Ancient Life-History of the Earth

A COMPREHENSIVE OUTLINE OF THE PRINCIPLES AND LEADING FACTS OF PALÆONTOLOGICAL SCIENCE

Page v PREFACE.

The study of Palæontology, or the science which is concerned with the living beings which flourished upon the globe during past periods of its history, may be pursued by two parallel but essentially distinct paths. By the one method of inquiry, we may study the anatomical characters and structure of the innumerable extinct forms of life which lie buried in the rocks simply as so many organisms, with but a slight and secondary reference to the time at which they lived. By the other method, fossil animals are regarded principally as so many landmarks in the ancient records of the world, and are studied historically and as regards their relations to the chronological succession of the strata in which they are entombed. In so doing, it is of course impossible to wholly ignore their structural characters, and their relationships with animals now living upon the earth; but these points are held to occupy a subordinate place, and to require nothing more than a comparatively general attention.

In a former work, the Author has endeavoured to furnish a summary of the more important facts of Page vi Palæontology regarded in its strictly scientific aspect, as a mere department of the great science of Biology. The present work, on the other hand, is an attempt to treat Palæontology more especially from its historical side, and in its more intimate relations with Geology. In accordance with this object, the introductory portion of the work is devoted to a consideration of the general principles of Palæontology, and the bearings of this science upon various geological problems—such as the mode of formation of the sedimentary rocks, the reactions of living beings upon the crust of the earth, and the sequence in time of the fossiliferous formations. The second portion of the work deals exclusively with Historical Palæontology, each formation being considered separately, as regards its lithological nature and subdivisions, its relations to other formations, its geographical distribution, its mode of origin, and its characteristic life-forms.

In the consideration of the characteristic fossils of each successive period, a general account is given of their more important zoological characters and their relations to living forms; but the technical language of Zoology has been avoided, and the aid of illustrations has been freely called into use. It may therefore be hoped that the work may be found to be available for the purposes of both the Geological and the Zoological student; since it is essentially an outline of Historical Palæontology, and the student of either of the above-mentioned sciences must perforce possess some knowledge of the last. Whilst primarily intended for students, it may be added that the method of treatment adopted has been so far untechnical as not to render the work useless to the general reader who may desire Page vii to acquire some knowledge of a subject of such vast and universal interest.

In carrying out the object which he has held before him, the Author can hardly expect, from the nature of the materials with which he has had to deal, that he has kept himself absolutely clear of errors, both of omission and commission. The subject, however, is one to which he has devoted the labour of many years, both in studying the researches of others and in personal investigations of his own; and he can only trust that such errors as may exist will be found to belong chiefly to the former class, and to be neither serious nor numerous. It need only be added that the work is necessarily very limited in its scope, and that the necessity of not assuming a thorough previous acquaintance with Natural History in the reader has inexorably restricted its range still further. The Author does not, therefore, profess to have given more than a merely general outline of the subject; and those who desire to obtain a more minute and detailed knowledge of Palæontology, must have recourse to other and more elaborate treatises.

Page ix CONTENTS.

The general objects or geological science—The older theories of catastrophistic and intermittent action—The more modern doctrines of continuous and uniform action—Bearing of these doctrines respectively on the origin or the existing terrestrial order—Elements or truth in Catastrophism—General truth of the doctrine of Continuity—Geological time.

Definition of Palæontology—Nature of Fossils—Different processes of fossilisation.

Aqueous and igneous rocks—General characters of the sedimentary rocks—Mode or formation of the sedimentary rocks—Definition of the term "formation"—Chief divisions of the aqueous rocks—Mechanically-formed rocks, their characters and mode of origin—Chemically and organically formed rocks—Calcareous rocks—Chalk, its microscopic structure and mode of formation—Limestone, varieties, structure, and origin—Phosphate of lime—Concretions—Sulphate of lime—Silica and siliceous deposits of various kinds—Greensands—Red clays—Carbon and carbonaceous deposits.

Chronological succession of the fossiliferous rocks—Tests or age of strata—Value of Palæontological evidence in stratigraphical Geology—General sequence of the great formations.

The breaks in the palæontological and geological record—Use of the term "contemporaneous" as applied to groups of strata—General sequence of strata and of life-forms interfered with by more or less extensive gaps—Unconformability—Phenomena implied by this—Causes of the imperfection of the palæontological record.

Conclusions to be drawn from fossils—Age of rocks—Mode of origin of any fossiliferous bed—Fluviatile, lacustrine, and marine deposits—Conclusions as to climate—Proofs of elevation and subsidence of portions of the earth's crust derived from fossils.

The biological relations of fossils—Extinction of life-forms—Geological range of different species—Persistent types of life—Modern origin of existing animals and plants—Reference of fossil forms to the existing primary divisions of the animal kingdom—Departure of the older types of life from those now in existence—Resemblance of the fossils of a given formation to those of the formation next above and next below—Introduction of new life-forms.

The Laurentian and Huronian periods—General nature, divisions, and geographical distribution of the Laurentian deposits—Lower and Upper Laurentian—Reasons for believing that the Laurentian rocks are not azoic based upon their containing limestones, beds of oxide of iron, and graphite—The characters, chemical composition, and minute structure of Eozoön Canadense—Comparison of Eozoön with existing Foraminifera—Archœosphœrinœ—Huronian formation—Nature and distribution of Huronian deposits—Organic remains of the Huronian—Literature.

The Cambrian period—General succession of Cambrian deposits in Wales—Lower Cambrian and Upper Cambrian—Cambrian deposits of the continent of Europe and North American—Life of the Cambrian period — Fucoids — Eophyton — Oldhamia — Sponges — Echinoderms — Annelides — Crustaceans — Structure of Trilobites—Brachiopods—Pteropods, Gasteropods, and Bivalves—Cephalopods—Literature.

The Lower Silurian period—The Silurian rocks generally—Limits of Lower and Upper Silurian—General succession, subdivisions, and characters of the Lower Silurian rocks of Wales—General succession, subdivisions, and characters of the Lower Silurian rocks of the North American continent—Life of the period — Fucoids — Protozoa — Graptolites — Structure of Graptolites — Corals — General structure of Corals — Crinoids — Cystideans — General characters of Cystideans — Annelides — Crustaceans — Polyzoa — Brachiopods — Bivalve and Univalve Molluscs—Chambered Cephalopods—General characters of the Cephalopoda—Conodonts.

The Upper Silurian period—General succession of the Upper Silurian deposits of Wales—Upper Silurian deposits of North America—Life of the Upper Silurian — Plants — Protozoa — Graptolites — Corals — Crinoids — General structure of Crinoids — Star-fishes — Annelides — Crustaceans — Eurypterids — Polyzoa — Brachiopods — Structure of Brachiopods — Bivalves and Univalves — Pteropods — Cephalopods — Fishes — Silurian literature.

The Devonian period—Relations between the Old Red Sandstone and the marine Devonian deposits—The Old Red Sandstone of Scotland—The Devonian strata of Devonshire—Sequence and subdivisions of the Devonian deposits of North America—Life of the period — Plants — Protozoa — Corals — Crinoids — Pentremites — Annelides — Crustaceans — Insects — Polyzoa — Brachiopods — Bivalves — Univalves — Pteropods — Cephalopods — Fishes — General divisions of the Fishes—Palæontological evidence as to the independent existence of the Devonian system as a distinct formation—Literature.

The Carboniferous period—Relations of Carboniferous rocks to Devonian—The Carboniferous Limestone or Sub-Carboniferous series—The Millstone-grit and the Coal-measures—Life of the period—Structure and mode of formation of Coal—Plants of the Coal.

Animal life of the Carboniferous period — Protozoa — Corals — Crinoids — Pentremites — Structure of Pentremites — Echinoids — Structure of Echinoidea — Annelides — Crustacea — Insects — Arachnids — Myriapods — Polyzoa — Brachiopods — Bivalves and Univalves — Cephalopods — Fishes — Labyrinthodont Amphibians—Literature.

The Permian period — General succession, characters, and mode of formation of the Permian deposits — Life of the period — Plants — Protozoa — Corals — Echinoderms — Annelides — Crustaceans — Polyzoa — Brachiopods — Bivalves — Univalves — Pteropods — Cephalopods — Fishes — Amphibians — Reptiles — Literature.

The Triassic period-—General characters and subdivisions of the Trias of the Continent of Europe and Britain—Trias of North America—Life of the period — Plants — Echinoderms — Crustaceans — Polyzoa — Brachiopods — Bivalves — Univalves — Cephalopods — Intermixture of Palæozoic with Mesozoic types of Molluscs — Fishes — Amphibians — Reptiles — Supposed footprints of Birds — Mammals — Literature.

The Jurassic period—General sequence and subdivisions of the Jurassic deposits in Britain—Jurassic rocks of North America—Life of the period — Plants — Corals — Echinoderms — Crustaceans — Insects — Brachiopods — Bivalves — Univalves — Pteropods — Tetrabranchiate Cephalopods — Dibranchiate Cephalopods — Fishes — Reptiles — Birds — Mammals — Literature.

The Cretaceous period—General succession and subdivisions of the Cretaceous rocks in Britain—Cretaceous rocks of North America—Life of the period — Plants — Protozoa — Corals — Echinoderms — Crustaceans — Polyzoa — Brachiopods — Bivalves — Univalves — Tetrabranchiate and Dibranchiate Cephalopods — Fishes — Reptiles — Birds — Literature.

The Eocene period—Relations between the Kainozoic and Mesozoic rocks in Europe and in North America—Classification of the Tertiary deposits—The sequence and subdivisions of the Eocene rocks of Britain and France—Eocene strata of the United States—Life of the period — Plants — Foraminifera — Corals — Echinoderms — Mollusca — Fishes — Reptiles — Birds — Mammals.

The Miocene period—Miocene strata of Britain—Of France—Of Belgium—Of Austria—Of Switzerland—Of Germany—Of Greece—Of India—Of North America—Of the Arctic regions—Life of the period—Vegetation of the Miocene period — Foraminifera — Corals — Echinoderms — Articulates — Mollusca — Fishes — Amphibians — Reptiles — Mammals.

The Pliocene period—Pliocene deposits of Britain—Of Europe—Of North America—Life of the period—Climate of the period as indicated by the Invertebrate animals—The Pliocene Mammalia—Literature relating to the Tertiary deposits and their fossils.

The Post-Pliocene period—Division of the Quaternary deposits into Post-Pliocene and Recent—Relations of the Post-Pliocene deposits of the northern hemisphere to the "Glacial period"—Pre-Glacial deposits—Glacial deposits—Arctic Mollusca in Glacial beds—Post-Glacial deposits—Nature and mode of formation of high-level and low-level gravels—Nature and mode of formation of cavern-deposits—Kent's Cavern-Post—Pliocene deposits of the southern hemisphere.

Life of the Post-Pliocene period—Effect of the coming on and departure of the Glacial period upon the animals inhabiting the northern hemisphere—Birds of the Post-Pliocene—Mammalia of the Post-Pliocene—Climate of the Post-Glacial period as deduced from the Post-Glacial Mammals—Occurrence of the bones and implements of Man in Post-Pliocene deposits in association with the remains of extinct Mammalia—Literature relating to the Post-Pliocene period.

The succession of life upon the globe—Gradual and successive introduction of life-forms—What is meant by "lower" and "higher" groups of animals and plants—Succession in time of the great groups of animals in the main corresponding with their zoological order—Identical phenomena in the vegetable kingdom—Persistent types of life—High organisation of many early forms—Bearings of Palæontology on the general doctrine of Evolution.

Page xv LIST OF ILLUSTRATIONS.

Page xix PART I.

Page 1 THE
ANCIENT LIFE-HISTORY
OF
THE EARTH

INTRODUCTION.

Under the general title of "Geology" are usually included at least two distinct branches of inquiry, allied to one another in the closest manner, and yet so distinct as to be largely capable of separate study. Geology,[1] in its strict sense, is the science which is concerned with the investigation of the materials which compose the earth, the methods in which those materials have been arranged, and the causes and modes of origin of these arrangements. In this limited aspect, Geology is nothing more than the Physical Geography of the past, just as Physical Geography is the Geology of to-day; and though it has to call in the aid of Physics, Astronomy, Mineralogy, Chemistry, and other allies more remote, it is in itself a perfectly distinct and individual study. One has, however, only to cross the threshold of Geology to discover that the field and scope of the science cannot be thus rigidly limited to purely physical problems. The study of the physical development of the earth throughout past ages brings us at once in contact with the forms of animal and vegetable life which peopled its surface in bygone epochs, and it is found impossible adequately to comprehend Page 2 the former, unless we possess some knowledge of the latter. However great its physical advances may be, Geology remains imperfect till it is wedded with Palæontology,[2] a study which essentially belongs to the vast complex of the Biological Sciences, but at the same time has its strictly geological side. Dealing, as it does, wholly with the consideration of such living beings as do not belong exclusively to the present order of things, Palæontology is, in reality, a branch of Natural History, and may be regarded as substantially the Zoology and Botany of the past. It is the ancient life-history of the earth, as revealed to us by the labours of palæontologists, with which we have mainly to do here; but before entering upon this, there are some general questions, affecting Geology and Palæontology alike, which may be very briefly discussed.

The working geologist, dealing in the main with purely physical problems, has for his object to determine the material structure of the earth, and to investigate, as far as may be, the long chain of causes of which that structure is the ultimate result. No wider or more extended field of inquiry could be found; but philosophical geology is not content with this. At all the confines of his science, the transcendental geologist finds himself confronted with some of the most stupendous problems which have ever engaged the restless intellect of humanity. The origin and primæval constitution of the terrestrial globe, the laws of geologic action through long ages of vicissitude and development, the origin of life, the nature and source of the myriad complexities of living beings, the advent of man, possibly even the future history of the earth, are amongst the questions with which the geologist has to grapple in his higher capacity.

These are problems which have occupied the attention of philosophers in every age of the world, and in periods long antecedent to the existence of a science of geology. The mere existence of cosmogonies in the religion of almost every nation, both ancient and modern, is a sufficient proof of the eager desire of the human mind to know something of the origin of the earth on which we tread. Every human being who has gazed on the vast panorama of the universe, though it may have been but with the eyes of a child, has felt the longing to solve, however imperfectly, "the riddle of the painful earth," and has, consciously or unconsciously, elaborated some sort of a theory as to the why and wherefore of what he sees. Apart from the profound and perhaps inscrutable problems which lie at the bottom of human existence, men have in all ages invented Page 3 theories to explain the common phenomena of the material universe; and most of these theories, however varied in their details, turn out on examination to have a common root, and to be based on the same elements. Modern geology has its own theories on the same subject, and it will be well to glance for a moment at the principles underlying the old and the new views.

It has been maintained, as a metaphysical hypothesis, that there exists in the mind of man an inherent principle, in virtue of which he believes and expects that what has been, will be; and that the course of nature will be a continuous and uninterrupted one. So far, however, from any such belief existing as a necessary consequence of the constitution of the human mind, the real fact seems to be that the contrary belief has been almost universally prevalent. In all old religions, and in the philosophical systems of almost all ancient nations, the order of the universe has been regarded as distinctly unstable, mutable, and temporary. A beginning and an end have always been assumed, and the course of terrestrial events between these two indefinite points has been regarded as liable to constant interruption by revolutions and catastrophes of different kinds, in many cases emanating from supernatural sources. Few of the more ancient theological creeds, and still fewer of the ancient philosophies, attained body and shape without containing, in some form or another, the belief in the existence of periodical convulsions, and of alternating cycles of destruction and repair.

That geology, in its early infancy, should have become imbued with the spirit of this belief, is no more than might have been expected; and hence arose the at one time powerful and generally-accepted doctrine of "Catastrophism." That the succession of phenomena upon the globe, whereby the earth's crust had assumed the configuration and composition which we find it to possess, had been a discontinuous and broken succession, was the almost inevitable conclusion of the older geologists. Everywhere in their study of the rocks they met with apparently impassable gaps, and breaches of continuity that could not be bridged over. Everywhere they found themselves conducted abruptly from one system of deposits to others totally different in mineral character or in stratigraphical position. Everywhere they discovered that well-marked and easily recognisable groups of animals and plants were succeeded, without the intermediation of any obvious lapse of time, by other assemblages of organic beings of a different character. Everywhere they found evidence that the earth's crust Page 4 had undergone changes of such magnitude as to render it seemingly irrational to suppose that they could have been produced by any process now in existence. If we add to the above the prevalent belief of the time as to the comparative brevity of the period which had elapsed since the birth of the globe, we can readily understand the general acceptance of some form of catastrophism amongst the earlier geologists.

As regards its general sense and substance, the doctrine of catastrophism held that the history of the earth, since first it emerged from the primitive chaos, had been one of periods of repose, alternating with catastrophes and cataclysms of a more or less violent character. The periods of tranquillity were supposed to have been long and protracted; and during each of them it was thought that one of the great geological "formations" was deposited. In each of these periods, therefore, the condition of the earth was supposed to be much the same as it is now—sediment was quietly accumulated at the bottom of the sea, and animals and plants flourished uninterruptedly in successive generations. Each period of tranquillity, however, was believed to have been, sooner or later, put an end to by a sudden and awful convulsion of nature, ushering in a brief and paroxysmal period, in which the great physical forces were unchained and permitted to spring into a portentous activity. The forces of subterranean fire, with their concomitant phenomena of earthquake and volcano, were chiefly relied upon as the efficient causes of these periods of spasm and revolution. Enormous elevations of portions of the earth's crust were thus believed to be produced, accompanied by corresponding and equally gigantic depressions of other portions. In this way new ranges of mountains were produced, and previously existing ranges levelled with the ground, seas were converted into dry land, and continents buried beneath the ocean—catastrophe following catastrophe, till the earth was rendered uninhabitable, and its races of animals and plants were extinguished, never to reappear in the same form. Finally, it was believed that this feverish activity ultimately died out, and that the ancient peace once more came to reign upon the earth. As the abnormal throes and convulsions began to be relieved, the dry land and sea once more resumed their relations of stability, the conditions of life were once more established, and new races of animals and plants sprang into existence, to last until the supervention of another fever-fit.

Such is the past history of the globe, as sketched for us, in alternating scenes of fruitful peace and revolutionary destruction, by the earlier geologists. As before said, we cannot Page 5 wonder at the former general acceptance of Catastrophistic doctrines. Even in the light of our present widely-increased knowledge, the series of geological monuments remains a broken and imperfect one; nor can we ever hope to fill up completely the numerous gaps with which the geological record is defaced. Catastrophism was the natural method of accounting for these gaps, and, as we shall see, it possesses a basis of truth. At present, however, catastrophism may be said to be nearly extinct, and its place is taken by the modern doctrine of "Continuity" or "Uniformity"—a doctrine with which the name of Lyell must ever remain imperishably associated.

The fundamental thesis of the doctrine of Uniformity is, that, in spite of all apparent violations of continuity, the sequence of geological phenomena has in reality been a regular and uninterrupted one; and that the vast changes which can be shown to have passed over the earth in former periods have been the result of the slow and ceaseless working of the ordinary physical forces—acting with no greater intensity than they do now, but acting through enormously prolonged periods. The essential element in the theory of Continuity is to be found in the allotment of indefinite time for the accomplishment of the known series of geological changes. It is obviously the case, namely, that there are two possible explanations of all phenomena which lie so far concealed in "the dark backward and abysm of time," that we can have no direct knowledge of the manner in which they were produced. We may, on the one hand, suppose them to be the result of some very powerful cause, acting through a short period of time. That is Catastrophism. Or, we may suppose them to be caused by a much weaker force operating through a proportionately prolonged period. This is the view of the Uniformitarians. It is a question of energy versus time and it is time which is the true element of the case. An earthquake may remove a mountain in the course of a few seconds; but the dropping of the gentle rain will do the same, if we extend its operations over a millennium. And this is true of all agencies which are now at work, or ever have been at work, upon our planet. The Catastrophists, believing that the globe is but, as it were, the birth of yesterday, were driven of necessity to the conclusion that its history had been checkered by the intermittent action of paroxysmal and almost inconceivably potent forces. The Uniformitarians, on the other hand, maintaining the "adequacy of existing causes," and denying that the known physical forces ever acted in past time with greater intensity than they do at present, are, equally of necessity, driven to the conclusion that Page 6 the world is truly in its "hoary eld," and that its present state is really the result of the tranquil and regulated action of known forces through unnumbered and innumerable centuries.

The most important point for us, in the present connection, is the bearing of these opposing doctrines upon the question, as to the origin of the existing terrestrial order. On any doctrine of uniformity that order has been evolved slowly, and, according to law, from a pre-existing order. Any doctrine of catastrophism, on the other hand, carries with it, by implication, the belief that the present order of things was brought about suddenly and irrespective of any pre-existent order; and it is important to hold clear ideas as to which of these beliefs is the true one. In the first place, we may postulate that the world had a beginning, and, equally, that the existing terrestrial order had a beginning. However far back we may go, geology does not, and cannot, reach the actual beginning of the world; and we are, therefore, left simply to our own speculations on this point. With regard, however, to the existing terrestrial order, a great deal can be discovered, and to do so is one of the principal tasks of geological science. The first steps in the production of that order lie buried in the profound and unsearchable depths of a past so prolonged as to present itself to our finite minds as almost in eternity. The last steps are in the prophetic future, and can be but dimly guessed at. Between the remote past and the distant future, we have, however, a long period which is fairly open to inspection; and in saying a "long" period, it is to be borne in mind that this term is used in its geological sense. Within this period, enormously long as it is when measured by human standards, we can trace with reasonable certainty the progressive march of events, and can determine the laws of geological action, by which the present order of things has been brought about.

The natural belief on this subject doubtless is, that the world, such as we now see it, possessed its present form and configuration from the beginning. Nothing can be more natural than the belief that the present continents and oceans have always been where they are now; that we have always had the same mountains and plains; that our rivers have always had their present courses, and our lakes their present positions; that our climate has always been the same; and that our animals and plants have always been identical with those now familiar to us. Nothing could be more natural than such a belief, and nothing could be further removed from the actual truth. On the contrary, a very slight acquaintance with geology shows us, in the words of Sir John Herschel, that Page 7 "the actual configuration of our continents and islands, the coast-lines of our maps, the direction and elevation of our mountain-chains, the courses of our rivers, and the soundings of our oceans, are not things primordially arranged in the construction of our globe, but results of successive and complex actions on a former state of things; that, again, of similar actions on another still more remote; and so on, till the original and really permanent state is pushed altogether out of sight and beyond the reach even of imagination; while on the other hand, a similar, and, as far as we can see, interminable vista is opened out for the future, by which the habitability of our planet is secured amid the total abolition on it of the present theatres of terrestrial life."

Geology, then, teaches us that the physical features which now distinguish the earth's surface have been produced as the ultimate result of an almost endless succession of precedent changes. Palæontology teaches us, though not yet in such assured accents, the same lesson. Our present animals and plants have not been produced, in their innumerable forms, each as we now know it, as the sudden, collective, and simultaneous birth of a renovated world. On the contrary, we have the clearest evidence that some of our existing animals and plants made their appearance upon the earth at a much earlier period than others. In the confederation of animated nature some races can boast of an immemorial antiquity, whilst others are comparative parvenus. We have also the clearest evidence that the animals and plants which now inhabit the globe have been preceded, over and over again, by other different assemblages of animals and plants, which have flourished in successive periods of the earth's history, have reached their culmination, and then have given way to a fresh series of living beings. We have, finally, the clearest evidence that these successive groups of animals and plants (faunæ and floræ) are to a greater or less extent directly connected with one another. Each group is, to a greater or less extent, the lineal descendant of the group which immediately preceded it in point of time, and is more or less fully concerned with giving origin to the group which immediately follows it. That this law of "evolution" has prevailed to a great extent is quite certain; but it does not meet all the exigencies of the case, and it is probable that its action has been supplemented by some still unknown law of a different character.

We shall have to consider the question of geological "continuity" again. In the meanwhile, it is sufficient to state that this doctrine is now almost universally accepted as the basis Page 8 of all inquiries, both in the domain of geology and that of palæontology. The advocates of continuity possess one immense advantage over those who believe in violent and revolutionary convulsions, that they call into play only agencies of which we have actual knowledge. We know that certain forces are now at work, producing certain modifications in the present condition of the globe; and we know that these forces are capable of producing the vastest of the changes which geology brings under our consideration, provided we assign a time proportionately vast for their operation. On the other hand, the advocates of catastrophism, to make good their views, are compelled to invoke forces and actions, both destructive and restorative, of which we have, and can have, no direct knowledge. They endow the whirlwind and the earthquake, the central fire and the rain from heaven, with powers as mighty as ever imagined in fable, and they build up the fragments of a repeatedly shattered world by the intervention of an intermittently active creative power.

It should not be forgotten, however, that from one point of view there is a truth in catastrophism which is sometimes overlooked by the advocates of continuity and uniformity. Catastrophism has, as its essential feature, the proposition that the known and existing forces of the earth at one time acted with much greater intensity and violence than they do at present, and they carry down the period of this excessive action to the commencement of the present terrestrial order. The Uniformitarians, in effect, deny this proposition, at any rate as regards any period of the earth's history of which we have actual cognisance. If, however, the "nebular hypothesis" of the origin of the universe be well founded—as is generally admitted—then, beyond question, the earth is a gradually cooling body, which has at one time been very much hotter than it is at present. There has been a time, therefore, in which the igneous forces of the earth, to which we owe the phenomena of earthquakes and volcanoes, must have been far more intensely active than we can conceive of from anything that we can see at the present day. By the same hypothesis, the sun is a cooling body, and must at one time have possessed a much higher temperature than it has at present. But increased heat of the sun would seriously alter the existing conditions affecting the evaporation and precipitation of moisture on our earth; and hence the aqueous forces may also have acted at one time more powerfully than they do now. The fundamental principle of catastrophism is, therefore, not wholly vicious; and we have reason to think that there must have been periods—very Page 9 remote, it is true, and perhaps unrecorded in the history of the earth—in which the known physical forces may have acted with an intensity much greater than direct observation would lead us to imagine. And this may be believed, altogether irrespective of those great secular changes by which hot or cold epochs are produced, and which can hardly be called "catastrophistic," as they are produced gradually, and are liable to recur at definite intervals.

Admitting, then, that there is a truth at the bottom of the once current doctrines of catastrophism, still it remains certain that the history of the earth has been one of law in all past time, as it is now. Nor need we shrink back affrighted at the vastness of the conception—the vaster for its very vagueness—that we are thus compelled to form as to the duration of geological time. As we grope our way backward through the dark labyrinth of the ages, epoch succeeds to epoch, and period to period, each looming more gigantic in its outlines and more shadowy in its features, as it rises, dimly revealed, from the mist and vapour of an older and ever-older past. It is useless to add century to century or millennium to millennium. When we pass a certain boundary-line, which, after all, is reached very soon, figures cease to convey to our finite faculties any real notion of the periods with which we have to deal. The astronomer can employ material illustrations to give form and substance to our conceptions of celestial space; but such a resource is unavailable to the geologist. The few thousand years of which we have historical evidence sink into absolute insignificance beside the unnumbered æons which unroll themselves one by one as we penetrate the dim recesses of the past, and decipher with feeble vision the ponderous volumes in which the record of the earth is written. Vainly does the strained intellect seek to overtake an ever-receding commencement, and toil to gain some adequate grasp of an apparently endless succession. A beginning there must have been, though we can never hope to fix its point. Even speculation droops her wings in the attenuated atmosphere of a past so remote, and the light of imagination is quenched in the darkness of a history so ancient. In time, as in space, the confines of the universe must ever remain concealed from us, and of the end we know no more than of the beginning. Inconceivable as is to us the lapse of "geological time," it is no more than "a mere moment of the past, a mere infinitesimal portion of eternity." Well may "the human heart, that weeps and trembles," say, with Richter's pilgrim through celestial space, "I will go no farther; for the spirit of man acheth with Page 10 this infinity. Insufferable is the glory of God. Let me lie down in the grave, and hide me from the persecution of the Infinite, for end, I see, there is none."

CHAPTER I.

The study of the rock-masses which constitute the crust of the earth, if carried out in the methodical and scientific manner of the geologist, at once brings us, as has been before remarked, in contact with the remains or traces of living beings which formerly dwelt upon the globe. Such remains are found, in greater or less abundance, in the great majority of rocks; and they are not only of great interest in themselves, but they have proved of the greatest importance as throwing light upon various difficult problems in geology, in natural history, in botany, and in philosophy. Their study constitutes the science of palæontology; and though it is possible to proceed to a certain length in geology and zoology without much palæontological knowledge, it is hardly possible to attain to a satisfactory general acquaintance with either of these subjects without having mastered the leading facts of the first. Similarly, it is not possible to study palæontology without some acquaintance with both geology and natural history.

Palæontology, then, is the science which treats of the living beings, whether animal or vegetable, which have inhabited the earth during past periods of its history. Its object is to elucidate, as far as may be, the structure, mode of existence, and habits of all such ancient forms of life; to determine their position in the scale of organised beings; to lay down the geographical limits within which they flourished; and to fix the period of their advent and disappearance. It is the ancient life-history of the earth; and were its record complete, it would furnish us with a detailed knowledge of the form and relations of all the animals and plants which have at any period flourished upon the land-surfaces of the globe or inhabited its waters; it would enable us to determine precisely their succession in time; and it would place in our hands an unfailing key to the problems of evolution. Unfortunately, from causes which will be subsequently discussed, the palæontological record is extremely imperfect, and our knowledge is interrupted Page 11 by gaps, which not only bear a large proportion to our solid information, but which in many cases are of such a nature that we can never hope to fill them up.

FOSSILS.—The remains of animals or vegetables which we now find entombed in the solid rock, and which constitute the working material of the palæontologist, are termed "fossils,"[3] or "petrifactions." In most cases, as can be readily understood, fossils are the actual hard parts of animals and plants which were in existence when the rock in which they are now found was being deposited. Most fossils, therefore, are of the nature of the shells of shell-fish, the skeletons of coral-zoophytes, the bones of vertebrate animals, or the wood, bark, or leaves of plants. All such bodies are more or less of a hard consistence to begin with, and are capable of resisting decay for a longer or shorter time—hence the frequency with which they occur in the fossil condition. Strictly speaking, however, by the term "fossil" must be understood "any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes" (Lyell). We shall find, in fact, that many of the objects which we have to study as "fossils" have never themselves actually formed parts of any animal or vegetable, though they are due to the former existence of such organisms, and indicate what was the nature of these. Thus the footprints left by birds, or reptiles, or quadrupeds upon sand or mud, are just as much proofs of the former existence of these animals as would be bones, feathers, or scales, though in themselves they are inorganic. Under the head of fossils, therefore, come the footprints of air-breathing vertebrate animals; the tracks, trails, and burrows of sea-worms, crustaceans, or molluscs; the impressions left on the sand by stranded jelly-fishes; the burrows in stone or wood of certain shell-fish; the "moulds" or "casts" of shells, corals, and other organic remains; and various other bodies of a more or less similar nature.

