposition of those rocks. The imperfection of the record is
+most pronounced for that long period of time, but in this respect the
+geologist is in the same condition as the student of human history,
+for the relics of the early stone age prove that man in that age had
+attained a fairly high state of civilisation, and the gap which
+separates palæolithic man from the first of our race is relatively
+speaking as great as that which divides the Cambrian period from the
+commencement of earth-history. Nevertheless, human history is a
+science which has made gigantic strides towards the solution of many
+problems connected with the development of man and civilisation, and
+similarly geology has advanced some way in its task of elucidating the
+history of our globe.
+
+The task of the stratigraphical geologist is two-fold. In the first
+place, he must establish the order of succession of the strata, for a
+correct chronology is of paramount importance to the student of
+earth-lore. The precautions which must be taken in making out the
+order of deposition of the rocks of any area, and correlating those of
+one area with those of another will be considered in the body of the
+work. When this task is completed, there yet remains the careful
+examination of all the information supplied by a study of the rocks of
+the crust, in order to ascertain the actual conditions which existed
+during the deposition of any stratum or group of strata. In practice,
+it is generally very difficult to separate these two departments of
+the labour of the stratigraphical geologist, and the two kinds of work
+are often done to a large extent simultaneously, or sometimes
+alternately. Frequently the general succession of the deposits
+comprising an important group is ascertained, and at the same time
+observations made concerning the physical characters of the deposits
+and the nature of their included organisms, which are sufficient to
+afford some insight into the general history of the period when these
+deposits were laid down; a more detailed classification of the same
+set of deposits may be subsequently made, and as the result of this,
+more minute observations as to the variations in the physical and
+biological conditions of the period are possible, which permit us to
+write a much more concise history of the period. So great has been the
+tendency to carry on work in a more and more detailed manner, that it
+is very difficult if not impossible to tell when any approach to
+finality is reached in the study of a group of strata in any area.
+Roughly speaking, we may state that our knowledge of a group of strata
+is obtained by three processes, or rather modifications of one
+process. The general order of succession is established by the
+pioneer, frequently as the result of work carried on through one or
+two seasons. Subsequently to this, a more minute subdivision of the
+rocks is possible as the result of labours conducted by one or more
+workers who are enabled to avail themselves of the work of the
+pioneer, and our knowledge of the rocks is largely increased thereby.
+But the minutiæ, often of prime importance, are supplied by workers
+who must spend a large portion of their time in the area where the
+work lies, and it is only in districts where work of this character
+has been performed, that our knowledge of the strata approaches
+completion. The strata of the Arctic regions, for example, have in
+many places been examined by pioneers, but a great deal remains to be
+done in those regions; the main subdivisions only have been defined in
+many cases, and our information concerning the physical history of
+Arctic regions in past times is comparatively meagre. To come nearer
+home--a few miles north of Cambridge lies the little patch of
+Corallian rock at Upware; it has been frequently visited, and a large
+suite of organic remains extracted from it, but no one has devoted the
+time to the collection of remains from this deposit which has been
+devoted to that of some other formations presently to be mentioned,
+and accordingly our knowledge of the fauna of that deposit is far from
+complete. Contrast with this the information we possess of the little
+seam known as the Cambridge Greensand, from which organic remains have
+been sedulously collected during the extensive operations which have
+been carried on for the extraction of the phosphatic nodules which
+occur in the seam. The suite of relics of the organisms of that period
+is accordingly far more perfect than in the case of many other beds,
+and indeed the large and varied collection of relics of the vertebrata
+of the period which furnish much information of value to the
+palæontologist would not have been gathered together, had not this
+seam been so carefully worked, and an important paragraph in the
+chapter bearing on the history of this period would have remained
+unknown to us. Again, two little patches of limestone of the same age,
+one in central England and the other in the island of Gothland, have
+been the objects of sedulous inquiry by local observers, and we find
+again that our knowledge of the physical history of the period, as
+regards these two regions, is exceptionally perfect. Special stress is
+laid upon this point, for in these days, when every county possesses
+its learned societies whose members are desirous of advancing in every
+possible way the progress of science, it is well to insist upon the
+importance of this detailed work which can only be done by those who
+have a large amount of time to devote to the rigorous examination of
+the rocks of a limited area.
+
+
+
+
+CHAPTER II.
+
+ACCOUNT OF THE GROWTH AND PROGRESS OF STRATIGRAPHICAL GEOLOGY.
+
+
+The history of the growth of a science is not always treated as an
+essential part of our knowledge of that science, and many text-books
+barely allude to the past progress of the science with which they
+deal. The importance of a review of past progress has, however,
+attracted the attention of many geologists, and Sir Charles Lyell, in
+his _Principles of Geology_, gave prominence to an historical sketch
+of the rise and progress of the science. Historical studies of this
+nature have more than an academic value; the very errors made by men
+in past times are useful as warnings to prevent those of the present
+day from going astray; the lines along which a science has progressed
+in the past may often be used as guides to indicate how work is to be
+conducted in the future; but perhaps the greatest lesson which is
+taught by a careful consideration of the rise and progress of a study
+is one which has a moral value, for he who pays attention to the
+growth of his science in past times, gains a reverence for the old
+masters, and at the same time learns that a slavish regard for
+authority is a dangerous thing. This is a lesson which is of the
+utmost importance to the student who wishes to advance his science,
+and will prevent him from paying too little attention to the work of
+those who have gone before him, whilst it will enable him to perceive
+that as great men have fallen into error through not having sufficient
+data at their disposal, he need not be unduly troubled should he find
+that conclusions which he has lawfully attained after consideration of
+evidence unknown to his predecessors clash with those which they
+adopted. Want of this historic knowledge has no doubt caused many
+workers to waste their time on work which has already been performed,
+but it has also led others to withhold important conclusions from
+their fellow-workers because they were supposed to be heterodox. In an
+uncertain science like geology one of the great difficulties is to
+keep an even balance between contempt and undue respect for authority,
+and assuredly a scientific study of the past history of a science will
+do much to enable a student to attain this end. It will be useful,
+therefore, at this point to give a brief account of the rise and
+progress of the study of stratigraphical geology, so far as that can
+be done without entering into technical details, at the same time
+recommending the student to survey the progress of this branch of our
+science for himself, after he has mastered the principles of the
+subject, and such details as are the property of all who have studied
+the science from the various text-books written for advanced students.
+
+William Smith, the 'Father of English Geology,' is rightly regarded as
+the founder of stratigraphical geology on a true scientific basis, but
+like all great discoverers, his work was foreshadowed by others,
+though so dimly, that this does not and cannot detract from his fame.
+It is desirable, however, to begin our historical review at a time
+somewhat further back than that at which Smith gave to the world his
+epoch-making map and memoirs.
+
+Before the eighteenth century, stratigraphical geology cannot be said
+to have existed as a branch of science--the way had not been prepared
+for it. Data had been accumulated which would have been invaluable if
+at the disposal of open-minded philosophers, but with few exceptions
+prejudice prevented the truth from becoming known. There were two
+great stumbling-blocks to the establishment of a definite system of
+stratigraphical geology by the writers of the Middle Ages, firstly,
+the contention that fossils were not the relics of organisms, and,
+secondly, when it was conceded that they represented portions of
+organisms which had once existed, the assertion that they had reached
+their present positions out of reach of the sea during the Noachian
+Deluge. For full details concerning the mischievous effects of these
+tenets upon the science the reader is referred to the luminous sketch
+of the growth of geology in the first four chapters of Sir Charles
+Lyell's _Principles of Geology_.
+
+The disposition of rocks in strata, and the occurrence of different
+fossils in different strata, was known to Woodward when he published
+his _Essay toward a Natural History of the Earth_ in 1695, and the
+valuable collections made by Woodward and now deposited in the
+Woodwardian Museum at Cambridge, show how fully he appreciated the
+importance of these facts, though he formed very erroneous conclusions
+from them, owing to the manner in which he drew upon his imagination
+when facts failed him, maintaining that fossils were deposited in the
+strata according to their gravity, the heaviest sinking first, and the
+lightest last, during the time of the universal deluge. The following
+extracts from Part II. of Woodward's book, show the position in which
+our knowledge of the strata stood at the end of the seventeenth
+century: "The Matter, subsiding ..., formed the _Strata_ of Stone, of
+Marble, of Cole, of Earth, and the rest; of which Strata, lying one
+upon another, the Terrestrial Globe, or at least as much of it as is
+ever displayed to view, doth mainly consist.... The Shells of those
+Cockles, Escalops, Perewinkles, and the rest, which have a greater
+degree of Gravity, were enclosed and lodged in the _Strata_ of Stone,
+Marble, and the heavier kinds of Terrestrial Matter: the lighter
+Shells not sinking down till afterwards, and so falling amongst the
+lighter Matter, such as Chalk, and the like ... accordingly we now
+find the lighter kinds of Shells, such as those of the _Echini_, and
+the like, very plentifully in Chalk.... Humane Bodies, the Bodies of
+Quadrupeds, and other Land-Animals, of Birds, of Fishes, both of the
+Cartilaginous, the Squamose, and Crustaceous kinds; the Bones, Teeth,
+Horns, and other parts of Beasts, and of Fishes: the Shells of
+Land-Snails: and the Shells of those River and Sea Shell-Fish that
+were lighter than Chalk &c. Trees, Shrubs, and all other Vegetables,
+and the Seeds of them: and that peculiar Terrestrial Matter whereof
+these consist, and out of which they are all formed, ... were not
+precipitated till the last, and so lay above all the former,
+constituting the supreme or outmost _Stratum_ of the Globe.... The
+said _Strata_, whether of Stone, of Chalk, of Cole, of Earth, or
+whatever other Matter they consisted of, lying thus each upon other,
+were all originally parallel: ... they were plain, eaven, and
+regular.... After some time the _Strata_ were broken, on all sides of
+the Globe: ... they were dislocated, and their Situation varied, being
+elevated in some places, and depressed in others ... the Agent, or
+force, which effected this Disruption and Dislocation of the
+_Strata_, was seated _within_ the Earth."
+
+Woodward's writings no doubt exercised a direct influence on the
+growth of our subject, but the indirect effects of his munificent
+bequest to the University of Cambridge and his foundation of the Chair
+of Geology in that University were even greater, for as will be
+pointed out in its proper place, two of the occupants of that chair
+played a considerable part in raising stratigraphical geology to the
+position which it now occupies.
+
+The discoveries which were made after the publication of Woodward's
+book and before the appearance of the map and writings of William
+Smith are given in the memoir of the latter author, written by his
+nephew, who formerly occupied the Chair of Geology at Oxford[1]. It
+would appear that the fact that "the strata, considered as definitely
+extended masses, were arranged one upon another in a certain _settled
+order_ or _series_" was first published by John Strachey in the
+_Philosophical Transactions_ for 1719 and 1725. "In a section he
+represents, in their true order, chalk, oolites, lias, red marls and
+coal, and the metalliferous rocks" of Somersetshire, but confines his
+attention to the rocks of a limited district.
+
+[Footnote 1: _Memoirs of William Smith, LL.D._ By J. Phillips, F.R.S.,
+F.G.S. 1844.]
+
+The Rev. John Michell published in the _Philosophical Transactions_
+for 1760 an "Essay on the Cause and Phænomena of Earthquakes," but
+Prof. Phillips gives proofs that Michell, who in 1762 became
+Woodwardian Professor, had before 1788 discovered (what he never
+published) the first approximate succession of the Mesozoic rocks, in
+the district extending from Yorkshire to the country about Cambridge.
+Michell's account was discovered written by Smeaton on the back of a
+letter dated 1788. The following is the succession as quoted in
+Phillips' memoir (p. 136):
+
+                                     Yards of thickness.
+   "Chalk                                 120
+    Golt                                   50
+    Sand of Bedfordshire                   10 to 20
+    Northamptonshire lime and Portland
+      lime, lying in several strata       100
+    Lyas strata                            78 to 100
+    Sand of Newark                        about 30
+    Red Clay of Tuxford, and several      100
+    Sherwood Forest pebbles and gravel     50 unequal
+    Very fine white sand                  uncertain
+    Roche Abbey and Brotherton limes      100
+    Coal strata of Yorkshire              --"
+
+The order of succession of the Cretaceous, Jurassic, Triassic and
+Permian beds will be readily recognised as indicated in this section,
+though the discovery of the detailed succession of the Jurassic rocks
+was reserved for Smith.
+
+In the year 1778, John Whitehurst published _An Inquiry into the
+Original State and Formation of the Earth_, containing an Appendix in
+which the general succession of the strata of Derbyshire is noted. The
+main points of interest are that the author clearly recognised the
+'toad-stones' of Derbyshire as igneous rocks, "as much a _lava_ as
+that which flows from Hecla, Vesuvius, or Ætna," though he believed
+that they were intrusive and not contemporaneous, and he also
+foreshadows the distinction between the solid strata and the
+superficial deposits,--"we may conclude," he says, "that all beds of
+sand and gravel are assemblages of adventitious bodies and not
+original _strata_: therefore wherever sand or gravel form the surface
+of the earth, they conceal the original _strata_ from our observation,
+and deprive us of the advantages of judging, whether coal or limestone
+are contained in the lower regions of the earth, and more especially
+in flat countries where the _strata_ do not basset."
+
+Werner, who was born in 1750, exercised more influence by his teaching
+than by his writings. His ideas of stratigraphical geology were
+somewhat vitiated by his theoretical views concerning the deposition
+of sediment from a universal ocean, in a definite order, beginning
+with granite, followed by gneiss, schists, serpentines, porphyries and
+traps, and lastly ordinary sediments. He recognised and taught that
+these rocks had a definite order "in which the remains of living
+bodies are successively accumulated, in an order not less determinate
+than that of the rocks which contain them[2]." The limited value of
+Werner's stratigraphical teaching is accounted for by Lyell, who
+remarks that "Werner had not travelled to distant countries; he had
+merely explored a small portion of Germany, and conceived and
+persuaded others to believe that the whole surface of our planet, and
+all the mountain-chains in the world, were made after the model of his
+own province," and the author of the _Principles_ justly calls
+attention to the great importance of travel to the geologist. Those
+who cannot travel extensively should at any rate pay special attention
+to the works published upon districts other than their own, and even
+at the present time, the writings of some British workers are apt to
+be marked by some of that 'insularity' which our neighbours regard as
+a national characteristic.
+
+[Footnote 2: Cuvier's _Eloge_.]
+
+It is now time to turn directly to the work of William Smith, who, of
+all men, exercised the most profound influence upon the study of
+stratigraphical geology and may indeed be regarded as the true founder
+of that branch of the science. The memoir of his life which was
+before mentioned is all too short to illustrate the methods of work
+which he followed, but in it we can trace his success to three
+things:--firstly, his 'eye for a country,' to use a phrase which is
+thoroughly understood by practical geologists, though it is hard
+to explain to others, inasmuch as it epitomises a number of
+qualifications of which the most important are, a ready recognition of
+the main geological features from some coign of vantage, an intuitive
+perception of what to note and what to neglect, and the power of
+storing up acquired information in the mind rather than the note-book,
+so that one may use it almost unconsciously for future work; secondly,
+ability to draw conclusions from his observations, and thirdly, and
+perhaps most important of all in its ultimate results, a facility for
+checking these conclusions by means of further observations, and
+dropping those which were clearly erroneous, whilst extracting the
+truth from those which contained a germ of truth mixed with error.
+
+Besides writers referred to above "some foreign writers, in particular
+Scilla and Rouelle, appear to have made very just comparisons of the
+natural associations of fossil shells, corals, &c. in the earth, with
+the groups of similar objects as they are found in the sea, and thus
+to have produced new proofs of the organic origin of these fossil
+bodies; but they give no sign of any knowledge of the _limitation of
+particular tribes of organic remains to particular strata_, of the
+_successive existence of different groups of organization_, on
+_successive beds of the antient sea_. Mr Smith's claim to this happy
+and fertile induction is clear and unquestionable[3]." We get a clue
+to the manner in which he arrived at his view in the following
+passage[4]:--"Accustomed to view the surfaces of the several strata
+which are met with near Bath uncovered in large breadths at once, Mr
+Smith saw with the distinctness of certainty, that 'each stratum had
+been in succession the bed of the sea'; finding in several of these
+strata abundance of the exuviae of marine animals, he concluded that
+these animals had lived and died during the period of time which
+elapsed between the formation of the stratum below and the stratum
+above, at or near the places where now they are imbedded; and
+observing that in the successively-deposited strata the organic
+remains were of different forms and structures--Gryphites in the lias,
+Trigoniæ in the inferior oolite, hooked oysters in the fuller's
+earth,--and finding these facts repeated in other districts, he
+inferred that each of the separate periods occupied in the formation
+of the strata was accompanied by a peculiar series of the forms of
+organic life, that these forms characterized those periods, and that
+the different strata could be identified in different localities and
+otherwise doubtful cases by peculiar imbedded organic remains[5]."
+
+[Footnote 3: _Memoir of William Smith_, p. 142.]
+
+[Footnote 4: _Ibid._ p. 141.]
+
+[Footnote 5: The work of Smith which directly bears upon the
+establishment of the law of identification of strata by included
+organisms is published in two treatises, entitled:--
+
+(i) _Strata identified by Organized Fossils_, 4to. (intended to
+comprise seven parts, of which four only were published), commenced in
+1816.
+
+(ii) _A Stratigraphical System of Organized Fossils_, compiled from
+the original Geological Collection deposited in the British Museum.
+4to. 1817.]
+
+William Smith seems to have recognised intuitively the truth of a law
+which was but dimly understood before his time,--the law of
+superposition, which may be thus stated: "of any two strata, the one
+which was originally the lower, is the older." This may appear
+self-evident but it was certainly not so. As the result of this
+recognition he established the second great stratigraphical law, with
+which his name will ever be linked, that strata are identifiable by
+their included organisms.
+
+Before Smith's time, geological maps were lithological rather than
+stratigraphical, they represented the different kinds of rocks seen
+upon the surface without regard to their age; since Smith
+revolutionised geology, the maps of a country composed largely of
+stratified rocks are essentially stratigraphical, but partly no doubt
+on account of adherence to old custom, partly on economic grounds, the
+majority of our stratigraphical maps are lithological rather than
+palæontological, that is the subdivisions of the strata represented
+upon the map are chosen rather on account of lithological
+peculiarities than because of the variations in their enclosed
+organisms. It is hardly likely that Government surveys will be allowed
+to publish palæontological maps, which will be almost exclusively of
+theoretical interest, and it remains for zealous private individuals
+to accomplish the production of such maps. When they are produced, a
+comparison of stratigraphical maps founded on lithological and
+palæontological considerations will furnish results of extreme
+scientific interest.
+
+Turning now from Smith's contributions to the science as a whole, we
+may now consider what he did for British geology. His geological map
+was published in 1815 and was described as follows:--"A Geological Map
+of England and Wales, with part of Scotland; exhibiting the
+Collieries, Mines, and Canals, the Marshes and Fen Lands originally
+overflowed by the Sea, and the varieties of Soil, according to the
+variations of the Substrata; illustrated by the most descriptive Names
+of Places and of Local Districts; showing also the Rivers, Sites of
+Parks, and principal Seats of the Nobility and Gentry, and the
+opposite Coast of France. By William Smith, Mineral Surveyor." The map
+was originally on the scale of five miles to an inch. In 1819 a
+reduced map was published, and in later years a series of county maps.
+He also published several geological sections, including one (in 1819)
+showing the strata from London to Snowdon.
+
+The student should compare Smith's map of the strata with one
+published in modern times in order to see how accurate was Smith's
+delineation of the outcrop of the later deposits of our island.
+
+The following table, taken from Phillips' memoir, p. 146, is also of
+interest as showing the development of Smith's work and the
+completeness of his classification in his later years, and as
+illustrating how much we are indebted to Smith for our present
+nomenclature, so much so that as Prof. Sedgwick remarked when
+presenting the first Wollaston Medal of the Geological Society to
+Smith, "If in the pride of our present strength, we were disposed to
+forget our origin, our very speech would bewray us: for we use the
+language which he taught us in the infancy of our science. If we, by
+our united efforts, are chiselling the ornaments and slowly raising up
+the pinnacles of one of the temples of nature, it was he who gave the
+plan, and laid the foundations, and erected a portion of the solid
+walls by the unassisted labour of his hands."[6]
+
+[Footnote 6: The reader may consult an interesting paper by Professor
+Judd, on "William Smith's Manuscript Maps," _Geological Magazine_,
+Decade IV. vol. IV. (1897) p. 439.]
+
+Comparative View of the Names and Succession of the Strata.
+
+  --------------------+-------------------------+--------------------------
+                      |                         | Improved table drawn up
+      Table drawn up  |  Table accompanying the | in 1815 and 1816 after
+         in 1799.     |  map, drawn up in 1812. | the first copies of the
+                      |                         | map had been issued.
+  --------------------+-------------------------+--------------------------
+                      | London Clay             |  1 London Clay
+                      | Clay or Brick-earth     |  2 Sand
+                      |                         |  3 Crag
+                      | Sand or light loam      |  4 Sand
+   1 Chalk            | Chalk                   |  5 Chalk { Upper
+                      |                         |          { Lower
+   2 Sand             | Green Sand              |  6 Green Sand
+                      | Blue Marl               |  7 Brick Earth
+                      | Purbeck Stone, Kentish {|  8 Sand
+                      |  Rag and Limestone     {|  9 Portland Rock
+                      |  of the vales          {| 10 Sand
+                      |  of Pickering and      {| 11 Oaktree Clay
+                      |  Aylesbury,            {| 12 Coral Rag and Pisolite
+                      | Iron Sand and Carstone {| 13 Sand
+   3 Clay             | Dark Blue Shale         | 14 Clunch Clay and Shale
+                      |                         | 15 Kelloway's Stone
+                      | Cornbrash               | 16 Cornbrash
+   4 Sand and Stone   |                         | 17 Sand and Sandstone
+   5 Clay             |                         |
+   6 Forest Marble    | Forest Marble Rock      | 18 Forest Marble
+                      |                         | 19 Clay over Upper
+                      |                         |     Oolite
+   7 Freestone        | Great Oolite Rock       | 20 Upper Oolite
+   8 Blue Clay       }|                         |
+   9 Yellow Clay     }|                         |
+  10 Fuller's Earth  }|                         | 21 Fuller's Earth and
+                     }|                         |     Rock
+  11 Bastard ditto   }|                         |
+      and Sundries   }|                         |
+  12 Freestone        | Under Oolite            | 22 Under Oolite
+  13 Sand             |                         | 23 Sand
+                      |                         | 24 Marlstone
+  14 Marl Blue        | Blue Marl               | 25 Blue Marl
+  15 Blue Lias        | Blue Lias               | 26 Blue Lias
+  16 White Lias       | White Lias              | 27 White Lias
+  17 Marlstone, Indigo|                         |
+      and Black Marls |                         |
+  18 Red Ground       | Red Marl and Gypsum     | 28 Red Marl
+  19 Millstone        | Magnesian Limestone     | 29 Redland Limestone
+                      | Soft Sandstone          |
+  20 Pennant Street  }|                         |
+  21 Grays           }| Coal Districts          | 30 Coal Measures
+  22 Cliff           }|                         |
+  23 Coal            }|                         |
+                      | Derbyshire Limestone    | 31 Mountain Limestone
+                      | Red and Dunstone        | 32 Red Rhab and Dunstone
+                      | Killas or Slate         | 33 Killas
+                      | Granite, Sienite and    | 34 Granite, Sienite and
+                      |  Gneiss                 |     Gneiss
+  --------------------+-------------------------+--------------------------
+
+The above table contains a very complete classification of the British
+Mesozoic rocks, one of the Tertiary strata which is less complete, and
+a preliminary division of the Palæozoic rocks into Permian (Redland
+Limestone), Carboniferous (Coal Measures and Mountain Limestone),
+Devonian (Red Rhab and Dunstone) and Lower Palæozoic (Killas).
+
+Since Smith's time the main work which has been done in classification
+is a fuller elucidation of the sequence of the Tertiary and Palæozoic
+Rocks, and this we may now consider.
+
+The Mesozoic rocks are developed in Britain under circumstances which
+render the application of the test of superposition comparatively
+simple, for the various subdivisions crop out on the surface over long
+distances, and the stratification is not greatly disturbed. With the
+Tertiary and Palæozoic Rocks it is otherwise, for some members of the
+former are found in isolated patches, whilst the latter have usually
+been much disturbed after their formation.
+
+Commencing with the Tertiary deposits we may note that "the first
+deposits of this class, of which the characters were accurately
+determined, were those occurring in the neighbourhood of Paris,
+described in 1810 by MM. Cuvier and Brongniart.... Strata were soon
+afterwards brought to light in the vicinity of London, and in
+Hampshire, which although dissimilar in mineral composition were
+justly inferred by Mr T. Webster to be of the same age as those of
+Paris, because the greater number of fossil shells were specifically
+identical[7]." It is to Lyell that we owe the establishment of a
+satisfactory classification of the Tertiary deposits which is the
+basis of later classifications. Recognising the difficulty of
+applying the ordinary test of superposition to deposits so scattered
+as are those of Tertiary age in north-west Europe, he in 1830,
+assisted by G. P. Deshayes, proposed a classification based on the
+percentage of recent mollusca in the various deposits. It may be
+noted, that although this method was sufficient for the purpose, it
+has been practically superseded, as the result of increase of our
+knowledge of the Tertiary faunas, by the more general method of
+identifying the various divisions by their actual fossils without
+reference to the number of living forms contained amongst them. The
+further study of the British Tertiary rocks was largely carried on by
+Joseph Prestwich, formerly Professor of Geology in the University of
+Oxford.
+
+[Footnote 7: Lyell, _Students' Elements of Geology_. 2nd Edition, p.
+118.]
+
+Amongst the Palæozoic rocks, it has been seen that the Permian,
+Carboniferous and some of the Devonian beds were recognised as
+distinct by Smith, though a large number of deposits now known to
+belong to the last named were thrown in with other rocks as 'killas.'
+The Devonian system was established and the name given to it in 1838
+by Sedgwick and Murchison, largely owing to the palæontological
+researches of Lonsdale. An attempt was subsequently made to abolish
+the system, but the detailed palæontological studies of R. Etheridge
+finally placed it upon a secure basis. The establishment of the
+Devonian system cleared the way for the right understanding of the
+Lower Palæozoic rocks, which Sedgwick and Murchison had commenced to
+study before the actual establishment of the Devonian system, and to
+these workers belongs the credit of practically completing what was
+begun by William Smith, namely, the establishment of the Geological
+Sequence of the British strata. The controversy which unfortunately
+marked the early years of the study of the British Lower Palæozoic
+Rocks is well-nigh forgotten, and in the future the names of Sedgwick
+and Murchison will be handed down together, in the manner which is
+most fitting.
+
+Our account of the growth of British Stratigraphical Geology is not
+yet complete. In 1854, Sir William Logan applied the term Laurentian
+to a group of rocks discovered in Canada, which occurred beneath the
+Lower Palæozoic Rocks. Murchison shortly afterwards claimed certain
+rocks in N.W. Scotland as being of generally similar age, and since
+then a number of geologists, most of whom are still living, have
+proved the occurrence of several large subdivisions of rocks in
+Britain, each of which is of pre-Palæozoic age.
+
+The above is a brief description of the growth of our knowledge of the
+order of succession of the strata which is the foundation of
+Stratigraphical Geology. A sketch of the manner in which the knowledge
+which has been obtained has been applied to the elucidation of the
+earth's history of different times would require far more space than
+can be devoted to it in a work like the present, but some idea of it
+may be gained from a study of the later chapters of the book. It will
+suffice here to remark, that to Godwin-Austen we owe the foundation of
+what may be termed the physical branch of Palæo-physiography, which is
+concerned with the restoration of the physical conditions of past
+ages, while Cuvier and Darwin have exerted the most influence on the
+study of Stratigraphical Palæontology.
+
+
+
+
+CHAPTER III.
+
+NATURE OF THE STRATIFIED ROCKS.
+
+
+The present constituents of the earth which are accessible for direct
+study are divisible into three parts. The inner portion, consisting of
+_rocks_, is known as the _lithosphere_; outside this, with portions of
+the lithosphere projecting through into the outermost part, is the
+_hydrosphere_, comprising the ocean, lakes, rivers, and all masses of
+water which rest upon the lithosphere in a liquid condition. The
+outermost envelope, which is continuous and unbroken is the
+_atmosphere_, in a gaseous condition. It is well known that some of
+the constituents of any one of these parts may be abstracted from it,
+and become a component of either of the others; thus the atmosphere
+abstracts aqueous vapour from the hydrosphere, and the lithosphere
+takes up water from the hydrosphere, and carbonic anhydride from the
+atmosphere.
+
+The nebular hypothesis of Kant and Laplace necessitates the former
+existence of the present solid portions of the lithosphere in a molten
+condition, and accordingly the first formed solid covering of the
+lithosphere, if this hypothesis be true, must have been formed from
+molten material, or in the language of Geology, it was an _igneous
+rock_. Consequently, the earliest _sedimentary rock_ was necessarily
+derived directly from an igneous rock, with possible addition of
+material from the early hydrosphere and atmosphere, and all
+subsequently formed sedimentary rocks have therefore been derived from
+igneous rocks (with the additions above stated) either directly, or
+indirectly through the breaking up of other sedimentary rocks which
+were themselves derived directly or indirectly from igneous rocks. The
+observations of geologists show that this supposition that the
+materials of sediments have been directly or indirectly obtained for
+the most part from once-molten rocks is in accordance with the
+observed facts, and so far their observations testify to the truth of
+the nebular hypothesis. This being the case, the study of the
+petrology of the igneous rocks is necessary, in order to arrive at a
+true understanding of the composition of the sedimentary ones. The
+igneous rocks are largely composed of four groups of minerals,
+viz.--quartz, felspars, ferro-magnesian minerals, and ores. Of these
+the quartz (composed of silica) yields particles of silica for the
+formation of sedimentary rocks; the felspars, which are double
+silicates of alumina and an alkali or alkaline earth, being prone to
+decomposition furnish silicate of alumina and compounds of soda,
+potash, lime, &c. The ferro-magnesian minerals (such as augite,
+hornblende and mica) may undergo a certain amount of decomposition,
+and yield compounds of iron, lime, &c. We may also have fragments of
+any of these minerals, and of the ore group in an unaltered condition.
+The composition of a sedimentary rock which has undergone no
+alteration after its formation will therefore depend upon the
+character of the rock from which it was derived, the chemical changes
+which take place in the materials which compose it, before they enter
+into its mass, and the mechanical sorting which they undergo prior to
+their deposition.
+
+In the above passage the terms igneous rock and sedimentary rock have
+been used, and it is necessary to give some account of the sense in
+which they were used.
+
+An _igneous_ rock is one which has been _consolidated_ from a state of
+_fusion_. It is not necessary to discuss here the exact significance
+of the word fusion, and whether certain rocks which are included in
+the igneous division were formed rather from solution at high
+temperature than from actual fusion. This point is of importance to
+the petrologist, but to the student of stratigraphical geology the
+term igneous rock may be used in its most comprehensive sense. These
+igneous rocks were consolidated either upon the surface of the
+lithosphere or in its interior.
+
+The other great group of rocks is one to which it is difficult to
+apply a satisfactory name. They have been termed by different writers,
+_sedimentary_, _stratified_, _derivative_, _aqueous_, and _clastic_,
+but no one of these terms is strictly accurate. The term _sedimentary_
+implies that they have settled down, at the bottom of a sheet of water
+for instance. It can hardly be maintained that limestones formed by
+organic agency, like the limestones of coral reefs, are sedimentary in
+the strict sense of the term, and an accumulation like surface-soil
+can only be called a sediment by straining the term. _Stratified_
+rocks are those which are formed in strata or layers, but many of the
+rocks which we are considering do not show layers on a small scale,
+and igneous rocks (such as lava-flows) are also found in layers,
+though such layers are not true strata in the sense in which the term
+is used by geologists; the term _stratified_ is perhaps the least open
+to objection of any of those named above. _Derivative_ implies that
+the fragments have been derived from some pre-existing rock, but as
+there are many ways in which fragments of one rock may be derived from
+another, the term is too comprehensive. _Aqueous_ rocks should be
+formed in water, and most of the class of rocks which we are
+considering have been so formed, but others such as sand-dunes and
+surface-soil have not. (The term Aerial or Æolian has been suggested
+to include these rocks which are thus separated from the Aqueous rocks
+proper; the objection to this is that the origin of these rocks is
+closely connected with that of the true Aqueous rocks, and moreover
+the group is too small to be raised to the dignity of a separate
+subdivision.) Lastly, the name _clastic_ has been given, because the
+rocks so called are formed by the _breaking up_ of pre-existing rocks.
+There are two objections to this name. In the first place, some rocks
+included under the head clastic are formed by solution of material and
+its consolidation from a state of solution by chemical or organic
+agency, though we may perhaps speak of rocks being broken up by
+chemical as well as by mechanical action. The most important objection
+is that many clastic rocks are formed by the breaking up of rocks
+subsequently to their formation, and it has been proposed that rocks
+of this nature should be termed _cataclastic_, while those which are
+formed by the breaking up of pre-existing rocks upon the earth's
+surface should be termed _epiclastic_; another group formed of
+materials broken up within the earth, and accumulated upon its surface
+as the result of ejection of fragmental material from volcanic vents
+being termed _pyroclastic_. This classification is scientific, and
+under special circumstances is extremely useful, but the older terms
+have been used so generally, and with this explanation their use is so
+unobjectionable, that they may be retained, and the term _stratified_
+will be generally used to indicate all rocks which are not of igneous
+origin or formed as mineral veins in the earth's interior.
+
+The division of rocks into _three_ great groups, the Igneous,
+Stratified and Metamorphic (the latter name being applied to those
+rocks which have undergone considerable alteration since their
+formation), is objectionable, since we have metamorphic igneous rocks
+as well as metamorphic stratified ones. The most convenient
+classification is as follows:--
+
+  A. Igneous       1. { Unaltered.
+                   2. { Metamorphic.
+
+  B. Stratified    1. { Unaltered.
+                   2. { Metamorphic.
+
+It must be distinctly understood that all geological phenomena must be
+taken into account by the stratigraphical geologist. The upheaval of
+strata, the production of jointing and cleavage in them, their
+intrusion by igneous material, their metamorphism, give indications of
+former physical conditions equally with the lithological characters of
+the strata, and their fossil contents. Nevertheless it is not proposed
+to give a full account of the various phenomena displayed by rocks;
+the student is referred to Text-books of General Geology for this
+information. It will be as well here, however, to point out in a few
+words the exact significance of the existence of strata in the
+lithosphere.
+
+The formation of strata and their subsequent destruction to supply
+material for fresh strata are due to three great classes of changes.
+Beginning with a portion of lithosphere composed of rock, it is found
+that rock is broken up by agents of denudation, as wind, rain, frost,
+rivers and sea. These agents perform their function mainly upon the
+portion of the lithosphere which projects through the hydrosphere to
+form _land_, and the land is the main area of denudation. The
+materials furnished by denudation are carried away, and owing to
+gravitation, naturally proceed from a higher to a lower level, often
+resting on the way, but if nothing else occurs, ultimately finding
+their way to the _sea_, where they are deposited as strata. The sea is
+the principal area for the reception of this material, and it is there
+accordingly that the bulk of stratified rock is formed. If nothing
+else occurred, in time the whole of the land would be destroyed, and
+the wreckage of the land deposited beneath the sea as stratified rock.
+As it is there is a third class of change, underground change, causing
+movements of the earth's crust (to use a term which can hardly be
+defined in few words but which is generally understood), and as the
+result of the relative uplift of portions of the earth's crust, the
+stratified rocks formed beneath the oceans are raised above its level,
+giving rise to new masses of land, which are once more ready for
+destruction by the agents of denudation. This cycle of change (all
+parts of which are ever proceeding simultaneously) is one of the
+utmost importance to the stratigraphical geologist.
+
+_Stratification_ is the rock-structure of prime importance in
+stratigraphical geology, and a few words must here be devoted to its
+consideration, leaving further details to be dealt with hereafter. The
+surface of the ocean-floor is, when viewed on a large scale, so level,
+that it may be considered practically horizontal, and accordingly in
+most places the materials which are laid down on the ocean-floor give
+rise to accumulations which at all times have a general horizontal
+surface (when the ocean-slopes depart markedly from horizontality the
+deposits tend to abut against these slopes rather than to lie with
+their upper surfaces parallel to their original angle). A practically
+horizontal surface of this character may give rise to a _plane of
+stratification_ (or _bedding-plane_) in more than one way. A pause may
+occur during which there is a cessation of the supply of material, so
+that the material which has already been accumulated has sufficient
+time to become partially consolidated before the deposition of fresh
+material upon it. In this way a want of coherence between the two
+masses is produced, along the plane of junction, which after
+consolidation of the deposits causes an actual divisional plane along
+which the two deposits may be separated. This is a plane of
+stratification. The pause may be produced in various ways, sometimes
+between successive high tides, at others as the result of physical
+changes which may have taken ages to happen. Again, after material of
+one kind has been deposited, say sand, some other substance such as
+clay may be accumulated on its upper surface, giving rise to a plane
+of stratification between two deposits of different lithological
+characters. If this occurs alone, there may be actual coherence
+between the two strata, so that it is erroneous to speak of a plane of
+stratification as if it were always one along which one deposit could
+be readily split from the other, though as a general though by no
+means universal rule, change from one kind of deposit to another is
+also marked by want of coherence between the two. The material between
+two planes of stratification forms a _stratum_ or _bed_, though if the
+deposit be very thin it is known as a _lamina_, and the planes are
+spoken of as _planes of lamination_ (no hard and fast line can be
+drawn between strata and laminæ; several of the latter usually occur
+in the space of an inch).
+
+A _stratum_ will have its upper and lower surface apparently parallel,
+though not really so, for no stratum extends universally round the
+earth, and many of them disappear at no great distance when traced in
+any direction. Parts of one stratum may be composed of different
+materials from other parts when traced laterally, thus one stratum may
+be found composed essentially of sand in one place, of mud in another,
+and of a mixture of the two in an intervening locality. Whatever be
+the composition of a stratum it dies out eventually, owing to the
+coming together of the upper and lower bounding planes of
+stratification. The stratum is thickest at some spot, from that spot
+it becomes thinner in all directions, until it disappears at last by
+the coalescence of the bounding-planes. This is spoken of as
+_thinning-out_. Strata, then, consist of lenticular masses of rock,
+separated from the underlying and overlying strata by planes of
+stratification. The shape of the lenticle may vary immensely, the
+thickness bearing no definite relationship to the horizontal extent.
+Some strata, many feet in thickness, may thin out and disappear
+completely in the course of a few yards, whilst others an inch or two
+in thickness may be traced horizontally for many miles. We often find
+thin strata of coal and limestone, extending for great distances,
+strata of mud thinning out more rapidly, and sandstones still more
+rapidly, but no universal rule connecting rapidity of thinning-out
+with composition of the strata can be laid down.
+
+Having seen what a stratum is, it now remains to speak of the
+composition of the stratified rocks. They have been classified
+according to their composition, and according to their origin.
+According to composition they have been divided into:
+
+    _Arenaceous_ rocks, composed essentially of grains of sand.
+
+    _Argillaceous_ rocks, composed essentially of particles of
+       mud.
+
+    _Calcareous_ rocks, composed essentially of particles of
+       carbonate of lime.
+
+    _Carbonaceous_ rocks, composed largely of hydrocarbon
+       compounds.
+
+    _Siliceous rocks_, composed essentially of silica not in the
+       form of grains;
+
+whilst according to their origin they have been separated into:--
+
+    _Mechanically-formed_ rocks, composed of fragments derived
+       from other rocks by mechanical fracture.
+
+    _Chemically-formed_ rocks, composed of particles which have
+       been chemically deposited from a state of solution.
+
+    _Organically-formed_ rocks, composed of materials which have
+       been derived from a state of solution or from the gaseous
+       condition by the agency of organisms.
+
+Whichever classification be adopted (and each is useful for special
+purposes), it must be noted that no hard and fast line can be drawn
+between one division and another. A rock may be partly arenaceous and
+partly calcareous, composed of a mixture of sand and lime, and the
+same rock may similarly be partly mechanically and partly organically
+formed, the sand being due to mechanical fracture, and the lime to the
+agency of organisms, and so with the other divisions.
+
+As many of the changes which have occurred in past times have been
+concerned in destruction and obliteration, whilst deposition is the
+cause of preservation, the study of deposits is peculiarly adapted for
+testing the truth of the grand principle of geology that the changes
+which have taken place in past times are generally speaking similar in
+kind and in intensity of action to those which are in progress at the
+present day, and a study of the modern deposits is specially important
+as throwing light upon the characters of those which have been formed
+in past times. It will be abundantly shown in the sequel that the
+deposits of the strata are in general comparable in all essential
+respects with those which are being formed at present, and accordingly
+they give most valuable indications as to the nature of the physical
+and other conditions under which they were laid down. The desert sand,
+the precipitate of the inland sea, the reef-limestone and many another
+deposit can thus be detected by an examination of their lithological
+characters, combined with consideration of other kinds of evidence.
+The petrology of the sedimentary rocks is still in its infancy, though
+much has already been done, but it offers a wide field of inquiry to
+the field-geologist and worker with the microscope[8].
+
+[Footnote 8: The student will do well to consult _The Challenger
+Report_ by Messrs Murray and Renard (1891), for information concerning
+many modern sediments, and Harker's _Petrology for Students_ Section
+D, for general information on the Petrology of the Stratified Rocks.]
+
+
+
+
+CHAPTER IV.
+
+THE LAW OF SUPERPOSITION.
+
+
+In a previous chapter this law was given as follows: "Of any two
+strata, the one which was originally the lower is the older;" the
+general truth of the law depends upon the fact that except under very
+exceptional circumstances the strata are deposited upon the surface of
+the lithosphere, and not beneath it. There are occasions where strata
+may be deposited beneath the lithosphere, but as a general rule the
+geologist will not be misled by such occurrences. In caverns,
+accumulations often occur which are newer than the strata over the
+cavern roof, and so long as caverns are formed in ordinary sedimentary
+rocks, no great difficulty will result from this exception to the law
+of superposition. When caverns occur beneath masses of land ice, the
+order of superposition may be misleading. A deposit may be formed on
+the surface of the ice, and subsequently to this a newer deposit may
+be laid down in a sub-glacial or englacial cavern; upon the melting of
+the ice the newer deposit would be found with the older one resting
+upon its surface.
+
+Apart from these exceptional cases, the law as stated holds good, but
+the reader will notice the insertion of the word 'originally' which
+requires some comment.
+
+A geologist speaks of one bed lying _upon_ another not only when the
+beds are horizontal, but when they are inclined at any angle, until
+they become vertical, so that until beds have been turned through an
+angle of 90° by earth-movement the test of superposition is
+applicable, but when they have been turned more than 90°, the stratum
+which was originally lower rests upon that which was originally above
+it, and in the case of these _inverted_ strata, the test of
+superposition is no longer applicable. It was formerly supposed that
+cases of inversion were comparatively rare and local, and that the
+test of superposition could therefore be generally applied with
+confidence, but it is now known that though this is generally true of
+such strata as have been subjected only to those widespread, fairly
+uniform movements which are spoken of as _epeirogenic_ or
+continent-forming, where the radius of each curve is very long,
+inversion is a frequent accompaniment of the more local _orogenic_ or
+mountain-forming movements, where the radius of a curve is short.
+Though orogenic movements are limited as compared with those of
+epeirogenic character, they often affect large tracts of country, in
+which case the apparent order of succession of the strata need not be
+the true one, and examples of inversion may be frequent[9].
+
+[Footnote 9: For a discussion of the principles of mountain-building
+see Heim, A., _Untersuchungen über den Mechanismus der
+Gebirgsbildung_, and Lapworth, C. "The Secret of the Highlands,"
+_Geological Magazine_, Decade II. vol. x. pp. 120, 193, 337.]
+
+It is not easy to lay down any definite rules for detecting inverted
+strata, where the top of an inverted arch is swept off by denudation
+or the bottom of an inverted trough concealed beneath the surface,
+beyond stating that if an easily recognised set of beds is obviously
+repeated in inverse order, inversion must have occurred, though even
+then it may not be clear which side of the fold shows the beds in
+original and which in inverted sequence. Suggestions are frequently
+made that ripple-marks and worm-tracks may be utilised in order to
+discover inversion, for the well-formed ripple-marks will appear
+convex on the upper surface of a bed which is not inverted, and we may
+note concave casts of these ripple-marks on the under surface of the
+overlying bed, whilst worm-tracks will appear concave on the upper
+surface, and their casts convex on the lower surface of the succeeding
+bed under similar conditions. In the case of inversion the occurrences
+will be the exact opposite to these. Unfortunately ripple-marks and
+worm-tracks may, as will appear in the sequel, be simulated by
+structures produced in quite a different way, and unless the observer
+is certain that he is confronted with true ripple-marks and
+worm-tracks he may be seriously misled. The geologist must take into
+account all the evidence at his disposal, when he is dealing with
+cases of possible inversion, but oftentimes he will after due
+consideration of all the phenomena be left in doubt unless he is able
+to supplement his observations on the succession of the strata by
+evidence derived from the included fossils.
+
+The test of superposition is most apt to be misleading when the strata
+have been affected by the faults known as reversed faults or
+thrust-planes.
+
+Reference to text-books will show that a fold consists of two parts,
+the arch and the trough, and that the two are connected by a common-,
+middle-, or partition-limb. In the case of an inverted fold, an
+=S=-shaped or sigmoidal structure is the result (Fig. 1 A).
+
+[Illustration: Fig. 1.
+
+A. A sigmoidal fold, showing a bed _xx_ in an overfold with arch
+(_a_), trough (_t_) and common limb _c_.
+
+B. A similar bed _xx_ affected by a thrust-plane _tt_ which replaces
+the common limb.]
+
+Here the portions of any bed (_xx_) which occur in the arch or trough
+are in normal position, and have not been moved round through an angle
+of 90°, whilst the portion which occurs in the common limb c has been
+moved round through an angle greater than 90° and is inverted, so that
+its former upper surface now faces downwards. In Fig. 1 B the common
+limb is replaced by a reversed fault, or thrust-plane, and the
+inverted portion of the bed seen in the common limb is therefore
+absent. An observer, applying the test of superposition, might suppose
+that the position of the bed _x_ on the left-hand side of the figure
+was a different bed from the portion which is seen on the right-hand
+side, instead of belonging to the same bed, and in this way, if a
+number of parallel thrust-planes affected one bed or a set of beds, he
+might be led to infer the occurrence of a great thickness of strata
+where there was in reality a slight thickness, or even one bed only
+repeated again and again by faulting. It is quite certain that
+exaggerated estimates of the thickness of deposits have frequently
+been made owing to the non-recognition of the occurrence of repetition
+as a consequence of the existence of thrust-planes.
+
+Where thrust-planes are suspected, it is well to look for some of the
+following features:
+
+(_a_) The strata of a country affected by thrust-planes often crop out
+as lenticular masses, thinning out rapidly in the direction of the
+strike[10]. This is true of beds thrown into sharp folds whether or
+not inverted, but the lenticles will be wider in a direction at right
+angles to that of the strike as compared with their length when
+inversion has not occurred. It is also true of beds which were
+originally deposited as lenticles, such as many massive sandstones,
+and as almost any kind of deposit may be formed originally as a
+lenticle, the test by itself is by no means sufficient as a proof of
+thrusting, though it is suggestive.
+
+[Footnote 10: For definitions of the terms dip, strike, outcrop and
+allied expressions, the reader is referred to a _Text-Book of
+Geology_.]
+
+(_b_) The _surfaces_ of the strata are often affected by the
+striations known as slickensides, and the joint-faces of gently
+inclined beds are also frequently marked by slickensides which often
+run in a nearly horizontal direction.
+
+(_c_) A parallel structure presenting the appearances characteristic
+of the mechanically-formed features of a foliated rock is often
+developed, and one or more of certain accompanying phenomena will
+probably be found, which will be noticed more fully in a later
+chapter.
+
+(_d_) Extension or stretching of the rocks will have been frequently
+produced, causing rupture, and the resulting fissures are usually
+filled with mineral-veins, though this occurrence is by no means
+characteristic of rocks which have been affected by thrust-planes.
+
+(_e_) Chemical changes may have occurred which have resulted in the
+reconstitution of some of the rock-constituents, which may crystallise
+where pressure is least, thus we often find rocks which have undergone
+movements of the type we are considering marked by the existence of
+sericitic films upon the surfaces.
+
+Another reservation must be made when considering the law of
+superposition. The test is only applicable for limited areas. Suppose
+we find a deposit of clay _a_ resting upon another deposit of
+limestone _b_ in the south of England, and can prove that the apparent
+succession is the true one, that is, that there has been no inversion;
+it is clear that the test of superposition is applicable in that area.
+Now, we may be able to trace the two deposits continuously across the
+country, one as a clay, the other as a limestone; so that when we
+reach the north of England we find the clay _a_ still reposing upon
+the limestone _b_. The test of superposition is applicable in that
+area also, the clay of the northern area being newer than the
+limestone of the same region. But, for reasons which will ultimately
+appear, it by no means follows that the clay of the north is newer
+than the limestone of the south, although the two deposits are
+continuously traceable with the same lithological characters; it may
+have been formed simultaneously with the limestone of the south, or
+even before it. Something more, therefore, than the test of
+superposition is necessary in order to make out the relative ages of
+continuous deposits in a wide region, and this is still truer in the
+case of deposits which are discontinuous, whether separated from one
+another by the sea, or by outcrops of older or newer rocks.
+
+A few words of warning may be added with reference to the detection of
+bedding-planes. A bedding-plane is one which separates two beds, and
+its existence is determined during the deposition of the beds. Many
+other planes are formed in rocks subsequently to their deposition, and
+it is not always easy to distinguish these from true bedding-planes.
+That even experienced observers may be led astray is shown by the fact
+that, of recent years, it has been proved that great masses of rock
+have been claimed as of sedimentary origin, and their apparent order
+of succession noted, which are in truth naught but irregular masses of
+intrusive igneous rocks affected by divisional planes which simulate
+bedding, produced in the rocks subsequently to their consolidation.
+Joints, faults, and cleavage-planes may all at times simulate planes
+of bedding, and it is frequently very difficult to distinguish them in
+the limited exposures with which a geologist has oftentimes to deal.
+It is easier to make suggestions for distinguishing bedding-planes
+from other planes which simulate them, than to apply the suggestions
+in practice, and the student of field geology will find that
+experience is the only guide, though after years of experience he may
+be confronted with cases where the evidence is insufficient to
+convince him that he is dealing with planes of stratification and not
+with some other structure.
+
+From what has been remarked, it will be inferred that the test of
+superposition though of prime importance to the geologist is
+frequently insufficient to enable him to ascertain the true order of
+succession of the strata, and he is compelled to supplement this test
+by some other. There are several useful physical tests which may
+frequently be applied. Thus, if a rock _a_ contains fragments of
+another rock _b_, _under such circumstances as to show that the
+fragments of_ b _were included in a during its deposition_, it is
+clear that _b_ is older than _a_. Here again, it will be found from
+what appears in a later chapter that the student is confronted with
+difficulties when actually examining rocks, for fragmental rocks of
+cataclastic origin, where the fragments have been formed as the result
+of fracture produced by earth-movements subsequently to the deposition
+of the rock, simulate epiclastic rocks in which the fragments were
+introduced during the accumulation of the deposits to so surprising a
+degree as sometimes to baffle the most experienced observer. Not only
+are the fragments of these cataclastic rocks broken up, but they may
+be further rounded so as to imitate in a remarkable manner the
+water-worn pebbles of an epiclastic conglomerate. Again, an older
+series of rocks may have had structures impressed upon them as the
+result of changes subsequent to their formation, and before the
+formation of a newer set which the latter therefore do not exhibit.
+Jointing, cleavage, and various metamorphic phenomena may thus be
+exhibited by the older rocks, but great care is required in applying
+this test, especially with a limited thickness of rocks, as one set
+may not exhibit the structures not because they were not in existence
+when the structures were developed, but because their nature is such
+that they were incapable of receiving or retaining the structures. For
+instance a mass of grit which is older than a mass of clay-slate may
+not be cleaved, because, although subjected to the pressure which
+produced the cleavage, it was of a nature not adapted to the
+development of cleavage structure.
+
+On the whole, application of tests dependent upon physical features of
+rocks, does not often supplement to any great extent the information
+supplied by ascertaining the order of superposition, and in all
+cases, where possible, every other kind of information should be
+supplemented, by that which is acquired after examination of the
+included organisms of the strata.
+
+
+
+
+CHAPTER V.
+
+THE TEST OF INCLUDED ORGANISMS.
+
+
+The second great law of the Stratigraphical Geologist is that
+fossiliferous strata are identifiable by their included organisms, in
+other words, that we can tell the geological age of deposits by
+examination of the fossils contained in them, though the determination
+of age must be given in more general terms in some cases than in
+others. Considerable misconception has arisen concerning the value of
+fossils as indices of age, and it is necessary therefore to discuss
+the significance of the law of identification of strata by their
+included organisms at some length.
+
+The comparison between fossils and medals has frequently been made and
+fossils have well been styled the "Medals of Creation"; and the
+significance of fossils as guides to the age of deposits may perhaps
+be made clearer if we pursue this comparison some way. In the first
+place there is clear indication of a gradual increase in the
+complexity of organisation of the fossils as one passes from the
+earlier to the later rocks, and accordingly the general facies of a
+fauna is likely to furnish a clue to the age of the rocks in which it
+is found, even though every species or even genus represented in the
+fauna was previously unknown to science. So an antiquary versed in the
+evolution of art or metallurgy, might detect the general age of a
+medal with whose image and superscription he was not acquainted. He
+would know that a medal struck in iron was formed subsequently to the
+bronze age, or that one formed of palladium appertained to the present
+century. But quite apart from any theoretical knowledge, an antiquary
+would find as the result of accumulated experience that certain medals
+are characteristic of certain periods; he would learn that the
+denarius is characteristic of a different period from that indicated
+by the coin of the Victorian era, even though he had no knowledge of
+the technicalities of numismatics. The same is the case with the
+geologist. He may not be influenced by any knowledge of the evolution
+of faunas and floras, but actual work amongst the rocks will show him
+that the trilobite is not found with the belemnite or the ichthyosaur
+with the elephant, save under exceptional circumstances, which only
+prove the rule, as for instance when worn bones of ichthyosaurs are
+washed from their proper strata into gravels with elephant-bones.
+
+It must be distinctly understood that the determination of fossils as
+characteristic of different periods is solely made as the result of
+experience. No _à priori_ reasoning may give one indication of the
+actual range in time of a species or genus; no one can say why
+_Discina_ has a long range in time, whilst that of the closely related
+_Trematis_ is very limited. This being the case, the greater the mass
+of evidence which is accumulated as to the range of a fossil, the
+greater will be the value of that fossil as a clue to the age of the
+deposit in which it is found. This is so important, that it requires
+more than mere notice. If a fossil is found in abundance in a group of
+strata _B_ in any one area, and is not found in an underlying group
+_A_ or overlying group _C_ in that area after prolonged search, we
+may confidently speak of the fossil as characteristic of the strata
+_B_ in that area, though without further work, the value of the fossil
+as a clue to age in other areas would be unproved. It may nevertheless
+happen, that after more prolonged search in _A_ or _C_, in the
+original area a few specimens of the fossil which has been spoken of
+as characteristic of _B_ may be found in one or other of them, in
+small quantity. The value of the fossil as one characteristic of _B_
+will be slightly diminished, though only slightly, as it is not likely
+to turn up in numbers in the strata _A_ or _C_ after the prolonged
+search. Should the fossil be found also to be characteristic of the
+strata _B_ in areas other than the original one, it becomes of more
+than local value, and if, after much study it is found to characterise
+the same strata over wide areas, the cumulative evidence now obtained
+will render the fossil peculiarly important to the stratigraphical
+geologist. The detection of characteristic fossils is not quite so
+simple as might be supposed from the above remarks, for examination of
+the position of one fossil will not prove the contemporaneity of beds
+in different places, to prove this, all the evidence at our disposal
+must be considered, for reasons which will be presently pointed out.
