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diff --git a/43963-0.txt b/43963-0.txt new file mode 100644 index 0000000..3ecbaa1 --- /dev/null +++ b/43963-0.txt @@ -0,0 +1,9792 @@ +*** 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. 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Fitzpatrick, M.A. + I. =Mechanics and Hydrostatics.= + II. =Heat and Optics.= + III. =Electricity and Magnetism.= _each_ 1_s._ + +_Other volumes are in preparation and will be announced shortly._ + + +_Press Opinions._ + + +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 +theme, and the author on having written a treatise cleverly conceived, +industriously and ably worked out, and on the whole, well written. + +_Brewer's Journal._ It will of course find a place in every brewer's +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, + Theoretical and Practical, for Colleges and Schools. By R. T. + 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 *** |