FOSSILISATION.—The term "fossilisation" is applied to all those processes through which the remains of organised beings may pass in being converted into fossils. These processes are numerous and varied; but there are three principal modes of fossilisation which alone need be considered here. In the first instance, the fossil is to all intents and purposes an actual portion of the original organised being—such as a bone, a shell, or a piece of wood. In some rare instances, as in the case of the body of the Mammoth discovered embedded in ice at the mouth of the Lena in Siberia, the fossil may be preserved Page 12 almost precisely in its original condition, and even with its soft parts uninjured. More commonly, certain changes have taken place in the fossil, the principal being the more or less total removal of the organic matter originally present. Thus bones become light and porous by the removal of their gelatine, so as to cleave to the tongue on being applied to that organ; whilst shells become fragile, and lose their primitive colours. In other cases, though practically the real body it represents, all the cavities of the fossil, down to its minutest recesses, may have become infiltrated with mineral matter. It need hardly be added, that it is in the more modern rocks that we find the fossils, as a rule, least changed from their former condition; but the original structure is often more or less completely retained in some of the fossils from even the most ancient formations.

In the second place, we very frequently meet with fossils in the state of "casts" or moulds of the original organic body. What occurs in this case will be readily understood if we imagine any common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded in clay or mud. If the clay were sufficiently soft and fluid, the first thing would be that it would gain access to the interior of the shell, and would completely fill up the space between the valves. The pressure, also, of the surrounding matter would insure that the clay would everywhere adhere closely to the exterior of the shell. If now we suppose the clay to be in any way hardened so as to be converted into stone, and if we were to break up the stone, we should obviously have the following state of parts. The clay which filled the shell would form an accurate cast of the interior of the shell, and the clay outside would give us an exact impression or cast of the exterior of the shell (fig. 1). We should have, then, Fig. 1

Fig. 1.—Trigonia longa, showing casts of the exterior and interior of the shell.—Cretaceous (Neocomian). two casts, an interior and an exterior, and the two would be very different to one another, since the inside of a shell is very unlike the outside. In the case, in fact, of many univalve shells, the interior cast or "mould" is so unlike the exterior cast, or unlike the shell itself, that it may be difficult to determine the true origin of the former.

It only remains to add that there is sometimes a further complication. If the rock be very porous and permeable by Page 13 water, it may happen that the original shell is entirely dissolved away, leaving the interior cast loose, like the kernel of a nut, within the case formed by the exterior cast. Or it may happen that subsequent to the attainment of this state of things, the space thus left vacant between the interior and exterior cast—the space, that is, formerly occupied by the shell itself—may be filled up by some foreign mineral deposited there by the infiltration of water. In this last case the splitting open of the rock would reveal an interior cast, an exterior cast, and finally a body which would have the exact form of the original shell, but which would be really a much later formation, and which would not exhibit under the microscope the minute structure of shell.

In the third class of cases we have fossils which present with the greatest accuracy the external form, and even sometimes the internal minute structure, of the original organic body, but which, nevertheless, are not themselves truly organic, but have been formed by a "replacement" of the particles of the primitive organism by some mineral substance. The most elegant example of this is afforded by fossil wood which has been "silicified" or converted into flint (silex). In such cases we have fossil wood which presents the rings of growth and fibrous structure of recent wood, and which under the microscope exhibits the minutest vessels which characterise ligneous tissue, together with the even more minute markings of the vessels (fig. 2). The whole, however, Fig. 2

Fig. 2.—Microscopic section of the silicified wood of a Conifer (Sequoia) cut in the long direction of the fibres. Post-tertiary? Colorado. (Original.) Fig. 3

Fig. 3.—Microscopic section of the wood of the common Larch (Abies larix), cut in the long direction of the fibres. In both the fresh and the fossil wood (fig. 2) are seen the discs characteristic of coniferous wood. (Original.) instead of being composed of the original carbonaceous matter of the wood, is now converted into flint. The only explanation that can be given Page 14 of this by no means rare phenomenon, is that the wood must have undergone a slow process of decay in water charged with silica or flint in solution. As each successive particle of wood was removed by decay, its place was taken by a particle of flint deposited from the surrounding water, till ultimately the entire wood was silicified. The process, therefore, resembles what would take place if we were to pull down a house built of brick by successive bricks, replacing each brick as removed by a piece of stone of precisely the same size and form. The result of this would be that the house would retain its primitive size, shape, and outline, but it would finally have been converted from a house of brick into a house of stone. Many other fossils besides wood—such as shells, corals, sponges, &c.—are often found silicified; and this may be regarded as the commonest form of fossilisation by replacement. In other cases, however, though the principle of the process is the same, the replacing substance may be iron pyrites, oxide of iron, sulphur, malachite, magnesite, talc, &c.; but it is rarely that the replacement with these minerals is so perfect as to preserve the more delicate details of internal structure.

CHAPTER II.

Fossils are found in rocks, though not universally or promiscuously; and it is therefore necessary that the palæontologist should possess some acquaintance with, at any rate, those rocks which yield organic remains, and which are therefore said to be "fossiliferous." In geological language, all the materials which enter into the composition of the solid crust of the earth, be their texture what it may—from the most impalpable mud to the hardest granite—are termed "rocks;" and for our present purpose we may divide these into two great groups. In the first division are the Igneous Rocks—such as the lavas and ashes of volcanoes—which are formed within the body of the earth itself, and which owe their structure and origin to the action of heat. The Igneous Rocks are formed primarily below the surface of the earth, which they only reach as the result of volcanic action; they are generally destitute of distinct "stratification," or arrangement in successive layers; and they do not contain fossils, except in the comparatively Page 15 rare instances where volcanic ashes have enveloped animals or plants which were living in the sea or on the land in the immediate vicinity of the volcanic focus. The second great division of rocks is that of the Fossiliferous, Aqueous, or Sedimentary Rocks. These are formed at the surface of the earth, and, as implied by one of their names, are invariably deposited in water. They are produced by vital or chemical action, or are formed from the "sediment" produced by the disintegration and reconstruction of previously existing rocks, without previous solution; they mostly contain fossils; and they are arranged in distinct layers or "strata." The so-called "aerial" rocks which, like beds of blown sand, have been formed by the action of the atmosphere, may also contain fossils; but they are not of such importance as to require special notice here.

For all practical purposes, we may consider that the Aqueous Rocks are the natural cemetery of the animals and plants of bygone ages; and it is therefore essential that the palæontological student should be acquainted with some of the principal facts as to their physical characters, their minute structure and mode of origin, their chief varieties, and their historical succession.

The Sedimentary or Fossiliferous Rocks form the greater portion of that part of the earth's crust which is open to our examination, and are distinguished by the fact that they are regularly "stratified" or arranged in distinct and definite layers or "strata." These layers may consist of a single material, as in a block of sandstone, or they may consist of different materials. When examined on a large scale, they are always found to consist of alternations of layers of different mineral composition. We may examine any given area, and find in it nothing but one kind of rock—sandstone, perhaps, or limestone. In all cases, however, if we extend our examination sufficiently far, we shall ultimately come upon different rocks; and, as a general rule, the thickness of any particular set of beds is comparatively small, so that different kinds of rock alternate with one another in comparatively small spaces.

As regards the origin of the Sedimentary Rocks, they are for the most part "derivative" rocks, being derived from the wear and tear of pre-existent rocks. Sometimes, however, they owe their origin to chemical or vital action, when they would more properly be spoken of simply as Aqueous Rocks. As to their mode of deposition, we are enabled to infer that the materials which compose them have formerly been spread out by the action of water, from what we see going on every day Page 16 at the mouths of our great rivers, and on a smaller scale wherever there is running water. Every stream, where it runs into a lake or into the sea, carries Fig. 4

Fig. 4.—Sketch of Carboniferous strata at Kinghorn, in Fife, showing stratified beds (limestone and shales) surmounted by an unstratified mass of trap. (Original.) with it a burden of mud, sand, and rounded pebbles, derived from the waste of the rocks which form its bed and banks. When these materials cease to be impelled by the force of the moving water, they sink to the bottom, the heaviest pebbles, of course, sinking first, the smaller pebbles and sand next, and the finest mud last. Ultimately, therefore, as might have been inferred upon theoretical grounds, and as is proved by practical experience, every lake becomes a receptacle for a series of stratified rocks produced by the streams flowing into it. These deposits may vary in different parts of the lake, according as one stream brought down one kind of material and another stream contributed another material; but in all cases the materials will bear ample evidence that they were produced, sorted, and deposited by running water. The finer beds of clay or sand will all be arranged in thicker or thinner layers or laminæ; and if there are any beds of pebbles these will all be rounded or smooth, just like the water-worn pebbles of any brook-course. In all probability, also, we should find in some of the beds the remains Page 17 of fresh-water shells or plants or other organisms which inhabited the lake at the time these beds were being deposited.

In the same way large rivers—such as the Ganges or Mississippi—deposit all the materials which they bring down at their mouths, forming in this way their "deltas." Whenever such a delta is cut through, either by man or by some channel of the river altering its course, we find that it is composed of a succession of horizontal layers or strata of sand or mud, varying in mineral composition, in structure, or in grain, according to the nature of the materials brought down by the river at different periods. Such deltas, also, will contain the remains of animals which inhabit the river, with fragments of the plants which grew on its banks, or bones of the animals which lived in its basin.

Nor is this action confined, of course, to large rivers only, though naturally most conspicuous in the greatest bodies of water. On the contrary, all streams, of whatever size, are engaged in the work of wearing down the dry land, and of transporting the materials thus derived from higher to lower levels, never resting in this work till they reach the sea.

Fig. 5.—Diagram to illustrate the formation of sedimentary deposits at the point where a river debouches into the sea. Lastly, the sea itself—irrespective of the materials delivered into it by rivers—is constantly preparing fresh Page 18 by its own action. Upon every coast-line the sea is constantly eating back into the land and reducing its component rocks to form the shingle and sand which we see upon every shore. The materials thus produced are not, however, lost, but are ultimately deposited elsewhere in the form of new stratified accumulations, in which are buried the remains of animals inhabiting the sea at the time.

Whenever, then, we find anywhere in the interior of the land any series of beds having these characters—composed, that is, of distinct layers, the particles of which, both large and small, show distinct traces of the wearing action of water—whenever and wherever we find such rocks, we are justified in assuming that they have been deposited by water in the manner above mentioned. Either they were laid down in some former lake by the combined action of the streams which flowed into it; or they were deposited at the mouth of some ancient river, forming its delta; or they were laid down at the bottom of the ocean. In the first two cases, any fossils which the beds might contain would be the remains of fresh-water or terrestrial organisms. In the last case, the majority, at any rate, of the fossils would be the remains of marine animals.

The term "formation" is employed by geologists to express "any group of rocks which have some character in common, whether of origin, age, or composition" (Lyell); so that we may speak of stratified and unstratified formations, aqueous or igneous formations, fresh-water or marine formations, and so on.

CHIEF DIVISIONS OF THE AQUEOUS ROCKS.

The Aqueous Rocks may be divided into two great sections, the Mechanically-formed and the Chemically-formed, including under the last head all rocks which owe their origin to vital action, as well as those produced by ordinary chemical agencies.

A. MECHANICALLY-FORMED ROCKS.—These are all those Aqueous Rocks of which we can obtain proofs that their particles have been mechanically transported to their present situation. Thus, if we examine a piece of conglomerate or puddingstone, we find it to be composed of a number of rounded pebbles embedded in an enveloping matrix or paste, which is usually of a sandy nature, but may be composed of carbonate of lime (when the rock is said to be a "calcareous conglomerate"). The pebbles in all conglomerates are worn and rounded by the action of water in motion, and thus show Page 19 that they have been subjected to much mechanical attrition, whilst they have been mechanically transported for a greater or less distance from the rock of which they originally formed part. The analogue of the old conglomerates at the present day is to be found in the great beds of shingle and gravel which are formed by the action of the sea on every coast-line, and which are composed of water-worn and well-rounded pebbles of different sizes. A breccia is a mechanically-formed rock, very similar to a conglomerate, and consisting of larger or smaller fragments of rock embedded in a common matrix. The fragments, however, are in this case all more or less angular, and are not worn or rounded. The fragments in breccias may be of large size, or they may be comparatively small (fig. 6); and the matrix may Fig. 6

Fig. 6.—Microscopic section of a calcareous breccia in the Lower Silurian (Coniston Limestone) of Shap Wells, Westmoreland. The fragments are all of small size, and consist of angular pieces of transparent quartz, volcanic ashes, and limestone embedded in a matrix of crystalline limestone. (Original.) be composed of sand (arenaceous) or of carbonate of lime (calcareous). In the case of an ordinary sandstone, again, we have a rock which may be regarded as simply a very fine-grained conglomerate or breccia, being composed of small grains of sand (silica), sometimes rounded, sometimes more or less angular, cemented together by some such substance as oxide of iron, silicate of iron, or carbonate of lime. A sandstone, therefore, like a conglomerate is a mechanically-formed rock, its component grams being equally the result of mechanical attrition and having equally been transported from a distance; and the same is true of the ordinary sand of the sea-shore, which is nothing more than an unconsolidated sandstone. Other so-called sands and sandstones, though equally mechanical in their origin, are truly calcareous in their nature, and are more or less entirely composed of carbonate of lime. Of this kind are the shell-sand so common on our coasts, and the coral-sand which is so largely formed in the neighbourhood of coral-reefs. In these cases the rock is composed of fragments of the skeletons of shellfish, and numerous other marine animals, together, in many instances, with the remains of certain sea-weeds (Corallines, Nullipores, &c,) which are endowed with the power of secreting Page 20 carbonate of lime from the sea-water. Lastly, in certain rocks still finer in their texture than sandstones, such as the various mud-rocks and shales, we can still recognise a mechanical source and origin. If slices of any of these rocks sufficiently thin to be transparent are examined under the microscope, it will be found that they are composed of minute grains of different sizes, which are all more or less worn and rounded, and which clearly show, therefore, that they have been subjected to mechanical attrition.

All the above-mentioned rocks, then, are mechanically-formed rocks; and they are often spoken of as "Derivative Rocks," in consequence of the fact that their particles can be shown to have been mechanically derived from other pre-existent rocks. It follows from this that every bed of any mechanically-formed rock is the measure and equivalent of a corresponding amount of destruction of some older rock. It is not necessary to enter here into a minute account of the subdivisions of these rocks, but it may be mentioned that they may be divided into two principal groups, according to their chemical composition. In the one group we have the so-called Arenaceous (Lat. arena, sand) or Siliceous Rocks, which are essentially composed of larger or smaller grains of flint or silica. In this group are comprised ordinary sand, the varieties of sandstone and grit, and most conglomerates and breccias. We shall, however, afterwards see that some siliceous rocks are of organic origin. In the second group are the so-called Argillaceous (Lat. argilla, clay) Rocks, which contain a larger or smaller amount of clay or hydrated silicate of alumina in their composition. Under this head come clays, shales, marls, marl-slate, clay-slates, and most flags and flagstones.

B. CHEMICALLY-FORMED ROCKS.—In this section are comprised all those Aqueous or Sedimentary Rocks which have been formed by chemical agencies. As many of these chemical agencies, however, are exerted through the medium of living beings, whether animals or plants, we get into this section a number of what may be called "organically-formed rocks." These are of the greatest possible importance to the palæontologist, as being to a greater or less extent composed of the actual remains of animals or vegetables, and it will therefore be necessary to consider their character and structure in some detail.

By far the most important of the chemically-formed rocks are the so-called Calcareous Rocks (Lat. calx, lime), comprising all those which contain a large proportion of carbonate Page 21 of lime, or are wholly composed of this substance. Carbonate of lime is soluble in water holding a certain amount of carbonic acid gas in solution; and it is, therefore, found in larger or smaller quantity dissolved in all natural waters, both fresh and salt, since these waters are always to some extent charged with the above-mentioned solvent gas. A great number of aquatic animals, however, together with some aquatic plants, are endowed with the power of separating the lime thus held in solution in the water, and of reducing it again to its solid condition. In this way shell-fish, crustaceans, sea-urchins, corals, and an immense number of other animals, are enabled to construct their skeletons; whilst some plants form hard structures within their tissues in a precisely similar manner. We do meet with some calcareous deposits, such as the "stalactites" and "stalagmites" of caves, the "calcareous tufa" and "travertine" of some hot springs, and the spongy calcareous deposits of so-called "petrifying springs," which are purely chemical in their origin, and owe nothing to the operation of living beings. Such deposits are formed simply by the precipitation of carbonate of lime from water, in consequence of the evaporation from the water of the carbonic acid gas which formerly held the lime in solution; but, though sometimes forming masses of considerable thickness and of geological importance, they do not concern us here. Almost all the limestones which occur in the series of the stratified rocks are, primarily at any rate, of organic origin, and have been, directly or indirectly, produced by the action of certain lime-making animals or plants, or both combined. The presumption as to all the calcareous rocks, which cannot be clearly shown to have been otherwise produced, is that they are thus organically formed; and in many cases this presumption can be readily reduced to a certainty. There are many varieties of the calcareous rocks, but the following are those which are of the greatest importance:—

Chalk is a calcareous rock of a generally soft and pulverulent texture, and with an earthy fracture. It varies in its purity, being sometimes almost wholly composed of carbonate of lime, and at other times more or less intermixed with foreign matter. Though usually soft and readily reducible to powder, chalk is occasionally, as in the north of Ireland, tolerably hard and compact; but it never assumes the crystalline aspect and stony density of limestone, except it be in immediate contact with some mass of igneous rock. By means of the microscope, the true nature and mode of formation of chalk can be determined with the greatest ease. In the case of the harder varieties, the examination can be conducted by means of Page 22 slices ground down to a thinness sufficient to render them transparent; but in the softer kinds the rock must be disintegrated under water, and the débris examined microscopically. When investigated by either of these methods, chalk is found to be a genuine organic rock, being composed of the shells or hard parts of innumerable marine animals of different kinds, some entire, some fragmentary, cemented together by a matrix of very finely granular carbonate of lime. Foremost amongst the animal remains which so largely compose chalk are the shells of the minute creatures which will be subsequently spoken of under the name of Foraminifera (fig. 7), and which, in spite of their Fig. 7

Fig. 7.—Section of Gravesend Chalk, examined by transmitted light and highly magnified. Besides the entire shells of Globigerina, Rotalia, and Textularia, numerous detached chambers of Globigerina are seen. (Original.) microscopic dimensions, play a more important part in the process of lime-making than perhaps any other of the larger inhabitants of the ocean.

As chalk is found in beds of hundreds of feet in thickness, and of great purity, there was long felt much difficulty in satisfactorily accounting for its mode of formation and origin. By the researches of Carpenter, Wyville Thomson, Huxley, Wallich, and others, it has, however, been shown that there is now forming, in the profound depths of our great oceans, a deposit which is in all essential respects identical with chalk, and which is generally known as the "Atlantic ooze," from its having been first discovered in that sea. This ooze is found at great depths (5000 to over 15,000 feet) in both the Atlantic and Pacific, covering enormously large areas of the sea-bottom, and it presents itself as a whitish-brown, sticky, impalpable mud, very like greyish chalk when dried. Chemical examination shows that the ooze is composed almost wholly of carbonate of lime, and microscopical examination proves it to be of organic origin, and to be made up of the remains of living beings. The principal forms of these belong to the Foraminifera, and the commonest of these are the irregularly-chambered shells of Globigerina, absolutely indistinguishable from the Globigerinœ which are so largely present in the chalk (fig. 8). Along with these occur fragments of the skeletons of other larger creatures, Page 23 and a certain proportion of the flinty cases of minute animal and vegetable organisms (Polycystina and Diatoms). Fig. 8

Fig. 8.—Organisms in the Atlantic Ooze, chiefly Foraminifera (Globigerina and Textularia), with Polycystina and sponge-spicules; highly magnified. (Original.) Though many of the minute animals, the hard parts of which form the ooze, undoubtedly live at or near the surface of the sea, others, probably, really live near the bottom; and the ooze itself forms a congenial home for numerous sponges, sea-lilies, and other marine animals which flourish at great depths in the sea. There is thus established an intimate and most interesting parallelism between the chalk and the ooze of modern oceans. Both are formed essentially in the same way, and the latter only requires consolidation to become actually converted into chalk. Both are fundamentally organic deposits, apparently requiring a great depth of water for their accumulation, and mainly composed of the remains of Foraminifera, together with the entire or broken skeletons of other marine animals of greater dimensions. It is to be remembered, however, that the ooze, though strictly representative of the chalk, cannot be said in any proper sense to be actually identical with the formation so called by geologists. A great lapse of time separates the two, and though composed of the remains of representative classes or groups of animals, it is only in the case of the lowly-organised Globigerinœ, and of some other organisms of little higher grade, that we find absolutely the same kinds or species of animals in both.

Limestone, like chalk, is composed of carbonate of lime, sometimes almost pure, but more commonly with a greater or less intermixture of some foreign material, such as alumina or silica. The varieties of limestone are almost innumerable, but the great majority can be clearly proved to agree with chalk in being essentially of organic origin, and in being more or less largely composed of the remains of living beings. In many instances the organic remains which compose limestone are so large as to be readily visible to the naked eye, and the rock is at once seen to be nothing more than an agglomeration of the skeletons, generally fragmentary, of certain marine animals, cemented together by a matrix of carbonate of lime. Page 24 This is the case, for example, with the so-called "Crinoidal Limestones" and "Encrinital Marbles" with which the geologist is so familiar, especially as occurring in great beds amongst the older formations of the earth's crust. These are seen, on weathered or broken surfaces, or still better in polished slabs (fig. 9), to be Fig. 9

Fig. 9.—Slab of Crinoidal marble, from the Carboniferous limestone of Dent, in Yorkshire, of the natural size. The polished surface intersects the columns of the Crinoids at different angles, and thus gives rise to varying appearances. (Original.) composed more or less exclusively of the broken stems and detached plates of sea-lilies (Crinoids). Similarly, other limestones are composed almost entirely of the skeletons of corals; and such old coralline limestones can readily be paralleled by formations which we can find in actual course of production at the present day. We only need to transport ourselves to the islands of the Pacific, to the West Indies, or to the Indian Ocean, to find great masses of lime formed similarly by living corals, and well known to everyone under the name of "coral-reefs." Such reefs are often of vast extent, both superficially and in vertical thickness, and they fully equal in this respect any of the coralline limestones of bygone ages. Again, we find other limestones—such as the celebrated "Nummulitic Limestone" (fig. 10), which sometimes attains a thickness of some thousands of feet—which are almost entirely made up of the shells of Foraminifera. In the case of the "Nummulitic Limestone," just mentioned, these shells are of large size, varying from the up to that of a Page 25 florin. There are, however, as we shall see, many other limestones, which are likewise largely made up of Foraminifera, but in Fig. 10

Fig. 10.—Piece of Nummulitic Limestone from the Great Pyramid. Of the natural size. (Original.) which the shells are very much more minute, and would hardly be seen at all without the microscope.

We may, in fact, consider that the great agents in the production of limestones in past ages have been animals belonging to the Crinoids, the Corals, and the Foraminifera. At the present day, the Crinoids have been nearly extinguished, and the few known survivors seem to have retired to great depths in the ocean; but the two latter still actively carry on the work of lime-making, the former being very largely helped in their operations by certain lime-producing marine plants (Nullipores and Corallines). We have to remember, however, that though the limestones, both ancient and modern, that we have just spoken of, are truly organic, they are not necessarily formed out of the remains of animals which actually lived on the precise spot where we now find the limestone itself. We may find a crinoidal limestone, which we can show to have been actually formed by the successive growth of generations of sea-lilies in place; but we shall find many others in which the rock is made up of innumerable fragments of the skeletons of these creatures, which have been clearly worn and rubbed by the sea-waves, and which have been mechanically transported to their present site. In the same way, a limestone may be shown to have been an actual coral-reef, by the fact that we find in it great masses of coral, growing in their natural position, and Page 26 exhibiting plain proofs that they were simply quietly buried by the calcareous sediment as they grew; but other limestones may contain only numerous rolled and water-worn fragments of corals. This is precisely paralleled by what we can observe in our existing coral-reefs. Parts of the modern coral-islands and coral-reefs are really made up of corals, dead or alive, which actually grew on the spot where we now find them; but other parts are composed of a limestone-rock ("coral-rock"), or of a loose sand ("coral-sand"), which is organic in the sense that it is composed of lime formed by living beings, but which, in truth, is composed of fragments of the skeletons of these living beings, mechanically transported and heaped together by the sea. To take another example nearer home, we may find great accumulations of calcareous matter formed in place, by the growth of shell-fish, such as oysters or mussels; but we can also find equally great accumulations on many of our shores in the form of "shell-sand," which is equally composed of the shells of molluscs, but which is formed by the trituration of these shells by the mechanical power of the sea-waves. We thus see that though all these limestones are primarily organic, they not uncommonly become "mechanically-formed" rocks in a secondary sense, the materials of which they are composed being formed by living beings, but having been mechanically transported to the place where we now find them.

Many limestones, as we have seen, are composed of large and conspicuous organic remains, such as strike the eye at once. Many others, however, which at first sight appear compact, more or less crystalline, and nearly devoid of traces of life, are found, when properly examined, to be also composed of the remains of various organisms. All the commoner limestones, in fact, from the Lower Silurian period onwards, can be easily proved to be thus organic rocks, if we investigate weathered or polished surfaces with a lens, or, still better, if we cut thin slices of the rock and grind these down till they are transparent. When thus examined, the rock is usually found to be composed of innumerable entire or fragmentary fossils, cemented together by a granular or crystalline matrix of carbonate of lime (figs. 11 and 12). When the matrix is granular, the rock is precisely similar to chalk, except that it is harder and less earthy in texture, whilst the fossils are only occasionally referable to the Foraminifera. In other cases, the matrix is more or less crystalline, and when this crystallisation has been carried to a great extent, the original organic nature of the rock may be greatly or completely obscured Page 27 thereby. Thus, in limestones which have been greatly altered or "metamorphosed" by the combined action of heat and pressure, all traces of organic remains become Fig. 11

Fig. 11.—Section of Carboniferous Limestone from Spergen Hill, Indiana, U.S., showing numerous large-sized Foraminifera (Endothyra) and a few oolitic grains; magnified. (Original.) Fig. 12

Fig 12.—Section of Coniston Limestone (Lower Silurian) from Keisler, Westmoreland; magnified. The matrix is very coarsely crystalline, and the included organic remains are chiefly stems of Crinoids. (Original.) annihilated, and the rock becomes completely crystalline throughout. This, for example, is the case with the ordinary white "statuary marble," slices of which exhibit under the microscope nothing but an aggregate of beautifully transparent crystals of carbonate of lime, without the smallest traces of fossils. There are also other cases, where the limestone is not necessarily highly crystalline, and where no metamorphic action in the strict sense has taken place, in which, nevertheless, the microscope fails to reveal any evidence that the rock is organic. Such cases are somewhat obscure, and doubtless depend on different causes in different instances; but they do not affect the important generalisation that limestones are fundamentally the product of the operation of living beings. This fact remains certain; and when we consider the vast superficial extent occupied by calcareous deposits, and the enormous collective thickness of these, the mind cannot fail to be impressed with the immensity of the period demanded for the formation of these by the agency of such humble and often microscopic creatures as Corals, Sea-lilies, Foraminifers, and Shell-fish.

Amongst the numerous varieties of limestone, a few are of such interest as to deserve a brief notice. Magnesian limestone or dolomite, differs from ordinary limestone in containing a certain proportion of carbonate of magnesia along with the carbonate of lime. The typical dolomites contain a large proportion Page 28 of carbonate of magnesia, and are highly crystalline. The ordinary magnesian limestones (such as those of Durham in the Permian series, and the Guelph Limestones of North America in the Silurian series) are generally of a yellowish, buff, or brown colour, with a crystalline or pearly aspect, effervescing with acid much less freely than ordinary limestone, exhibiting numerous cavities from which fossils have been dissolved out, and often assuming the most varied and singular forms in consequence of what is called "concretionary action." Examination with the microscope shows that these limestones are composed of an aggregate of minute but perfectly distinct crystals, but that minute organisms of different kinds, or fragments of larger fossils, are often present as well. Other magnesian limestones, again, exhibit no striking external peculiarities by which the presence of magnesia would be readily recognised, and though the base of the rock is crystalline, they are replete with the remains of organised beings. Thus many of the magnesian limestones of the Carboniferous series of the North of England are very like ordinary limestone to look at, though effervescing less freely with acids, and the microscope proves them to be charged with the remains of Foraminifera and other minute organisms.

Marbles are of various kinds, all limestones which are sufficiently hard and compact to take a high polish going by this name. Statuary marble, and most of the celebrated foreign marbles, are "metamorphic" rocks, of a highly crystalline nature, and having all traces of their primitive organic structure obliterated. Many other marbles, however, differ from ordinary limestone simply in the matter of density. Thus, many marbles (such as Derbyshire marble) are simply "crinoidal limestones" (fig. 9); whilst various other British marbles exhibit innumerable organic remains under the microscope. Black marbles owe their colour to the presence of very minute particles of carbonaceous matter, in some cases at any rate; and they may either be metamorphic, or they may be charged with minute fossils such as Foraminifera (e.g., the black limestones of Ireland, and the black marble of Dent, in Yorkshire).