+
+As the result of accumulated knowledge, we can now compile lists of
+characteristic fossils of the major subdivisions of the strata, which
+are of world-wide utility and as our knowledge increases, we are
+enabled to subdivide the strata into minor divisions of more than
+local value.
+
+_What is a fossil?_ Before discussing the value of fossils as aids to
+the stratigraphical geologist, it may be well to make a few
+observations as to what constitutes a fossil. It is difficult to give
+any concise definition, and as is often the case in geology, an
+explanatory paragraph is of more value than a mere definition. The
+term fossil was originally applied to anything dug up from the rocks
+of the earth's crust, and was used with reference to inorganic objects
+as well as organic remains, for instance minerals were spoken of as
+fossils. It is now applied essentially though not exclusively to
+relics of former organisms, though one still reads of fossil
+rain-drops, fossil sun-cracks, and so on. Furthermore, the relics need
+not necessarily be parts of the organism, the track of a worm or a
+bird's nest if embedded in the strata would be termed a fossil. It is
+generally agreed that no sharp line can be drawn between recent and
+fossil organic remains which is based upon the degree of
+mineralisation (or as it was sometimes termed petrifaction) of the
+relics, for many true fossils have not undergone mineralisation,
+subsequent to their entombment.
+
+It has been suggested that the name fossil should be applied to
+organic remains which have been entombed by some process other than
+human agency, but this restriction is undesirable. The stone-implement
+of the river gravels is as genuine a fossil as the ammonite extracted
+from the chalk, and the human relics of very recent date may give
+information of a character quite similar to that supplied by other
+remains, for instance, the occurrence of moa-bones in New Zealand in
+accumulations below those containing biscuit-tins and jam-pots has
+been used as a geological argument pointing to the extinction of the
+moa before the arrival of Europeans in New Zealand. The biscuit-tin
+here serves all the purposes of a fossil, and there is no valid reason
+why it should not be spoken of as such.
+
+This statement brings one to consider another method which has been
+adopted in order to separate fossil organisms from recent ones,
+namely the time-test. This again is inapplicable, for no line can be
+drawn between the shell which was buried in yesterday's tidal deposit
+and that which has lain in the strata through geological ages, and
+each may be equally useful to the geologist.
+
+Whilst, then, we can give no definition of fossil which is likely to
+meet with general acceptance, the term can be so used, as not to give
+rise to any doubts as to its meaning, and it is generally applicable
+to any organic relics which have been embedded in any deposit or
+accumulation by any agent human or otherwise.
+
+_Mode of occurrence of fossils._ It will not be out of place to say a
+few words as to the way in which fossils are found in strata, as beds
+are often inferred to be unfossiliferous, because of ignorance of
+methods which should be pursued in searching for organic relics. It is
+unnecessary to dilate upon the actual modes of preservation of
+organisms, which is treated of fully in other works. In the first
+place, it is rash to assert that any deposit is unfossiliferous
+because no fossils have been found in it, even after prolonged search.
+The Llanberis slates had been eagerly searched for fossils for many
+years without result, but that the search was not exhaustive was
+proved by the discovery of trilobites in them some years ago. Seekers
+after fossils are rather prone to confine their attention to strata
+which are already known to be fossiliferous than to pay much attention
+to those which have hitherto yielded no organic remains.
+
+Some kinds of deposits are more often fossiliferous than others.
+Limestones which are frequently largely of organic origin, are often
+rich in remains, and muddy deposits more frequently furnish fossils
+than those of a purely sandy nature. The difference in the yield is
+not necessarily due to the original inclusion of more remains in one
+rock than in another, but is often caused by the obliteration of
+former relics owing to changes which have taken place in the rocks
+subsequently to their deposition. No sedimentary rock must be regarded
+as unfossiliferous, however unfitted it appears for the preservation
+of fossils. The writer has seen fossils, not only in coarse
+conglomerates, rocks which frequently contain no traces of organisms,
+but in deposits composed largely of specular iron ore, and even in
+intrusive igneous rocks, though in the latter case, the inclusion of
+fossils was due to circumstances which cannot have occurred with
+frequency.
+
+In sandy strata, the substance of the fossils has often been
+completely removed, leaving hollow casts, which may be almost or quite
+unrecognisable. In these circumstances, much information may be
+obtained by taking impressions of the casts in modelling wax or some
+other material. The importance of this process may be judged from the
+results it yielded to Mr Clement Reid in the case of the fossils of
+the Pliocene deposits occurring in pipe-like hollows in the Cretaceous
+rocks of Kent and the discovery of the remarkable reptiles described
+by Mr E. T. Newton from the Triassic sandstones of Elgin.
+
+In argillaceous rocks which have been affected by the processes
+producing cleavage, the fossils may be distorted beyond recognition or
+owing to the difficulty of breaking the rocks along the original
+planes of deposition, may remain invisible. Under such circumstances,
+small nodules of sandy or calcareous nature may sometimes be found
+included in the argillaceous deposits and may perhaps yield fossils.
+Oftentimes, also, where the argillaceous rock is in close proximity
+to a harder rock, such as massive grit, the argillaceous rock in
+close contiguity to the hard rock may escape the impress of
+cleavage-structure, and fossils may be readily extracted from rocks in
+this position when not obtainable from other parts of the deposit. It
+was under these circumstances that the trilobites alluded to above
+were obtained from the Llanberis slates.
+
+The fossils of calcareous rocks are often very obvious, but difficult
+to extract, as they break across when the rock is fractured. They are
+frequently obtainable in a perfect condition when the rock is
+weathered. Occasionally they may be extracted from certain
+argillaceous limestones if the limestone be heated to redness, and
+suddenly plunged into cold water. Fossils are often found in a state
+which enables them to be readily extracted when a limestone is
+coarsely crystalline, though they cannot be extracted in a perfect
+condition when the same limestone is in a different state.
+
+Many microzoa, which are invisible in rocks, even when viewed through
+a lens, may be found in microscopic sections of calcareous and
+silicious rocks, and plant structures may be detected under similar
+circumstances in the case of carbonaceous rocks.
+
+Various special methods of extracting fossils from rocks have been
+described by different writers, many of which are very complex, and
+require much time. The mechanical action of the sand-blast and the
+solvent action of various acids as hydrochloric and hydrofluosilicic
+have been found of use upon different occasions[11]. The various
+processes which have been utilised in order to extract and develop
+fossils can, however, be best learned by information obtainable from
+curators of palæontological collections, and by actual experience,
+and there is yet much information to be acquired as to the manner of
+extracting fossils from various kinds of rocks.
+
+[Footnote 11: For information concerning use of acids see especially
+Wiman, C. "Ueber die Graptoliten," _Bull. Geol. Inst._, Upsala, No. 4,
+vol. II. Part II.]
+
+_Relative value of fossils to the Stratigraphical Geologist._ It has
+been hinted above that no general rule as to the relative value of
+fossils as guides to the age of strata can be laid down, and that the
+ascertainment of their relative value is largely the result of actual
+experience. It may be noted, however, that organisms which possess
+hard parts are naturally more important to the geologist than those
+which do not, as few traces of the latter are preserved in the fossil
+state, and even when preserved are usually too obscure to be of much
+practical use. Of the organisms which do possess hard parts, different
+groups have been utilised to a different degree, and one group will be
+more or less important than another, according to the use to which it
+is applied. Groups of organisms which have a long range in time are
+naturally useful for the identification of large subdivisions of the
+strata, whilst those which have had a shorter range are valuable when
+separating minor subdivisions.
+
+Again, as the bulk of the sedimentary deposits has been formed beneath
+the waters of the ocean, relics of marine organisms are naturally more
+useful than those of freshwater ones. Other things being equal, the
+more easily the organism is recognisable, and the more abundant are
+its remains, the greater its value to the stratigraphical geologist,
+and as the remains of invertebrates are usually found in greater
+quantities and in more readily recognisable condition than those of
+the vertebrates, they have been used more extensively as indices of
+age. Of the invertebrates, the mollusca are often very abundant, their
+remains are adapted for preservation, and their characteristics have
+been extensively studied, and accordingly they have been and are of
+great use to the geologist. Of other groups, the graptolites, corals,
+echinids, brachiopods, and trilobites have been very largely utilised.
+The Lower Palæozoic strata have been divided into numerous groups,
+each characterised by definite forms of graptolites, and a similar use
+has been made of the ammonites in the case of the Mesozoic rocks. It
+is not to be inferred that these groups of organisms are naturally
+more useful than other groups, on account of the extent to which they
+have been used; we can merely state that they have been proved to be
+useful as the result of prolonged study; when other groups have
+received equal attention, they may well be found to be equally useful
+for the purposes which we have in view.
+
+_Contemporaneity and Homotaxis._ From what has been already stated, it
+will be recognised that the ages of the various fossiliferous rocks of
+the geological column[12] in any one area can be identified with
+greater or less degree of certainty by reference to their included
+organisms, the various subdivisions being marked by the possession of
+characteristic fossils, and it will be naturally and rightly inferred
+that the greater the number of characteristic fossils of any one
+deposit, the more certain is the identification of that deposit. In
+practice, geologists are wont to ascertain the age of the strata after
+consideration of all the fossils found therein, some of which may be
+actually characteristic whilst many may come up from the strata
+below, or pass into those above. Having ascertained the order of
+succession and fossil contents of the strata in various regions, it is
+the task of the geologist to compare the strata of these two regions,
+and this task is fraught with considerable difficulty. Much
+controversy has arisen as to the degree of accuracy with which strata
+of remote regions can be correlated, and the subject is one which
+requires full consideration.
+
+[Footnote 12: Although the rocks do not always lie on one another in
+regular succession, it is often convenient to speak of them as though
+they did, and as though a column of strata could be carved out in any
+region consisting of horizontal bands of deposit one above another. We
+speak of such an ideal arrangement as constituting a 'geological
+column.']
+
+Suppose that a series of strata which we will call _A_, _B_, and _C_
+is found in any one area, each member of which contains characteristic
+fossils which enable it to be recognised in that area, and we will
+further suppose that in another area a series of strata _A´_, _B´_,
+and _C´_ is discovered, of which _A´_ has the fauna of _A_ in the
+former area, and similarly _B´_ the fauna of _B_, and _C´_ that of
+_C_.
+
+It cannot be assumed that the stratum _A_ is therefore contemporaneous
+with _A´_, _B_ with _B´_, and _C_ with _C´_, but on the other hand, it
+must not be assumed that they are not contemporaneous. This is a
+statement which requires some comment. It has been urged that if the
+deposits _A_ and _A´_ in different localities contain the same fauna,
+this is a proof that the two are not contemporaneous, for some time
+must have elapsed in order to allow of the migration of the organisms
+from one area to another, it being justifiably assumed that they did
+not originate simultaneously in the two areas. But everything depends
+on the time taken for migration as compared with the period of
+existence of the fauna. If the former was extremely short as compared
+with the latter it may be practically ignored, for we might then speak
+of the strata as contemporaneous, just as a historian would rightly
+speak of events in the same way which occurred upon the same
+afternoon, though one might have happened an hour before the other.
+Let us then glance at the evidence which we have at our disposal,
+which bears upon this matter.
+
+The objection to identification of strata with similar faunas as
+contemporaneous was urged by Whewell, Herbert Spencer, and Huxley, and
+the latter suggested the term Homotaxis or similarity of arrangement
+as applicable to groups of strata in different areas, in which a
+similar succession of faunas was traceable, maintaining that though
+not contemporaneous the strata might be spoken of as homotaxial.
+Huxley went so far as to assert that "for anything that geology or
+palæontology are able to show to the contrary, a Devonian fauna and
+flora in the British Islands may have been contemporaneous with
+Silurian life in North America, and with a Carboniferous fauna and
+flora in Africa[13]," a statement which few if any living geologists
+will endorse. If the statement be true, and the fauna which we speak
+of as Devonian, when present be always found (as it is) above that
+which we in Britain know as Silurian and below that which we term
+Carboniferous, the faunas must have originated independently in the
+three centres, and disappeared before the appearance of the next
+fauna, or having originated at the same centre, each must have
+migrated in the same direction, spread over the world, and become
+extinct as it reached the point or line from which it started. Suppose
+for instance a fauna _A_ originates at the meridian of Greenwich,
+migrates eastward, and dies out again when it once more reaches
+Greenwich, that _B_ and _C_ do the same, at a later period, then the
+fauna _B_ will always be found above _A_ and _C_ above _B_, but if
+_B_ did not become extinct when it reached the Greenwich meridian, it
+would continue its eastward course, and _C_ having in the meantime
+started on its first round, the fossils of the fauna _B_ would be
+found both above and below those of _C_. It will be shown below that
+cases of recurrence do occur, but nowhere do we find a Silurian fauna
+above a Devonian one, or a Devonian one above one belonging to the
+Carboniferous, nor is the fauna of a great group of rocks found in one
+region above the fauna of another group, and in another region below
+the same. And this is true not only of the faunas of one major
+division, such as those of the Silurian and Carboniferous periods, but
+also of the faunas of many minor subdivisions into which the large
+ones are separated, for instance we do not find the Llandovery fauna
+of the Silurian period which in Britain is found below the Wenlock
+fauna embedded elsewhere in strata above the Wenlock. I have
+simplified the statement by assuming that the faunas are identical in
+the different localities, and exactly similar throughout the whole
+thickness of the containing strata, which is naturally not the case,
+but the additional complexity does not conceal the truth of what has
+been stated. In the absence of actual inversion of well-marked faunas,
+only one explanation is possible, namely, that the time for migration
+of forms is so short as compared with the entire period during which
+the forms existed, that it may be practically ignored, and the strata
+containing similar faunas may be therefore spoken of truthfully as
+contemporaneous and not merely homotaxial[14].
+
+[Footnote 13: Huxley, T. H. "Geological Contemporaneity and Persistent
+Types of Life," being the Anniversary Address to the Geological
+Society for 1862; reprinted in _Lay Sermons, Addresses and Reviews_.]
+
+[Footnote 14: For fuller discussion of this matter see a paper by the
+Author 'On Homotaxis,' _Proc. Camb. Phil. Soc._, vol. VI. Part II. p.
+74.]
+
+_Apparent anomalies in the distribution of fossils._ There are several
+occurrences which have tended to augment the distrust frequently felt
+concerning the value of fossils as indices of the age of the beds in
+which they occur, which may be here considered.
+
+Though the greater number of fossil remains belonged to organisms
+which lived during the time of accumulation of the deposits in which
+they are now embedded, this is by no means universally the case, and
+the occurrence of _remanié_ fossils, which have been derived from
+deposits more ancient than the ones in which they are now found is far
+from being a rare event. The existence of remains of this nature in
+the superficial drifts and river-gravels of our own country has long
+been recognised, and no one would suppose that the _Gryphæa_ and other
+shells furnished by these gravels had lived contemporaneously with the
+species of _Corbicula_, _Unio_ and other molluscs which are part of
+the true fauna of the gravels. In this case the water-worn nature of
+the remains is a good index to their origin, but in other cases, it is
+by no means an infallible guide, for we sometimes find on the one hand
+that remains of organisms proper to the deposits in which they occur
+are water-worn, whilst on the other the relics of _remanié_ fossils
+are not. The now well-known gault fossils of the Cambridge Greensand
+at the base of the chalk were not always recognised as having been
+derived from older beds, and there are certain fossils found in
+nodules in the Cretaceous rocks of Lincolnshire, which still form a
+subject for difference of opinion, for while some writers maintain
+that they belong to the deposits in which they are now found, others
+suppose that the nodules have been washed out of earlier beds.
+
+Occasionally we find forms which occurring in a set of beds _A_ in an
+area, are absent from the overlying beds _B_, and appear again in the
+succeeding deposits _C_. Such cases of _recurrence_ are by no means
+rare, though many supposed instances of recurrence have been recorded
+as the result of stratigraphical or palæontological errors. The best
+examples have been noted by Barrande among the Lower Palæozoic
+deposits of Bohemia. The stage _D_ of Bohemia consists of five
+'bandes' or subdivisions, the lowest (_d_ 1), central (_d_ 3) and
+uppermost (_d_ 5) divisions are mainly argillaceous, whilst the second
+(_d_ 2) and fourth (_d_ 4) are essentially arenaceous. Some of the
+forms found in _d_ 1, _d_ 3 and _d_ 5 have not been found in _d_ 2 and
+_d_ 4. The best-known example is the trilobite _Æglina rediviva_. It
+is clear that this and other forms did not become extinct during the
+deposition of the strata of _d_ 2 and _d_ 4, though they may have
+disappeared temporarily from the Bohemian area, or else lingered on in
+such diminished numbers that their remains have not been discovered.
+The range of the organism is in fact right through the deposits of the
+stage _D_, and the discontinuity of distribution is not a real
+anomaly; it may be compared to some extent with cases of discontinuous
+distribution in space. It is needless to remark that the whole fauna
+does not disappear for a time and then reappear, but only a few out of
+the many forms which compose it. The comparative rarity of examples of
+recurrence after long intervals is an indication that the
+palæontological record as it is termed is not so imperfect as some
+suppose, for if our knowledge of fossils were very imperfect, we
+should expect cases of apparent recurrence to be common, as the result
+of the non-detection of fossils in the intermediate beds. One of the
+most marked cases of apparent recurrence known some years ago was the
+reappearance of a genus of trilobite _Ampyx_ in Ludlow rocks, found in
+the Bala rocks, but not in the Llandovery or Wenlock strata. It has
+since been discovered in Llandovery beds, and its eventual discovery
+in beds of Wenlock age may be regarded as certain. A supposed case of
+recurrence which would have been remarkable, that of the disappearance
+of _Phillipsia_ in Ordovician rocks, its entire absence in those of
+Silurian age, and its reappearance in the Devonian, has broken down,
+for the supposed Ordovician form has been shown to belong to an
+entirely different group of trilobites from that containing the genus
+_Phillipsia_, and it has been therefore renamed _Phillipsinella_.
+
+Many apparent anomalies of distribution have been explained as due to
+migration, but it is doubtful whether any one of these supposed
+anomalies is actual and not due to errors in determining the position
+of the beds or the nature of their included fossils. Some of the
+supposed anomalies have already been shown to be due to error, and the
+others will almost certainly be cleared up. In speaking of anomalies
+of distribution, the geologist can only be guided by experience as to
+what constitutes an anomaly. For instance the existence of a complete
+fauna in any one place in the beds of a system above that to which it
+is elsewhere confined would be regarded as anomalous and as probably
+due to error, whilst the reappearance of several forms in beds of a
+system higher than that in which they had hitherto been found, could
+hardly be considered as an anomaly. A geologist would suspect the
+statement that after the disappearance of an Ordovician fauna in an
+area and its replacement by a Silurian fauna, the Ordovician fauna
+reappeared for a time, but would not regard the statement that a
+Cenomanian fauna partly reappeared in the Chalk Rock with surprise.
+
+The existence of a Silurian fauna in Ordovician times was maintained
+by Barrande in the case of the Bohemian basin. Lenticular patches of
+Silurian rocks having the lithological characters of the Silurian
+strata are found in the Ordovician beds of that region, and they
+contain fossils specifically identical with those of the Silurian
+rocks. Barrande explained this appearance as due to the existence of a
+fauna in other regions resembling the Silurian fauna of Bohemia,
+during the Ordovician period, when the normal Ordovician fauna of
+Bohemia inhabited that area. He supposed that in parts of the basin,
+when favourable conditions arose, _colonies_ of the foreign fauna
+settled for a time, but did not get a permanent footing in the basin
+until the commencement of Silurian times. The theory of colonies has
+now been rejected for the Bohemian area, and the phenomena shown to be
+due to repetition of strata by folding and faulting, but it is a
+theory which is again and again advocated in order to explain
+apparently anomalous phenomena in other areas, and these apparent
+anomalies which are so explained, must be regarded with grave
+suspicion.
+
+The various complexities alluded to in the foregoing pages increase
+the difficulty experienced by the geologist in correlating strata in
+different areas by their included organisms, but no one of them
+disproves the possibility of making these correlations, which can be
+carried on to a greater or less extent according to the nature of the
+faunas.
+
+A good deal of misconception has arisen concerning the geographical
+distribution of former faunas, owing to the tendency to compare them
+exclusively with the littoral faunas of the present day. These
+littoral faunas have a comparatively limited geographical
+distribution, the forms of one marine province often differing
+considerably from those of an adjoining one, and still more widely
+from one which is remote, so that anyone confronted with the relics
+of faunas from the existing Australian and European seas, would find
+no indications furnished by identity of species that the faunas were
+contemporaneous. Recent researches have shown, however, that the
+creatures whose remains are deposited at some distance from the
+coast-line have a much stronger resemblance to one another than the
+littoral organisms have, if the fauna of two distant areas be
+compared. It is still a moot point which will be discussed in a later
+chapter, how far the deep-sea deposits of modern times are represented
+amongst the strata of the geological column by deposits of similar
+origin. But it is certain that many of the ancient strata are not
+littoral deposits, and it will be found that it is by comparison of
+the faunas of the deeper-water deposits that the geologist correlates
+the strata of remote regions: where shallow water deposits are formed,
+the faunas differ markedly in different regions, and these
+shallow-water forms can only be correlated owing to their occurrence
+between deeper-water strata. Thus if strata _A_, _B_ and _C_ be found
+in one area, and the fauna of _A_ and _C_ are deep-water forms, those
+of _B_ being shallow-water forms, and in another area beds _A´_
+contain the same fauna as _A_, and _C´_ the same fauna as _C_ whilst
+the fauna of _B´_ is different from that of _B_, we can nevertheless
+correlate the strata _B_ and _B´_ (if they be conformable with the
+underlying and overlying beds), because of the identity of age of the
+associated beds in the two areas. It will possibly be found that the
+strata _A_ and _C_ can be further subdivided into _A_{1}_, _A_{2}_,
+... &c. _C_{1}_, _C_{2}_, ... by the existence of minor faunas, which
+are comparable in the two cases, but such subdivisions may not be
+established in the case of the beds _B_ and _B´_.
+
+To take actual examples:--The Llandovery beds of Dumfriesshire can be
+subdivided into several minor divisions each of which can be
+recognised in the Lake District of England, and to a large extent in
+Scandinavia and elsewhere, for the deposits in these areas are of
+deep-water character, and the sub-faunas of the subdivisions are
+similar in the different areas, but the Llandovery rocks of the Welsh
+borderland are shallow-water deposits, with a different fauna from
+that of the deep-water deposits of this age, and can only be stated to
+be contemporaneous with the Llandovery rocks elsewhere, because the
+deeper-water faunas of the underlying Bala rocks and overlying Wenlock
+rocks of the Welsh borders are respectively similar to those of the
+Bala and Wenlock rocks of the other regions. The shallow-water
+Llandoveries of the Welsh borders have only been separated into two
+divisions, upper and lower, and have not been split up into a number
+of subdivisions, each characterised by a sub-fauna, and each
+comparable with one of the subdivisions of Dumfriesshire, Lakeland and
+the other regions where the deep-water facies is found.
+
+It will be seen that though the principle of William Smith that strata
+can be recognised by their included organisms has been extended since
+his time, and shown to apply to far smaller subdivisions of the strata
+than was suspected, the method of application is the same, and is more
+or less successful according to the amount of evidence which is
+accumulated in support of it.
+
+
+
+
+CHAPTER VI.
+
+METHODS OF CLASSIFICATION OF THE STRATA.
+
+
+Earth-history like human history is the record of an unbroken chain of
+events. The agents which have produced geological phenomena have been
+in operation since the earth came into existence. Accordingly a
+perfect earth-history would be written as a continuous narrative, just
+as would a complete history of the human race. The historian of man
+finds it not only convenient but necessary to divide the epoch of
+which he is writing into periods of time, and so does the geologist,
+and in each case the division is necessarily more or less arbitrary.
+It is true that in writing the history or geology of a country, marked
+events stand out which form a convenient means of making divisions,
+but the marked events occurring in one country are not likely to take
+place simultaneously with those of another country, and consequently a
+classification of this character is only locally applicable.
+
+The classification which is at present used by geologists was
+originally founded upon definite principles, and although our
+principles of classification have, as will appear, been somewhat
+altered subsequently, it has been found more convenient to modify the
+original classification than to adopt a new one in its entirety.
+
+The largest divisions into which the strata of the geological column
+were separated were instituted because of the supposed extinction of
+faunas, and sudden or rapid replacement by other faunas of an entirely
+different character. This supposed rapid extinction and replacement is
+now known to have been only apparent and due to observation in
+restricted areas, and it is doubtful whether the three great divisions
+founded upon them are not rather mischievous than useful, as tending
+to disseminate wrong notions.
+
+Moreover there is considerable diversity of opinion as to the terms to
+be adopted. The rocks were formerly divided into Primary, Secondary,
+and Tertiary. Owing chiefly to the use of the term Primary in another
+sense, the alternative titles Palæozoic, Mesozoic and Cainozoic (or
+Cænozoic) were suggested, and though the term Primary has been
+definitely abandoned in favour of Palæozoic, the words Secondary and
+Tertiary are used extensively as synonyms of Mesozoic and Cainozoic.
+It was soon perceived that the period of time included in the
+Palæozoic age was much longer than the combined periods of Secondary
+and Tertiary ages, and it was proposed to group the latter under one
+title Neozoic, whilst another suggestion was to split the Palæozoic
+age into an earlier Proterozoic and later Deuterozoic division. The
+interest excited by the advent of man is probably the cause of the
+attempt to establish a Quaternary division, which some hold to be a
+minor subdivision of the Tertiary, whilst others would separate it
+altogether. The terms Palæozoic, Mesozoic (or Secondary) and Cainozoic
+(or Tertiary) are now used so generally that any attempt to abolish
+them would be doomed to failure, but it must be remembered that they
+are purely arbitrary expressions, and the other terms which are not in
+general use, might be dropped with advantage.
+
+The other subdivisions have been used somewhat loosely, and although
+an attempt has been made by the International Geological Congress to
+restrict certain names to subdivisions of varying degrees of value, it
+will probably be found best to allow of a certain elasticity in the
+use of terms, merely agreeing that they shall be used as nearly as
+possible with the signification assigned to them by the Congress.
+According to this classification, and apart from the division into
+Palæozoic, Mesozoic and Cainozoic, the strata of the geological column
+are grouped into _Systems_, which are subdivided into _Series_, and
+the series are further split up into _Stages_. A number of
+chronological terms were also suggested, of equivalent importance,
+thus the beds of a _system_ would be deposited during a _Period_,
+those of a _series_ during an _Epoch_, and those of a _stage_ during
+an _Age_[15].
+
+[Footnote 15: The chronological words have been used so loosely that
+it is doubtful whether any good will come of trying to restrict their
+use, and Sir A. Geikie has pointed out the confusion which would arise
+if the term _group_ be employed for the largest divisions (Palæozoic,
+&c.). The terms _System_, _Series_ and _Stage_ may well be employed in
+the senses suggested by the Congress.]
+
+The rocks of the Geological Column were originally divided into
+systems, owing to the occurrence of marked physical and
+palæontological breaks between the rocks of two adjacent systems,
+except in cases where a complete change occurred locally in the
+lithological characters of the rocks of two systems which were in
+juxtaposition: it is necessary to consider for awhile the nature of
+these breaks.
+
+The most apparent physical break is where the rocks of one set of
+deposits rest unconformably upon the rocks of another one, indicating
+that the older set has been uplifted and to some extent eroded before
+the deposition of the strata of the newer set. This uplift and erosion
+signifies a change from oceanic to continental conditions in the area
+in which unconformity is found on a large scale, and accordingly a
+long period of time would elapse during which the continental surface
+would not receive deposits, so that the highest rocks of the
+underlying system would be considerably older than the lowest rocks of
+the one which succeeds it. Such a break may be obviously utilised for
+purposes of classification, but as some areas of the earth's surface
+must have been occupied by the waters of the ocean when other regions
+formed land, deposit in some areas must constantly have occurred
+simultaneously with denudation in others, and any classification
+founded upon the existence of unconformities will therefore have a
+purely local value.
+
+Another, and less apparent physical break, which will also be locally
+applicable, may be due to the depression of an area to so great a
+depth that little or no deposit was formed upon the ocean floor there
+during the period of great depression; but as a break of this
+character is difficult to detect, the existence of unconformities has
+alone been practically utilised as a means of separating strata into
+systems owing to marked physical change, except in the cases where the
+lithological character of the strata completely changes, as between
+the Triassic and Jurassic rocks of England.
+
+[Illustration: Fig. 2.]
+
+Palæontological breaks or breaks in the succession of organisms are in
+many cases, the result of physical breaks, and accordingly it is often
+possible to separate one set of strata from another by the existence
+of a combined physical and palæontological break between them. It is
+by no means necessary however that a physical break should be
+accompanied by a break in succession of the organisms, and the latter
+may also occur without the former. It was once maintained that a
+palæontological break was due to the complete and sudden extinction of
+a fauna and its entire replacement by a new one, but this is far from
+true, and accordingly the breaks differ in degree. Study of the strata
+shows that when the succession is not to any extent interrupted, the
+species do not appear simultaneously, but come in at different
+horizons, and they disappear in the same way. In Figure 2 let _A_
+represent a set of conformable strata _ab ... k_, and suppose the
+vertical lines represent the ranges of the various species found in
+these strata. It will be seen that of 27 species whose range is shown
+only 2 pass through the whole thickness, so that the fauna of _k_ is
+very different from the fauna of _a_, nevertheless the fauna of each
+stratum is closely similar to that of the underlying as well as to
+that of the overlying stratum, and though most of the species of _k_
+are different from those of _a_, this need not be the case with the
+genera. The fauna of the set of strata would contain every species
+whose range is represented, and for convenience' sake it might be said
+to be composed of sub-faunas, one of which occurs in each division
+_ab_ ..., but the separation into sub-faunas would be artificial and
+merely for convenience' sake, for there is no break between any two
+sub-faunas. Turning now to _B_ (Fig. 2), an attempt is made there to
+show what happens when there has been a physical break, resulting in
+the denudation of the strata _ghik_, and the deposition of another set
+_op_ ... unconformably upon those deposits of the earlier set which
+have not been denuded. As the result of this we note, first, that the
+relics of organisms which existed in the area during the deposition of
+_ghik_, and were entombed in those strata, are destroyed by the
+processes of denudation, and a large number of organisms which lived
+long after the deposition of _f_, and disappeared not simultaneously
+but at different times during the period when denudation was in
+operation, seem to become extinct simultaneously at the top of _f_,
+though, if we could visit an area which was receiving sediment during
+the period of denudation, we should find them dying out in the rocks
+of that region at different levels. Furthermore, whilst denudation is
+going on, a longer or shorter period of time elapses, during which the
+upheaved area receives no deposit, and accordingly no organisms which
+lived during that period are preserved in the upheaved area. During
+this time a set of deposits _lmn_ may have been laid down elsewhere,
+and besides the gradual disappearance of some of the organisms of _ab
+... k_, there will have been a gradual appearance of new species.
+When the upheaved area is once more submerged, a new set of deposits
+_op_ ... is accumulated in it, and the species which gradually
+appeared in adjoining regions will now migrate to it, and will seem to
+come in simultaneously at the bottom of _o_; accordingly we may find
+that there is not a single species which passes through from _f_ to
+_o_ and the palæontological break in this area is complete, though it
+is clear that it only implies local change, and that we may and indeed
+must find intermediate forms in other regions which fill up the gap.
+
+As an illustration of the local character of a palæontological break
+we may cite the case of the Carboniferous and Permian systems of
+Britain. These rocks are separated from one another in our area by a
+physical and palæontological break, but in parts of India, and other
+places, we find a group of rocks now known as the Permo-Carboniferous
+rocks which contain a fauna intermediate in character between those of
+the Permian and Carboniferous systems, and a study of this fauna shows
+that the hiatus which exists locally is filled by the species
+contained in the Permo-Carboniferous rocks.
+
+A palæontological break may, like a physical one, result from
+depression of the ocean-floor to so great a depth, that no organisms
+are preserved there during the period of great depression, and the
+remarks made concerning a depression of this nature when speaking of
+physical breaks will apply here also.
+
+A local palæontological break may result owing to physical changes
+without the production of an unconformity in the area, or its
+submergence to a great depth, or if an unconformity is found, the
+break may be more marked owing to other physical changes. The
+difference between the Upper and Lower Carboniferous faunas is very
+marked in England, where the Upper Carboniferous beds were deposited
+under physical conditions different from those of the Lower
+Carboniferous, and accordingly the corals, crinoids and other
+open-water animals which flourished in Lower Carboniferous times are
+rare or altogether absent in the higher rocks. Where the change of
+conditions did not occur to a great extent as in parts of Spain and
+North America, the similarity between the two faunas is much more
+pronounced. Again, there is an unconformity between the Cretaceous and
+Eocene beds of England, which is accompanied by a palæontological
+break, but this break is more pronounced owing to difference of
+physical conditions, for we find abundance of gastropods in the lower
+Tertiary beds, and a rarity of these shells at the top of the chalk of
+England, though where physical conditions were favourable for the
+growth of gastropods, their shells are found in the higher strata of
+chalk age, and the palæontological break is not so apparent.
+
+A palæontological break may occur also as the result of climatic
+change, though actual instances of this occurrence are much more
+difficult to detect owing to the general absence of any evidence of
+climatic change other than that supplied by the organisms themselves.
+Still, when no physical break exists, and the lithological characters
+of a group of sediments remain constant throughout, indicating the
+prevalence of similar physical conditions through the period of
+deposition of the sediments, if the fauna suddenly changes, there must
+have been cause for the change, and in the absence of any other cause
+which is likely to produce the change, alteration of the character of
+the climate may be suspected.
+
+It follows from the observations which have been made, that although
+the rocks of the Geological Column may be divided into systems owing
+to the existence of physical and palæontological breaks, and this
+classification may be and has been applied generally, the line of
+demarcation between the rocks of two systems will be a purely
+conventional one, where there is no break, and, to avoid confusion,
+that line when once drawn should be adopted by everyone, unless good
+cause can be shown for its abandonment.
+
+The subdivision of systems into series has been conducted in a manner
+generally similar to that in which large masses of strata have been
+grouped into systems, with the exception that actual breaks need not
+occur. The subdivision was usually made on account of marked
+differences in the lithological characters or fossil contents of the
+rocks of the various series, and frequently the lithological
+characters as well as the fossil contents are dissimilar; taking the
+rocks of the Silurian system of the typical Silurian area as an
+example, we find the Llandovery rocks largely arenaceous, the Wenlock
+rocks largely calcareo-argillaceous, and the Ludlow rocks
+argillaceo-arenaceous, whilst the fauna of the Wenlock rocks differs
+from that of the Llandovery rocks below and also from that of the
+Ludlow rocks above. The Llandovery, Wenlock and Ludlow therefore
+constitute three series of the Silurian system, but the lines of
+demarcation between these series are nevertheless conventional, for it
+has been suggested that a more natural division, as far as the British
+rocks are concerned, could be made by drawing a line, not as at
+present at the base of the Ludlow, but in the middle of that series as
+now defined, and uniting the Lower Ludlow beds with the Wenlock strata
+to form a single series.
+
+The same process as that adopted in the case of series has been
+essentially pursued in subdividing these into stages. Each stage is
+usually different from that above and below in its lithological
+characters, fossil contents, or both, though the difference is usually
+less in degree than that which has been utilised for the demarcation
+of series. A stage is often, though not always, composed of deposits
+of one kind of sediment, and is furthermore frequently characterised
+by the possession of one or, it may be, two, three or more
+characteristic fossils. Thus the Wenlock series is divided in the
+typical area into Woolhope limestone, Wenlock shale, and Wenlock
+limestone, and the very names given to these stages indicate that each
+is largely composed of one kind of material. Their fossils are also to
+some extent different, though the difference between them is not
+likely to be of so marked a nature as that which exists between the
+faunas of separate series.
+
+It will be seen that the system differs from the series and the series
+from the stage in degree rather than in kind, and no hard line can be
+drawn between divisions of different degrees of magnitude. It follows
+therefore that frequently a mass of sediment which one author will
+consider sufficiently important to constitute a system will be defined
+by another as a series, and similarly a series of one writer may
+become a stage of another.
+
+The student of Stratigraphical Geology will find the expression
+'fossil zone' occurring over and over again in geological literature,
+and as the term has been used somewhat vaguely by many writers and is
+apt to be misunderstood, it will be useful to notice the expression at
+some length.
+
+Strictly speaking the term zone (a belt or girdle), when applied to
+distribution of fossils, should refer to the belt of strata through
+which a fossil or group of fossils ranges. Generally speaking, the
+expression is used in connexion with one fossil; thus we speak of the
+zone of _Coenograptus gracilis_, the zone of _Cidaris florigemma_ and
+the zone of _Belemnites jaculum_, though sometimes it is used with
+reference to more than one species, as the zone of Micrasters and the
+_Olenellus_ zone. The term has been used not of a belt of strata but
+of a group of organisms[16], and zones defined as "assemblages of
+organic remains of which one abundant and characteristic form is
+chosen as an index," but if it be agreed that the term should be
+applied to strata and not to organisms this might be modified and the
+definition run:--'Zones are belts of strata, each of which is
+characterised by an assemblage of organic remains of which one
+abundant and characteristic form is chosen as an index.'
+
+[Footnote 16: See H. B. Woodward, "On Geological Zones," _Proc. Geol.
+Assoc._, vol. XII. Part 7, p. 295, and vol. XII. Part 8, p. 313.]
+
+It has been objected that the subdivision of strata into zones has
+been pushed too far, but this is merely because in the establishment
+of zones, workers find it easier to work out the successive zones
+where the strata are thin and presumably deposited with extreme
+slowness, than where they are much thicker and have been rapidly
+accumulated, and accordingly, as the subdivision of strata into zones
+is a recent event, geological literature contains many more references
+to thin zones than to those of great thickness. Where an abundant and
+characteristic form (which is chosen as an index) of an assemblage of
+organic remains ranges through a great thickness of deposit, there is
+no objection to speaking of the whole as a zone, and it cannot be
+divided. To give some idea of the variations in the thickness of
+strata through which these abundant and characteristic forms will
+range, I append a list of the zones of graptolites which have been
+established amongst the Silurian rocks of English Lakeland and the
+thickness of each (which in the case of the thicker deposits is
+naturally only approximate):--
+
+                                         Thickness.
+             Zone of                  Feet.    Inches.
+  _Monograptus leintwardinensis_      5000        0
+  _Monograptus bohemicus_             5000        0
+  _Monograptus Nilssoni_              1000        0
+  _Cyrtograptus Murchisoni_           1000        0
+  _Monograptus crispus_                 22        0
+  _Monograptus turriculatus_            60        0
+  _Rastutes maximus_                    25        0
+  _Monograptus spinigerus_               3        0
+  _Monograptus Clingani_                 3        0
+  _Monograptus convolutus_               7        6
+  _Monograptus argenteus_                0        8
+  _Monograptus fimbriatus_               7        6
+  _Dimorphograptus confertus_           25        0
+  _Diplograptus acuminatus_              2        6
+
+It must not be supposed that each of the subdivisions in the above
+list is of equal importance, and has occupied approximately the same
+length of time for its formation, but a study of the strata proves by
+various kinds of evidence that the deposits in which the
+characteristic forms range through a small thickness of rock were on
+the whole deposited much more slowly than where the range is
+continuous through a great thickness of deposit.
+
+The geological systems, as originally founded, were not very
+accurately separated from one another except locally. A comprehensive
+view of the characters of a system was taken, and accordingly the
+lines of demarcation between the same systems adopted by workers in
+different countries were by no means necessarily at or near the same
+geological horizon. As the result of more recent work, the
+establishment of fossil zones has been growing apace, and though many
+of these are seen to have only local significance, it is found as the
+result of experience that many of them are widely spread and occur in
+the same order in different localities; accordingly the remarks that
+have been made concerning the contemporaneity of strata apply to these
+zones also. After a study of this kind, a much more accurate
+comparison of strata is possible, and correlation of strata can be
+carried on to a much greater extent than when the systems were only
+roughly subdivided by reference to breaks, differences of lithological
+character, and general comparison of the faunas; accordingly whilst
+largely retaining the old names, the old method of classification is
+being partly superseded, and the included faunas alone are utilised to
+establish accurate correlations of the strata in various parts of the
+world. How far this correlation can be carried on remains to be seen,
+for the work though well advanced has by no means reached completion,
+and predictions as to the ultimate issue are useless without the
+experience by means of which only the work can be done. The difference
+between the methods of classification is well shown by an examination
+of the old and new divisions of the chalk. It was formerly roughly
+divided mainly by lithological characters into Chalk Marl, Lower Chalk
+without flints, Middle Chalk with few flints and Upper Chalk with many
+flints, but no two observers would probably agree as to where the
+deposit with few flints ceased and that with many commenced. The chalk
+is now separated on palæontological grounds into Cenomanian, Turonian,
+Senonian and Danian, and the superiority of the new method to the old
+is practically shown by the abandonment of the old classification
+except for very rough purposes, and the general acceptance of the new
+one. Many other examples might be given, but this one will suffice. In
+the case of some of the systems, the Carboniferous for example, the
+old classification founded upon lithological characters is largely
+extant, and it has been inferred therefore that no accurate
+subdivisions of the Carboniferous rocks can be made by reference to
+the faunas, owing to the rapidity with which the deposits were
+accumulated. It is by no means certain because the work has not been
+done that it cannot be done, and the experience obtained from a study
+of other strata in which subdivisions have been established by
+reference to the fauna would lead one to suppose that the
+non-establishment of subdivisions of the Carboniferous strata is due
+to our want of knowledge rather than to their non-existence.
+
+The establishment of a classification on palæontological lines by no
+means does away with the necessity for local classifications on a
+lithological basis, and it has already been remarked that important
+results will follow from a comparison of the classifications of
+sediments founded on the two lines, results which have hitherto
+largely escaped our attention owing to the existence of a cumbrous
+classification attained by the application sometimes of one method, at
+other times of the alternative one.
+
+
+
+
+CHAPTER VII.
+
+SIMULATION OF STRUCTURES.
+
+
+Although it is easy to give an account of the structures which are of
+importance to the student of the stratified rocks, actual observation
+of these structures is frequently attended with difficulties owing to
+the close imitation of one structure by another, and the past history
+of the science shows that erroneous conclusions have been reached
+again and again on account of the incorrect interpretation of
+structures.
+
+Simulation of organisms has frequently been the cause of error.
+Inorganic substances take on the form of organisms with various
+degrees of closeness. The dendritic markings produced by
+efflorescences of oxide of manganese are familiar to all, and as the
+name implies, they simulate, to some extent, plant remains. More
+complex chemical changes have resulted in the production of
+rock-masses in which, not the outward form alone but, the internal
+structure of organisms is reproduced with more or less approach to
+fidelity, as the rocks which contain the supposed organisms described
+as _Eozoon bohemicum_, _E. bavaricum_, and, we may add, _E.
+canadense_. Mechanical changes in rocks subsequent to their formation
+may also cause the simulation of organisms by inorganic substances.
+Prof. Sollas has given reasons for considering the structure
+described as _Oldhamia_ to be inorganic, and in the Carboniferous
+Sandstones of Little Haven, Pembrokeshire, every stage in the
+formation of tubular bodies resembling worm-tubes, as the result of
+complex folding of the strata, may be observed, whilst in other cases
+we find imitation of worm-tracks, as has been observed before.
+
+It is when one inorganic structure is simulated by another that the
+stratigraphical geologist is most likely to be led astray, and
+accordingly it is worth noting some cases where this has occurred, as
+a warning, for it must not be supposed that the cases here noted are
+the only ones which are likely to occur.
+
+It has been seen that the existence of bedding-planes is of prime
+importance to the geologist, and their detection is a matter of
+supreme moment. Under ordinary circumstances there is no great
+difficulty in distinguishing bedding-planes from other planes, but the
+importance of discovering them is often greatest when the difficulty
+is most pronounced. In rocks which have undergone no great amount of
+disturbance the planes of stratification are often marked by their
+regular parallelism, the separation of layers having different
+lithological characters by these planes, the arrangement of the longer
+axes of pebbles parallel to them, and the occurrence of fossils and
+also of rain-prints, ripple-marks and other structures produced during
+deposition, upon the surfaces of the strata, but none of these
+appearances is necessarily conclusive, especially in areas where the
+rocks have been subjected to orogenic movements. In regularly-jointed
+rocks, jointing may well be mistaken for bedding, and there is often
+great difficulty in discriminating between bedding and cleavage,
+especially when the exposures of rock are of small extent. Fossils may
+be dragged out along planes at an angle to the true bedding, pebbles
+will be compressed by cleavage so that their longer axes do not remain
+parallel to the bedding-planes but now lie parallel to the
+superinduced planes of cleavage, and a structure closely resembling
+'ripple-mark' may be produced on planes other than those of original
+bedding, as the result of puckering. The alternation of rocks having
+different lithological characters may also be misleading. Intrusion of
+dykes along cleavage-planes, followed by decomposition of the
+dyke-rock causing it to resemble a sediment, and formation of mineral
+veins along the same planes, may give rise to an apparent succession
+of rocks of different lithological characters which could easily
+mislead an observer and cause him to mistake the cleavage-planes for
+planes of stratification. In rocks which have undergone great lateral
+pressure, the beds of different lithological character may be folded
+in such a way as to give very erroneous ideas of the true dip of the
+rock on a large scale. In Fig. 3 the dip of the rocks in a small
+exposure might appear to be in the direction indicated by the
+unfeathered arrow, whilst the true dip of the strata as a whole,
+leaving the minor foldings out of account, is in the direction of the
+feathered arrow, at the inclination represented by the dotted line.
+The minor folds in a case like that represented may extend upwards for
+scores or even hundreds of feet, so that an error as to the direction
+and amount of dip may be made, even if the observer faces a cliff of
+considerable height.
+
+[Illustration: Fig. 3.]
+
+False-bedding on a large scale may be a cause of error. In the Penrith
+Sandstone of Cumberland, the planes of deposition are often found
+dipping in one direction in a large quarry, but inspection of a wider
+area shows that this is not the true dip of the beds as a whole, but
+merely a local dip due to deposition on a slope, and any one
+attempting to calculate the total thickness of the beds by reference
+to these divisional planes might be seriously led astray. A reference
+to Fig. 4 will explain this. The lines _AA´_, _BB´_ are the true
+bedding-planes cut across in the section, whilst the lines sloping to
+the right from _xx_ are only lines of false-bedding on a large scale.
+An exaggerated estimate of the thickness of the deposit would be made
+by measuring the thickness of each of these stratula from _A_ to _A´_
+and adding these thicknesses together, whereas the actual thickness of
+the middle bed is the distance between _A_ and _B_ or _A´_ and _B´_.
+
+[Illustration: Fig. 4.]
+
+When rocks have been affected by thrust-planes, the simulation of
+bedding may be carried out to a very full extent. Not only do the
+major thrust-planes resemble bedding-planes but the minor thrusts
+produce an appearance of divisional planes separating stratula or
+laminæ, and a close approximation to false-bedding is the result. To
+this structure Prof. Bonney has given the name
+'pseudo-stromatism[17].' It may be developed in rocks of all kinds,
+whether possessing original planes of stratification or not, and as a
+result of its existence the geologist may be seriously misled, not
+merely by mistaking the direction of the strata, but also the nature
+of the rock, for we may find it produced in an unstratified glacial
+till, and in a massive igneous rock, and in each case the resulting
+rock will resemble a sedimentary deposit, and of course the observer
+may be confirmed in his erroneous opinion by the formation of apparent
+fossils, ripple-marks or other objects which he might expect to
+discover in sediments. As illustrative examples, reference may be made
+to a number of schistose rocks, in which the planes of discontinuity
+(which are in truth planes of foliation) have been taken for
+bedding-planes and the rocks claimed as sedimentary though they are in
+reality igneous; for instance many of the rocks of the Laurentian of
+Canada, of the Hebridean of the North West Highlands, and some of the
+ancient rocks of Anglesey.
+
+[Footnote 17: Bonney, T. G., _Quart. Journ. Geol. Soc._, vol. XLII.
+_Proc._ p. 65.]
+
+A foliated structure may, as is now well known, be simulated by a
+structure developed in a rock prior to its consolidation. The
+similarity of flow structure of some lavas to the foliated structure
+of a schist was long ago pointed out by Darwin and Scrope, and recent
+work has proved that parallel structure due to differential movement
+prior to consolidation may be developed in plutonic rocks, as shown
+by Lieut.-General McMahon in the Himalayan granites, and by Lawson
+amongst the plutonic rocks of the Rainy Lake Region; and as the
+foliated structure may be mistaken for original stratification the
+same may occur, and has occurred, when dealing with this
+flow-structure.
+
+This is not the place to discuss the truth of the old theory of
+progressive metamorphism, in which it was maintained that a gradual
+passage could be traced between ordinary sediments and plutonic rocks,
+but it may be pointed out that much of the evidence which was relied
+upon to prove the theory was fallacious and due to the confusion of
+the parallel structure set up in plutonic rocks prior to, or
+subsequent to, consolidation, with original stratification. Recent
+study of metamorphic rocks has proved that the parallel structures
+developed in the rocks of an area which has undergone metamorphism may
+be produced by three distinct processes; they may be original planes
+of deposition, or formed in a solid rock subsequently to its
+formation, or in an igneous rock before its consolidation, and
+although it is sometimes possible to separate the structures produced
+by these processes, this is not always the case[18]. When a plutonic
+rock contains large phenocrysts and an eye-structure is developed in
+it, it may simulate a conglomerate, the rounded phenocrysts being
+taken for pebbles[19]. Still closer simulation of an epiclastic
+conglomerate may be produced in other ways and will be referred to
+immediately.
+
+[Footnote 18: It must be noticed that the rock in which parallel
+structure is produced before consolidation, if it undergoes no further
+change, though often associated with metamorphic rocks, is not itself
+metamorphic. The term _gneiss_ applied to these rocks is a misnomer,
+unless the term be used even more vaguely than it is at present.]
+
+[Footnote 19: See Lehmann, _Untersuchungen über die Entstehung der
+Altkrystallinischen Schiefergesteine mit besonderer Bezugnahme auf das
+Sächsische Granulitgebirge_, Plate XI. fig. 1.]
+
+We have already seen that the existence of unconformities has been
+utilised in the demarcation of large divisions of strata in various
+regions, and whether they be utilised in this manner or not, their
+detection is a matter of importance to the stratigraphical geologist,
+as they afford information concerning the occurrence of great physical
+changes during their production. These unconformities may also be
+closely simulated by structures produced in very different manner.
+
+The occurrence of an unconformity implies the denudation of one set of
+beds before the deposition of another set upon them, and accordingly
+the denuded edges of the lower set will somewhere abut against the
+lower surface of the lowest deposit or deposits of the overlying
+set[20]. The existence of an unconformity may often be detected in
+section, but when the unconformity is upon a large scale this may not
+be possible, but it will be discovered by mapping the strata and will
+be apparent on a map owing to the deposits of the lower set of beds
+abutting against the others. This is well seen where the Permian rocks
+of Durham, Yorkshire, and Nottinghamshire rest upon different members
+of the underlying Carboniferous series, and will be noticed on any
+good geological map of England. But a similar effect may be caused by
+a fault, so that mere inspection of a map or even of the strata in the
+field and discovery of one set of beds ending off against another does
+not prove unconformity. When the fault is a normal one, with low hade
+(that is, having a fissure approaching the vertical position), the
+outcrop of the fault-fissure will approximate to a straight line if
+the fault has a straight course, even if the ground be very uneven,
+whereas, if the plane of unconformity has not been tilted to a high
+angle from its original horizontal position, it will crop out in a
+sinuous manner across uneven ground, in a way similar to that of beds
+which are nearly horizontal, so that though the general trend of the
+outcrop of the plane of unconformity may be fairly straight, its
+deviation from a straight line will be frequent and marked, as seen in
+the case of the Permian unconformity above referred to. But if the
+unconformable junction has been highly inclined its outcrop will
+resemble that of a normal fault, or if the fault be a thrust-plane
+with high hade, the outcrop of this will resemble that of an
+unconformable junction which has not been greatly tilted from its
+original horizontal position. In these cases we require more evidence
+before we can decide whether we are dealing with an unconformable
+junction or a faulted one.
+
+[Footnote 20: An unconformity may be simulated or an actual
+unconformity rendered apparently more important, as the result of
+underground solution of the underlying strata subsequently to the
+deposition of the upper set upon them, and any insoluble materials in
+the underlying strata may be left as an apparent pebble-bed at the
+base of the upper beds. This is seen at the junction of the Tertiary
+beds with the chalk near London. Subterranean water has dissolved the
+upper part of the chalk, increasing the unconformity which naturally
+exists between chalk and Tertiary beds, and the insoluble flint of the
+dissolved chalk is left as a layer of 'green-coated flint' at the base
+of the Tertiary deposits.]
+
+The lowest deposits of the newer set of strata lying above an
+unconformity have probably been laid down in water near the
+shore-line. As the unconformity, if large, implies elevation above the
+sea-level, the deposits first formed after this elevation has ceased,
+and depression commenced, will necessarily be littoral in character
+and possibly of beach-formation, and accordingly we often find that an
+unconformity is marked by the existence of an epiclastic conglomerate
+immediately above the plane of unconformity and, although this need
+not be continuous, it is usually found somewhere along the line of
+junction. The conglomeratic base of the Lowest Carboniferous strata
+when they repose upon the upturned edges of the Lower Palæozoic rocks
+of the dales of West Yorkshire is well known, and may be cited as an
+example. The association of conglomerates with unconformities is
+indeed so frequent that its possible occurrence will always be
+suspected and sought by the geologist. Unfortunately the result of
+recent observation is to show that along thrust-planes of which the
+outcrop simulates those of unconformable junctions, the difficulty of
+discrimination may be increased by the existence of cataclastic rocks
+which bear a close resemblance to epiclastic conglomerates, and which
+may be and have been styled conglomerates. It is well known that
+fragments of the adjoining rocks are knocked into a fault-fissure
+during the occurrence of the movements which cause the fault, to
+constitute a _fault-breccia_, and as the result of the abrasion of
+these fragments by chemical or mechanical agency, the angular
+fragments may become rounded and converted into rounded pebble-like
+bodies, when the rock is changed into a _fault-conglomerate_. Fig. 5,
+from a photograph kindly supplied by Prof. W. W. Watts, shows a stage
+in the formation of a conglomerate of this nature from a
+fault-breccia; the fragment on the right remains angular, whilst those
+on the left have become much more rounded. The illustration is from a
+case described by Mr Lamplugh occurring in the slaty rocks of the Isle
+of Man, and Mr Lamplugh's paper[21] furnishes the reader with
+references to other examples of the production of similar rocks. No
+general rule can be laid down for distinguishing the true from the
+apparent unconformity, for the attendant phenomena will differ in each
+case; but if a fault-conglomerate should be suspected, the observer
+should try to ascertain whether fragments of a newer rock are imbedded
+in an older one, which sometimes occurs; he should note the existence
+of extensive slickensiding along the plane of junction and along
+planes of faulting, though the existence of these, implying as it does
+the occurrence of differential movement along the plane, does not
+prove that the movement was necessarily great, or that it did not take
+place along a plane of original unconformity; above all, he should
+look for structures such as mylonitic structure, pseudo-stromatism,
+development of new minerals, crushing out and stretching of fossils
+and fragments and, in short, for any structure which is familiar to
+him as a result of orogenic movements.
+
+[Footnote 21: Lamplugh, G. W., "On the Crush-Conglomerates of the Isle
+of Man," _Quart. Journ. Geol. Soc._, vol. LI. p. 563.]
+
+[Illustration: Fig. 5.]