"Oolitic" limestones, or "oolites," as they are often called, are of interest both to the palæontologist and geologist. The peculiar structure to which they owe their name is that the rock is more or less entirely composed of spheroidal or oval grains, which vary in size from the head of a small pin or less up to the size of a pea, and which may be in almost immediate contact with one another, or may be cemented together by a Page 29 more or less abundant calcareous matrix. When the grains are pretty nearly spherical and are in tolerably close contact, the rock looks very like the roe of a fish, and the name of "oolite" or "egg-stone" is in allusion to this. When the grains are of the size of peas or upwards, the rock is often called a "pisolite" (Lat. pisum, a pea). Limestones having this peculiar structure are especially abundant in the Jurassic formation, which is often called the "Oolitic series" for this reason; but essentially similar limestones occur not uncommonly in the Silurian, Devonian, and Carboniferous formations, and, indeed, in almost all rock-groups in which limestones are largely developed. Whatever may be the age of the formation in which they occur, and whatever may be the size of their component "eggs," the structure of oolitic limestones is fundamentally the same. All the ordinary oolitic limestones, namely, consist of little spherical or ovoid "concretions," as they are termed, cemented together by a larger or smaller amount of crystalline carbonate of lime, together, in many instances, with numerous organic remains of different kinds Fig. 13

Fig. 13.—Slice of oolitic limestone from the Jurassic series (Coral Rag) of Weymouth; magnified. (Original.) (fig. 13). When examined in polished slabs, or in thin sections prepared for the microscope, each of these little concretions is seen to consist of numerous concentric coats of carbonate of lime, which sometimes simply surround an imaginary centre, but which, more commonly, have been successively deposited round some foreign body, such as a little crystal of quartz, a cluster of sand-grains, or a minute shell. In other cases, as in some of the beds of the Carboniferous limestone in the North of England, where the limestone is highly "arenaceous," there is a modification of the oolitic structure. Microscopic sections of these sandy limestones (fig. 14) show numerous generally angular or oval grains of silica or flint, each of which is commonly surrounded by a thin coating of carbonate of lime, or sometimes by several such coats, the whole being cemented together along with the shells of Foraminifera and other minute fossils by a matrix of crystalline calcite. As compared with typical oolites, the concretions in these limestones are usually much more irregular in shape, Page 30 often lengthened out and almost cylindrical, at other times angular, the central nucleus Fig. 14

Fig. 14.—Slice of arenaceous and oolitic limestone from the Carboniferous series of Shap, Westmoreland; magnified. The section also exhibit Foraminifera and other minute fossils. (Original.) being of large size, and the surrounding envelope of lime being very thin, and often exhibiting no concentric structure. In both these and the ordinary oolites, the structure is fundamentally the same. Both have been formed in a sea, probably of no great depth, the waters of which were charged with carbonate of lime in solution, whilst the bottom was formed of sand intermixed with minute shells and fragments of the skeletons of larger marine animals. The excess of lime in the sea-water was precipitated round the sand-grams, or round the smaller shells, as so many nuclei, and this precipitation must often have taken place time after time, so as to give rise to the concentric structure so characteristic of oolitic concretions. Finally, the oolitic grains thus produced were cemented together by a further precipitation of crystalline carbonate of lime from the waters of the ocean.

Phosphate of Lime is another lime-salt, which is of interest to the palæontologist. It does not occur largely in the stratified series, but it is found in considerable beds [4] in the Laurentian formation, and less abundantly in some later rock-groups, whilst it occurs abundantly in the form of nodules in parts of the Cretaceous (Upper Greensand) and Tertiary deposits. Phosphate of lime forms the larger proportion of the earthy matters of the bones of Vertebrate animals, and also occurs in less amount in the skeletons of certain of the Invertebrates (e.g., Crustacea). It is, indeed, perhaps more distinctively than carbonate of lime, an organic compound; and though the formation of many known deposits of phosphate of Page 31 lime cannot be positively shown to be connected with the previous operation of living beings, there is room for doubt whether this salt is not in reality always primarily a product of vital action. The phosphatic nodules of the Upper Greensand are erroneously called "coprolites," from the belief originally entertained that they were the droppings or fossilised excrements of extinct animals; and though this is not the case, there can be little doubt but that the phosphate of lime which they contain is in this instance of organic origin.[5] It appears, in fact, that decaying animal matter has a singular power of determining the precipitation around it of mineral salts dissolved in water. Thus, when any animal bodies are undergoing decay at the bottom of the sea, they have a tendency to cause the precipitation from the surrounding water of any mineral matters which may be dissolved in it; and the organic body thus becomes a centre round which the mineral matters in question are deposited in the form of a "concretion" or "nodule." The phosphatic nodules in question were formed in a sea in which phosphate of lime, derived from the destruction of animal skeletons, was held largely in solution; and a precipitation of it took place round any body, such as a decaying animal substance, which happened to be lying on the sea-bottom, and which offered itself as a favourable nucleus. In the same way we may explain the formation of the calcareous nodules, known as "septaria" or "cement stones," which occur so commonly in the London Clay and Kimmeridge Clay, and in which the principal ingredient is carbonate of lime. A similar origin is to be ascribed to the nodules of clay iron-stone (impure carbonate of iron) which occur so abundantly in the shales of the Carboniferous series and in other argillaceous deposits; and a parallel modern example is to be found in the nodules of manganese, which were found by Sir Wyville Thomson, in the Challenger, to be so numerously scattered over the floor of the Pacific at great depths. In accordance with this mode of origin, it is exceedingly common to find in the centre of all these nodules, both old and new, some organic body, such as a bone, a shell, or a tooth, which acted as the original nucleus of precipitation, and Page 32 was thus preserved in a shroud of mineral matter. Many nodules, it is true, show no such nucleus; but it has been affirmed that all of them can be shown, by appropriate microscopical investigation, to have been formed round an original organic body to begin with (Hawkins Johnson).

[Footnote 4: Apart from the occurrence or phosphate of lime in actual beds in the stratified rocks, as in the Laurentian and Silurian series, this salt may also occur disseminated through the rock, when it can only be detected by chemical analysis. It is interesting to note that Dr Hicks has recently proved the occurrence of phosphate of lime in this disseminated form in rocks as old as the Cambrian, and that in quantity quite equal to what is generally found to be present in the later fossiliferous rocks. This affords a chemical proof that animal life flourished abundantly in the Cambrian seas.]

[Footnote 5: It has been maintained, indeed, that the phosphatic nodules so largely worked for agricultural purposes, are in themselves actual organic bodies or true fossils. In a few cases this admits of demonstration, as it can be shown that the nodule is simply an organism (such as a sponge) infiltrated with phosphate of lime (Sollas); but there are many other cases in which no actual structure has yet been shown to exist, and as to the true origin of which it would be hazardous to offer a positive opinion.]

The last lime-salt which need be mentioned is gypsum, or sulphate of lime. This substance, apart from other modes of occurrence, is not uncommonly found interstratified with the ordinary sedimentary rocks, in the form of more or less irregular beds; and in these cases it has a palæontological importance, as occasionally yielding well-preserved fossils. Whilst its exact mode of origin is uncertain, it cannot be regarded as in itself an organic rock, though clearly the product of chemical action. To look at, it is usually a whitish or yellowish-white rock, as coarsely crystalline as loaf-sugar, or more so; and the microscope shows it to be composed entirely of crystals of sulphate of lime.

We have seen that the calcareous or lime-containing rocks are the most important of the group of organic deposits; whilst the siliceous or flint-containing rocks may be regarded as the most important, most typical, and most generally distributed of the mechanically-formed rocks. We have, however, now briefly to consider certain deposits which are more or less completely formed of flint; but which, nevertheless, are essentially organic in their origin.

Flint or silex, hard and intractable as it is, is nevertheless capable of solution in water to a certain extent, and even of assuming, under certain circumstances, a gelatinous or viscous condition. Hence, some hot-springs are impregnated with silica to a considerable extent; it is present in small quantity in sea-water; and there is reason to believe that a minute proportion must very generally be present in all bodies of fresh water as well. It is from this silica dissolved in the water that many animals and some plants are enabled to construct for themselves flinty skeletons; and we find that these animals and plants are and have been sufficiently numerous to give rise to very considerable deposits of siliceous matter by the mere accumulation of their skeletons. Amongst the animals which require special mention in this connection are the microscopic organisms which are known to the naturalist as Polycystina. These little creatures are of the lowest possible grade of organisation, very closely related to the animals which we have previously spoken of as Foraminifera, but differing in the fact that they secrete a shell or skeleton composed of flint instead of lime. The Polycystina occur abundantly in our present seas; Page 33 and their shells are present in some numbers in the ooze which is found at great depths in the Atlantic and Pacific oceans, being easily recognised by their exquisite shape, their glassy transparency, the general presence of longer or shorter spines, and the sieve-like perforations in the walls. Both in Barbadoes and in the Nicobar islands occur geological formations which are composed of the flinty skeletons of these microscopic animals; the deposit in the former locality attaining a great thickness, and having been long known to workers with the microscope under the name of "Barbadoes earth" (fig. 15).

In addition to flint-producing animals, we have also the great group of fresh-water and marine microscopic plants known as Fig. 15

Fig. 15.—Shells of Polycystina from "Barbadoes earth;" greatly magnified. (Original.) Fig. 16

Fig 16.—Cases of Diatoms in the Richmond "Infusorial earth;" highly magnified. (Original.) Diatoms, which likewise secrete a siliceous skeleton, often of great beauty. The skeletons of Diatoms are found abundantly at the present day in lake-deposits, guano, the silt of estuaries, and in the mud which covers many parts of the sea-bottom; they have been detected in strata of great age; and in spite of their microscopic dimensions, they have not uncommonly accumulated to form deposits of great thickness, and of considerable superficial extent. Thus the celebrated deposit of "tripoli" ("Polir-schiefer") of Bohemia, largely worked as polishing-powder, is composed wholly, or almost wholly, of the flinty cases of Diatoms, of which it is calculated that no less than forty-one thousand millions go to make up a single cubic inch of the stone. Another celebrated deposit is the so-called "Infusorial earth" of Richmond in Virginia, where there is a stratum in places thirty feet thick, composed almost entirely of the microscopic shells of Diatoms.

Nodules or layers of flint, or the impure variety of flint Page 34 known as chert, are found in limestones of almost all ages from the Silurian upwards; but they are especially abundant in the chalk. When these flints are examined in thin and transparent slices under the microscope, or in polished sections, they are found to contain an abundance of minute organic bodies—such as Foraminifera, sponge-spicules, &c.—embedded in a siliceous basis. In many instances the flint contains larger organisms—such as a Sponge or a Sea-urchin. As the flint has completely surrounded and infiltrated the fossils which it contains, it is obvious that it must have been deposited from sea-water in a gelatinous condition, and subsequently have hardened. That silica is capable of assuming this viscous and soluble condition is known; and the formation of flint may therefore be regarded as due to the separation of silica from the sea-water and its deposition round some organic body in a state of chemical change or decay, just as nodules of phosphate of lime or carbonate of iron are produced. The existence of numerous organic bodies in flint has long been known; but it should be added that a recent observer (Mr Hawkins Johnson) asserts that the existence of an organic structure can be demonstrated by suitable methods of treatment, even in the actual matrix or basis of the flint.[6]

[Footnote 6: It has been asserted that the flints of the chalk are merely fossil sponges. No explanation of the origin of flint, however, can be satisfactory, unless it embraces the origin of chert in almost all great limestones from the Silurian upwards, as well as the common phenomenon of the silicification of organic bodies (such as corals and shells) which are known with certainty to have been originally calcareous.]

In addition to deposits formed of flint itself, there are other siliceous deposits formed by certain silicates, and also of organic origin. It has been shown, namely—by observations carried out in our present seas—that the shells of Foraminifera are liable to become completely infiltrated by silicates (such as "glauconite," or silicate of iron and potash). Should the actual calcareous shell become dissolved away subsequent to this infiltration—as is also liable to occur—then, in place of the shells of the Foraminifera, we get a corresponding number of green sandy grains of glauconite, each grain being the cast of a single shell. It has thus been shown that the green sand found covering the sea-bottom in certain localities (as found by the Challenger expedition along the line of the Agulhas current) is really organic, and is composed of casts of the shells of Foraminifera. Long before these observations had been made, it had been shown by Professor Ehrenberg that the green sands of various geological formations are composed mainly of the internal casts of the shells of Foraminifera, and Page 35 we have thus another and a very interesting example how rock-deposits of considerable extent and of geological importance can be built up by the operation of the minutest living beings.

As regards argillaceous deposits, containing alumina or clay as their essential ingredient, it cannot be said that any of these have been actually shown to be of organic origin. A recent observation by Sir Wyville Thomson would, however, render it not improbable that some of the great argillaceous accumulations of past geological periods may be really organic. This distinguished observer, during the cruise of the Challenger, showed that the calcareous ooze which has been already spoken of as covering large areas of the floor of the Atlantic and Pacific at great depths, and which consists almost wholly of the shells of Foraminifera, gave place at still greater depths to a red ooze consisting of impalpable clayey mud, coloured by oxide of iron, and devoid of traces of organic bodies. As the existence of this widely-diffused red ooze, in mid-ocean, and at such great depths, cannot be explained on the supposition that it is a sediment brought down into the sea by rivers, Sir Wyville Thomson came to the conclusion that it was probably formed by the action of the sea-water upon the shells of Foraminifera. These shells, though mainly consisting of lime, also contain a certain proportion of alumina, the former being soluble in the carbonic acid dissolved in the sea-water, whilst the latter is insoluble. There would further appear to be grounds for believing that the solvent power of the sea-water over lime is considerably increased at great depths. If, therefore, we suppose the shells of Foraminifera to be in course of deposition over the floor of the Pacific, at certain depths they would remain unchanged, and would accumulate to form a calcareous ooze; but at greater depths they would be acted upon by the water, their lime would be dissolved out, their form would disappear, and we should simply have left the small amount of alumina which they previously contained. In process of time this alumina would accumulate to form a bed of clay; and as this clay had been directly derived from the decomposition of the shells of animals, it would be fairly entitled to be considered an organic deposit. Though not finally established, the hypothesis of Sir Wyville Thomson on this subject is of the greatest interest to the palæontologist, as possibly serving to explain the occurrence, especially in the older formations, of great deposits of argillaceous matter which are entirely destitute of traces of life.

It only remains, in this connection, to shortly consider the rock-deposits in which carbon is found to be present in Page 36 greater or less quantity. In the great majority of cases where rocks are found to contain carbon or carbonaceous matter, it can be stated with certainty that this substance is of organic origin, though it is not necessarily derived from vegetables. Carbon derived from the decomposition of animal bodies is not uncommon; though it never occurs in such quantity from this source as it may do when it is derived from plants. Thus, many limestones are more or less highly bituminous; the celebrated siliceous flags or so-called "bituminous schists" of Caithness are impregnated with oily matter apparently derived from the decomposition of the numerous fishes embedded in them; Silurian shales containing Graptolites, but destitute of plants, are not uncommonly "anthracitic," and contain a small percentage of carbon derived from the decay of these zoophytes; whilst the petroleum so largely worked in North America has not improbably an animal origin. That the fatty compounds present in animal bodies should more or less extensively impregnate fossiliferous rock-masses, is only what might be expected; but the great bulk of the carbon which exists stored up in the earth's crust is derived from plants; and the form in which it principally presents itself is that of coal. We shall have to speak again, and at greater length, of coal, and it is sufficient to say here that all the true coals, anthracites, and lignites, are of organic origin, and consist principally of the remains of plants in a more or less altered condition. The bituminous shales which are found so commonly associated with beds of coal also derive their carbon primarily from plants; and the same is certainly, or probably, the case with similar shales which are known to occur in formations younger than the Carboniferous. Lastly, carbon may occur as a conspicuous constituent of rock-masses in the form of graphite or black-lead. In this form, it occurs in the shape of detached scales, of veins or strings, or sometimes of regular layers;[7] and there can be little doubt that in many instances it has an organic origin, though this is not capable of direct proof. When present, at any rate, in quantity, and in the form of layers associated with stratified rocks, as is often the case in the Laurentian formation, there can be little hesitation in regarding it as of vegetable origin, and as an altered coal.

[Footnote 7: In the Huronian formation at Steel River, on the north shore of Lake Superior, there exists a bed of carbonaceous matter which is regularly interstratified with the surrounding rocks, and has a thickness of from 30 to 40 feet. This bed is shown by chemical analysis to contain about 50 per cent of carbon, partly in the form of graphite, partly in the form of anthracite; and there can be little doubt but that it is really a stratum of "metamorphic" coal.]

Page 37 CHAPTER III.

The physical geologist, who deals with rocks simply as rocks, and who does not necessarily trouble himself about what fossils they may contain, finds that the stratified deposits which form so large a portion of the visible part of the earth's crust are not promiscuously heaped together, but that they have a certain definite arrangement. In each country that he examines, he finds that certain groups of strata lie above certain other groups; and in comparing different countries with one another, he finds that, in the main, the same groups of rocks are always found in the same relative position to each other. It is possible, therefore, for the physical geologist to arrange the known stratified rocks into a successive series of groups, or "formations," having a certain definite order. The establishment of this physical order amongst the rocks introduces, however, at once the element of time, and the physical succession of the strata can be converted directly into a historical or chronological succession. This is obvious, when we reflect that any bed or set of beds of sedimentary origin is clearly and necessarily younger than all the strata upon which it rests, and older than all those by which it is surmounted.

It is possible, then, by an appeal to the rocks alone, to determine in each country the general physical succession of the strata, and this "stratigraphical" arrangement, when once determined, gives us the relative ages of the successive groups. The task, however, of the physical geologist in this matter is immensely lightened when he calls in palæontology to his aid, and studies the evidence of the fossils embedded in the rocks. Not only is it thus much easier to determine the order of succession of the strata in any given region, but it becomes now for the first time possible to compare, with certainty and precision, the order of succession in one region with that which exists in other regions far distant. The value of fossils as tests of the relative ages of the sedimentary rocks depends on the fact that they are not indefinitely or promiscuously scattered through the crust of the earth,—as it is conceivable that they might be. On the contrary, the first and most firmly established law of Palæontology is, that particular kinds of fossils are confined to particular rocks, and particular groups of fossils are confined to particular groups of rocks. Page 38 Fossils, then, are distinctive of the rocks in which they are found—much more distinctive, in fact, than the mere mineral character of the rock can be, for that commonly changes as a formation is traced from one region to another, whilst the fossils remain unaltered. It would therefore be quite possible for the palæontologist, by an appeal to the fossils alone, to arrange the series of sedimentary deposits into a pile of strata having a certain definite order. Not only would this be possible, but it would be found—if sufficient knowledge had been brought to bear on both sides—that the palæontological arrangement of the strata would coincide in its details with the stratigraphical or physical arrangement.

Happily for science, there is no such division between the palæontologist and the physical geologist as here supposed; but by the combined researches of the two, it has been found possible to divide the entire series of stratified deposits into a number of definite rock-groups or formations, which have a recognised order of succession, and each of which is characterised by possessing an assemblage of organic remains which do not occur in association in any other formation. Such an assemblage of fossils, characteristic of any given formation, represents the life of the particular period in which the formation was deposited. In this way the past history of the earth becomes divided into a series of successive life-periods, each of which corresponds with the deposition of a particular formation or group of strata.

Whilst particular assemblages of organic forms characterise particular groups of rocks, it may be further said that, in a general way, each subdivision of each formation has its own peculiar fossils, by which it may be recognised by a skilled worker in Palæontology. Whenever, for instance, we meet with examples of the fossils which are known as Graptolites, we may be sure that we are dealing with Silurian rocks (leaving out of sight one or two forms doubtfully referred to this family). We may, however, go much farther than this with perfect safety. If the Graptolites belong to certain genera, we may be quite certain that we are dealing with Lower Silurian rocks. Furthermore, if certain special forms are present, we may be even able to say to what exact subdivision of the Lower Silurian series they belong.

As regards particular fossils, however, or even particular classes of fossils, conclusions of this nature require to be accompanied by a tacit but well-understood reservation. So far as Page 39 our present observation goes, none of the undoubted Graptolites have ever been discovered in rocks later than those known upon other grounds to be Silurian; but it is possible that they might at any time be detected in younger deposits. Similarly, the species and genera which we now regard as characteristic of the Lower Silurian, may at some future time be found to have survived into the Upper Silurian period. We should not forget, therefore, in determining the age of strata by palæontological evidence, that we are always reasoning upon generalisations which are the result of experience alone, and which are liable to be vitiated by further and additional discoveries.

When the palæontological evidence as to the age of any given set of strata is corroborated by the physical evidence, our conclusions may be regarded as almost certain; but there are certain limitations and fallacies in the palæontological method of inquiry which deserve a passing mention. In the first place, fossils are not always present in the stratified rocks; many aqueous rocks are unfossiliferous, through a thickness of hundreds or even thousands of feet of little-altered sediments; and even amongst beds which do contain fossils, we often meet with strata of many feet or yards in thickness which are wholly destitute of any traces of fossils. There are, therefore, to begin with, many cases in which there is no palæontological evidence extant or available as to the age of a given group of strata. In the second place, palæontological observers in different parts of the world are liable to give different names to the same fossil, and in all parts of the world they are occasionally liable to group together different fossils under the same title. Both these sources of fallacy require to be guarded against in reasoning as to the age of strata from their fossil remains. Thirdly, the mere fact of fossils being found in beds which are known by physical evidence to be of different ages, has commonly led palæontologists to describe them as different species. Thus, the same fossil, occurring in successive groups of strata, and with the merely trivial and varietal differences due to the gradual change in its environment, has been repeatedly described as a distinct species, with a distinct name, in every bed in which it was found. We know, however, that many fossils range vertically through many groups of strata, and there are some which even pass through several formations. The mere fact of a difference of physical position ought never to be taken into account at all in considering and determining the true affinities of a fossil. Fourthly, the results of experience, instead of being an assistance, are sometimes liable to operate as a source of error. When once, Page 40 namely, a generalisation has been established that certain fossils occur in strata of a certain age, palæontologists are apt to infer that all beds containing similar fossils must be of the same age. There is a presumption, of course, that this inference would be correct; but it is not a conclusion resting upon absolute necessity, and there might be physical evidence to disprove it. Fifthly, the physical geologist may lead the palæontologist astray by asserting that the physical evidence as to the age and position of a given group of beds is clear and unequivocal, when such evidence may be, in reality, very slight and doubtful. In this way, the observer may be readily led into wrong conclusions as to the nature of the organic remains—often obscure and fragmentary—which it is his business to examine, or he may be led erroneously to think that previous generalisations as to the age of certain kinds of fossils are premature and incorrect. Lastly, there are cases in which, owing to the limited exposure of the beds, to their being merely of local development, or to other causes, the physical evidence as to the age of a given group of strata may be entirely uncertain and unreliable, and in which, therefore, the observer has to rely wholly upon the fossils which he may meet with.

In spite of the above limitations and fallacies, there can be no doubt as to the enormous value of palæontology in enabling us to work out the historical succession of the sedimentary rocks. It may even be said that in any case where there should appear to be a clear and decisive discordance between the physical and the palæontological evidence as to the age of a given series of beds, it is the former that is to be distrusted rather than the latter. The records of geological science contain not a few cases in which apparently clear physical evidence of superposition has been demonstrated to have been wrongly interpreted; but the evidence of palæontology, when in any way sufficient, has rarely been upset by subsequent investigations. Should we find strata containing plants of the Coal-measures apparently resting upon other strata with Ammonites and Belemnites, we may be sure that the physical evidence is delusive; and though the above is an extreme case, the presumption in all such instances is rather that the physical succession has been misunderstood or misconstrued, than that there has been a subversion of the recognised succession of life-forms.

We have seen, then, that as the collective result of observations made upon the superposition of rocks in different localities, from their mineral characters, and from their included Page 41 fossils, geologists have been able to divide the entire stratified series into a number of different divisions or formations, each characterised by a general uniformity of mineral composition, and by a special and peculiar assemblage of organic forms. Each of these primary groups is in turn divided into a series of smaller divisions, characterised and distinguished in the same way. It is not pretended for a moment that all these primary rock-groups can anywhere be seen surmounting one another regularly.[8] There is no region upon the earth where all the stratified formations can be seen together; and, even when most of them occur in the same country, they can nowhere be seen all succeeding each other in their regular and uninterrupted succession. The reason of this is obvious. There are many places—to take a single example—where one may see the the Silurian rocks, the Devonian, and the Carboniferous rocks succeeding one another regularly, and in their proper order. This is because the particular region where this occurs was always submerged beneath the sea while these formations were being deposited. There are, however, many more localities in which one would find the Carboniferous rocks resting unconformably upon the Silurians without the intervention of any strata which could be referred to the Devonian period. This might arise from one of two causes: 1. The Silurians might have been elevated above the sea immediately after their deposition, so as to form dry land during the whole of the Devonian period, in which case, of course, no strata of the latter age could possibly be deposited in that area. 2. The Devonian might have been deposited upon the Silurian, and then the whole might have been elevated above the sea, and subjected to an amount of denudation sufficient to remove the Devonian strata entirely. In this case, when the land was again submerged, the Carboniferous rocks, or any younger formation, might be deposited directly upon Silurian strata. From one or other of these causes, then, or from subsequent disturbances and denudations, it happens that we can Page 42 rarely find many of the primary formations following one another consecutively and in their regular order.

[Footnote 8: As we have every reason to believe that dry land and sea have existed, at any rate from the commencement of the Laurentian period to the present day, it is quite obvious that no one of the great formations can ever, under any circumstances, have extended over the entire globe. In other words, no one of the formations can ever have had a greater geographical extent than that of the seas of the period in which the formation was deposited. Nor is there any reason for thinking that the proportion of dry land to ocean has ever been materially different to what it is at present, however greatly the areas of sea and land may have changed as regards their place. It follows from the above, that there is no sufficient basis for the view that the crust of the earth is composed of a succession of concentric layers, like the coats of an onion, each layer representing one formation.]

In no case, however, do we ever find the Devonian resting upon the Carboniferous, or the Silurian rocks reposing on the Devonian. We have therefore, by a comparison of many different areas, an established order of succession of the stratified formations, as shown in the subjoined ideal section of the crust of the earth (fig. 17).

The main subdivisions of the stratified rocks are known by the following names:—

1.	Laurentian.
2.	Cambrian (with Huronian?).
3.	Silurian.
4.	Devonian or Old Red Sandstone.
5.	Carboniferous.
6.	Permian	} New Red Sandstone.
7.	Triassic
8.	Jurassic or Oolitic.
9.	Cretaceous.
10.	Eocene.
11.	Miocene.
12.	Pliocene.
13.	Post-tertiary.

Page 43 IDEAL SECTION OF THE CRUST OF THE EARTH.

Fig. 17.

Page 44 Of these primary rock divisions, the Laurentian, Cambrian, Silurian, Devonian, Carboniferous, and Permian are collectively grouped together under the name of the Primary or Palœozoic rocks (Gr. palaios, ancient; zoe, life). Not only do they constitute the oldest stratified accumulations, but from the extreme divergence between their animals and plants and those now in existence, they may appropriately be considered as belonging to an "Old-Life" period of the world's history. The Triassic, Jurassic, and Cretaceous systems are grouped together as the Secondary or Mesozoic formations (Gr. mesos, intermediate; zoe, life); the organic remains of this "Middle-Life" period being, on the whole, intermediate in their characters between those of the palæozoic epoch and those of more modern strata. Lastly, the Eocene, Miocene, and Pliocene formations are grouped together as the Tertiary or Kainozoic rocks (Gr. kainos, new; zoe, life); because they constitute a "New-Life" period, in which the organic remains approximate in character to those now existing upon the globe. The so-called Post-Tertiary deposits are placed with the Kainozoic, or may be considered as forming a separate Quaternary system.

CHAPTER IV.

The term "contemporaneous" is usually applied by geologists to groups of strata in different regions which contain the same fossils, or an assemblage of fossils in which many identical forms are present. That is to say, beds which contain identical, or nearly identical, fossils, however widely separated they may be from one another in point of actual distance, are ordinarily believed to have been deposited during the same period of the earth's history. This belief, indeed, constitutes the keystone of the entire system of determining the age of strata by their fossil contents; and if we take the word "contemporaneous" in a general and strictly geological sense, this belief can be accepted as proved beyond denial. We must, however, guard ourselves against too literal an interpretation of the word "contemporaneous," and we must bear in mind the enormously-prolonged periods of time with which the geologist has to deal. When we say that two groups of Page 45 strata in different regions are "contemporaneous," we simply mean that they were formed during the same geological period, and perhaps at different stages of that period, and we do not mean to imply that they were formed at precisely the same instant of time.

A moment's consideration will show us that it is only in the former sense that we can properly speak of strata being "contemporaneous;" and that, in point of fact, beds containing the same fossils, if occurring in widely distant areas, can hardly be "contemporaneous" in any literal sense; but that the very identity of their fossils is proof that they were deposited one after the other. If we find strata containing identical fossils within the limits of a single geographical region—say in Europe—then there is a reasonable probability that these beds are strictly contemporaneous, in the sense that they were deposited at the same time. There is a reasonable probability of this, because there is no improbability involved in the idea of an ocean occupying the whole area of Europe, and peopled throughout by many of the same species of marine animals. At the present day, for example, many identical species of animals are found living on the western coasts of Britain and the eastern coasts of North America, and beds now in course of deposition off the shores of Ireland and the seaboard of the state of New York would necessarily contain many of the same fossils. Such beds would be both literally and geologically contemporaneous; but the case is different if the distance between the areas where the strata occur be greatly increased. We find, for example, beds containing identical fossils (the Quebec or Skiddaw beds) in Sweden, in the north of England, in Canada, and in Australia. Now, if all these beds were contemporaneous, in the literal sense of the term, we should have to suppose that the ocean at one time extended uninterruptedly between all these points, and was peopled throughout the vast area thus indicated by many of the same animals. Nothing, however, that we see at the present day would justify us in imagining an ocean of such enormous extent, and at the same time so uniform in its depth, temperature, and other conditions of marine life, as to allow the same animals to flourish in it from end to end; and the example chosen is only one of a long and ever-recurring series. It is therefore much more reasonable to explain this, and all similar cases, as owing to the migration of the fauna, in whole or in part, from one marine area to another. Thus, we may suppose an ocean to cover what is now the European area, and to be peopled by certain species of animals. Beds of sediment—clay, sands, and limestones—will be deposited over the sea-bottom, and Page 46 will entomb the remains of the animals as fossils. After this has lasted for a certain length of time, the European area may undergo elevation, or may become otherwise unsuitable for the perpetuation of its fauna; the result of which would be that some or all of the marine animals of the area would migrate to some more suitable region. Sediments would then be accumulated in the new area to which they had betaken themselves, and they would then appear, for the second time, as fossils in a set of beds widely separated from Europe. The second set of beds would, however, obviously not be strictly or literally contemporaneous with the first, but would be separated from them by the period of time required for the migration of the animals from the one area into the other. It is only in a wide and comprehensive sense that such strata can be said to be contemporaneous.