+
+The effects of thrusting not only give rise to appearances suggestive
+of unconformity, but naturally also to a simulation of overlap. The
+thrust-planes are often parallel to original bedding-planes for some
+distance, but must cut across them sooner or later, producing
+lenticular masses which might be supposed to be due to the thinning
+out of beds as the result of cessation of deposition in a lateral
+direction.
+
+Attention has already been directed to the deceptive appearance of
+great thickness of strata which is due to repetition of one stratum or
+set of strata by a series of thrust-planes, so that there is no actual
+inversion of any part of a bed. When masses of limestone are affected
+in this way, the thrust-planes may become sealed up, as the result of
+chemical change, and a compact irregular mass of limestone devoid of
+any definite divisional planes may be the consequence, and beds of
+grit sometimes exhibit the same feature to some extent.
+
+Enough has been said to show that simulation of one structure by
+another has frequently occurred in rocks in so marked a degree as to
+render mistakes easy; and that these examples of 'mimicry' in the
+inorganic world are particularly frequent in rocks which have been
+subjected to great orogenic movements. The student will do well to
+acquaint himself with the macroscopic and microscopic structures
+which may be taken as characteristic of the rocks which have been thus
+affected, some of which can usually be detected with ease, and when he
+discovers them he may suspect that many phenomena which appear
+explicable in one way were in reality produced in a different one, for
+it is frequently very true of a region in which the rocks have been
+violently squeezed, stretched and broken that 'things are not what
+they seem.'
+
+
+
+
+CHAPTER VIII.
+
+GEOLOGICAL MAPS AND SECTIONS.
+
+
+The writer does not propose to give an account of the intricacies of
+geological mapping, for their right consideration requires a separate
+treatise[22]; all he desires is to call attention to some of the uses
+of geological maps as a means of conveying information. A geological
+map may be looked upon as an attempt to express as far as possible in
+two dimensions phenomena which possess three dimensions; this can be
+done to some extent on the actual surface of the map, by conventional
+signs, still more fully, by supplementing the map with sections; but
+best of all by a geological model, which is cut across in various
+directions in order to show the underground structure as well as that
+of the surface.
+
+[Footnote 22: The student is recommended to consult in particular,
+Appendix I. "On Geological Surveying" in _The Student's Manual of
+Geology_, by J. B. Jukes (Third Edition, Edited by A. Geikie), p. 747,
+and _Outlines of Field Geology_, by Sir A. Geikie (Macmillan and
+Co.).]
+
+The ordinary geological map is one which shows the outcrop of the
+strata, subdivided according to age, as they would be seen upon the
+surface of the earth after stripping off the superficial
+accumulations, and it is to be feared that the term 'geological map'
+is associated in the minds of most students with a map of this
+character and of no other. Nevertheless, a great many most important
+observations other than those connected with the order of succession
+of the strata are capable of representation upon a geological map, and
+the possession of a large number of maps of any area upon the geology
+of which a person is engaged--each map to be used for recording
+observations of a particular kind--will save much writing in
+note-books and, what is of more importance, will allow him to compare
+observations which have been made at different times at a glance,
+instead of causing him to search through a series of note-books.
+Still, however well furnished with maps, the geologist will find a
+note-book essential[23].
+
+[Footnote 23: As a result of some experience, the writer recommends
+every student to acquire some skill in the use of the pencil, and if
+to such a degree that he can combine artistic effect with accuracy, so
+much the better. An acquaintance with photography is invaluable: often
+the possession of a camera would enable a section to be recorded,
+which is otherwise lost to science.]
+
+The earliest geological maps represented the variations in the surface
+soils, or at most the general lithological characters of the rocks
+which by their decay furnished the materials for the soils. We have
+seen that the first chronological map was due to William Smith, and
+most subsequent English geological maps have been based upon his map
+of the strata of England and Wales. The order of succession of the
+strata is represented in these maps to some extent by the use of
+arrows to indicate the direction of dip of the strata, though this is
+not an unerring guide where strata are reversed, and accordingly the
+addition of a legend at the side of the map may be looked upon as
+essential to the correct understanding of the map itself. The legend
+is usually in the form of a section of a column, the strata being
+arranged in right order, the oldest at the base and the newest at the
+summit, the colours by which the strata are indicated being similar to
+those placed upon the map. Other information besides the mere order of
+succession of the strata may appear in the legend; thus their relative
+and actual thicknesses can be indicated if the column is drawn to some
+definite scale, and a brief description of the lithological characters
+of the rocks may well be appended to the side of the column. On the
+actual maps it is customary to exhibit the outcrop of the junctions of
+all igneous rocks as well as of the sedimentary ones: the nature of
+the metamorphism which sedimentary rocks have undergone at the contact
+with igneous ones may be and often is indicated by suitable signs; the
+position of faults is shown, and often also that of metalliferous
+veins, the nature of the ore in the latter being further indicated in
+some suitable manner, as by giving the recognised symbol for the
+metal; and in many maps an attempt is made to show the variations in
+dip and strike of the cleavage-planes.
+
+The Geological Survey of the United Kingdom publishes two sets of
+maps, one showing the 'solid geology' and the other the 'superficial
+geology.' It is easier to understand these terms than to define them,
+for in Britain there is a sharp line between the two everywhere except
+near Cromer. The maps showing the superficial geology represent
+gravels, glacial drifts and other incoherent accumulations of
+geologically recent origin, which to a greater or less extent mask the
+strata below which are usually composed of more or less solidified
+material. The maps showing the solid geology display the outcrops of
+these strata, though it is usual to insert alluvium upon these maps,
+as it is often impossible to trace the junction-lines of the strata
+below it. Attention has already been directed to the fact that these
+maps of solid geology, though chronological, that is, having the
+strata represented according to age, are founded largely upon
+lithological differences, rather than upon included organisms; and it
+has been stated that for theoretical purposes two sets of
+chronological maps, one founded upon lithological differences, the
+other upon difference of fossil organisms, would be extremely
+valuable.
+
+Other phenomena are often best represented upon separate maps, for if
+all observations are crowded upon one map the result will be very
+confusing. Special glacial maps showing the contour of the country,
+with the portions between the contour lines coloured differently
+according to altitude, say the country between sea-level and 500 feet
+light green, that between 500 and 1000 dark green, that between 1000
+and 1500 light brown and so on, exhibiting the direction of all
+observed glacial striae, the distribution of boulders so far as it is
+possible, and any other glacial phenomena which can be noted upon the
+map, will be valuable to the student of glaciation[24].
+
+[Footnote 24: For examples see Tiddeman, R. H., "Evidence for the
+Ice-Sheet in North Lancashire and the adjacent parts of Yorkshire and
+Westmorland," _Quart. Journ. Geol. Soc._, vol. XXVIII. pl. XXX., and
+Goodchild, J. G., "Glacial Phenomena of the Eden Valley" &c., _Quart.
+Journ. Geol. Soc._, vol. XXXI. pl. II.; and for a map of distribution
+of boulders, Ward, J. C., "Geology of the Northern Part of the English
+Lake District" (_Mem. Geol. Survey_), pl. IV.]
+
+Various structural features may be well displayed on separate maps.
+The trend of the axes of folds will be useful, and may be accompanied
+by other information of cognate character[25]; maps of the
+distribution of joint planes may be given in combination with those
+showing the folding of the strata if it be desired to exhibit the
+relationship between these; or with the physical features of the
+country, if the dependence of physical features upon joint structure
+be under consideration[26]. Much information concerning cleavage may
+be acquired from a map showing anticlinal and synclinal axes of
+cleavage[27], or the actual strike of the cleavage over different
+parts of a map may be represented, and its relationship to the
+geological structure of the district exhibited[28].
+
+[Footnote 25: See Bertrand, M., "Sur le Raccordement des Bassins
+Houillers du nord de la France et du sud de l'Angleterre," _Annales
+des Mines_, Jan. 1893, Plate 1.]
+
+[Footnote 26: See Daubrée, A., _Études Synthétiques de Géologie
+Expérimentale_, 1^{ère} Partie, Plates III.-VI., for an example of the
+latter, which is also interesting as showing the utility of a map on
+transparent paper super-posed on another, when illustrating the
+connexion between two sets of structures.]
+
+[Footnote 27: Ward, J. C., _Geology of the Northern Part of the
+English Lake District_, Plate IX.]
+
+[Footnote 28: Harker, Alfred, "The Bala Volcanic Series of
+Caernarvonshire" (_Sedgwick Essay_ for 1888), Fig. 5.]
+
+Maps exhibiting changes in physical geography appertain to the
+geologist as well as to the geographer. The position of ancient
+beaches, former lakes, representation of the changes in the courses of
+rivers and kindred phenomena may be shown upon maps, and will prove
+useful[29].
+
+[Footnote 29: For examples of maps of this kind, see Kjerulf, Th.,
+_Die Geologie des südlichen und mittleren Norwegen_.]
+
+A perusal of the maps to which reference has been made above will give
+the student some notion of the extent to which maps may be utilised to
+represent geological structures, and may suggest other methods by
+which they may be utilised.
+
+A geological section is usually drawn in order to exhibit the lie of
+the rocks, as it would be seen if a vertical cutting were made in that
+part of the earth's crust which is under consideration. The character
+of the section will depend upon circumstances. The Geological Survey
+of Great Britain issues two kinds of sections which are usually spoken
+of as vertical sections and horizontal sections, though each is in
+truth a vertical section; but whereas in the former the horizontal
+distance represented is small as compared with the thickness of the
+strata, in the latter the rocks of a considerable horizontal extent of
+country are exhibited in the section, and the section is not carried
+down to a great depth below the earth's surface. There is no essential
+difference between the two kinds of section, and often sections are
+drawn which cannot be definitely classed as belonging to either kind,
+but in extreme cases the vertical section is a representation of the
+order of succession as it would appear if the rocks were horizontal,
+no matter how disturbed they may be in reality; whereas the horizontal
+section represents the strata as they actually occur, with all the
+folds and faults by which they are affected. The accompanying figure
+(Fig. 6) represents a horizontal section on the left side of the
+figure with a vertical section of the same rocks on the right side.
+
+[Illustration: Fig. 6.]
+
+Vertical sections are extremely useful when it is desirable to compare
+variations in the strata over wide extents of country: this can be
+done by drawing a series of columns of the strata, each showing in
+vertical section the lithological characters and thicknesses of the
+strata in one place, whilst the relationship between the strata of
+two different places may be indicated by joining the beds of the same
+age by dotted lines as shown in Fig. 7[30].
+
+[Illustration: Fig. 7.]
+
+[Footnote 30: It is useful to adopt conventional symbols for the
+representation of strata of different lithological characters, and so
+far as possible to adhere to the same kind of symbol for any one kind
+of deposit. Those which are generally in use, are rough pictorial
+representations of the characters of the deposits, as shown in Fig. 7.
+The conglomerate is indicated by circular marks representing
+cross-sections of the pebbles, a breccia by triangular marks
+signifying that the fragments are angular and not rounded; a sandstone
+is indicated by dots to represent the grains of sand; a mud, clay or
+shale by continuous or broken horizontal lines, which reproduce the
+appearance of the planes of lamination so frequent in beds of this
+composition; a limestone is usually marked by the use of regular
+horizontal lines illustrating the pronounced bedding, with vertical
+lines at intervals to represent the regular jointing which occurs in
+so many limestones: the nature of the bedding may be further shown by
+drawing the lines comparatively far apart when the limestone is a
+thick-bedded one, nearer together when it is thin-bedded. Igneous
+rocks are represented by crosses or irregular V-shaped marks,
+illustrating the absence of stratification and presence of joints.
+
+Volcanic ashes are sometimes represented by dots, at other times by
+signs somewhat similar to those which are used for true igneous rocks.
+Sedimentary rocks which are composed of more than one kind of material
+may be further shown by a combination of two symbols, thus the
+existence of a sandy clay may be shown by means of a combination of
+horizontal lines and dots, and so with other combinations. The
+practical geologist should become accustomed to the use of these
+symbols in his note-book; if used, they will save much writing.
+
+These symbols are used in some of the later illustrations to this
+book.]
+
+The horizontal section is one which is in constant use by the
+practical geologist: the results of the first traverse of a district
+may be jotted down in his note-book in the form of a horizontal
+section (with accompanying notes), and the written memoir on the
+geology of any district composed largely of stratified rocks will
+almost certainly require illustration by means of these sections.
+Perhaps nothing more clearly marks the careful observer than the
+nature of the sections which he makes, and geological literature is
+too frequently marred by the publication of slovenly sections. A badly
+drawn section not only offends the eye, it may and frequently does
+convey inaccurate information.
+
+[Illustration: Fig. 8.]
+
+In the above figure (Fig. 8) taken from Sir Henry de la Beche's
+"Sections and Views Illustrative of Geological Phænomena," Plate II.,
+the lower drawing represents a section drawn to true scale, while that
+above shows one which is exaggerated. The student who saw this would
+infer that the uppermost beds on the left side of the upper section
+rested unconformably upon the dotted beds beneath, and once abutted
+against them in that portion of the figure where the beds have been
+removed by denudation in the deep valley, whereas an examination of
+the section drawn to true scale shows that the unconformity does not
+exist (although there is one at the base of the deposits marked by
+dots), and that there is room for the higher deposits to pass above
+those marked by dots at the place where the former have been removed
+by denudation. Whenever possible, horizontal sections should be drawn
+to true scale, the vertical heights being on the same scale as the
+horizontal distances. Sections which are so drawn represent the nature
+of the surface of the country as well as the relationship of the
+strata, and often illustrate in a marked degree the influence which
+the character of the strata has exerted upon the nature of the
+superficial features of a country. If it be impossible to draw a
+section in which the elevations and horizontal distances are
+represented upon a true scale, the former ought to be drawn on a scale
+which is a multiple of the latter; thus the vertical heights may be
+shown on 2, 3, or 4 or more times the scale chosen for the horizontal
+distances; when this is done, it will often be necessary to show the
+strata with an exaggerated dip, and accordingly the exaggerated
+section loses some of its value, though if vertical and horizontal
+scales bear some definite proportion it will still be more valuable
+than a rough diagram which is not drawn to any scale.
+
+Section-drawing cannot be satisfactorily accomplished without some
+practice, and the student is strongly advised to acquire the art of
+drawing good sections; the writer can assert as the result of
+considerable experience in the conduct of examinations of all kinds,
+that slovenly sections are the rule in candidates' papers, and good
+sections very rarely appear. Study of the six-inch maps and horizontal
+sections (drawn on the same scale) of the Geological Survey of the
+United Kingdom will enable the student to familiarise himself with
+admirable sections, and it should be his aim to produce sections like
+these. He is recommended to take some of these six-inch maps which
+show contour-lines as well as the disposition of the strata, and to
+draw sections on the scale of six inches to the mile, vertical and
+horizontal, exhibiting the proper outline of the ground and the
+arrangement of the strata, and afterwards to compare them with the
+published sections. The sections should be drawn as far as possible at
+right angles to the general strike of the strata. Some datum-line is
+taken for the base of the section (say sea-level) and offsets drawn
+vertically from this where the section crosses a contour-line or
+recorded height. The height is marked on these offsets; thus if a
+recorded height of 2700 feet (just over half a mile) occurred on the
+line of section a height of somewhat over three inches is marked on
+the offset, and so with the other points where the section crosses
+contours or recorded heights. By joining these points on the offsets,
+giving the connecting lines curves similar to those which are likely
+to occur in nature, the general character of the surface of the ground
+is represented. The geology of the district is next shown. Wherever a
+dip is marked on the map, the direction and amount of dip is shown by
+a short line on the section, and where dips are not actually seen
+along the line of section, the dips which are nearest to that line on
+the map must be considered, and marked on the section. The lines of
+junction between the various deposits shown by different colours upon
+the map are inserted on the section as short lines, the inclination
+being judged by study of the nearest dips; faults and igneous rocks
+must be marked off, and any indication of the hade of the fault or the
+slope of the edges of the igneous rock which the map affords will be
+taken into account. The section will then appear somewhat as shown in
+the following figure:
+
+[Illustration: Fig. 9.]
+
+and sufficient indication of the trend of the rocks will be obtained
+to shew that they form portions of curves which may then be filled in
+as shown in Fig. 10 and the section will be complete.
+
+[Illustration: Fig. 10.]
+
+It will be noticed that the small dyke of igneous rock on the right of
+the main dyke is joined to it lower down, though no indication of this
+is given along the line of section; but the requisite information for
+this and evidence of the existence of the small dyke proceeding from
+the left-hand side of the main one may be obtained by the study of
+the rocks in a valley on one side or other of the line of section.
+
+After the student has become conversant with the nature of geological
+maps and sections, and has read Sir A. Geikie's _Outlines of Field
+Geology_, he should on no account omit to learn something of the art
+of making geological maps, by going into the field and attempting to
+produce a map, for the art of geological surveying does not come
+naturally to any one, and some acquaintance with the methods of
+surveying is a necessity to everyone who wishes to make original
+geological observations, though all cannot expect to afford the time
+and acquire the skill necessary for the production of maps vying with
+the detailed maps of the Government Survey. Before actually attempting
+to draw lines on a map on his own account, he will do well to tramp
+over a portion of a district with the published geological map in his
+hands, selecting a country which is not characterised by great
+intricacy of geological structure, and he can then attempt to
+represent the geology of another portion of the same district without
+consulting the published map. Of all the districts of Britain with
+which he is acquainted the writer believes that the basin of the river
+Ribble, in the neighbourhood of the town of Settle in the West Riding
+of Yorkshire, is best adapted for studying field geology in the way
+suggested above, for the main geological features are marked by
+extreme simplicity, and the exposures are good, whilst the presence of
+an important fault-system and of a great unconformity relieve the area
+from monotony. Anyone who stands on the summit of Ingleborough or
+Penyghent will grasp the main features of a portion of the district
+without any difficulty, for it lies beneath his feet like a geological
+model, and when the student has mastered and mapped in the leading
+features, he can find bits of country with geology of varying degrees
+of complexity amongst the Lower Palæozoic rocks of the valleys which
+run down to Ingleton, Clapham, Austwick and Settle.
+
+The biologist is supplied with laboratories at home and abroad, where
+he may study his science under the best conditions. Would that some
+munificent person would found, in a district like that referred to
+above, a geological station where Cambridge students would have the
+means of acquiring a knowledge of field-geology under conditions more
+favourable than those presented by the flats around the sluggish Cam!
+
+
+
+
+CHAPTER IX.
+
+EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED.
+
+
+The establishment of the order of succession of the strata, and the
+correlation of strata of different areas merely pave the way for the
+geologist. To write the history of the earth during various geological
+ages, he has to ascertain the physical and climatic conditions which
+prevailed during the successive geological periods, and to study the
+various problems connected with the life of each period. In the
+present chapter an attempt will be made to illustrate the methods
+which have been pursued in order to write to the fullest degree which
+is compatible with our present knowledge, the earth-history of various
+ages of the past. In making this attempt, the physical and climatic
+conditions may be first considered, and their consideration followed
+by that of the changes in the faunas, though it will frequently be
+necessary to refer to one set of conditions as illustrative of the
+other.
+
+It will be assumed here that the great principle of geology, that the
+modern changes of the earth and its inhabitants are illustrative of
+past changes, is rigidly true. Reference will be made to this
+principle in a later chapter, but it is sufficient to state here that
+the study of the sediments which have been deposited from the
+commencement of Lower Palæozoic times to the times in which we now
+live bear the marks of having been formed under physical conditions,
+which, in the main, are similar in kind to those which prevail upon
+some part of the surface of the lithosphere at the present day.
+
+One of the most important inferences of the stratigrapher relates to
+the existence of marine or terrestrial conditions over an area at any
+particular time, and we may, in the first place, consider the evidence
+which supplies us with a clue to this subject.
+
+It has been previously stated that the ocean is essentially the
+theatre of deposition, the land that of destruction, and accordingly,
+the presence of deposit as a general rule indicates the evidence of
+marine conditions during the formation of those deposits, though this
+is not universally the case. Again, as denudation is practically
+confined to the land areas, and the shallow-waters at their margins,
+unconformity on a large scale gives evidence of the existence of
+terrestrial conditions in the area in which it is developed, during
+its production. Accordingly a mass of deposit separated from deposits
+above and below by marked unconformities shows the alternation of
+terrestrial conditions (during which the unconformity was produced)
+and marine conditions (during which the deposits were laid down). The
+deposits formed after an unconformity has been developed will
+naturally be of shallow-water character, as will also be those of the
+period immediately preceding the incoming of conditions which will
+cause the occurrence of another unconformity, and between these two
+shallow-water periods will occur a period when deeper-water conditions
+probably prevailed. We can therefore not only divide the history of
+any particular area into a series of chapters, of which every two
+successive ones will describe a continental period and a marine one,
+but each marine period may be divided into three phases--a
+shallow-water phase at the commencement, an intermediate deeper-water
+phase, and a shallow-water phase at the end. These phases are
+frequently complicated by the occurrence of a host of minor changes,
+but on eliminating these, the effects of the three great phases are
+shown by study of the nature of the strata, and their recognition does
+much to simplify the detailed study of the stratigraphical geology of
+various parts of the earth's surface.
+
+In discriminating between terrestrial conditions and marine ones, the
+existence of unconformities is of great importance in marking
+terrestrial conditions and is often the only available evidence, for
+no accumulations or deposits formed on the land may be preserved to
+testify to the terrestrial conditions[31]. When terrestrial deposits
+and accumulations do occur, they are extremely important, and it is
+necessary to allude to the points wherein they differ from marine
+deposits.
+
+[Footnote 31: The term terrestrial is used above in opposition to
+marine, to include the conditions prevalent above sea-level. The term
+continental would be better if it did not exclude insular conditions.
+Accordingly deposits formed in rivers, and fresh-water and salt-water
+lakes are spoken of as terrestrial.]
+
+Apart from organic contents, the mechanically formed deposits of
+rivers and lakes resemble in general characters the shallow-water
+deposits of the ocean, though they are usually less widely
+distributed. It is the accumulations which have actually been formed
+as æolian rocks, or those which have been laid down as chemical
+precipitates in salt-lakes which, by study of lithological characters,
+furnish the most convincing evidence of their terrestrial origin.
+
+Many æolian accumulations may be looked upon as soils, if the term
+soil be used in a special sense to refer to the accumulations which
+are produced as the result of the excess of disintegration over
+transportation in an area, whilst others are due to transport which
+has not been sufficiently effective to carry the material to the sea.
+When the weathered material accumulates above the weathered rock, it
+depends chiefly upon climate whether the disintegrated rock becomes
+mingled with much decayed organic matter forming humus. If this
+organic matter exists in quantity, the probability is that the
+accumulation is a terrestrial one, though this is by no means
+necessarily the case, for under exceptional circumstances a good deal
+of humus may be deposited in the sea, as beneath the mangrove-swamps
+which line the coasts of some regions, and to go further back, in the
+case of the Cromer Forest series of Pliocene times, or some coals,
+such as the Wigan Cannel Coal of the Carboniferous strata.
+
+In addition to the work of water, which affects both land and
+sea-deposits, the land is especially characterised by the operations
+of wind and frost upon it, for these produce results which may
+frequently serve to differentiate a land-accumulation from a deposit
+laid down beneath sea-level. The effect of wind in rounding the grains
+of sand which are blown by it is well-known, and samples of the
+'millet-seed' sands of desert regions are preserved in most museums.
+The greater rounding which characterises wind-borne as compared with
+water-borne sand grains is due, in great measure, to the greater
+friction between the grains when carried by the air than when swept
+along by the water. Under favourable circumstances water-worn grains
+may become rounded, especially when agitated by gentle currents
+sweeping over a shoal[32]; but a large mass of sand, in which most of
+the grains have undergone much rounding so as to give rise to
+'millet-seed' sand, will nevertheless be probably formed by
+wind-action except where a marine deposit is formed of material
+largely derived from an earlier æolian one. The effect of frost is to
+split rocks into fragments which are more or less angular before they
+are subjected to water-action. The broken fragments are prone to
+collect on slopes as screes, and as any scree-material falling into
+the sea is likely to become rounded except under conditions which
+rarely prevail, the existence of much scree-material in a rock
+suggests its terrestrial origin. Glaciers gave rise to terrestrial
+moraines, which may occasionally be identified as land-accumulations
+by mere inspection of their physical characters, but all geologists
+are aware of the difficulties with which they are confronted when they
+attempt to discriminate between terrestrial and marine glacial
+deposits.
+
+[Footnote 32: Cf. Hunt, A. R., "The Evidence of the Skerries Shoal on
+the wearing of Fine Sands by Waves," _Trans. Devon. Assoc._, 1887,
+vol. XIX. p. 498.]
+
+The existence of much material amongst the stratified rocks which has
+been precipitated from a state of solution is an indication of the
+terrestrial origin of the rocks, which were laid down on the floors of
+the inland seas, separated more or less completely from the open
+ocean; for the waters of the ocean are capable of retaining in
+solution all of the material which is brought down to them, and
+accordingly precipitates of carbonate of lime, rock-salt, gypsum and
+other compounds formed from solution, are only formed on a large scale
+in inland lakes, though they may be formed to some extent when the
+water of a lagoon is only slightly connected with that of the open
+ocean, and the evaporation is great, for instance in the lagoons of
+coral reefs. Certain physical features often mark the deposits of
+chemical origin, cubical or hopper-crystals of rock-salt may be
+dissolved, and the hollow afterwards filled with mud, so that the rock
+surfaces are sometimes marked with pseudomorphs of mud after
+rock-salt. Sun-cracks and rain-prints impressed on the rock are not
+actual indications of terrestrial origin of the rocks on which they
+are found, for the shallow-water muds of an estuary may be deposited
+in the sea and yet exposed to the action of the air at low tide, but
+they mark very shallow-water deposits which have been exposed to the
+atmosphere immediately after their formation if not during the time
+they were formed, and they frequently occur amongst the deposits of
+inland lakes.
+
+It will be observed that the characters of the terrestrial
+accumulations serve to distinguish them to some extent from the marine
+ones, but they also enable one to detect to some degree the actual
+conditions under which the accumulation was produced, whether on the
+mountain-slope, or in the plain, the desert or the fen, the river-bank
+or the lake-floor.
+
+The conditions of formation of the marine deposits may be
+distinguished within certain limits with ease, by examination of their
+physical characters, for the near-shore deposits will generally be
+coarser and contain more mechanically-transported material than the
+sediments which accumulate at a greater distance from the shore,
+though it is not safe to infer that deposits are formed away from the
+shore on account of the absence of mechanically-transported sediments.
+In districts where the mechanically-transported material is rapidly
+deposited, organic deposits of great purity may form close to the
+coast-line; for instance, when the rivers of a country end in fjords,
+the mechanical sediments are deposited in the fjords, and the sea
+around the coast is free from this sediment, and there the organisms
+can build up deposits of great purity; and a similar thing may happen
+when the rivers on one side of a country have short courses, and do
+not carry down much sediment, which occurs when the watershed is near
+the coast. On the one hand, clay may be formed in considerable purity
+near the coast, where the supply of mud is so great that the organisms
+existing there can do little in the way of contribution to the mass of
+the deposit, or it may be formed on the other hand in great depths of
+the ocean, where the supply of sediment is extremely small, but where
+all the organic tests become dissolved; as the characters of the deep
+sea clays are mainly negative, a geologist examining the rocks of the
+geological column would have much difficulty in distinguishing a
+deep-water clay from a shallow-water one by its lithological
+characters only. In cases of difficulty, information of importance is
+likely to be furnished by examination of the relative thickness of
+equivalent deposits in adjoining areas, for if we find a mass of clay
+a few feet thick in one region represented by hundreds of feet of clay
+and limestone in another, the former mass probably accumulated slowly
+and at some distance from the land; again, the uniformity of
+lithological characters of a deposit over a very wide area is a
+possible indication of its formation away from land, but this is not a
+safe guide, for reasons which will eventually appear, unless it can be
+shown that the deposit is everywhere of the same age.
+
+A clue to climatic conditions is frequently furnished by the physical
+characters of accumulations, especially terrestrial ones. The
+accumulations containing a large percentage of hydrocarbons have
+probably been formed under fairly temperate and moist climatic
+conditions, whilst the existence of millet-seed sandstones associated
+with chemical deposits points to desert conditions and inland lakes,
+requiring a dry climate and probably a warm one. Glaciated surfaces
+and glacial deposits of course indicate a low temperature. Some
+geologists profess that occasionally they can even determine the
+direction of the prevailing winds during past periods, by examination
+of the character of ripple-marks, rain-pits and other features, though
+it is doubtful whether much reliance can be placed upon these obscure
+indications.
+
+Useful as is the physical evidence supplied by deposits, as an index
+to the conditions under which they were formed, it is usually only
+supplementary to the evidence derived from a study of the fossils.
+Fossils when present in the rocks, usually supply considerable
+information concerning the prevalent conditions during the deposition
+of the rocks. By them we can not only separate marine from terrestrial
+deposits, but also freshwater deposits from æolian accumulations; each
+kind of deposit will generally contain the remains of organisms which
+existed under the conditions prevalent in the area of formation of the
+rock, though it is of course a frequent thing for a terrestrial
+creature or plant to be washed into a freshwater area or into the sea.
+In an æolian deposit, the invertebrate remains may be those of any
+air-breathing forms, as insects, galley-worms, spiders, scorpions and
+molluscs. The land-molluscs are all univalve. Of vertebrates, we may
+find the bones and teeth of amphibians, reptiles, birds and mammals.
+Occasionally freshwater or even marine forms may be found in an æolian
+deposit, but they will be exceptional. Marine shells are often blown
+amongst the sand-grains of the coastal dunes, and seagulls and other
+birds frequently carry marine organisms far inland.
+
+The creatures frequenting fresh water differ from those of the land
+and of the sea. The most abundant vertebrate remains will be those of
+fishes, and of the invertebrates we find mollusca preponderate. The
+variety of molluscs is not so great as in the case of marine faunas.
+The bivalves always possess two muscular scars on each valve (except
+adult _Mulleria_); whilst many marine shells as the oyster have only
+one muscular scar on each valve. (See Fig. 11.)
+
+[Illustration: Fig. 11.
+
+_A._ Monomyary shell with one scar.
+
+_B._ Dimyary shell with two scars.]
+
+These scars mark the attachment of the adductor muscles, for drawing
+the valves together, and the shells with only one impression on each
+valve are called _monomyary_, those with two impressions _dimyary_.
+The discovery of monomyary shells indicates with tolerable certainty
+the marine character of the deposit in which they are found, though
+their absence cannot be taken as proof of freshwater origin. The
+beaks or umbones of the bivalves are often corroded in freshwater
+deposits, as may be seen by examining shells of the common freshwater
+mussel. "All univalve shells of land and freshwater species, with the
+exception of _Melanopsis_ and _Achatina_, which has a slight
+indentation, have entire mouths; and this circumstance may often serve
+as a convenient rule for distinguishing freshwater from marine strata;
+since if any univalves occur of which the mouths are not entire, we
+may presume that the formation is marine[33]."
+
+[Footnote 33: Lyell's _Students' Elements of Geology_, Second Edition
+(1874), Chap. III. A good account of the differences between
+freshwater and marine organisms, from which some of the facts here
+cited are extracted, will be there found.]
+
+[Illustration: Fig. 12.
+
+_A._ Holostomatous shell.
+
+_B._ Siphonostomatous shell.]
+
+In Fig. 12 _A_ shows a freshwater shell (_Vivipara_) with entire
+mouth, whilst _B_ exhibits the shell of a marine gastropod
+(_Pleurotoma_) with a notched mouth. The entire-mouthed shells are
+called _holostomatous_ whilst those which are notched, the notch being
+often prolonged into a canal, are termed _siphonostomatous_.
+
+Many groups of invertebrates are seldom or never found in fresh water.
+Of exclusively or nearly exclusively marine creatures we may name the
+foraminifera, radiolaria, sponges with a hard framework, most hydrozoa
+which secrete hard parts, corals, echinoderms, cirripedes, king-crabs,
+locust-shrimps, most polyzoa, brachiopods, pteropods, heteropods, and
+cephalopods. Of extinct groups, the graptolites and trilobites seem to
+have been entirely confined to the sea.
+
+In the modern and comparatively modern deposits, the forms frequently
+belong to existing genera, and we get fairly conclusive evidence of
+the conditions of deposit by determination of the genera. The
+terrestrial (including freshwater) molluscs have mostly a long range
+in time. We find pulmoniferous gastropods of living genera in the
+Carboniferous period, one (_Dendropupa_) belongs to a subgenus of the
+modern land-shell _Pupa_, the other (_Zonites_) to a subgenus of the
+snail group _Helix_. Many freshwater molluscs as _Unio_, _Cyclas_, and
+_Physa_ are found amongst the secondary rocks, and give a clue to the
+origin of the deposits which contain them. Many extinct genera are
+closely allied to modern genera, and their mode of existence may be
+assumed with fair certainty. With all these guides, we may sometimes
+be left in doubt as to the conditions of deposit when organisms are
+few in number; thus, it is yet a matter for discussion whether the Old
+Red Sandstone and many of the deposits of the Coal Measures of Britain
+were of freshwater or marine origin.
+
+In considering the possibility of fossils having been carried from
+land to water or _vice versa_, it will be remembered that generally
+speaking they are more readily transferred from a higher to a lower
+level, so we are more likely to find remains of land-animals and
+plants in fresh water or the sea, and relics of freshwater animals and
+plants in the sea, than of marine or freshwater animals and plants in
+land, or marine organisms in fresh water. River-gravels and lacustrine
+deposits are especially prone to contain a considerable intermixture
+of land-forms with those proper to the station.
+
+Fossils supply much information concerning the depth and distance from
+land at which the deposits were laid down. When portions of the
+ocean-water have been separated to form inland lakes, the water
+becomes saltier than that of the open ocean, if the evaporation is
+greater than the supply of fresh water, and the life of the inland sea
+undergoes change under the unfavourable conditions set up. Many forms
+disappear altogether, and those which survive tend to become stunted,
+and the shells of many of the mollusca are abnormally thin; the fauna
+of an inland sea though it may have abundance of individuals is apt to
+be characterised by paucity of species.
+
+Turning now to the faunas of the open oceans, it is found that in
+addition to latitude, the distribution of organisms is affected by
+depth, and by the nature of the sea-floor, and accordingly we find
+different organisms in different areas; and in examining the same area
+the organisms inhabiting different depths are not all the same, and at
+the same depth some kinds of animals have different _stations_ from
+those of others, one creature being confined to a sandy floor, another
+to a muddy one, and so on[34]. The oceans have been divided into 18
+_provinces_, each of which is more or less characterised by the
+possession of peculiar forms which are termed _endemic_, in contrast
+to the _sporadic_ forms which are widely distributed. In any area
+which is margined by a coast line, the molluscs are distributed in
+zones which were formerly classed as follows:--the _littoral_ zone
+between tide marks, the _laminarian_ zone from low water to fifteen
+fathoms, the _coralline_ zone between fifteen and fifty fathoms, and
+the _deep-sea coral_ zone from fifty fathoms to one hundred fathoms or
+more; this last depth was once supposed to mark the limit of the
+downward extension of marine life, but as the result of modern
+deep-sea soundings we know that organisms extend to a much greater
+depth, and the deep-sea fauna, owing to uniformity of conditions over
+wide areas, contains fewer endemic forms in proportion to the sporadic
+ones than the shallow-water[35]. The deep-sea deposits entomb the
+remains of these deep-sea organisms and also of numerous _pelagic_
+organisms which live upon the surface of the ocean, whose remains sink
+to the ocean-floor after death. Amongst the deposits of the deeper
+parts of the ocean, we find many which are almost exclusively composed
+of the tests of foraminifera, radiolaria and pteropods, the spicules
+of sponges, and the frustules of diatoms; and accordingly the
+existence of foraminiferal, pteropodan, radiolarian, and diatomaceous
+oozes, amongst the strata of the geological column, has been taken by
+some as indicating the prevalence of deep-sea conditions during the
+formation of those deposits: as the purity of a calcareous ooze
+depends upon the absence of mechanical sediment, or volcanic dust, and
+as the component organisms of these oozes are pelagic forms which live
+near the continents as well as in the open oceans, the presence of
+calcareous oozes implies the existence of a _clear_ sea during their
+deposition but not necessarily of a deep one, for if the sea-area be
+far away from land masses, or if the sediment be strained off in
+fjords, calcareous oozes may be formed in shallow water. The existence
+of pure radiolarian or diatomaceous deposits is better evidence of
+deep water, for if they were formed in shallow water we should expect
+an intermixture of calcareous tests, whereas these are dissolved
+whilst sinking into the extreme depths of the ocean. As the deep-sea
+creatures are under very different conditions from those of shallower
+waters, we might expect marked structural differences between the deep
+and shallow-water creatures: one such difference has been emphasized,
+namely the occurrence of animals which are blind or have enormously
+developed eyes in the great depths of the sea, where the only light is
+due to phosphorescent organisms. This is well seen in the case of many
+recent crustacea, and has been noted by Suess in the case of the
+trilobites of some beds which he accordingly infers to be of
+deep-water origin, and it is interesting to find that these creatures
+are found in deposits which give independent evidence of an open-water
+origin. The _Æglinæ_ of the Ordovician strata are frequently furnished
+with enormous eyes, and they are often accompanied by blind
+trilobites, and in Bohemia the blind and large-eyed forms are
+sometimes different species of the same genus, for instance
+_Illænus_[36].
+
+[Footnote 34: For an account of the distribution of one group of
+organisms see Woodward, S. P., _A Manual of the Mollusca_, from which
+many of the following observations are taken.]
+
+[Footnote 35: For an account of the deep-sea fauna, see Hickson, S.
+J., _The Fauna of the Deep Sea_, 1894.]
+
+[Footnote 36: Suess, E., _Das Antlitz der Erde_, 2^{er}. Bd., p. 266.]
+
+As one would naturally expect, the actual depth at which deposits were
+formed can generally be calculated with a greater degree of certainty
+amongst the newer rocks than amongst the older ones. In the case of
+the Pliocene Crags, the depth in fathoms may be confidently given. In
+the Cretaceous rocks attempts have been made to give numerical
+estimates of the depths at which different accumulations were formed,
+but some differences of opinion have arisen in the case of these
+rocks. In the Palæozoic rocks, only a rough idea of the general depth
+can usually be obtained, and no attempt to calculate the depth in
+fathoms is likely to be even approximately correct in the present
+state of our knowledge.
+
+The comminution of fossils has sometimes been taken as an indication
+of shallower water origin of the deposits which contain them, but
+although the hard parts of organisms in a broken condition have
+frequently been shattered by the action of the waves, they may also be
+broken at great depths by predaceous creatures, and in many instances
+the fracture is the result of earth-movements occurring subsequently
+to the formation of the deposits.
+
+Turning now to the difference in organisms which results from
+difference of station, it will be sufficient to give a quotation from
+Woodward's _Manual of the Mollusca_ as an illustration:--"In Europe
+the characteristic genera of _rocky_ shores are _Littorina_,
+_Patella_, and _Purpura_; of sandy beaches, _Cardium_, _Tellina_,
+_Solen_; gravelly shores, _Mytilus_; and on muddy shores, _Lutraria_
+and _Pullastra_. On rocky coasts are also found many species of
+_Haliotis_, _Siphonaria_, _Fissurella_, and _Trochus_; they occur at
+various levels, some only at the high-water line, others in a middle
+zone, or at the verge of low-water. _Cypræa_ and _Conus_ shelter under
+coral-blocks, and _Cerithium_, _Terebra_, _Natica_ and _Pyramidella_
+bury in sand at low-water, but may be found by tracing the marks of
+their long burrows (Macgillivray)[37]."
+
+[Footnote 37: Woodward, S. P., _A Manual of the Mollusca_, p. 151.]
+
+The geologist will naturally select sporadic forms rather than endemic
+ones in comparing the strata of different areas, but how far
+differences in faunas are the result of existence at different times,
+and how far they are due to difference of conditions affecting
+contemporaneous organisms can only be discovered as the result of
+accurate observation. The main points to be regarded when comparing
+the successive faunas of different regions have been noticed in this
+and the preceding chapters, and it has been shown that as the evidence
+is cumulative, it requires the collection of a large number of facts
+obtained by observation of the strata before accurate inferences can
+be drawn.
+
+The indications of climatic conditions furnished by organisms require
+some consideration. In the comparatively recent deposits it is not
+difficult to get some notion of the prevalent climatic conditions when
+the fossils belong to forms closely related to modern genera. The
+existence of the arctic birch and arctic willow, and of shells
+belonging to species now living north of the British Isles, in
+deposits of comparatively recent date in Britain would afford
+convincing evidence of the occurrence of colder climatic conditions
+than those which are now prevalent in the area, even if the evidence
+were not confirmed as it is, by physical proof of glaciation in
+deposits of the same age. Nevertheless, even in these recent beds, we
+have a useful warning, by finding species of elephant and rhinoceros
+associated with northern forms like the lemming, glutton, and musk-ox.
+We know that the species of elephant and rhinoceros (the mammoth and
+woolly rhinoceros) were provided with thick coverings which would
+enable them to resist the severity of an arctic climate, but had not
+these coverings been found, we might have been puzzled by the
+association of forms whose nearest allies are sub-tropical with others
+of arctic character. As we go back in time and deal with earlier
+deposits, the ascertainment of the climatic conditions becomes more
+difficult, as the fossils mostly belong to extinct species, genera or
+even families.
+
+In these circumstances, it is very dangerous to draw conclusions as to
+climatic conditions from examination of a few forms, but when we find
+that plants and animals, terrestrial and marine forms, vertebrates and
+invertebrates alike point to the same conclusion, as in the London
+Clay, where all the fossils belong to forms allied to those now living
+under sub-tropical conditions, the state of the climate may be
+inferred with considerable certainty[38]. The character of the fossils
+must be taken into account rather than their size. There was a
+tendency amongst geologists to believe that large organisms probably
+indicate warm conditions. Recent researches in arctic seas have
+dispelled this belief. Marine algæ of enormous size are found in the
+cold seas, and the size of creatures, abundance of individuals and
+variety of forms in the arctic faunas of some regions is very
+noteworthy. In the Kara Sea, for instance, a variety of creatures were
+dredged up during the voyage of the Vega, and Baron Nordenskjöld makes
+the following pertinent remarks about them: "For the science of our
+time, which so often places the origin of a northern form in the
+south, and _vice versa_, as the foundation of very wide theoretical
+conclusions, a knowledge of the types which can live by turns in
+nearly fresh water of a temperature of +10°, and in water cooled down
+to -2·7° and of nearly the same salinity as that of the Mediterranean,
+must have a certain interest. The most remarkable were, according to
+Dr Stuxberg, the following: a species of Mysis, _Diastylis Rathkei_
+Kr., _Idothea entomon_ Lin., _Idothea Sabinei_ Kr., two species of
+Lysianassida, _Pontoporeia setosa_ Stbrg., _Halimedon brevicalcar_
+Goës, an Annelid, a Molgula, _Yoldia intermedia_ M. Sars, _Yoldia_ (?)
+_arctica_ Gray, and a Solecurtus[39]. "The temperatures were taken by
+a centigrade thermometer. Again we read of the results of dredging off
+Cape Chelyuskin. "The yield of the trawling was extraordinarily
+abundant; large asterids, crinoids, sponges, holothuria, a gigantic
+sea-spider (Pycnogonid), masses of worms, crustacea, etc. _It was the
+most abundant yield that the trawl-net at any one time brought up
+during the whole of our voyage round the coast of Asia_, and this from
+the sea off the northern extremity of that continent[40]."
+
+[Footnote 38: For a discussion as to the value of plants as indices of
+climate see Seward, A. C., Sedgwick Essay for 1892.]
+
+[Footnote 39: Nordenskjöld, A. E., _The Voyage of the Vega_, Vol. I.
+Chap. IV.]
+
+[Footnote 40: _Ibid._ Chap. VII.]
+
+Amongst the marine invertebrates reef-building corals and mollusca
+perhaps furnish the best evidence of climatic conditions. The
+coral-reefs of the Jurassic rocks with large gastropods and
+lamellibranchs clustered around them have been appealed to in proof of
+the existence of sub-tropical conditions during their formation;
+further back in time we find evidence of climate furnished by the
+fossils of the Silurian rocks of the Isle of Gothland in the Baltic
+Sea. Of these, Lindström writes "_The fauna had a tropical character_.
+In consideration of the great numbers of Pleurotomariae, Trochi,
+Turbinidae and the large Pteropods the assumption of a tropical
+character of the fauna may seem justifiable[41]."
+
+[Footnote 41: Lindström, G., _On the Silurian Gastropoda and Pteropoda
+of Gotland_, Stockholm, 1884, p. 33.]
+
+Structure may give some indication of climate even though the organism
+is not allied to living species. The bark of trees in arctic regions
+is often thicker than in more temperate regions, and the leaves of
+arctic plants often have special characters to enable them to resist
+the long periods during which they are deprived of water, though the
+fact that desert-plants frequently shew similar modifications deprives
+this test of any particular value except as a means of corroborating
+conclusions reached from other evidence[42]. The shells of arctic
+mollusca may become stunted, but this is not by any means universal,
+and the same result may be brought about by other abnormal conditions,
+as for instance the increase of salt in a water area by evaporation.
+
+[Footnote 42: For an account of the modifications of the leaves of
+arctic plants, see Warming, Eug., _Om Grønlands Vegetation_,
+Meddelelser om Grønland, 12th part, p. 105.]
+
+On the whole, an examination of the evidence available for
+ascertaining the character of climate by reference to included
+organisms, shews that inferences may be drawn within certain limits,
+but that the task is a difficult one not unaccompanied by danger, and
+every kind of available evidence derived from a study of physical
+phenomena and the included organisms should be utilised before any
+conclusion is drawn.
+
+The likelihood of accurate inference is increased by comparing the
+faunas of various areas; should they seem to indicate a progressive
+lowering of climate when passing from lower to higher latitudes, it is
+probable that the indication is correct. The student is referred to a
+paper by the late Professor Neumayr for an account of the existence of
+climatic zones during the Mesozoic Period[43].
+
+[Footnote 43: Neumayr, M., "Ueber klimatische Zonen während der Jura-
+und Kreidezeit," _Denkschrift. der Math.-Naturwissensch. Classe der k.
+Akad. der Wissenschaften_, Bd. XLVII. Vienna, 1883.]
+
+
+
+
+CHAPTER X.
+
+EVIDENCES OF CONDITIONS UNDER WHICH STRATA WERE FORMED, CONTINUED.
+
+
+In the preceding chapter, attention was drawn to the indications as to
+conditions of deposition furnished by the sediments of any one
+locality, and only passing reference was made to variation in the
+nature of the sediments and their organic contents, when the deposits
+are traced laterally from place to place; some attention must now be
+paid to this matter.
+
+It is sometimes inferred that, whereas similarity of organisms is a
+dangerous guide in correlating the strata of two areas, accurate
+correlations may be made, if the deposits can be traced continuously
+through the intervening interval; no doubt the task is simplified when
+this can be done, but the continuity of deposit of one particular
+composition is no more proof of contemporaneity than the occurrence of
+the same fossils continuously through the interval, imbedded in strata
+of different character, indeed probably not so much so. The existence
+of widespread masses of conglomerate, which are not found as linear
+strips, but which extend in all directions, is in itself an indication
+of this; the Oldhaven pebble bed for instance, in the Tertiary rocks
+of the London basin, is very widely distributed. We cannot suppose
+that coastal conditions prevailed far away from the shore-line, and
+accordingly when a conglomerate occurs in a widespread sheet, and not
+in a linear strip, this is indicative that the deposit has not been
+formed continuously but that strip has been added to strip along an
+advancing or receding shore line, and if this happens with
+conglomerates, it must occur also in the case of other deposits.
+
+[Illustration: Fig. 13.]
+
+In fig. 13[44] let _A_ represent a shore line of a continent which is
+undergoing gradual elevation. A deposit of pebbles _a_ will be formed
+against the coast, one of sand _b_ further away, then one of mud _c_
+and lastly limestone _d_, may be formed in the open sea away from
+land. Naturally there may be intermingling of two kinds of deposit at
+the junctions, but for the sake of simplicity this may be disregarded.
+During the accumulation of the deposits _a_, _b_, _c_, _d_, certain
+sporadic forms may be distributed throughout all the deposits, and
+some of them may become extinct before the deposition of these beds is
+completed, if the process is carried out on a large scale; we may
+speak of the characteristic fossils of this period as fauna I. As the
+result of elevation or of mere silting up of the sea-margin, or of
+both combined, the next mass of pebble-deposit will be laid down
+further away from the original shore, for the shore line will now be
+at _A´_ and not at _A_, and it will partly overlap the mass of sand
+_b_; the sand _b_^1 will also be deposited somewhat further out and
+partly overlap the mud _c_, and similarly the mud _c_^{1} will partly
+overlie the limestone _d_. During the formation of _a_^{1}, _b_^{1},
+_c_^{1}, _d_^{1}, other sporadic forms belonging to a fauna II may
+replace those of the first fauna. In the same way _a_^{2}, _b_^{2},
+_c_^{2}, _d_^{2} will be deposited, and in the meantime a new fauna
+III may arise and replace II. So the process will go on until we
+finally have a group of deposits lying one over the other, consisting
+of a basal accumulation of limestone, succeeded by mud, sandstone and
+pebble-beds in succession. Each of these will be continuous, though
+the inner part of the pebble-deposit was formed long before the outer
+part of the limestone, which is nevertheless beneath a mass of
+pebble-deposit continuous with that formed first, and the various
+deposits will be separated by fairly horizontal planes _x_, _y_, _z_,
+which might be regarded as bedding planes, but which are not so,
+strictly speaking. The true bedding planes will occur at a slight
+angle to these planes of separation, for the structure resembles false
+bedding on a gigantic scale, but of course, the lines separating two
+masses of similar deposit will be practically horizontal and parallel
+to the planes of demarcation of two distinct kinds of material. The
+lines separating two faunas would, under the conditions postulated,
+run approximately parallel to the planes of separation of adjoining
+deposits of the same lithological character but would pass from
+conglomerate, through sandstone, mud and limestone, as indicated by
+the lines 1, 2, 3, ... and the deposits between adjoining lines would
+be contemporaneous[45]. In nature, complications will arise, owing to
+the gradual appearance and disappearance of forms, and the existence
+of endemic species in contemporaneous deposits formed in different
+stations and having different lithological characters.
+
+[Footnote 44: The writer gratefully acknowledges his indebtedness to
+Prof. Lapworth for some of his views concerning deposition of strata.]
+
+[Footnote 45: The lines 1, 2, 3 ... are incorrectly drawn in the
+figure. Line 1 should be drawn so as to separate _a_, _b_, _c_, _d_
+from _a_^{1}, _b_^{1}, _c_^{1}, _d_^{1}, line 2 to separate _a_^{1},
+_b_^{1}, _c_^{1}, _d_^{1} from _a_^{2}, _b_^{2}, _c_^{2}, _d_^{2}, and
+so with the others.]
+
+If elevation ceased and were succeeded by depression, the exact
+opposite would occur, and the pebble beds would be overlain by
+sandstones, these by muds, and lastly limestones would appear. It
+follows that during a marine phase occurring between two
+unconformities we should have a =V=-shaped accumulation of deposits
+with the apex pointing to the part of the shore line which was last
+submerged before the commencement of elevation, as shewn in fig. 14,
+though the beds of the apex will in most cases be denuded during the
+re-emergence.
+
+[Illustration: Fig. 14.]
+
+Indications of the non-coincidence of the planes separating faunas and
+those which separate deposits of one lithological character from those
+of another have already been detected, for instance the 'greensand'
+condition of the Cretaceous period occurs in some places during the
+existence of one fauna, and in others during that of another, though
+the planes have not been traced continuously. Mr Lamplugh has
+furnished another example amongst the Cretaceous rocks of Yorkshire
+and Lincolnshire, but as has already been observed, a great deal
+remains to be done in this direction, and geologists are much in want
+of two sets of stratigraphical maps, in one of which the lines are
+drawn with reference to the differences of lithological character,
+whilst in the other they separate different faunas.
+
+The student will notice the normal recurrence of deposits in definite
+order; conglomerate succeeded by sandstone, mud and limestone, in a
+sinking area, and limestone succeeded by mud, sandstone and
+conglomerate in a rising area. Naturally many instances of departure
+from this rule are seen, owing to local conditions, but on a large
+scale, it is very frequently noted, and recognition of this will
+enable the student to remember the variations in the lithological
+characters of the deposits more easily, than if he simply acquired
+them from a text-book without taking heed as to their significance.
+
+Upon the variations in the lithological characters of deposits and of
+their faunas, when the beds are traced laterally depends very largely
+the successful ascertainment of the existence of former coast-lines,
+the restoration of which constitutes an important part, of
+Palæo-physiography, concerning which some observations may here be
+made[46]. If a set of deposits having different lithological
+characters can be proved to be contemporaneous, the coarser detrital
+accumulations will point to the approach to a coast-line, and the
+actual position of the coast during the period of accumulation of the
+deposits may be very accurately fixed. The pebble-beds at the base of
+the Cambrian rocks of Llanberis indicate the existence of a
+coast-line in that position during the accumulation of those
+pebble-beds. Similar pebble-beds occur at St David's, at the base of
+the Cambrian, but it is impossible in the case of these rapidly
+accumulated sediments to say that two deposited so far away from one
+another were actually contemporaneous, and therefore although we
+might draw a line through Llanberis and St David's to indicate the
+old coast-line of the period, it does not follow that the actual
+beach existed simultaneously at the positions indicated. The
+palæo-physiographer, however, attempts to restore the physical
+conditions of greater thicknesses of deposit; for instance, the
+distribution of land and sea during Lower Carboniferous times over the
+area now occupied by the British Isles is often taken to illustrate
+the methods of restoration of ancient features, and all admit that the
+lithological and palæontological characters of the rocks indicate a
+shallowing of the Carboniferous sea when passing northwards towards
+Scotland. For conveying an idea of the restorations to the student, it
+is almost imperative to portray the distribution of land and sea upon
+a map, and this can only be done by drawing definite lines. It must be
+distinctly understood that these lines are necessarily only an
+approximation to the actual position of the ancient shore-lines, which
+must have shifted again and again during the long period occupied by
+the accumulation of the Lower Carboniferous strata, so that a true
+idea of the positions of the Lower Carboniferous shore-lines could
+only be obtained by placing on a series of maps the successive
+shore-lines of different parts of the Lower Carboniferous period, and
+taking a composite photograph of these, which would appear as a wide
+belt of shaded portion of the map with no definite boundaries. The
+utmost that the maker of palæo-physiographical maps can expect to
+indicate, when dealing with considerable thicknesses of strata, is an
+approximation to the mean position of the shore-lines of the period
+when these strata were deposited. This is extremely valuable in
+enabling the student to understand the significance of the variations
+in the characters of the strata and their organic contents, if he
+distinctly recognises the generalised nature of the map. Examination
+of any two palæo-physiographical maps of the same period by different
+authors will shew wide divergences in the details, but a general
+resemblance of the main features. The reader will do well to consult
+Prof. Hull's restoration of the physical features of Old Red Sandstone
+and Lower Carboniferous Times on Plate VI. of his _Contributions to
+the Physical History of the British Isles_, and compare it with the
+map drawn by Prof. Green (_Coal: its History and Uses_, by Profs.
+Green, Miall, Thorpe, Rücker, and Marshall, Fig. 3, p. 38), which will
+be found to bear out this statement.
+
+[Footnote 46: On this subject, the student may consult Prof. E. Hull's
+_Contributions to the Physical History of the British Isles_.]
+
+Valuable as the published maps of palæo-physiography are as an aid to
+the student in understanding the significance of the variations of
+characters amongst the sediments, he will do well to supplement them
+by maps which he fills in for himself. He is recommended to procure a
+number of outline maps of England, or of the British Isles, and when
+studying in detail the characters of the British sedimentary rocks
+formed during the various periods, to place a blank map by his side
+when beginning the study of each period or important portion of a
+period. On this map he should jot down the geographical distribution
+of the different kinds of sediments, using the conventional signs
+indicated at p. 90: thus, in the case of the Lower Carboniferous
+rocks he would place the conventional sign for limestone in
+Derbyshire, a combination of those for limestone and shale in
+Yorkshire, and would add to these the sandstone sign in
+Northumberland. He should also note the general character of the
+fossils, using abbreviations for such terms as fresh-water fossils,
+shallow-sea fossils, deep-water fossils. After reading the account of
+the group of rocks in a comprehensive text-book, and inserting his
+notes on the map, he should proceed to insert the probable position of
+the coast-lines. He should also take notes of any indications of
+contemporaneous volcanic action, though these might well be inserted
+on a separate map. If this course be pursued, the student will not
+only have the significance of the variations amongst the strata
+impressed upon his mind, but he will have a means of obtaining at a
+glance the distribution of sediments and faunas of different kinds in
+the British area during the principal geological periods. On another
+set of maps he may indicate the axes of the orogenic movements which
+have occurred at different times, and when his various maps are
+completed, he will have the materials for the construction of a
+general account of the various geological processes which have been
+concerned with the building of the British area.
+
+When an area like Britain has been studied, the student may proceed to
+construction of maps of wider regions, and he will find that in doing
+this, new sets of facts must be taken into consideration, as for
+instance the occurrence of different faunas on opposite sides of
+once-existing continental masses, and the problems connected with the
+present distribution of the faunas and floras. For an instance of the
+importance of the former distribution of life the reader may consult
+the twelfth section of the first part of Professor Suess' _Das
+Antlitz der Erde_, whilst a good account of the value of recent
+geographical distribution of organisms in supplying a clue to former
+distribution of land and sea will be found in Mr A. R. Wallace's
+_Island Life_, Chapter xxii.
+
+Should the method suggested above be adopted, the student is likely to
+acquire a much more coherent idea of the significance of the facts of
+stratigraphical geology than can be obtained by a mere perusal of the
+accounts of the strata given in those portions of the various
+text-books which are devoted to a consideration of the stratigraphical
+branch of the science.
+
+
+
+
+CHAPTER XI.
+
+THE CLASSIFICATION OF THE STRATIFIED ROCKS.
+
+
+In the succeeding chapters, a general account of the characters of the
+Geological Deposits of different periods will be given, for the
+purposes of illustrating the principles to the consideration of which
+the earlier chapters have been devoted. It is not proposed to enter
+into a description of numberless details, which would only confuse the
+student who wished to grasp the main principles, for many facts have
+been recorded which it is necessary to notice in a comprehensive
+text-book treating of stratigraphical geology, though their full
+significance is not yet grasped. The writer, while noting the main
+characters of the various subdivisions of the different
+stratigraphical systems, will assume that this work is used in
+conjunction with some recognised text-book. The stratigraphical
+portion of Sir A. Geikie's _Class Book of Geology_ gives an admirable
+general account of the British Strata, while the larger text-book by
+the same author has a condensed though very full account of the rocks
+of the stratigraphical column in all parts of the world, and this is
+supplemented by numerous references to the original works wherein
+further descriptions may be found. The English edition of Prof. E.
+Kayser's _Text-Book of Comparative Geology_, edited by P. Lake, is
+also well adapted to the wants of the student, and an excellent
+account of the strata is given in Mr A. J. Jukes-Browne's _Handbook of
+Historical Geology_, which may be read with the same author's
+_Building of the British Isles_.
+
+The reader who refers to different text-books will be struck with the
+variations of nomenclature even amongst the larger stratigraphical
+divisions, for two authors seldom subdivide the geological column into
+the same number of rock-systems. The following classification will be
+here adopted:--
+
+  Groups.              Systems.
+
+                  { Recent
+                  { Pleistocene
+  Cainozoic or    { Pliocene
+  Tertiary        { Miocene
+                  { Oligocene
+                  { Eocene
+
+                  { Cretaceous
+  Mesozoic or     { Jurassic
+  Secondary       { Triassic
+
+                  { Permian
+                  { Permo-Carboniferous
+                  { Carboniferous
+  Palæozoic       { Devonian
+                  { Silurian
+                  { Ordovician
+                  { Cambrian.
+
+  Precambrian.
+
+A few remarks may be given as to the reason for adopting this
+classification.
+
+It is not for a moment suggested that the Systems have the same value,
+if the time taken for their accumulation be alone considered. The beds
+classified as Recent, for example, were probably accumulated during a
+lapse of time far shorter than that occupied for the deposit of some
+of the series or even stages of a system like the Silurian, but the
+recent rocks acquire a special significance from the fact that we are
+living in the period, and the Cainozoic rocks as a whole are capable
+of greater subdivision than the earlier groups, on account of the
+greater ease with which they can be studied, owing to the small amount
+of disturbance which they have usually undergone when compared with
+that which has affected older rocks, and the closer resemblance of
+their faunas and floras to those of existing times.
+
+With reference to the groups, the writer has already commented upon
+the use of the terms Palæozoic, Mesozoic and Cainozoic; below the
+lowest Palæozoic rocks (those of the Cambrian system) lie a group of
+rocks which have been variously spoken of as Azoic, Eozoic, and
+Archæan. There is an objection to the use of any one of these words in
+this sense; the objection in the case of the first two is that the
+term is theoretical and probably incorrect, whilst the word Archæan,
+otherwise suitable, has also been used in a more restricted sense. In
+these circumstances the term Precambrian will be used when referring
+to any rocks which were formed below Palæozoic times, though no doubt
+when this obscure group of rocks is more thoroughly understood a
+satisfactory classification will be applied to it.
+
+Taking the other groups into account, the lower systems of the
+Palæozoic group will be found to vary greatly according to the views
+of different writers; some make only one system, the Silurian, others
+two, the Cambrian and Silurian. The three systems are here adopted,
+not only because the one, Silurian, is too unwieldy on account of its
+size and requires subdivision (and the Cambrian and Silurian however
+defined, will be found to be of very unequal importance, whereas the
+three systems adopted are of fairly equal value), but especially
+because when the term Ordovician is used, the significance of the
+other terms Cambrian and Silurian is at once understood.
+
+An attempt has been made to shew that the Devonian system is
+non-existent, but the result of modern research is to shew that the
+rocks placed in this system are worthy of the distinction, both from
+their importance and from the distinctness of the fauna from those of
+the underlying and overlying systems.
+
+The Permo-Carboniferous system is adopted, because an important group
+of deposits has recently been brought to light which were not
+represented either in the Permian or Carboniferous system as
+originally defined.
+
+Some authors have advocated the union of the Permian and Triassic
+systems into one system placed at the base of the Mesozoic group. This
+is unnecessary, and would depart from the classification originally
+proposed, which is to be deprecated, unless there is any strong reason
+for it.
+
+The Mesozoic systems are classified according to the method generally
+adopted. Were a fresh classification to be proposed, a portion of the
+Cretaceous system might be included with the Jurassic rocks, but it is
+better to adhere to the old classification.
+
+The divisions of the Cainozoic rocks are hardly systems in the sense
+in which the term is used in the case of the older rocks, but the
+reason for using these smaller subdivisions has already been
+mentioned. The addition of the Oligocene to the original divisions
+suggested by Lyell has been found useful, and the term will be used
+in this work.
+
+The reasons for the adoption of the particular minor subdivisions
+(series and stages) in the following chapters will frequently appear
+when the rocks of the various systems are described, and need not be
+further alluded to in this place.
+
+Although most geologists describe the stratified rocks in ascending
+sequence beginning with the oldest, and proceeding towards the newest,
+others, and notably Lyell, adopted the opposite method and commenced
+with an account of the newest beds. The argument generally used for
+the latter method is that it is easier to work from the study of the
+known to that of the less known, and as the faunas of the newest rocks
+are most like the existing faunas, the student would more readily
+follow a description of the rocks in the order which is opposite to
+that in which they were deposited.
+
+In practice, the study of the sediments in their proper order, that
+is, in the order of deposit, will not be found to task the student to
+any great extent, especially if, as is very desirable, he has studied
+the main facts and principles of Palæontology before commencing the
+study of the rock-systems in detail. There is one reason for beginning
+with the study of the older sediments which outweighs any reasons
+which can be advanced against it, namely that the events of any period
+produce their effect not only upon the strata of that period, but also
+on those of succeeding periods.
+
+The task of the stratigraphical geologist is really to learn the
+evolution of the earth, in its changes from the simple to the more
+complex conditions, and it is quite obvious that it is unnatural to
+attempt any study of evolution by working backward. For this reason
+the study of the sediments will be here made in the order which is
+usually adopted, by passing from the older to the newer, and from the
+simple to the more complex.
+
+The British strata will be mainly considered, though references will
+frequently be made to their foreign equivalents, and a fuller account
+of the latter will be added when the British strata are abnormal, as
+are those of Triassic times, and also when a period is not represented
+amongst the strata of the British Isles, as for instance, the
+Permo-Carboniferous and Miocene periods.
+
+The student is recommended to refer constantly to good geological maps
+of the British Isles, of Europe, and of the world. Of maps of the
+British Isles, mention may be made of Sir A. Ramsay's geological map
+of England, Sir A. Geikie's map of Scotland, and his map of the
+British Isles, J. G. Goodchild's map of England and Wales, a map of
+Europe by W. Topley and one of the world reduced from that by J.
+Marcou, accompanying the first and second volumes of the late Sir J.
+Prestwich's _Geology_. For special purposes more detailed maps will be
+studied, including the one-inch maps of H. M. Geological Survey, and
+the index map on a smaller scale. Lastly, for an account of British
+Geology, reference must be made to H. B. Woodward's _Geology of
+England and Wales_, where the British formations are described in
+order, and to W. J. Harrison's _Geology of the Counties of England and
+Wales_, where the stratigraphical geology of the country is given
+under the head of the different counties, which are taken in
+alphabetical order.
+
+In concluding this chapter, it is hardly necessary to say that every
+opportunity of studying the characters of the deposits and their
+fossils in the field should be eagerly seized, and that much
+information may be acquired even on a railway journey, especially as
+to the influence which the deposits exert upon the scenery of a
+region[47].
+
+[Footnote 47: In the first edition of H. B. Woodward's _Geology of
+England and Wales_, an account of the geology of the main lines of
+English railways is given, which is omitted in the later edition. It
+is well worth consulting by those who take a long journey, and it will
+be found useful to take a geological map with one on the journey so as
+to discover when one is passing from one formation to another.]
+
+
+
+
+CHAPTER XII.
+
+THE PRECAMBRIAN ROCKS.
+
+
+Study of a geological map of the world will shew that extensive
+regions, such as parts of Scandinavia, many tracts of Central Europe,
+a large area in Canada, and a considerable portion of Brazil and the
+adjoining countries are occupied by crystalline schists, which
+underlie the oldest known sedimentary strata in those places. These
+crystalline schists form the floor upon which the sediments
+constituting the bulk of the geological column rest, and it is
+necessary that we should know something of the character of this
+floor. Other rocks which can be definitely proved to be of Precambrian
+age are often found associated with the crystalline schists, and these
+associated rocks have often undergone more or less alteration
+subsequently to their formation. The difference between the coarser
+types of crystalline schists and these associated rocks is sometimes
+so marked that geologists have necessarily paid attention to it, and
+separated the two groups of rocks; the term Archæan has been used by
+some geologists to include the crystalline schists, and Eparchæan for
+the associated rocks of known Precambrian age, but though this
+separation may sometimes be effected, there are cases when it is
+impossible to draw any sharp line of demarcation between 'Archæan'
+and 'Eparchæan' types.
+
+In the present state of our knowledge, a chronological classification
+of the Precambrian rocks when applied to wide and distant regions is
+destined to break down, and it will be convenient if we consider at
+some length the features of the Precambrian rocks of a particular
+region, and apply the knowledge thus gained to a study of Precambrian
+rocks of other areas, and to a consideration of our knowledge of the
+Precambrian rocks as a whole. In doing so, the term 'crystalline
+schists' will be used somewhat vaguely with reference to a complex of
+schistose rocks of which the mode of origin cannot be fully
+determined. We may take our own country as a region where a good
+development of the Precambrian rocks occurs.
+
+A few explanatory remarks concerning the mode of detection of
+Precambrian rocks may not be amiss. If any true organisms have been
+hitherto discovered amongst the rocks formed before Cambrian times
+they are valueless as a means of correlating rocks, and accordingly
+lithological characters only are available in attempting to correlate
+the rocks of one area with those of another. Those who have read the
+preceding chapters will have gathered that comparisons founded on
+similarity of lithological character are not so valuable as those made
+after careful scrutiny of the fossils of strata, but they are by no
+means valueless, and when the rocks of two areas which are not far
+distant from one another present close lithological resemblances,
+their general contemporaneity may be inferred with some degree of
+certainty.
+
+It is only when we get the lowest Cambrian strata overlying earlier
+rocks that we have absolute proof of the Precambrian age of the
+latter, and it is necessary, therefore, that we should have some
+definite lower limit to the rocks of the Cambrian system. It is now
+generally agreed that that limit shall be drawn at the base of a group
+of rocks containing what is known as the _Olenellus_-fauna, which will
+be considered at greater length in the next chapter, and it will be
+well, if the term Cambrian be not in future applied to any rocks
+beneath the ones containing the relics of this fauna, for otherwise
+there is danger of the indefinite downward extension of the Cambrian
+system. We need not be surprised to find great thicknesses of rock
+below the rocks containing the _Olenellus_-fauna, and passing upwards
+with complete conformity into those rocks; nevertheless, if it can be
+shewn that the _Olenellus_-fauna had not appeared during the
+deposition of the underlying group, the rocks of that group should be
+termed Precambrian. A case of this nature has not yet been detected in
+our area, and all the rocks which have been proved to be Precambrian
+in Britain are separated from the overlying Cambrian rocks by a
+physical break, though that break is not necessarily very large, and
+in some districts is probably of little importance. Hitherto the
+_Olenellus_-fauna has been detected in Ross, Warwickshire, Shropshire,
+Worcestershire and probably in Pembrokeshire, and the rocks underlying
+the _Olenellus_-beds in those counties can be proved to be Precambrian
+(i.e. if the _Olenellus_-age of the Pembrokeshire rocks be ultimately
+established, and the researches of Dr Hicks tend to prove that it will
+almost certainly be done). It will be convenient if we take the
+instances where the age of the rocks can be proved with certainty or
+with a considerable degree of probability first, and then consider the
+examples of rocks which are found below Cambrian strata, though these
+have not hitherto yielded the _Olenellus_-fauna, concluding with a
+notice of rocks which have been claimed to be of Precambrian age on
+account of their lithological characters, though they are not now seen
+to be immediately succeeded by strata appertaining to the Cambrian
+system.
+
+Commencing with the region where we have the greatest development of
+the known Precambrian rocks, namely Ross, Sutherland and the Hebrides,
+we may explain the general relationship of the rocks by means of a
+generalised section (fig. 15).
+
+[Illustration: Fig. 15.]
+
+The lowest rocks _a_ are crystalline schists, they are succeeded by a
+set of arenaceous rocks _b_ known as the Torridonian beds, which rest
+unconformably upon the upturned edges of the crystalline schists,
+whilst the Cambrian rocks, _c_, rest with another unconformity
+sometimes upon the partly denuded Torridonian beds, or where the
+latter have been completely removed, as on the right side of the
+figure, directly upon the crystalline schists, thus presenting an
+example of unconformable overlap. The occurrence of the
+_Olenellus_-fauna in the basement beds of the Cambrian system near
+Loch Maree, proves the Precambrian age of the Torridonian strata,
+whilst the unconformable junction between the latter and the
+crystalline schists indicates that we are here dealing with two
+distinct sets of Precambrian rocks, one of Eparchæan and the other of
+Archæan type.
+
+The crystalline schists consist of rocks of very varied lithological
+characters, some with gneissose, and others with schistose structure,
+and they vary in degree of acidity from ultrabasic rocks to those of
+acid composition. Most of them exhibit parallel structures, which in
+many cases can be shewn to have been impressed on the rocks
+subsequently to their consolidation, though this need not have
+occurred and probably did not occur with some of them, especially the
+granitoid gneisses. The researches of the members of H. M. Geological
+Survey have shewn that many of these rocks were originally intrusive
+igneous rocks, though it is not yet known into what rocks those which
+were first consolidated were injected, and the origin of the bulk of
+the schists still remains to be elucidated. Subsequently to their
+consolidation and before the deposition of the earliest Torridonian
+rocks they were subjected to more than one set of earth-movements,
+which folded them and impressed a series of parallel structures upon
+many of them; and accordingly we find that the pebbles of the
+crystalline schists which are found amongst the basal conglomerates of
+the Torridonian rocks consist of fragments which had undergone the
+alteration caused by these earth-movements before they were denuded
+from their parent-rocks[48].
+
+[Footnote 48: For an account of these rocks, their characters, and the
+effects of earth movement upon them, the reader should consult a
+"Report on the Recent Work of the Geological Survey in the North-West
+Highlands of Scotland": _Quart. Journ. Geol. Soc._, vol. XLIV. p.
+378.]
+
+The Torridonian system is composed of rocks which are largely of
+arenaceous character, the most prominent beds being formed of red
+sandstones, and the bulk of the fragments in them have clearly been
+derived by denudation from the crystalline schists, many of the beds
+being composed of arkose, where the quartz is mixed with a large
+proportion of felspar and often of ferro-magnesian minerals. The
+deposits are clearly sedimentary, and are as little altered as many
+strata of much more recent origin, only possessing structures produced
+by metamorphic action under exceptional circumstances. The detailed
+researches of the geological surveyors prove that the rocks of this
+system have a much greater thickness and are of more varied
+lithological characters than was previously supposed. The total
+thickness of the strata is over 10,000 feet, and the sandstones are
+associated with deposits of a muddy character, and with occasional
+bands of limestone; in these circumstances the discovery of fossils
+would excite no surprise, and in 1891 Sir A. Geikie announced the
+detection of "traces of annelids and some more obscure remains of
+other organisms in these strata," which have not yet been
+described[49]. These Torridonian strata furnish us with the most
+satisfactory group of Precambrian sediments yet detected in
+Britain[50].
+
+[Footnote 49: An account of the subdivisions and lithological
+characters of the rocks of the Torridonian System will be found in the
+_Annual Report of the Geological Survey of the United Kingdom_ for
+1893.]
+
+[Footnote 50: It has been recently maintained that some of the
+Torridonian rocks are of Æolian origin.]
+
+In the south-east Highlands is a great mass of crystalline schists of
+a less gneissose character than that of the north-west, to which Sir
+A. Geikie has applied the name Dalradian. Many of these schists will
+be found by examination of the geological map of Scotland to be
+separable into divisions, which by means of their lithological
+characters can be traced long distances across the country, and they
+present all the characters of sedimentary rocks, though they are
+associated with intrusive igneous rocks, and have undergone great
+metamorphic changes since their formation. Cambrian rocks have not yet
+been discovered immediately above them, though they are clearly older
+than Ordovician times, but the existence of rocks associated with them
+along their north-west borders, which in lithological characters
+closely resemble some of the rocks of the crystalline schists of the
+north-west Highlands, indicates the probability of their general
+Precambrian age. In some instances, the extreme types of metamorphism
+which they exhibit are the result of the kind of action usually termed
+pyrometamorphic as has been shewn by Mr G. Barrow[51].
+
+[Footnote 51: Barrow, G. "On an Intrusion of Muscovite-biotite gneiss
+in the S.E. Highlands of Scotland, and its accompanying metamorphism."
+_Quart. Journ. Geol. Soc._, vol. XLIX. p. 330.]
+
+In England and Wales the rocks which have been shewn or inferred to be
+Precambrian, when not intrusive, are largely of volcanic origin. The
+most satisfactory example of the occurrence of the _Olenellus_-fauna
+is that of the Cambrian Comley sandstone of Shropshire, which rests
+unconformably upon a set of rocks termed by Dr Callaway the Uriconian
+rocks; the latter are essentially volcanic, and strongly resemble
+Precambrian rocks of other British areas. There is also strong reason
+to suppose that the sediments to which the name Longmyndian has been
+applied, which have been described by the Rev. J. F. Blake, are of
+Precambrian age, for, as Professor Lapworth has pointed out, the three
+great subdivisions of the Cambrian system are present in the area
+under consideration, and the rocks of each are entirely different from
+those of the adjoining Longmynd area. In Shropshire therefore we meet
+with one set of volcanic rocks, and another set consisting of
+sedimentary rocks, of which the former is certainly, the latter
+almost certainly of Precambrian age, and as the Longmyndian rocks are
+in a comparatively unaltered condition, consisting of normal
+sediments, we may well expect the discovery of fossils in them
+also[52]. The _Olenellus_-fauna has been found near Nuneaton in
+Warwickshire in beds which unconformably succeed volcanic rocks, the
+Caldecote series of Prof. Lapworth, and the latter are therefore of
+Precambrian age[53]. A few fossils belonging to the _Olenellus_-fauna
+have occurred in the oldest Cambrian rocks of the Malvern district,
+and these rocks rest unconformably upon those of an old ridge which is
+therefore composed of Precambrian rocks. The rocks of this ridge are
+largely of intrusive igneous origin, though parallel structures have
+been impressed upon them as the result of subsequent deformation, but
+some of the rocks are almost certainly of contemporaneous volcanic
+origin[54]. In the Wrekin ridge, igneous and pyroclastic rocks are
+found succeeded unconformably by Cambrian rocks which resemble those
+of the Malvern and Nuneaton districts, and probably belong to the
+period of existence of the _Olenellus_-fauna, and these igneous and
+pyroclastic rocks are presumably of Precambrian age, and the
+contemporaneous rocks constitute Dr Callaway's typical Uriconian
+group. Volcanic ashes and breccias are accompanied by devitrified
+pitchstones and intruded granitic rocks, which may or may not be all
+of the same general age[55]. The rocks which have been claimed as
+Precambrian in Pembrokeshire and in Caernarvonshire have the same
+general characters as those of the Wrekin ridge. Pyroclastic rocks
+underlie the oldest Cambrian rocks, with discordance between the two,
+and associated with these pyroclastic rocks are quartz felsites which
+according to some are of contemporaneous nature whilst others maintain
+their intrusive origin. In each county granites are found which are
+now generally recognised to be intrusive, though there seems to be no
+doubt as to their being of the same general age as the rocks with
+which they are associated, and therefore presumably Precambrian. The
+Pembrokeshire rocks are marked by the occurrence of a certain amount
+of metamorphism, probably of more than one kind, which has converted
+pyroclastic volcanic rocks into sericitic-schists and quartz-felsites
+into hälleflintas[56]. The term Pebidian given by Dr Hicks to the
+contemporaneous volcanic fragmental rocks should be retained, and if
+these rocks be eventually shewn to be contemporaneous with similar
+volcanic rocks of other districts, may be applied generally, as it has
+priority over other terms as Uriconian and Caldecote series. The term
+Dimetian was applied to rocks known to be intrusive, and must be
+dropped as a chronological term, whilst the existence of an Arvonian
+system separate from the Pebidian system is not fully proved.
+
+[Footnote 52: The reader may consult a paper by Prof. Lapworth "On
+_Olenellus Callavei_ and its geological relationships," _Geol. Mag._
+Dec III. vol. VIII. p. 529, for information concerning the
+relationship of the _Olenellus_ beds of Shropshire to the more ancient
+rocks; the Uriconian rocks are described by Dr Callaway in a series of
+papers, especially in the _Quarterly Journal of the Geological
+Society_, vol. XXXV. p. 643, vol. XXXVIII. p. 119, vol. XLII. p. 481
+and vol. XLVII. p. 109, whilst the lithological characters of the
+Longmyndian rocks are described by the Rev. J. F. Blake (_Quart.
+Journ. Geol. Soc._, vol. XLVI. p. 386).]
+
+[Footnote 53: See Lapworth, C., "On the sequence and systematic
+position of the Cambrian rocks of Nuneaton," _Geol. Mag._ Dec III.
+vol. III. p. 319; and Waller, T. H., "Preliminary Note on the Volcanic
+and Associated Rocks of the neighbourhood of Nuneaton," _ibid._ p.
+322.]
+
+[Footnote 54: For details concerning the rocks of the Malvern Hills
+see papers by Callaway in the _Quarterly Journal of the Geological
+Society_, vol. XXXVI. p. 536, XLIII. p. 525, XLV. p. 475, and XLIX. p.
+398, and a paper by Prof. A. H. Green, _ibid._ vol. LVI. p. 1.]
+
+[Footnote 55: Callaway, C., _Quart. Journ. Geol. Soc._, vol. XXXV. p.
+643.]
+
+[Footnote 56: The Pembrokeshire area is of interest as the probable
+existence of Precambrian rocks in Britain was first indicated on good
+evidence in this county. The general structure of the district is
+fairly simple, consisting of Cambrian rocks beneath which Precambrian
+rocks are exposed in at least two ridges of which the northerly and
+more important one runs through St Davids. The rocks of the St Davids
+ridge consist of a binary granite (granitoidite), felsites, and
+volcanic ashes and breccias of intermediate composition. Much
+diversity of opinion has existed, and to some extent still exists as
+to questions of detail, and a very extensive literature has been
+devoted to these rocks. Amongst the numerous papers which treat of
+them, the student may consult the following:--Hicks, H., _Quart.
+Journ. Geol. Soc._, vol. XXXIII. p. 229, XXXIV. p. 147, XXXV. p. 285,
+XL. p. 507, XLII. p. 351, Geikie, A., _ibid._ vol. XXXIV. p. 261,
+Blake, J. F., _ibid._ vol. XL. p. 294, and Morgan, C. Ll., _ibid._
+vol. XLVI. p. 241. Much of the matter contained in these papers is
+controversial, and need not be fully read by those who merely wish to
+obtain a general account of the rocks of the district.]
+
+In Caernarvonshire two ridges are found, the one running from Bangor
+to Caernarvon, and the other through Llanberis lake. The rocks of
+these are generally similar to those of St Davids, and as the lowest
+Cambrian rocks of the area closely resemble those of St Davids, the
+Precambrian age of the rocks of these ridges is rendered highly
+probable, though until the discovery of the _Olenellus_-fauna in the
+area, it cannot be regarded as proved[57].
+
+[Footnote 57: These rocks are described by T. M^{c}K. Hughes, _Quart.
+Journ. Geol. Soc._, vol. XXXIV. p. 137, and XXXV. p. 682; by Prof. T.
+G. Bonney, _ibid._ vol. XXXIV. p. 144; and by Dr Hicks, _ibid._ vol.
+XXXV. p. 295.]
+
+The actual position of the similar rocks of Anglesey has not been so
+clearly fixed, as the rocks associated with them are of Ordovician
+age, but their resemblance to the rocks of the adjoining regions
+renders their Precambrian age highly probable. It is interesting to
+find in association with the rocks which resemble those of
+Caernarvonshire, others which Sir A. Geikie recognises as quite
+similar to some existing amongst the crystalline schists of the
+north-west Highlands of Scotland, and when these ancient rocks of
+Anglesey have been mapped in detail, they will probably be found to
+present greater variety than is afforded by any Precambrian rocks of
+Great Britain occurring S. of the Scotch border[58].
+
+[Footnote 58: Papers upon the old rocks of Anglesey will be found in
+many volumes of the _Quarterly Journal of the Geological Society_; see
+especially Hicks, vol. XXXV. p. 295, Callaway, vol. XXXVI. p. 536,
+XXXVII. p. 210, and Blake, XLIV. p. 463.]
+
+Of rocks whose age is more uncertain, but which are probably of
+Precambrian age, those of Charnwood Forest in Leicestershire may first
+be noticed. They are largely of pyroclastic origin, and from their
+likeness to similar rocks of proved Precambrian age, they are very
+probably of this age, as suggested by Messrs Hill and Bonney[59]. A
+group of crystalline schists is found in the south of Cornwall,
+especially near the Lizard, and similar rocks are found in the Channel
+Isles. As their relationship to newer rocks is not clear, little can
+be said about them, which has not already been noticed in mentioning
+the crystalline schists of other regions[60].
+
+[Footnote 59: Hill and Bonney, _Quart. Journ. Geol. Soc._, vol.
+XXXIII. p. 754, XXXIV. p. 199 and XLVII. p. 78; see also Watts, W. W.,
+_Rep. Brit. Assoc._ for 1896, p. 795.]
+
+[Footnote 60: For an account of the Volcanic History of Britain in
+Precambrian times, see Sir A. Geikie, Presidential Address to the
+Geological Society, _Quart. Journ. Geol. Soc._, vol. XLVII. p. 63.]
+
+The Precambrian rocks of the European continent consist largely of
+crystalline schists which in their general aspects recall those of the
+north-west Highlands of Scotland. Important masses are found in
+Bavaria, Bohemia, France, Spain, Scandinavia and Russia. The
+Scandinavian and Russian rocks of Archæan type are in places succeeded
+by the _Olenellus_-bearing beds of the Cambrian rocks, and rocks of
+Eparchæan character are not extensively developed, though certain
+Norwegian rocks may be the equivalents of the Torridonian rocks of
+Scotland, and other rocks of this type are found in places in Sweden.
+In Bohemia and in Brittany Precambrian strata of Eparchæan type have
+been discovered, and this type probably occurs elsewhere in Europe.
+
+The North American rocks require some notice, for it was in Canada
+that the existence of Precambrian rocks was first recognised, and the
+term Laurentian, originally applied to an Archæan type of Precambrian
+rocks in Canada, was subsequently adopted in speaking of many
+Precambrian rocks elsewhere, though it is now wisely restricted to the
+type of rock in the original area to which the name was first given.
+These Laurentian rocks acquired a special, interest on account of the
+occurrence in their limestones of a supposed reef-building
+foraminifer, _Eozoon canadense_, but detailed study of its structure
+and mode of occurrence has convinced most geologists that the
+structure is inorganic.
+
+The Laurentian rocks of the typical Laurentide region are largely
+crystalline schists associated with massive crystalline rocks. The
+attempt to separate them chronologically into a Lower and Upper
+division was premature, as shewn by the fact that many of them, upon
+detailed study, prove to be intrusive igneous rocks. In the
+neighbourhood of Lake Huron, a set of sedimentary rocks overlying the
+Archæan rocks is of Eparchæan type, consisting to a great extent of
+volcanic rocks, clay-slates and schists with intrusive igneous rocks;
+it has been termed the Huronian System, and this term has also been
+extensively applied to other Eparchæan types found elsewhere, but
+should be restricted to the rocks of the Huron district. A number of
+other rocks of Eparchæan type have been discovered in various parts of
+North America, and have been grouped together under the title of
+Algonkian, a name proposed for them by Dr C. D. Walcott, and an
+attempt has been made to arrange them in chronological order, though
+in the absence of fossils, the rocks of different districts can only
+be so arranged by reference to lithological characters; nevertheless a
+detailed study of the Eparchæan and some of the more finely
+crystalline schistose rocks points to the existence of a number of
+divisions of sedimentary rocks of Precambrian age, some of which may
+attain to the dignity of forming separate systems[61]. By far the most
+instructive development of American Precambrian rocks has been found
+in the Rainy Lake region of Canada, and it is the subject of a special
+memoir by Dr A. C. Lawson[62]. The Archæan rocks of the region are
+divided into a lower Laurentian and an upper division, which is
+further subdivided into the Coutchiching series below and the Keewatin
+series above, though the rocks of the Keewatin series are largely of
+Eparchæan character. The Laurentian rocks of this region resemble
+those of the Laurentide area, and consist of highly crystalline
+schistose and gneissose rocks associated with compact rocks. The
+Coutchiching series consists of mica schists and grey laminated
+gneisses, which appear to have been of sedimentary origin, altered by
+subsequent metamorphic action, while the Keewatin series, which
+reposes sometimes upon the rocks of the Coutchiching series (when the
+junction is an unconformable one), sometimes upon the Laurentian
+rocks, is formed of pyroclastic rocks and lava flows with intercalated
+sedimentary rocks; some of the Keewatin rocks are highly metamorphosed
+but others have undergone little or no metamorphic change. The most
+important point in connexion with these rocks of the Rainy Lake Region
+has reference to the relationship between the Laurentian rocks and
+those of the Coutchiching and Keewatin series. Lawson demonstrates the
+igneous nature of the Laurentian rocks, and brings forward evidence of
+various kinds that they were formed "by the fusion of the basement or
+floor upon which the formations of the upper division of the Archæan
+were originally deposited. With the fusion of this floor it seems
+probable that portions of the superincumbent strata, which once formed
+integral parts of either the Coutchiching series or the Keewatin, have
+also been absorbed into the general magma, and reappeared on
+crystallization as Laurentian gneiss. This fusion, however, only
+extended up to a certain uneven surface, which surface constitutes the
+demarcation between the present upper and lower Archæan. Above this
+surface, or upper limit of fusion, the formation of the Coutchiching
+and Keewatin series retained their stratiform or bedded disposition,
+and rested as a crust of hard and brittle rocks upon the magma,
+subject to its metamorphosing influences[63]."
+
+[Footnote 61: A large number of classifications have been proposed for
+the Archæan rocks of America; the most plausible one is given in Sir
+A. Geikie's _Text Book of Geology_, Third Edition, p. 716.]
+
+[Footnote 62: Lawson, A. C., _Report on the Geology of the Rainy Lake
+Region_. Montreal, 1888.]
+
+[Footnote 63: Lawson, _op. cit._ p. 139.]
+
+We may now pass briefly in review the evidence which has been so far
+obtained as to the mode of formation of the various Precambrian rocks.
+The existence of a very varied fauna amongst the earliest Cambrian
+strata has been commented upon by many geologists, and according to
+accepted explanations of the origin of that fauna, an enormous period
+of time elapsed before the deposition of the earliest Cambrian strata.
+During portions of that long period, the undoubtedly clastic rocks of
+Eparchæan type were deposited, and probably many others which are now
+so altered by metamorphism, like some of the Coutchiching rocks of
+Canada, that their original clastic origin can only be inferred and
+not directly proved. Volcanic activity was very rife during the
+deposition of some of these Eparchæan rocks, though perhaps not more
+so than during the formation of some of the Lower Palæozoic Rocks. All
+attempts to prove the occurrence of organisms in Precambrian strata
+have hitherto failed, for no undoubted fossil has been described which
+is unhesitatingly accepted as of Precambrian age, notwithstanding the
+many asserted occurrences of such fossils. That fossils will
+eventually be discovered is more than probable, and their
+non-detection at the present time is in no way very surprising, when
+we remember the long time that elapsed after the existence of
+stratified rocks below the Upper Palæozoic rocks had been recognised,
+before definite faunas were discovered in them. The determination of
+the Precambrian age of stratified rocks is recent, and now that this
+determination has been made, the search for fossils will be more
+eager, and is likely to be rewarded by their discovery. Furthermore,
+experience shows that when fossils are discovered in rocks of unknown
+age, there is a tendency to refer those rocks to some known period,
+and consequently we may actually possess Precambrian fossils, out of
+beds which have been erroneously referred to the Cambrian or a later
+period.
+
+Another important question is that of the metamorphism of a large
+number of Precambrian rocks, and here again recent research tends to
+show that the metamorphism is not of a kind different from that which
+occurred after the end of Precambrian times; the discovery of
+crystalline schists in Norway, Kirkcudbrightshire and Westmorland
+amongst Lower Palæozoic rocks, which resemble those of Archæan masses
+in all respects except in the extent of area which they cover, shows
+that similar processes to those which occurred in Precambrian times
+went on during later periods, though perhaps not on so large a scale.
+The great extent of these metamorphic rocks of Precambrian age can
+hardly be due in any great degree to the longer time during which they
+have been subjected to metamorphic influence, for there is evidence
+that much of the change took place in Precambrian times, far more than
+has occurred since, and it is a significant fact that these old rocks
+are more extensively penetrated by intrusive igneous masses than those
+of later periods; here again we find that much of the intrusion
+actually occurred in Precambrian times. The greater extent of
+intrusion and metamorphism amongst these Precambrian rocks than
+amongst later sediments indicates some differences of conditions in
+the case of Precambrian and later times. If besides intrusion, actual
+fusion of floors of Precambrian rocks occurred, we may well suppose
+that the earlier records of the rocks are for ever lost to us, the
+earliest sediments having been fused, but that the history of life
+upon our earth is to be revealed to us first in so late a stage as
+that of Cambrian times is highly improbable, and we may look forward
+with confidence to laying bare the records of the rocks composing the
+geological column some way below the Cambrian portion of the column.
+
+Upon this foundation of igneous rock, sediment and volcanic material,
+formed in Precambrian times, whose history we have only begun to
+study, was laid down the great mass of sediment which the geologist
+has more completely studied, where abundant traces of life are
+preserved, and concerning whose history we can gain a greater insight
+than is permitted us in the case of the old Foundation Stones.
+
+
+
+
+CHAPTER XIII.
+
+CYCLES OF CHANGE IN THE BRITISH AREA.
+
+
+Before studying in further detail the strata of the geological column,
+it will be convenient to deal with the great physical changes which
+have occurred in the British area from Precambrian times to the
+present day, as this will clear the way for a right appreciation of
+the main variations in the characters and distribution of the strata.
+
+At the end of Precambrian times there was a general upheaval of the
+British area, and this we may speak of as the First Continental
+Period. It was followed by depression and extensive sedimentation,
+proceeding more or less continuously though with local interruptions
+through Lower Palæozoic times, so that so far as Britain is concerned
+we may speak of Lower Palæozoic times as constituting the First Marine
+Period. Extensive upheaval gave rise to continental tracts and
+mountain chains, and deposits of abnormal character (as compared with
+ordinary marine deposits) at the end of Lower Palæozoic times;--the
+Devonian period was one of elevation and denudation, and we may
+therefore refer to it as the Second Continental Period. This was
+followed by depression and sedimentation in Carboniferous times, and
+these Carboniferous times constitute the Second Marine Period.
+Elevation gave rise to continental tracts and mountain chains at the
+end of Carboniferous times, and here again we find proofs of extensive
+denudation and the formation of abnormal deposits:--the Permo-Triassic
+period is the Third Continental Period. Depression set in during early
+Jurassic times and continued throughout the Mesozoic and the early
+part of Tertiary times, which form the Third Marine Period.
+Disturbances culminating in Miocene times once more produced
+terrestrial conditions. In this, the Fourth Continental Period, we are
+still living.
+
+From what has been previously written it will be seen that each of the
+marine periods should be marked by an early and late shallow-water
+phase, separated by an intervening marine phase, and the importance of
+the phases will depend upon the length of time during which they
+existed, and will differ markedly in different cases, whilst the
+distinctness of the middle phase from the upper and lower, will depend
+upon the magnitude of the maximum submergence.
+
+During the first marine period submergence was comparatively rapid,
+and the shallow-water phase only lasted through very early Cambrian
+times in most regions, whilst the deep-water phase, complicated by
+many minor upheavals, extended through the main part of Cambrian,
+Ordovician and Silurian times, and was replaced by the later
+shallow-water phase at the end of Silurian times.
+
+The second marine period again was ushered in by rapid submergence, so
+that the shallow-water phase was brief, and the main mass of the Lower
+Carboniferous strata was deposited in deep water; but, unlike the
+first marine period, the second was characterised by the occurrence of
+a long interval of time marking the later shallow-water phase, during
+which the whole of the Upper Carboniferous strata were deposited. The
+Carboniferous Marine Period is the simplest of the three with which we
+have to deal, as the local oscillations occurring on a fairly large
+scale for such movements were less frequent than was the case during
+the first and third marine periods.
+
+The third marine period had a long shallow-water phase at the
+commencement, with many minor oscillations, causing great variation in
+the character of the deposits and frequent minor unconformities. This
+shallow-water phase existed throughout Jurassic and Lower Cretaceous
+times. The deep-water phase existed during the deposition of the Upper
+Cretaceous deposits, and was succeeded by the second shallow-water
+phase, when the early Tertiary strata were accumulated.
+
+The difference between the elevations which accompanied the
+Continental Periods and those which have been alluded to as minor
+elevations is no doubt one of degree, but in considering the British
+strata only no confusion is likely to arise on this account, as the
+difference was here very great.
+
+The events which occurred during the continental periods are of
+extreme importance to the geologist. Every great upheaval was
+accompanied by crumpling and stiffening of portions of the earth's
+crust, and a definite trend was given to the strata as the result of
+these movements. It is to the earth-movements of the four great
+continental periods that the present structure of the British Isles is
+largely due, and in any attempt to restore the physical history of our
+islands considerable attention must be paid to the changes which were
+produced in the stratified rocks during these periods of
+earth-movement.
+
+
+
+
+CHAPTER XIV.
+
+THE CAMBRIAN SYSTEM.
+
+
+_Classification._ The rocks of the Cambrian system when found reposing
+on Precambrian rocks in Britain are always separated from the latter
+by an unconformity. The typical development of the rocks of the
+system, as the name implies, is in the hilly region of Caernarvonshire
+and Merionethshire in North Wales, and they are also well represented
+in South Wales, the border counties between England and Wales, and the
+North-West Highlands of Scotland. Two distinct classifications of the
+Cambrian rocks of Britain are in use, the original one founded on
+variations of lithological character, whilst the second depends upon
+faunistic differences, but the original lithological classification
+has been to some extent modified to make it locally correspond with
+the classification based upon palæontological grounds. The following
+table will shew the differences:--
+
+  Lithological Classification.          Palæontological Classification.
+
+  Tremadoc Slate Series[64]              Beds with Intermediate Fauna
+
+  Lingula Flags Series                  Beds with _Olenus_ Fauna
+
+  Menevian beds (formerly included   }
+     in Lingula Flags)               }  Beds with _Paradoxides_ Fauna
+               } Formerly grouped    }
+  Solva beds   } together as Harlech
+  Caerfai beds } or Llanberis beds      Beds with _Olenellus_ Fauna
+
+[Footnote 64: In accordance with the custom usually observed in
+Britain, the Tremadoc slates are placed in the Cambrian system; most
+continental geologists place them in the succeeding Ordovician system.
+The matter is not an important one, as the fauna is an intermediate
+one between that of the Lingula Flags and that of the Arenig series of
+the Ordovician system, and the beds are true beds of passage. As the
+lithological classification is essentially British, it will be as well
+to retain the Tremadoc Slates in the Cambrian system.]
+
+The original lithological classification was essentially the result of
+Prof. Sedgwick's work in North Wales, while the classification
+according to faunas is the outcome of the researches of Dr Hicks in
+South Wales.
+
+_Description of the Strata._ The Cambrian rocks of North Wales occur
+in two complex anticlines, separated by an intermediate syncline of
+Ordovician strata occupying the Snowdonian hills. The southerly or
+Harlech anticline forms a part of Merionethshire to the east of
+Harlech, whilst the northern one is developed around Bangor and
+Llanberis. The South Welsh Cambrian rocks are chiefly found on either
+side of the Pembrokeshire axis of Precambrian rocks which runs through
+St David's. As the corresponding rocks of the two regions were
+deposited in bathymetrical zones of much the same depth, it will be
+convenient to give a general account of the rocks of the two regions
+at the same time, leaving the student to acquire information of the
+detailed variations in the larger text-books and in special
+memoirs[65].
+
+[Footnote 65: A general account of the Cambrian, Ordovician and
+Silurian rocks will be found in the Sedgwick Essay for 1883, _A
+Classification of the Cambrian and Silurian Rocks_, though the use of
+a cumbrous nomenclature therein will tend to confuse the reader. For a
+detailed account of the Cambrian rocks of North Wales the reader is
+referred to the Geological Survey Memoir, _The Geology of North
+Wales_, by Sir A. Ramsay (2nd edition), he may also consult Belt, T.,
+"On the Lingula Flags or Festiniog Group of the Dolgelly district,"
+_Geol. Mag._, Dec I. vol. IV. pp. 493, 536, vol. V. p. 5. The geology
+of the Cambrian rocks is described in a series of Memoirs in the
+_Quarterly Journal of the Geological Society_ by Dr H. Hicks; the
+following should be consulted: Harkness, R. and Hicks, H., "On the
+Ancient Rocks of the St David's Promontory, South Wales, and their
+Fossil Contents," vol. XXVII. p. 384; Hicks, H., "On some Undescribed
+Fossils from the Menevian Group," vol. XXVIII. p. 173; and "On the
+Tremadoc Rocks in the neighbourhood of St David's, South Wales, and
+their Fossil Contents," vol. XXIX. p. 39. See also Hicks, "The
+Classification of the Eozoic and Lower Palæozoic Rocks of the British
+Isles," _Popular Science Review_, New Series, vol. V., and Hicks,
+"Life-zones in the Lower Palæozoic Rocks," _Geol. Mag._ Dec IV. vol.
+I. pp. 368, 399 and 441.]
+
+The strata of the Caerfai and Solva groups show the prevalence of the
+shallow-water phase almost uninterruptedly through the whole of the
+time occupied by their accumulation in the Welsh areas. They consist
+chiefly of basal conglomerates, succeeded by alternations of grits and
+shales, though the latter are often converted into slates, owing to
+the subsequent production of cleavage. The basal conglomerates of the
+Caerfai beds are frequently marked by the existence of enormous
+pebbles, composed of fragments of the rocks of the underlying
+Precambrian groups, and the possibility of the occurrence of glacial
+action during their accumulation as advocated by Dr Hicks must be
+taken into account. Above these beds are various coloured grits, with
+alternations of muddy sediments often coloured red[66]. The Solva
+group consists of massive grits, of various colours, also with
+alternations of mud, which have prevalent purple and green hues. The
+great thickness of the strata of the Caerfai and Solva Series, which
+sometimes exceeds 10,000 feet, must also be noted.
+
+[Footnote 66: In giving this description the red (Glyn) slates of
+North Wales are treated as belonging to the Caerfai series, though
+this correlation depends on lithological characters only at present.]
+
+The Menevian beds consist essentially of very fine, well laminated
+black and grey muds, which are of a texture favourable for the
+production of a somewhat regular jointing, causing the rock to break
+into small rectangular blocks. They are thin, not exceeding 600 feet
+in thickness, and indicate the incoming of the general deep-water
+phase of the Lower Palæozoic epoch. The Lingula Flags mark a local
+return to shallower water conditions, especially in the central
+portion. The total thickness is over 3,000 feet, of which the lower
+stage (locally the Maentwrog series) is over 500 feet, and consists of
+blackish muds, the middle (Festiniog stage[67]) is about 2,000 feet
+thick, and is composed chiefly of shallower water gritty flags, whilst
+the upper (Dolgelly) stage is of about the same thickness as the lower
+stage and has similar lithological characters.
+
+[Footnote 67: The term Festiniog has been used for the whole Lingula
+Flag series as well as for the middle stage. It will be well to use it
+with reference to the stage only.]
+
+The Tremadoc Slates are about 1,000 feet thick. They are divided into
+a lower and upper stage, of about equal thickness, and are essentially
+composed of iron-stained slates, with a considerable admixture of
+calcareous matter in some parts of South Wales, when they furnish the
+nearest approach to a limestone which has been found amongst the Welsh
+Cambrian strata. They were probably formed in a fairly deep sea.
+
+Much pyroclastic rock and some lava flows are intercalated amongst the
+Welsh Cambrian sediments. Tuffs are formed in the lower beds of St
+David's, and lavas and ashes have been found amongst the Lingula Flags
+and Tremadoc Slates of North Wales, while the Lingula Flags of South
+Wales have furnished several bands of ash to the north of
+Haverfordwest. Much of the material of the grits and muds may be
+derived from volcanic rocks, though how far this is so cannot be
+stated in the absence of information obtained by detailed petrological
+examination of the rocks.
+
+The various isolated outcrops of Cambrian strata amongst the counties
+of the Welsh borders and adjoining Midland counties indicate a great
+thinning of the Cambrian rocks in this direction.
+
+The probable equivalents of the Caerfai rocks occur at Nuneaton,
+Comley, and on the flanks of the Wrekin and Malvern hills. The thin
+basal conglomerates are succeeded by quartzites, and sometimes red
+calcareous sandstones (Comley sandstone). These rocks are succeeded by
+thin arenaceous and calcareous beds which represent either the Solva
+or Menevian beds of Wales. The Lingula Flags are represented by the
+Malvern Shales of the Malvern area and the Stockingford Shales of
+Nuneaton, whilst the Tremadoc Slates have as their equivalents the
+Shineton Shales. The exact thicknesses of these deposits do not seem
+to have been recorded, but Prof. Lapworth observes that in central
+Shropshire "the Comley and Shineton groups which ... have a collective
+thickness of perhaps less than 3,000 feet, we have apparently a
+condensed epitome of the entire Cambrian system as at present
+generally defined."
+
+The Cambrian rocks of the North-west Highlands consist of a thin
+conglomerate succeeded by grits and flags with shaley beds, and above
+these a mass of limestone, which may represent some of the Ordovician
+deposits as well as those of Cambrian age. Pending a complete
+description of the faunas of these rocks, it is sufficient to state
+that the only fauna which has hitherto been described in detail
+indicates the existence of Lowest Cambrian rocks. Further remarks will
+be made on this head when describing the character of the Cambrian
+faunas. The Cambrian rocks of the North-west Highlands are also very
+thin as compared with those of Wales, so that the Highland and Welsh
+borderland regions appear to have existed as a deeper sea area than
+that which is indicated by the Cambrian rocks of Wales, an inference
+which is to some extent borne out by study of the Cambrian rocks of
+extra-British areas, to which we may now turn.
+
+The principal European developments of Cambrian rock are found in
+Scandinavia, Russia, Bohemia and Spain, and of these the Scandinavian
+one is by far the most fully developed, as there is a complete
+sequence in the rocks of that peninsula. They occur both in Norway and
+Sweden, but the Swedish exposures are the most interesting in most
+respects, especially those of Westrogothia and Scania. The rocks are
+of no great thickness, and consist essentially of black carbonaceous
+shales, with inconstant bands of impure black limestone composed
+almost entirely of the remains of trilobites or more rarely of
+brachiopods. These Alum Shales, as they are termed, rest unconformably
+upon Precambrian rocks, and have arenaceous and conglomeratic deposits
+at the base. In Russia the rocks are still further attenuated, and
+have not yielded the relics of so many faunas as have been found in
+the Scandinavian Cambrian rocks.
+
+The Bohemian development is incomplete, owing apparently to an
+unconformity at the base of the overlying Ordovician rocks, while the
+Spanish deposits which seem fairly thick and composed largely of
+mechanical sediments have not been worked out in very great detail.
+
+The American development of Cambrian rocks resembles the European one
+in many striking particulars, and as in the case of Europe, there are
+lateral variations in the lithological characters of the rocks, though
+in the opposite direction, the shallow-water deposits occurring on the
+east coast, and the deep-water deposits further west.
+
+The general distribution of the different types of Cambrian strata in
+Europe and North America has been accounted for on the supposition
+that in Cambrian times a tract of land lay over much of the present
+site of the North Atlantic Ocean, and that the detritus of that land
+formed the shallow-water accumulations of Wales and the east of
+Canada, whilst further away from it were deposited the open-sea
+accumulations of Scandinavia and Russia on one side and of the more
+westerly regions of North America on the other, as indicated in Fig.
+16.
+
+[Illustration: Fig. 16.
+
+  P. Precambrian Rocks.
+  A. Land.
+  X, X´. Sea level.
+  BB´. Shore deposits.
+  CC´. Deep-water deposits.
+  DD´. Abyssal deposits.
+]
+
+_The Cambrian Faunas._ The Cambrian Period has been termed the age of
+trilobites, for they are the dominant forms of the time, but they are
+associated with many other forms of invertebrata; indeed all the great
+groups of this division are represented in the earliest Cambrian
+fauna. Dr C. D. Walcott records representatives of Spongiae, Hydrozoa,
+Echinodermata, Annelida, Brachiopoda, Lamellibranchiata, Gastropoda,
+Pteropoda, Crustacea and Trilobita as occurring in the _Olenellus_
+beds of North America and other groups are represented in the rocks of
+this age in the Old World. The Cambrian trilobites as a whole are of
+more generalised types than those of the later systems which furnish
+their remains, as indicated especially by the looseness of the body,
+and the large number of body rings in many of the genera, while the
+tail or pygidium was small and formed of only a few coalesced
+segments, as pointed out by Barrande. In the later trilobites the test
+is more compact, there are on the whole fewer body rings, as more of
+these have become fused into a tail which is therefore larger than
+that of the average tail of the Cambrian trilobite.
+
+Taking the faunas in order, the oldest or _Olenellus_ fauna has
+furnished a great variety of forms in the North-west Highlands of
+Scotland, Shropshire, Scandinavia, Esthonia, Sardinia, Canada, and
+Newfoundland, whilst representative species of the fauna have been
+recorded also from Worcestershire, Warwickshire, Pembrokeshire, India,
+China, and Australia.
+
+The dominant form is the trilobite of the genus or group _Olenellus_,
+which contains a great variety of species referable to three or four
+divisions which have been ranked as separate genera by some writers.
+Associated with _Olenellus_ are trilobites belonging to other genera,
+which are found in higher deposits, though there represented by
+different species.
+
+Brachiopods are fairly abundant, especially those provided with a
+horny shell; of these, the genus _Kutorgina_ is widely distributed.
+
+The zoological relationships of several of the fossils of this horizon
+are as yet doubtful. The Archæocyathinæ show affinities with certain
+corals; a number of tests, included in the genus _Hyolithes_ and its
+allies are doubtfully referred to the Pteropods, and the position of
+the genus _Volborthella_ is uncertain. Special attention is directed
+to these doubtful relationships, as it is possible that a number of
+'generalised forms' of organisms occur in these strata[68].
+
+[Footnote 68: For an account of the _Olenellus_ fauna see Walcott, C.
+D., "The Fauna of the Lower Cambrian or Olenellus Zone," _Tenth Annual
+Report of the Director of the United States Geological Survey_,
+Washington, 1890. It is possible that some of the fossils mentioned in
+that report belong to strata above that containing _Olenellus_.]
+
+It should be noticed here that faunas have been discovered which are
+possibly of earlier date than the _Olenellus_ fauna, as they do not
+correspond with it, or with those of newer strata. One, the _Neobolus_
+fauna of the Salt Range of India, occurs in beds below those with
+_Olenellus_, though it is not yet clear that _Olenellus_ will not be
+eventually discovered associated with it, whilst the other, the
+_Protolenus_ fauna of Canada, is of unknown age[69].
+
+[Footnote 69: For an account of the _Neobolus_ beds see Noetling, F.,
+"On the Cambrian Formation of the Eastern Salt Range," _Records Geol.
+Survey, India_, vol. XXVII. p. 71, and for the Protolenus fauna
+consult a paper by Matthew, G. F., "The _Protolenus_ Fauna," _Trans.
+New York Acad. of Science_, 1895, vol. XIV. p. 101.]
+
+The _Olenellus_ beds are succeeded by beds containing the
+_Paradoxides_ fauna, which have been found in North and South Wales,
+Shropshire, Scandinavia, Bohemia, Spain, and North and South America.
+_Olenellus_ and its allies became extinct (or else so scarce that no
+relics of them have been discovered in the _Paradoxides_ beds) before
+the commencement of the deposition of the strata containing the
+_Paradoxides_ fauna, and few genera pass from the beds with the one
+fauna to that containing the other. The _Paradoxides_ fauna existed
+for a considerable period, and the beds have been divided into a
+series of zones characterised by different species of _Paradoxides_,
+thus
+
+Dr Hicks records the following zones in Pembrokeshire[70]:--
+
+  Zone of _Paradoxides_  _Davidis_   }  Menevian.
+    "             "      _Hicksii_   }
+
+    "             "      _Aurora_    }
+    "             "      _Solvensis_ }  Solva.
+    "             "      _Harknessi_ }
+
+[Footnote 70: The order here as elsewhere is _ascending_, i.e. the
+newest deposit is placed at the top.]
+
+Dr Tullberg divides the _Paradoxides_ beds of Scania into thirteen
+zones, though only a few of these are characterised by definite
+species of _Paradoxides_. The _Olenellus_ beds have not yet been
+divided into zones, though this will probably be the outcome of
+further study[71].