It is impossible to enter further into this subject here; but it may be taken as certain that beds in widely remote geographical areas can only come to contain the same fossils by reason of a migration having taken place of the animals of the one area to the other. That such migrations can and do take place is quite certain, and this is a much more reasonable explanation of the observed facts than the hypothesis that in former periods the conditions of life were much more uniform than they are at present, and that, consequently, the same organisms were able to range over the entire globe at the same time. It need only be added, that taking the evidence of the present as explaining the phenomena of the past—the only safe method of reasoning in geological matters—we have abundant proof that deposits which are actually contemporaneous, in the strict sense of the term, do not contain the same fossils, if far removed from one another in point of distance. Thus, deposits of various kinds are now in process of formation in our existing seas, as, for example, in the Arctic Ocean, the Atlantic, and the Pacific, and many of these deposits are known to us by actual examination and observation with the sounding-lead and dredge. But it is hardly necessary to add that the animal remains contained in these deposits—the fossils of some future period—instead of being identical, are widely different from one another in their characters.

We have seen, then, that the entire stratified series is capable of subdivision into a number of definite rock-groups or "formations," each possessing a peculiar and characteristic assemblage of fossils, representing the "life" of the "period" in which the formation was deposited. We have still to inquire shortly how it came to pass that two successive formations should Page 47 thus be broadly distinguished by their life-forms, and why they should not rather possess at any rate a majority of identical fossils. It was originally supposed that this could be explained by the hypothesis that the close of each formation was accompanied by a general destruction of all the living beings of the period, and that the commencement of each new formation was signalised by the creation of a number of brand-new organisms, destined to figure as the characteristic fossils of the same. This theory, however, ignores the fact that each formation—as to which we have any sufficient evidence—contains a few, at least, of the life-forms which existed in the preceding period; and it invokes forces and processes of which we know nothing, and for the supposed action of which we cannot account. The problem is an undeniably difficult one, and it will not be possible here to give more than a mere outline of the modern views upon the subject. Without entering into the at present inscrutable question as to the manner in which new life-forms are introduced upon the earth, it may be stated that almost all modern geologists hold that the living beings of any given formation are in the main modified forms of others which have preceded them. It is not believed that any general or universal destruction of life took place at the termination of each geological period, or that a general introduction of new forms took place at the commencement of a new period. It is, on the contrary, believed that the animals and plants of any given period are for the most part (or exclusively) the lineal but modified descendants of the animals and plants of the immediately preceding period, and that some of them, at any rate, are continued into the next succeeding period, either unchanged, or so far altered as to appear as new species. To discuss these views in detail would lead us altogether too far, but there is one very obvious consideration which may advantageously receive some attention. It is obvious, namely, that the great discordance which is found to subsist between the animal life of any given formation and that of the next succeeding formation, and which no one denies, would be a fatal blow to the views just alluded to, unless admitting of some satisfactory explanation. Nor is this discordance one purely of life-forms, for there is often a physical break in the successions of strata as well. Let us therefore briefly consider how far these interruptions and breaks in the geological and palæontological record can be accounted for, and still allow us to believe in some theory of continuity as opposed to the doctrine of intermittent and occasional action.

Page 48 In the first place, it is perfectly clear that if we admit the conception above mentioned of a continuity of life from the Laurentian period to the present day, we could never prove our view to be correct, unless we could produce in evidence fossil examples of all the kinds of animals and plants that have lived and died during that period. In order to do this, we should require, to begin with, to have access to an absolutely unbroken and perfect succession of all the deposits which have ever been laid down since the beginning. If, however, we ask the physical geologist if he is in possession of any such uninterrupted series, he will at once answer in the negative. So far from the geological series being a perfect one, it is interrupted by numerous gaps of unknown length, many of which we can never expect to fill up. Nor are the proofs of this far to seek. Apart from the facts that we have hitherto examined only a limited portion of the dry land, that nearly two-thirds of the entire area of the globe is inaccessible to geological investigation in consequence of its being covered by the sea, that many deposits can be shown to have been more or less completely destroyed subsequent to their deposition, and that there may be many areas in which living beings exist where no rock is in process of formation, we have the broad fact that rock-deposition only goes on to any extent in water, and that the earth must have always consisted partly of dry land and partly of water—at any rate, so far as any period of which we have geological knowledge is concerned. There must, therefore, always have existed, at some part or another of the earth's surface, areas where no deposition of rock was going on, and the proof of this is to be found in the well-known phenomenon of "unconformability." Whenever, namely, deposition of sediment is continuously going on within the limits of a single ocean, the beds which are laid down succeed one another in uninterrupted and regular sequence. Such beds are said to be "conformable," and there are many rock-groups known where one may pass through fifteen or twenty thousand feet of strata without a break—indicating that the beds had been deposited in an area which remained continuously covered by the sea. On the other hand, we commonly find that there is no such regular succession when we pass from one great formation to another, but that, on the contrary, the younger formation rests "unconformably," as it is called, either upon the formation immediately preceding it in point of time, or upon some still older one. The essential physical feature of this unconformability is that the beds of the younger formation rest upon a worn and eroded surface formed by the Page 49 beds of the older series (fig. 18); and a moment's consideration will show us what this indicates. It indicates, Fig. 18

Fig. 18.—Section showing strata of Tertiary age (a) resting upon a worn and eroded surface of White Chalk (b), the stratification of which is marked by lines of flint. beyond the possibility of misconception, that there was an interval between the deposition of the older series and that of the newer series of strata; and that during this interval the older beds were raised above the sea-level, so as to form dry land, and were subsequently depressed again beneath the waters, to receive upon their worn and wasted upper surface the sediments of the later group. During the interval thus indicated, the deposition of rock must of necessity have been proceeding more or less actively in other areas. Every unconformity, therefore, indicates that at the spot where it occurs, a more or less extensive series of beds must be actually missing; and though we may sometimes be able to point to these missing strata in other areas, there yet remains a number of unconformities for which we cannot at present supply the deficiency even in a partial manner.

It follows from the above that the series of stratified deposits is to a greater or less extent irremediably imperfect; and in this imperfection we have one great cause why we can never obtain a perfect series of all the animals and plants that have lived upon the globe. Wherever one of these great physical gaps occurs, we find, as we might expect, a corresponding break in the series of life-forms. In other words, whenever we find two formations to be unconformable, we shall always find at the same time that there is a great difference in their fossils, and that many of the fossils of the older formation do not survive into the newer, whilst many of those in the newer are not known to occur in the older. The cause of this is, obviously, Page 50 that the lapse of time, indicated by the unconformability, has been sufficiently great to allow of the dying out or modification of many of the older forms of life, and the introduction of new ones by immigration.

Apart, however, altogether, from these great physical breaks and their corresponding breaks in life, there are other reasons why we can never become more than partially acquainted with the former denizens of the globe. Foremost amongst these is the fact that an enormous number of animals possess no hard parts of the nature of a skeleton, and are therefore incapable, under any ordinary circumstances, of leaving behind them any traces of their existence. It is true that there are cases in which animals in themselves completely soft-bodied are nevertheless able to leave marks by which their former presence can be detected: Thus every geologist is familiar with the winding and twisting "trails" formed on the surface of the strata by sea-worms; and the impressions left by the stranded carcases of Jelly-fishes on the fine-grained lithographic slates of Solenhofen supply us with an example of how a creature which is little more than "organised sea-water" may still make an abiding mark upon the sands of time. As a general rule, however, animals which have no skeletons are incapable of being preserved as fossils, and hence there must always have been a vast number of different kinds of marine animals of which we have absolutely no record whatever. Again, almost all the fossiliferous rocks have been laid down in water; and it is a necessary result of this that the great majority of fossils are the remains of aquatic animals. The remains of air-breathing animals, whether of the inhabitants of the land or of the air itself, are comparatively rare as fossils, and the record of the past existence of these is much more imperfect than is the case with animals living in water. Moreover, the fossiliferous deposits are not only almost exclusively aqueous formations, but the great majority are marine, and only a comparatively small number have been formed by lakes and rivers. It follows from the foregoing that the palæontological record is fullest and most complete so far as sea-animals are concerned, though even here we find enormous gaps, owing to the absence of hard structures in many great groups; of animals inhabiting fresh waters our knowledge is rendered still further incomplete by the small proportion that fluviatile and lacustrine deposits bear to marine; whilst we have only a fragmentary acquaintance with the air-breathing animals which inhabited the earth during past ages.

Lastly, the imperfection of the palæontological record, due Page 51 to the causes above enumerated, is greatly aggravated, especially as regards the earlier portion of the earth's history, by the fact that many rocks which contained fossils when deposited have since been rendered barren of organic remains. The principal cause of this common phenomenon is what is known as "metamorphism"—that is, the subjection of the rock to a sufficient amount of heat to cause a rearrangement of its particles. When at all of a pronounced character, the result of metamorphic action is invariably the obliteration of any fossils which might have been originally present in the rock. Metamorphism may affect rocks of any age, though naturally more prevalent in the older rocks, and to this cause must be set down an irreparable loss of much fossil evidence. The most striking example which is to be found of this is the great Laurentian series, which comprises some 30,000 feet of highly-metamorphosed sediments, but which, with one not wholly undisputed exception, has as yet yielded no remains of living beings, though there is strong evidence of the former existence in it of fossils.

Upon the whole, then, we cannot doubt that the earth's crust, so far as yet deciphered by us, presents us with but a very imperfect record of the past. Whether the known and admitted imperfections of the geological and palæontological records are sufficiently serious to account satisfactorily for the deficiency of direct evidence recognisable in some modern hypotheses, may be a matter of individual opinion. There can, however, be little doubt that they are sufficiently extensive to throw the balance of evidence decisively in favour of some theory of continuity, as opposed to any theory of intermittent and occasional action. The apparent breaks which divide the great series of the stratified rocks into a number of isolated formations, are not marks of mighty and general convulsions of nature, but are simply indications of the imperfection of our knowledge. Never, in all probability, shall we be able to point to a complete series of deposits, or a complete succession of life linking one great geological period to another. Nevertheless, we may well feel sure that such deposits and such an unbroken succession must have existed at one time. We are compelled to believe that nowhere in the long series of the fossiliferous rocks has there been a total break, but that there must have been a complete continuity of life, and a more or less complete continuity of sedimentation, from the Laurentian period to the present day. One generation hands on the lamp of life to the next, and each system of rocks is the direct offspring of those which preceded it in time. Though there Page 52 has not been continuity in any given area, still the geological chain could never have been snapped at one point, and taken up again at a totally different one. Thus we arrive at the conviction that continuity is the fundamental law of geology, as it is of the other sciences, and that the lines of demarcation between the great formations are but gaps in our own knowledge.

CHAPTER V.

We have already seen that geologists have been led by the study of fossils to the all-important generalisation that the vast series of the Fossiliferous or Sedimentary Rocks may be divided into a number of definite groups or "formations," each of which is characterised by its organic remains. It may simply be repeated here that these formations are not properly and strictly characterised by the occurrence in them of any one particular fossil. It may be that a formation contains some particular fossil or fossils not occurring out of that formation, and that in this way an observer may identify a given group with tolerable certainty. It very often happens, indeed, that some particular stratum, or sub-group of a series, contains peculiar fossils, by which its existence may be determined in various localities. As before remarked, however, the great formations are characterised properly by the association of certain fossils, by the predominance of certain families or orders, or by an assemblage of fossil remains representing the "life" of the period in which the formation was deposited.

Fossils, then, enable us to determine the age of the deposits in which they occur. Fossils further enable us to come to very important conclusions as to the mode in which the fossiliferous bed was deposited, and thus as to the condition of the particular district or region occupied by the fossiliferous bed at the time of the formation of the latter. If, in the first place, the bed contain the remains of animals such as now inhabit rivers, we know that it is "fluviatile" in its origin, and that it must at one time have either formed an actual riverbed, or been deposited by the overflowing of an ancient stream. Secondly, if the bed contain the remains of shellfish, minute crustaceans, or fish, such as now inhabit lakes, Page 53 we know that it is "lacustrine," and was deposited beneath the waters of a former lake. Thirdly, if the bed contain the remains of animals such as now people the ocean, we know that it is "marine" in its origin, and that it is a fragment of an old sea-bottom.

We can, however, often determine the conditions under which a bed was deposited with greater accuracy than this. If, for example, the fossils are of kinds resembling the marine animals now inhabiting shallow waters, if they are accompanied by the detached relics of terrestrial organisms, or if they are partially rolled and broken, we may conclude that the fossiliferous deposit was laid down in a shallow sea, in the immediate vicinity of a coast-line, or as an actual shore-deposit. If, again, the remains are those of animals such as now live in the deeper parts of the ocean, and there is a very sparing intermixture of extraneous fossils (such as the bones of birds or quadrupeds, or the remains of plants), we may presume that the deposit is one of deep water. In other cases, we may find, scattered through the rock, and still in their natural position, the valves of shells such as we know at the present day as living buried in the sand or mud of the sea-shore or of estuaries. In other cases, the bed may obviously have been an ancient coral-reef, or an accumulation of social shells, like Oysters. Lastly, if we find the deposit to contain the remains of marine shells, but that these are dwarfed of their fair proportions and distorted in figure, we may conclude that it was laid down in a brackish sea, such as the Baltic, in which the proper saltness was wanting, owing to its receiving an excessive supply of fresh water.

In the preceding, we have been dealing simply with the remains of aquatic animals, and we have seen that certain conclusions can be accurately reached by an examination of these. As regards the determination of the conditions of deposition from the remains of aerial and terrestrial animals, or from plants, there is not such an absolute certainty. The remains of land-animals would, of course, occur in "sub-aerial" deposits—that is, in beds, like blown sand, accumulated upon the land. Most of the remains of land-animals, however, are found in deposits which have been laid down in water, and they owe their present position to the fact that their former owners were drowned in rivers or lakes, or carried out to sea by streams. Birds, Flying Reptiles, and Flying Mammals might also similarly find their way into aqueous deposits; but it is to be remembered that many birds and mammals habitually spend a great part of their time in the water, and that these might therefore be naturally expected to present themselves as fossils in Page 54 Sedimentary Rocks. Plants, again, even when undoubtedly such as must have grown on land, do not prove that the bed in which they occur was formed on land. Many of the remains of plants known to us are extraneous to the bed in which they are now found, having reached their present site by falling into lakes or rivers, or being carried out to sea by floods or gales of wind. There are, however, many cases in which plants have undoubtedly grown on the very spot where we now find them. Thus it is now generally admitted that the great coal-fields of the Carboniferous age are the result of the growth in situ of the plants which compose coal, and that these grew on vast Fig. 19

Fig. 19.—Erect Tree containing Reptilian remains. Coal-measures, Nova Scotia. (After Dawson.) marshy or partially submerged tracts of level alluvial land. We have, however, distinct evidence of old land-surfaces, both in the Coal-measures and in other cases (as, for instance, in the well-known "dirt-bed" of the Purbeck series). When, for example, we find the erect stumps of trees standing at right angles to the surrounding strata, we know that the surface through which these send their roots was at one time the surface of the dry land, or, in other words, was an ancient soil (fig. 19).

In many cases fossils enable us to come to important conclusions as to the climate of the period in which they lived but only a few instances of this can be here adduced. As fossils in the majority of instances are the remains of marine animals, it is mostly the temperature of the sea which can alone be determined in this way; and it is important to remember that, owing to the existence of heated currents, the marine climate of a given area does not necessarily imply a correspondingly warm climate in the neighbouring land. Land-climates can only be determined by the remains of land-animals or land-plants, and these are comparatively rare as fossils. It is also important to remember that all conclusions on this Page 55 head are really based upon the present distribution of animal and vegetable life on the globe, and are therefore liable to be vitiated by the following considerations:—

a. Most fossils are extinct, and it is not certain that the habits and requirements of any extinct animal were exactly similar to those of its nearest living relative.

b. When we get very far back in time, we meet with groups of organisms so unlike anything we know at the present day as to render all conjectures as to climate founded upon their supposed habits more or less uncertain and unsafe.

c. In the case of marine animals, we are as yet very far from knowing the exact limits of distribution of many species within our present seas; so that conclusions drawn from living forms as to extinct species are apt to prove incorrect. For instance, it has recently been shown that many shells formerly believed to be confined to the Arctic Seas have, by reason of the extension of Polar currents, a wide range to the south; and this has thrown doubt upon the conclusions drawn from fossil shells as to the Arctic conditions under which certain beds were supposed to have been deposited.

d. The distribution of animals at the present day is certainly dependent upon other conditions beside climate alone; and the causes which now limit the range of given animals are certainly such as belong to the existing order of things. But the establishment of the present order of things does not date back in many cases to the introduction of the present species of animals. Even in the case, therefore, of existing species of animals, it can often be shown that the past distribution of the species was different formerly to what it is now, not necessarily because the climate has changed, but because of the alteration of other conditions essential to the life of the species or conducing to its extension.

Still, we are in many cases able to draw completely reliable conclusions as to the climate of a given geological period, by an examination of the fossils belonging to that period. Among the more striking examples of how the past climate of a region may be deduced from the study of the organic remains contained in its rocks, the following may be mentioned: It has been shown that in Eocene times, or at the commencement of the Tertiary period, the climate of what is now Western Europe was of a tropical or sub-tropical character. Thus the Eocene beds are found to contain the remains of shells such as now inhabit tropical seas, as, for example, Cowries and Volutes; and with these are the fruits of palms, and the remains of other tropical plants. It has been shown, Page 56 again, that in Miocene times, or about the middle of the Tertiary period, Central Europe was peopled with a luxuriant flora resembling that of the warmer parts of the United States, and leading to the conclusion that the mean annual temperature must have been at least 30° hotter than it is at present. It has been shown that, at the same time, Greenland, now buried beneath a vast ice-shroud, was warm enough to support a large number of trees, shrubs, and other plants, such as inhabit temperate regions of the globe. Lastly, it has been shown upon physical as well as palæontological evidence, that the greater part of the North Temperate Zone, at a comparatively recent geological period, has been visited with all the rigours of an Arctic climate, resembling that of Greenland at the present day. This is indicated by the occurrence of Arctic shells in the superficial deposits of this period, whilst the Musk-ox and the Reindeer roamed far south of their present limits.

Lastly, it was from the study of fossils that geologists learnt originally to comprehend a fact which may be regarded as of cardinal importance in all modern geological theories and speculations—namely, that the crust of the earth is liable to local elevations and subsidences. For long after the remains of shells and other marine animals were for the first time observed in the solid rocks forming the dry land, and at great heights above the sea-level, attempts were made to explain this almost unintelligible phenomenon upon the hypothesis that the fossils in question were not really the objects they represented, but were in truth mere lusus naturœ, due to some "plastic virtue latent in the earth." The common-sense of scientific men, however, soon rejected this idea, and it was agreed by universal consent that these bodies really were remains of animals which formerly lived in the sea. When once this was admitted, the further steps were comparatively easy, and at the present day no geological doctrine stands on a firmer basis than that which teaches us that our present continents and islands, fixed and immovable as they appear, have been repeatedly sunk beneath the ocean.

Page 57 CHAPTER VI.

Not only have fossils, as we have seen, a most important bearing upon the sciences of Geology and Physical Geography, but they have relations of the most complicated and weighty character with the numerous problems connected with the study of living beings, or in other words, with the science of Biology. To such an extent is this the case, that no adequate comprehension of Zoology and Botany, in their modern form, is so much as possible without some acquaintance with the types of animals and plants which have passed away. There are also numerous speculative questions in the domain of vital science, which, if soluble at all, can only hope to find their key in researches carried out on extinct organisms. To discuss fully the biological relations of fossils would, therefore, afford matter for a separate treatise; and all that can be done here is to indicate very cursorily the principal points to which the attention of the palæontological student ought to be directed.

In the first place, the great majority of fossil animals and plants are "extinct"—that is to say, they belong to species which are no longer in existence at the present day. So far, however, from there being any truth in the old view that there were periodic destructions of all the living beings in existence upon the earth, followed by a corresponding number of new creations of animals and plants, the actual facts of the case show that the extinction of old forms and the introduction of new forms have been processes constantly going on throughout the whole of geological time. Every species seems to come into being at a certain definite point of time, and to finally disappear at another definite point; though there are few instances indeed, if there are any, in which our present knowledge would permit us safely to fix with precision the times of entrance and exit. There are, moreover, marked differences in the actual time during which different species remained in existence, and therefore corresponding differences in their "vertical range," or, in other words, in the actual amount and thickness of strata through which they present themselves as fossils. Some species are found to range through two or even three formations, and a few have an even more extended life. More commonly the species which begin in the Page 58 commencement of a great formation die out at or before its close, whilst those which are introduced for the first time near the middle or end of the formation may either become extinct, or may pass on into the next succeeding formation. As a general rule, it is the animals which have the lowest and simplest organisation that have the longest range in time, and the additional possession of microscopic or minute dimensions seems also to favour longevity. Thus some of the Foraminifera appear to have survived, with little or no perceptible alteration, from the Silurian period to the present day; whereas large and highly-organised animals, though long-lived as individuals, rarely seem to live long specifically, and have, therefore, usually a restricted vertical range. Exceptions to this, however, are occasionally to be found in some "persistent types," which extend through a succession of geological periods with very little modification. Thus the existing Lampshells of the genus Lingula are little changed from the Lingulœ which swarmed in the Lower Silurian seas; and the existing Pearly Nautilus is the last descendant of a clan nearly as ancient. On the other hand, some forms are singularly restricted in their limits, and seem to have enjoyed a comparatively brief lease of life. An example of this is to be found in many of the Ammonites—close allies of the Nautilus—which are often confined strictly to certain zones of strata, in some cases of very insignificant thickness.

Of the causes of extinction amongst fossil animals and plants, we know little or nothing. All we can say is, that the attributes which constitute a species do not seem to be intrinsically endowed with permanence, any more than the attributes which constitute an individual, though the former may endure whilst many successive generations of the latter have disappeared. Each species appears to have its own life-period, its commencement, its culmination, and its gradual decay; and the life-periods of different species may be of very different duration.

From what has been said above, it may be gathered that our existing species of animals and plants are, for the most part, quite of modern origin, using the term "modern" in its geological acceptation. Measured by human standards, the majority of existing animals (which are capable of being preserved as fossils) are known to have a high antiquity; and some of them can boast of a pedigree which even the geologist may regard with respect. Not a few of our shellfish are known to have commenced their existence at some point of the Tertiary period; one Lampshell Page 59 (Terebratulina caput-serpentis) is believed to have survived since the Chalk; and some of the Foraminifera date, at any rate, from the Carboniferous period. We learn from this the additional fact that our existing animals and plants do not constitute an assemblage of organic forms which were introduced into the world collectively and simultaneously, but that they commenced their existence at very different periods, some being extremely old, whilst others may be regarded as comparatively recent animals. And this introduction of the existing fauna and flora was a slow and gradual process, as shown admirably by the study of the fossil shells of the Tertiary period. Thus, in the earlier Tertiary period, we find about 95 per cent of the known fossil shells to be species that are no longer in existence, the remaining 5 per cent being forms which are known to live in our present seas. In the middle of the Tertiary period we find many more recent and still existing species of shells, and the extinct types are much fewer in number; and this gradual introduction of forms now living goes on steadily, till, at the close of the Tertiary period, the proportions with which we started may be reversed, as many as 90 or 95 per cent of the fossil shells being forms still alive, while not more than 5 per cent may have disappeared.

All known animals at the present day may be divided into some five or six primary divisions, which are known technically as "sub-kingdoms." Each of these sub-kingdoms [9] may be regarded as representing a certain type or plan of structure, and all the animals comprised in each are merely modified forms of this common type. Not only are all known living animals thus reducible to some five or six fundamental plans of structure, but amongst the vast series of fossil forms no one has yet been found—however unlike any existing animal—to possess peculiarities which would entitle it to be placed in a new sub-kingdom. All fossil animals, therefore, are capable of being referred to one or other of the primary divisions of the animal kingdom. Many fossil groups have no closely-related group now in existence; but in no case do we meet with any grand structural type which has not survived to the present day.

[Footnote 9: In the Appendix a brief definition is given of the sub-kingdoms, and the chief divisions of each are enumerated.]

The old types of life differ in many respects from those now upon the earth; and the further back we pass in time, the more marked does this divergence become. Thus, if we were to compare the animals which lived in the Silurian seas with Page 60 those inhabiting our present oceans, we should in most instances find differences so great as almost to place us in another world. This divergence is the most marked in the Palæozoic forms of life, less so in those of the Mesozoic period, and less still in the Tertiary period. Each successive formation has therefore presented us with animals becoming gradually more and more like those now in existence; and though there is an immense and striking difference between the Silurian animals and those of to-day, this difference is greatly reduced if we compare the Silurian fauna with the Devonian; that again with the Carboniferous; and so on till we reach the present.

It follows from the above that the animals of any given formation are more like those of the next formation below, and of the next formation above, than they are to any others; and this fact of itself is an almost inexplicable one, unless we believe that the animals of any given formation are, in part at any rate, the lineal descendants of the animals of the preceding formation, and the progenitors, also in part at least, of the animals of the succeeding formation. In fact, the palæontologist is so commonly confronted with the phenomenon of closely-allied forms of animal life succeeding one another in point of time, that he is compelled to believe that such forms have been developed from some common ancestral type by some process of "evolution." On the other hand, there are many phenomena, such as the apparently sudden introduction of new forms throughout all past time, and the common occurrence of wholly isolated types, which cannot be explained in this way. Whilst it seems certain, therefore, that many of the phenomena of the succession of animal life in past periods can only be explained by some law of evolution, it seems at the same time certain that there has always been some other deeper and higher law at work, on the nature of which it would be futile to speculate at present.

Not only do we find that the animals of each successive formation become gradually more and more like those now existing upon the globe, as we pass from the older rocks into the newer, but we also find that there has been a gradual progression and development in the types of animal life which characterise the geological ages. If we take the earliest-known and oldest examples of any given group of animals, it can sometimes be shown that these primitive forms, though in themselves highly organised, possessed certain characters such as are now only seen in the young of their existing representatives. In technical language, the early forms of life in some Page 61 instances possess "embryonic" characters, though this does not prevent them often attaining a size much more gigantic than their nearest living relatives. Moreover, the ancient forms of life are often what is called "comprehensive types"—that is to say, they possess characters in combination such as we nowadays only find separately developed in different, groups of animals. Now, this permanent retention of embryonic characters and this "comprehensiveness" of structural type are signs of what a zoologist considers to be a comparatively low grade of organisation; and the prevalence of these features in the earlier forms of animals is a very striking phenomenon, though they are none the less perfectly organised so far as their own type is concerned. As we pass upwards in the geological scale, we find that these features gradually disappear, higher and ever higher forms are introduced, and "specialisation" of type takes the place of the former comprehensiveness. We shall have occasion to notice many of the facts on which these views are based at a later period, and in connection with actual examples. In the meanwhile, it is sufficient to state, as a widely-accepted generalisation of palæontology, that there has been in the past a general progression of organic types, and that the appearance of the lower forms of life has in the main preceded that of the higher forms in point of time.

Page 63 PART II.

Page 65 CHAPTER VII.

The Laurentian Rocks constitute the base of the entire stratified series, and are, therefore, the oldest sediments of which we have as yet any knowledge. They are more largely and more typically developed in North America, and especially in Canada, than in any known part of the world, and they derive their title from the range of hills which the old French geographers named the "Laurentides." These hills are composed of Laurentian Rocks, and form the watershed between the valley of the St Lawrence river on the one hand, and the great plains which stretch northwards to Hudson Bay on the other hand. The main area of these ancient deposits forms a great belt of rugged and undulating country, which extends from Labrador westwards to Lake Superior, and then bends northwards towards the Arctic Sea. Throughout this extensive area the Laurentian Rocks for the most part present themselves in the form of low, rounded, ice-worn hills, which, if generally wanting in actual sublimity, have a certain geological grandeur from the fact that they "have endured the battles and the storms of time longer than any other mountains" (Dawson). In some places, however, the Laurentian Rocks produce scenery of the most magnificent character, as in the great gorge cut through them by the river Saguenay, where they rise at times into vertical precipices 1500 feet in height. In the famous group of the Adirondack mountains, also, in the state of New York, they form elevations no less than 6000 feet above the level of the sea. As a general rule, the character of the Laurentian region is that of a rugged, rocky, rolling country, often Page 66 densely timbered, but rarely well fitted for agriculture, and chiefly attractive to the hunter and the miner.

As regards its mineral characters, the Laurentian series is composed throughout of metamorphic and highly crystalline rocks, which are in a high degree crumpled, folded, and faulted. By the late Sir William Logan the entire series was divided into two great groups, the Lower Laurentian and the Upper Laurentian, of which the latter rests unconformably upon the truncated edges of the former, and is in turn unconformably overlaid by strata of Huronian and Cambrian age (fig. 20).

The Lower Laurentian series attains the enormous thickness Fig. 20

Fig. 20.—Diagrammatic section of the Laurentian Rocks in Lower Canada. a Lower Laurentian; b Upper Laurentian, resting unconformably upon the lower series; c Cambrian strata (Potsdam Sandstone), resting unconformably on the Upper Laurentian. of over 20,000 feet, and is composed mainly of great beds of gneiss, altered sandstones (quartzites), mica-schist, hornblende-schist, magnetic iron-ore, and hæmatite, together with masses of limestone. The limestones are especially interesting, and have an extraordinary development—three principal beds being known, of which one is not less than 1500 feet thick; the collective thickness of the whole being about 3500 feet.

The Upper Laurentian series, as before said, reposes unconformably upon the Lower Laurentian, and attains a thickness of at least 10,000 feet. Like the preceding, it is wholly metamorphic, and is composed partly of masses of gneiss and quartzite; but it is especially distinguished by the possession of great beds of felspathic rock, consisting principally of "Labrador felspar."

Though typically developed in the great Canadian area already spoken of, the Laurentian Rocks occur in other localities, both in America and in the Old World. In Britain, the so-called "fundamental gneiss" of the Hebrides and of Sutherlandshire is probably of Lower Laurentian age, and the "hypersthene rocks" of the Isle of Skye may, with great probability, be regarded as referable to the Upper Laurentian. In other localities in Great Britain (as in St David's, South Wales; the Malvern Hills; and the North of Ireland) occur ancient metamorphic deposits which also are probably referable to the Laurentian series. The so-called "primitive gneiss" of Norway appears to belong to the Laurentian, and the Page 67 ancient metamorphic rocks of Bohemia and Bavaria may be regarded as being approximately of the same age.