+
+[Footnote 71: The _Paradoxides_ fauna is described in the following
+works: Britain, Hicks, H. and Salter J. W., _Quart. Journ. Geol.
+Soc._, vol. XXIV. p. 510, XXV. p. 51, XXVII. p. 173, and Hicks, H. and
+Harkness, R., _ibid._ vol. XXVII. p. 384; Scandinavia, Angelin, N. P.,
+_Palæontologia Scandinavica_; Brögger, W. C., _Nyt Magazin for
+Naturvidenskaberne_, vol. XXIV., Linnarsson, G., _Sveriges Geologiska
+Undersökning_, Ser. C. No. 35; Bohemia, Barrande, J., _Système
+Silurien du centre de la Bohême_; Spain, Prado, C. de, "Sur
+l'existence de la faune Primordiale dans la chaîne Cantabrique suivie
+de la description des Fossiles par MM. de Verneuil et Barrande,"
+_Bull. Soc. Geol. France_, 2 Series, vol. XVII. p. 516; America,
+Walcott, C. D., _Bull. U. S. Geol. Survey_: "The Cambrian Faunas of
+North America," and Matthew, G. F., _Trans. Roy. Soc. Canada_, 1882
+and succeeding years.]
+
+The strata with _Paradoxides_ are succeeded by those with the _Olenus_
+fauna, characterised by the genus _Olenus_ and a large number of
+allied genera or sub-genera as some prefer to term them. The genus
+_Olenus_ (_sensu stricto_) is very abundant in the lower part of the
+series, whilst the allied forms are more abundant in the upper beds.
+The genus _Paradoxides_ and its associates disappeared before the
+deposition of these strata containing _Olenus_ and its allies, and
+indeed the complete change in the character of the faunas in Europe is
+very remarkable. The _Olenus_ fauna has been found in North Wales,
+Pembrokeshire, Warwickshire, Worcestershire, and abroad in Scandinavia
+and Canada. It is interesting to note among the fossils of the
+_Olenus_ beds the occurrence of a graptolite which is associated with
+_Olenus_ in Scandinavia; this is the earliest recorded appearance of a
+group which is destined to play so important a role amongst the
+fossils of the succeeding system[72]. The following zones have been
+detected by Dr S. A. Tullberg amongst the _Olenus_ beds of Scania:--
+
+  Zone of _Acerocare ecorne_.
+     "    _Dictyograptus flabelliformis_.
+     "    _Cyclognathus micropygus_.
+     "    _Peltura scarabæoides_.
+     "    _Eurycare camuricorne_.
+     "    _Parabolina spinulosa_.
+     "    _Ceratopyge_ sp.
+     "    _Olenus_ (proper).
+     "    _Leperditia_.
+     "    _Agnostus pisiformis_.
+
+[Footnote 72: For descriptions of the _Olenus_ fauna consult the
+following:--Wales, Belt, T., _Geol. Mag._ Dec. I. vol. V. p. 5, and
+Salter, J. W., _Decades Geol. Survey_, Decade II. Pl. IX. and Decade
+XI. Pl. VIII.; Scandinavia, Angelin, N. P., _Palæontologia
+Scandinavica_, and Brögger, W. C., _Die Silurischen Etagen 2 und 3 im
+Kristianiagebiet und auf Eker_; Canada, Matthew, G. F., "Illustrations
+of the Fauna of the St John Group, No. VI.," _Trans. Roy. Soc.
+Canada_, 1891.]
+
+The beds with _Dictyograptus flabelliformis_ form a wonderfully
+constant horizon at or near the top of the _Olenus_ beds. They are
+found in North Wales, the Border Counties between Wales and England,
+France, Scandinavia, Russia and Canada.
+
+The passage fauna of the beds which are the equivalents of the
+Tremadoc Slates may be spoken of as the _Ceratopyge_ fauna, for
+_Ceratopyge forficula_, a remarkable species of trilobite,
+characterises it in Scandinavia, and will probably be found
+elsewhere. _Ceratopyge_ beds have been found in North and South Wales,
+Shropshire, Scandinavia, Bavaria and North America, and in each case
+the fauna is intermediate in character between that of the Cambrian
+and that of the Ordovician system, containing the loosely-formed
+trilobites of the former with the more compact ones of the latter. The
+genus _Bryograptus_, a many-branched graptolite, also appears to
+characterise this fauna[73].
+
+[Footnote 73: For accounts of the Tremadoc Slates Fauna in England and
+Wales see Ramsay, A. C., _Geology of North Wales_, Appendix; Hicks,
+H., _Quart. Journ. Geol. Soc._, vol. XXIX. p. 39; Callaway, C.,
+_ibid._ vol. XXXIII. p. 652, whilst many of the foreign fossils are
+noticed in Brögger's _Die Silurischen Etagen 2 und 3_ and Barrande's
+_Faune silurienne des Environs de Hof en Bavière_.]
+
+The faunas of the Cambrian rocks have not been studied in sufficient
+detail, with reference to the physical surroundings of the organisms,
+to throw much light upon the conditions under which the strata were
+deposited, though the evidence obtained from an examination of the
+lithological characters of the deposits is generally corroborated by
+study of the organic contents.
+
+
+
+
+CHAPTER XV.
+
+THE ORDOVICIAN SYSTEM.
+
+
+_Classification._ The Ordovician strata were originally divided into
+series by Sedgwick as follows:--
+
+  Upper Bala,
+  Middle Bala,
+  Lower Bala,
+  Arenig.
+
+The Arenig series was at one time included by some writers with the
+Lower Bala under the name Llandeilo, but the word Llandeilo is now
+used in the sense of Sedgwick's Lower Bala. The Middle Bala is often
+spoken of as Caradoc, but the terms Bala and Caradoc are sometimes
+used interchangeably. As much confusion attaches to the use of the
+name Bala without explanation, the alternative titles have been
+largely adopted, and as the series are well defined there is no
+objection to their use, save that some expression is wanted equivalent
+to Upper Bala. The local term Ashgill shales was originally applied by
+Mr W. Talbot Aveline to beds of this age in Lakeland, and I have
+elsewhere suggested the use of this name for the whole series in that
+region; its use may well be extended to the series which is developed
+in many parts of Britain and the continent. The terms which will be
+used here, therefore, for the different series of the Ordovician
+system are the following:--
+
+  Ashgill Series (= Upper Bala)
+  Caradoc    "   (= Middle " )
+  Llandeilo  "   (= Lower  " )
+  Arenig     "
+
+Adopting a palæontological classification, we may speak of the Arenig
+and Llandeilo beds as those containing the _Asaphus_ fauna, whilst the
+Caradoc and Ashgill beds possess the _Trinucleus_ fauna; this is the
+terminology employed by Angelin for the equivalent strata of Sweden.
+It must be noted that here the names applied are not those of
+absolutely characteristic genera, as was the case with those adopted
+for naming the Cambrian faunas, for both _Asaphus_ and _Trinucleus_
+range through the beds of the system; but whereas _Asaphus_ is most
+abundant in the beds of the two lower series, _Trinucleus_ occurs most
+frequently in those of the two upper series.
+
+_Description of the strata._ The Ordovician rocks are found over large
+tracts in North and South Wales, in the counties on the Welsh border,
+in Lakeland and the outlying districts in the Southern Uplands of
+Scotland, and in detached areas in Ireland. There are three main types
+of deposit:--(i) the volcanic type, in which the ordinary sediments
+are associated with a large amount of contemporaneous volcanic matter,
+(ii) the black shale type, with a fauna consisting largely of
+graptolites, and (iii) the ordinary sedimentary type, in which we find
+alternations of grits, shales, and more or less impure limestones. We
+also find developments which are intermediate between any two or even
+all three of these types. The first type is characteristically
+developed in Caernarvonshire and Merionethshire, the second in the
+Dumfriesshire Uplands, and the third in the Girvan district of
+Ayrshire. The variation in the thickness of these three types of
+deposit is shown in the accompanying sections of the Caernarvon,
+Merioneth, Moffat and Girvan regions (see Fig. 17).
+
+[Illustration: Fig. 17.
+
+Showing the variations in the characters of the Ordovician deposits of
+the three principal types.
+
+  Scale 1 in. = 1000 feet.
+
+  A = Arenig.  L = Llandeilo.  C = Caradoc.
+
+The thickness of the Arenig rocks of the Scotch areas is unknown.]
+
+The North Welsh area gives two different developments of the
+Ordovician strata, one of which is much less volcanic than the other.
+In the Merioneth-Caernarvon area, two great masses of volcanic rock
+form the Aran and Arenig hills of Merioneth and the Snowdonian group
+of Caernarvon. The former are of Arenig, the latter of Caradoc age.
+The Merionethshire volcanic rocks consist of a great thickness of
+lavas and ashes of intermediate composition (anderites), associated
+with sandy and muddy sediments of no great vertical depth. The
+Llandeilo beds of this area are chiefly of the nature of black shales,
+while the Caradoc series is represented by volcanic lavas and ashes of
+acid composition (felsites) with a few thin interbedded sediments. A
+calcareous ash forming the summit of Snowdon is of importance as being
+on the same horizon as a limestone (the Bala limestone) found in the
+other North Welsh area. The Ashgill series is not represented in
+Snowdonia.
+
+In the other North Welsh tract, around Bala Lake, the volcanic matter
+is much less conspicuous. The Arenig rocks are not seen nearer than
+the Arenig mountains which form the western boundary of this second
+tract. The Llandeilo beds consist of shaley deposits with a
+well-marked limestone, the Llandeilo limestone, in the centre, whilst
+the Caradoc beds consist chiefly of muddy sediments with some thin
+ashes and a limestone, the Bala limestone, at the top. The Ashgill
+series contains a basal limestone, the Rhiwlas limestone, succeeded by
+shales, and another thin limestone called the Hirnant limestone at the
+summit.
+
+In South Wales the Arenig beds[74] are chiefly composed of slates, and
+are divisible into an upper and lower group. The total thickness is
+about 2000 feet. The Llandeilo beds contain three series:--
+
+  Upper Llandeilo Slates 1000
+  Llandeilo Limestone     200
+  Lower Llandeilo Slates  800.
+
+[Footnote 74: A remarkable fauna, fairly well represented in Britain
+and exceedingly well developed on the continent, exists in the
+Uppermost Arenig and Lower Llandeilo beds, and it is well separated
+from the dominant Arenig fauna below and Llandeilo fauna above. To the
+beds which contain it Dr Hicks has given the name Llanvirn series.]
+
+The Caradoc beds consist of black graptolitic shales of no great
+thickness, succeeded by an impure limestone on the horizon of the Bala
+limestone, while the Ashgill series like that of North Wales is
+separated into upper and lower limestone stages with an intervening
+stage composed of shales.
+
+The deposits of the Welsh borderland are well developed in Shropshire,
+where there is practically a repetition of the Caernarvon-Merioneth
+development, with variations in detail. The Arenig and Caradoc
+volcanic rocks are not so thick as those of the Welsh district, but
+are nevertheless of considerable importance[75].
+
+[Footnote 75: For information concerning these beds see Lapworth, C.
+and Watts, W. W., "The Geology of South Shropshire," _Proc. Geol.
+Assoc._, vol. XIII. p. 297.]
+
+In the hilly region of Cumberland, Westmorland, and the adjoining
+parts of Yorkshire the succession differs from that of any of the
+Welsh regions, for the great period of volcanicity was during the
+formation of the Llandeilo rocks, and there were merely sporadic
+outbursts in Arenig and Caradoc times. The Arenig rocks consist of
+black shales with interstratified beds of coarser sediment, and some
+thin lavas and ashes of intermediate type. The Llandeilo series is
+represented by a very great thickness of volcanic rocks, varying in
+composition from basic to acid lavas, with associated pyroclastic
+rocks. The rocks of the Caradoc period largely consist of impure
+limestone with associated argillaceous rocks, and contemporaneous
+volcanic rocks of acid character. A marked unconformity is found
+locally in the centre of these. The Ashgill series consists of a basal
+limestone with shales above, and there is evidence that volcanic
+activity had not become extinct during the deposition of the rocks of
+this series.
+
+Passing on to Scotland, the graptolitic type is admirably shown in the
+southern Uplands of the neighbourhood of Moffat, Dumfriesshire. The
+base of the Ordovician system has not been found, but the lowest
+series seems to be represented by shales with a graptolite possibly of
+Arenig age. Above this are volcanic beds succeeded by a group of black
+shales known as the Moffat shales. They are only about six hundred
+feet in thickness, and yet represent much of the Ordovician and part
+of the Silurian strata as developed elsewhere. The beds belonging to
+the Ordovician system are divided into two series, the Glenkiln shales
+below and the Hartfell shales above. The former consist of intensely
+black muds with few fossils save graptolites, and a deposit of chert
+at the base which is composed of radiolaria. The graptolites of the
+black shales are Upper Llandeilo forms, but the thin deposit of
+radiolarian chert may represent the rest of the Llandeilo period and
+part of the Arenig period also. The Hartfell shales are also usually
+black graptolite shales with lighter deposits nearly barren of organic
+remains; they represent the Caradoc and Ashgill series and pass
+conformably into the deposits of Silurian age[76]. The ordinary
+sedimentary type of Ordovician rocks is found in Ayrshire, though a
+few thin graptolitic seams are intercalated with the conglomerates
+and shelly sands, clays and limestones of the region, which is
+therefore peculiarly valuable as affording a means of comparison of
+the shelly type with the graptolitic type of Ordovician deposits. The
+Arenig series consists of black shales with graptolites, and these
+rocks are succeeded by a volcanic group which is probably of Llandeilo
+age. Above these volcanic beds, as in Dumfriesshire, we find three
+great divisions, two of which are of Ordovician, the third of Silurian
+age. The Ordovician divisions are respectively termed the Barr series,
+which is the equivalent of the Glenkiln shales, and the Ardmillan
+series above, equivalent to the Hartfell shales[77].
+
+[Footnote 76: The Moffat beds are described in a paper by Prof.
+Lapworth entitled "The Moffat Series" in the _Quarterly Journal of the
+Geological Society_, vol. XXXIV. p. 239. This paper, which is a
+masterpiece of detailed work, has furnished a clue to many problems.
+Few students will be able to follow the numerous details, and for
+general information concerning the beds they are recommended to read
+another paper by the same author "On the Ballantrae Rocks of South
+Scotland," _Geol. Mag._ Dec. III. vol. VI. p. 20. An account of the
+radiolarian cherts by Dr G. J. Hinde will be found in the _Annals and
+Magazine of Natural History_ for July, 1890, p. 40.]
+
+[Footnote 77: See Lapworth, C., "The Girvan Succession," _Quart.
+Journ. Geol. Soc._, vol. XXXVIII. p. 537, and also the paper on the
+Ballantrae Rocks referred to in the preceding footnote. The latter
+paper should be carefully read by all students of the stratigraphy of
+the Lower Palæozoic Rocks.]
+
+It is interesting to find that in the north of Ireland the rocks
+generally coincide in characters with those which are found along the
+same line of strike in Great Britain; thus, the Girvan type appears in
+Londonderry, Tyrone and Fermanagh, the Moffat type in County Down, and
+the Lake District type in the counties of Dublin and Kildare.
+
+On the continent the volcanic material which plays so important a part
+in the constitution of the Ordovician accumulations of Britain is
+practically absent, and the strata are largely composed of
+accumulations of shale and limestone with occasional coarser deposits.
+In Scandinavia, the Arenig beds consist of limestones with a few
+shales, the Llandeilo deposits are largely calcareous, those of
+Caradoc age are partly calcareous and towards the top usually
+argillaceous, while the equivalents of the British Ashgill series are
+calcareous at the base and argillaceous at the summit. In Russia the
+calcareous matter preponderates over the argillaceous material.
+
+Ordovician strata are also found in Belgium, France, Bohemia, and
+other places, and are largely composed of mechanical sediments of
+varying degrees of fineness mixed occasionally with some calcareous
+matter.
+
+The variation in the characters of the Ordovician strata of Britain
+points to accumulation in a fairly deep sea, usually at some distance
+from the land, but dotted over with volcanoes which often rose above
+the water, causing the addition of much volcanic material to the
+ordinary sediments, and the existence of minor unconformities at
+different horizons along their flanks. As these unconformities are not
+always associated with volcanic material it is obvious that uplifts
+must have occurred occasionally during the deposition of the rocks;
+one important uplift is indicated by the occurrence of an unconformity
+in the Arenig rocks of Wales, while another is seen amongst the
+Caradoc rocks of the Welsh borders. On the whole, however, the period
+was one of slow subsidence, the deposition of material generally
+keeping pace with this subsidence, and accordingly there is a great
+uniformity of characters amongst the strata over wide areas. The
+probable continuation through the Ordovician period of the tract of
+land over the present site of the N. Atlantic ocean which as we have
+reason to suppose existed during Cambrian times, is indicated by
+similar changes of lithological character amongst the strata when
+traced from Britain eastward to Russia in both Cambrian and Ordovician
+times, and the continuance of these conditions over the American area
+is also indicated by study of the variations amongst the American
+Ordovician deposits.
+
+_The Ordovician Faunas._ The Ordovician period has justly been termed
+the Period of Graptolites, which are the dominant forms of the time,
+and continue in abundance throughout the period. The abundance of
+graptolites in black shales associated with few other organisms has
+often been noted. It appears to be due to a large extent to the slow
+accumulation of the graptolitic deposits, allowing an abundance of
+these creatures to be showered upon the ocean floor, after death, for
+the evidence derived from detailed examination of their structure
+points to their existence as floating organisms. The tests of other
+creatures largely calcareous may well have been dissolved before
+reaching the sea-floor. In support of the view that these black shales
+are abysmal deposits may be noted the singular persistence of their
+lithological characters over wide areas, their replacement by much
+greater thicknesses of normal sediments along the ancient coast-lines,
+the frequent occurrence together of blind trilobites with those having
+abnormally large eyes when these creatures are associated with
+graptolites in the black shales, and lastly the interstratification of
+the black shales with radiolarian cherts similar to the modern abysmal
+radiolarian oozes. If this be so, we ought to find graptolites in
+marine deposits of all kinds, and indeed they are found there, though
+largely masked by the mass of sediment and the hosts of other included
+fossils, so that their discovery is rendered much more difficult than
+when they occur in the black shales,--a state of things which is
+familiar in the case of other pelagic organisms as _Globigerinæ_,
+radiolaria, and pteropods, whose tests abound in the abysmal deposits
+and are comparatively rare in those of terrigenous origin[78].
+
+[Footnote 78: The importance of the graptolites as indices of the
+geological age will be seen by perusal of Prof. Lapworth's paper "On
+the Geological Distribution of the Rhabdophora," _Ann. and Mag. Nat.
+Hist._, Ser. 5, vol. III. (1897).]
+
+The characters of the Ordovician trilobites have already been noticed.
+These organisms are abundant, and occur in sediments of all kinds. Of
+other groups, the significance of the radiolaria has been referred to
+above. Corals occasionally form reef-like masses of limestone as in
+the limestones of the Caradoc epoch; the echinoderms are well
+represented, cystids being locally abundant; of the crustacea, many
+remains of tests of phyllocarida have been recorded; the brachiopods
+are very abundant, and of the mollusca, lamellibranchs, gastropods and
+cephalopods all occur with frequency though none of these groups is
+very prevalent. Certain forms have been referred to pteropods though
+with doubt, and other shells seem to be referable to the heteropods.
+The existence of vertebrates during Ordovician times is not, in the
+opinion of many geologists, proved, though remains of fishes have been
+recorded from the Ordovician strata of North America; but it is
+desirable that more evidence of this occurrence should be given[79].
+
+[Footnote 79: Walcott, C. D., "Preliminary Notes on the Discovery of a
+Vertebrate Fauna in Silurian (Ordovician) Strata," _Bulletin Geol.
+Soc. America_, vol. III. p. 153.]
+
+The distribution of the Ordovician faunas like that of the sediments
+points to the prevalence of open ocean conditions over wide areas
+during the period, with occasional approaches to land, which was often
+of a volcanic nature. Around this land clustered the ordinary
+invertebrates, building up coral-reefs and shell-banks, whilst away in
+the open oceans the graptolites floated, almost alone, and sank to the
+ocean floor after death.
+
+
+
+
+CHAPTER XVI.
+
+THE SILURIAN SYSTEM AND THE CHANGES WHICH OCCURRED IN BRITAIN AT THE
+CLOSE OF SILURIAN TIMES.
+
+
+_Classification._ The Silurian system was originally divided by its
+founder, Sir R. I. Murchison, into three series, as follows:--
+
+  Ludlow Series
+  Wenlock    "
+  Llandovery "
+
+The term May Hill, proposed by Sedgwick, is sometimes used as
+synonymous with Llandovery. This classification omits a somewhat
+important set of beds intercalated between those of the Llandovery and
+Wenlock series known as the Tarannon shales, and in Britain if we were
+to classify afresh, it would be more convenient to include some of the
+beds formerly referred to the Ludlow in the Wenlock. I shall, however,
+adopt the old and well-established classification, adding the term
+Tarannon to Llandovery, and speaking of the Llandovery-Tarannon
+series. The nature of the two classifications is shown in the
+following table:
+
+                             Old             New           Palæontological
+         Stages.         Classification.  Classification.  Classification.
+
+  1 Upper Ludlow       }                }                }
+  2 Aymestry Limestone }  Ludlow        }  Downtonian    }
+  3 Lower Ludlow       }                                 }  Fauna
+                                        }                }  with
+  4 Wenlock Limestone  }                }                }  _Encrinurus_
+  5 Wenlock Shale      }  Wenlock       }  Salopian      }
+  6 Woolhope Limestone }                }                }
+
+  7 Tarannon Shales                     }                }  Fauna
+  8 Upper Llandovery   }  Llandovery    }  Valentian     }  with
+  9 Lower Llandovery   }                }                }  _Harpes_
+
+[Illustration: Fig. 18.
+
+L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.]
+
+_Description of the strata._ Lithologically the Silurian deposits of
+Britain form a continuation of those of the Ordovician period, with a
+local interruption due to the elevation of portions of Wales and the
+Welsh borders at the close of Ordovician times. Elsewhere we find a
+predominance of shales passing into grits at the top of the system,
+the change indicating the incoming of the shallow-water phase before
+the commencement of the second continental period. Particular stress
+is laid upon the predominant shaley character of the beds, for, on
+account of the richness and variety of the faunas of the calcareous
+rocks, greater attention is naturally paid to them in geological
+works, and the student may get a false idea of their relative
+importance. An attempt is made below (Fig. 18) to give a general idea
+of the variations in lithological characters of the Silurian rocks in
+different parts of Britain.
+
+The Silurian strata are mostly found in the same localities as those
+which furnish exposures of the rocks of Ordovician age.
+
+The development in the typical Silurian region of the Welsh borders is
+characterised by the abundance of calcareous matter which is found
+there as compared with that which exists in the other British
+localities.
+
+The Llandovery strata are sandy, often conglomeratic, with a fair
+amount of calcareous matter in places. The arenaceous nature is
+undoubtedly due to the proximity of land caused by local upheaval at
+the end of Ordovician times, and the Upper Llandovery rocks sometimes
+rest unconformably on the Lower ones, at other times on Ordovician,
+Cambrian, or even Precambrian rocks. The Tarannon shales are light
+green shales with intercalated grits. The Wenlock series consists of a
+group of shales separating a lower, very inconstant, earthy limestone
+from an upper, more constant, thicker and purer limestone. The latter,
+the Wenlock limestone, is composed of fragments and perfect specimens
+of various fossils, and the fragmentary nature of many of the shells
+indicates the occurrence of wave-action and probable formation in
+shallow water, in some places against coral-reefs.
+
+The Lower Ludlow beds consist of sandy shales; they are separated from
+the Upper Ludlow beds by an impure limestone, the Aymestry limestone.
+The Upper Ludlow beds consist mainly of grits and flags, often
+coloured red towards the summit.
+
+In North Wales the Llandovery beds occasionally present the shelly
+arenaceous types of deposit as near Llangollen, at other times as near
+Conway, Corwen, and in Anglesey, the graptolitic shale type. They
+also rest unconformably upon the Ordovician rocks in this area. The
+Tarannon shales resemble those of the border county. The Wenlock
+series consists essentially of shales, while the Ludlow development
+differs from that of the borders in its greater thickness and the
+absence of any calcareous band in the centre. In Central Wales the
+graptolitic type of the Llandovery-Tarannon series is found, but the
+graptolite-bearing shales of the Llandovery epoch are thin beds
+occurring between grits and flags no doubt deposited in shallow water,
+and this division of the series is of very great thickness.
+
+In South Wales the Silurian rocks are very similar to those of the
+Welsh borders, save that the calcareous deposits are fewer and
+thinner.
+
+The Lake District Silurian strata generally resemble those of North
+Wales. The Llandovery-Tarannon rocks are of the graptolite-shale type,
+intercalated with fine grits in the case of the beds of Tarannon age.
+The Wenlock beds consist of shales, and the Ludlow beds of gritty
+shales beneath, and massive flags and grits at the summit. These
+Ludlow beds are here of great thickness (certainly not less than 7000
+feet) and were obviously accumulated for the most part in shallow
+water.
+
+The Llandovery-Tarannon rocks of Southern Scotland show the two types
+which prevailed in the Moffat and Girvan areas in later Ordovician
+times. The Llandovery beds of Moffat are known as the Birkhill shales,
+and are very thin. The representatives of the Tarannon shales,
+however, the Gala beds, consist mainly of grits, and attain a great
+thickness. In the Girvan area, the Llandovery beds are of the shelly
+type. Here as at Moffat and in the Lake District there is perfect
+conformity between the beds of Ordovician and those of Silurian age,
+and accordingly it is instructive to note the completeness of the
+palæontological break, especially in the Moffat district. The higher
+Silurian beds of Southern Scotland present a general resemblance to
+those of North Wales and the Lake District[80].
+
+[Footnote 80: For descriptions of the Silurian beds of the typical
+region see Lapworth and Watts, _Proc. Geol. Assoc._, vol. XIII. p.
+297, those of Wales are described by Lake and Groom, _Quart. Journ.
+Geol. Soc._, vol. XLIX. p. 426, and Lake, _ibid._ vol. LI. p. 9. A
+description of those of Lakeland will be found in the Memoir of the
+Geological Survey "The Geology of the Country around Kendal, etc."
+while the Scotch Rocks are described in Lapworth's papers on Moffat
+and Girvan.]
+
+On the European continent we find indications of conditions similar to
+those which prevailed during the Ordovician period; the strata become
+much thinner and more calcareous in Scandinavia, and still thinner in
+the Baltic provinces of Russia, where they consist very largely of
+calcareous matter. In central Europe the greater abundance of
+calcareous matter, compared with that which is found in the Ordovician
+strata of that region, points to a change in physical conditions which
+became still more marked after Silurian times.
+
+In North America, the succession is very similar to that of Britain,
+the calcareous development of the Silurian rocks being found around
+Niagara, but towards the close of Silurian times the shallow-water
+phase became marked in places by the deposition of chemical
+precipitates which indicate the separation of a portion of the late
+Silurian ocean from the main mass during the period of formation of
+these abnormal deposits.
+
+The conditions of Silurian times, until the advent of the
+shallow-water phase, recall those of Ordovician times and point to a
+wide expanse of ocean at some distance from the land, though the
+earliest deposits become arenaceous where they were deposited against
+an old land surface formed by the elevation of the Welsh Ordovician
+rocks, which were denuded to supply this material. One marked
+difference existed between the physical conditions of our area during
+Ordovician and Silurian times, for the volcanic activity which was
+rife during Ordovician times almost ceased during Silurian times,
+except in the region now occupied by the extreme south-west of
+Ireland, and accordingly volcanic material does not appreciably
+contribute to the formation of the Silurian deposits. The shallowness
+of the sea-floor at times is marked by the occurrence of masses of
+reef-building corals in the limestones, and these probably indicate
+the prevalence of a fairly warm climate, an inference supported by the
+nature of the Gastropod fauna of Gothland, as noticed in Chap. IX.
+
+The shallow-water phase commences fairly simultaneously over the whole
+area at the beginning of the deposition of the Lower Ludlow rocks, and
+becomes more marked in the Upper Ludlow rocks, being most noticeable
+at their extreme summit, when a change occurred which will be
+considered at the conclusion of this chapter.
+
+_The Silurian Faunas[81]._ The Silurian period has been termed the
+period of Crinoids, and this group of creatures certainly contained a
+great variety of very remarkable forms, which are specially numerous
+in the Wenlock Limestone of the Welsh borders, Gothland, and North
+America, but many of the rocks of the system display few traces of
+these organisms. The trilobites and graptolites still contribute
+largely to the fauna, the latter becoming very scarce at the summit of
+the system, though a few specimens have been detected in the rocks of
+the succeeding system. The trilobites belong to few genera though
+these are mostly more highly organised than those of the Ordovician
+period. The genus _Harpes_ has been taken as fairly characteristic of
+the lower part of the system in Sweden, and it occurs there abundantly
+in places in Britain, whilst _Encrinurus_ is more abundant in the
+upper series, but both of these genera range from higher Ordovician
+beds into the Devonian. Mention has already been made of the corals.
+Brachiopods are very abundant, and Mollusca appear with considerable
+frequency. The appearance of true insects is of importance,
+cockroaches have been recorded from Silurian rocks and a number of
+other insects have lately been recorded from Canada[82]. Eurypterids
+occur in considerable abundance in the higher parts of the system, as
+do also the remains of fish.
+
+[Footnote 81: For an account of the Silurian faunas the student may
+consult Sir R. I. Murchison's _Silurian System_ or the shorter
+_Siluria_ and Lapworth's paper on the Geological Distribution of the
+Rhabdophora.]
+
+[Footnote 82: See an article by Dr G. F. Matthew, "Description of an
+extinct Palæozoic Insect and a review of the Fauna with which it
+occurs," _Bulletin_ XV. _of the Natural History Society of New
+Brunswick_. The Silurian Rocks of the Little River Group of St John,
+New Brunswick, have yielded species of land snails, two doubtful
+saw-bugs, several arachnids, and myriopods, two insects of the order
+Thysanura (Spring-tails), and eight Palæodictyoptera.]
+
+The close of Silurian times ushered in the second continental period
+in Britain when a large part of our area and the adjoining areas to
+the north and north-east were uplifted to form land, which in the case
+of our area was interpenetrated by watery tracts, whose exact nature
+is still a subject of dispute. Accordingly the deposits which were
+formed during this period are local and in some cases abnormal, but
+they will be considered in the next chapter. Simultaneously with the
+formation of these deposits, uplift of the sea-floor converted wider
+and wider areas into land, and this land underwent considerable
+denudation, so that the tops of the anticlines were worn away. The
+general trend of the anticlines was east-north-east and
+west-south-west, and accordingly a series of mountain chains possessed
+that direction, for the epeirogenic movements were accompanied by
+orogenic ones. Between the regions of uplifts were depressions in
+which sediments accumulated. The principal axes of uplift in our area
+range through the North of Scotland towards Scandinavia, across the
+Southern Scotch Uplands to the North of Ireland, through the Lake
+District and through Wales. As the result of lateral pressure, a
+cleavage structure was impressed on many of the Lower Palæozoic rocks,
+the strike of the rocks extended in the direction of the ridges and
+depressions, and the rocks as a whole became considerably compacted
+and hardened, thus producing one of the most important portions of the
+framework of our island, for although the ancient mountain chains were
+largely denuded during their elevation, and their stumps were
+afterwards covered by later deposits, upon the removal of these, the
+ancient stumps were once more exposed as fairly rigid masses which do
+not yield greatly to denuding influences, and accordingly stand out as
+the most important upland regions of Britain at the present day.
+
+It is interesting to notice, as an illustration of the now well
+established fact that successive earth movements often occur in the
+same direction, that the axes of the folds produced during this second
+continental (Devonian) period, run parallel with the lines separating
+tracts of different lithological characters. It has been seen that
+the Ordovician and Silurian rocks of the Southern Uplands continue
+into Ireland, and that the beds of similar characters run in belts
+having a general east-north-east and west-south-west trend, which
+accordingly must have been the direction of the coast-line parallel to
+which they were deposited, and as that coast-line was due to uplift,
+the movement which produced it would naturally produce foldings with
+east-north-east and west-south-west trend. This is one of many cases
+where the lines separating belts of rock having different lithological
+characters run parallel to axial lines of folds which have been
+produced in the rocks at a later period.
+
+As the result of the existence of land over parts of north-west Europe
+in Devonian times, it is comparatively rare to find a passage from
+normal Silurian rocks into normal Devonian ones; there is often an
+unconformity above the Silurian strata. As we proceed southwards
+towards central Europe, where the epeirogenic and orogenic movements
+died out, this is not the case, and we get complete conformity between
+marine sediments of the Silurian and Devonian periods.
+
+
+
+
+CHAPTER XVII.
+
+THE DEVONIAN SYSTEM.
+
+
+_Classification._ As a result of the movements which were briefly
+described in the last chapter, two types of Devonian deposit are found
+in the British Isles, and are called respectively the Devon type and
+the Old Red Sandstone type. The latter rocks, formerly divided into
+three divisions, are now separated into two only, the upper and lower
+Old Red Sandstone, and the exact relation of these to the different
+subdivisions of the rocks of Devon type remains to be settled. The
+Devon type itself has given rise to much difference of opinion, two
+local classifications have been applied, one for the rocks of North
+Devon and another for those of South Devon. The classification which
+has been most generally adopted is as follows:--
+
+                          N. Devon.               S. Devon[83].
+
+                   { Pilton Beds              { Entomis Slates
+  Upper Devonian   { Cucullæa (Marwood)       { Goniatite Limestones
+  (Clymenian)      { Beds                     { and Slates
+                   { Pickwell Down Sandstone  { Massive Limestones
+
+  Middle Devonian  { Morte Slates             { Middle Devonian
+     (Eifelian)    { Ilfracombe Beds          {  Limestones
+                                              { Ashprington Volcanic
+                                              {  Series
+                                              { Eifelian Slates and
+                                              {  Shaly Limestones
+
+                                              { Lower Devonian
+  Lower Devonian   { Hangman Grits            {  Slates
+    (Coblenzian)   { Lynton Slates            { Lincombe and Warberry
+                   { Foreland Grits           {  Grits and
+                                              {  Meadfoot Sands
+
+[Footnote 83: An account of the South Devon rocks by Mr Ussher will be
+found in the _Quart. Journ. Geol. Soc._, vol. XLVI. p. 487; from it
+the above classification of the rocks of S. Devon is taken.]
+
+The division into Lower Middle and Upper Devonian is generally
+adopted, though the alternative titles given to these divisions are
+not always used with the same signification, and the distribution of
+the different local stages given in the above classifications is
+usually adopted in the main, though a detailed comparison of the
+Devonian beds of North and South Devon is still attended with
+difficulty.
+
+More than once an attempt has been made to prove that the apparent
+succession of the North Devon rocks, which is that given in the above
+table, is not the true one, and of recent years Dr Hicks has obtained
+a number of fossils from the Morte Slates which had hitherto yielded
+none, and he believes that these fossils indicate that the Morte
+Slates are on a lower horizon than the beds on which they rest.
+Whatever be the ultimate verdict, we can, at any rate, say that the
+"Devonian Question," as it is termed, is not settled[84].
+
+[Footnote 84: See Hicks, H., "On the Morte Slates and Associated Beds
+in North Devon and West Somerset," _Quart. Journ. Geol. Soc._, vols.
+LII. p. 254, LIII. p. 438.]
+
+_Description of the Strata._ The general variations in the
+lithological characters of the deposits of Devonian age will be seen
+from the accompanying figure which represents the deposits of Britain
+as they occurred from north to south before they had been affected by
+subsequent earth-movements (Fig. 19). The conventional signs which are
+used are similar to those which have been used in other parts of this
+work, and will save description of the section.
+
+[Illustration: Fig. 19.
+
+  A.   Lower Palæozoic and Precambrian Rocks.
+  N.S. North of Scotland       }
+  C.V. Central valley of ditto } Old Red Sandstone Type.
+  W.   Wales                   }
+  N.D. North Devon } Devon Type.
+  S.D. South Devon }
+]
+
+The ridges separate different deposits of Devonian rocks, which were
+possibly deposited in isolated areas, though there was probably
+connexion between them at any rate at times.
+
+The Old Red Sandstone type consists to a large extent, as the name
+implies, of sandstones which are coloured red by a deposit of peroxide
+of iron around the sand grains. They are separable into a lower and
+upper division with an unconformity often occurring between them. The
+lower Old Red passes down in places into the Silurian rocks with
+perfect conformity, and the upper Old Red similarly passes up into the
+Carboniferous strata. The existence of pebble beds at different
+horizons is a noteworthy feature. They are frequently found at or near
+the base of the two divisions. The sandstones of the lower division
+are often accompanied by flagstones, while the red sandstones of the
+upper division usually have deposits of yellow and brown sandstone
+intercalated between them. Inconstant beds of limestone, known as
+cornstones, are found in both divisions, and Prof. Sollas has shown
+that some of these, at any rate, are true mechanical deposits, formed
+by the destruction of pre-existing strata of limestone and the
+deposition of the resulting fragments from a state of suspension. In
+Scotland a great thickness of volcanic material of various kinds is
+associated with the two divisions. For the sake of simplicity this is
+omitted from Fig. 19[85]. It is not known how far normal sediments are
+associated with the Old Red Sandstone type of deposit. The existence
+of some in South Wales is suggested by evidence supplied by the late
+Mr J. W. Salter.
+
+[Footnote 85: For an account of these and all other British volcanic
+rocks the reader is referred to Sir A. Geikie's work on _The Ancient
+Volcanoes of Great Britain_. Macmillan and Co., 1897.]
+
+The Devon type, as will be seen in the figure, consists of rocks which
+are to a great extent of normal character. We find in Devonshire
+alternations of sandstones, shales and limestones, but even here, red
+sandstones, which are comparable with those of the Old Red type occur
+in diminished amount: the Foreland Grits and Pickwell Down Sandstones
+are both coloured red, and are like the sandstones formed further
+north. The recognition of this fact induces one to believe that the
+contrast between the two types of rock which are found at a short
+distance from one another on opposite sides of the Bristol Channel is
+not so marked as one is sometimes led to suppose.
+
+The rocks of North Devon differ from those of South Devon chiefly
+owing to the amount of calcareous sediment found in the two areas, for
+limestones occur in South Devon to a great extent, and in North Devon
+there is a comparative poverty of this kind of sediment. Here, again,
+the apparent difference is possibly greater than the real one. The
+North Devon limestones have in places been stretched out after their
+formation and thus rendered thinner, and the highly-cleaved limestones
+are occasionally mistaken for shales, while in South Devon there is
+evidence of thickening of the limestones by folding subsequently to
+their deposition. Allowing for these changes, however, there is still
+a marked diminution in the amount of coarse mechanical sediments and
+increase in the quantity of calcareous matter as one passes from North
+to South Devon, and this prepares one for the condition of things met
+with on parts of the continent, where the mechanical sediments become
+finer and thinner on the whole as one travels southward, until, when
+we reach the Bohemian area, the Devonian rocks are found to be largely
+composed of calcareous sediments.
+
+It is interesting to find that in North America the two types of
+Devonian strata recur, and present characters generally similar to
+those which they possess upon this side of the Atlantic.
+
+Passing now to a consideration of the conditions under which the
+Devonian rocks were deposited, we may examine the bearing of the
+character of the strata as a whole, and then proceed to more detailed
+consideration of the nature and conditions of deposits of the two
+types.
+
+The gradual increase in calcareous matter and dying out of mechanical
+sediments as one travels southward points to recession from land in
+that direction, and we have already seen that the epeirogenic and
+orogenic movements of this continental period elevated the Silurian
+sea-floor in the north, and gave rise to a Northern Continent, while
+oceanic conditions continued further South, and allowed the
+accumulation of sediments lying conformably upon those of Silurian
+age, and giving indications of the prevalence of physical conditions
+during Devonian times which were in the main similar to those of the
+preceding Silurian period.
+
+In the shallow waters adjoining the land of the Northern Continent the
+Old Red Sandstones were laid down, and the exact conditions under
+which they were accumulated is a matter of some interest. The late Sir
+Andrew Ramsay gave reasons for supposing that many red deposits were
+accumulated in the waters of inland lakes, which underwent rapid
+evaporation, and his views have been applied, with much corroborative
+evidence by Sir A. Geikie, to account for the red sandstones of
+Devonian age, which he believes to have been accumulated in a series
+of inland lakes, though others hold a different opinion, and consider
+that the Old Red Sandstone waters had a direct connexion with those of
+the open ocean; the question is too intricate to be discussed at
+length here. Besides the difference of physical characters of the two
+types of strata, the difference in the nature of their included
+organisms is significant. The ordinary invertebrates, as corals,
+crinoids, brachiopods and molluscs are extremely rare in the Old Red
+Sandstone, which contains remarkable remains of Agnatha fishes and
+eurypterids, and although these are also found associated with a true
+marine fauna in Russia, Germany and Bohemia, the rarity or apparent
+absence of the ordinary marine invertebrates, though only negative
+evidence, which is proverbially dangerous, must be regarded.
+
+The North Devon rocks are sediments which might well be accumulated on
+the shores of a continent, while those of South Devon, with their
+abundant coral reefs, and other organic limestones were no doubt
+deposited in a clearer sea, at a greater distance from the land, and
+the clear water deposits of Germany and still more of Bohemia, were
+accumulated in the open ocean. It is interesting to note in these
+Bohemian deposits abundance of shells of a Pteropod _Styliola_ which
+has been proved by Prof. H. A. Nicholson to form masses of limestone
+in the Devonian system of Canada. The modern distribution of the
+Pteropoda suggests the open ocean character of the deposits which
+contain them even so far back as Devonian times, though one cannot
+conclude that these deposits are really analogous to the so-called
+Pteropod ooze of modern seas which, as a matter of fact, is largely
+composed of foraminiferal tests with a considerable percentage of
+pteropod shells.
+
+_The Devonian flora and faunas._ The plant remains in the Lower
+Palæozoic rocks are few in number. Some undoubted terrestrial plants
+have been discovered, but the prevalent flora of lower Palæozoic
+times, so far as yet known, was one consisting of Algæ. In Devonian
+times we begin to meet with a number of Cryptogams of higher type,
+allied to those which form the dominant flora of the succeeding
+period. The fauna is in many ways remarkable. The Devonian period has
+been termed the period of ganoid fishes, and the remarkable remains,
+so graphically described by the late Hugh Miller, are indeed
+peculiarly characteristic of Devonian times, but they are largely
+though by no means exclusively entombed in rocks of the Old Red
+Sandstone type[86]. The Devon type of rock contains a great abundance
+and variety of the problematical group, the Stromatoporoids, which
+contribute extensively to the formation of many of the limestones,
+and although these organisms are not by any means confined to Devonian
+strata, their abundance and variety therein might lead one to speak of
+the period as that of Stromatoporoids. The remains of corals are very
+abundant in the limestones, and, as already stated, frequently give
+rise to true reef-masses. The graptolites, as remarked in the previous
+chapter, disappear in the rocks of the Devonian period, and as only
+one or two fragments have been found, we may assert that the group was
+practically extinct at the end of Silurian times, though species of
+one genus, _Monograptus_, lingered for a short time in greatly
+diminished quantity. The trilobites which played so important a part
+amongst the faunas of Lower Palæozoic times still occur fairly
+abundantly amongst the rocks of the Devonian system, and there is a
+very interesting point to be noticed in connexion with them. They seem
+to have become practically extinct in the succeeding Carboniferous
+period, where few genera are found, and the decadence of the group
+began in Devonian times. In these circumstances it is interesting to
+note the tendency displayed by the creatures to possess spiny
+coverings. It is true that _Acidaspis_, the most spinose of all
+trilobites, is abundant in Ordovician and Silurian strata, and that
+other spinose trilobites are found there, but the peculiarity of the
+Devonian trilobites is, that genera which were previously smooth, or
+rarely possessing one or few spines, are found represented by
+extremely spinose species in these beds,--the spines being developed
+from all parts of the test, sometimes as a fringe to head or tail,
+sometimes as prominent projections from glabella and neck segment, and
+frequently in rows down the body segments. Besides _Acidaspis_, we
+find spinose species of _Phacops_, _Homalonotus_, _Cyphaspis_,
+_Bronteus_ and _Encrinurus_ in Devonian strata, and the occurrence of
+these forms is so frequent and world-wide, that one might perhaps
+infer with confidence that an unknown fauna containing many spiny
+trilobites was of Devonian age.
+
+[Footnote 86: For an account of these see A. S. Woodward's _Vertebrate
+Palæontology_.]
+
+The abundance of Eurypterids has been previously noted. Occurring as
+they do in Silurian rocks, they are far more abundant in those of
+Devonian age, and are found indifferently in sediments of Old Red and
+Devon types. Of air breathers, several insects have been found in the
+strata of different parts of the world.
+
+The ordinary marine faunas are otherwise intermediate in character
+between those of the Silurian and Carboniferous periods, but there are
+several characteristic Devonian genera, and no one who is acquainted
+with the peculiarity of the Devonian fauna would deny to the Devonian
+strata the right to rank as a separate system, containing a fauna as
+well marked in its way as that of the Silurian system below or that of
+the Carboniferous above. Special stress is laid upon this point
+because it has been suggested that the Devonian system should be
+abolished, and its strata either divided between the Silurian and
+Carboniferous systems or referred exclusively to the latter
+system[87].
+
+[Footnote 87: The literature of the fauna of the Devonian rocks is a
+rich one. For an account of the Devonian rocks of Britain, the reader
+may consult the Monograph of the Devonian Fossils of the South of
+England by Rev. G. F. Whidbourne, which is now appearing in the series
+of Monographs of the Palæontographical Society, and in the
+publications of the same Society he will find a Monograph of the
+Eurypterids from the pen of Dr Henry Woodward. The richest Devonian
+fauna is undoubtedly that of the Bohemian area, for the work of Dr E.
+Kayser has conclusively proved that the stages _F_, _G_ and _H_ of
+that basin, formerly referred to the Silurian, are of Devonian age,
+and an excellent idea of the richness of the Devonian fauna may be
+obtained by studying the descriptions of the fossils from those stages
+which have appeared and are appearing in Barrande's classic work.]
+
+
+
+
+CHAPTER XVIII.
+
+THE CARBONIFEROUS SYSTEM.
+
+
+_The Classification._ The British rocks of the Carboniferous system
+have been classified according to their lithological characters, but
+as the classification has been altered from time to time, we may use
+that which seems most acceptable to the majority of British geologists
+at the present day. According to this, the beds are grouped as
+below:--
+
+                      {                  { Ardwick Stage
+  Upper Carboniferous { Coal Measures    { Pennant Stage
+                      {                  { Gannister Stage
+                      { Millstone Grit
+  Lower Carboniferous { Carboniferous (Mountain) Limestone
+                      { Series.
+
+The Lower Carboniferous beds have been further subdivided into:--
+
+    Yoredale Series or Upper Limestone Shales,
+    Mountain Limestone,
+    Lower Limestone Shales, with Sandstones and Conglomerates,
+
+but as these lithological types are found to be very variable when
+traced laterally for comparatively short distances, it is found more
+satisfactory to use the terms in a purely lithological sense rather
+than with chronological significance.
+
+The somewhat abnormal development of the higher portions of the
+Carboniferous rocks of Britain renders the local classification only
+partially applicable in other regions, and as our knowledge
+progresses, a palæontological classification will probably be adopted.
+This has already been done with the more purely open-water sediments
+of Russia and Eastern Asia, where the development of the beds is more
+normal. There the rocks are classified as under:--
+
+    Upper Carboniferous or Gshellian,
+    Middle Carboniferous or Moscovian,
+    Lower Carboniferous,
+
+and as this classification has already been found to be applicable
+over rather wide areas, it is almost certain that, as in the case of
+the rocks of other systems, it will prove more serviceable than one
+which is mainly (though not quite exclusively) based upon vertical
+variation of lithological characters, especially as the Carboniferous
+rocks over large tracts in North America possess faunas which are
+similar to those which have been discovered in Russia, Eastern Asia
+and North Africa.
+
+_Description of the strata._ The variations in the lithological
+characters and fossil contents of the British Carboniferous strata
+when traced from north to south have been so frequently described, and
+utilised as a means of illustrating the indications as to local
+variations in physical conditions which are supplied by those strata,
+that little need be said upon the subject. The restoration of the
+physical geography of Carboniferous times over the British area will
+be found in a chapter by the late Professor Green in the work upon
+_Coal_ by various professors at the Yorkshire College of Science and
+also in Prof. Hull's _Physical History of the British Isles_. Some
+modifications must be made in these restorations as the result of
+recent research, the principal being caused by discoveries amongst the
+Carboniferous rocks of Devonshire.
+
+Taking the strata in vertical succession, we find evidence of the
+occurrence of a complete marine period (the second great marine
+period) between the second and third continental periods. The first
+shallow-water phase over a great portion of the British Isles is
+marked by thin terrigenous sediments, indicating that the period was a
+brief one; it was followed by the deep-water phase, probably of some
+length, lasting through the greater part of the remainder of Lower
+Carboniferous times; while the concluding shallow-water phase was
+lengthy as compared with that of the beginning of the period, and is
+marked by the accumulation of the great thickness of deposits
+belonging to the Millstone Grit and Coal Measures. There is no doubt,
+however, that in some parts of the British area minor changes produced
+local terrestrial conditions during the period, and accordingly we
+find that the deepest water deposits of the system in Britain are
+succeeded by an unconformable junction with the sediments of the upper
+portion of the system.
+
+The general change in the lithological characters of the beds of the
+Lower Carboniferous division when traced from south to north is shewn
+in the following diagram (Fig. 20).
+
+It will be seen that the land and open sea areas were in the
+respective positions which they occupied during Devonian times, but
+that as the result of greater submergence, with which the accumulation
+of sediment did not keep pace, the shallow-water marine deposits of
+Devonian age are in Devon replaced by open-sea deposits[88], while
+shallow-water marine deposits further north replace the anomalous
+deposits which were found there during the Devonian period.
+
+[Footnote 88: The Radiolarian Cherts of the Lower Carboniferous rocks
+of Devon, and the associated sediments, together with the unconformity
+between these and the Upper Carboniferous beds are described by Messrs
+Hinde and Fox, Quart. _Journ. Geol. Soc._, vol. LI. p. 609.]
+
+[Illustration: Fig. 20.
+
+    _a._ Radiolarian cherts of Devon.
+    _b._ Mountain limestone of Central England.
+    _c._ Mechanical sediments of Northern England.
+    _d._ Freshwater deposits of Southern Scotland.
+    O.R. Older rocks.
+]
+
+Owing to the accumulation of thick masses of sediment, the Lower
+Carboniferous sea of the north of England appears to have been largely
+silted up, and although the organic deposits of the south are so thin
+that they did not render the sea shallow in that region, the general
+level of the Lower Carboniferous floor of the south was also uplifted,
+and actually converted into land, as the result of the upward movement
+which took place in Devonshire and tracts of France; and owing to
+silting up in the north, and elevation in the south, a general plane
+surface was produced over very extensive areas, not only in Britain
+but upon the Continent, upon which the peculiar deposits and
+accumulations of Upper Carboniferous times were laid down, sometimes
+in shallow water, sometimes upon the land, and often under conditions
+which cannot at present be determined with accuracy. That the deposits
+of the Millstone Grit and Coal Measure epochs were to a large extent
+laid down in water is admitted by all, and in the case of many of the
+deposits of the Millstone Grit, and some thin deposits of the Coal
+Measures, it is equally clear that the water area was part of an
+expanse of ocean, for we find marine fossils, as corals, crinoids, and
+cephalopods, in these beds. Associated with them in the Coal Measures
+are other beds in which the ordinary Carboniferous genera of marine
+invertebrates are absent, and their place is taken by shells which
+bear much resemblance to the modern fresh-water mussel, and it has
+been maintained with good reason that as the ordinary marine forms are
+rarely or never mixed with those resembling recent fresh-water shells,
+the latter are truly fresh-water[89]. If this be so, many of the
+mechanically formed sediments of the Coal Measures were of fresh-water
+origin, laid down in shallow lagoon-like expanses, probably shut off
+from the main ocean by a narrow portion of intervening land, which was
+occasionally destroyed, thus permitting incursions of salt-water when
+some of the ordinary marine invertebrates of the period obtained a
+temporary footing in the area.
+
+[Footnote 89: For further information upon this subject the student
+should consult the Introduction to a Monograph on _Carbonicola_,
+_Anthracomya_ and _Naiadites_ (the shells in question) by Dr Wheelton
+Hind, being one of the Monographs of the Palæontographical Society.]
+
+There is not only a difference of opinion as to the mode of
+accumulation of many of the mechanical sediments of the Coal Measures,
+but also as to that of the coal-seams which accompanied them. Two
+different theories have been put forward to account for these
+coal-seams, which are usually spoken of as the drift theory and the
+growth-in-place theory. According to the former, in its extreme
+application, coal is an aqueous deposit formed by the settlement of
+drifted masses of vegetation upon the floor of a water-tract, while
+those who push the growth-in-place theory to its extreme limits
+maintain that coal is the result of growth of vegetation upon the
+actual site where the coal is now found. Much apparently conflicting
+evidence has been advanced by the advocates of the two hypotheses, and
+special cases of coal-formation have been appealed to by each in
+support of their views; thus the existence of coal composed largely of
+bodies which resemble the spores of modern lycopods,--objects of so
+resinous a nature that they float on the surface until they are
+decomposed,--is cited by the upholders of the growth-in-place theory,
+while the supporters of the other hypothesis can point with equal
+force to the occurrence of the finely divided carbonaceous mud
+containing remains of fishes which gives rise to cannel coal in some
+places. One of the main assertions in support of the growth-in-place
+theory was that of the supposed universality of 'underclays' or old
+surface soils beneath all coal-seams, but though these are common,
+they are far from universal. It is impossible to do justice in small
+compass to this question of coal-formation, but it may be pointed out
+that much of the difference of opinion can be understood if it be
+remembered that the term 'coal' is rather a popular term which has
+been admitted into scientific terminology, and therefore used somewhat
+loosely, than a strictly scientific term applied to a definite
+substance, and accordingly, just as at the present day we find
+carbonaceous substances growing in one place on land to form peat, in
+other places on a tract sometimes dry and sometimes submerged, to form
+the carbonaceous deposits of the cypress-swamps, and once more
+accumulated beneath the shallows of a sea as a sediment to form the
+carbonaceous muds of the ocean margins where the mangroves grow, so
+the diverse substances which are included under the general term coal
+may have accumulated in one place on land, in another beneath water,
+and in a third on an area alternately dry and submerged. This is not a
+question of great importance; the important point is that
+accumulations of vegetation on a fairly large scale are found at the
+present day on plains, for even if they grow on mountain regions, the
+deposits are readily denuded before they are covered up, and also it
+must be noted that a moist climate is necessary for the growth of much
+vegetation. The conclusion that the accumulations of coaly matter were
+formed on plains is borne out by their great horizontal extent as
+compared with their thickness, and it is now generally agreed that the
+coal vegetation which is found in the normal coal-measures was
+essentially a swamp vegetation.
+
+An attempt has been made to prove that an upland vegetation of very
+different character existed contemporaneously with it, but reasons
+will be given in the sequel for concluding that this supposed upland
+Carboniferous flora is everywhere of later date.
+
+The later shallow-water phase of Carboniferous times, as already
+stated, was unusually long, it was also very widespread, and appears
+to have been accompanied over wide areas by humid conditions during
+its continuance, and accordingly the marsh conditions which existed
+during Upper Carboniferous times were probably on a larger scale than
+that of similar conditions before or after. Special stress is laid
+upon this fact, as it is a good illustration of the view which seems
+to be gaining ground, that every period possessed peculiar conditions
+never to be repeated, which must have left their impress upon the
+character of the sediments.