By some geological writers the ancient and highly metamorphosed sediments of the Laurentian and the succeeding Huronian series have been spoken of as the "Azoic rocks" (Gr. a, without; zoe, life); but even if we were wholly destitute of any evidence of life during these periods, this name would be objectionable upon theoretical grounds. If a general name be needed, that of "Eozoic" (Gr. eos, dawn; zoe, life), proposed by Principal Dawson, is the most appropriate. Owing to their metamorphic condition, geologists long despaired of ever detecting any traces of life in the vast pile of strata which constitute the Laurentian System. Even before any direct traces were discovered, it was, however, pointed out that there were good reasons for believing that the Laurentian seas had been tenanted by an abundance of living beings. These reasons are briefly as follows:—(1) Firstly, the Laurentian series consists, beyond question, of marine sediments which originally differed in no essential respect from those which were subsequently laid down in the Cambrian or Silurian periods. (2) In all formations later than the Laurentian, any limestones which are present can be shown, with few exceptions, to be organic rocks, and to be more or less largely made up of the comminuted debris of marine or fresh-water animals. The Laurentian limestones, in consequence of the metamorphism to which they have been subjected, are so highly crystalline (fig. 21) that the microscope fails to detect Fig. 21

Fig. 21.—Section of Lower Laurentian Limestone from Hull, Ottawa; enlarged five diameters. The rock is very highly crystalline, and contains mica and other minerals. The irregular black masses in it are graphite. (Original.) any organic structure in the rock, and no fossils beyond those which will be spoken of immediately have as yet been discovered in them. We know, however, of numerous cases in which limestones, of later age, and undoubtedly organic to begin with, have been rendered so intensely crystalline by metamorphic action that all traces of organic structure have been obliterated. We have therefore, by analogy, the strongest possible ground for believing that the vast beds of Laurentian limestone have been originally organic in their origin, and primitively composed, in the main, of the calcareous skeletons Page 68 of marine animals. It would, in fact, be a matter of great difficulty to account for the formation of these great calcareous masses on any other hypothesis. (3) The occurrence of phosphate of lime in the Laurentian Rocks in great abundance, and sometimes in the form of irregular beds, may very possibly be connected with the former existence in the strata of the remains of marine animals of whose skeleton this mineral is a constituent. (4) The Laurentian Rocks contain a vast amount of carbon in the form of black-lead or graphite. This mineral is especially abundant in the limestones, occurring in regular beds, in veins or strings, or disseminated through the body of the limestone in the shape of crystals, scales, or irregular masses. The amount of graphite in some parts of the Lower Laurentian is so great that it has been calculated as equal to the quantity of carbon present in an equal thickness of the Coal-measures. The general source of solid carbon in the crust of the earth is, however, plant-life; and it seems impossible to account for the Laurentian graphite, except upon the supposition that it is metamorphosed vegetable matter. (5) Lastly, the great beds of iron-ore (peroxide and magnetic oxide) which occur in the Laurentian series interstratified with the other rocks, point with great probability to the action of vegetable life; since similar deposits in later formations can commonly be shown to have been formed by the deoxidising power of vegetable matter in a state of decay.

In the words of Principal Dawson, "anyone of these reasons might, in itself, be held insufficient to prove so great and, at first sight, unlikely a conclusion as that of the existence of abundant animal and vegetable life in the Laurentian; but the concurrence of the whole in a series of deposits unquestionably marine, forms a chain of evidence so powerful that it might command belief even if no fragment of any organic or living form or structure had ever been recognised in these ancient rocks." Of late years, however, there have been discovered in the Laurentian Rocks certain bodies which are believed to be truly the remains of animals, and of which by far the most important is the structure known under the now celebrated name of Eozoön. If truly organic, a very special and exceptional interest attaches itself to Eozoön, as being the most ancient fossil animal of which we have any knowledge; but there are some who regard it really a peculiar form of mineral structure, and a severe, protracted, and still unfinished controversy has been carried on as to its nature. Into this controversy it is wholly unnecessary to enter here; and it will be sufficient to briefly explain the structure of Eozoön, as elucidated by the elaborate and masterly investigations of Carpenter Page 69 and Dawson, from the standpoint that it is a genuine organism—the balance of evidence up to this moment inclining decisively to this view.

The structure known as Eozoön is found in various localities in the Lower Laurentian limestones of Canada, in the form of isolated masses or spreading layers, which are composed of thin alternating laminæ, arranged more or less concentrically (fig. 22). The laminæ of these masses are usually of different colours Fig. 22

Fig. 22.—Fragment of Eozoön, of the natural size, showing alternate laminæ of loganite and dolomite. (After Dawson.) and composition; one series being white, and composed of carbonate of lime—whilst the laminæ of the second series alternate with the preceding, are green in colour, and are found by chemical analysis to consist of some silicate, generally serpentine or the closely-related "loganite." In some instances, however, all the laminæ are calcareous, the concentric arrangement still remaining visible in consequence of the fact that the laminæ are composed alternately of lighter and darker coloured limestone.

When first discovered, the masses of Eozoön were supposed to be of a mineral nature; but their striking general resemblance to the undoubted fossils which will be subsequently spoken of under the name of Stromatopora was recognised by Sir William Logan, and specimens were submitted for minute examination, first to Principal Dawson, and subsequently to Dr W. B. Carpenter. After a careful microscopic examination, these two distinguished observers came to the conclusion that Eozoön was truly organic, and in this opinion they were afterwards corroborated by other high authorities (Mr W. K. Parker, Professor Rupert Jones, Mr H. B. Brady, Professor Gümbel, &c.) Stated briefly, the structure of Eozoön, as exhibited by the microscope, is as follows:—

Page 70 The concentrically-laminated mass of Eozoön is composed of numerous calcareous layers, representing the original skeleton of the organism (fig. 23, b). These calcareous layers Fig. 23

Fig. 23.—Diagram of a portion of Eozoön cut vertically. A, B, C, Three tiers of chambers communicating with one another by slightly constricted apertures: a a, The true shell-wall, perforated by numerous delicate tubes; b b. The main calcareous skeleton ("intermediate skeleton"); c, Passage of communication ("stolon-passage") from one tier of chambers to another; d, Ramifying tubes in the calcareous skeleton. (After Carpenter.) serve to separate and define a series of chambers arranged in successive tiers, one above the other (fig. 23, A, B, C); and they are perforated not only by passages (fig. 23, c), which serve to place successive tiers of chambers in communication, but also by a system of delicate branching canals (fig. 23, d). Moreover, the central and principal portion of each calcareous layer, with the ramified canal-system just spoken of, is bounded both above and below by a thin lamina which has a structure of its own, and which may be regarded as the proper shell-wall (fig. 23, a a). This proper wall forms the actual lining of the chambers, as well as the outer surface of the whole mass; and it is perforated with numerous fine vertical tubes (fig. 24, a a), opening into the chambers and on to the surface by corresponding fine pores. From the resemblance of this tubulated layer to similar structures in the shell of the Nummulite, it is often spoken of as the "Nummuline layer." The chambers are sometimes piled up one above the other in an irregular manner; but they are more commonly arranged in regular tiers, the separate chambers being marked off from one another by projections of the wall in the form of partitions, which are so far imperfect as to allow of a free communication between contiguous chambers. In the original condition of the organism, all these chambers, of course, must have been filled with living-matter; but they are found in the present state of the fossil to be generally filled with some silicate, such as serpentine, which not only fills the actual chambers, but has also penetrated the minute tubes of the proper wall and the branching canals of the intermediate skeleton. In some cases Page 71 the chambers are simply filled with crystalline carbonate of lime. When the originally porous fossil has been permeated by a silicate, Fig. 24

Fig. 24.—The animal of Nonionina, one of the Foraminifera, after the shell has been removed by a weak acid; b, Gromia, a single-chambered Foraminifer (after Schultze), showing the shell surrounded by a network of filaments derived from the body substance. it is possible to dissolve away the whole of the calcareous skeleton by means of acids, leaving an accurate and beautiful cast of the chambers and the tubes connected with them in the insoluble silicate.

The above are the actual appearances presented by Eozoön when examined microscopically, and it remains to see how far they enable us to decide upon its true position in the animal kingdom. Those who wish to study this interesting subject in detail must consult the admirable memoirs by Dr W. B. Carpenter and Principal Dawson: it will be enough here to indicate the results which have been arrived at. The only animals at the present day which possess a continuous calcareous skeleton, perforated by pores and penetrated by canals, are certain organisms belonging to the group of the Foraminifera. We have had occasion before to speak of these animals, and as they are not conspicuous or commonly-known forms of life, it may be well to say a few words as to the structure of the living representatives of the group. The Foraminifera are all inhabitants of the sea, and are mostly of small or even microscopic dimensions. Their bodies are composed Page 72 of an apparently structureless animal substance of an albuminous nature ("sarcode"), of a gelatinous consistence, transparent, and exhibiting numerous minute granules or rounded particles. The body-substance cannot be said in itself to possess any definite form, except in so far as it may be bounded by a shell; but it has the power, wherever it may be exposed, of emitting long thread-like filaments ("pseudopodia"), which interlace with one another to form a network (fig. 25, b). These Fig. 25

Fig. 25.—The animal of Nonionina, one of the Foraminifera, after the shell has been removed by a weak acid; b, Gromia, a single-chambered Foraminifer (after Schultze), showing the shell surrounded by a network of filaments derived from the body substance. filaments can be thrown out at will, and to considerable distances, and can be again retracted into the soft mass of the general body-substance, and they are the agents by which the animal obtains its food. The soft bodies of the Foraminifera are protected by a shell, which is usually calcareous, but may be composed of sand-grains cemented Page 73 together; and it may consist of a single chamber (fig. 26, a), or of many chambers arranged in different ways (fig. 26, b-f). Sometimes the shell has but Fig. 26

Fig. 26.—Shells of living Foraminifera. a, Orbulina universa, in its perfect condition, showing the tubular spines which radiate from the surface of the shell; b, Globigerina bulloides, in its ordinary condition, the thin hollow spines which are attached to the shell when perfect having been broken off; c, Textularia variabilis; d, Peneroplis planatus; e, Rotalia concamerata; f, Cristellaria subarcuatula. [Fig. a is after Wyville Thomson; the others are after Williamson. All the figures are greatly enlarged. one large opening into it—the mouth; and then it is from this aperture that the animal protrudes the delicate net of filaments with which it seeks its food. In other cases the entire shell is perforated with minute pores (fig. 26, e), through which the soft body-substance gains the exterior, covering the whole shell with a gelatinous film of animal matter, from which filaments can be emitted at any point. When the shell consists of many chambers, all of these are placed in direct communication with one another, and the actual substance of the shell is often traversed by minute canals filled with living matter (e.g., in Calcarina and Nummulina). The shell, therefore, may be regarded, in such cases, as a more or less completely porous calcareous structure, Page 74 filled to its minutest internal recesses with the substance of the living animal, and covered externally with a layer of the same substance, giving off a network of interlacing filaments.

Such, in brief, is the structure of the living Foraminifera; and it is believed that in Eozoön we have an extinct example of the same group, not only of special interest from its immemorial antiquity, but hardly less striking from its gigantic dimensions. In its original condition, the entire chamber-system of Eozoön is believed to have been filled with soft structureless living matter, which passed from chamber to chamber through the wide apertures connecting these cavities, and from tier to tier by means of the tubuli in the shell-wall and the branching canals in the intermediate skeleton. Through the perforated shell-wall covering the outer surface the soft body-substance flowed out, forming a gelatinous investment, from every point of which radiated an interlacing net of delicate filaments, providing nourishment for the entire colony. In its present state, as before said, all the cavities originally occupied by the body-substance have been filled with some mineral substance, generally with one of the silicates of magnesia; and it has been asserted that this fact militates strongly against the organic nature of Eozoön, if not absolutely disproving it. As a matter of fact, however—as previously noticed—it is by no means very uncommon at the present day to find the shells of living species of Foraminifera in which all the cavities primitively occupied by the body-substance, down to the minutest pores and canals, have been similarly injected by some analogous silicate, such as glauconite.

Those, then, whose opinions on such a subject deservedly carry the greatest weight, are decisively of opinion that we are presented in the Eozoön of the Laurentian Rocks of Canada with an ancient, colossal, and in some respects abnormal type of the Foraminifera. In the words of Dr Carpenter, it is not pretended that "the doctrine of the Foraminiferal nature of Eozoön can be proved in the demonstrative sense;" but it may be affirmed "that the convergence of a number of separate and independent probabilities, all accordant with that hypothesis, while a separate explanation must be invented for each of them on any other hypothesis, gives it that high probability on which we rest in the ordinary affairs of life, in the verdicts of juries, and in the interpretation of geological phenomena generally."

It only remains to be added, that whilst Eozoön is by far the most important organic body hitherto found in the Laurentian, and has been here treated at proportionate length, other Page 75 traces of life have been detected, which may subsequently prove of great interest and importance. Thus, Principal Dawson has recently described under the name of Archœosphœrinœ certain singular rounded bodies which he has discovered in the Laurentian limestones, and which he believes to be casts of the shells of Foraminifera possibly somewhat allied to the existing Globigerinœ. The same eminent palæontologist has also described undoubted worm-burrows from rocks probably of Laurentian age. Further and more extended researches, we may reasonably hope, will probably bring to light other actual remains of organisms in these ancient deposits.

THE HURONIAN PERIOD.

The so-called Huronian Rocks, like the Laurentian, have their typical development in Canada, and derive their name from the fact that they occupy an extensive area on the borders of Lake Huron. They are wholly metamorphic, and consist principally of altered sandstones or quartzites, siliceous, felspathic, or talcose slates, conglomerates, and limestones. They are largely developed on the north shore of Lake Superior, and give rise to a broken and hilly country, very like that occupied by the Laurentians, with an abundance of timber, but rarely with sufficient soil of good quality for agricultural purposes. They are, however, largely intersected by mineral veins, containing silver, gold, and other metals, and they will ultimately doubtless yield a rich harvest to the miner. The Huronian Rocks have been identified, with greater or less certainty, in other parts of North America, and also in the Old World.

The total thickness of the Huronian Rocks in Canada is estimated as being not less than 18,000 feet, but there is considerable doubt as to their precise geological position. In their typical area they rest unconformably on the edges of strata of Lower Laurentian age; but they have never been seen in direct contact with the Upper Laurentian, and their exact relations to this series are therefore doubtful. It is thus open to question whether the Huronian Rocks constitute a distinct formation, to be intercalated in point of time between the Laurentian and the Cambrian groups; or whether, rather, they should not be considered as the metamorphosed representatives of the Lower Cambrian Rocks of other regions.

As regards the fossils of the Huronian Rocks, little can be said. Some of the specimens of Eozoön Canadense which have Page 76 been discovered in Canada are thought to come from rocks which are probably of Huronian age. In Bavaria, Dr Gümbel has described a species of Eozoön under the name of Eozoön Bavaricum, from certain metamorphic limestones which he refers to the Huronian formation. Lastly, the late Mr Billings described, from rocks in Newfoundland apparently referable to the Huronian, certain problematical limpet-shaped fossils, to which he gave the name of Aspidella.

LITERATURE.

Amongst the works and memoirs which the student may consult with regard to the Laurentian and Huronian deposits may be mentioned the following:[10]—

The above list only includes some of the more important memoirs which may be consulted as to the geological and chemical features of the Laurentian and Huronian Rocks, and as to the true nature of Eozoön. Those who are desirous of studying the later phases of the controversy with regard to Eozoön must consult the papers of Carpenter, Carter, Dawson, King & Rowney, Hahn, and others, in the 'Quart. Journ. of the Geological Society,' the 'Proceedings of the Royal Irish Academy,' the 'Annals of Natural History,' the 'Geological Magazine,' &c. Dr Carpenter's 'Introduction to the Study of the Foraminifera' should also be consulted.

[Footnote 10: In this and in all subsequently following bibliographical lists, not only is the selection of works and memoirs quoted necessarily extremely limited; but only such have, as a general rule, been chosen for mention as are easily accessible to students who are in the position of being able to refer to a good library. Exceptions, however, are occasionally made to this rule, in favour of memoirs or works of special historical interest. It is also unnecessary to add that it has not been thought requisite to insert in these lists the well-known handbooks of geological and palæontological science; except in such instances as where they contain special information on special points.]

Page 77 CHAPTER VIII.

The traces of life in the Laurentian period, as we have seen, are but scanty; but the Cambrian Rocks—so called from their occurrence in North Wales and its borders ("Cambria ")—have yielded numerous remains of animals and some dubious plants. The Cambrian deposits have thus a special interest as being the oldest rocks in which occur any number of well-preserved and unquestionable organisms. We have here the remains of the first fauna, or assemblage of animals, of which we have at present knowledge. As regards their geographical distribution, the Cambrian Rocks have been recognised in many parts of the world, but there is some question as to the precise limits of the formation, and we may consider that their most typical area is in South Wales, where they have been carefully worked out, chiefly by Dr Henry Hicks. In this region, in the neighbourhood of the promontory of St David's, the Cambrian Rocks are largely developed, resting upon an ancient ridge of Pre-Cambrian (Laurentian?) strata, and overlaid by the lowest beds of the Lower Silurian. The subjoined sketch-section (fig. 27) exhibits in a general manner the succession of strata in this locality.

From this section it will be seen that the Cambrian Rocks in Wales are divided in the first place into a lower and an upper group. The Lower Cambrian is constituted at the base by a great series of grits, sandstones, conglomerates, and slates, which are known as the "Longmynd group," from their vast development in the Longmynd Hills in Shropshire, and which attain in North Wales a thickness of 8000 feet or more. The Longmynd beds are succeeded by the so-called "Menevian group," a series of sandstones, flags, and grits, about 600 feet in thickness, and containing a considerable number of fossils. The Upper Cambrian series consists in its lower portion of nearly 5000 feet of strata, principally shaly and slaty, which are known as the "Lingula Flags," from the great abundance in them of a shell referable to the genus Lingula. These are followed by 1000 feet of dark shales and flaggy sandstones, which are known as the "Tremadoc slates," from their occurrence near Tremadoc in North Wales; and these in turn are surmounted, apparently quite conformably, by the basement beds of the Lower Silurian.

Page 78 GENERALIZED SECTION OF THE CAMBRIAN ROCKS IN WALES
Fig. 27.
Fig. 27

The above may be regarded as giving a typical series of the Cambrian Rocks in a typical locality; but strata of Cambrian age are known in many other regions, of which it is only possible here to allude to a few of the most important. In Scandinavia occurs a well-developed series of Cambrian deposits, representing both the lower and upper parts of the Page 79 formation. In Bohemia, the Upper Cambrian, in particular, is largely developed, and constitutes the so-called "Primordial zone" of Barrande. Lastly, in North America, whilst the Lower Cambrian is only imperfectly developed, or is represented by the Huronian, the Upper Cambrian formation has a wide extension, containing fossils similar in character to the analogous strata in Europe, and known as the "Potsdam Sandstone." The subjoined table shows the chief areas where Cambrian Rocks are developed, and their general equivalency:

TABULAR VIEW OF THE CAMBRIAN FORMATION.

Britain.

Europe.

America.

Upper Cambrian.

a.	Tremadoc Slates.

a.	Primordial zone of Bohemia.

a.	Potsdam Sandstone.

b.	Lingula Flags.

b.	Paradoxides Schists, Olenus Schists, and Dictyonema schists of Sweden.

b.	Acadian group of New Brunswick.

Lower Cambrian.

a.	Longmynd Beds.

a.	Fucoidal Sandstone of Sweden.

Huronian Formation?

b.	Llanberis Slates.

b.	Eophyton Sandstone of Sweden.

c.	Harlech Grits.

d.	Oldhamia Slates of Ireland.

e.	Conglomerates and Sandstones of Sutherlandshire?

f.	Menevian Beds.

Like all the older Palæozoic deposits, the Cambrian Rocks, though by no means necessarily what would be called actually "metamorphic," have been highly cleaved, and otherwise altered from their original condition. Owing partly to their indurated state, and partly to their great antiquity, they are usually found in the heart of mountainous districts, which have undergone great disturbance, and have been subjected to an enormous amount of denudation. In some cases, as in the Longmynd Hills in Shropshire, they form low rounded elevations, largely covered by pasture, and with few or no elements of sublimity. In other cases, however, they rise into bold and rugged mountains, girded by precipitous cliffs. Industrially, the Cambrian Rocks are of interest, if only for the reason that the celebrated Welsh slates of Llanberis are derived from highly-cleaved beds of this age. Taken as a whole, the Cambrian formation is essentially composed of arenaceous and Page 80 muddy sediments, the latter being sometimes red, but more commonly nearly black in colour. It has often been supposed that the Cambrians are a deep-sea deposit, and that we may thus account for the few fossils contained in them; but the paucity of fossils is to a large extent imaginary, and some of the Lower Cambrian beds of the Longmynd Hills would appear to have been laid down in shallow water; as they exhibit rain-prints, sun-cracks, and ripple-marks—incontrovertible evidence of their having been a shore-deposit. The occurrence, of innumerable worm-tracks and burrows in many Cambrian strata is also a proof of shallow-water conditions; and the general absence of limestones, coupled with the coarse mechanical nature of many of the sediments of the Lower Cambrian, maybe taken as pointing in the same direction.

The life of the Cambrian, though not so rich as in the succeeding Silurian period, nevertheless consists of representatives of most of the great classes of invertebrate animals. The coarse sandy deposits of the formation, which abound more particularly towards its lower part, naturally are to a large extent barren of fossils; but the muddy sediments, when not too highly cleaved, and especially towards the summit of the group, are replete with organic remains. This is also the case, in many localities at any rate, with the finer beds of the Potsdam Sandstone in America. Limestones are known to occur in only a few areas (chiefly in America), and this may account for the apparent total absence of corals. It is, however, interesting to note that, with this exception, almost all the other leading groups of Invertebrates are known to have come into existence during the Cambrian period.

Of the land-surfaces of the Cambrian period we know nothing; and there is, therefore, nothing surprising in the fact that our acquaintance with the Cambrian vegetation is confined to some marine plants or sea-weeds, often of a very obscure and problematical nature. The "Fucoidal Sandstone" of Sweden, and the "Potsdam Sandstone" of North America, have both yielded numerous remains which have been regarded as markings left by sea-weeds or "Fucoids;" but these are highly enigmatical in their characters, and would, in many instances, seem to be rather referable to the tracks and burrows of marine worms. The first-mentioned of these formations has also yielded the curious, furrowed and striated stems which have been described as a kind of land-plant under the name of Eopkyton (fig. 28). It cannot be said, however, that the vegetable origin of these singular bodies has been satisfactorily proved. Lastly, there are found in certain green Page 81 and purple beds of Lower Cambrian age at Bray Head, Wicklow, Ireland, some very remarkable fossils, which are well known under Fig. 28

Fig. 28.—Fragment of Eophyton Linneanum, a supposed land-plant. Lower Cambrian, Sweden, of the natural size. the name of Oldhamia, but the true nature of which is very doubtful. The commonest form of Oldhamia (fig. 29) consists of a thread-like stem or axis, from which spring at regular intervals bundles of short filamentous branches in a fan-like manner. In the locality where it occurs, the fronds of Oldhamia are very abundant, and are spread over the surfaces of the strata in tangled layers. That it is organic is certain, and that it is a calcareous sea-weed is probable; but it may possibly belong to the sea-mosses (Polyzoa), or to the sea-firs (Sertularians).

Amongst the lower forms of animal life (Protozoa), we find the Sponges represented by the curious bodies, composed of netted fibres, to which the name of Protospongia has been given (fig. 32, a); and the comparatively gigantic, conical, or Page 82 cylindrical fossils termed Archœocyathus by Mr Billings are certainly referable either to the Foraminifera Fig. 29

Fig. 29.—A portion of Oldhamia antiqua, Lower Cambrian, Wicklow, Ireland, of the natural size. (After Salter.) or to the Sponges. The almost total absence of limestones in the formation may be regarded as a sufficient explanation of the fact that the Foraminifera are not more largely and unequivocally represented; though the existence of greensands in the Cambrian beds of Wisconsin and Tennessee may be taken as an indication that this class of animals was by no means wholly wanting. The same fact may explain the total absence of corals, so far as at present known.

The group of the Echinodermata (Sea-lilies, Sea-urchins, and their allies) is represented by a few forms, which are principally of interest as being the earliest-known examples of the class. It is also worthy of note that these precursors of a group which subsequently attains such geological importance, are referable to no less than three distinct orders—the Crinoids or Sea-lilies, represented by a species of Dendrocrinus; the Cystideans by Protocystites; and the Star-fishes by Palasterina and some other forms. Only the last of these groups, however, appears to occur in the Lower Cambrian.

The Ringed-worms (Annelida), if rightly credited with all the remains usually referred to them, appear to have swarmed in the Cambrian seas. Being soft-bodied, we do not find the actual worms themselves in the fossil condition, but we have, nevertheless, abundant traces of their existence. In some cases we find vertical burrows of greater or less depth, often expanded towards their apertures, in which the worm must have actually lived (fig. 30), as various species do at the present day. In these cases, the tube must have been rendered more or less permanent by receiving a coating of mucus, or perhaps a genuine membranous secretion, from the body of the animal; and it may be found quite empty, or occupied by a cast of sand or mud. Of this nature are the burrows which have been described under the names of Scolithus and Scolecoderma, and probably the Histioderma of the Lower Cambrian Page 83 of Ireland. In other cases, as in Arenicolites (fig. 32, b), the worm seems to have inhabited a double Fig. 30

Fig. 30.—Annelide-burrows (Scolithus linearus) from the Potsdam Sandstone of Canada, of the natural size. (After Billings.) burrow, shaped like the letter U, and having two openings placed close together on the surface of the stratum. Thousands of these twin-burrows occur in some of the strata of the Longmynd, and it is supposed that the worm used one opening to the burrow as an aperture of entrance, and the other as one of exit. In other cases, again, we find simply the meandering trails caused by the worm dragging its body over the surface of the mud. Markings of this kind are commoner in the Silurian Rocks, and it is generally more or less doubtful whether they may not have been caused by other marine animals, such as shellfish, whilst some of them have certainly nothing whatever to do with the worms. Lastly, the Cambrian beds often show twining cylindrical bodies, commonly more or less matted together, and not confined to the surfaces of the strata, but passing through them. These have often been regarded as the remains of sea-weeds, but it is more probable that they represent casts of the underground burrows of worms of similar habits to the common lob-worm (Arenicola) of the present day.

The Articulate animals are numerously represented in the Cambrian deposits, but exclusively by the class of Crustaceans. Some of these are little double-shelled creatures, resembling our living water-fleas (Ostracoda). A few are larger forms, and belong to the same group as the existing brine-shrimps and fairy-shrimps (Phyllopoda). One of the most characteristic Page 84 of these is the Hymenocaris vermicauda of the Lingula Flags (fig. 32, d). By far the larger number of the Cambrian Crustacea belong, however, to the remarkable and wholly extinct group of the Trilobites. These extraordinary animals must have literally swarmed in the seas of the later portion of this and the whole of the succeeding period; and they survived in greatly diminished numbers till the earlier portion of the Carboniferous period. They died out, however, wholly before the close of the Palæozoic epoch, and we have no Crustaceans at the present day which can be considered as their direct representatives. They have, however, relationships of a more or less intimate character with the existing groups of the Phyllopods, the King-crabs (Limulus), and the Isopods ("Slaters," Wood-lice, &c.) Indeed, one member of the last-mentioned order, namely, the Serolis of the coasts of Patagonia, has been regarded as the nearest living ally of the Trilobites. Be this as it may, the Trilobites possessed a skeleton which, though capable of undergoing almost endless variations, was wonderfully constant in its pattern of structure, and we may briefly describe here the chief features of this.

The upper surface of the body of a Trilobite was defended by a strong shell or "crust," partly horny and partly calcareous in its composition. This shell (fig. 31) generally exhibits a very distinct "trilobation" or division into three longitudinal lobes, one central and two lateral. It also exhibits a more important and more fundamental division into three transverse portions, which are so loosely connected with one another as very commonly to be found separate. The first and most anterior of these divisions is a shield or buckler which covers the head; the second or middle portion is composed of movable rings covering the trunk ("thorax "); and the third is a shield which covers the tailor "abdomen." The head-shield (fig. 31, e) is generally more or less semicircular in shape; and its central portion, covering the stomach of the animal, is usually strongly elevated, and generally marked by lateral furrows. A little on each side of the head are placed the eyes, which are generally crescentic in shape, and resemble the eyes of insects and many existing Crustaceans in being "compound," or made up of numerous simple eyes aggregated together. So excellent is the state of preservation of many specimens of Trilobites, that the numerous individual lenses of the eyes have been uninjured, and as many as four hundred have been counted in each eye of some forms. The eyes may be supported upon prominences, but they are never carried on movable stalks (as they are in the existing lobsters and crabs); and Page 85 in some of the Cambrian Trilobites, such as the little Agnosti (fig. 31 g), the animal was blind. The lateral portions of the head-shield are usually separated from the central portion by a peculiar Fig. 31

Fig. 31.—Cambrian Trilobites: a, Paradoxides Bohemicus, reduced in size; b, Ellipsocephalus Hoffi; c, Sao hirsuta; d, Conocorypke Sultzeri (all the above, together with fig. g, are from the Upper Cambrian or "Primordial Zone" of Bohemia); e, Head-shield of Dikellocephalus Celticus, from the Lingula Flags of Wales; f, Head-shield of Conocoryphe Matthewi, from the Upper Cambrian (Acadian Group) of New Brunswick; g, Agnostus rex, Bohemia; h, Tail-shield of Dikellocephalus Minnesotensis, from the Upper Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, and Dale Owen.) line of division (the so-called "facial suture") on each side; but this is also wanting in some of the Cambrian species. The backward angles of the head-shield, also, are often prolonged into spines, which sometimes reach a great length. Following the head-shield behind, we have a portion of the body which is composed of movable segments or "body-rings," and which is technically called the "thorax," Ordinarily, this region is strongly trilobed, and each ring consists of a central convex portion, and of two flatter side-lobes. The number of body-rings in the thorax is very variable (from two to twenty-six), but is fixed for the adult forms of each group of the Trilobites. The young forms have much fewer rings than the full-grown ones; and it is curious to find that the Cambrian Page 86 Trilobites very commonly have either a great many rings (as in Paradoxides, fig. 31, a), or else very few (as in Agnostus, fig. 31, g). In some instances, the body-rings do not seem to have been so constructed as to allow of much movement, but in other cases this region of the body is so flexible that the animal possessed the power of rolling itself up completely, like a hedgehog; and many individuals have been permanently preserved as fossils in this defensive condition. Finally, the body of the Trilobite was completed by a tail-shield (technically termed the "pygidium"), which varies much in size and form, and is composed of a greater or less number of rings, similar to those which form the thorax, but immovably amalgamated with one another (fig. 31, h).