+
+Though the conditions above described were widespread, they were
+naturally not universal, and accordingly in many parts of the world,
+as previously stated, we find true marine deposits of Upper
+Carboniferous times, though even these were sometimes replaced during
+part of the epoch, by conditions which were favourable for the
+formation of coal-seams in those places. Interruption in the
+continuance of a humid temperate climate over the regions of
+North-West Europe is also suggested by the discovery of deposits which
+are maintained to be of glacial origin amongst the Coal Measures of
+France[90].
+
+[Footnote 90: For an account of the numerous volcanic products see Sir
+A. Geikie's work on "The Ancient Volcanoes of Great Britain."]
+
+_The Floras and Faunas._ The flora of the Carboniferous rock is so
+noteworthy that the period has been termed the Period of Cryptogams;
+the remains of ferns, horsetails, and clubmosses predominate, and many
+of the forms reached a gigantic size. Though the floras of the various
+stages are marked by a general resemblance, there are differences
+which enable the palæobotanist to ascertain the stratigraphical
+position of the beds by reference to the included plant remains, and a
+considerable number of successive floras have been described[91]. The
+invertebrate fauna does not differ on the whole very greatly from that
+of Devonian times, though the trilobites are now becoming rare, and
+the mollusca assume a more prominent position as compared with the
+brachiopods. Corals occur in abundance in the calcareous deposits of
+the period, and frequently give rise to sheets of reef-formation, but
+the foraminifera and crinoidea certainly play the principal part as
+limestone-producers, and the influence of the latter in giving rise to
+great masses of limestone which are frequently used for ornamental
+purposes is too well known to need more than passing reference. The
+air-breathers have also been detected in greater abundance, though
+they are rare, when we consider the comparatively favourable
+conditions for their preservation presented by the Coal Measure rocks.
+Myriopods, arachnids, insects and pulmoniferous gastropods have
+however been found with tolerable frequency. The danger of arguing
+from imperfect data is well illustrated by the great addition to our
+knowledge of the insect-fauna of these times due to the exploration of
+the beds of one small coal-field, that of Commentry in France, of
+which the insects have been described by M. C. Brongniart. The
+vertebrates are represented by a considerable variety of fishes, and
+less abundant though tolerably numerous remains of Amphibia, which
+occur in the Carboniferous rocks of the North of England, Ireland,
+France, North America and elsewhere.
+
+[Footnote 91: Consult Kidston, R., "On the Various Divisions of the
+Carboniferous Rocks as determined by their Fossil Flora," _Proc. Roy.
+Phys. Soc. Edin._, vol. XII. p. 183.]
+
+The existence of definite zones of organisms in the case of the
+Carboniferous rocks has been denied, and it appears to be considered
+by some that the Carboniferous rocks were accumulated so rapidly as
+compared with rocks of some other systems that the fauna remained very
+similar throughout. It is very doubtful if this was so. In the case of
+other systems, the division into zones has only been accomplished by
+means of more detailed researches than those which have been conducted
+amongst the Carboniferous rocks of Britain: again, the occurrence of
+successive floras suggests that there may have been a similar
+succession amongst the faunas, and finally we find that zonal division
+has been carried on to some extent amongst the Carboniferous strata
+of other regions. The following classification of the Russian type of
+sediment may prove useful, as an indication of the possibility of more
+detailed separation of our own beds:--
+
+                        { Beds with _Spirifera fascigera_, _Spiriferina_
+       Gshellian        {   _Saranae_, &c.
+  (with _Fusulina_ and  { Beds with _Producta cora_, _P. uralica_,
+    _Archimedipora_)    {   _Camarophoria crumena_, &c.
+                        { Beds with _Syringopora parallela_ and
+                        {   _Spirifera striata_.
+
+     Moscovian          { Stage of _Spirifera mosquensis_.
+
+                        { Stage of _Spirifera Kleini_.
+                        { Coals, Sandstones and Shales with _Noeggerathia_
+  Lower Carboniferous   {   _tenuistriata_ and _Producta_
+                        {   _gigantea_.
+                        { Stage of _Producta mesoloba_.
+
+The marine fauna of the Upper Carboniferous beds, which is so poorly
+represented in Britain, but is well developed in Spain, Russia, Asia
+and North America, is largely characterised by the abundance of
+foraminifers of the genus _Fusulina_ and _Fusulinella_ and of bryozoa
+of the genus _Archimedipora_. It is very desirable that the truly
+marine fauna of the _Spirorbis_ limestone and other marine bands of
+the British Coal Measures should be carefully studied to see if they
+present any close relationship with that of the Gshellian beds[92].
+
+[Footnote 92: A good idea of the general characters of the
+Carboniferous fauna of Britain will be obtained from an examination of
+Professor Phillips' _Geology of Yorkshire_, Part I., and Mr (now Sir
+F.) M^{c}Coy's _Carboniferous Fossils of Ireland_, while the nature of
+the European fauna is well illustrated in Prof. de Koninck's
+well-known work _Description des animaux fossiles qui se trouvent dans
+le terrain carbonifère de Belgique_. For an account of the characters
+of the marine fauna of the Upper Carboniferous rocks the reader should
+consult the work on Geology and Palæontology published by the
+Geological Survey of the State of Illinois in 1866.]
+
+
+
+
+CHAPTER XIX.
+
+THE CHANGES WHICH OCCURRED DURING THE THIRD CONTINENTAL PERIOD IN
+BRITAIN; AND THE FOREIGN PERMO-CARBONIFEROUS ROCKS.
+
+
+At the close of Carboniferous times a marked change took place in the
+nature of the earth-movements. The prevalent depression which occurred
+over the British and adjoining regions during Carboniferous times was
+replaced by upward movement, accompanied by orogenic folds, which once
+more brought on continental conditions and developed a series of
+mountain ranges. The change is marked even at the close of
+Carboniferous times by the abnormal red sandstones of the uppermost
+part of the Carboniferous system which are found around Whitehaven in
+Cumberland and Rotherham in Yorkshire, as the Whitehaven Sandstone and
+Rotherham Red Rock. These movements continued through Permian and
+Triassic times, and it is to them and to the climatic conditions of
+the periods, that the anomalous nature of the Permo-Triassic deposits
+is largely due, as will be shewn in the succeeding chapters. At
+present it is our purpose to call attention to the effect of these
+movements upon the sediments which had been deposited previously to
+their occurrence.
+
+Over the British area, two different systems of orogenic movement can
+be detected, producing folds of which the axes run approximately at
+right angles to one another. One of these, of which the Pennine system
+is the best representative in Britain, caused the production of
+elevations having axes in a general north and south direction, and we
+may therefore speak of it as the Pennine system of movement, while the
+other, which gave rise to folds running in an east and west direction,
+is well represented in the Mendip Hills, and may be therefore termed
+the Mendip system, though it is more widely known as the Hercynian
+system, as, on the Continent, the rocks which are greatly affected by
+it form the foundations of the region occupied by the ancient
+Hercynian forest.
+
+The effects of these systems were in the main similar; they resulted
+in the uplift of parallel belts of country to form hill-ranges with
+intervening lowlands, but when studied in detail the movements are
+seen to be of a different character. The Pennine system of movements
+was of a type which is familiar to the geologists as developed in the
+Great Basin Region of the western territories of North America, and
+produced what is spoken of as Basin-Range structure. The movements
+were of the nature of direct uplift, causing fracture, only
+accompanied by folding in a minor degree, and accordingly the hills
+are composed of terraced scarps, with one gently sloping side, and one
+steep scarp-side, the latter on the upthrow side of the fault, as seen
+in fig. 21.
+
+In the Mendip system, the folds were of the Alpine type, which is a
+familiar product of lateral pressure, consisting essentially of
+overfolds, though these are often complicated by reversed faults.
+
+Of the Pennine system, the Pennine Chain itself furnishes the most
+noteworthy example in Britain, but we have indications of other folds
+of this system, such as that which runs from the Lake District to the
+Ayrshire coast, which is partly concealed as the result of other
+movements, and a still more marked one, in the rocks of the Malvern
+Hills.
+
+[Illustration: Fig. 21.
+
+_a a´_. One stratum displaced by faults _f f_. _h._ Hills.]
+
+The Mendip system is well shewn in the Mendip Hills, but the remains
+of a still more important anticline are seen in South Devon and
+Cornwall, separated from the Mendip Hills by the great syncline of
+Devon. Another parallel anticline runs from Lancashire to Yorkshire at
+right angles to the Pennine Chain and separates the coal-field of
+Cumberland and that of Northumberland and Durham, from those of South
+Lancashire, and Yorkshire, Notts, and Derbyshire.
+
+On the European continent the Ural Chain is the most important uplift
+of the system of which the Pennine Chain forms a minor representative,
+while the Hercynian system has caused the compression and stiffening
+of many of the Carboniferous and earlier rocks which now rise to the
+surface in many parts of central Europe.
+
+The extensive continental area which was the result of these uplifts
+not only determined the formation of abnormal deposits, but allowed
+the occurrence of a long period of time subsequently to the close of
+the Carboniferous period, of which few deposits now exposed in Europe
+are representative, and we must accordingly seek other regions in
+order to find typical representatives of this _Permo-Carboniferous_
+period, of which the strata developed in the Salt Range of India have
+been most carefully worked, especially by Dr Waagen, though marine
+sediments of the period are known elsewhere, as in Spitsbergen, the
+Ural Mountains, China and Australasia; and a group of somewhat
+anomalous sediments of this age in parts of India, Australia and South
+America is of peculiar interest, on account of the insight as to the
+climatic conditions of the times which it affords.
+
+_The Permo-Carboniferous Rocks._ In the Salt Range of the North-West
+of India an interesting series of sandstones alternating with
+limestones rests unconformably upon lower rocks. The sandstones are
+known as the Speckled Sandstones, while the limestones are termed the
+_Productus_ Limestones. The Lower and Middle Speckled Sandstones are
+succeeded by the Lower _Productus_ Limestone which is separated from
+the Lower division of the Middle _Productus_ Limestone by the Upper
+Speckled Sandstone; these are all of the Permo-Carboniferous period,
+while the upper part of the Middle _Productus_ Limestone and the Upper
+_Productus_ Limestone belongs to the Permian period. The fossils,
+largely invertebrates, are intermediate in character between those of
+Carboniferous and Permian ages. Similar fossils are found in the
+marine Permo-Carboniferous beds of the other areas which have been
+named above. The Lower Speckled Sandstone is of interest on account of
+the occurrence of boulder-beds within it, and this division of the
+sandstone has been correlated with the lowest (Talchir) stage of the
+Permo-Carboniferous strata of other parts of India, while the other
+Speckled Sandstones and those divisions of _Productus_ Limestone which
+are referred to the Permo-Carboniferous are correlated with the higher
+divisions of other parts.
+
+Special mention is made of the Talchir division, on account of the
+occurrence therein of boulder beds which have long been known, and
+whose glacial origin was inferred by Dr W. T. Blanford forty years
+ago. The accumulations shew signs of having been deposited in water,
+but the existence of large subangular, sometimes striated boulders
+therein, which must have come from distant sources, and the occasional
+occurrence of striated rock surfaces on the strata upon which the
+Talchir beds repose unconformably points to ice-action; this would not
+be so very remarkable if it were an isolated case, though sufficiently
+so, from the comparative nearness of the region to the equator; but
+researches conducted in different parts of the southern hemisphere
+have brought to light similar, and sometimes even more striking
+evidences of glacial action in widely distinct regions[93]. In
+Australia they have been found in New South Wales, Victoria,
+South Australia, East Australia and Tasmania; the Dwyka
+boulder-conglomerates of South Africa and certain deposits of similar
+character discovered by Prof. Derby in Southern Brazil have been
+referred to the same period, and their glacial origin has also been
+inferred. This widespread distribution of deposits which are generally
+contemporaneous, of which the glacial origin may now be taken as
+established, is extremely remarkable, and must be taken into careful
+consideration by those who put forward theories framed to account for
+former climatic changes.
+
+[Footnote 93: The reader will find an excellent account of the
+Permo-Carboniferous glacial deposits in a paper by Prof. Edgworth
+David, entitled "Evidences of Glacial Action in Australia in
+Permo-Carboniferous Time" (_Quart. Journ. Geol. Soc._ Vol. LII. p.
+289). In this paper other glacial beds besides those of Australia are
+noticed.]
+
+_The Flora and Fauna._ The flora of the Permo-Carboniferous beds has
+caused as much discussion as the question concerning the origin of the
+boulder-deposits. In the southern hemisphere, the Permo-Carboniferous
+rocks of those countries which have yielded boulder-beds also contain
+remains of a flora which is now known as the _Glossopteris_ flora,
+from the prevailing genus, which is associated with other genera, such
+as _Gangamopteris_. These fossils appear to be ferns, though their
+modern allies have not been indicated with certainty; associated with
+them are rare cycads and conifers. The _Glossopteris_ flora is
+markedly contrasted with the Coal-Measure flora of the northern
+hemisphere with its giant lycopods. Moreover _Glossopteris_ appears in
+the northern hemisphere in rocks of later date than the
+Permo-Carboniferous period. It has been suggested that the
+_Glossopteris_ flora originated in a continent in the southern
+hemisphere, on which the boulder beds were also formed in isolated
+water areas, and that some of the forms migrated northwards. To this
+continent the name Gondwanaland has been applied by Prof. Suess, from
+the _Gondwana_ series of the Permo-Carboniferous rocks of India, in
+which the _Glossopteris_ flora is found, and it has also been
+maintained that the southern _Glossopteris_ flora was contemporaneous
+with the northern flora of ordinary Coal-measure type, though whether
+this was so to any extent remains to be proved, for the beds
+containing the _Glossopteris_ flora are distinctly newer than any
+which have furnished a typical northern Coal-measure flora. In
+any case, the change of floras between Coal Measure and
+Permo-Carboniferous times is very marked, and when taken in connexion
+with the widespread glacial deposits, is one of the most striking
+phenomena displayed by the rocks of the stratified column[94].
+
+[Footnote 94: For an account of the Glossopteris flora and its
+geological relations, consult Seward, A. C., _Science Progress_,
+January, 1897, p. 178.]
+
+The fauna has already been noticed. It consists of brachiopods, some
+of which are of peculiar genera. The general similarity of the faunas
+in regions so remote as Spitsbergen, the Ural Mountains, India, and
+New South Wales, indicates an extensive sea during the period. It can
+hardly be supposed that the fauna of Permo-Carboniferous times has
+been completely described, for the fossils of one or two areas only
+have been made known to us with any degree of fulness, and when the
+Permo-Carboniferous and marine Permian faunas are as well known as
+those of Triassic times (and the latter have only been fully described
+very recently) there is no doubt that the important break which was at
+one time supposed to exist between Palæozoic and Mesozoic faunas will
+be filled in satisfactorily[95].
+
+[Footnote 95: The Permo-Carboniferous beds are described in Messrs
+Medlicott and Blanford's _Geology of India_, second edition (edited by
+Mr R. D. Oldham), and figures of some of the important fossils given
+therein. For fuller information the reader should refer to Waagen's
+account of the Salt Range Fossils and Feistmantel's description of the
+plants in the _Memoirs of the Geological Survey of India_.]
+
+
+
+
+CHAPTER XX.
+
+THE PERMIAN SYSTEM.
+
+
+_Classification._ It has already been observed that as the result of
+the Pennine and Mendip systems of earth-movement, the Carboniferous
+rocks of Britain are succeeded by a marked unconformity, and that the
+rocks of the succeeding Permian and Triassic systems of Britain shew
+an abnormal development. The principal areas where Permian rocks are
+found are on either side of the Pennine Chain in the North of England,
+but sporadic exposures of rocks of this age are found in some of the
+Midland and Southern counties. The Permian rocks have been well
+studied in Germany, and the German names are sometimes adopted in
+Britain, and the following comparison will prove useful:--
+
+           Britain.               Germany.
+    Magnesian Limestone       Magnesian Limestone } Zechstein.
+    Marl Slate                Kupferschiefer      }
+    Lower Permian Sandstones  Rothliegende.
+
+The term Zechstein has been applied in a somewhat different sense by
+different writers, but the one given in the table appears to find most
+favour.
+
+In a region which was essentially continental, considerable variations
+in the lithological characters of the rocks may be expected, when the
+strata are traced laterally, but we nevertheless find that the
+differences are not so great as was formerly supposed to be the case
+when certain red sandstones lying above recognised Permian strata in
+the district on the west side of the Pennine Chain towards its
+northern extremity were also referred to the Permian; these sandstones
+(the St Bees Sandstones) are now generally admitted to be of Triassic
+age, and comparison between the rocks on opposite sides of the Pennine
+Chain is much simplified, as seen below.
+
+        West side.                           East side.
+  Thin Magnesian Limestones and Marls    Magnesian Limestone
+  Hilton Shales                          Marl Slate
+  Penrith Sandstone and Brockrams        Lower Permian Sandstones.
+
+_Description of the Strata._ On the east side of the Pennine Chain,
+the Lower Permian sandstone is an inconstant deposit often consisting
+of yellow false-bedded arenaceous strata. The Marl Slate is an
+argillaceous shale, often containing bituminous matter, and yielding
+several fish-remains and some plants; it is usually only a few feet in
+thickness. The Magnesian Limestone is typically developed in Durham as
+a yellow or greyish limestone containing a variable percentage of
+carbonate of magnesia; when traced southward, it alters its
+characters, becoming mixed with mechanical deposits, and some chemical
+precipitates in places, so that at Mansfield it appears as a red
+sandstone with grains cemented by a mixture of carbonates of lime and
+magnesia; and, like the rest of the Permian strata, it has disappeared
+when we reach Nottingham. In addition to the southward thinning of the
+Permian beds of this area, there is some evidence of their
+disappearance in a westerly direction, though, as the present strike
+of the beds is nearly north and south, the indications of this are
+less convincing.
+
+On the east side of the Pennine Chain, the main difference observable
+is the relative thickness of the major divisions. The Lower Permian
+sandstones have thickened out considerably, while the reputed
+representatives of the Magnesian Limestone are thin. The Penrith
+sandstone is of considerable interest. It contains in places, as near
+Appleby, thick deposits of breccia consisting of angular fragments
+chiefly composed of Carboniferous Limestone, which in many cases have
+undergone subsequent dolomitisation, embedded in a matrix of red
+sandstone. This breccia is known as brockram. Many beds of the Penrith
+sandstone are composed of crystalline grains of sand, due to
+deposition of silica in crystalline continuity with the quartz of the
+original grain after the formation of the deposit; of more
+significance, for our present purpose, is the presence of other
+accumulations of the sand, in which the individual grains often
+approach the form of spheres, thus resembling the 'millet-seed' sands
+of modern desert regions. The Hilton shales are grey sandy shales,
+with plant remains, and above them are variable deposits including
+thin magnesian limestones which have yielded no fossils.
+
+The isolated Permian deposits of the midland and southern counties of
+England consist of red marls and sandstones with occasional breccias,
+and in the absence of fossils, their exact position in the Permian
+series is still unknown.
+
+The German Permian rocks resemble those of Britain, especially as seen
+in Durham, in many particulars, and give indications of formation
+under physical and climatic conditions generally similar to those
+which were then prevalent in the British area. At Stassfurt, in
+Germany, the less soluble constituents of ocean water are accompanied
+by a great variety of salts:--chlorides, sulphates and borates; and
+the very soluble salts of potassium and magnesium known as the Abraum
+salts are found in abundance as well as the less soluble salts of
+sodium and calcium. The occurrence of these very soluble salts is so
+infrequent on a large scale among the rocks of the Geological Column,
+and the matter is one of so great theoretical import, that it is
+necessary to take special note of their presence in the Permian
+strata.
+
+The frequent existence of chemical deposits in the Permian Rocks of
+N.W. Europe, the formation of red sandstones, and the dolomitisation
+of limestone beds and fragments of pre-existing limestones point to
+inland seas of a Caspian character, while the evaporation necessary
+for the formation of the precipitates also indicates a fairly warm
+temperature. The presence of millet-seed sands, in very lenticular
+patches, suggesting former sand-dunes, and the occurrence in places of
+breccias (like some parts of the brockram) almost devoid of matrix,
+piled up against pre-existing cliffs, recalling screes of modern
+times, give almost certain evidence of the occurrence of land tracts
+most probably of desert character, during part of the period of
+accumulation of the materials of the Permian rocks. The fossil
+evidence supports this view, and geologists are mostly agreed that the
+Permian rocks of north-west Europe were accumulated in an area of
+desert character, occupied in part by inland seas, though there is
+much difference of opinion as to the extent of these seas, some
+geologists holding that a number of isolated sheets of water were
+necessary to produce the distribution and character of the
+accumulations. It is still a vexed question with British geologists
+how far the Pennine ridge stood up as land during the period, but
+leaving this and other minor considerations out of account, it may be
+noted that the similarity of deposits in the different areas, whether
+we examine the order of succession, the lithological characters or the
+included fossils, suggests communication between the water tracts of
+different regions, though this communication need not have been more
+than a series of straits, or comparatively narrow belts of water[96].
+
+[Footnote 96: It should be mentioned that some writers have inferred
+the evidence of glacial conditions over parts of the British area, on
+account of the resemblance of some of the Permian breccias to recent
+glacial deposits. The question is still _sub judice_. It is not
+necessarily opposed to the existence of desert conditions, if the
+mountains were sufficiently high, for the Wahsatch regions adjoining
+the Basin Region of N. America have been glaciated.]
+
+The extensive development of Permian and Triassic rocks with
+terrestrial characters in the southern hemisphere also, and the
+absence of newer deposits in many places, suggests that the land areas
+of these times in that hemisphere have largely remained such ever
+since, in which case, the Permo-Triassic series of movements produced
+a marked direct effect upon our present continental areas, and at any
+rate produced an indirect one upon the British land tracts.
+
+The presence of anomalous deposits of Permian age over wide areas need
+not be surprising, but it would be indeed remarkable if no ordinary
+marine type of Permian rocks was known, and the researches of recent
+years have proved that this type is extensively developed, in Eastern
+Europe, Asia, and North America, where Permian rocks consisting of
+limestones, with a greater or less admixture of mechanical deposits,
+occur in some abundance. The studies of Waagen and others in India
+have given us the farthest insight into the nature of these beds.
+Below is a general classification taken from Waagen's work:--
+
+       Salt Range.                       Germany.
+
+  Base of Trias                 }
+  Unfossiliferous Shale and     }
+    Sandstone                   } Passage Beds into Trias
+  Top Beds of Upper _Productus_ }
+    Limestone                   }
+
+  Cephalopoda Beds of Upper     } Gypsum Beds
+    _Productus_ Limestone       }
+
+  Middle Division of Upper      }
+    _Productus_ Limestone       } Zechstein (in restricted sense)
+  Lower Division of Upper       }
+    _Productus_ Limestone       }
+
+  Upper Division of Middle      } Weissliegende and Kupferschiefer
+    _Productus_ Limestone       }
+
+  Middle Division of Middle     } Rothliegende.
+    _Productus_ Limestone       }
+
+It will be seen that in the Salt Range there is a complete passage
+from the Permo-Carboniferous strata through the Permian into the
+Trias, and the detailed work which has been carried out by Waagen and
+others amongst the rocks of the Salt Range must make this, for the
+present at all events, the type area for the marine development of the
+strata of Permo-Carboniferous and Permian ages.
+
+_The Permian flora and fauna._ The Permian flora presents some
+difficulties. The flora of the Zechstein consists largely of ferns and
+conifers, but that of the Rothliegende of Germany has been compared
+with that of the Carboniferous, and if a true Permian flora of the
+northern hemisphere has many forms of Carboniferous affinities, the
+presence of the Glossopteris flora in Permo-Carboniferous rocks of
+more southerly regions seems to imply its origin there and _slow_
+migration northwards. It must be noted, however, that the Rothliegende
+has been divided by some geologists into an upper and lower division,
+of which the lower is actually referred to the Carboniferous system.
+All that can be now said is, that our knowledge of the floras of
+Permo-Carboniferous and Permian times is still incomplete, and that
+the difficulties will no doubt be cleared up as the result of further
+work.
+
+The invertebrate fauna of the north-west European Permian deposits is
+chiefly noticeable on account of the paucity of species, though
+individuals are often abundant. The shells are also sometimes stunted
+and occasionally distorted. These characters bear out the supposition
+that the aqueous deposits were laid down in inland seas of Caspian
+character and not in the open ocean. Polyzoa, brachiopods, and
+lamellibranchs predominate, but other groups are found. The
+vertebrates consist of forms of fish, amphibia and reptiles, and the
+Permian rocks are the earliest strata in which the remains of true
+Reptilia are known to occur with certainty. The Reptiles belong to the
+orders Anomodontia (Theromora) and Rhynchocephalia, of which the
+former is exclusively Permian and Triassic, while the latter is
+abundant in the strata of those periods, but is represented at the
+present day by the genus _Sphenodon_ of New Zealand. The Amphibia
+belong to the order Labyrinthodontia which ranges from Carboniferous
+to Lower Jurassic, but the members of the order are most abundant in
+Permian and Triassic strata, and these periods may be spoken of as the
+Periods of Labyrinthodonts.
+
+A few words must be said of the fauna of the truly marine Permian
+beds. It is much richer than that of the abnormal deposits of
+north-western Europe, and its study is important as furnishing another
+link between Palæozoic and Mesozoic life. Many Palæozoic genera pass
+up into the Permian rocks, and, as will be ultimately seen, several
+occur in those of the Triassic system, and one or two even in the
+basal Jurassic strata, though Mesozoic forms predominate in the Lower
+Jurassic Rocks, and there is a fairly equal admixture of forms usually
+considered as Palæozoic and of those generally regarded as Mesozoic in
+Triassic rocks, while the Palæozoic forms still predominate over the
+Mesozoic in the Permian strata. Along with these characteristic
+Palæozoic genera, it is interesting to find representatives of more
+than one genus of the tribe of Ammonites, which is to take so
+prominent a place in the fauna of the Mesozoic rocks, amongst the true
+marine Permian sediments of India and other areas. The announcement of
+the contemporaneity of ammonites with fossils regarded as exclusively
+palæozoic was received with considerable doubt, but this
+contemporaneity is now clearly established, and need not be regarded
+as in any way anomalous.
+
+With the deposition of the Permian rocks, Palæozoic time comes to an
+end, but as already remarked there is no marked and sudden change to
+characterise it. Had our classification been originally founded on
+study of the Indian Rocks instead of those of Britain, and similar
+terms adopted, the line of demarcation between Palæozoic and Mesozoic
+rocks would probably have been drawn below the Permo-Carboniferous
+deposits, and if it had been based on study of other areas, perhaps
+elsewhere. The palæontological break is purely local, and it is of the
+utmost importance that it should be recognised as such, and that it
+should not be considered that division into Palæozoic and Mesozoic
+implies some great and widespread change which occurred between the
+times covered by the deposits of each of these great divisions[97].
+
+[Footnote 97: The Permian Fossils of Britain are described by
+Professor King in the Monographs of the Palæontographical Society (the
+Brachiopods by Dr Davidson in the Monographs of the same Society). For
+a general account of the marine type the student may consult the
+second edition of Messrs Medlicott and Blanford's _Geology of India_.
+For information concerning the Permian volcanic rocks see Sir A.
+Geikie's _Ancient Volcanoes of Great Britain_.]
+
+
+
+
+CHAPTER XXI.
+
+THE TRIASSIC SYSTEM.
+
+
+_Classification._ The term Triassic has been applied to these rocks on
+account of the threefold division into which those of Germany
+naturally fall. These three divisions are:--
+
+    Keuper,
+    Muschelkalk,
+    Bunter;
+
+but above the Keuper beds we find a group of deposits of some
+importance, which shew affinities with both Triassic and Jurassic
+rocks, which may be looked upon as true passage beds, though they are
+generally placed in the Triassic System. They are known as Rhætic or
+locally in Britain as Penarth Beds. The Muschelkalk is usually
+considered to be unrepresented in Britain, and accordingly the British
+deposits may be, and are usually grouped as under:--
+
+  Rhætic or Penarth beds
+  Keuper                  { Keuper Marls
+                          { Keuper Sandstones
+  [Muschelkalk] absent
+                          { Upper Red and Mottled Sandstones
+  Bunter                  { Bunter Pebble Beds
+                          { Lower Red and Mottled Sandstones.
+
+The threefold grouping has been applied more or less universally, but
+when used outside the north-west European area, it loses its
+significance, as the conditions which enable one to differentiate the
+rocks of the three divisions were naturally only prevalent over a
+limited area.
+
+_Description of the strata._ The British Triassic rocks possess a
+certain sameness as regards their general characters, consisting
+mainly of mechanical sediments coloured red by peroxide of iron, with
+occasional chemical precipitates of rock-salt and gypsum. They have a
+wider distribution over Britain than have the Permian rocks, and the
+lithological characters of the different subdivisions do not as a rule
+vary to a remarkable degree when traced laterally. The differences in
+detail in the characters of the various deposits are noteworthy, and
+an explanation of the exact origin of some of these abnormal deposits
+which will satisfy everyone is not yet forthcoming. Leaving the
+details out of consideration for the moment, and looking at the
+general aspect of the deposits, the prevalence of conditions generally
+similar to those which existed over the British Isles in the preceding
+Permian period is decidedly indicated by the nature of the strata,
+though the continental conditions appear to have been more widely
+established over our area, as shewn by the general absence of any
+calcareous deposits resembling the Magnesian Limestone. We find
+chemical precipitates, millet-seed sandstones, and scree-like breccias
+in the British Triassic rocks as well as in those of Permian age, and
+the paucity of a marine invertebrate fauna in the Triassic rocks of
+Britain is even more apparent than in the Permian strata. It is only
+at the extreme close of the Triassic period, during the deposition of
+the rocks which are admitted on all hands to be of Rhætic age, that
+we note the incoming of those marine conditions over our area, which
+prevailed so extensively, with few local exceptions, during the
+remainder of the Mesozoic and the early part of Tertiary times; the
+Rhætic beds, in fact, mark the commencement of the third marine
+period. Referring to the strata in further detail, we may proceed to
+consider the character of the different subdivisions in the order of
+their formation, commencing as usual with the oldest. The Bunter
+deposits rest in places upon those of Permian age with an unconformity
+at the junction, but as these unconformities occur frequently among
+the British Triassic rocks, it is doubtful whether this unconformity
+marks more than very local change of physical conditions. The lower
+and upper divisions of the Bunter sandstone consist of false-bedded
+red and variegated sandstones, and there is no great difficulty in
+explaining their formation in desert areas with tracts of water, but
+the great change which marks the appearance and disappearance of the
+middle division, the Bunter pebble beds, requires some explanation,
+for the contrast between the lithological characters of the rocks of
+this division and those of the rocks appertaining to the preceding and
+succeeding division is very marked. The matrix differs, but the main
+difference is the abundance of pebbles, mostly of fairly uniform size,
+well rounded, and largely consisting of liver-coloured quartzite. Much
+difference of opinion exists as to the exact origin of these pebble
+beds, and the source of the pebbles, but without entering into this
+vexed question, it may be remarked that the agency of rivers has been
+somewhat generally invoked to account for their transport, and the
+conditions during their accumulation need not have been very different
+from those which are now found in northern India where the torrential
+rivers of the south side of the Himalayan chains debouch upon the
+plain, and spread an abundant deposit of well-worn pebbles over the
+finer silts which were previously laid down thereon.
+
+The junction of the Bunter and Keuper beds requires a short notice. It
+is usually if not always an unconformable one in Britain, and it is
+generally assumed that the absence of the Muschelkalk of the Continent
+is due to the presence of land undergoing denudation in Britain during
+the time when the Muschelkalk was elsewhere deposited, though it is
+quite possible that the Muschelkalk epoch is represented in Britain
+not only by the time which elapsed when the unconformity was being
+impressed on the rocks, but also during the true deposition of the
+upper part of the Bunter beds, or the lower part of the Keuper, or
+both.
+
+The Keuper sandstones and marls contain a great development of
+chemical deposits, of millet-seed sands, and of many other features
+pointing to desert conditions, such as sun-cracks, tracks of animals
+impressed upon a rapidly drying surface, and pseudomorphs of mud after
+rock salt in the form of cubes and hopper-crystals; furthermore we
+find the scree-like breccias at different horizons of the Keuper beds
+where they abut against the old Mendip ridge composed largely of
+mountain-limestone which furnished the fragments, as was the case with
+the brockrams abutting against the Pennine ridge. It must be noted
+that the chemical precipitates of Triassic age consist of the less
+soluble substances dissolved in ocean water, namely, gypsum and rock
+salt, whilst the more deliquescent potash and magnesia salts are not
+represented in Britain.
+
+Turning to these continental beds, we get evidence of a general
+approach to open sea conditions as we pass away from Britain in a
+south-easterly direction as roughly shewn in the following diagram
+(fig. 22), where _B_ represents the Bunter beds, _M_ the Muschelkalk,
+and _K_ the Keuper.
+
+[Illustration: Fig. 22.]
+
+It will be seen that the mechanical sediments gradually die out and
+become replaced by calcareous material as one passes from Britain
+towards Switzerland; the Muschelkalk is very thin in the east of
+France and thickens out in Germany, while in Switzerland Keuper,
+Muschelkalk and Bunter are alike largely represented by calcareous
+deposits, and the mechanical deposits are chiefly argillaceous, the
+only important sandstone being situated at the extreme base of the
+Bunter series.
+
+The marine development of the Triassic system is naturally the one
+which is most widely spread, though full appreciation of its
+importance has only taken place as the result of researches in distant
+climes of recent years. It is found in southern Europe, in
+Spitsbergen, in considerable tracts of Asia, including India, and
+along the Pacific coast region of North America, and everywhere
+possesses much the same characters.
+
+It will be seen from the above remarks that the physical conditions
+which prevailed in the continental area of Triassic times which is now
+partly occupied by the British Isles are most closely represented by
+those of the desert regions of central Asia, hemmed in by the
+mountain ranges which intercept the vapour-laden winds of the oceans,
+and cause them to precipitate the great bulk of their vapour on the
+seaward slopes of the mountains, so that they blow over the deserts as
+dry winds, causing the fall of any large amount of rain to be a rare
+though by no means unknown event in the desert regions.
+
+_Flora and Fauna of the Period._ The Triassic flora is essentially
+similar to that of the higher Permian strata, though many of the
+genera are different.
+
+The invertebrate fauna of the British deposits is, as might be
+expected, very poor until the beds of the Rhætic series are reached.
+In the beds below the Rhætics, the principal invertebrate remains are
+the tests of the crustacean genus _Estheria_, though a few obscure
+lamellibranch shells have been recorded. The vertebrate fauna is of
+great interest. A number of fishes have been found, the most
+remarkable of which is the genus _Ceratodus_, occurring in the Rhætic
+beds of Britain and lower Triassic strata of foreign countries. It is
+closely related to the Barramunda of the Queensland rivers belonging
+to the order Dipnoi. As in the Permian strata, abundance of
+Labyrinthodont amphibians have been discovered, and the reptiles
+belong to the orders Anomodontia and Rhynchocephalia. In the Rhætic
+beds of Britain and in still lower Triassic beds abroad the orders
+Ichthyopterygia and Sauropterygia (represented by _Ichthyosaurus_ and
+_Plesiosaurus_) are found.
+
+The Triassic rocks also yield the earliest known mammals, the best
+known, _Microlestes_, occurring in the Triassic rocks of Britain and
+the Continent. These mammals are now placed in a subclass Metatheria
+of the order Monotremata.
+
+The marine invertebrate fauna of the normal Triassic rocks presents
+some points of considerable interest. As already remarked, the fauna
+may be looked upon as a passage fauna between that of Palæozoic and
+that of Mesozoic times, the number of Palæozoic forms which pass into
+the Trias being approximately comparable with those which appear here
+and range upwards into higher Mesozoic strata. This may be well seen
+by examining the table given in Chapter XXI. of the Second Edition of
+Sir Charles Lyell's _Student's Elements of Geology_, in which three
+columns shew the genera of Mollusca common to older rocks, those
+characteristic of the Trias, and those common to newer rocks. Amongst
+the first are _Orthoceras_, _Bactrites_, _Loxonema_, _Murchisonia_,
+and _Euomphalus_, in the second column are _Ceratites_, _Halobia_
+(_Daonella_), _Koninckina_, and _Myophoria_, and in the third,
+Ammonites, _Cerithium_, _Opis_, _Plicatula_ and _Thecidium_[98].
+
+[Footnote 98: It has been seen that some of the Ammonites appear
+earlier, namely, in Permian strata. _Myophoria_ is extremely abundant
+in the Trias, but ranges into newer strata.]
+
+The Ammonites are largely utilised in the case of the Mesozoic strata
+for separation of these strata into zones, each zone being
+characterised by some species of Ammonite, and the researches of
+Mojsisovics have proved that this zonal subdivision, long adopted for
+Jurassic rocks, is also applicable to those of Triassic age[99]. He
+gives the following table of the classification of the Triassic rocks
+of the Mediterranean Province, which is reproduced, as it is founded
+upon Palæontological evidence, and will probably be widely adopted.
+
+[Footnote 99: von Mojsisovics, Dr E., "Faunistische Ergebnisse aus der
+Untersuchung der Ammoneen-faunen der Mediterranen Trias." _Abhandl.
+der k. k. Geologisch. Reichsanstalt_, VI. Band 2 Abtheilung. Vienna,
+1893.]
+
+     Series                                    Zonal Divisions
+  --------------+-------------------+--------------------------------------
+  Rhætic        |                   |  1. Zone of _Avicula Contorta_
+  --------------+-------------------+--------------------------------------
+                |                   |  2. Zone of _Sirenites Argonautae_
+                | Upper Juvavic     |  3. Zone of _Pinnacoceras
+                |                   |       Metternichi_
+  Juvavic       | Middle Juvavic    |  4. Zone of _Cyrtopleurites
+                |                   |       bicrenatus_
+                |                   |  5. Zone of _Cladiscites ruber_
+                | Lower Juvavic     |  6. Zone of _Sagenites Giebeli_
+  --------------+-------------------+--------------------------------------
+                | Upper Carnic      |  7. Zone of _Tropites subbullatus_
+  Carnic        | Middle Carnic     |  8. Zone of _Trachyceras Aonoides_
+                | Lower Carnic      |  9. Zone of _Trachyceras Aon_
+  --------------+-------------------+--------------------------------------
+                | Upper Noric       | 10. Zone of _Protrachyceras
+  Noric         |                   |       Archelaus_
+                | Lower Noric       | 11. Zone of _Protrachyceras Curionii_
+  --------------+-------------------+--------------------------------------
+                | Upper Muschelkalk | 12. Zone of _Ceratiles trinodosus_
+  Muschelkalk   |                   |
+                | Lower Muschelkalk | 13. Zone of _Ceratiles binodosus_
+  --------------+-------------------+--------------------------------------
+  Buntsandstein | Werfener Schichten| 14. Zone of _Tirolites Cassianus_
+  --------------+-------------------+--------------------------------------
+
+
+
+
+CHAPTER XXII.
+
+THE JURASSIC SYSTEM.
+
+
+The Jurassic rocks were formerly separated on account of differences
+of lithological character into Oolites and Lias, but it was apparent
+that the Oolites were more important than the Lias, and a fourfold
+division was made into:--
+
+  Upper or Portland Oolites }
+  Middle or Oxford Oolites  } = Malm
+  Lower or Bath Oolites       = Dogger
+    Lias.
+
+The Lias strata have also been spoken of as the Black Jura, the Lower
+Oolites and part of the Oxford Oolites as Brown Jura, and the rest of
+the Oxford Oolites with the Portland Oolites as White Jura.
+
+As the outcome of a detailed study of the faunas of the Jurassic
+rocks, a further subdivision has been made, partly based upon the
+original British series, but the divisions are defined with greater
+accuracy, so that they are applicable over wider areas. They are as
+follows:--
+
+                 { Purbeckian
+  Upper Oolites  { Portlandian
+                 { Kimmeridgian
+
+                 { Corallian
+  Middle Oolites { Oxfordian
+                 { Callovian
+
+  Lower Oolites  { Bathonian
+                 { Bajocian
+
+                 { Toarcian
+  Lias           { Liassian
+                 { Sinemurian.
+
+Many of these series have been still farther subdivided into smaller
+stages, and the whole differentiated into a number of zones
+characterised by different forms of Ammonites. Dr E. von Mojsisovics
+gives thirty-two Ammonite zones, of which fourteen occur in the Lias,
+eight in the Lower Oolites, six in the Middle Oolites, and four in the
+Upper Oolites.
+
+_Characters of the strata._ The whole of the Jurassic rocks and also
+those of Lower Cretaceous age may be regarded as having been deposited
+during the first shallow water phase of the third marine period, but
+this shallow water phase is represented by strata which are varied
+owing to numerous marine changes resulting in the production of land
+at times, and estuarine conditions, shallow water, marine conditions,
+and somewhat deeper sea conditions respectively at other times, and
+accordingly the strata of the British Isles vary greatly when traced
+laterally. That the uplifts of the Permo-Triassic periods produced
+some effect on the nature and distribution of the Jurassic rocks is
+certain, but it is not quite clear how far the ridges produced by
+these uplifts were submerged and denuded during the deposition of the
+main portion of the Jurassic strata.
+
+Viewed broadly, the Jurassic rocks of Britain may be regarded as
+consisting of three great clay deposits, the Lias, Oxford and
+Kimmeridge Clays, alternating with the deposits of variable
+lithological characters, which compose the Bajocian, Bathonian,
+Corallian, Portlandian and Purbeckian subdivisions. This essentially
+argillaceous character of a large part of the deposits of Jurassic age
+is often overlooked, as, owing to their sameness and the comparative
+paucity of organisms constituting the faunas in the clays, their
+description in text-books can be given at much shorter length than
+that of the more variable and highly fossiliferous deposits which
+separate the clays. The following figure (Fig. 23) roughly represents
+the nature of the different divisions of the rocks of this system when
+traced across England from south-west to north-east.
+
+[Illustration: Fig. 23.
+
+Vertical scale: 1 in. = about 1000 feet.]
+
+It will be seen that the greatest variations in lithological character
+occur in the Bathonian and Bajocian beds, and it will be of interest
+to give some account of the principal variations and to attempt to
+account for them. In so doing it will be convenient to consider the
+four major divisions of the Jurassic rocks separately, and to enter
+into particulars concerning the local classification applied to the
+rocks of these divisions.
+
+_The Lias._ The British Lias deposits are divided into the Lower Lias,
+the Marlstone, and the Upper Lias corresponding in general terms only
+with the Sinemurian, Liassian, and Toarcian. The Marlstone is
+separated from the Upper and Lower Lias on account of the greater
+percentage of carbonate of lime which it contains, so that the bands
+of argillaceous limestone are much more marked in the Marlstone than
+in the upper and lower divisions, which consist chiefly of clay. The
+three divisions possess very much the same characters throughout the
+country, though the presence of the Mendip ridge and its continuation
+beneath London is marked by the attenuation of this and succeeding
+strata, and by the conglomeratic character of some of the Liassic
+strata where they abut against it. The British Lias, as a whole, seems
+to have been deposited in a fairly shallow sea at no great distance
+from the land. It passes down conformably into the Rhætic beds, indeed
+the zone of Ammonites (_Aegoceras_) _planorbis_, referred by British
+geologists to the Lower Lias is included by some continental writers
+with the Rhætic beds, and the plane of demarcation here as in other
+cases is conventional.
+
+_The Lower Oolites._ Of all the British strata, these perhaps cause
+most trouble to the learner, on account of the different nomenclature
+applied to the rocks in different parts of England, and the rapid
+variations in lithological character, when the beds are traced
+laterally. The following divisions are usually adopted for the beds of
+the south-western counties where the most marked marine development
+occurs:--
+
+    Cornbrash,
+    Forest Marble,
+    Great Oolite (with Bradford Clay),
+    Fuller's Earth,
+    Inferior Oolite.
+
+Of these divisions, the uppermost one, the Cornbrash, though thin,
+retains its characters with great constancy across the island. Of the
+others the Forest Marble may be looked upon as a local development of
+the upper portion of the Great Oolite, and the Fuller's Earth is a
+local deposit, so that the Inferior Oolite and Great Oolite constitute
+the important divisions of the Lower Oolites. The variations in the
+characters of the rocks may be best shown in tabular form.
+
+ -----------------+------------------+-------------------+-----------------
+ Gloucestershire, |      South       |        N.         |
+       &c.        | Northamptonshire | Northamptonshire  |    Yorkshire
+                  |                  |    and Lincoln    |
+ -----------------+------------------+-------------------+-----------------
+     Cornbrash    |    Cornbrash     |     Cornbrash     |    Cornbrash
+ -----------------+------------------+-------------------+-----------------
+ Great Oolite     | Great Oolite     | Great Oolite Clay |
+                  | (Upper part)     | Great Oolite      | Upper
+                  |                  |   Limestone       |
+                  |                  | Upper             | Estuarine
+                  | Northamptonshire | Estuarine         |
+  ............... | ................ | ................. | ................
+                  |                  | Series            |   Series
+                  |                  | Lincolnshire      | Scarbro'
+                  |                  |   Limestone       |   Limestone
+                  | Sands            |                   | Middle Estuarine
+                  |                  |                   |   Series
+ Inferior Oolite  |                  | Lower Estuarine   | Millepore Oolite
+                  |                  |   Series          |
+                  |                  |                   | Lower Estuarine
+                  |                  |                   |   Series
+ -----------------+------------------+-------------------+-----------------
+    Upper Lias    |    Upper Lias    |     Upper Lias    |    Upper Lias
+ -----------------+------------------+-------------------+-----------------
+
+     The dotted line shows roughly the division between Bathonian
+     and Bajocian.
+
+The changes may be explained very simply if we leave out of account
+for the moment the development of Lincolnshire Limestone, with its
+equivalent the Scarbro' Limestone, and the Millepore series. The beds
+in Gloucestershire and other south-western counties are essentially
+marine; whilst in Northamptonshire and Lincolnshire estuarine
+conditions set in after the deposition of the Upper Lias, and
+continued throughout the deposition of the Bajocian and Lower
+Bathonian beds, being replaced by marine conditions during the
+formation of the Upper Bathonian strata, and still further north in
+Yorkshire the estuarine conditions generally prevailed throughout
+Bajocian and Bathonian times. These changes point to the existence of
+land towards the north. The general simplicity is modified by
+temporary prevalence of marine conditions twice over (during the
+deposition of the Millepore Oolite and the Scarbro' Limestone) in
+Yorkshire, and once (during the deposition of the Lincolnshire
+Limestone) in Lincolnshire.
+
+Certain local deposits have not been noticed, but two of them merit
+brief reference. At the base of the Great Oolite of Oxfordshire is an
+estuarine deposit of finely laminated mechanical sediment mixed with
+calcareous matter known as the Stonesfield Slate, especially
+interesting on account of its fossils, while a bed with similar
+lithological characters but with a different fauna occurring at the
+base of the Lincolnshire Limestone (of Bajocian age) is termed the
+Collyweston Slate. Neither of these deposits is a slate in the true
+sense of the word, as they have not been affected by cleavage
+subsequently to their accumulation, but each has been somewhat
+extensively used for roofing purposes.
+
+The Middle Oolites are much less complicated though considerable
+variations arise with respect to the Corallian Rocks. The Oxfordian
+with Callovian consist chiefly of clay, though the Callovian of the
+south of England is represented by calcareous sandstone, with a
+peculiar fauna which seems to be represented in the lower part of the
+Oxford Clay further north, though this Callovian fauna has not been
+everywhere recognised.
+
+The Corallian of the southern counties consists of limestones with
+calcareous grits, the limestones being often largely composed of the
+remains of reef-building corals, and a similar development of the
+rocks of this series is found in Yorkshire, while a local development
+of the same character is found at Upware in Cambridgeshire, though in
+the other parts of the Fenland counties the Corallian is represented
+by an argillaceous deposit with Corallian fossils known as the
+Ampthill Clay.
+
+The Upper Oolites have a tolerably constant base, the Kimmeridge Clay,
+usually consisting of laminated bituminous argillaceous material, but
+the Portlandian and Purbeckian divisions vary greatly, and are only
+locally developed, though their absence in some parts of central
+England is no doubt due to unconformity.
+
+The Portlandian rocks of the south of England consist of limestones
+and sandstones which pass further northward into shallower water
+mechanical deposits often charged with iron hydrate, and the beds
+disappear in Oxfordshire. The Purbeckian rocks of the south are also
+limited as regards area of exposure: they consist of estuarine
+deposits with some terrestrial accumulations of the nature of old
+surface soils. Representations of the Portlandian and Purbeckian beds
+are found in Lincolnshire and Yorkshire, as arenaceous deposits in the
+former county and argillaceous ones in the latter. Both are marine
+deposits of a northern type, developed elsewhere in northern European
+and circumpolar regions, and in these counties we find a complete
+passage from the Jurassic rocks through the Cretaceous rocks, but the
+exact lines of demarcation between the different series of the passage
+beds are difficult to define.
+
+The foreign Jurassic rocks of Europe and of some parts of Asia
+strongly resemble in general characters those which have been
+described above as occurring in Britain. One of the most remarkable
+features of the Jurassic rocks as a whole, is the absence of the Lias
+over wide areas, the continental period which in Britain existed in
+Permo-Triassic times is elsewhere frequently replaced by one of
+Liassic age.
+
+The Jurassic and Cretaceous rocks are of interest on account of the
+evidence which they supply as to the existence of climatic zones in
+these periods, which run fairly parallel with those at present
+existing. The late Dr Neumayr in a paper already cited divides the
+world during later Mesozoic times into four distinct climatic zones,
+equatorial, north and south temperate and boreal zones (the
+corresponding austral zone is not known owing no doubt to the
+extensive sea of South Polar regions and our general ignorance of its
+lands). In Europe the Mediterranean Province belongs to the equatorial
+zone, the Middle European to the North temperate zone, and the Russian
+or Boreal to the Boreal zone. The last-named is marked partly by
+negative characters, the absence of certain Ammonite-genera and of
+coral reefs being noticeable, whilst the lamellibranch _Aucella_ is
+very frequent. In the North temperate zone, certain Ammonite genera as
+_Aspidoceras_ and _Oppelia_ are abundant and there are also extensive
+coral-reefs. The Equatorial zone is marked by the Ammonite-genera
+_Phylloceras_ and _Lytoceras_ and by the _Diphya_ group of
+_Terebratulæ_. It is of special interest to note that the fauna of the
+South temperate bears closer relationship to that of the North
+temperate than to that of the intermediate Equatorial zone.
+
+_Jurassic floras and faunas._ The Jurassic flora is very similar in
+its characters to that of the Lower Cretaceous rocks, and the two
+taken together afford a decided contrast with that of later Palæozoic
+times, and also with that which succeeds them in the Upper Cretaceous
+rocks, which bears a marked resemblance to the existing flora. Cycads
+predominate, accompanied by conifers, and a fair number of ferns and
+Equisetaceæ.
+
+The Jurassic fauna is specially noteworthy on account of the character
+of the vertebrata, but some notice of the invertebrates must also be
+taken. The abundance of corals in the Temperate zones has already been
+pointed out, but the mollusca form the bulk of the invertebrate fauna,
+lamellibranchs, gastropods and cephalopods being all abundant; of the
+last-named the ammonites and belemnites contribute most largely. The
+vertebrates include remains of fishes, amphibia, reptiles, birds and
+mammals. The Jurassic reptilia furnish representatives of some modern
+orders as the Chelonia and Crocodilia, but the most important orders
+are essentially characteristic of later Mesozoic times and their
+representatives abound in the Jurassic strata. These are the
+Sauropterygia (including the Plesiosaurs), the Ichthyopterygia
+(including the Ichthyosaurs), the Dinosauria, and the Pterosauria
+commonly known as Pterodactyls. No birds have hitherto been discovered
+in the British Jurassic rocks, but the Solenhofen Slate of Bavaria (of
+Kimmeridgian age) has furnished the celebrated _Archæopteryx
+macrura_, which is not only placed in a family but also in an order by
+itself, the order Saururæ. Many remains of mammals have been extracted
+from the estuarine deposits of Stonesfield, and the old surface soils
+of the Purbeckian beds; representatives of the Monotremata are
+furnished by the _Plagiaulacidæ_ and _Tritylodontidæ_, the former
+family containing the genus _Plagiaulax_ of the Purbeck Beds and the
+latter, _Stereognathus_ of the Stonesfield slate. The Marsupialia are
+represented by the _Amphitheridæ_, _Spalacotheridæ_ and
+_Triconodontidæ_. Some forms have been referred to the Insectivora,
+but there is still disagreement concerning the correctness of this
+reference.
+
+Before dismissing the subject of the Jurassic fossils, attention may
+be called to a feature which has been frequently commented upon,
+namely, the general resemblance of the flora and fauna of Jurassic
+times to the modern Australian fauna and flora. The explanation which
+has been offered to account for this resemblance has been given in a
+preceding chapter, where it was stated that Mr A. R. Wallace
+considers, after review of the geological and biological evidence,
+that Australia was severed from the adjoining continental lands in
+Mesozoic times, and that the higher forms of life which on the larger
+continents have replaced the earlier and lower forms have not
+succeeded in obtaining a footing in Australia, which therefore
+furnishes us with a local survival of a once widespread fauna. In
+connection with this matter the actual existence of the genus
+_Trigonia_ (a form peculiarly abundant in Jurassic strata and
+characteristic of Mesozoic strata in Britain) in the Australian sea is
+of considerable interest.[100]
+
+[Footnote 100: A good account of the British Jurassic rocks will be
+found in Mr H. B. Woodward's Memoir on "The Jurassic Rocks of
+Britain." _Mem. Geol. Survey_, 1893--.]
+
+
+
+
+CHAPTER XXIII.
+
+THE CRETACEOUS SYSTEM.
+
+
+_Classification._ The rocks of the Cretaceous system are conveniently
+divided into Upper and Lower Cretaceous. The following classification
+has been widely used for the British deposits, and is founded on
+lithological characters:
+
+             { Upper Chalk with flints      }
+             { Middle Chalk with few flints } Chalk
+  Upper      { Lower Chalk without flints   }
+  Cretaceous { Chalk Marl                   }
+             { Upper Greensand
+             { Gault
+
+             { Lower Greensand
+  Lower      { Wealden
+  Cretaceous { Hastings sands
+
+As the result of examination of the faunas, a more generally
+applicable classification has been established and is now largely
+adopted. It is as follows:
+
+  Danian     }
+  Senonian   } Upper Cretaceous
+  Turonian   }
+  Cenomanian }
+
+  Albian     }
+  Aptian     }Lower Cretaceous.
+  Neocomian  }
+
+In this classification the Neocomian practically represents the
+Wealden and Hastings beds, the Aptian the Lower Greensand and the
+Albian the Gault, placed according to this classification in the Lower
+Cretaceous, while the Upper divisions represent the strata above the
+Gault, consisting essentially of Chalk in England.
+
+
+_Description of the Strata._
+
+(i) _The Neocomian and Aptian Beds._ In the south of England the Lower
+Cretaceous beds succeed the Jurassic rocks with little or no break,
+and the type of the lower beds is similar to that of the beds
+deposited during the Purbeck age, consisting of estuarine deposits of
+variable characters, chiefly arenaceous below (the Hastings sands) and
+argillaceous above (the Wealden series), though impure limestones are
+found, largely composed of the shells of the freshwater _Paludina_,
+and much ironstone is developed in places. At the close of Neocomian
+times, the freshwater conditions in southern England were replaced by
+marine conditions and the Lower Greensand strata with their marine
+fauna were deposited in the Aptian sea. The Neocomian and Aptian beds
+thin out westward, and much more rapidly to the northward, so that
+both divisions disappear against the now buried ridge which forms a
+continuation of the Mendip axis. North of this they appear in another
+form. At first the highest Aptian beds alone are developed as shore
+deposits. Passing into Norfolk lower beds come in until in
+Lincolnshire we get a complete development of the Neocomian and Aptian
+beds with a marine facies, though of fairly shallow water character,
+whilst in Yorkshire the two divisions are represented by a deeper
+water clay, forming the Upper portion of the Speeton series. There is
+a consensus of opinion in favour of the Neocomian beds of southern
+Britain having been laid down in an estuary of a river flowing from
+the west over a continent now destroyed. To the north of this river
+stood the London ridge of the Palæozoic rocks, the northern borders of
+which formed the coast line off which were deposited the sediments of
+Neocomian and Aptian ages which occur in northern England. Before the
+deposition of the Albian beds a considerable upheaval of some parts of
+Britain occurred, and an unconformity separates the higher Cretaceous
+beds from older strata of Cretaceous and Jurassic ages, thus
+complicating the major phases by local changes in the characters of
+the strata.
+
+(ii) _The Albian and higher Cretaceous Beds._ The commencement of the
+deep-water phase of the third marine period may be said to occur in
+Albian times in Britain, reaching its maximum during the deposition of
+the chalk. The existence of a deeper sea towards the north of England
+is indicated by the characters of the Albian and newer strata. The
+Albian beds of gault consist of a stiff clay in southern England,
+replaced by coarser mechanical sediments towards the west. As one
+passes north from the London ridge (which exerted its influence in
+Albian times, after which it was finally buried in sediment) the gault
+thins out, and becomes gradually replaced by calcareous deposit when
+it is known as the red chalk which replaces the gault in northern
+Norfolk, Lincolnshire and Yorkshire.
+
+A local unconformity separating the chalk and gault in parts of East
+Anglia points to another local uplift with its accompanying
+complications in the characters of the strata. After the uplift had
+ceased, general depression must have occurred, and the various
+divisions of the chalk were accumulated in a fairly open sea, though,
+for reasons to be given presently, this was probably of no great
+lateral extent, save when united with the open ocean, probably in a
+manner similar to the connexion between the Gulf of Mexico and the
+Atlantic.
+
+The general variations in the lithological characters of the various
+members of the Cretaceous system will probably be rendered clearer by
+reference to the accompanying diagram (fig. 24) representing the
+variations when traced across England from south to north[101].
+
+[Footnote 101: For information concerning the British Cretaceous beds,
+see Topley and Foster, "Geology of the Weald," _Mem. Geol. Survey_,
+1875; Bristow and Strahan, "Geology of the Isle of Wight," _Mem. Geol.
+Survey_, 1889; Lamplugh, "On the Speeton Clay," _Q. J. G. S._, vol.
+XLV. p. 575, and "The Speeton Series in Yorkshire and Lincolnshire,"
+_ibid._, vol. LII. p. 179; Barrois "Recherches sur le Terrain Crétacé
+supérieur de l'Angleterre et d'Irlande," Lille, 1876; and various
+papers by Messrs Hill and Jukes-Browne, in the _Quarterly Journal of
+the Geological Society_ and _Geological Magazine_ of recent years. For
+the Scotch deposits consult a paper by Prof. Judd, _Q. J. G. S._, vol.
+XXXIV. p. 736, and for those of Ireland, see Hume, _Q. J. G. S._, vol.
+LII. p. 540.]
+
+[Illustration: Fig. 24.
+
+  Ch. Chalk.
+  Al. Albian.
+
+  Ap. Aptian.
+  N. Neocomian.
+
+  J. Jurassic.
+]
+
+The clue to the physical geography of Britain during Cretaceous times
+is furnished to a considerable extent by study of the foreign
+deposits. In Northern Europe the Cretaceous beds of England are met
+with in Northern France, and there the characters are generally
+speaking similar to those of our British deposits. In Germany
+shallower water conditions prevailed, the lower beds gradually
+disappear, and the upper beds are replaced by mechanical sediments of
+various degrees of coarseness, becoming on the whole coarser, as one
+travels eastward, so that in Saxony the chalk is partly replaced by
+arenaceous deposits (the 'Quader' sandstones) which are responsible
+for the remarkable scenery of the Elbe district above Dresden. In
+passing northwards, indications of similar change are noted in the
+deposits of Denmark and Scania, whilst to the south, we get a complete
+change in the character of the rocks, after crossing the Loire in
+France, and a similar change is observable in districts lying further
+east. Furthermore, as will be noted more fully in a subsequent
+paragraph, the character of the Upper Cretaceous flora indicates the
+existence of a large tract of land lying to the north and north-west
+of Europe, so that it would appear that the Cretaceous rocks of
+Northern Europe were deposited in a gulf-like expansion of a western
+ocean, bounded on the north by Scandinavia, on the west by eastern
+Germany, and on the south by a ridge running eastward from the mouth
+of the Loire[102]. We may speak of this gulf as the Chalk gulf. To the
+south of the presumed ridge the character of the strata alters, and
+also that of the included organisms. This southern type of Cretaceous
+rocks is one which is very widely spread, being found in Europe south
+of the Loire, and of the Alps, and in Greece and Turkey, while it
+also occurs in the northern parts of Africa. The beds of this type are
+traceable through Asia Minor into India and to the shores of the
+Indian Ocean, indicating the existence of a widespread Cretaceous
+ocean, which is sometimes spoken of as the Hippurite-limestone sea,
+for reasons which will eventually appear. The deposits are largely
+formed of hard limestone which is very different in its character from
+the soft chalk of the northern gulf.
+
+[Footnote 102: The reader will find the existence of this gulf
+maintained and supported by a considerable mass of detail in Mr A. R.
+Wallace's _Island Life_.]
+
+The climatic conditions which prevailed during Cretaceous times were
+apparently similar in most respects to those of the preceding Jurassic
+period, and as already stated the climatic zones which Neumayr defined
+for Jurassic times are also maintained by him to have existed during
+the Cretaceous period. The existence of cold has sometimes been
+inferred from the presence of large foreign blocks in the chalk,
+especially at its base, but if these are due to the transport, they
+might well be caused by masses of floating ice, which are often found
+at considerable distances from the coast in temperate regions after
+the break-up of the frost which succeeds an unusually hard winter. The
+flora and fauna are not suggestive of severe conditions.
+
+_The Cretaceous flora and fauna._ It has been noted in the last
+chapter that the gymnospermous flora of the Jurassic period, in which
+cycads form a considerable percentage of the whole flora, was
+prevalent in Lower Cretaceous times. In the Upper Cretaceous rocks
+this flora is replaced by one which consists to a large extent of
+dicotyledonous angiosperms. These are found in the Upper Cretaceous
+rocks of Europe and North America, and as the researches of botanists
+indicate their origin in circumpolar regions, their arrival in Europe
+is an additional argument in favour of the existence of an extensive
+northern continent, sending a prolongation to the southward in eastern
+Europe.
+
+The invertebrate fauna bears considerable resemblance to that of
+Jurassic times, and many of the dominant Jurassic genera are also
+found in Cretaceous rocks. A most interesting feature is connected
+with the character and geographical distribution of the Ammonites. In
+Europe they are almost exclusively confined to the deposits of the
+northern gulf, and before their final disappearance they undergo many
+changes of form. We find the discoid spiral shells of earlier times,
+but these are accompanied by shells which are straight, curved,
+boat-shaped, and coiled into various helicoid spirals, sometimes
+having the whorls in contact, while at other times they are separate.
+
+In the chalk of Britain gastropods are on the whole rare, and this
+fact serves to emphasize the palæontological break which occurs
+between the Cretaceous and Tertiary rocks; but when conditions were
+favourable, as during the deposition of some of the strata of the
+Middle Chalk, gastropods are abundant, and some are related to
+Tertiary genera, so that we may assume that the palæontological break
+alluded to is exaggerated by the difference of conditions which
+prevailed during the deposition of the earliest Tertiary and latest
+Cretaceous sediments.
+
+In the Cretaceous deposits of the southern sea, where the Ammonite
+tribe is almost unknown, the remarkable family of the lamellibranchs
+known as the Hippuritidæ furnish the dominant invertebrates of the
+period, and the representatives of this family are exceedingly scarce
+amongst the Cretaceous strata of the northern gulf, though they are
+found on two or three horizons.
+
+Of vertebrates, the most interesting are the reptiles. The families
+which predominate in Jurassic times have many representatives amongst
+the Cretaceous strata also, but the order Squamata is represented by
+the sub-order Pythonomorpha, which is characteristic of the Cretaceous
+rocks. The best known representative is the gigantic _Mosasaurus_.
+Lastly, we have the remarkable toothed birds or Odontornithes, now
+placed in different orders, the genus _Hesperornis_ being the only
+representative of the sub-order Odontolcæ of the Ratitæ, whilst
+_Ichthyornis_ and allied forms are placed in the sub-order Odontormæ
+of the Carinatæ.
+
+
+
+
+CHAPTER XXIV.
+
+THE EOCENE ROCKS.
+
+
+_Classification._ The Eocene Beds of the south of England have been
+subdivided according to the variations in their lithological
+characters, and the subdivisions have received local names. The
+following classification is generally adopted, though the different
+subdivisions are by no means of equal value:
+
+  Upper Eocene  { Upper Bagshot Beds
+                { Barton Beds
+
+  Middle Eocene   Bracklesham Beds
+
+                { Lower Bagshot Beds
+                { London Clay[103]
+  Lower Eocene  { Oldhaven Beds             } Lower London
+                { Woolwich and Reading Beds } Tertiary Strata
+                { Thanet Sands              }
+
+[Footnote 103: Some writers place the London Clay in the Middle
+Eocene.]
+
+The deposits vary greatly when traced abroad, and the exact
+equivalents of the minor subdivisions of the British rocks can seldom
+be ascertained at any distance from England, though the division into
+Upper, Middle, and Lower Eocene can be made over wide areas.
+
+_Description of the strata._ The character of the strata of Europe and
+Asia indicates the persistence of the northern gulf and southern
+ocean of Cretaceous times in Eocene times also, though the area of
+each had shrunk in the meantime, owing to the physiographical changes
+which occurred at the end of Cretaceous times, giving rise to more
+extended land areas, and producing a shallow water phase over wide
+extents of ocean,--the final shallow water phase of the third and last
+great marine period of the British area. It is difficult to ascertain
+the exact importance of the physical break between Cretaceous and
+Eocene rocks in the south-east of England, owing to the subterranean
+solution of the upper part of the chalk, subsequently to the
+deposition of the Eocene strata, but the contraction of the Cretaceous
+gulf is shown in several ways, one of the most significant being the
+distribution of Cretaceous and Eocene rocks in the south-west of
+England. The existence of an outlier of Cretaceous rock at Buckland
+Brewer in North Devon, only three miles from the Atlantic Ocean,
+indicates the former extension westward of the Upper Cretaceous beds,
+while the occurrence of an outlier of Eocene rocks at Bovey Tracey in
+South Devon, resting not on Cretaceous but on Palæozoic rocks, shows
+that there was an uplift after the deposition of the Cretaceous rocks
+and before the Eocene rocks were deposited there, and that during the
+period of uplift the Cretaceous rocks were removed.
+
+Owing to these physical changes, the Eocene rocks of Britain are
+mainly mechanical sediments, some, as the Oldhaven beds, being
+composed of coarse pebbles over a fairly wide district, while some of
+the earlier Eocene rocks are estuarine or fluvio-marine.
+
+The Eocene rocks of Britain occur in four areas, namely, the London
+Basin, the Hampshire Basin, the Bovey Tracey outlier, and the
+north-east of Ireland and western Isles of Scotland. The deposits of
+the three southern areas may be considered together, and give general
+indications of an approach to land when passing westward. The Lower
+London Tertiary strata are fluvio-marine at the east end of the London
+Basin; they become shallower water deposits when traced westward, and
+begin to disappear. The London Clay is an estuarine deposit, which is
+generally supposed to have been laid down at the mouth of a large
+river flowing from the west. It is absent in the Bovey Tracey outlier.
+
+Local disturbances caused the existence of a shallow water region in
+the east during the deposition of the Middle and Upper Eocene
+deposits, and accordingly the well-marked marine deposits which form
+the representatives of these divisions in Hampshire are replaced by
+the Bagshot beds of the London Basin, consisting chiefly of coarse
+mechanical sediments with a poor marine fauna, but even in the west
+shallow water prevailed at times during the accumulation of various
+plant-bearing strata. The Middle Eocene beds only are found in the
+Bovey Tracey outlier, though the Upper Eocene beds may originally have
+been laid down in that area, and subsequently denuded.
+
+The fourth area displays a very different succession of Eocene strata,
+and one of extreme interest. Mechanical sediments and plant-bearing
+clays and lignites alternate with a vast accumulation of basaltic
+lavas, indicating the outbreak of the volcanic forces in the British
+area, after a period of quiescence which lasted through the greater
+part of Mesozoic times. The region in which these lavas were poured
+out was probably a land area during the greater part of the period of
+volcanic activity, but the horizontal lie of the lava flows and their
+wide extent indicate the existence of a flat tract of country,
+gradually raised into a plateau by the accumulation of sheet over
+sheet of basalt. How far this plateau extended it is impossible to
+say. The distribution of the lavas at the present day is somewhat
+limited in our isles, but there is no sign of dying out at the present
+margins of the accumulations, and they have probably escaped
+denudation in these regions, as maintained by Professor Judd, on
+account of the faults which have depressed them, while the portions
+which were not depressed have been removed by denudation. Two views as
+to the origin of the lavas have been put forward: according to Prof.
+Judd, they were poured forth from gigantic volcanoes, while Sir A.
+Geikie maintains that they represent portions of massive or fissure
+eruptions, the molten rock having welled out from great cracks in the
+earth, which are now filled by once molten rock in the form of dykes.
+As these dykes extend far away from the present volcanic plateau, one
+actually extending to the Yorkshire coast, we may well believe,
+whatever was the origin of the sheets of lava, that they were formerly
+spread far away from their present terminations[104]. Without entering
+here into a discussion of the exact nature of extrusion of these
+igneous sheets, it will suffice to say that all the evidence points to
+the formation of extensive plateaux, which must have presented a
+fairly uniform surface, similar to that which is still found
+characterising the volcanic districts of the western territories of
+North America.
+
+[Footnote 104: Prof. Judd's views will be found in a series of papers
+by him on the "Secondary Rocks of Scotland," _Quart. Journ. Geol.
+Soc._, vol. XXIX. p. 95, XXX. p. 220, XXXIV. p. 660, while Sir A.
+Geikie's explanation is advanced in a paper in the _Transactions of
+the Royal Society of Edinburgh_, vol. XXXV.; see also the same
+author's _Ancient Volcanoes of Great Britain_.]
+
+The Eocene rocks of the north-west of Europe possess characters very
+similar to those of the south of England, and there are indications
+that the northern gulf had diminished in extent towards the east as
+well as towards the west.
+
+Passing to southern Europe, Central Asia and northern Africa, we find
+the conditions of Cretaceous times reproduced, and an extensive series
+of marine deposits extends very widely over these regions, the most
+persistent deposit being a mass of limestone of Middle Eocene age,
+which is almost entirely composed of the tests of Nummulites, whence
+the development is known as the Nummulitic Limestone facies, and we
+may speak of the ocean as the Nummulitic Limestone Sea. The incoming
+of shallow water conditions marked by accumulation of coarse
+mechanical sediments towards the end of the Eocene period in some
+parts of the southern European area indicates the setting in, even
+then, of those continental conditions which culminated during the
+Miocene period.
+
+In North America we get similar evidence of the contractions of the
+oceans which in Mesozoic times occupied large expanses of our present
+continents.
+
+The climatic conditions of Eocene times have been noticed in passing
+in chapter IX., and evidence was given to prove the prevalence of a
+warmer climate over the British area than that which now exists. A
+study of the floras of various parts of the northern hemisphere
+suggests that climatic zones, whose lines of demarcation ran
+practically parallel with the Equator, existed in Eocene times also,
+though further information upon this subject is desirable.
+
+_The Eocene flora and fauna._ The flora of prevalent dicotyledonous
+angiosperms, which appeared in Upper Cretaceous times, also marks the
+Eocene and later deposits, but a study of the floras indicates that
+the differentiation which now marks off the floras of different areas
+from one another had not occurred to so great an extent as at the
+present time. The existence of a rich flora in the Eocene beds of
+circumpolar regions in the northern hemisphere should be noted, though
+perhaps its importance has been somewhat exaggerated.
+
+The invertebrate fauna shows an approximation to that of the present
+day. The remarkable ammonite fauna of Mesozoic times has disappeared,
+and gastropods and lamellibranchs predominate, many of the forms
+belonging to existing genera, though very rarely to existing species.
+The Nummulites are the most characteristic Eocene fossils, and the
+period may be spoken of as the Nummulitic Period, though it is now
+known that Nummulites are not confined to the Eocene strata.
+
+The vertebrate fauna is very noteworthy. The fishes and reptiles are
+closely related to existing forms, and the orders of reptiles which
+predominated in Mesozoic times have completely disappeared. But the
+mammals are the most interesting vertebrates of the Eocene period.
+Instead of the lowly organised forms of Mesozoic times, we find
+representatives of many orders, including the highest, the Primates.
+The generalised forms which serve as links between groups which are
+now separated to a considerable extent are of particular importance.
+They have been detected in Eocene rocks of various regions, though the
+most complete series have been obtained from the Eocene rocks of North
+America and made known to us through the numerous memoirs of
+Professors Cope and Marsh[105].
+
+[Footnote 105: The Eocene floras of Britain are described by Mr J.
+Starkie Gardner and Baron von Ettingshausen in the _Monographs of the
+Palæontographical Society_; other Monographs of the same Society
+contain an account of the Eocene Mollusca by Mr F. E. Edwards and Mr
+S. V. Wood. An idea of the generalised forms of Mammalia may be
+obtained by perusal of that portion of Nicholson and Lydekker's
+_Manual of Palæontology_ in which the latter author treats of the
+Mammalia, and in this connexion the reader will do well to read Prof.
+Huxley's "Lecture on Fossil Horses," reprinted in his _American
+Addresses_.]
+
+
+
+
+CHAPTER XXV.
+
+THE OLIGOCENE AND MIOCENE PERIODS.
+
+
+(i) _The Oligocene Beds._
+
+_Classification._ The Oligocene Beds of Britain are classified as
+follows:--
+
+  Upper    Wanting
+
+  Middle   Hempstead Beds
+
+         { Bembridge Beds
+  Lower  { Osborne Beds
+         { Headon Beds
+
+_Description of the strata._ Little need be said of the deposits of
+this period, either in Britain or abroad, except to remark that they
+show the further spread of continental conditions over the regions now
+occupied by land. The British deposits are now seen in the Hampshire
+Basin only, and have been spoken of as the fluvio-marine series, as
+many of the strata were laid down in continental sheets of water,
+while the true marine sediments are thin and infrequent.
+
+The lithological characters of deposits formed under these conditions
+naturally vary greatly, consisting of different kinds of mechanical
+sediments occasionally mixed with thin freshwater marls and
+limestones. On the Continent similar conditions prevailed, though the
+occurrence of fairly wide tracts of level surface is indicated by the
+widespread distribution of beds of brown coal or lignite, and the
+coarse and thick Oligocene 'nagelfluh' of Switzerland points to the
+elevation of mountain ranges in the neighbourhood.
+
+_The flora and fauna._ The remarks made concerning the Eocene flora
+and fauna are generally applicable to those of Oligocene times, except
+that the Oligocene fossils bear a still closer resemblance to living
+forms, and the Nummulites are no longer dominant.
+
+(ii) _The Miocene Period._ Beds of Miocene age are wanting in Britain,
+and on the Continent they occur in isolated basins deposited in
+gulf-like prolongations of the ocean, never very far from land. A
+description of the strata and their fossil contents would be of little
+use for our present purposes, and the remarks made concerning the
+Oligocene beds will apply to the Miocene strata also.
+
+The period was mainly remarkable on account of the important physical
+changes which occurred, to which we must devote some consideration.
+Commencing with the British area, we find in the south evidence of the
+separation of the London and Hampshire Basins at this time, for the
+Oligocene beds of Hampshire are tilted up on the south side of an
+anticline, which separates the Hampshire Basin from that of London,
+indicating that the movement was post-Miocene, while in Kent, beds of
+Pliocene age rest on the denuded top of the chalk, showing that the
+elevation and denudation which accompanied it were pre-Pliocene; the
+great Wealden anticline is thus seen to be of Miocene age. On the
+north side of the London Basin the line of demarcation between Eocene
+and Mesozoic beds runs approximately parallel to the strike of the
+latter in that part of Britain, and this points to the elevation of
+the Mesozoic strata which gave them their present south-easterly dip
+about the same period, though in the absence of Oligocene rocks it
+cannot be definitely stated that the movement was altogether
+post-Oligocene. The present physical geography of considerable parts
+of Britain must date from Miocene times.
+
+Important as the changes were in Britain, they were slight as compared
+with those which affected Europe and many parts of Asia. The great
+mountain chains of the Old World received their maximum uplift during
+this great period of earth-movement, and orogenic structures were
+impressed upon the rocks of many regions, for the Tertiary Mountain
+Chains of the Old World have an Alpine structure impressed upon them
+as the result of intense lateral pressure, accordingly we find the
+Eocene strata lifted far above their original level to heights of
+8,000 feet in the Alps and over 12,000 feet in the Himalayas. Away
+from these marked uplifts epeirogenic movements caused the
+disappearance of the seas of earlier Eocene times, so that towards the
+close of the Miocene Period, the main features of the Eurasian
+continent were much as they are now. The present drainage-systems must
+have originated at the same time, and the sculpture of our continent
+has been carried on more or less continuously by subaerial agents from
+Miocene times to the present day. That any addition to the total area
+of land was made is doubtful. The land which appears to have existed
+to the west of Britain during Cretaceous and Eocene times finally
+disappeared beneath the waters of the Atlantic Ocean, and the movement
+probably gave rise to the prominent submarine feature which now exists
+at some distance from the coast of Ireland. A great marine period is
+now existent in our ocean areas, but so far as the existing
+continents are concerned, we are living on the fourth continental
+period which practically came into existence in Miocene times.
+
+The strike of the uplifted strata naturally coincides on the whole
+with the axes of the major uplifts, and accordingly we find the
+Mesozoic and early Tertiary strata folded around axes which have a
+prevalent east and west direction, with others which have a trend at
+right angles to this. The strike of the British Mesozoic rocks seems
+to have been determined by each of these sets of movements, so that
+although it is east and west in the south of England, it runs north
+and south in the eastern counties north of the Thames.
+
+In America, although epeirogenic movements had occurred before Miocene
+times, with the formation of wide continental tracts, these appear to
+have been of the nature of plains, diversified by extensive inland
+sheets of water, and uplift of orogenic character converted these
+plains into uneven tracts in Miocene times. Many of the movements in
+America, which like those of Europe are still progressing with
+enfeebled power, differ from those of Eurasia, giving rise to raised
+monoclinal blocks rather than to violent folds of Alpine character, as
+seen in the western territories of North America, and as proved also
+by the differential movements which are now known to affect the
+Atlantic coast of that continent.
+
+Accompanying these changes in the earth's crust were others which
+affected the climate, at any rate locally. The warm climate of Eocene
+times gradually gave way to a cooler climate in Oligocene times, and
+this lowering of temperature was still further advanced in Miocene
+times, though there is evidence that the temperature of those parts of
+Europe which have strata representative of the Miocene period was
+higher than it is at the present day.
+
+Owing to the changes which occurred in Miocene times, the area of
+sedimentation was extensively shifted to our present oceans, and
+accordingly we find that the times subsequent to those of the Miocene
+uplifts are marked by scattered accumulations of continental
+character, with a few insignificant marine strata seldom found far
+inland from the present coast-lines.
+
+
+
+
+CHAPTER XXVI.
+
+THE PLIOCENE BEDS.
+
+
+_Classification._ The Italian Pliocene Beds which have long been known
+have been divided into three stages, to which names have been applied
+which are somewhat widely used, though the division of the British
+deposits into the same three stages has not been made. The stages
+are:--
+
+  Astian.
+
+  Plaisancean.
+
+  Zanclean.
+
+The classification of the British deposits may be made as follows:--
+
+  Cromer "Forest" Series.
+
+  Weybourne Crag and Bure Valley Beds.
+
+  Chillesford Crag.
+
+  Norwich Crag and Red Crag.
+
+  Upper Coralline Crag.
+
+  Lower Coralline Crag.
+
+As the English deposits are somewhat scattered it is difficult to make
+out the exact order of succession, but the above shows the
+classification which is adopted by the best authorities, the Norwich
+Crag (or Fluvio-marine Crag as it is sometimes termed) being now
+supposed to represent the upper portion of the Red Crag.
+
+_Description of the strata._ The British deposits are chiefly found in
+the counties of Norfolk and Suffolk, but isolated patches have been
+detected in Kent and at St Erth in Cornwall; while the inclusion of
+Pliocene fossils in the glacial deposits of Aberdeenshire and on the
+west coasts and islands of Great Britain suggests the occurrence of
+Pliocene beds beneath sea-level, around the British coasts, at no
+great distance from the land.
+
+The term 'Crag' has been applied to shelly sands of which the British
+Pliocene beds are largely composed. The oldest British Pliocene strata
+are supposed to be the Lenham Beds, occurring in 'pipes' on the Chalk
+of the North Downs, which are referred to the Lower Coralline Crag,
+and some writers believe that the St Erth beds of Cornwall are of
+similar age[106]. The former are ferruginous sands, and the latter
+shelly sands and clays. The higher beds of the Coralline Crag are
+found in Suffolk, and are largely calcareous, being made of remains of
+polyzoa, molluscs, and other invertebrates. They were probably
+deposited in deeper water than the rest of the British Pliocene
+strata, and contain a far larger percentage of carbonate of lime. The
+Red Crag consists of ferruginous shelly sands, of the nature of
+sand-banks, formed near land; while the Norwich Crag is of a still
+more littoral character, and contains remains of land shells and the
+bones of mammalia mingled with the marine shells of the coast. The
+higher Pliocene deposits are also coastal accumulations, even the
+so-called Forest bed being a deposit and not a true surface soil, as
+proved by the observations of Mr Clement Reid. At the summit of the
+Cromer 'Forest' Series, however, is a true freshwater bed. These
+British deposits appear to have been laid down on a coast line which
+formed one side of the estuary of a large river, of which the present
+Rhine is the 'betrunked' portion (to use a term introduced by Prof. W.
+M. Davis)[107].
+
+[Footnote 106: See Clement Reid, _Nature_, 1886, p. 342; and Kendall
+and Bell, _Quart. Journ. Geol. Soc._, vol. XLII. p. 201.]
+
+[Footnote 107: See a paper by Mr F. W. Harmer, "On the Pliocene
+Deposits of Holland, and their relationship to the English and Belgian
+Crags," _Quart. Journ. Geol. Soc._, vol. LII. p. 748.]
+
+On the European continent, marine Pliocene beds are found in Belgium
+and Italy. The former deposits greatly resemble our Crags, whilst the
+latter are of interest on account of the mixture of volcanic beds with
+marine sediments in Sicily, showing that the formation of Etna
+commenced in Pliocene times. Various deposits formed in inland basins
+are found in France and Germany, but the most remarkable occur in the
+Vienna basin, where Caspian conditions prevailed over large areas, and
+the ordinary strata alternate with chemical deposits of which the
+best-known are the celebrated rock salt masses of Wieliczka, near
+Cracow. At the same time volcanic activity was rife to the south of
+the Carpathian mountains. Other deposits, which are partly referable
+to the Pliocene period, occur in Greece at Pikermi, and in India in
+the Siwalik hills; these are celebrated for their remarkable mammals,
+as are the Pliocene strata of the Western territories of North
+America. The occurrence of marked earth-movements since Pliocene times
+is indicated by the nature of the deposits of Barbadoes, where
+radiolarian cherts have furnished two echinids which are described by
+Dr Gregory as deep-sea forms. These beds were once referred to the
+Miocene period, but there is good reason for assigning them to a later
+date, and correlating them with the Pliocene beds of other areas, in
+which case there must have been a considerable uplift in this region
+since Pliocene times, a fact of great theoretical importance.
+
+The climatic conditions of Pliocene times show steady fall of
+temperature. The early Pliocene beds of Britain were deposited during
+the prevalence of warmer temperatures than those which now exist in
+the same area, but during later Pliocene times, the temperature was at
+first similar to that now prevailing, and afterwards distinctly
+colder, and we find in the upper Pliocene beds the remains of
+organisms of a northern type. In the uppermost deposit of the Cromer
+'Forest' Series, the arctic birch and arctic willow indicate the
+commencement of the cold which culminated in the succeeding 'Great Ice
+Age.'
+
+_The flora and fauna._ Little need be said of the Pliocene fossils:
+the flora approaches that of present times, and the invertebrates are
+in most cases specifically identical with those now living. The
+vertebrates alone differ markedly from living forms, being chiefly of
+extinct species, and in many cases belonging to extinct genera. It is
+interesting to find that the mammalian fauna of Pliocene times
+resembles the existing fauna of the area in which the beds are found,
+a fact long ago observed by Darwin. Thus the European Pliocene mammals
+are like existing European forms, whilst in Australia the mammalian
+terrestrial fauna consists of Marsupials, and in South America there
+are Edentata of Pliocene age[108].
+
+[Footnote 108: The Pliocene fauna of Britain is described by Mr
+Searles V. Wood in the _Monographs of the Palæontographical
+Society_.]
+
+
+
+
+CHAPTER XXVII.
+
+THE PLEISTOCENE ACCUMULATIONS.
+
+
+_Classification._ The term Pleistocene, as used here, is approximately
+equivalent to the expressions 'Glacial Period' and 'Great Ice Age' of
+some writers; but I have adopted it in preference to these
+expressions, because it may eventually be possible to define the
+Pleistocene period in such a manner as to give the term a strictly
+chronological meaning, whereas the other terms indicate the existence
+of climatic conditions which must have ceased in some areas sooner
+than in others. At present, climatic change gives us the best means
+for separating the accumulations formed subsequently to the Pliocene
+period over large parts of the Eurasian land-tract, and the most
+convenient division of these continental accumulations is to refer
+them to three periods, viz.:--
+
+  The Forest Period (in which we are now living).
+
+  The Steppe Period.
+
+  The Glacial Period.
+
+Some of the accumulations which were formed during the Steppe period
+are included in the Pleistocene period by many writers, but I prefer
+to treat of them as post-Pleistocene.
+
+In the present state of our knowledge of the glacial deposits any
+attempt to make a classification applicable over very wide areas is
+doomed to failure, and the very principles upon which the
+classification should be based are a subject of disagreement. The most
+promising basis for classification is founded on alternate recession
+and advance of land-ice, though the proofs that advance takes place
+simultaneously over very wide areas are not yet forthcoming. Dr J.
+Geikie in the last edition of his work _The Great Ice Age_ adopts four
+periods of glaciation, with intervening periods of recession, and this
+division accords with the observations of many foreign geologists. In
+order to understand the method of classification upon this basis, a
+few words concerning glacial deposits in general will not be out of
+place. Glacial accumulations may be divided into three classes:--(i)
+true glacial accumulations, formed on, in, and under the ice, and left
+behind upon its recession, (ii) marine glacial deposits, laid down in
+the sea, when floating ice is extensively found on its surface, and
+(iii) fluvio-glacial deposits, laid down by streams which come from
+the ice. The two former indicate glacial conditions, while the
+occurrence of fluvio-glacial deposits overlain by true glacial
+deposits indicates an advance of land-ice, for the fluvio-glacial
+deposits are accumulated in front of those which are truly glacial.
+Accordingly if we find alternations of glacial and fluvio-glacial
+deposits on a large scale, we may fairly infer the alternation of
+periods of great glaciation with others when the ice diminished, or in
+other words of glacial and interglacial periods. There is, however, in
+many cases great difficulty in distinguishing glacial deposits from
+marine glacial ones, while some of the true glacial deposits formed
+_in_ the ice (englacial deposits) cannot readily be distinguished from
+those of fluvio-glacial origin. Furthermore, as the terminal moraines
+of land-ice often rest upon other true glacial deposits, it is often
+difficult to know whether we are dealing with the products of one or
+two glaciations over limited areas. The test of superposition is often
+applicable, and one is enabled to obtain some clue as to the relative
+order of events. In England at least three periods of glaciation seem
+to be indicated by the glacial deposits. On the east coast the Cromer
+Forest Series is succeeded by the Cromer Till, and in Yorkshire the
+Basement Clay occupies a similar position with regard to the overlying
+glacial accumulations to that of the Cromer Till. Whether these
+deposits be marine or terrestrial, and the evidence is not yet
+sufficient to settle this question to the satisfaction of all
+geologists, there is no doubt that they are glacial. Above them, in
+East Anglia, lies the Contorted Drift, the origin of which is still a
+moot point, and it is overlain by the great Chalky Boulder Clay, which
+extends far and wide over East Anglia, the Midland Counties and into
+Yorkshire. Evidence has been adduced to connect this with the _till_
+or boulder clay which spreads over the upland districts of the north
+of England at the foot of the main hill-systems. This set of deposits
+indicates a second glaciation. As the upland till is often ploughed
+out by glaciers which have left their traces in the form of moraines
+in our upland regions, we seem here to have evidence of a third
+glaciation, which naturally leaves no traces in the southern
+districts, and the exact age of this cannot be ascertained in the
+absence of fossil evidence, though we may provisionally refer it to
+the Pleistocene period.
+
+Another attempt has been made to classify the glacial deposits, on the
+supposition that there have been periods of elevation and depression
+of the land during Pleistocene times. Some writers advocate one
+interglacial period when the land was depressed to an extent of 1400
+and perhaps 2000 feet, while others have advocated the occurrence of a
+number of such interglacial marine periods. The evidence for the
+supposed oscillations is furnished by the existence of shell-bearing
+sands associated with boulder clays at high levels, the best known
+being on Moel Tryfan in Caernarvonshire, near Macclesfield in
+Cheshire, and near Oswestry in Shropshire. As many geologists believe
+that these shells have been carried to their present position by ice
+in a way which it is not our province to discuss here, we may dismiss
+this method of classification as based upon events which cannot be
+proved to have occurred. In the present state of our knowledge, it is
+indeed best to avoid, as far as possible, classifications which are
+intended to be applicable over wide regions, and to devote our
+attention to local details, gradually piecing together the evidence
+which is obtained as the result of exhaustive examination of each
+separate area[109].
+
+[Footnote 109: The glacial literature of our own island only, is so
+extensive that the student may well be bewildered when he attempts to
+grapple with it. He is recommended to read the following general
+works:
+
+J. Geikie, _The Great Ice Age_. 3rd Edition, 1894.
+
+H. Carvill Lewis, _The Glacial Geology of Great Britain and Ireland_.
+1894.
+
+G. F. Wright, _Man and the Glacial Period_, 1892, and _The Ice Age in
+North America_, 1890.
+
+Sir C. Lyell, _Antiquity of Man_. 4th Edition, 1873.
+
+For the glacial geology of special regions the following papers may be
+consulted:
+
+_The Lake District and adjoining neighbourhood._ E. H. Tiddeman,
+"Evidence for the Ice Sheet in North Lancashire &c." _Quart. Journ.
+Geol. Soc._, vol. XXVIII. p. 471. J. G. Goodchild, "Glacial Phenomena
+of the Eden Valley &c." _Quart. Journ. Geol. Soc._, vol. XXXI. p. 55,
+and J. C. Ward, _Mem. Geol. Survey_, "The Geology of the Northern half
+of the Lake District."
+
+_Yorkshire._ G. W. Lamplugh, "Drift of Flamborough Head," _Quart.
+Journ. Geol. Soc._, vol. XLVII. p. 384.
+
+_Lincolnshire._ A. J. Jukes-Browne, _Quart. Journ. Geol. Soc._, vol.
+XXXV. p. 397 and XLI. p. 114.
+
+_East Anglia._ Clement Reid, _Mem. Geol. Survey_, "The Geology of the
+district around Cromer."
+
+_North Wales._ T. McK. Hughes, "Drifts of the Yale of Clwyd" &c.
+_Quart. Journ. Geol. Soc._, vol. XLIII. p. 73, and A. Strahan,
+"Glaciation of South Lancashire, Cheshire, and the Welsh Border,"
+_ibid._, vol. XLII. p. 486.
+
+_Switzerland._ C. S. du Riche Preller, "On Fluvio-glacial and
+Interglacial Deposits in Switzerland," _Quart. Journ. Geol. Soc._,
+vol. LI. p. 369 and "On Glacial Deposits, Preglacial Valleys and
+Interglacial Lake formations in Sub-Alpine Switzerland," _ibid._, vol.
+LII. p. 556.
+
+The reader will find references to other works on the Glacial Geology
+of other districts by consulting the general works referred to on the
+preceding page.]
+
+The foregoing remarks will convince the student that any attempt to
+show the distribution of land and sea during any part of the glacial
+period is not likely to meet with general acceptance, as so much
+depends upon the terrestrial or marine origin of the deposits of the
+lowlands, and the mode of formation of the shell-bearing drifts of
+high levels. The occurrence of elevation to a greater height than that
+which our country at present possesses during portions at any rate of
+the glacial period has been inferred on general grounds, but direct
+evidence in favour of it is furnished by the existence of a number of
+ancient valleys on the land around our coasts, whose floors are often
+considerably below sea-level, while the valleys are now completely
+filled up with glacial accumulations, except where they have been
+partially re-excavated by streams which for some distance run above
+the courses of the ancient streams.
+
+The climatic conditions of glacial times can only be briefly touched
+upon in this place. If the periods of advance can be proved to be
+contemporaneous over wide areas, this points to alternations of colder
+and warmer periods, or at any rate of drier and wetter periods, though
+local advance may be due to a number of causes. It must be borne in
+mind that with the temperature remaining the same, advance of ice can
+be brought about by increased precipitation of aqueous vapour in the
+form of snow.
+
+The question of the cause of the glacial period is one that only
+indirectly affects the stratigraphical geologist until he has
+accumulated sufficient evidence to indicate the cause. It must suffice
+to observe that the extremely plausible hypothesis of Croll (for which
+the student should consult Dr Croll's _Climate and Time_) does not
+explain the apparent gradual lowering of climate throughout Tertiary
+times till the cold culminated in the Pleistocene period, and the
+student will do well to remain in suspense concerning the cause of the
+Ice Age until further evidence has been brought to bear upon it.
+
+_The glacial flora and fauna._ The glacial deposits naturally yield
+few traces of life, except those which have been derived from other
+deposits, and we are dependent for our information concerning the
+fauna and flora of the glacial period upon the remains furnished by
+the interglacial deposits. Unfortunately it is very hard to ascertain
+which deposits are interglacial, and many which have been claimed as
+such are either preglacial or postglacial. The meagre evidence which
+we possess points to the existence of an arctic fauna or flora in
+Britain during the prevalence of this glacial period. A question which
+has received much attention of recent years is that of the existence
+of preglacial or interglacial man, on which much has been written. The
+existence of man in glacial times is probable, but it is the opinion
+of many of those who are most competent to form a judgment, that it
+has not been proved in the only conclusive way, namely, by the
+discovery of relics of man in deposits which are directly overlain by
+glacial deposits, or which at any rate are demonstrably older than
+glacial deposits[110].
+
+[Footnote 110: On the question of preglacial and interglacial man, see
+W. Boyd Dawkins, _Early Man in Britain_; H. Hicks, _Quart. Journ.
+Geol. Soc._, vol. XLII. p. 3, XLIV. p. 561, and XLVIII. p. 453; T.
+McK. Hughes, _ibid._, vol. XLIII. p. 73; Sir J. Evans, _Presidential
+Address to British Assoc._ 1897.]
+
+
+
+
+CHAPTER XXVIII.
+
+THE STEPPE PERIOD.
+
+
+The occurrence of a period marked by dry climate over wide areas of
+the Eurasian continent, and possibly also in North America, is
+evidenced by the widespread distribution of an accumulation known as
+_loess_, concerning the origin of which there has been much difference
+of opinion, though that it was formed subsequently to the glacial
+period seems to be generally admitted, inasmuch as it is largely
+composed of rearranged glacial mud. The formation of the loess as a
+steppe-deposit was first advocated by Baron von Richthofen, and his
+views were supported by Nehring after study of the loess-fauna.
+Richthofen's explanation of the loess as due to the spread of dust by
+wind in a dry region is becoming widely accepted, and it necessitates
+the widespread occurrence of steppe conditions, as the loess has a
+very extensive geographical range, and may be truly regarded as the
+normal continental deposit of Eurasia during the period immediately
+succeeding the glacial period. In our own country, as the sea cannot
+have been far distant during these times the normal loess is not
+found, but several accumulations occur, which on stratigraphical and
+palæontological grounds must be regarded as synchronous with the
+formation of the loess. These are certain rubble-drifts of the
+southern counties, the older river-gravels of southern England, and
+some of the older cave deposits of various parts of England. It is
+doubtful whether any classification into minute subdivisions can be
+adopted for them, though Prof. Boyd Dawkins has advocated their
+separation into an older age of River Drift Man, and a newer period of
+Cave Man, on account of the evidences of a lower state of civilisation
+afforded by examination of the River Drift implements when compared
+with those fashioned by Cave Man. Roughly speaking, the Steppe period
+corresponds with the period during which Palæolithic man existed, at
+any rate in north-west Europe, and we may speak of the Steppe period
+as the Palæolithic period, without asserting that Palæolithic man
+necessarily disappeared at the time when the climate changed and
+caused the replacement of Steppe conditions by others favourable to
+forest-growth.
+
+_Description of the accumulations._ The loess consists of unstratified
+calcareous mud or dust, with a peculiar vertical fracture, and is
+interesting rather on account of the nature of its fossils and of its
+distribution than for its lithological characters. As it is not found
+in Britain it is not necessary to say much about it, but merely to
+refer to the published descriptions[111].
+
+[Footnote 111: An account of Richthofen's views by that author will be
+found in the _Geological Magazine_, Dec. 2, vol. IX. (1882), p. 293,
+and the fauna of the loess is described by Nehring (_Ibid._, p. 570).]
+
+The British deposits require some notice, as their characters and mode
+of occurrence are of some significance. Along the south coast are
+deposits of coarse rubble which have yielded some organic remains,
+which have been described by Mr Clement Reid[112], who also discusses
+their origin. The rock, also known as the Elephant Bed, consists of
+angular fragments of flint and chalk, and seems to have been produced
+by streams which were able to flow over the surface of the chalk when
+it was frozen. Many other similar deposits in the south of England,
+which are found on the open surface, may have had a similar origin.
+
+[Footnote 112: C. Reid, "Origin of Dry Chalk Valleys and of Coombe
+Rock," _Quart. Journ. Geol. Soc._, vol. XLIII. p. 364.]
+
+The Palæolithic river-gravels are found at various distances above
+present river-levels, and are the surviving relics of alluvial
+deposits which were laid down when the rivers ran at a higher level
+than they now do. That they are newer than the main glacial drifts of
+the region in which they occur is indicated by the frequent presence
+in them of boulders derived from the drift. Their antiquity is shown
+by the physical changes which have occurred since their deposition
+(there having been sufficient time since then to allow of the
+excavation of some river-valleys to a depth of over one hundred feet
+beneath their former level), and also by the character of the included
+mammals which will presently be referred to. The deposits vary in
+coarseness, like those of modern alluvial flats, from the coarse
+gravels of the river-beds to the fine loams and marls of the
+flood-plains. They are found, in Britain, with their typical mammalian
+remains, south-east of a line drawn from the mouth of the Tees to the
+Bristol Channel.
+
+The cave-deposits have a wider distribution than those which have just
+been noticed, being also found to the north-west of the
+above-mentioned line in Yorkshire, and in North and South Wales. In
+the south of England they are found as far east as Ightham in Kent,
+and in a westerly direction to Torquay and Tenby. The Ightham deposits
+occur in fissures and consist of materials which were apparently
+introduced from above by river action[113]. The cave-deposits of
+limestone areas are sometimes found in fissures, but at other times in
+caverns with a fairly horizontal floor, on which the various
+accumulations lie in order of formation. The deposits vary in
+character and may be divided into three groups, though accumulations
+of intermediate character are found; the first group consists of
+cave-earths and cave-breccias--formed by weathering of the limestone,
+and the retention of the insoluble residue, as a more or less
+ferruginous mud, mixed with angular fragments of limestone, and with
+the remains of creatures which inhabited the caves; the second group
+consists of true deposits laid down under water, as gravels, sands,
+and laminated clays; while the third is composed of limestone
+deposited from solution in water, in the form of stalagmite[114].
+
+[Footnote 113: The Ightham fissures and their contents are described
+by Messrs Abbot and Newton, _Quart. Journ. Geol. Soc._, vol. L. pp.
+171 and 188.]
+
+[Footnote 114: The reader should consult Prof. W. Boyd Dawkins' works
+on _Cave Hunting_ and _Early Man in Britain_, for information
+concerning the Cave Deposits. See also Sir C. Lyell, _Antiquity of
+Man_; Sir J. Evans, _Ancient Stone Implements of Great Britain_, and
+Sir J. Lubbock, _Prehistoric Times_. In these works references will be
+found to papers by Messrs Pengelly, Magens Mello, Tiddeman and others
+on the Caves of Devon, Derbyshire and Yorkshire. References have
+already been made to papers upon the Caverns of North Wales.]
+
+The organic contents of the Palæolithic period are of much interest,
+and it is desirable to discuss their character before making further
+observations upon the physical conditions of the period.
+
+_The Palæolithic flora and fauna._ The plants of some of the earlier
+deposits of the age we are considering show the prevalence of cold
+conditions during their accumulation, for instance the Arctic birch
+and Arctic willow are found in the accumulations beneath the
+implement-bearing Palæolithic deposits of Hoxne in Suffolk[115]. The
+invertebrate fauna consists essentially of the remains of molluscs.
+The loess molluscs are chiefly pulmoniferous gastropods which lived
+upon the land, though swamp forms are occasionally associated with
+them. The palæolithic river-gravels have yielded numerous land- and
+freshwater-molluscs of living species, though some which are abundant
+in the British gravels are now extinct in Britain, e.g. _Cyrena
+(Cobicula) fluminalis_ and _Unio littoralis_. Marine deposits of this
+age are occasionally found, as at March, in Cambridgeshire, where the
+fauna closely resembles that of our present sea-shores.
+
+[Footnote 115: These beds are described by Messrs Reid and Ridley,
+_Geol. Mag._ Dec. III. vol. V. p. 441. See also C. Reid on the
+"History of the Recent Flora of Britain," _Annals of Botany_, vol. II.
+No. 8, Aug. 1888.]
+
+The vertebrate remains are much more remarkable, and it is not quite
+clear that the association of forms whose living allies now live under
+widely different conditions has been satisfactorily explained. The
+river-gravels and cave-deposits contain remains of temperate forms, as
+the bison, and brown bear, associated with those of northern forms, as
+the mammoth, woolly rhinoceros, glutton, reindeer, and musk ox, and
+also with those whose living allies are inhabitants of warmer regions,
+like the lion, hyæna, and hippopotamus. One of the most remarkable
+creatures is the sabre-toothed lion or _Machairodus_, remains of which
+have been discovered in Kent's Cavern, Torquay, and in the caves of
+Cresswell Crags, Derbyshire.
+
+The loess fauna consists of characteristic steppe animals, such as the
+jerboa, Saiga antelope and steppe-porcupine, and it is interesting to
+find an indication of this fauna in the Ightham fissures.
+
+The first undoubted relics of mankind are found in the Palæolithic
+deposits, which are very widely spread over the Eurasian continent.
+They consist mainly of implements of bone and stone, the latter being
+chipped, but never ground or polished, though both bone and stone
+implements are frequently ornamented with engraved figures. The
+cave-deposits have furnished implements of a higher type than those
+usually found in the river-drifts, but the latter are also found in
+caverns in deposits beneath those containing the higher type, hence
+the division of the period into two minor periods, that of river-drift
+man, and that of cave-man[116].
+
+[Footnote 116: Concerning this matter, the reader should consult Prof.
+Boyd Dawkins' _Early Man in Britain_. Sir J. Prestwich has argued in
+favour of the existence of a group of implements found on the plateau
+south of the Thames of an age antecedent to that of the ordinary
+river-drift implements. See _Quart. Journ. Geol. Soc._, vol. XLV. p.
+270.]
+
+There are several questions of interest connected with the Palæolithic
+fauna, three of which deserve some notice here. The absence of the
+relics of the Palæolithic mammalia and of the human implements in the
+river-gravels north-west of the line drawn between the Tees and
+Bristol Channel, and the presence of the mammalian remains in the
+caverns of that area requires some explanation. One such explanation
+assumes that the relics were destroyed in the open country to the
+north-west of that line, owing to glaciation, but it is not by any
+means universally accepted.
+
+Another difficulty which in the opinion of some writers has not been
+fully cleared up is the mixture of apparently southern forms like the
+Hippopotamus, with others of northern character like the Musk ox,
+under such conditions as to show that the creatures lived in the
+British area contemporaneously. Seasonal migration might account for
+it, but the wide belt of overlap of apparent northern and southern
+forms requires something more, though secular changes of climate might
+shift the belt of seasonal overlap from one place to another, causing
+the entire belt of overlap to extend over a considerable distance.
+
+The third, and perhaps most important difficulty is the abrupt change
+from the Palæolithic type of implement to the Neolithic type,
+characteristic of the next period. Some implements, as those of the
+kitchen-middens of Denmark, and those found at Brandon and Cissbury in
+this country, have been appealed to as intermediate in character, but
+evidence has been brought forward to show that each set is truly
+Neolithic, the one being the implements of the lowly fisher-folk who
+lived contemporaneously with the makers of the highly finished
+polished implements of Denmark, while the others are unfinished
+implements thrown away during the manufacture on account of flaws or
+accidental fractures. The difficulty is increased when we take into
+account the great physical and faunistic changes which occurred
+between Palæolithic and Neolithic times.
+
+The country was undoubtedly more elevated than it is at present during
+portions if not during the whole of Palæolithic times, as shown by the
+appearance of the great mammals in Britain, the discovery of their
+remains beneath sea-level, and especially the occurrence of remains in
+the caverns of rocky islands such as those of the Bristol Channel,
+where they could not possibly have existed unless the present islands
+were connected with the mainland.
+
+The fossils of the times between the Glacial period and the Neolithic
+period indicate variations of climatic conditions. Upon this point I
+cannot do better than quote the words of Sir John Evans in his
+Presidential Address to the British Association at Toronto[117]. "At
+Hoxne the interval between the deposit of the Boulder clay and of the
+implement-bearing beds is distinctly proved to have witnessed at least
+two noteworthy changes in climate. The beds immediately reposing on
+the clay are characterised by the presence of alder in abundance, of
+hazel, and yew, as well as by that of numerous flowering plants
+indicative of a temperate climate very different from that under which
+the Boulder clay itself was formed. Above these beds characterised by
+temperate plants, comes a thick and more recent series of strata, in
+which leaves of the dwarf Arctic willow and birch abound, and which
+were in all probability deposited under conditions like those of the
+cold regions of Siberia and North America.
+
+"At a higher level, and of more recent date than these--from which
+they are entirely distinct--are the beds containing the Palæolithic
+implements, formed in all probability under conditions not essentially
+different from those of the present day."
+
+[Footnote 117: _Report Brit. Assoc._ for 1897, p. 13.]
+
+
+
+
+CHAPTER XXIX.
+
+THE FOREST PERIOD.
+
+
+Subsequently to Palæolithic times, the physical conditions over
+Eurasia changed greatly, and at the commencement of Neolithic times
+the conditions were favourable for the growth of forests over wide
+regions of that continent. At the commencement of the Forest period
+the physical conditions were very much the same as they are at
+present, though minor changes have of course taken place since then,
+including probably a submergence of large parts of Britain to a depth
+of about fifty feet beneath its former level, as indicated by the
+existence of Neolithic submerged forests round many parts of our
+coast-lines.
+
+The Forest period may be best subdivided for local purposes by
+reference to the civilisation of mankind at different times, and in
+this way we obtain the following divisions:
+
+  Historic Iron age.
+  Prehistoric Iron age.
+  Bronze age.
+  Neolithic age.
+
+A classification may also be based upon changes in the flora. In
+Denmark the peat deposits of this age are divisible into five layers,
+characterised by different dominant forms of trees. These are as
+follows in descending order:
+
+  Fifth layer:   Beech ... Iron age
+  Fourth layer:  Alder
+  Third layer:   Oak ... Bronze age
+  Second layer:  Scotch Firs ... Neolithic age
+  Lowest layer:  Poplar.
+
+In our own country the forest growth has been much interfered with by
+man, but the lower fenland peat gives a good example of the material
+formed by forest growth. It is not necessary to touch on the various
+accumulations which are now being formed in different parts of our
+island, except to remark that the deposits of the Forest period give
+indications of earth-movements on a small scale, which is well seen in
+the fenland, where the forest peat is covered in places by a "buttery
+clay" with _Scrobicularia piperata_ indicating submergence, and above
+this is a marsh peat.
+
+The flora and fauna of the Forest period are practically those of the
+present day, though the larger forms of mammalia have disappeared one
+by one. The Irish elk and _Bos primogenius_ probably became extinct
+early in the period, while as far as Britain is concerned the wolf,
+bear, and beaver have disappeared within historic times.
+
+The relics of man deserve passing notice. The Neolithic period is
+characterised by the absence of metal instruments, though those made
+of stone were much more highly finished than those of Palæolithic
+times, and were often ground and polished. The first metal which was
+largely worked was bronze, which gradually replaced stone, though
+stone was extensively used in the Bronze age, as indicated by the
+imitation of bronze implements in stone. The Bronze age in turn was
+replaced by the Prehistoric iron age; at first, when iron was scarce,
+bronze implements were merely tipped with iron, but ultimately the one
+metal was practically replaced by the other.
+
+The date of the Palæolithic period is unknown; no approximate date can
+be satisfactorily assigned to it, but various calculations, founded on
+different data, have been made as to the age of the Neolithic period,
+and several of them agree in placing it at about 7000 years from the
+present time.
+
+It will be seen that no sudden and violent change marks the incoming
+of the human race, which to the geologist is but one of a large number
+of events which have followed each other in unbroken sequence, and
+accordingly the thread of the story where abandoned by the geologist
+is taken up by the antiquary, and passed on by him to the
+historian[118].
+
+[Footnote 118: The student may obtain information concerning the
+Neolithic age in Britain in Boyd Dawkins's _Early Man in Britain_; Sir
+J. Evans' _Early Stone Implements of Great Britain_, and Sir J.
+Lubbock's _Prehistoric Times_. In the latter work he will find a good
+account of the Neolithic remains of Denmark and of the Swiss Lake
+dwellings. For information concerning the Bronze age he should consult
+Evans' _Ancient Bronze Implements of Great Britain_. The varied Danish
+antiquities of Neolithic and Bronze ages are figured in H. P. Madsen's
+_Antiquités Préhistoriques du Danemark_. The Prehistoric fauna of the
+fenlands is described in Sir R. Owen's _History of British Fossil
+Mammals and Birds_.]
+
+
+
+
+CHAPTER XXX.
+
+REMARKS ON VARIOUS QUESTIONS.
+
+
+There are many problems connected with geology which can only be
+solved by detailed study of the stratified rocks, and when solved the
+principles of the science will be more fully elucidated. In the
+present state of our knowledge some of these problems are ripe for
+discussion, others can merely be indicated, while others again have
+probably remained hidden, though it will be the task of the geologist
+of the future to clear them up. Among the many questions which demand
+knowledge of stratigraphical geology for their right understanding are
+the following, which will be briefly considered in this chapter:--the
+changes in the position of land and sea in past times, and the growth
+of continents; the replacement of a school of uniformitarianism by one
+of evolutionism; and the duration of geological time.
+
+_Changes in the position of land and sea._ Certain physicists have
+arrived at the conclusion that the general position of our oceans and
+continents was determined at a very early period in the earth's
+history, and that the changes which have occurred in their position
+since then have been comparatively insignificant. The wide extent of
+land over which stratified rocks are distributed at once indicates
+that from the point of view of the geologist the changes have been
+very important, and it is worth inquiring whether they are not
+sufficiently important to prove that the primitive oceans and
+continents have undergone so much alteration as to be unrecognisable.