The under surface of the body in the Trilobites appears to have been more or less entirely destitute of hard structures, with the exception of a well-developed upper lip, in the form of a plate attached to the inferior side of the head-shield in front. There is no reason to doubt that the animal possessed legs; but these structures seem to have resembled those of many living Crustaceans in being quite soft and membranous. This, at any rate, seems to have been generally the case; though structures which have been regarded as legs have been detected on the under surface of one of the larger species of Trilobites. There is also, at present, no direct evidence that the Trilobites possessed the two pairs of jointed feelers ("antennæ") which are so characteristic of recent Crustaceans.

The Trilobites vary much in size, and the Cambrian formation presents examples of both the largest and the smallest members of the order. Some of the young forms may be little bigger than a millet-seed, and some adult examples of the smaller species (such as Agnostus) may be only a few lines in length; whilst such giants of the order as Paradoxides and Asaphus may reach a length of from one to two feet. Judging from what we actually know as to the structure of the Trilobites, and also from analogous recent forms, it would seem that these ancient Crustaceans were mud-haunting creatures, denizens of shallow seas, and affecting the soft silt of the bottom rather than the clear water above. Whenever muddy sediments are found in the Cambrian and Silurian formations, there we are tolerably sure to find Trilobites, though they are by no means absolutely wanting in limestones. They appear to have crawled out upon the sea-bottom, or burrowed in the yielding mud, with the soft under surface directed downwards; and it is probable that they really derived their nutriment from the organic matter contained in the ooze amongst which they Page 87 lived. The vital organs seem to have occupied the central lobe of the skeleton, by which they were protected; and a series of delicate leaf-like paddles, which probably served as respiratory organs, would appear to have been carried on the under surface of the thorax. That they had their enemies may be regarded as certain; but we have no evidence that they were furnished with any offensive weapons, or, indeed, with any means of defence beyond their hard crust, and the power, possessed by so many of them, of rolling themselves into a ball. An additional proof of the fact that they for the most part crawled along the sea-bottom is found in the occurrence of tracks and markings of various kinds, which can hardly be ascribed to any other creatures with any show of probability. That this is the true nature of some of the markings in question cannot be doubted at all; and in other cases no explanation so probable has yet been suggested. If, however, the tracks which have been described from the Potsdam Sandstone of North America under the name of Protichnites are really due to the peregrinations of some Trilobite, they must have been produced by one of the largest examples of the order.

As already said, the Cambrian Rocks are very rich in the remains of Trilobites. In the lowest beds of the series (Longmynd Rocks), representatives of some half-dozen genera have now been detected, including the dwarf Agnostus and the giant Paradoxides. In the higher beds, the number both of genera and species is largely increased; and from the great comparative abundance of individuals, the Trilobites have every right to be considered as the most characteristic fossils of the Cambrian period,—the more so as the Cambrian species belong to peculiar types, which, for the most part, died out before the commencement of the Silurian epoch.

All the remaining Cambrian fossils which demand any notice here are members of one or other division of the great class of the Mollusca, or "Shell-fish" properly so called. In the Lower Cambrian Rocks the Lamp-shells (Brachiopoda) are the principal or sole representatives of the class, and appear chiefly in three interesting and important types—namely, Lingulella, Discina, and Obolella. Of these the last (fig. 32, i) is highly characteristic of these ancient deposits; whilst Discina is one of those remarkable persistent types which, commencing at this early period, has continued to be represented by varying forms through all the intervening geological formations up to the present day. Lingulella (fig. 32, c), again, is closely allied to the existing "Goose-bill" Lamp-shell (Lingula anatina), and thus presents us with another example of an extremely long-lived Page 88 type. The Lingulellœ and their successors; the Lingulœ, are singular in possessing a shell which is of a horny texture, and contains but a small proportion of calcareous matter. In the Upper Cambrian Rocks, the Lingulellœ become much more abundant, the broad satchel-shaped species known as L. Davisii (fig. 32, e) being so abundant that one of the great divisions of the Cambrian is termed the "Lingula Flags." Here, also, we meet for the first time with examples of the genus Orthis (fig. 32, f, k, l) Fig. 32

Fig. 32.—Cambrian Fossils: a, Protospongia fenestrata, Menevian Group; b, Arenicolites didymus, Longmynd Group; c, Lingulella ferruginea, Longmynd and Menevian, enlarged; d, Hymenocaris vermicauda, Lingula Flags; e, Lingulella Davisii, Lingula Flags; f, Orthis lenticularis, Lingula Flags; g, Theca Davidii, Tremadoc Slates; h, Modiolopsis Solvensis, Tremadoc Slates; i, Obolela sagittalis, interior of valve, Menevian; j, Exterior of the same; k, Orthis Hicksii, Menevian; l, Cast of the same; m, Olenus micrurus, Lingula Flags. (Alter Salter, Hicks, and Davidson.) a characteristic Palæozoic type of the Brachiopods, which is destined to undergo a vast extension in later ages.

Of the higher groups of the Mollusca the record is as yet but scanty. In the Lower Cambrian, we have but the thin, fragile, dagger-shaped shells of the free-swimming oceanic Molluscs or "Winged-snails" (Pteropoda), of which the most characteristic is the genus Theca (fig. 32, g). In the Upper Cambrian, in addition to these, we have a few Univalves (Gasteropoda), and, thanks to the researches of Dr Hicks, quite a small assemblage of Bivalves (Lamellibranchiata), though these are mostly of no great dimensions (fig. 32, h). Of the chambered Cephalopoda (Cuttle-fishes and their allies), we have but Page 89 few traces; and these wholly confined to the higher beds of the formation. We meet, however, with examples of the wonderful genus Fig. 33

Fig. 33.—Fragment of Dictyonema sociale, considerably enlarged, showing the horny branches, with their connecting cross-bars, and with a row of cells on each side. (Original.) Orthoceras, with its straight, partitioned shell, which we shall find in an immense variety of forms in the Silurian rocks. Lastly, it is worthy of note that the lowest of all the groups of the Mollusca—namely, that of the Sea-mats, Sea-mosses, and Lace-corals (Polyzoa)—is only doubtfully known to have any representatives in the Cambrian, though undergoing a large and varied development in the Silurian deposits.

An exception, however, may with much probability be made to this statement in favour of the singular genus Dictyonema (fig. 33), which is highly characteristic of the highest Cambrian beds (Tremadoc Slates). This curious fossil occurs in the form of fan-like or funnel-shaped expansions, composed of slightly-diverging horny branches, which are united in a net-like manner by numerous delicate cross-bars, and exhibit a row of little cups or cells, in which the animals were contained, on each side. Dictyonema has generally been referred to the Graptolites; but it has a much greater affinity with the plant-like Sea-firs (Sertularians) or the Sea-mosses (Polyzoa), and the balance of evidence is perhaps in favour of placing it with the latter.

LITERATURE.

The following are the more important and accessible works and memoirs which may be consulted in studying the stratigraphical and palæontological relations of the Cambrian Rocks:—

In the above list, allusion has necessarily been omitted to numerous works and memoirs on the Cambrian deposits of Sweden and Norway, Central Europe, Russia, Spain, and various parts of North America, as well as to a number of important papers on the British Cambrian strata by various well-known observers. Amongst these latter may be mentioned memoirs by Prof. Phillips, and Messrs Salter, Hicks, Belt, Plant, Homfray, Ash, Holl, &c.

CHAPTER IX.

The great system of deposits to which Sir Roderick Murchison applied the name of "Silurian Rocks" reposes directly upon the highest Cambrian beds, apparently without any marked unconformity, though with a considerable change in the nature of the fossils. The name "Silurian" was originally proposed by the eminent geologist just alluded to for a great series of strata lying below the Old Red Sandstone, and occupying districts in Wales and its borders which were at one time inhabited by the "Silures," a tribe of ancient Britons. Deposits of a corresponding age are now known to be largely developed in other parts of England, in Scotland, and in Ireland, in North America, in Australia, in India, in Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and in various other regions of less note. In some regions, as in the neighbourhood of St Petersburg, the Silurian strata are found not only to have preserved their original horizontality, but also to have retained almost unaltered their primitive soft and incoherent nature. In other regions, as in Scandinavia and many Page 91 parts of North America, similar strata, now consolidated into shales, sandstones, and limestones, may be found resting with a very slight inclination on still older sediments. In a great many regions, however, the Silurian deposits are found to have undergone more or less folding, crumpling, and dislocation, accompanied by induration and "cleavage" of the finer and softer sediments; whilst in some regions, as in the Highlands of Scotland, actual "metamorphism" has taken place. In consequence of the above, Silurian districts usually present the bold, rugged, and picturesque outlines which are characteristic of the older "Primitive" rocks of the earth's crust in general. In many instances, we find Silurian strata rising into mountain-chains of great grandeur and sublimity, exhibiting the utmost diversity of which rock-scenery is capable, and delighting the artist with endless changes of valley, lake, and cliff. Such districts are little suitable for agriculture, though this is often compensated for by the valuable mineral products contained in the rocks. On the other hand, when the rocks are tolerably soft and uniform in their nature, or when few disturbances of the crust of the earth have taken place, we may find Silurian areas to be covered with an abundant pasturage or to be heavily timbered.

Under the head of "Silurian Rocks," Sir Roderick Murchison included all the strata between the summit of the "Longmynd." beds and the Old Red Sandstone, and he divided these into the two great groups of the Lower Silurian and Upper Silurian. It is, however, now generally admitted that a considerable portion of the basement beds of Murchison's Silurian series must be transferred—if only upon palæontological grounds—to the Upper Cambrian, as has here been done; and much controversy has been carried on as to the proper nomenclature of the Upper Silurian and of the remaining portion of Murchison's Lower Silurian. Thus, some would confine the name "Silurian" exclusively to the Upper Silurian, and would apply the name of "Cambro-Silurian" to the Lower Silurian, or would include all beds of the latter age in the "Cambrian" series of Sedgwick. It is not necessary to enter into the merits of these conflicting views. For our present purpose, it is sufficient to recognise that there exist two great groups of rocks between the highest Cambrian beds, as here defined, and the base of the Devonian or Old Red Sandstone. These two great groups are so closely allied to one another, both physically and palæontologically, that many authorities have established a third or intermediate group (the "Middle Silurian"), by which a Page 92 passage is made from one into the other. This method of procedure involves disadvantages which appear to outweigh its advantages; and the two groups in question are not only generally capable of very distinct stratigraphical separation, but at the same time exhibit, together with the alliances above spoken of, so many and such important palæontological differences, that it is best to consider them separately. We shall therefore follow this course in the present instance; and pending the final solution of the controversy as to Cambrian and Silurian nomenclature, we shall distinguish these two groups of strata as the "Lower Silurian" and the "Upper Silurian."

The Lower Silurian Rocks are known already to be developed in various regions; and though their general succession in these areas is approximately the same, each area exhibits peculiarities of its own, whilst the subdivisions of each are known by special names. All, therefore, that can be attempted here, is to select two typical areas—such as Wales and North America and to briefly consider the grouping and divisions of the Lower Silurian in each.

In Wales, the line between the Cambrian and Lower Silurian is somewhat ill-defined, and is certainly not marked by any strong unconformity. There are, however; grounds for accepting the line proposed, for palæontological reasons, by Dr Hicks, in accordance with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become the highest of the Cambrian deposits of Britain. If we take this view, the Lower Silurian rocks of Wales and adjoining districts are found to have the following general succession from below upwards (fig. 34):—

1. The Arenig Group.—This group derives its name from the Arenig mountains, where it is extensively developed. It consists of about 4000 feet of slates, shales, and flags, and is divisible into a lower, middle, and upper division, of which the former is often regarded as Cambrian under the name of "Upper Tremadoc Slates."

2. The Llandeilo Group.—The thickness of this group varies from about 4000 to as much as 10,000 feet; but in this latter case a great amount of the thickness is made up of volcanic ashes and interbedded traps. The sedimentary beds of this group are principally slates and flags, the latter occasionally with calcareous bands; and the whole series can be divided into a lower, middle, and upper Llandeilo division, of which the last is the most important. The name of "Llandeilo" is derived from the town of the same name in Wales, where strata of this age were described by Murchison.

Page 93 3. The Caradoc or Bala Group.—The alternative names of this group are also of local origin, and are derived, the one from Caer Caradoc in Shropshire, the other from Bala in Wales, strata of this age occurring in both localities. The series is divided into a lower and upper group, the latter chiefly composed of shales and flags, and the former of sandstones and shales, together with the important and interesting calcareous band known as the "Bala Limestone." The thickness of the entire series varies from 4000 to as much as 12,000 feet, according as it contains more or less of interstratified igneous rocks.

4. The Llandovery Group (Lower Llandovery of Murchison).—This series, as developed near the town of Llandovery, in Caermarthenshire, consists of less than 1000 feet of conglomerates, sandstones, and shales. It is probable, however, that the little calcareous band known as the "Hirnant Limestone," together with certain pale-coloured slates which lie above the Bala Limestone, though usually referred to the Caradoc series, should in reality be regarded as belonging to the Llandovery group.

The general succession of the Lower Silurian strata of Wales and its borders, attaining a maximum thickness (along with contemporaneous igneous matter) of nearly 30,000 feet, is diagramatically represented in the annexed sketch-section (fig. 34):—

Page 94 GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF WALES.
Fig. 34.
Fig. 34

In North America, both in the United States and in Canada, the Silurian rocks are very largely developed, and may be Page 95 regarded as constituting an exceedingly full and typical series of the deposits of this period. The chief groups of the Silurian rocks of North America are as follows, beginning, as before, with the lowest strata, and proceeding upwards (fig. 35):—

1. Quebec Group.—This group is typically developed in the vicinity of Quebec, where it consists of about 5000 feet of strata, chiefly variously-coloured shales, together with some sandstones and a few calcareous bands. It contains a number of peculiar Graptolites, by which it can be identified without question with the Arenig group of Wales and the corresponding Skiddaw Slates of the North of England. It is also to be noted that numerous Trilobites of a distinct Cambrian facies have been obtained in the limestones of the Quebec group, near Quebec. These fossils, however, have been exclusively obtained from the limestones of the group; and as these limestones are principally calcareous breccias or conglomerates, there is room for believing that these primordial fossils are really derived, in part at any rate, from fragments of an upper Cambrian limestone. In the State of New York, the Graptolitic shales of Quebec are wanting; and the base of the Silurian is constituted by the so-called "Calciferous Sand-rock" and "Chazy Limestone."[11] The first of these is essentially and typically calcareous, and the second is a genuine limestone.

[Footnote 11: The precise relations of the Quebec shales with Graptolites (Levis Formation) to the Calciferous and Chazy beds are still obscure, though there seems little doubt but that the Quebec Shales are superior to the Calciferous Sand-rock.]

2. The Trenton Group.—This is an essentially calcareous group, the various limestones of which it is composed being known as the "Bird's-eye," "Black River," and "Trenton" Limestones, of which the last is the thickest and most important. The thickness of this group is variable, and the bands of limestone in it are often separated by beds of shale.

3. The Cincinnati Group (Hudson River Formation[12]).—This group consists essentially of a lower series of shales, often black in colour and highly charged with bituminous matter (the "Utica Slates "), and of an upper series of shales, sandstones, Page 96 and limestones (the "Cincinnati" rocks proper). The exact parallelism of the Trenton and Cincinnati groups with the subdivisions of the Welsh Silurian series can hardly be stated positively. Probably no precise equivalency exists; but there can be no doubt but that the Trenton and Cincinnati groups correspond, as a whole, with the Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic section (fig. 35) gives a general idea of the succession of the Lower Silurian rocks of North America:— GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.
Fig. 35.
Fig. 35

[Footnote 12: There is some difficulty about the precise nomenclature of this group. It was originally called the "Hudson River Formation;" but this name is inappropriate, as rocks of this age hardly touch anywhere the actual Hudson River itself, the rocks so called formerly being now known to be of more ancient date. There is also some want of propriety in the name of "Cincinnati Group," since the rocks which are known under this name in the vicinity of Cincinnati itself are the representatives of the Trenton Limestone, Utica Slates, and the old Hudson River group, inseparably united in what used to be called the "Blue Limestone Series."].

Page 97 Of the life of the Lower Silurian period we have record in a vast number of fossils, showing that the seas of this period were abundantly furnished with living denizens. We have, however, in the meanwhile, no knowledge of the land-surfaces of the period. We have therefore no means of speculating as to the nature of the terrestrial animals of this ancient age, nor is anything known with certainty of any land-plants which may have existed. The only relics of vegetation upon which a positive opinion can be expressed belong to the obscure group of the "Fucoids," and are supposed to be the remains of sea-weeds. Some of the fossils usually placed under this head are probably not of a vegetable Fig. 36

Fig. 36.—Licrophycus Ottawaensis a "Fucoid," from the Trenton Limestone (Lower Silurian) of Canada. (After Billings.) nature at all, but others (fig. 36) appear to be unquestionable plants. The true affinities of these, however, are extremely dubious. All that can be said is, that remains which appear to be certainly vegetable, Page 98 and which are most probably due to marine plants, have been recognised nearly at the base of the Lower Silurian (Arenig), and that they are found throughout the series whenever suitable conditions recur.

The Protozoans appear to have flourished extensively in the Lower Silurian seas, though to a large extent under forms which are still little understood. We have here for the first time the appearance of Foraminifera of the ordinary type—one of the most interesting observations in this collection being that made by Ehrenberg, who showed that the Lower Silurian sandstones of the neighbourhood of St Petersburg contained casts in glauconite of Foraminiferous shells, some of which are referable to the existing genera Rotalia and Texularia. True Sponges, belonging to that section of the group in which the skeleton is calcareous, are also not unknown, one of the Fig. 37

Fig. 37.—Astylospongia prœmorsa, cut vertically so as to exhibit the canal-system in the interior. Lower Silurian, Tennessee. (After Ferdinand Rœmer.) most characteristic genera being Astylospongia (fig. 37). In this genus are included more or less globular, often lobed sponges, which are believed not to have been attached to foreign bodies. In the form here figured there is a funnel-shaped cavity at the summit; and the entire mass of the sponge is perforated, as in living examples, by a system of canals which convey the sea-water to all parts of the organism. The canals by which the sea-water gains entrance open on the exterior of the sphere, and those by which it again escapes from the sponge open into the cup-shaped depression at the summit.

The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera Stromatopora and Receptaculites, the structure of which can merely be alluded to here. The specimens of Stromatopora (fig. 38) occur as hemispherical, pear-shaped, globular, or irregular masses, often of very considerable size, and sometimes demonstrably attached to foreign bodies. In their structure these masses consist of numerous thin calcareous laminæ, usually arranged concentrically, and separated by narrow interspaces. These interspaces are generally crossed by numerous vertical calcareous pillars, giving the Page 99 vertical section of the fossil a lattice-like appearance. There are also usually minute pores in the concentric laminæ, by which the successive interspaces are Fig. 38

Fig. 38.—A small and perfect specimen of Stromatopora rugosa, of the natural size, from the Trenton Limestone of Canada. (After Billings.) placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-canals of the Sponges. Upon the whole, though presenting some curious affinities to the calcareous Sponges, Stromatopora is perhaps more properly regarded as a gigantic Foraminifer. If this view be correct, it is of special interest as being probably the nearest ally of Eozoön, the general appearance of the two being strikingly similar, though their minute structure is not at all the same. Lastly, in the fossils known as Receptaculites and Ischadites we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic Foraminifera. Their structure is very complex; but fragments are easily recognised by the fact that the exterior is covered with numerous rhomboidal calcareous plates, closely fitting together, and arranged in peculiar intersecting curves, presenting very much the appearance of the engine-turned case of a watch.

Passing next to the sub-kingdom of Cœlenterate animals (Zoophytes, Corals, &c.), we find that this great group, almost or wholly absent in the Cambrian, is represented in Lower Page 100 Silurian deposits by a great number of forms belonging on the one hand to the true Corals, and en the other hand to the singular family of the Graptolites. If we except certain plant-like fossils which probably belong rather to the Sertularians or the Polyzoans (e.g., Dictyonema, Dendrograptus, &c.), the family of the Graptolites may be regarded as exclusively Silurian in its distribution. Not only is this the case, but it attained its maximum development almost upon its first appearance, in the Arenig Rocks; and whilst represented by a great variety of types in the Lower Silurian; it only exists in the Upper Silurian in a much diminished form. The Graptolites (Gr. grapho, I write; lithos, stone) were so named by Linnæus, from the resemblance of some of them to written or pencilled marks upon the stone, though the great naturalist himself did not believe them to be true fossils at all. They occur as linear or leaf-like bodies, sometimes simple, sometimes compound and branched; and no doubt whatever can be entertained as to their being the skeletons of composite organisms, or colonies of semi-independent animals united together by a common fleshy trunk, similar to what is observed in the colonies of the existing Sea-firs (Sertularians). This fleshy trunk or common stem of the colony was protected by a delicate horny sheath, and it gave origin to the little flower-like "polypites," which constituted the active element of the whole assemblage. These semi-independent beings were, in turn, protected each by a little horny cup or cell, directly connected with the common sheath below, and terminating above in an opening through which the polypite could protrude its tentacled head or could again withdraw itself for safety. The entire skeleton, again, was usually, if not universally, supported by a delicate horny rod or "axis," which appears to have been hollow, and which often protrudes to a greater or less extent beyond one or both of the extremities of the actual colony.

The above gives the elementary constitution of any Graptolite, but there are considerable differences as to the manner in which these elements are arranged and combined. In some forms the common stem of the colony gives origin to but a single row of cells on one side. If the common stem is a simple, straight, or slightly-curved linear body, then we have the simplest form of Graptolite known (the genus Monograptus); and it is worthy of note that these simple types do not come into existence till comparatively late (Llandeilo), and last nearly to the very close of the Upper Silurian. In other cases, whilst there is still but a single row of cells, the colony may consist of two of these simple stems springing from a Page 101 common point, as in the so-called "twin Graptolites" (Didymograptus, fig. 40). This type is Fig. 39

Fig. 39.—Dichograptus octobrachiatus, a branched, "unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig). (After Hall.) entirely confined to the earlier portion of the Lower Silurian period (Arenig and Llandeilo). In other cases, again, there may be four of such stems springing from a central point (Tetragraptus). Lastly, there are numerous complex forms (such as Dichograptus, Loganograptus, &c.) in which there are eight or more of these simple branches, all arising from a common centre (fig. 39), which is sometimes furnished with a singular horny disc. These complicated branching forms, as well as the Tetragrapti, are characteristic of the horizon of the Arenig group. Similar forms, often specifically identical, are found at this horizon in Wales, in the great series of the Skiddaw Slates of the north of England, in the Quebec group in Canada, in equivalent beds in Sweden, and in certain gold-bearing slates of the same age in Victoria in Australia.

In another great group of Graptolites (including the genera Diplograptus, Dicranograptus, Climacograptus, &c.) the common stem of the colony gives origin, over part or the whole or its length, to two rows of cells, one on each side (fig. 41). These "double-celled" Graptolites are highly characteristic of the Lower Silurian deposits; and, with an exception more apparent than Page 102 real in Bohemia, they are exclusively confined to strata of Lower Silurian age, and are not known to occur in the Upper Silurian. Fig. 40

Fig. 40.—Central portion of the colony of Didymegraptus divaricatus, Upper Llandeilo, Dumfresshire. (Original.) Lastly, there is a group of Graptolites (Phyllograptus, fig. 42) in which the colony is leaf-like in form, and is composed Fig. 41

Fig. 41.—Examples of Diplograptus pristis, showing variations in the appendages at the base. Upper Llandeilo, Dumfriesshire. (Original.) Fig. 42

Fig. 42.—Group of individuals of Phyllograptus typus, from the Quebec group of Canada. (After Hall.) One of the four rows of cells is hidden on the under surface. of four rows of cells springing in a cross-like Page 103 manner from the common stem. These forms are highly characteristic of the Arenig group.

The Graptolites are usually found in dark-coloured, often black shales, which sometimes contain so much carbon as to become "anthracitic." They may be simply carbonaceous; but they are more commonly converted into iron-pyrites, when they glitter with the brilliant lustre of silver as they lie scattered on the surface of the rock, fully deserving in their metallic tracery the name of "written stones." They constitute one of the most important groups of Silurian fossils, and are of the greatest value in determining the precise stratigraphical position of the beds in which they occur. They present, however, special difficulties in their study; and it is still a moot point as to their precise position in the zoological scale. The balance of evidence is in favour of regarding them as an ancient and peculiar group of the Sea-firs (Hydroid Zoophytes), but some regard them as belonging rather to the Sea-mosses (Polyzoa). Under any circumstances, they cannot be directly compared either with the ordinary Sea-firs or the ordinary Sea-mosses; for these two groups consist of fixed organisms, whereas the Graptolites were certainly free-floating creatures, living at large in the open sea. The only Hydroid Zoophytes or Polyzoans which have a similar free mode of existence, have either no skeleton at all, or have hard structures quite unlike the horny sheaths of the Graptolites.

The second great group of Cœlenterate animals (Actinozoa) is represented in the Lower Silurian rocks by numerous Corals. These, for obvious reasons, are much more abundant in regions where the Lower Silurian series is largely calcareous (as in North America) than in districts like Wales, where limestones are very feebly developed. The Lower Silurian Corals, though the first of their class, and presenting certain peculiarities, may be regarded as essentially similar in nature to existing Corals. These, as is well known, are the calcareous skeletons of animals—the so-called "Coral-Zoophytes"—closely allied to the common Sea-anemones in structure and habit. A simple coral (fig. 43) consists of a calcareous cup embedded in the soft tissues of the flower-like polype, and having at its summit a more or less deep depression (the "calice") in which the digestive organs are contained. The space within the coral is divided into compartments by numerous vertical calcareous plates (the "septa"), which spring from the inside of the wall of the cup, and of which some generally reach the centre. Compound corals, again (fig. 44), consist of a greater or less number of structures similar in structure to the above, Page 104 but united together in different ways into a common mass. Simple corals, therefore, are the Fig. 43

Fig. 43.—Zaphrentis Stokesi, a simple "cup-coral," Upper Silurian, Canada. (After Billings.) Fig. 44

Fig. 44.—Upper surface of a mass of Strombodes pentagonus. Upper Silurian, Canada. (After Billings.) skeletons of single and independent polypes; whilst compound corals are the skeletons of assemblages or colonies of similar polypes, living united with one another another as an organic community.

In the general details of their structure, the Lower Silurian Corals do not differ from the ordinary Corals of the present day. The latter, however, have the vertical calcareous plates of the coral ("septa") arranged in multiples of six or five; whereas the former have these structures arranged in multiples of four, and often showing a cross-like disposition. For this reason, the common Lower Silurian Corals are separated to form a distinct group under the name of Rugose Corals or Rugosa. They are further distinguished by the fact that the cavity of the coral ("visceral chamber") is usually subdivided by more or less numerous horizontal calcareous plates or partitions, which divide the coral into so many tiers or storeys, and which are known as the "tabulæ" (fig. 45).

In addition to the Rugose Corals, the Lower Silurian rocks contain a number of curious compound corals, the tubes of which have either no septa at all or merely rudimentary ones, but which have the transverse partitions or "tabulæ" very highly developed. These are known as the Tabulate Corals; and recent researches on some of their existing allies (such as Heliopora) have shown that they are really allied to Page 105 the modern Sea-pens, Organ-pipe Corals, and Red Coral, rather than to the typical stony Corals. Amongst the characteristic Rugose Corals of the Lower Silurian Fig. 45

Fig. 45.—Columnaria alveolata, a Rugose compound coral, with imperfect septa, but having the corallites partitioned off into storeys by "tabulæ." Lower Silurian, Canada. (After Billings.) may be mentioned species belonging to the genera Columnaria, Favistella, Streptelasma, and Zaphrentis; whilst amongst the "Tabulate" Corals, the principal forms belong to the genera Chœtetes, Halysites (the Chain-coral), Constellaria, and Heliolites. These groups of the Corals, however, attain a greater development at a later period, and they will be noticed more particularly hereafter.

[Footnote 13: The genus Caryocrinus is sometimes regarded as properly belonging to the Crinoids, but there seem to be good reasons for rather considering it as an abnormal form of Cystidean.]

Passing onto higher animals, we find that the class of the Echinodermata is represented by examples of the Star-fishes (Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct group of the Cystideans (Cystoidea), with one or two of the Brittle-stars (Ophiuroidea)—the Sea-urchins (Echinoidea) being still wanting. The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of. In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly entitled to the name of "Crinoidal limestones," being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12). Allied to the Crinoids are the singular creatures which are known as Cystideans (fig. 46). These are generally composed of a globular or ovate body (the "calyx"), supported upon a short stalk (the "column"), by which the organism was usually attached to some foreign body. The body was enclosed by closely-fitting calcareous plates, accurately Page 106 jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane. The Fig. 46

Fig. 46.—Group of Cystideans. A, Caryocrinus ornatus,[13] Upper Silurian, America; B, Pleurocystites squamosus, showing two short "arms," Lower Silurian, Canada; C, Pseudocrinus bifasciatus, Upper Silurian, England; D, Lepadocrinus Gebhartii, Upper Silurian, America. (After Hall, Billings, and Salter.) chief distinction which strikes one in comparing the Cystideans with the Crinoids is, that the latter are always furnished, as will be subsequently seen, with a beautiful crown of branched and feathery appendages, springing from the summit of the calyx, and which are composed of innumerable calcareous plates or joints, and are known as the "arms." In the Cystideans, on the other hand, there are either no "arms" at all, or merely short, unbranched, rudimentary arms. The Cystideans are principally, and indeed nearly exclusively, Silurian fossils; and though occurring in the Upper Silurian in no small numbers, they are pre-eminently characteristic of the Llandeilo-Caradoc period of Lower Silurian time. They commenced their existence, so far as known, in the Upper Cambrian; and though examples are not absolutely unknown Page 107 in later periods, they are pre-eminently characteristic of the earlier portion of the Palæozoic epoch.

The Ringed Worms (Annelides) are abundantly represented in the Lower Silurian, but principally by tracks and burrows similar in essential respects to those which occur so commonly in the Cambrian formation, and calling for no special comment. Much more important are the Articulate animals, represented as heretofore, wholly by the remains of the aquatic group of the Fig. 47

Fig. 47.—Lower Silurian Crustaceans. a, Asaphus tyrannus, Upper Llandeilo; b. Ogygia Buchii, Upper Llandeilo; c, Trinucleus concentricus, Caradoc; d, Caryocaris Wrightii, Arenig (Skiddaw Slates); e, Beyrichia complicata, natural size and enlarged, Upper Llandeilo and Caradoc; f, Primitia strangulata, Caradoc: g. Head-shield of Calymene Blumenbachii, var. brevicapitata, Caradoc; h, Head-shield of Triarthrus Becki (Utica Slates), United States: i, Shield of Leperditia Canadensis, var. Josephiana, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the front. (After Salter, M'Coy, Rupert Jones, and Dana.) Crustaceans. Amongst these are numerous little bivalved forms—such as species of Primitia (fig. 47, f), Page 108 Beyrichia (fig. 47, e), and Leperditia (fig. 47, i and j). Most of these are very small, varying from the size of a pin's head up to that of a hemp seed; but they are sometimes as large as a small bean (fig. 47, i), and they are commonly found in myriads together in the rock. As before said, they belong to the same great group as the living Water-fleas (Ostracoda). Besides these, we find the pod-shaped head-shields of the shrimp-like Phyllopods—such as Caryocaris (fig. 47, d) and Ceratiocaris. More important, however, than any of these are the Trilobites, which may be considered as attaining their maximum development in the Lower Silurian. The huge Paradoxides of the Cambrian have now disappeared, and with them almost all the principal and characteristic "primordial" genera, save Olenus and Agnostus. In their place we have a great number of new forms—some of them, like the great Asaphus tyrannus of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera—Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illœnus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis, and Homalonotus, a few of them passing upwards under new forms into the Upper Silurian.

Coming next to the Mollusca, we find the group of the Sea-mosses and Sea-mats (Polyzoa) represented now by quite a number of forms. Amongst these are examples of the true Lace-corals (Retepora and Fenestella), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to shells and corals (Hippothoa, Alecto, &c.); but perhaps the most characteristic forms belong to the genus Ptilodictya (figs. 48 and 49). In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides. Each of these little chambers or "cells" was originally tenanted by a minute animal, and the whole thus constituted a compound organism or colony.

The Lamp-shells or Brachiopods are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of "Age of Brachiopods" Page 109 has with justice been applied to the Silurian period as a whole. It would be impossible here to enter into details as to the many Fig. 48

Fig. 48.—Ptilodictya falciformis. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged. Trenton Limestone and Cincinnati Group, America. (Original.) Fig. 49

Fig. 49.—A, Ptilodictya acuta; B. Ptilodictya Schafferi. a, Fragment, of the natural size; b, Portion, enlarged to show the cells. Cincinnati Group of Ohio and Canada. (Original.) different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera Orthis, Strophomena, and Leptœna for illustration, as being specially characteristic of this period, Fig. 50

Fig. 50.—Lower Silurian Brachiopods. a and a', Orthis biforata, Llandeilo-Caradoc, Britain and America: b, Orthis flabellulum, Caradoc, Britain: c, Orthis subquadrata, Cincinnati Group, America; c', Interior of the dorsal valve of the same: d, Strophomena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.) though not exclusively confined to it. The numerous shells which belong to the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, Page 110 and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes, Fig. 51

Fig. 51.—Lower Silurian Brachiopods, a, Strophomena alternata, Cincinnati Group, America; b, Strophomena filitexta, Trenton and Cincinnati Groups, America; c, Orthis testudinaria, Caradoc, Europe, and America; d, d', Orthis plicateila, Cincinnati Group, America; e, e', e'', Leptœna sericea, Llandeilo and Caradoc, Europe and America. (After Meek, Hall, and the Author.) generally more or less convex, and marked with radiating ribs or lines. The valves of the shell are united to one another by teeth and sockets, and there is a straight hinge-line. The beaks are also separated by a distinct space ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, passed a muscular cord attaching the shell to some foreign object. The genus Strophomena (fig. 50, d, and 51, a and b) is very like Orthis in general character; but the shell is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate. In Leptœna, again (fig. 51, e), the shell is like Strophomena in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave. Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and Athyris; but none of these can claim the importance to which the three previously-mentioned groups are entitled.

The remaining Lower Silurian groups of Mollusca can be but briefly glanced at here. The Bivalves (Lamellibranchiata) find numerous representatives, belonging to such genera as Page 111 Modiolopsis, Ctenodonta, Orthonota, Palœarca, Lyrodesma,
Fig. 52.—Murchisonia gracilis, Trenton Limestone, America. (After Billings.) Ambonychia,and Cleidophorus. The Univalves (Gasteropoda) are also very numerous, the two most important genera being Murchisonia (fig. 52) and Pleurotomaria. In both these groups the outer lip of the shell is notched; but the shell in the former is elongated and turreted, whilst in the latter it is depressed. The curious oceanic Univalves known as the Heteropods are also very abundant, the principal forms belonging to Bellerophon and Maclurea. In the former (fig. 53) there is a symmetrical convoluted shell, like that of the Pearly Nautilus in shape, but without any internal partitions, and having the aperture often expanded and notched behind. The species of Maclurea (fig. 54) are found both in North America and in Scotland, and are exclusively confined to the Lower Silurian period, so far as known. They have the shell coiled into a flat spiral, the mouth being furnished with a very curious, thick, and solid lid or "operculum." The Lower Silurian Pteropods, or "Winged snails," are numerous, and belong principally to the genera Theca, Conularia, and Tentaculites, the last-mentioned of these often being extremely abundant in certain strata.

Lastly, the Lower Silurian Rocks have yielded a vast number of chambered shells, referable to animals which belong to the same great division as the Cuttle-fishes (the Cephalopoda), and of which the Pearly Nautilus is the only living representative at the present day. In this group of Cephalopods the animal Fig. 53

Fig. 53.—Different views of Bellerophon Argo, Trenton Limestone, Canada. (After Billings.) possesses a well-developed external shell, which is divided into chambers by shelly partitions ("septa"). The animal lives in the last-formed and largest chamber of the shell, to which Page 112 it is organically connected by muscular attachments. The head is furnished with long muscular processes or "arms," and can be Fig. 54

Fig. 54.—Different views of Maclurea crenulata, Quebec Group, Newfoundland. (After Billings.) protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or "gills;" hence all these forms are grouped together under the name of the "Tetrabranchiate" Cephalopods (Gr. tetra, four; bragchia, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered; their "arms" are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills. For this last reason they are termed the "Dibranchiate" Cephalopods (Gr. dis, twice; bragchia, gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size. The principal Lower Silurian genus is the well-known and widely-distributed Orthoceras (fig. 55). The shell in this genus agrees with that of the existing Pearly Nautilus, in consisting of numerous chambers separated by shelly partitions (or septa), the latter being perforated by a tube which runs the whole length of the shell after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its shell. The various air-chambers, though the siphuncle passes through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it. The Orthoceras Page 113 further agrees with the Pearly Nautilus in the fact that the partitions or septa separating the different air-chambers are Fig. 55

Fig. 55.—Fragment of Orthoceras crebriseptum, Cincinnati Group, North America, of the natural size. The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.) Fig. 56

Fig. 56.—[14] Restoration of Orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel") by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle. simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it. In the Nautilus, however, the shell is coiled into a flat spiral; whereas in Orthoceras the shell is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front. The chief objections to the belief that the animal of the Orthoceras was essentially like that of the Pearly Nautilus are—the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of Page 114 the shell. Thus, some Orthocerata have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity. These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own shell. To some, this difficulty has appeared so great that they prefer to believe that the Orthoceras did not live in its shell at all, but that its shell was an internal skeleton similar to what we shall find to exist in many of the true Cuttle-fishes. There is something to be said in favour of this view, but it would compel us to believe in the existence in Lower Silurian times of Cuttle-fishes fully equal in size to the giant "Kraken" of fable. It need only be added in this connection that the Lower Silurian rocks have yielded the remains of many other Tetrabranchiate Cephalopods besides Orthoceras. Some of these belong to Cyrtoceras, which only differs from Orthoceras in the bow-shaped form of the shell; others belong to Phragmoceras, Lituites, &c.; and, lastly; we have true Nautili, with their spiral shells, closely resembling the existing Pearly Nautilus.

[Footnote 14: This illustration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]

Whilst all the sub-kingdoms of the Invertebrate animals are represented in the Lower Silurian rocks, no traces of Vertebrate animals have ever been discovered in these ancient deposits, unless the so-called "Conodonts" found by Pander in vast numbers in strata of this age [15] in Russia should prove to be really of this nature. These problematical bodies are of microscopic size, and have the form of minute, conical, tooth-shaped spines, with sharp edges, and hollow at the base. Their original discoverer regarded them as the horny teeth of fishes allied to the Lampreys; but Owen came to the conclusion that they probably belonged to Invertebrates. The recent investigation of a vast number of similar but slightly larger bodies, of very various forms, in the Carboniferous rocks of Ohio, has led Professor Newberry to the conclusion that these singular fossils really are, as Pander thought, the teeth of Cyclostomatous fishes. The whole of this difficult question has thus been reopened, and we may yet have to record the first advent of Vertebrate animals in the Lower Silurian.

[Footnote 15: According to Pander, the "Conodonts" are found not only in the Lower Silurian beds, but also in the "Ungulite Grit" (Upper Cambrian), as well as in the Devonian and Carboniferous deposits of Russia. Should the Conodonts prove to be truly the remains of fishes, we should thus have to transfer the first appearance of vertebrates to, at any rate, as early a period as the Upper Cambrian.]

Page 115 CHAPTER X.

Having now treated of the Lower Silurian period at considerable length, it will not be necessary to discuss the succeeding group of the Upper Silurian in the same detail—the more so, as with a general change of species the Upper Silurian animals belong for the most part to the same great types as those which distinguish the Lower Silurian. As compared, also, as regards the total bulk of strata concerned, the thickness of the Upper Silurian is generally very much below that of the Lower Silurian, indicating that they represent a proportionately shorter period of time. In considering the general succession of the Upper Silurian beds, we shall, as before, select Wales and America as being two regions where these deposits are typically developed.

In Wales and its borders the general succession of the Upper Silurian rocks may be taken to be as follows, in ascending order (fig. 57):—

(1) The base of the Upper Silurian series is constituted by a series of arenaceous beds, to which the name of "May Hill Sandstone" was applied by Sedgwick. These are succeeded by a series of greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes of great thickness; and these two groups of beds together form what may be termed the "May Hill Group" (Upper Llandovery of Murchison). Though not very extensively developed in Britain, this zone is one very well marked by its fossils; and it corresponds with the "Clinton Group" of North America, in which similar fossils occur. In South Wales this group is clearly unconformable to the highest member of the subjacent Lower Silurian (the Llandovery group); and there is reason to believe that a similar, though less conspicuous, physical break occurs very generally between the base of the Upper and the summit of the Lower Silurian.

(2) The Wenlock Group succeeds the May Hill group, and constitutes the middle member of the Upper Silurian. At its base it may have an irregular limestone ("Woolhope Limestone"), and its summit may be formed by a similar but thicker calcareous deposit ("Wenlock Limestone"); but the bulk of the group is made up of the argillaceous and shaly strata known as the "Wenlock Shale." In North Wales the Wenlock group is, represented by a great accumulation of flaggy and gritty strata (the "Denbighshire Flags and Grits"), and similar beds (the Page 116 "Coniston Flags" and "Coniston Grits") take the same place in the north of England.

(3) The Ludlow Group is the highest member of the Upper Silurian, and consists typically of a lower arenaceous and shaly series (the "Lower Ludlow Rock") a middle calcareous member (the "Aymestry Limestone"), and an upper shaly and sandy series (the "Upper Ludlow Rock" and "Downton Sandstone"). At the summit, or close to the summit, of the Upper Ludlow, is a singular stratum only a few inches thick (varying from an inch to a foot), which contains numerous remains of crustaceans and fishes, and is well known under the name of the "bone-bed." Finally, the Upper Ludlow rock graduates invariably into a series of red sandy deposits, which, when of a flaggy character, are known locally as the "Tile-stones." These beds are probably to be regarded as the highest member of the Upper Silurian; but they are sometimes looked upon as passage-beds into the Old Red Sandstone, or as the base of this formation. It is, in fact, apparently impossible to draw any actual line of demarcation between the Upper Silurian and the overlying deposits of the Devonian or Old Red Sandstone series. Both in Britain and in America the Lower Devonian beds repose with perfect conformity upon the highest Silurian beds, and the two formations appear to pass into one another by a gradual and imperceptible transition.

The Upper Silurian strata of Britain vary from perhaps 3000 or 4000 feet in thickness up to 8000 or 10,000 feet. In North America the corresponding series, though also variable, is generally of much smaller thickness, and may be under 1000 feet. The general succession of the Upper Silurian deposits of North America is as follows:—

(1) Medina Sandstone.—This constitutes the base of the Upper Silurian, and consists of sandy strata, singularly devoid of life, and passing below in some localities into a conglomerate ("Oneida Conglomerate"), which is stated to contain pebbles derived from the older beds, and which would thus indicate an unconformity between the Upper and Lower Silurian.

(2) Clinton Group.—Above the Medina sandstone are beds of sandstone and shale, sometimes with calcareous bands, which constitute what is known as the "Clinton Group." The Medina and Clinton groups are undoubtedly the equivalent of the "May Hill Group" of Britain, as shown by the identity of their fossils.

Page 117 GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND SHROPSHIRE.
Fig. 57.
Fig. 57

(3) Niagara Group.—This group consists typically of a series of argillaceous beds ("Niagara Shale") capped by limestones ("Niagara Limestone"); and the name of the group is derived from the fact that it is over limestones of this age that the Niagara river is precipitated to form the great Falls. In places the Niagara group is wholly calcareous, and it is continued upwards into a series of marls and sandstones, with beds of salt and masses of gypsum (the "Salina Group"), or into a series of magnesian limestones ("Guelph Limestones"). The Niagara group, as a whole, corresponds unequivocally with the Wenlock group of Britain.

(4) Lower Helderberg Group.—The Upper Silurian period in North America was terminated by the deposition of a series of calcareous beds, which derive the name of "Lower Helderberg" from the Helderberg mountains, south of Albany, and Page 118 which are divided into several zones, capable of recognition by their fossils, and known by local names (Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone, Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As a whole, this series may be regarded as the equivalent of the Ludlow group of Britain, though it is difficult to establish any precise parallelism. The summit of the Lower Heiderberg group is constituted by a coarse-grained sandstone (the "Oriskany Sandstone"), replete with organic remains, which have to a large extent a Silurian facies. Opinions differ as to whether this sandstone is to be regarded as the highest bed of the Upper Silurian or the base of the Devonian. We thus see that in America, as in Britain, no other line than an artificial one can be drawn between the Upper Silurian and the overlying Devonian.

As regards the life of the Upper Silurian period, we have, as before, a number of so-called "Fucoids," the true vegetable nature of which is in many instances beyond doubt. In addition to these, however, we meet for the first time, in deposits of this age, with the remains of genuine land-plants, though our knowledge of these is still too scanty to enable us to construct any detailed picture of the terrestrial vegetation of the period. Some of these remains indicate the existence of the remarkable genus Lepidodendron—a genus which played a part of great importance in the forests of the Devonian and Carboniferous periods, and which may be regarded as a gigantic and extinct type of the Club-mosses (Lycopodiaceœ). Near the summit of the Ludlow formation in Britain there have also been found beds charged with numerous small globular bodies, which Dr Hooker has shown to be the seed-vessels or "sporangia" of Club-mosses. Principal Dawson further states that he has seen in the same formation fragments of wood with the structure of the singular Devonian Conifer known as Prototaxites. Lastly, the same distinguished observer has described from the Upper Silurian of North America the remains of the singular land-plants belonging to the genus Psilophyton, which will be referred to at greater length hereafter.

The marine life of the Upper Silurian is in the main constituted by types of animals similar to those characterising the Lower Silurian, though for the most part belonging to different species. The Protozoans are represented principally by Stromatopora and Ischadites, along with a number of undoubted sponges (such as Amphispongia, Astrœospongia, Astylospongia, and Palœomanon).

Amongst the Cœlenterates, we find the old group of Graptolites now verging on extinction. Individuals still Page 119 remain numerous, but the variety of generic and specific types has now become greatly reduced. All the branching and complex forms of the Arenig, the twin-Graptolites Fig. 1

Fig. 58.—A, Monograptus priodon, slightly enlarged. B, Fragment of the same viewed from behind. C, Fragment of the same viewed in front, showing the mouths of the cellules. D, Cross-section of the same. From the Wenlock Group (Coniston Flags of the North of England). (Original.) and Dicranograpti of the Llandeilo, and the double-celled Diplograpti and Climacograpti of the Bala group, have now disappeared. In their place we have the singular Retiolites, with its curiously-reticulated skeleton; and several species of the single-celled genus Monograptus, of which a characteristic species (M. Priodon) is here figured. If we remove from this group the plant-like Dictyonemœ, which are still present, and which survive into the Devonian, no known species of Graptolite has hitherto been detected in strata higher in geological position than the Ludlow. This, therefore, presents us with the first instance we have as yet met with of the total disappearance and extinction of a great and important series of organic forms.

The Corals are very numerously represented in the Upper Silurian rocks some of the limestones (such as the Wenlock Limestone) being often largely composed of the skeletons of these animals. Almost all the known forms of this period belong to the two great divisions of the Rugose and Tabulate corals, the former being represented by species of Zaphrentis, Omphyma, Cystiphyllum, Strombodes, Acervularia, Cyathophyllum, &c.; whilst the latter belong principally to the genera Favosites, Chœtetes, Halysites, Syringopora, Heliolites, and Plasmopora. Amongst the Rugosa, the first appearance of the great and important genus Cyathophyllum, so characteristic of the Palæozoic period, is to be noted; and amongst the Tabulata we have similarly the first appearance, in force at any rate, of the widely-spread genus Favosites—the "Honeycomb-corals." The "Chain-corals" (Halysites), figured below (fig. 59), are also very common examples of the Tabulate corals during this period, though they occur likewise in the Lower Silurian.

Page 120 Amongst the Echinodermata, all those orders which have hard parts capable of ready preservation are more or less largely Fig. 59

Fig. 59.—a, Halysites catenularia, small variety, of the natural size; b, Fragment of a large variety of the same, of the natural size; c, Fragment of limestone with the tubes of Halysites agglomerata, of the natural size; d, Vertical section of two tubes of the same, showing the tabulæ, enlarged. Niagara Limestone (Wenlock), Canada. (Original.) represented. We have no trace of the Holothurians or Sea-cucumbers; but this is not surprising, as the record of the past is throughout almost silent as to the former existence of these soft-bodied creatures, the scattered plates and spicules in their skin offering a very uncertain chance of preservation in the fossil condition. The Sea-urchins (Echinoids) are said to be represented by examples of the old genus Palœchinus. The Star-fishes (Asteroids) and the Brittle-stars (Ophiuroids) are, comparatively speaking, largely represented; the former by species of Palasterina (fig. 60), Palœaster (fig. 60), Palœocoma (fig. 60), Petraster, Glyptaster, and Lepidaster—and the latter by species of Protaster (fig. 61), Palœodiscus, Acroura, and Eucladia. The singular Cystideans, or "Globe Crinoids," with their globular or ovate, tesselated bodies (fig. 46, A, C, D,), are also not uncommon in the Upper Silurian; and if they do not become finally extinct here, they certainly survive the close of this period by but a very brief time. By far the most important, however, of the Upper Silurian Echinodenns, are the Sea-lilies or Crinoids. The limestones of this period are often largely composed of the fragmentary columns and detached Page 121 plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded Fig. 60

Fig. 60.—Upper Silurian Star-fishes. 1, Palasterina primœva, Lower Ludlow; 2, Paloeaster Ruthveni, Lower Ludlow; 3, Palœocoma Colvini, Lower Ludlow. (After Salter.) perhaps the most exquisitely-preserved examples of this group with which we are as yet acquainted. However varied in their forms, these beautiful organisms consist of a globular, ovate, or Fig. 61

Fig. 61.—A, Protaster Sedgwickii, showing the disc and bases of the arms; B, Portion of an arm, greatly enlarged. Lower Ludlow. (After Salter.) pear-shaped body (the "calyx"), supported upon a longer or shorter jointed stem (or "column"). The body is covered externally with an armour of closely-fitting calcareous plates (fig. 62), and its upper surface is protected by similar but smaller plates more loosely connected by a leathery integument. From the upper surface of the body, round its margin, springs a series of longer or shorter flexible processes, composed of innumerable calcareous joints or pieces, movably united with one Page 122 another. The arms are typically five in number; but they generally subdivide at least once, sometimes twice, and they are furnished with similar but Fig. 62

Fig. 62.—Upper Silurian Crinoids. a, Calyx and arms of Eucalyptocrinus polydactylus, Wenlock Limestone; b, Ichthyocrinus lœvis, Niagara Limestone, America; c, Taxocrinus tuberculatus, Wenlock Limestone. (After M'Coy and Hall.) more slender lateral branches or "pinnules," thus giving rise to a crown of delicate feathery plumes. The "column" is the stem by which the animal is attached permanently to the bottom of the sea; and it is composed of numerous separate plates, so jointed together that whilst the amount of movement between any two pieces must be very limited, the entire column acquires more or less flexibility, allowing the organism as a whole to wave backwards and forwards on its stalk. Into the exquisite minutiœ of structure by which the innumerable parts entering into the composition of a single Crinoid are adapted for their proper purposes in the economy of the animal, it is impossible to enter here. No period, as before said, has yielded examples of greater beauty than the Upper Silurian, the principal genera represented being Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, Glyptocrinus, Crotalocrinus, and Edriocrinus.

The tracks and burrows of Annelides are as abundant in the Upper Silurian strata as in older deposits, and have just as commonly been regarded as plants. The most abundant forms are the cylindrical, twisted bodies (Planolites), which are Page 123 so frequently found on the surfaces of sandy beds, and which have been described as the stems of sea-weeds. These fossils (fig. 63), however, can be nothing more, in most Fig. 63

Fig. 63.—Planolites vulgaris, the filled-up burrows of a marine worm. Upper Silurian (Clinton Group), Canada. (Original.) cases, than the filled-up burrows of marine worms resembling the living Lob-worms. There are also various remains which belong to the group of the tube-inhabiting Annelides (Tubicola). Of this nature are the tubes of Serpulites and Cornultites, and the little spiral discs of Spirorbis Lewisii.

Amongst the Articulates, we still meet only with the remains of Crustaceans. Besides the little bivalved Ostracoda—which here are occasionally found of the size of beans—and various Phyllopods of different kinds, we have an abundance of Trilobites. These last-mentioned ancient types, however, are now beginning to show signs of decadence; and though still individually numerous, there is a great diminution in the number of generic types. Many of the old genera, which flourished so abundantly in Lower Silurian seas, have now died out; and the group is represented chiefly by species of Cheirurus, Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Illœnus, Calymene, Homalonotus, and Phacops—the last of these, one of the Page 124 highest and most beautiful of the groups of Trilobites, attaining here its maximum of development. In the annexed illustration (fig. 64) some of the characteristic Upper Silurian Trilobites are Fig. 64

Fig. 64.—Upper Silurian Trilobites. a, Cheirurus bimucronatus, Wenlock and Caradoc; b, Phacops longicaudatus, Wenlock, Britain, and America; c, Phacops Downingiœ, Wenlock and Ludlow; d, Harpes ungula, Upper Silurian, Bohemia. (After Salter and Barrande.) represented—all, however, belonging to genera which have their commencement in the Lower Silurian period. In addition to the above, the Ludlow rocks of Britain and the Lower Helderberg beds of North America have yielded the remains of certain singular Crustaceans belonging to the extinct order of the Eurypterida. Some of these wonderful forms are not remarkable for their size; but others, such as Pterygotus Anglicus (fig. 65), attain a length of six feet or more, and may fairly be considered as the giants of their class. The Eurypterids are most nearly allied to the existing King-crabs (Limuli), and have the anterior end of the body covered with a great head-shield, carrying two pairs of eyes, the one simple and the other compound. The feelers are converted into pincers, whilst the last pair of limbs have their bases covered with spiny teeth so as to act as jaws, and are flattened and widened out towards their extremities so as to officiate as swimming-paddles. The hinder extremity of the body is composed of thirteen rings, which have no legs attached to them; and the last segment of the tail is either a flattened plate or a Page 125 narrow, sword-shaped spine. Fragments of the skeleton are easily recognised by the peculiar scale-like markings with Fig. 65

Fig. 65.—Pterygotus Anglicus, viewed from the under side, reduced in size, and restored. c c, The feelers (antennæ), terminating in nipping-claws; o o, Eyes; m m, Three pairs of jointed limbs, with pointed extremities; n n, Swimming-paddles, the bases of which are spiny and act as jaws. Upper Silurian, Lanarkshire. (After Henry Woodward.) which the surface is adorned, and which look not at all unlike the scales of a fish. The most famous locality for these great Crustaceans is Lesmahagow, in Lanarkshire, where many different species have been found. The true King-crabs (Limuli) of existing seas also appear to have been represented by at least one form (Neolimulus) in the Upper Silurian.

Coming to the Mollusca, we note the occurrence of the same great groups as in the Lower Silurian. Amongst the Sea-mosses (Polyzoa), we have the ancient Lace-corals (Fenestella and Retepora), with the nearly-allied Glauconome, and species of Ptilodictya (fig. 66); whilst many forms often referred here may probably have to be transferred to the Corals, just as some so-called Corals will ultimately be removed to the present group.

The Brachiopods continued to flourish during the Upper Silurian Period in immense numbers and under a greatly increased variety of forms. The three prominent Lower Silurian genera Orthis, Strophomena, and Leptœna are still well represented, though they have lost their former preeminence. Amongst the numerous types which have now come upon the scene for the first time, or which have now a special development, are Spirifera and Pentamerus. In the first of these (fig. 69. b, c), one of the valves of the shell (the dorsal) is furnished in its interior with a pair of great calcareous spires, which served for the support of the long and fringed fleshy processes or "arms" which were attached to the sides of the mouth.[16] In the genus Pentamerus (fig. 70) the Page 126 shell is curiously subdivided in its interior by calcareous plates. The Pentameri commenced their existence at the very close of the Lower Silurian (Llandovery), and Fig. 66

Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped frond of Rhinopora verrucosa; 1a, Portion of the surface of the same, enlarged; 2 and 2a, Phœnopora ensiformis, of the natural size and enlarged; 3 and 3a, Helopora fragilis, of the natural size and enlarged; 4 and 4a, Ptilodictya raripora, of the natural size and enlarged. The specimens are all from the Clinton Formation (May Hill Group) of Canada. (Original.) survived to the close of the Upper Silurian; but they are specially characteristic of the May Hill and Wenlock groups, both in Britain and in other regions. One species, Pentamerus galeatus, is common to Sweden, Britain, and America. Amongst the remaining Upper Silurian Brachiopods are the extraordinary Page 127 Trimerellids; the old and at the same time modern Lingulœ, Discinœ, and Craniœ; together with many species of Atrypa (fig. 68, e), Fig. 68

Fig. 68.—Upper Silurian Brachiopods. a a', Leptocœlia plano-convexa, Clinton Group, America; b b', Rhynchonella neglecta, Clinton Group, America; c, Rhynchonella cuneata, Niagara Group, America, and Wenlock Group, Britain; d d', Orthis elelgantula, Llandeilo to Ludlow, America and Europe; e e', Atrypa hemispherica, Clinton Group, America, and Llandovery and May Hill Groups, Britain; f f', Atrypa congesta, Clinton Group, America; g g', Orthis Davidsoni, Clinton Group, America. (After Hall, Billings, and the Author.) Leptocœlia (fig. 68, a), Rhynchonella (fig. 68, b, c), Meristella (fig. 69, a, e, f), Athyris, Retzia, Chonetes, &c.

[Footnote 16: In all the Lamp-shells the mouth is provided with two long fleshy organs, which carry delicate filaments on their sides, and which are usually coiled into a spiral. These organs are known as the "arms," and it is from their presence that the name of "Brachiopoda" is derived (Gr. brachion, arm; podes, feet). In some cases the arms are merely coiled away within the shell, without any support; but in other cases they are carried upon a more or less elaborate shelly loop, often spoken of as the "carriage-spring apparatus." In the Spirifers, and in other ancient genera, this apparatus is coiled up into a complicated spiral (fig. 67). It is these "arms," with or without Fig. 67

Fig. 67.—Spirifera hysterica. The right-hand figure shows the interior of the dorsal valve with the calcareous spires for the support of the arms. the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]

Fig. 69.—a a', Meristella intermedia, Niagara Group, America; b, Spirifera Niagarensis, Niagara Group, America; c c', Spirifera crispa, May Hill to Ludlow, Britain, and Niagara Group, America; d, Strophomena (Streptorhynchus) subplana, Niagara Group, America; e, Meristella naviformis, Niagara Group, America; f, Meristella cylindrica, Niagara Group, America. (After Hall, Billings, and the Author.)

The higher groups of the Mollusca are also largely represented in the Upper Silurian. Apart from some singular types, Page 128 such as the huge and thick-shelled Megalomi of the American Wenlock formation, the Bivalves (Lamellibranchiata) present little of Fig. 70

Fig. 70.—Pentamerus Knightii. Wenlock and Ludlow. The right-hand figure shows the internal partitions of the shell. special interest; for though sufficiently numerous, they are rarely well preserved, and their true affinities are often uncertain. Amongst the most characteristic genera of this period may be mentioned Cardiola (fig. 71, A and C) and Pterinea Fig. 71

Fig. 71.—Upper Silurian Bivalves. A, Cardiola interrupta, Wenlock and Ludlow; B, Pterinea subfalcata, Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and M'Coy.) (fig. 71, B), though the latter survives to a much later date. The Univalves (Gasteropoda) are very numerous, and a few characteristic forms are here figured (fig. 72). Of these, no genus is perhaps more characteristic than Euomphalus (fig. 72, b), with its flat discoidal shell, coiled up into an oblique spiral, and deeply hollowed out on one side; but examples of this group are both of older and of more modern date. Another very extensive genus, especially in America, is Platyceras (fig. 72, a and f), with its thin fragile shell—often hardly coiled up at all—its minute spire, and its widely-expanded, often sinuated mouth. The British Acroculiœ should probably be placed here, and the group has with reason been regarded as allied to the Violet-snails (Ianthina) of the open Atlantic. The Page 129 species of Platyostoma (fig. 72, h) also belong to the same family; and the entire group is continued throughout the Devonian into the Carboniferous. Amongst other well-known Upper Silurian Gasteropods are species of the genera Holopea (fig. 72, g), Holopella (fig. 72. e), Fig. 72

Fig. 72.—Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helderberg, America; b, Euomphalus discors, Wenlock, Britain; c, Holopella obsoleta Ludlow, Britain; d, Platyschisma helicites, Upper Ludlow, Britain; e, Holopella gracilior, Wenlock, Britain; f, Platyceras multisinuatum, Lower Helderberg, America; g, Holopea subconica, Lower Helderberg, America; h, h', Platyostoma Niagarense, Niagara Group, America. (After Hall, M'Coy, and Salter.) Platyschisma (fig. 72, d), Cyclonema, Pleurotomaria, Murchisonia, Trochonema, &c. The oceanic Fig. 73

Fig. 73.—Tentaculites ornatus. Upper Silurian of Europe and North America. Univalves (Heteropods) are represented mainly by species of Bellerophon; and the Winged Snails, or Pteropods, can still boast of the gigantic Thecœ and Conulariœ, which characterise yet older deposits. The commonest genus of Pteropoda, however, is Tentaculites (fig. 73), which clearly belongs here, though it has commonly been regarded as the tube of an Annelide. The shell in this group is a conical tube, usually adorned with prominent transverse rings, and often with finer transverse or longitudinal striæ as well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces.