+Some authorities, while recognising the great changes which have
+occurred in the relative position of land and sea during those periods
+of which geologists have direct information, suppose that the changes
+took place to a large degree in certain 'critical areas' bordering the
+more stable areas of permanent ocean on the one side and permanent
+land on the other.
+
+In discussing the question of general permanence of land and ocean
+regions it will be convenient to commence with a study of the present
+land areas, and at the outset we may take into consideration the
+present distribution of marine sediment over different parts of the
+land, using the last edition of M. Jules Marcou's geological map of
+the world for the purpose[119]. A glimpse at this map indicates that
+more than half of the land areas are occupied by rocks which are as
+yet unknown (many of which _may_ be marine sediments), or by
+crystalline schists of which the mode of origin has not yet been fully
+explained, so that a large part of Central Asia, the interior of
+Africa, and of South America may have existed as land from very early
+times, and the same may be said of smaller portions of Europe and
+North America. Actual observation of a geological map therefore
+indicates the possibility that about half of the land surfaces may
+have existed as such through very long periods, but though there is a
+possibility of this, the probability is not very great. The unknown
+regions, as remarked above, may consist to a considerable extent of
+marine sediments, and the existence of isolated patches of late
+Palæozoic and of Mesozoic strata in the heart of Central Asia, points
+to the submergence of much wider regions than those in which these
+isolated patches have been found. Again, the character of the
+sediments when they abut against the crystalline schists frequently
+proves that these sediments once extended further over the crystalline
+schists, and have since been removed by denudation, so that even if we
+assume that the crystalline schists are all of very early date, and
+not necessarily formed in any case from marine sediments, we cannot
+suppose that all the area occupied by them has existed as land for
+long periods of time. On the other hand, the major part of Europe and
+North Africa, extensive tracts in Asia, the greater part of Australia,
+a very large part of North America and considerable tracts of South
+America give proofs of having been occupied by the oceans in Palæozoic
+and later times.
+
+[Footnote 119: A reduced copy of this map will be found opposite the
+title-page of the first volume of Prof. Prestwich's _Geology_.]
+
+It may be answered that most of these regions containing marine
+sediments occur in critical areas, which have undergone a certain
+amount of oscillation owing to earth-movements, and that the interior
+parts of the great continental masses have been practically
+stationary. But if these lands had been land-areas through geological
+ages they must have been acted upon by the agents of subaerial
+denudation, throughout these ages, and long ago reduced to
+peneplains[120] unless the action of these subaerial agents was
+counteracted by that of elevating forces, but if these forces were
+sufficient to counteract the action of subaerial denudation through
+countless ages, they were also sufficient to raise extensive tracts
+of land above sea-level, and materially to alter the distribution of
+land and sea, and if elevation could go on to this extent, why not
+also depression?
+
+[Footnote 120: A term proposed by Prof. W. M. Davis for a nearly level
+surface of subaerial denudation, as opposed to a plain of marine
+denudation.]
+
+Proceeding a step further, and examining the character of the
+sediments as well as their geographical distribution, we find
+further evidence of great crust-movements. It has been urged that
+deep-water sediments do not occur amongst the strata found on the
+continents,--that there are no representatives of the abysmal deposits
+of recent ocean floors amongst the strata of the geological
+column[121], but the researches of the last two decades have brought
+to light foraminiferal and radiolarian deposits, pteropodal deposits,
+and possibly deep-sea clays, which are comparable with those in
+process of formation at great depths in existing oceans, and though
+the proofs of their deep-sea origin are not always as full as might be
+desired in the case of the older rocks[122], we can speak with greater
+certainty when we examine those of Tertiary age, and if the deep-sea
+accumulations of this late date can be uplifted above sea-level, this
+is much more likely to have occurred with those of past times. When a
+deposit like the radiolarian rock of Barbadoes, the deep-water
+character of which has been conclusively proved, can be elevated into
+land since Miocene or possibly Pliocene times, it is evident that the
+crust-movements have been sufficient to produce the most profound
+changes in the distribution of land and sea during the long ages which
+are known to us. Another argument against the occurrence of extensive
+changes has been derived from an examination of those islands which
+are spoken of as oceanic islands. Strictly speaking an oceanic island
+is one in which the present fauna and flora give indications of their
+introduction by transport across intervening sea, and no indications
+of the existence of forms of life which inhabited it when it was once
+united to a continent; it may be inferred with a considerable degree
+of certainty that these islands have been isolated for long periods of
+time. It has been stated that these oceanic islands never contain
+marine sediments of any considerable degree of antiquity, and that
+there are therefore no traces of former continents over those wide
+tracts of ocean which are occupied by oceanic islands. The evidence is
+of a negative character. The islands would be less likely to exhibit
+ancient sediments than continents, for being near the ocean, they
+would be readily submerged, and the older deposits masked by newer
+ones, though this need not necessarily account for the entire absence
+of ancient rocks amongst them. The danger of the argument lies in the
+fact that we do not yet know how far these old rocks really are
+absent, as the geology of the oceanic isles has not been fully
+explored from this point of view, and already several cases of the
+asserted presence of ancient rocks on these islands have been
+recorded.
+
+[Footnote 121: See Mr A. R. Wallace's _Island Life_.]
+
+[Footnote 122: See chapter IX.]
+
+The argument derived from the present distribution of organisms is far
+too complex to be discussed here, and the student is recommended to
+read a masterly review of the evidence in Dr W. T. Blanford's
+Presidential Address to the Geological Society in 1890, on the
+question of the Permanence of Ocean Basins[123]. After reviewing the
+evidence furnished by a study of modern distribution he concludes that
+it "is far too contradictory to be received as proof of the permanence
+of oceans and continents."
+
+[Footnote 123: _Quart. Journ. Geol. Soc._, vol. XLVI., _Proc._, p.
+59.]
+
+The existence of former extensive land tracts over regions now
+occupied by sea is naturally more difficult to prove than that of sea
+over land, as we depend upon inference rather than actual observation
+to a much greater degree than when considering the permanence of
+continents, nevertheless a considerable amount of indirect evidence in
+favour of the existence of widespread land tracts over our present
+ocean regions has been accumulated and will be briefly noticed. We may
+take first the evidence derived from the nature of sediments, and
+afterwards that which has been acquired by studying distribution of
+organisms in past times.
+
+The indications of existence of an extensive tract of continent over
+the North Atlantic Ocean, during Palæozoic times have already been
+considered, and it was seen that the thinning out of the Palæozoic
+sediments when traced away from the present Atlantic borders in an
+easterly direction over Europe and in a westerly one over North
+America pointed to the existence of this Palæozoic 'Atlantis,' as
+maintained by Prof. Hull in his work, "Contributions to the Physical
+History of the British Isles." This writer gives some reasons for
+supposing that the continental mass began to break up towards the end
+of Palæozoic times, though it is not clear that complete replacement
+of land by sea occurred, and the nature of the Wealden deposits has
+been pointed to as evidence of the existence of an extensive tract of
+land to the west of Britain during the Cretaceous period.
+
+The Palæontological evidence in favour of destruction of ancient
+continental areas and their replacement by the sea is more
+satisfactory than that which is based on physical grounds. The
+distribution of the Glossopteris flora of the Permo-Carboniferous
+period points to the former existence of a great southern continent,
+including the sites of Australia, India, South Africa and South
+America,--the Gondwanaland of Prof. E. Suess[124].
+
+[Footnote 124: On this question and that of the other destroyed
+continental areas noted here, see W. T. Blanford's _Presidential
+Address_, _loc. cit._]
+
+Again, a study of Jurassic and Cretaceous faunas has led
+palæontologists to conclude that there was a connexion betwixt S.
+Africa and India in Mesozoic times across a portion of the area now
+occupied by the Indian Ocean, and also between S. Africa and S.
+America, and these inferences are supported by study of the
+distribution of existing forms.
+
+The sudden appearance of the Dicotyledonous Angiosperms in Upper
+Cretaceous rocks has also been used as evidence of destruction of
+considerable tracts of land subsequently to Upper Cretaceous times,
+and there is a certain amount of evidence in favour of the existence
+of this land in the north polar region, in an area now largely
+occupied by water, though relics of it are left, as the Faroe Isles,
+Spitsbergen, Novaya Zembla and Franz Josef Land.
+
+I cannot conclude the consideration of the question of permanence of
+oceans and continents more fitly than by quoting from Dr Blanford's
+address. He says, "There is no evidence whatever in favour of the
+extreme view accepted by some physicists and geologists that every
+ocean-bed now more than 1000 fathoms deep has always been ocean, and
+that no part of the continental area has ever been beneath the deep
+sea. Not only is there clear proof that some land-areas lying within
+continental limits have at a comparatively recent date been submerged
+over 1000 fathoms, whilst sea-bottoms now over 1000 fathoms deep must
+have been land in part of the Tertiary era, but there are a mass of
+facts both geological and biological in favour of land-connexion
+having formerly existed in certain cases across what are now broad and
+deep ocean[125]."
+
+[Footnote 125: _Loc. cit._, _Proc._ p. 107.]
+
+_Growth of continents._ Whatever view as to the general permanence of
+continents and oceans be ultimately established, the occurrence of
+widespread changes in the position of land and sea is indisputable,
+and it is of interest for us to consider the nature of these changes
+in the formation of continents. Prof. J. D. Dana has put forward a
+hypothesis of growth of continents by a process of accretion, causing
+diminution in the oceanic areas, which at the same time became deeper:
+such growth need not always take place in exactly the same way, and
+study of the distribution of the strata of the North American
+continent suggests that the growth there was endogenous, the older
+rocks lying to the west and north forming a horseshoe shaped continent
+enclosing a gulf-like prolongation of the Atlantic, which became
+contracted by deposition and uplift in successive geological periods,
+though it is still partly existent as the Gulf of Mexico. The Eurasian
+continent, especially its western portion, suggests more irregular
+growth around scattered nuclei of older rocks, though the process is
+not completed, and many gulf-like prolongations, as the Baltic and the
+Mediterranean, still remain as water-tracts, which have not yet been
+added to the continents.
+
+Although extensive additions to continents may be and no doubt are
+often largely due to epeirogenic movements, the influence of orogenic
+movements on continent-formation is very pronounced. As the result of
+orogenic movements, the rocks of portions of the earth's crust become
+greatly compressed, and give rise to masses which readily resist
+denudation; moreover, these comparatively rigid masses, as shown by M.
+Bertrand, tend to undergo elevation along the same lines as those
+which formed the axes of previous elevations, and accordingly after a
+continental area has undergone denudation for a considerable period,
+the uplands consist of rocks which have undergone orogenic
+disturbance, while the tracts of ground which are occupied by rocks
+which have not suffered disturbances of this character, even if
+originally uplifted far above sea-level, tend to be destroyed, and
+ultimately occupied by tracts of ocean. Stumps of former mountain
+chains may be again and again established as nuclei of continents and
+as every period of orogenic movement will add to the number of these
+nuclei, the continental areas must in course of time become more
+complex in structure. Moreover, as some areas are affected by orogenic
+movements to a greater extent than others, the complexity of different
+continental masses will vary. Thus, western Europe has been affected
+by orogenic movements during many periods since the commencement of
+Cambrian times and its structure is extremely complex, while the
+central and western parts of Russia have not been subjected to violent
+orogenic disturbances since Cambrian times, and accordingly we find
+the structure of that area comparatively simple; the greater part of
+Africa seems to have escaped these movements since remote times, and
+the structure of that continent is extremely simple when compared with
+the Eurasian continental tract. It need hardly be stated that the
+formation of extensive chains composed of volcanic material, by
+accumulation of lavas and ashes on the earth's surface, may give and
+often has given rise to more rigid tracts, which will bring about the
+same effects as those produced by orogenic disturbance as illustrated
+on a small scale by the Lower Palæozoic volcanic rocks of Cambria and
+Cumbria.
+
+_Uniformitarianism and Evolution._ According to the extreme
+uniformitarian views held by some geologists, the agents which are in
+operation at the present day are similar in kind and in intensity to
+those which were at work in past times, though no geologist will be
+found who is sufficiently bold to assert that this holds true for all
+periods of the earth's history, but only for those of which the
+geologist has direct information derived from a study of the rocks,
+and he is content to follow his master Hutton in ignoring periods of
+which he cannot find records amongst the rocks. The modern geologist,
+however, while rightly regarding the rocks as his principal source of
+information finds that he cannot afford to ignore the evidence
+furnished by the physicist, chemist, astronomer and biologist, which
+throws light upon the history of periods far earlier than those of
+which he has any records preserved amongst the outer portions of the
+earth itself, just as the modern historian is not content with written
+records, but must turn to the 'prehistoric' archæologist and geologist
+for information concerning the history of early man upon the earth.
+Interpreting the scope of geology in this general way, rigid
+uniformitarianism must be abandoned. Assuming that the tenets of the
+evolutionist school are generally true, the question is, how far does
+this affect the geologist in his study of those periods of which we
+have definite records amongst the rocks? This is a question which
+cannot readily be answered at the present day, for our study of the
+rocks is not sufficiently far advanced to enable us to point out
+effects amongst the older rocks which were clearly caused by agents
+working with greater intensity than they do at present, but as, on
+the other hand, we cannot prove that these effects are due to agents
+working with no greater intensity than that which now marks these
+operations, it is unphilosophical to assume the latter. No student of
+science at the present day would state that because there has been no
+observed case of incoming of fresh species within the time that man
+has actually observed the present faunas and floras, the hypothesis of
+evolution of organisms is disproved, for the time of observation has
+been too short, and similarly the time which has elapsed since the
+formation of, say, the Cambrian rocks may have been too short, as
+compared with the time which has elapsed since the formation of the
+earth, to allow of any important change in the operation of the
+geological agents.
+
+Leaving out of account, for the moment, the actual evidence which has
+been derived from a study of the rocks, we may briefly consider the
+theoretical grounds upon which the substitution of an evolutionist
+school of geology for one of uniformity has been suggested[126]. The
+principal sources of energy which have exerted an influence upon
+geological changes are the heat received from the sun and that given
+off from the earth itself, both of which must have diminished in
+quantity throughout geological ages. To the former source we largely
+owe climatic changes and the operations of denudation, and accordingly
+of deposition; to the latter, those of earth-movement and vulcanicity.
+It by no means follows that because the agents were once potentially
+more powerful than now, they would necessarily produce greater
+effects, for that depends to some extent upon the various conditions
+which prevailed at different times. To give an example:--if there had
+at any time been a universal ocean of considerable depth, however
+active the agents of denudation were then, they could produce no
+effect whatever, having nothing to work upon; to take a less extreme
+case, if our continents at any past time were smaller and less
+elevated than at present, agents of denudation working with greater
+intensity than that of the present agents need not necessarily have
+produced a greater amount of denudation than that which is going on at
+the present day. Again, let us consider vulcanicity: "It is as
+certain," says Lord Kelvin, "that there is less volcanic energy in the
+whole earth than there was a thousand years ago, as it is that there
+is less gunpowder in a 'Monitor' after she has been seen to discharge
+shot and shell, whether at a nearly equable rate or not, for five
+hours without receiving fresh supplies than there was at the beginning
+of the action." But it does not follow that the manifestations of
+volcanic activity were necessarily more violent in early geological
+times than now, for the degree of violence would be affected by other
+things than the volcanic energy, such as the thickness of the earth's
+crust.
+
+[Footnote 126: The student may consult an interesting article by Prof.
+Sollas bearing on this subject. See _Geol. Mag._ Dec. 2, vol. IV. p.
+1.]
+
+And now, let us consider briefly the characters of the rocks of the
+crust, to see if they throw any light upon this question. The earliest
+sediments of which we have any certain knowledge resemble in a
+striking manner those formed at the present day, and they seem to have
+been formed under very much the same conditions, though further work
+may show that there were somewhat different conditions which did
+produce definite differences in the characters of the earlier
+strata[127]. Our knowledge of earth-movement and vulcanicity which
+took place in past times is still too small to enable us to draw any
+certain conclusions connected with the subject under discussion from
+it. Perhaps the most suggestive indication of one set of conditions
+having been generally similar in those early periods of which we have
+definite records amongst the rocks is furnished by study of past
+climate. If we accept the nebular hypothesis as a starting point, we
+must admit that in the early stages of the earth's history the
+temperature of the surface, which would then be largely dependent upon
+the amount of heat given out from the earth itself as well as upon
+that received from the sun, must have been much higher than it is at
+the present day, and indeed the mere diminution of the amount of heat
+received from the sun would probably be sufficient to account for a
+very marked lowering of the temperature. Besides this change of
+temperature, resulting in gradual lowering of temperature over the
+whole earth's surface, we have other changes dependent upon different
+conditions, as proved by the fact, that there have been alternations
+of glacial and genial periods. If the general temperature had been
+very high in the early periods of which we have actual records, the
+oscillations would not be sufficient to produce a lowering of
+temperature sufficient to cause glacial periods, whereas if it had not
+been appreciably higher than now, glacial periods might be produced.
+This may be represented diagrammatically.
+
+[Footnote 127: On this matter see Teall, J. J. H., 'Presidential
+Address to Section C,' _Report of the British Association_, 1893.]
+
+Let _a_ represent the temperature at the commencement of earth-history
+and _b_ that necessary for glaciation, and _bc_ the lapse of time
+between then and now. The curved line indicates the gradual fall in
+temperature due to diminution of the amount of heat, while the zigzag
+line represents the oscillations due to secular climatic changes. If
+the Cambrian period x occurred comparatively soon after the
+commencement of earth-history as shown in fig. _A_, no glaciation
+could be produced, even during periods when secular changes caused
+colder conditions than the mean, whereas if the Cambrian period
+occurred at a time very remote from the commencement of earth-history
+as shown in _B_, glacial conditions could be produced then as now, for
+the mean temperature, as shown by the distance of the curve from the
+line _bc_, would be practically as it now is. The studies of the last
+few decades have brought into prominence the occurrence of glacial
+periods in remote times, probably in early Palæozoic times; and as far
+as the mean temperature of the earth's surface is concerned, it would
+appear, from the knowledge in our possession, that matters were not
+very different in those early times from what they now are.
+
+[Illustration: Fig. 25.]
+
+Some further remarks will be made in subsequent paragraphs concerning
+the period of the earth's history at which the geologist is first
+furnished with definite records, but in the meantime it may be
+observed that the geologist will do well, when working amongst the
+strata, to consider that the more active operation of agents, even in
+times of which he has definite knowledge, may have produced effects
+which he should be prepared to discover, as their discovery would be
+of considerable importance, and that he should not be content to infer
+that because it has been proved that agents operating with the same
+intensity as that which they have at present, _may_ have produced all
+the effects which he can actually observe, they therefore necessarily
+_did_ produce them.
+
+_Recurrences._ Absolute uniformity of conditions is impossible, even
+in a single area. Every change which takes place upon the earth
+produces conditions somewhat dissimilar from those which previously
+existed, and these will leave their effects upon the physiography of
+the area. For this reason, assuming that the conditions have gradually
+changed from simpler to more complex, every period of time will have
+been marked by conditions which never prevailed before or afterwards,
+and these will leave their impress upon the deposits of the period. It
+is doubtful for instance, as already remarked, whether the exact
+conditions which gave rise to the extensive deposits of vegetable
+matter in Carboniferous times which now form coal, ever occurred to a
+like extent in previous or subsequent periods, and accordingly, though
+we have deposits of coal of other ages, none are so extensive as those
+of the Coal Measures. Again, as the strata of one period are largely
+composed of denuded particles of pre-existing strata, which were
+derived directly or indirectly from igneous rock, the soluble material
+existing in the igneous rocks must have been gradually eliminated
+unless restored by other processes, and we might expect to find that
+early sediments have, on the whole, a larger proportion of soluble
+silicates than the later ones.
+
+Besides these changes, there are physical changes which are recurrent,
+and cause conditions generally similar to pre-existing ones to occur
+in an area after an interval of dissimilar ones. We have seen that
+deposits tend to vary according to the distance from the coast,
+limestone being succeeded by mud, this by sand and gravel, and after
+subsidence the sand and gravel are succeeded by mud, and that by
+limestone. These changes will produce some effect upon the organisms,
+and the recurrence of organisms is a well-known event, of which cases
+have been cited in a former chapter.
+
+Again we find, as already pointed out, recurrence of climatic changes,
+with alternation of glacial and warmer periods, and these may have
+been very widespread, and would influence the other physical
+conditions, as well as the distribution of the organisms. Vulcanicity
+may have been more rife at some periods than others, for instance
+there seems, in the present imperfect state of our knowledge, evidence
+of enfeebled vulcanicity in later Mesozoic times, and of its renewed
+activity in Tertiary times. Again, orogenic movements seem to have
+occurred more extensively at some times than others, as for instance
+in early upper Palæozoic times, at the end of the Palæozoic epoch, and
+in early Tertiary times, though this may also be an apparent and not
+an actual truth, due to imperfect knowledge. In any case, in limited
+areas, there seem to have been alternations of periods of uplift
+accompanied by marked orogenic movements, and of widespread
+depression, accompanied by sedimentation.
+
+The subject of rhythmic recurrence is worthy of further study. This
+recurrence in combination with evolutionary change may account for the
+apparent marked difference between Cambrian and Precambrian times, a
+difference which strikes some geologists as being too great to be
+accounted for as due to our ignorance only.
+
+_Organic evolution._ This subject is too wide for more than passing
+notice in a work of this character. The evidence of Palæontology is of
+extreme importance to the biologist, and indeed, the way in which
+evolution of organisms has occurred can only be actually demonstrated
+by reference to Palæontology, and the study of Palæontology has
+already given much information concerning the lines on which evolution
+has proceeded in different groups of organisms. It must be remembered
+that the major divisions of the invertebrata were in existence in very
+early times; indeed representatives of most of them are found in the
+rocks containing the earliest known fauna, that of the _Olenellus_
+beds of Cambrian age. If our present views as to evolution be correct,
+there is no doubt that the period which elapsed between the appearance
+of life upon the globe and the existence of the _Olenellus_ fauna must
+have been very great, possibly, as Huxley suggested, much greater than
+that which has elapsed between early Cambrian times and the present
+day. If this be so, however probable it is that we shall carry our
+knowledge of ancient faunas far back beyond Cambrian times, it is
+extremely improbable that we shall ever get traces of the very
+earliest faunas which occupied our earth.
+
+_Geological time._ Various attempts have been made to give numerical
+estimates of the lapse of time which occurred since the earth was
+formed, or since the earliest known rocks were deposited. These
+attempts may be classed under two heads, namely, those made by
+physicists, mainly on evidence obtained otherwise than by a study of
+the rocks, and those made by geologists by calculating the mean rate
+of denudation and deposition of the rocks, and estimating the average
+thickness of the rocks of the geological column.
+
+The calculations of physicists as to the age of the earth vary:--Lord
+Kelvin assigned 20,000,000 years as the minimum and 100,000,000 as the
+maximum duration of geological time. Prof. Tait has halved Lord
+Kelvin's minimum period, while Prof. G. Darwin admits the possibility
+of the lapse of 500,000,000 years.
+
+The estimates made by geologists, which will appeal more directly to
+the geological student, also vary considerably, though they bear some
+proportion to those which have been put forward by the physicists.
+Prof. S. Haughton[128] assigned a period of 200,000,000 years for the
+accumulation of the rocks of the geological column; Mr Clifton
+Ward[129] one of 62,000,000 years, after studying the rocks of the
+English Lake District, and allowing for the gaps in the succession; Mr
+A. R. Wallace[130] further lowers the time for the formation of the
+column to 28,000,000 years; Sir A. Geikie[131] gives 73,000,000 years
+as the minimum and 680,000,000 as the maximum; while Mr J. G.
+Goodchild has lately[132] estimated the period at over 700,000,000
+years.
+
+[Footnote 128: _Nature_, vol. XVIII. p. 268.]
+
+[Footnote 129: Ward, J. C., 'The Physical History of the English Lake
+District,' _Geol. Mag._ Dec 2, vol. VI. p. 110.]
+
+[Footnote 130: Wallace, A. R., _Island Life_, Chap. X.]
+
+[Footnote 131: Geikie, Sir A., 'Presidential Address to the British
+Association,' _Report Brit. Assoc._, 1892.]
+
+[Footnote 132: Goodchild, J. G., _Proc. Roy. Soc. Edinburgh_, vol.
+XIII. p. 259.]
+
+Interesting as these figures are, they probably convey little to the
+ordinary reader, and it is doubtful whether the geologist is really
+affected by them to any extent when picturing to himself the vast
+duration of geological time. One numerical estimate probably does
+impress him, namely that made by Croll as to the date of the Great Ice
+Age, for if the Ice Age be so remote as Croll imagined, the
+commencement of earth-history must be inconceivably more remote; as
+Croll's estimate is not generally accepted, it is doubtful how far
+geologists are thus influenced, and probably the fact which does
+impress them most, leaving fossils out of account, is the very little
+change which has occurred in historic or even in prehistoric times as
+compared with the vast changes which are familiar to them after
+studying the strata of the geological column.
+
+It is, after all, the succession of varied faunas which really gives
+students of the rocks the most convincing proof of the vast periods of
+geological time. If anyone doubts this assertion, let him consider
+what impression would be made upon him by observing the several
+thousand feet of strata of the column if none of them contained any
+organisms. Cognisant as he is of the slow rate of change of existing
+organisms, the fact that fauna has succeeded fauna in past times
+brings home to him in an unmistakeable manner the great antiquity of
+the earliest fossiliferous rocks, and as our detailed knowledge of
+these faunas increases the impression of great lapse of time is
+intensified. And if the earliest fossiliferous rocks be of such vast
+antiquity, and, as has been remarked, the period of their formation is
+comparatively recent with reference to the actual commencement of
+earth-history, the latter must indeed be inconceivably remote, and
+numerical estimates can do but little to familiarise us with the
+significance of the vast time which has rolled by since the world's
+birthday.
+
+
+
+
+INDEX.
+
+
+  Abraum salts, 212
+  Æolian rocks, 24, 99, 100
+  Age, definition of, 60
+  Albian series, 236, 238
+  Algonkian rocks, 144
+  Ampthill clay, 232
+  Angelin, N. P., 161, 162, 165
+  Aptian series, 236, 237
+  Aqueous rocks, 24
+  Archæan rocks, 132
+  Ardmillan series, 170
+  Ardwick stage, 192
+  Arenaceous rocks, 29
+  Arvonian rocks, 141
+  Asaphus fauna, 165
+  Ashgill series, 164, 165, 167-169
+  Ashprington series, 184
+  Astian series, 256
+  Atlantis, 283
+  Aveline, W. T., 164
+  Aymestry limestone, 175, 176
+
+  Bagshot beds, 244, 246
+  Bajocian series, 227, 231
+  Bala limestone, 167
+  Bala series, 164
+  Barr series, 170
+  Barrande, J., 53, 55, 159, 161, 163
+  Barrois, C., 239
+  Barrow, G., 138
+  Barton beds, 244
+  Bath oolites, 226
+  Bathonian series, 227, 231
+  Bed, 27
+  Bedding plane, 27
+  Bell, A., 257
+  Belt, T., 153, 162
+  Bembridge beds, 251
+  Bertrand, M., 87, 286
+  Birkhill shales, 177
+  Black Jura, 226
+  Blake, J. F., 138-140
+  Blanford, W. T., 206, 208, 217, 282, 284
+  Bonney, T. G., 76, 141, 142
+  Boulder clay, 262
+  Bracklesham beds, 244
+  Bradford clay, 230
+  Break, palæontological, 61;
+    physical, 60
+  Bristow, H., 239
+  Brockram, 211
+  Brögger, W. C., 161-163
+  Brongniart, H., 18
+  Brongniart, C., 200
+  Bronze age, 275-277
+  Brown Jura, 226
+  Bunter sandstone, 218, 220-222
+  Bure valley beds, 256
+  Buttery clay, 276
+
+
+  Caerfai beds, 152, 154, 156
+  Calcareous rocks, 29
+  Caldicote series, 139
+  Callaway, C., 138-140
+  Callovian series, 227, 232
+  Cambrian faunas, 158-163
+  Cambrian system, 152-163
+  Caradoc series, 165, 168-171
+  Carbonaceous rocks, 29
+  Carboniferous fauna and flora, 199-201
+  Carboniferous limestone, 192, 194, 195
+  Carboniferous system, 192-201
+  Carnic beds, 225
+  Cataclastic rocks, 24
+  Cave man, 268
+  Cenomanian series, 236
+  Ceratopyge fauna, 162
+  Chalk, 236, 238, 239
+  Chalk marl, 236
+  Chemically-formed rocks, 29, 101
+  Chillesford crag, 256
+  Chronological terms, 60
+  Clastic rocks, 24
+  Climatic conditions, 103, 112, 290, 291
+  Climatic zones, in Jurassic times, 233;
+    in Cretaceous times, 241
+  Clymenian beds, 183
+  Coal, 196-199
+  Coal measures, 192;
+    mode of formation of, 195-199
+  Coblenzian beds, 184
+  Collyweston slate, 231
+  Colonies, theory of, 55
+  Contemporaneity of strata, 48
+  Continents, growth of, 285-287
+  Cope, E., 249
+  Corallian series, 227, 232
+  Coralline crag, 256, 257
+  Cornbrash, 230
+  Cornstones, 186
+  Coutchiching series, 144
+  Crags, 256-259
+  Cretaceous fauna and flora, 241-243
+  Cretaceous system, 236-243
+  Croll, J., 265, 295, 296
+  Cromer Forest series, 100, 256, 259
+  Cromer till, 262
+  Cucullæa beds, 183
+  Cuvier, Baron G., 18, 20
+
+
+  Dalradian rocks, 137
+  Dana, J. D., 285
+  Danian series, 236
+  Darwin, C., 20, 76
+  Darwin, G., 295
+  Daubrée, A., 88
+  David, T. W. E., 206
+  Davis, W. M., 258, 280
+  Dawkins, W. B., 266, 268, 270, 272, 277
+  Deep-sea deposits, 109
+  De Hayes, G. P., 19
+  De la Beche, Sir H., 92
+  Deposition, order of, 37, 116
+  Derivative rocks, 23
+  Devonian flora and fauna, 189-191
+  Devonian system, 183-191
+  Dictyograptus fauna, 162
+  Dimetian rocks, 141
+  Dogger, 226
+  Downtonian beds, 175
+  Dwyka conglomerate, 206
+
+
+  Edwards, F. E., 250
+  Eifelian beds, 184
+  Encrinurus fauna, 185
+  Englacial deposits, 261
+  Entomis slates, 183
+  Eocene fauna and flora, 248, 249
+  Eocene rocks, 244-250
+  Eozoon canadense, 143
+  Eparchæan rocks, 132
+  Epeirogenic movements, 32
+  Epiclastic rocks, 24;
+    simulation by cataclastic rocks, 38, 80
+  Epoch, definition of, 60
+  Estuarine series, 230
+  Etheridge, R., 19
+  Ettingshausen, Baron von, 250
+  Evans, Sir J., 266, 270, 274, 277
+  Evolution, 287, 293
+
+
+  Feistmantel, O., 208
+  Fenland, 276
+  Fluvio-glacial deposits, 261
+  Foreland grits, 184
+  Forest marble, 230
+  Forest period, 260, 275-277
+  Fossils, 42;
+    strata identifiable by, 40;
+    mode of occurrence of, 44;
+    relative value of, 47;
+    remanié, 52;
+    geographical distribution of, 55;
+    as indicative of physical conditions, 104
+  Fossil zone, 67
+  Foster, C. Le N., 239
+  Fox, H., 195
+  Freshwater deposits, 104;
+    distinction from marine, 105
+  Fuller's earth, 230
+  Fusulina beds, 201
+
+
+  Gala beds, 177
+  Gannister stage, 192
+  Gardner, J. S., 250
+  Gault, 236, 238
+  Geikie, Sir A., 60, 84, 95, 125, 130, 137, 141, 142, 144, 186, 188,
+    199, 247, 295
+  Geikie, J., 263
+  Girvan type, 170
+  Glacial deposits, permo-carboniferous, 206;
+    Pleistocene, 260-266
+  Glacial period, 260-266
+  Glenkiln shales, 169, 170
+  Glossopteris flora, 207, 208, 214
+  Godwin-Austen, R. A. C., 20
+  Gondwana series, 207
+  Gondwanaland, 207, 284
+  Goniatite beds, 183
+  Goodchild, J. G., 87, 130, 263, 295
+  Great ice age, 295, 296
+  Great oolite, 230, 231
+  Gregory, J. G., 258
+  Green, A. H., 122, 139, 193
+  Greensand, Lower, 236;
+    Upper, 236
+  Groom, T. T., 178
+  Gshellian beds, 193, 201
+
+
+  Hampshire basin, 245
+  Hangman grits, 184
+  Harker, A., 30, 88
+  Harkness, R., 161
+  Harmer, F. W., 258
+  Harpes fauna, 175
+  Harrison, W. J., 130
+  Hartfell shales, 169, 170
+  Hastings sands, 236, 237
+  Haughton, S., 295
+  Headon beds, 251
+  Heim, A., 32
+  Hempstead beds, 251
+  Hercynian systems of folds, 203
+  Hicks, H., 134, 141, 153, 154, 160, 161, 163, 167, 184, 266
+  Hickson, S. J., 109
+  Hill, A., 239
+  Hill, E., 142
+  Hilton shales, 210, 211
+  Hind, W., 196
+  Hinde, G. J., 169, 195
+  Hippurite limestone, 241, 242
+  Hirnant limestone, 167
+  Homotaxis, 48
+  Hughes, T. McK., 141, 264, 266
+  Hull, E., 120, 122, 193, 283
+  Hume, W. F., 239
+  Hunt, A. R., 101
+  Huronian system, 143
+  Hutton, J., 287
+  Huxley, T. H., 50, 250
+
+
+  Igneous rocks, 21-23
+  Ilfracombe beds, 184
+  Inferior oolite, 230
+  Inverted strata, 32;
+    detection of, 32
+  Iron age, 275, 276
+
+
+  Judd, J. W., 239, 247
+  Jukes, J. B., 84
+  Jukes-Browne, A. J., 126, 239, 264
+  Jurassic beds, 225
+  Jurassic fauna and flora, 234, 235
+  Jurassic system, 226-235
+
+
+  Kayser, E., 125, 191
+  Keewatin series, 144
+  Kelvin, Lord, 289
+  Kendall, P., 257
+  Keuper beds, 218, 221, 222
+  Kidston, B., 199
+  Kimmeridge clay, 232
+  Kimmeridgian series, 226
+  King, W., 217
+  Kjerulf, Th., 88
+  Koninck, L. de, 201
+  Kupferschiefer, 209
+
+
+  Lake, P., 126, 178
+  Lamina, 27
+  Lamplugh, G. W., 80, 119, 264
+  Lapworth, C., 32, 138, 139, 156, 168-170, 173, 178, 179
+  Laurentian rocks, 143
+  Lawson, A. C., 144, 145
+  Lehmann, J., 77
+  Lenham beds, 257
+  Lewis, H. C., 263
+  Lias, 226, 229
+  Liassian series, 227, 229
+  Lincolnshire limestone, 230, 231
+  Lincombe and Warberry grits, 184
+  Lindström, G., 114
+  Lingula flags, 152, 155, 156
+  Linnarsson, J. G. O., 161
+  Llandeilo limestone, 167
+  Llandeilo series, 165, 167
+  Llandovery series, 174-177
+  Loess, 267
+  Logan, Sir W., 20
+  London Basin, 245
+  London clay, 113, 244, 246
+  Longmyndian rocks, 138
+  Lower London Tertiary beds, 244, 246
+  Lubbock, Sir J., 270, 277
+  Ludlow series, 174-176
+  Lydekker, R., 250
+  Lyell, Sir C., 6, 12, 19, 106, 129, 224, 263, 270
+  Lynton slates, 184
+
+
+  McCoy, Sir F., 201
+  McMahon, C. A., 77
+  Madsen, H. P., 277
+  Magnesian Limestone, 209-211
+  Malm, 226
+  Maps, geological, 84, 130;
+    use of, 86, 120, 121
+  Marcou, J., 130, 279
+  Marine deposits, 102;
+    nature of fossils in, 107
+  Marl slate, 209, 210
+  Marlstone, 229
+  Marsh, O. C., 249
+  Marwood beds, 183
+  Matthew, G. F., 160-162, 180
+  Meadfoot sands, 184
+  Mechanically formed rocks, 29, 102
+  Mello, J. M., 270
+  Mendip system of folds, 203
+  Menevian beds, 152, 154, 156, 161
+  Metamorphic rocks, 25
+  Miall, L. C., 122
+  Michell, J., 10, 11
+  Millepore oolite, 230, 231
+  Miller, H., 189
+  Millet seed sands, 100
+  Millstone grit, 192
+  Miocene period, 252-255
+  Moffat shales, 169, 177
+  Mojsisovics, E. von, 224, 227
+  Morgan, C. Ll., 141
+  Morte slates, 184
+  Moscovian beds, 193, 301
+  Mountain limestone, 192
+  Murchison, Sir R. I., 19, 20, 174, 179
+  Murray, Sir J., 30
+  Muschelkalk, 218, 221, 222
+
+
+  Nehring, A., 267, 268
+  Neobolus fauna, 160
+  Neocomian series, 236-238
+  Neolithic age, 275-277
+  Neumayr, M., 115, 233
+  Newton, E. T., 45
+  Nicholson, H. A., 189, 250
+  Noachian Deluge, 8
+  Noetling, F., 160
+  Nordenskjöld, A. E., 113, 114
+  Noric beds, 225
+  Northamptonshire sands, 230
+  Norwich crag, 256, 257
+  Nummulitic limestone, 248
+
+
+  Old red sandstone, 183, 185, 186, 188, 191
+  Oldham, R. D., 208
+  Oldhaven beds, 244, 245
+  Olenellus fauna, 134, 153, 156-160
+  Olenus fauna, 152, 161, 162
+  Oligocene beds, 251, 252
+  Oligocene fauna and flora, 252
+  Oolite, 226
+  Ordovician faunas, 172, 173
+  Ordovician system, 164-173
+  Organically formed rocks, 29, 102, 109
+  Orogenic movements, 32
+  Osborne beds, 257
+  Owen, Sir R., 277
+  Oxford clay, 232
+  Oxford oolite, 226
+  Oxfordian series, 227, 232
+
+
+  Palæolithic fauna and flora, 270-274
+  Palæolithic man, 268, 272-274
+  Palæolithic period, 267-274
+  Palæontological break, 61
+  Palæo-physiography, 120
+  Paradoxides fauna, 152, 160, 161
+  Peat deposits, 275, 276
+  Pebble beds of Bunter, 218
+  Pebidian rocks, 140
+  Pengelly, W., 270
+  Pennant stage, 192
+  Pennine system of folds, 203
+  Penrith sandstone, 75, 210, 211
+  Period, definition of, 60
+  Permanence of ocean basins, 278-285
+  Permian fauna and flora, 214-216
+  Permian system, 209-217
+  Permo-carboniferous fauna and flora, 207, 208
+  Permo-carboniferous glacial deposits, 206
+  Permo-carboniferous period, 205-208
+  Phillips, J., 10, 11, 201
+  Physical break, 60
+  Pickwell Down sandstone, 183
+  Pilton beds, 183
+  Plaisancean series, 256
+  Planes of lamination, 27
+  Planes of stratification, 27
+  Pleistocene fauna and flora, 265, 266
+  Pleistocene period, 260-266
+  Pliocene fauna and flora, 259
+  Pliocene period, 256-259
+  Portland oolites, 226
+  Portlandian series, 226, 232
+  Prado, C. de, 161
+  Precambrian rocks, 132;
+    mode of formation of, 146
+  Preller, C. S. du R., 264
+  Prestwich, Sir J., 19, 130, 279
+  Productus limestones, 205, 206, 214
+  Protolenus fauna, 160
+  Pseudo-stromatism, 76
+  Purbeckian series, 226, 232
+  Pyroclastic rocks, 24
+
+
+  Quader sandstone, 240
+
+
+  Ramsay, Sir A. C., 130, 153, 163, 188
+  Reading beds, 244
+  Recurrences, 292
+  Red crag, 256, 257
+  Reid, C., 45, 257, 264, 268, 271
+  Renard, A., 30
+  Reversed fault, 34
+  Rhætic beds, 218
+  Rhiwlas limestone, 167
+  Richthofen, Baron von, 267, 268
+  Ridley, H. N., 271
+  River drift man, 268
+  Rotherham red rock, 202
+  Rothliegende, 209
+  Rouelle, 13
+
+
+  St Bees sandstone, 210
+  St Erth beds, 257
+  Salopian beds, 175
+  Salter, J. W., 161, 162, 186
+  Scarbro' limestone, 230, 231
+  Schists, crystalline, 76, 77, 133, 147
+  Scilla, A., 13
+  Screes, 101
+  Scrope, G. P., 76
+  Sections, geological, 84;
+    use of, 88
+  Sedimentary rocks, 23
+  Sedgwick, A., 16, 19, 20, 153, 174
+  Senonian series, 236
+  Series, definition of, 60
+  Seward, A. C., 113, 208
+  Sigmoidal structure, 33
+  Siliceous rocks, 29
+  Silurian faunas, 179, 180
+  Silurian system, 174-182
+  Simulation of structures, 72
+  Sinemurian series, 227, 229
+  Smith, W., 8, 12-18, 57, 85
+  Soil, 100
+  Solenhofen slate, 234
+  Sollas, W. J., 288
+  Solva beds, 152, 154, 156, 161
+  Speckled sandstone, 205, 206
+  Speeton series, 238
+  Spencer, H., 50
+  Spirorbis limestone, 201
+  Stages, definition of, 60
+  Steppe period, 260, 267-274
+  Stonesfield slate, 231
+  Strachey, J., 10
+  Strahan, A., 239, 264
+  Strata, 27;
+    classification of, 58, 125
+  Stratification, 26
+  Stratified rocks, 23;
+    composition of, 28;
+    origin of, 29;
+    classification of, 28, 125;
+    symbols to represent, 90
+  Stratigraphical geology, aim of, 1;
+    W. Smith, founder of, 8, 12-18
+  Suess, E., 110, 123, 207, 284
+  Superposition, law of, 31
+  Surveying, geological, 84
+  Systems, definition of, 60
+
+
+  Talchir stage, 205, 206
+  Tarannon shales, 174-177
+  Teall, J. J. H., 289
+  Terrestrial rocks, 99
+  Thanet sands, 244
+  Thinning out, 28
+  Thrust plane, 34;
+    detection of, 35, 82
+  Tiddeman, B. H., 87, 263, 270
+  Till, 262
+  Time, geological, 294-296
+  Toarcian series, 227, 229
+  Topley, W., 130, 239
+  Torridonian beds, 135-137
+  Tremadoc slates, 152, 155, 162, 163
+  Triassic fauna and flora, 223-225
+  Triassic system, 218-225;
+    ammonite zones of, 225
+  Trinucleus fauna, 165
+  Tullberg, S. A., 162
+  Turonian series, 236
+
+
+  Unconformity, 60, 78, 98
+  Underclays, 197
+  Uniformitarianism, 287-292
+  Uriconian rocks, 138
+  Ussher, W. A. E., 183
+
+
+  Valentian beds, 175
+  Verneuil, E. P. de, 161
+  Volcanic rocks, Cambrian, 155;
+    Carboniferous, 199;
+    Devonian, 184, 186;
+    Eocene, 246, 247;
+    Ordovician, 165-170;
+    Precambrian, 146
+  Vulcanicity, 289
+
+
+  Waagen, W., 213, 214
+  Walcott, C. D., 144, 158, 160, 161, 173
+  Wallace, A. R., 124, 235, 240, 281, 295
+  Ward, J. C., 87, 88, 263, 295
+  Warming, E., 115
+  Watts, W. W., 142, 168, 178
+  Wealden beds, 236, 237
+  Webster, T., 18
+  Weissliegende, 214
+  Wenlock limestone, 175, 176
+  Wenlock series, 174-177
+  Wenlock shale, 175-177
+  Werfener Schichten, 225
+  Werner, A. G., 12
+  Weybourne crag, 256
+  Whewell, W., 50
+  Whidbourne, G. F., 91
+  White Jura, 226
+  Whitehaven sandstone, 202
+  Whitehurst, J., 11, 12
+  Wiman, C., 46
+  Wood, S. V., 250, 259
+  Woodward, H., 191
+
+  Woodward, H. B., 68, 130, 131
+  Woodward, J., 8-10
+  Woodward, S. P., 108, 111
+  Woolhope limestone, 175
+  Woolwich beds, 244
+  Wright, G. F., 263
+
+
+  Yoredale series, 192
+
+
+  Zanclean series, 256
+  Zechstein, 209
+  Zone, fossil, 67;
+    ammonite, 225, 237;
+    graptolite, 69
+
+
+CAMBRIDGE: PRINTED BY J. AND C. F. CLAY, AT THE UNIVERSITY PRESS.
+
+
+
+
+Cambridge Natural Science Manuals.
+
+
+BIOLOGICAL SERIES.
+
+General Editor, A. E. Shipley, M.A.
+
+=Elementary Palæontology--Invertebrate=
+  H. Woods, M.A., F.G.S.                 6_s._
+
+=Elements of Botany=
+  F. Darwin, M.A., F.R.S.          4_s._ 6_d._
+
+=Practical Physiology of Plants=
+  F. Darwin, & E. H. Acton, M.A.   4_s._ 6_d._
+
+=Practical Morbid Anatomy=
+  H. D. Rolleston, M.D., F.R.C.P.
+  & A. A. Kanthack, M.D., M.R.C.P.       6_s._
+
+=Zoogeography=
+  F. E. Beddard, M.A., F.R.S.            6_s._
+
+=Flowering-Plants and Ferns=
+  J. C. Willis, M.A. In two vols. 10_s._ 6_d._
+
+=The Vertebrate Skeleton=
+  S. H. Reynolds, M.A.            12_s._ 6_d._
+
+=Fossil Plants=
+  A. C. Seward, M.A., F.G.S.           2 vols.
+                                Vol. I. 12_s._
+
+=Outlines of Vertebrate Palæontology=
+  A. S. Woodward.                       14_s._
+
+=The Soluble Ferments and Fermentation=
+  J. Reynolds Green, Sc.D., F.R.S.      12_s._
+
+=Zoology=
+  E. W. MacBride, M.A. and
+  A. E. Shipley, M.A.         [_In the Press._
+
+
+PHYSICAL SERIES.
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+General Editor, R. T. Glazebrook, M.A., F.R.S.
+
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+                                 R. T. Glazebrook, M.A., F.R.S.       5_s._
+    "        "
+        in two separate parts     "        "                   _each_ 3_s._
+
+  =Mechanics and  Hydrostatics=
+                                 R. T. Glazebrook, M.A., F.R.S. 8_s._ 6_d._
+      "               "
+        in three separate parts
+          Part I. =Dynamics=                 "        "               4_s._
+           "  II. =Statics=                  "        "               3_s._
+           " III. =Hydrostatics=             "        "               3_s._
+
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+                                 W. C. D. Whetham, M.A.         7_s._ 6_d._
+                                                           [_Out of print._
+
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+                                 R. T. Glazebrook, M.A., F.R.S.
+                                                          _In Preparation._
+
+
+GEOLOGICAL SERIES.
+
+  =Petrology for Students=
+                               A. Harker, M.A., F.G.S.          7_s._ 6_d._
+
+  =Handbook to the Geology of Cambs=
+                               F. R. C. Reed, M.A.              7_s._ 6_d._
+
+  =The Principles of Stratigraphical Geology=
+                               J. E. Marr, M.A.                       6_s._
+
+  =Crystallography=            Prof. W. J. Lewis, M.A.        14_s._ _Net._
+
+         *       *       *       *       *
+
+  =Laboratory Note-Books of= }           { L. R. Wilberforce, M.A., and
+        =Elementary Physics= }           { T. C. Fitzpatrick, M.A.
+            I. =Mechanics and Hydrostatics.=
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+
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+BIOLOGICAL SERIES.
+
+     =A Manual and Dictionary of the Flowering Plants and Ferns.=
+     Morphology, Natural History and Classification.
+     Alphabetically arranged. By J. C. Willis, M.A., Gonville and
+     Caius College. In Two Volumes. Crown 8vo. With Illustrations.
+     10_s._ 6_d._
+
+_Bookman._ One of the most useful books existing for students of
+botany.... The student who has this book and the chances which Kew, or
+even one of the smaller gardens, affords him, will make a steady and
+really scientific progress.
+
+     =Elements of Botany.= By F. Darwin, M.A., F.R.S. Second
+     Edition. Crown 8vo. With numerous Illustrations. 4_s._ 6_d._
+
+_Journal of Education._ A noteworthy addition to our botanical
+literature.
+
+     =Practical Physiology of Plants.= By F. Darwin, M.A., F.R.S.,
+     Fellow of Christ's College, Cambridge, and Reader in Botany
+     in the University, and E. H. Acton, M.A., late Fellow and
+     Lecturer of St John's College, Cambridge. With Illustrations.
+     Second Edition. Crown 8vo. 4_s._ 6_d._
+
+_Nature._ The authors are much to be congratulated on their work,
+which fills a serious gap in the botanical literature of this country.
+
+     =Zoogeography.= By F. E. Beddard, M.A., F.R.S. With Maps.
+     6_s._
+
+_Daily Chronicle._ Although included in the series of Cambridge
+Natural Science Manuals, and therefore designed chiefly for students
+of biology, Mr Beddard deals with his subject in a clear and graphic
+way that should commend his book to the general reader interested in
+the question. His style, while never lacking dignity, avoids the
+dulness which too often accompanies that virtue.
+
+     =Elementary Palæontology--Invertebrate.= By Henry Woods,
+     M.A., F.G.S. With Illustrations. Crown 8vo. Second Edition.
+     6_s._
+
+_Nature._ As an introduction to the study of palæontology Mr Woods's
+book is worthy of high praise.
+
+     =Outlines of Vertebrate Palæontology= for the use of Students
+     of Zoology. By A. S. Woodward, Assistant Keeper in the
+     Department of Geology in the British Museum. Demy 8vo. 14_s._
+
+_Athenæum._ Mr Woodward, in studying vertebrate fossils for the
+purpose of this volume, takes the biological view, and has designed
+his work primarily for the assistance of students of vertebrate
+morphology and zoology. Mindful, however, of the geological side of
+the subject, he has introduced a chapter on the succession of the
+vertebrate faunas, offering a brief but convenient summary of the
+distribution of vertebrate life throughout geologic time. The author
+is to be congratulated on having produced a work of exceptional value,
+dealing with a difficult subject in a thoroughly sound manner.
+
+     =The Vertebrate Skeleton.= By S. H. Reynolds, M.A., Trinity
+     College. Crown 8vo. 12_s._ 6_d._
+
+_British Medical Journal._ A volume which will certainly take its
+place amongst the standard text-books of the day.
+
+
+BIOLOGICAL SERIES.
+
+     =Practical Morbid Anatomy.= By H. D. Rolleston, M.D.,
+     F.R.C.P., Fellow of St John's College, Cambridge, Assistant
+     Physician and Lecturer on Pathology, St George's Hospital,
+     London, and A. A. Kanthack, M.D., M.R.C.P., Lecturer on
+     Pathology, St Bartholomew's Hospital, London. Crown 8vo.
+     6_s._
+
+_British Medical Journal._ This manual can in every sense be most
+highly recommended, and it should supply what has hitherto been a real
+want.
+
+     =The Soluble Ferments and Fermentation.= By J. Reynolds
+     Green, Sc.D., F.R.S., Professor of Botany to the
+     Pharmaceutical Society of Great Britain. Demy 8vo. 12_s._
+
+_Nature._ It is not necessary to recommend the perusal of the book, to
+all interested in the subject since it is indispensable to them, and
+we will merely conclude by congratulating the Cambridge University
+Press on having added to their admirable series of Natural Science
+Manuals an eminently successful work on so important and difficult a
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+industriously and ably worked out, and on the whole, well written.
+
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+library, and will be a work much studied and much pondered over by the
+thoughtful and highly trained men who now represent our profession.
+
+
+PHYSICAL SERIES.
+
+     =Mechanics and Hydrostatics.= An Elementary Text-book,
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+     Glazebrook, M.A., F.R.S., Fellow of Trinity College,
+     Cambridge, Director of the National Physical Laboratory. With
+     Illustrations. Crown 8vo. 8_s._ 6_d._
+
+  Also in separate parts.
+  Part I. =Dynamics.= 4_s._
+  Part II. =Statics.= 3_s._
+  Part III. =Hydrostatics.= 3_s._
+
+_Knowledge._ We cordially recommend Mr Glazebrook's volumes to the
+notice of teachers.
+
+_Practical Teacher._ We heartily recommend these books to the notice
+of all science teachers, and especially to the masters of Organised
+Science Schools, which will soon have to face the question of simple
+practical work in physics, for which these books will constitute an
+admirable introduction if not a complete _vade mecum_.
+
+     =Heat and Light.= An Elementary Text-book, Theoretical and
+     Practical, for Colleges and Schools. By R. T. Glazebrook,
+     M.A., F.R.S. Crown 8vo. 5_s._ The two parts are also
+     published separately.
+
+  =Heat.= 3_s._
+  =Light.= 3_s._
+
+_Journal of Education._ We have no hesitation in recommending this
+book to the notice of teachers.
+
+_Practical Photographer._ Mr Glazebrook's text-book on "Light" cannot
+be too highly recommended.
+
+
+GEOLOGICAL SERIES.
+
+     =Handbook to the Geology of Cambridgeshire.= For the use of
+     Students. By F. R. Cowper Reed, M.A., F.G.S., Assistant to
+     the Woodwardian Professor of Geology. With Illustrations.
+     Crown 8vo. 7_s._ 6_d._
+
+_Nature._ The geology of Cambridgeshire possesses a special interest
+for many students.... There is much in Cambridgeshire geology to
+arouse interest when once an enthusiasm for the science has been
+kindled, and there was need of a concise hand-book which should
+clearly describe and explain the leading facts that have been made
+known.... The present work is a model of what a county geology should
+be.
+
+     =The Principles of Stratigraphical Geology.= By J. E. Marr,
+     M.A., Fellow of St John's College, Cambridge. Crown 8vo.
+     6_s._
+
+_Nature._ The work will prove exceedingly useful to the advanced
+student; it is full of hints and references, gathered during the
+author's long experience as a teacher and observer, and which will be
+valuable to all who seek to interpret the history of our stratified
+formations.
+
+_University Extension Journal._ Mr Marr is an old University Extension
+lecturer, and his book, which is distinguished by the lucidity and
+thoroughness which characterise all his work, cannot fail to be of
+service to University Extension students who are making a serious
+study of Geology.
+
+     =Crystallography.= By W. J. Lewis, M.A., Professor of
+     Mineralogy in the University of Cambridge. Demy 8vo. 14_s._
+     net.
+
+_Athenæum._ Prof. Lewis has written a valuable work.... The present
+work deserves to be welcomed not only as a greatly needed help to
+advanced students of mineralogy, but as a sign that the study itself
+maintains an honoured place in the university Science Course.
+
+_Nature._ The author and the University Press may be congratulated on
+the completion of a treatise worthy of the subject and of the
+University.
+
+     =Petrology for Students.= An Introduction to the Study of
+     Rocks under the Microscope. By A. Harker, M.A., F.G.S.,
+     Fellow of St John's College, and Demonstrator in Geology
+     (Petrology) in the University of Cambridge. Crown 8vo. Second
+     Edition, Revised. 7_s._ 6_d._
+
+_Nature._ No better introduction to the study of petrology could be
+desired than is afforded by Mr Harker's volume.
+
+
+  London: C. J. CLAY AND SONS,
+  CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
+  AVE MARIA LANE
+  AND
+  H. K. LEWIS, 136, GOWER STREET, W.C.
+  _Medical Publisher and Bookseller._
+
+
+
+
+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).
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of The Principles of Stratigraphical
+Geology, by J. E. Marr
+
+*** END OF THE PROJECT GUTENBERG EBOOK 43963 ***