Page 130 The last and highest group of the Mollusca—that of the Cephalopoda—is still represented only by Tetrabranchiate forms; but the abundance and variety of these is almost beyond belief. Many hundreds of different species are known, chiefly belonging to the straight Orthoceratites, but the slightly-curved Cyrtoceras is only little less common. There are also numerous forms of the genera Phragmoceras, Ascoceras, Gyroteras, Lituites, and Nautilus. Here, also, are the first-known species of the genus Goniatites—a group which attains considerable importance in later deposits, and which is to be regarded as the precursor of the Ammonites of the Secondary period.

Finally, we find ourselves for the first time called upon to consider the remains of undoubted vertebrate animals, in the Fig. 74

Fig. 74.—Head-shield of Pteraspis Banksii, Ludlow rocks. (After Murchison.) form of Fishes. The oldest of these remains, so far as yet known, are found in the Lower Ludlow rocks, and they consist of the bony head-shields or bucklers of certain singular armoured fishes belonging to the group of the Ganoids, represented at the present day by the Sturgeons, the Gar-pikes of North America, and a few other less familiar forms. The principal Upper Silurian genus of these is Pteraspis, and the annexed illustration (fig. 74) will give some idea of the extraordinary form of the shield covering the head in these ancient fishes. The remarkable stratum near the top of the Ludlow formation known as the "bone-bed" has also yielded the remains of shark-like fishes. Some of these, for which the name of Onchus has been proposed, are in the form of compressed, slightly-curved spines (fig. 75, A), which would appear to be of the nature of the strong defensive spines implanted in front of certain of the fins in many living fishes. Besides these, have been found fragments of prickly skin Fig. 75

Fig. 75.—A, Spine of Onchus tenuistriatus; B, Shagreen-scales of Thelodus. Both from the "bone-bed" of the Upper Ludlow rocks. (After Murchison.) or shagreen (Sphagodus), along with minute cushion-shaped bodies (Thelodus, fig. 75, B), which Page 131 are doubtless the bony scales of some fish resembling the modern Dog-fishes. As the above mentioned remains belong to two distinct, and at the same time highly-organised, groups of the fishes, it is hardly likely that we are really presented here with the first examples of this great class. On the contrary, whether the so-called "Conodonts" should prove to be the teeth of fishes or not, we are justified in expecting that unequivocal remains of this group of animals will still be found in the Lower Silurian. It is interesting, also, to note that the first appearance of fishes—the lowest class of vertebrate animals—so far as known to us at present, does not take place until after all the great sub-kingdoms of invertebrates have been long in existence; and there is no reason for thinking that future discoveries will materially affect the relative order of succession thus indicated.

LITERATURE.

From the vast and daily-increasing mass of Silurian literature, it is impossible to do more than select a small number of works which have a classical and historical interest to the English-speaking geologist, or which embody researches on special groups of Silurian animals—anything like an enumeration of all the works and papers on this subject being wholly out of the question. Apart, therefore, from numerous and in many cases extremely important memoirs, by various well-known observers, both at home and abroad, the following are some of the more weighty works to which the student may refer in investigating the physical characters and succession of the Silurian strata and their fossil contents:—

CHAPTER XI.

Between the summit of the Ludlow formation and the strata which are universally admitted to belong to the Carboniferous series Page 133 is a great system of deposits, to which the name of "Old Red Sandstone" was originally applied, to distinguish them from certain arenaceous strata which lie above the coal ("New Red Sandstone"). The Old Red Sandstone, properly so called, was originally described and investigated as occurring in Scotland and in South Wales and its borders; and similar strata occur in the south of Ireland. Subsequently it was discovered that sediments of a different mineral nature, and containing different organic remains, intervened between the Silurian and the Carboniferous rocks on the continent of Europe, and strata with similar palæontological characters to these were found occupying a considerable area in Devonshire. The name of "Devonian" was applied to these deposits; and this title, by common usage, has come to be regarded as synonymous with the name of "Old Red Sandstone." Lastly, a magnificent series of deposits, containing marine fossils, and undoubtedly equivalent to the true "Devonian" of Devonshire, Rhenish Prussia, Belgium, and France, is found to intervene in North America between the summit of the Silurian and the base of the Carboniferous rocks.

Much difficulty has been felt in correlating the true "Devonian Rocks" with the typical "Old Red Sandstone"—this difficulty arising from the fact that though both formations are fossiliferous, the peculiar fossils of each have only been rarely and partially found associated together. The characteristic crustaceans and many of the characteristic fishes of the Old Red are wanting in the Devonian; whilst the corals and marine shells of the latter do not occur in the former. It is impossible here to enter into any discussion as to the merits of the controversy to which this difficulty has given origin. No one, however, can doubt the importance and reality of the Devonian series as an independent system of rocks to be intercalated in point of time between the Silurian and the Carboniferous. The want of agreement, both lithologically and palæontologically, between the Devonian and the Old Red, can be explained by supposing that these two formations, though wholly or in great part contemporaneous, and therefore strict equivalents, represent deposits in two different geographical areas, laid down under different conditions. On this view, the typical Devonian rocks of Europe, Britain, and North America are the deep-sea deposits of the Devonian period, or, at any rate, are genuine marine sediments formed far from land. On the other hand, the "Old Red Sandstone" of Britain and the corresponding "Gaspé Group" of Eastern Page 134 Canada represent the shallow-water shore-deposits of the same period. In fact, the former of these last-mentioned deposits contains no fossils which can be asserted positively to be marine (unless the Eurypterids be considered so); and it is even conceivable that it represents the sediments of an inland sea. Accepting this explanation in the meanwhile, we may very briefly consider the general succession of the deposits of this period in Scotland, in Devonshire, and in North America.

In Scotland the "Old Red" forms a great series of arenaceous and conglomeratic strata, attaining a thickness of many thousands of feet, and divisible into three groups. Of these, the Lower Old Red Sandstone reposes with perfect conformity upon the highest beds of the Upper Silurian, the two formations being almost inseparably united by an intermediate series of "passage-beds." In mineral nature this group consists principally of massive conglomerates, sandstones, shales, and concretionary limestones; and its fossils consist chiefly of large crustaceans belonging to the family of the Eurypterids, fishes, and plants. The Middle Old Red Sandstone consists of flagstones, bituminous shales, and conglomerates, sometimes with irregular calcareous bands; and its fossils are principally fishes and plants. It may be wholly wanting, when the Upper Old Red seems to repose unconformably upon the lower division of the series. The Upper Old Red Sandstone consists of conglomerates and grits, along with a great series of red and yellow sandstones—the fossils, as before, being fishes and remains of plants. The Upper Old Red graduates upwards conformably into the Carboniferous series.

The Devonian rocks of Devonshire are likewise divisible into a lower, middle, and upper division. The Lower Devonian or Lynton Group consists of red and purple sandstones, with marine fossils, corresponding to the "Spirifer Sandstein" of Germany, and to the arenaceous deposits (Schoharie and Cauda-Galli Grits) at the base of the American Devonian. The Middle Devonian or Ilfracombe Group consists of sandstones and flags, with calcareous slates and crystalline limestones, containing many corals. It corresponds with the great "Eifel Limestone" of the Continent, and, in a general way, with the Corniferous Limestone and Hamilton group of North America. The Upper Devonian or Pilton Group, lastly, consists of sandstones and calcareous shales which correspond with the "Clymenia Limestone" and "Cypridina Shales" of the Continent, and with the Chemung and Portage groups of Page 135 North America. It seems quite possible, also, that the so-called "Carboniferous Slates" of Ireland correspond with this group, and that the former would be more properly regarded as forming the summit of the Devonian than the base of the Carboniferous.

In no country in the world, probably, is there a finer or more complete exposition of the strata intervening between the Silurian and Carboniferous deposits than in the United States. The following are the main subdivisions of the Devonian rocks in the State of New York, where the series may be regarded as being typically developed (fig. 67):—

(1) Cauda-Galli Grit and Schoharie Grit.—Considering the "Oriskany Sandstone" as the summit of the Upper Silurian, the base of the Devonian is constituted by the arenaceous deposits known by the above names, which rest quite conformably upon the Silurian, and which represent the Lower Devonian of Devonshire. The Cauda-Galli Grit is so called from the abundance of a peculiar spiral fossil (Spirophyton cauda-Galli), which is of common occurrence in the Carboniferous rocks of Britain, and is supposed to be the remains of a sea-weed.

(2) The Corniferous or Upper Helderberg Limestone.—A series of limestones usually charged with considerable quantities of siliceous matter in the shape of hornstone or chert (Lat. cornu, horn). The thickness of this group rarely exceeds 300 feet; but it is replete with fossils, more especially with the remains of corals. The Corniferous Limestone is the equivalent of the coral-bearing limestones of the Middle Devonian of Devonshire and the great "Eifel Limestone" of Germany.

(3) The Hamilton Group—consisting of shales at the base ("Marcellus shales"); flags, shales, and impure limestones ("Hamilton beds") in the middle; and again a series of shales ("Genesee Slates") at the top. The thickness of this group varies from 200 to 1200 feet, and it is richly charged with marine fossils.

(4) The Portage Group.—A great series of shales, flags, and shaly sandstones, with few fossils.

(5) The Chemung Group.—Another great series of sandstones and shales, but with many fossils. The Portage and Chemung groups may be regarded as corresponding with the Upper Devonian of Devonshire. The Chemung beds are succeeded by a great series of red sandstones and shales—the Page 136 "Catskill Group"—which pass conformably upwards into the Carboniferous, and which may perhaps be regarded as the equivalent of the great sandstones of the Upper Old Red in Scotland.

Throughout the entire series of Devonian deposits in North America no unconformability or physical break of any kind has hitherto been detected; nor is there any marked interruption to the current of life, though each subdivision of the series has its own fossils. No completely natural line can thus be indicated, dividing the Devonian in this region from the Silurian on the one hand, and the Carboniferous on the other hand. At the same time, there is the most ample evidence, both stratigraphical and palæontological, as to the complete independence of the American Devonian series as a distinct life-system between the older Silurian and the later Carboniferous. The subjoined section (fig. 76) shows diagrammatically the general succession of the Devonian rocks of North America.

As regards the life of the Devonian period, we are now acquainted with a large and abundant terrestrial flora—this being the first time that we have met with a land vegetation capable of reconstruction in any fulness. By the researches of Gœppert, Unger, Dawson, Carruthers, and other botanists, a knowledge has been acquired of a large number of Devonian plants, only a few of which can be noticed here. As might have been anticipated, the greater number of the vegetable remains of this period have been obtained from such shallow-water deposits as the Old Red Sandstone proper and the Gaspè series of North America, and few traces of plant-life occur in the strictly marine sediments. Apart from numerous remains, mostly of a problematical nature, referred to the comprehensive group of the Sea-weeds, a large number of Ferns have now been recognised, some being, of the ordinary plant-like type (Pecopteris, Neuropteris, Alethopteris, Sphenopteris, &c.), whilst others belong to the gigantic group of the "Tree-ferns" (Psaronius, Caulopteris, &c.) Besides these there is an abundant development of the singular extinct types of the Lepidodendroids, the Sigillarioids, and the Calamites, all of which attained their maximum in the Carboniferous. Of these, the Lepidodendra may be regarded as gigantic, tree-like Club-mosses (Lycopodiaceœ); the Calamites are equally gigantic Horse-tails (Equisetaceœ); and the Sigillarioids, equally huge in size, in some respects hold a position intermediate between the Club-mosses and the Pines (Conifers). The Devonian rocks have Page 137 GENERALIZED SECTION OF THE DEVONIAN ROCKS OF NORTH AMERICA.
Fig. 76.
Fig. 76

also yielded traces of many other plants (such as Annularia, Asterophyllites, Cardiocarpon, &c.), which acquire a greater pre-dominance in the Carboniferous period, and which will be spoken of in discussing the structure of the plants of the Coal-measures. Upon the whole, the one plant which may be considered as specially characteristic of the Devonian (though not confined to this series) is the Psilophyton (fig. 77) of Dr Dawson. These singular plants have slender branching stems, with sparse needle-shaped leaves, the young stems being at first coiled up, crosier-fashion, like the young fronds of ferns, whilst the old branches carry numerous spore-cases. The Page 138 stems and branches seem to have attained a height of two or three feet; and they sprang from prostrate "root-stocks" or creeping stems. Upon the whole, Fig. 77

Fig. 77.—Restoration of Psilophyton princeps. Devonian, Canada. (After Dawson.) Principal Dawson is disposed to regard Psilophyton as a "generalised type" of plants intermediate between the Ferns and the Club-mosses. Lastly, the Devonian deposits have yielded the remains of the first actual trees with which we are as yet acquainted. About the nature of some of these (Ormoxylon and Dadoxylon) no doubt can be entertained, since their trunks not only show the concentric rings of growth characteristic of exogenous trees in general, but their woody tissue exhibits under the microscope the "discs" which are characteristic of the wood of the Pines and Firs (see fig. 2). The singular genus Prototaxites, however, which occurs in an older portion of the Devonian series than the above, is not in an absolutely unchallenged position. By Principal Dawson it is regarded as the trunk of an ancient Conifer—the most ancient known; but Mr Carruthers regards it as more probably the stem of a gigantic sea-weed. The trunks of Prototaxites (fig. 78, A) vary from one to three feet in diameter, and exhibit concentric rings of growth; but its woody fibres have not hitherto been clearly demonstrated to possess discs. Before leaving the Devonian vegetation, it may be mentioned that the hornstone or chert so abundant in the Corniferous limestone of North America has been shown to contain the remains of various microscopic plants (Diatoms and Desmids). We find also in the same siliceous material the singular spherical bodies, with radiating spines, which occur so abundantly in the chalk flints, and which are termed Xanthidia. These may be regarded Page 139 as probably the spore-cases of the minute plants known as Desmidiœ.

Fig. 78.—A, Trunk of Prototaxites Logani, eighteen inches in diameter, as seen in the cliff near L'Anse Brehaut, Gaspé; B, Two wood-cells showing spiral fibres and obscure pores, highly magnified. Lower Devonian, Canada. (After Dawson)

The Devonian Protozoans have still to be fully investigated. True Sponges (such as Astrtœospongia, Sphœrospongia, &c.) are not unknown; but by far the commonest representatives of this sub-kingdom in the Devonian strata are Stromatopora and its allies. These singular organisms (fig. 79) are not only very abundant in some of the Devonian limestones—both in the Old World and the New—but they often attain very large dimensions. However much they may differ in minor details, the general structure of these bodies is that of numerous, concentrically-arranged, thin, calcareous laminæ, separated by narrow interspaces, which in turn are crossed by numerous delicate vertical pillars, giving the whole mass a cellular structure, and dividing it into innumerable minute quadrangular compartments. Many of the Devonian Stromatoporœ also exhibit on their surface the rounded openings of canals, which can hardly have served any other purpose than that of permitting the sea-water to gain ready access to every part of the organism.

No true Graptolites have ever been detected in strata of Page 140 of Devonian age; and the whole of this group has become extinguished—unless we refer here the still surviving Dictyonemœ. The Cœlenterates, however, Fig. 79

Fig. 79.—a, Part of the under surface of Stromatopora tuberculata, showing the wrinkled basement membrane and the openings of water-canals, of the natural size; b, Portion of the upper surface of the same, enlarged; c, Vertical section of a fragment, magnified to show the internal structure. Corniferous Limestone, Canada. (Original.) are represented by a vast number of Corals, of beautiful forms and very varied types. The marbles of Devonshire, the Devonian limestones of the Eifel and of France, and the calcareous strata of the Corniferous and Hamilton groups of America, are often replete with the skeletons of these organisms—so much so as to sometimes entitle the rock to be considered as representing an ancient coral-reef. In some instances the Corals have preserved their primitive calcareous composition; and if they are embedded in soft shales, they may weather out of the rock in almost all their original perfection. In other cases, as in the marbles of Devonshire, the matrix is so compact and crystalline that the included corals can only be satisfactorily studied by means of polished sections. In other cases, again, the corals have been more or less completely converted into flint, as in the Corniferous limestone of North America. When this is the case, they often come, by the action of the weather, to stand out from Page 141 the enclosing rock in the boldest relief, exhibiting to the observer the most minute details of their organization. As before, the principal Fig. 80

Fig. 80.—Cystiphyllum vesiculosum, showing a succession of cups produces by budding from the original coral. Of the natural size. Devonian, America and Europe. (Original.) Fig. 81

Fig. 81—Zaphrentis cornicula, of the natural size. Devonian, America. (Original.)
Fig. 82

Fig. 82—Heliophyllum exiguum, viewed from in front and behind. Of the natural size. Devonian, Canada. (Original.) representatives of the Corals are still referable to the groups of the Rugosa and Tabulata. Amongst the Rugose group we find a vast number of simple "cup-corals," generally known by the quarrymen as "horns," from their shape. Of Page 142 the many forms of these, the species of Cyathophyllum, Heliophyllum (fig. 82), Zaphrentis (fig. 81), and Cystiphyllum (fig. 80), are perhaps those most abundantly represented—none of these genera, however, except Heliophyllum, being peculiar to the Devonian period. There are also numerous compound Rugose corals, such as species of Eridophyllum, Diphyphyllum, Syringopora, Phillipsastrœa, and some of the forms of Cyathophyllum and Crepidophyllum (fig. 83). Some of these compound corals attain a very large size, and form of Fig. 83

Fig. 83.—Portion of a mass of Crepidophyllum Archiaci, of the natural size. Hamilton Formation, Canada. (After Billings.) themselves regular beds, which have an analogy, at any rate, with existing coral-reefs, though there are grounds for believing that these ancient types differed from the modern reef-builders in being inhabitants of deep water. The "Tabulate Corals" are hardly less abundant in the Devonian rocks than the Rugosa; and being invariably compound, they hardly yield to the latter in the dimensions of the aggregations which they sometimes form.

The commonest, and at the same time the largest, of these are the "honeycomb corals," forming the genus Favosites (figs. 84, 85), which derive both their vernacular and their technical names from their great likeness to masses of petrified honeycomb. The most abundant species are Favosites Gothlandica and F. Hemispherica, both here figured, which form masses sometimes not less than two or three feet in diameter. Whilst Favosites has acquired a popular name by its honey-combed appearance, the resemblance of Michelinia to a fossilised Page 143 wasp's nest with the comb exposed is hardly less striking, and has earned for it a similar recognition from the non-scientific Fig. 84

Fig. 84.—Portion of a mass of Favosites Gothlandica, of the natural size. Upper Silurian and Devonian of Europe and America. (Original.) Billings. Fig. 85

Fig. 85.—Fragment of Favosites hemispherica, of the natural size. Upper Silurian and Devonian of America. (After Billings.) public. In addition to these, there are numerous branching or plant-like Tabulate Corals, often of the most graceful form, which are distinctive of the Devonian in all parts of the world.

The Echinoderms of the Devonian period call for little special notice. Many of the Devonian limestones are "crinoidal;" and the Crinoids are the most abundant and widely-distributed representatives of their class in the deposits of this period.

The Cystideans, with doubtful exceptions, have not been recognised in the Devonian; and their place is taken by the allied group of the "Pentremites," which will be further spoken of as occurring in the Carboniferous rocks. On the other hand, the Star-fishes, Brittle-stars, and Sea-urchins are all continued by types more or less closely allied to those of the preceding Upper Silurian.

Of the remains of Ringed-worms (Annelides), the most numerous and the most interesting are the calcareous envelopes of some small tube-inhabiting species. No one who has visited the seaside can have failed to notice the little spiral tubes of the existing Spirorbis growing attached to shells, or covering the fronds of the commoner Sea weeds (especially Fucus serratus). These tubes are inhabited by a small Annelide, and structures of a similar character occur not uncommonly from the Upper Silurian upwards. In the Devonian rocks, Spirorbis is an extremely common fossil, growing in hundreds attached to the outer surface of corals and shells, and appearing Page 144 in many specific forms (figs. 86 and 87); but almost all the known Fig. 86

Fig. 87.—a, Spirobois omphalodes, natural size and enlarged. Devonian, Europe and America; b, Spirorbis Arkonensis, of the natural size and enlarged; c, The same, with the tube twisted in the reverse direction. Devonian, America. (Original.)
Fig. 87

Fig. 88.—a b, Spirorbis laxus, enlarged, Upper Silurian, America; c, Spirorbis spinulifera, of the natural size and enlarged, Devonian, Canada. (After Hall and the Author.) examples are of small size, and are liable to escape a cursory examination.

The Crustaceans of the Devonian are principally Eurypterids and Trilobites. Some of the former attain gigantic dimensions, and the quarrymen in the Scotch Old Red give them the name of "seraphim" from their singular scale-like ornamentation. The Trilobites, though still sufficiently abundant in some localites, have undergone a yet further diminution since the close of the Upper Silurian. In both America and Europe quite a number of generic types have survived from the Silurian, but few or no new ones make their appearance during this period Fig. 88

Fig. 88.—Devonian Trilobites; a, Phacops latifrons, Devonian of Britain, the Continent of Europe, and South America; b, Homalonotus armatus, Europe; c, Phacops (Trimerocephalus) lœvis, Europe; d, Head-shield of Phacops (Portlockia) granulatus, Europe. (After Salter and Burmeister.) in either the Old World or the New. The species, however, are distinct; and the Page 145 principal forms belong to the genera Phacops (fig. 88, a, c, d), Homalonotus (fig. 88, b), Proetus, and Bronteus. The species figured above under the name of Phacops latifrons (fig. 88, a), has an almost world-wide distribution, being found in the Devonian of Britain, Belgium, France, Germany, Russia, Spain, and South America; whilst its place is taken in North America by the closely-allied Phacops rana. In addition to the Trilobites, the Devonian deposits have yielded the remains of a number of the minute Ostracoda, such as Entomis ("Cypridina"), Leperditia, &c., which sometimes occur in vast numbers, as in the so-called "Cypridina Slates" of the German Devonian. There are also a few forms of Phyllopods (Estheria). Taken as a whole, the Crustacean fauna of the Devonian period presents many alliances with that of the Upper Silurian, but has only slight relationships with that of the Lower Carboniferous.

Besides Crustaceans, we meet here for the first time with the remains of air-breathing Articulates, in the shape of Insects. So far, these have only been obtained from the Devonian rocks of North America, and they indicate the existence of at least four generic types, all more or less allied to the existing May-flies (Ephemeridœ). One of these interesting primitive insects, namely, Platephemera antiqua (fig. 89), appears to have measured five inches in expanse of wing; Fig. 89

Fig. 89.—Wing of Platephemera antiqua Devonian, America. (After Dawson.) and another (Xelloneura antiquorum) has attached to its wing the remains of a "stridulating-organ" similar to that possessed by the modern Grasshoppers—the instrument, as Principal Dawson remarks, of "the first music of living things that Geology as yet reveals to us."

Amongst the Mollusca, the Devonian rocks have yielded a great number of the remains of Sea-mosses (Polyzoa). Some of these belong to the ancient type Ptilodictya, which seems to disappear here, or to the allied Clathropora (fig. 90), with its fenestrated and reticulated fronds. We meet also with the graceful and delicate stems of Ceriopora (fig. 91).

The majority of the Devonian Polyzoa belong, however, to the great and important Palæozoic group of the Lace-corals (Fenestella, figs. 92 and 94, Retepora, fig. 93, Polypora, and their allies). In all these forms there is a horny skeleton, of a Page 146 fan-like or funnel-shaped form, which grew attached by its base to some foreign body. The frond consists of slightly-diverging or nearly parallel branches, which are Fig. 90

Fig. 90.—Fragment of Clathropora intertexta, of the natural size and enlarged. Devonian, Canada. (Original.) Fig. 91

Fig. 91.—Fragment of Ceriopora Hamiltonensis, of the natural size and enlarged. Devonian, Canada. (Original.) either united by delicate cross-bars, or which bend alternately from side to side, and become directly united with one another at short intervals—in either case giving origin to numerous oval or oblong perforations, which communicate to the whole Fig. 92

Fig. 92.—Fragment of Fenestella magnifica, of the natural size and enlarged. Devonian, Canada. (Original.) Fig. 93

Fig. 93.—Fragment of Retepora Phillipsi, of the natural size and enlarged. Devonian, Canada. (Original.)
Fig. 94

Fig. 94.—Fragment of Fenestella cribrosa, of the natural size and enlarged. Dovonian, Canada. (Original.) plant-like colony a characteristic netted and lace-like appearance. On one of its surfaces—sometimes the internal, sometimes the external—the frond carries a number of minute chambers or Page 147 "cells," which are generally borne in rows on the branches, and of which each originally contained a minute animal.

The Brachiopods still continue to be represented in great force through all the Devonian deposits, though not occurring in the true Old Red Sandstone. Besides such old types as Orthis, Strophomena, Lingula, Athyris, and Rhynchonella, we find some entirely new ones; whilst various types which only commenced their existence in the Upper Silurian, now undergo a great expansion and development. This last is especially the case with the two families of the Spiriferidœ and the Produclidœ. The Spirifers, in particular, are especially characteristic of the Devonian, both in the Old and New Worlds—some of the most typical forms, such as Spirifera mucronata (fig. 96), having the shell "winged," or with the Fig. 95

Fig. 95.—Spirifera sculptilis. Devonian, Canada. (After Billings.) Fig. 96

Fig. 96.—Spirifera mucronata. Devonian, America. (After Billings.) lateral angles prolonged to such an extent as to have earned for them the popular name of "fossil-butterflies." The closely-allied Spirifera disjunda occurs in Britain, France, Spain, Belgium, Germany, Russia, and China. The family of the Productidœ commenced to exist in the Upper Silurian, in the genus Chonetes, and we shall hereafter find it culminating in the Carboniferous in many forms of the great genus Producta[17] itself. In the Devonian period, there is an intermediate state of things, the genus Chonetes being continued in new and varied types, and the Carboniferous Produdœ being represented by many forms of the allied group Productella. Amongst other well-known Devonian Brachiopods may be mentioned the two long-lived and persistent types Atrypa reticularis (fig. 97) and Strophomena rhomboidalis (fig. 98). The former of these commences in the Upper Silurian, but is more abundantly developed in the Devonian, having a geographical range that is nothing less than world-wide; whilst the latter commences in the Lower Silurian, Page 148 and, with an almost equally cosmopolitan range, survives into the Carboniferous period.

[Footnote 17: The name of this genus is often written Productus, just as Spirifera is often given in the masculine gender as Spirifer (the name originally given to it). The masculine termination to these names is, however, grammatically incorrect, as the feminine noun cochlea (shell) is in these cases understood.]

Fig. 97.—Atrypa reticularis. Upper Silurian and Devonian of Europe and America. (After Billings.)

The Bivalves (Lamellibranchiata) of the Devonian call for no special comment, the genera Pterinea and Megalodon being, perhaps, the most noticeable. The Univalves Fig. 98

Fig. 98.—Strophomena rhomboidalis. Lower Silurian, Upper Silurian, and Devonian of Europe and America. (Gasteropods), also, need not be discussed in detail, though many interesting forms of this group are known. The type most abundantly represented, especially in America, is Platyceras (fig. 99), comprising thin, wide-mouthed shells, Fig. 99

Fig. 99.—Different views of Platyceras dumosum, of the natural size. Devonian, Canada. (Original.) probably most nearly allied to the existing "Bonnet-limpets," and sometimes attaining very considerable dimensions. We may also note the continuance of the genus Euomphalus, with its discoidal spiral shell. Amongst the Heteropods, the survival of Bellerophon is to be recorded; and in the "Winged-snails," or Pteropods, we find new forms of the old genera Tentaculites and Conularia Page 149 (fig. 100). The latter, with its fragile, conical, and often beautifully ornamented shell, is especially noticeable.

The remains of Cephalopoda are far from uncommon in the

Fig. 100.—Conularia ornata, of the natural size. Devonian, Europe. Devonian deposits, all the known forms being still Tetrabranchiate. Besides the ancient types Orthoceras and Cyrtoceras, we have now a predominance of the spirally-coiled chambered shells of Goniatites and Clymenia. In the former of these the shell is shaped like that of the Nautilus; but the partitions between the chambers ("septa") are more or less lobed, folded, or angulated, and the "siphuncle" runs along the back or convex side of the shell—these being characters which approximate Goniatites to the true Ammonites of the later rocks. In Clymenia, on the other hand, whilst the shell (fig. 101) is coiled into a flat spiral, and the partitions or septa are simple or only slightly lobed, there is still this difference, as compared with the Nautilus, that the tube of the siphuncle is placed on the inner or concave side of the shell. The

Fig. 101.—Clymenia Sedgwickii. Devonian, Europe. species of Clymenia are exclusively Devonian in Page 150 their range; and some of the limestones of this period in Germany are so richly charged with fossils of this genus as to have received the name of "Clymenien-kalk."

The sub-kingdom of the Vertebrates is still represented by Fishes only; but these are so abundant, and belong to such varied types, that the Devonian period has been appropriately called the "Age of Fishes." Amongst the existing fishes there are three great groups which are of special geological importance, as being more or less extensively represented in past time. These groups are: (1) The Bony Fishes (Teleostei), comprising most existing fishes, in which the skeleton is more or less completely converted into bone; the tail is symmetrically lobed or divided into equal moieties; and the scales are usually thin, horny, flexible plates, which overlap one another to a greater or less extent. (2) The Ganoid Fishes (Ganoidei), comprising the modern Gar-pikes, Sturgeons, &c., in which the skeleton usually more or less completely retains its primitive soft and cartilaginous condition; the tail is generally markedly unsymmetrical, being divided into two unequal lobes; and the scales (when present) have the form of plates of bone, usually covered by a layer of shining enamel. These scales may overlap; or they may be rhomboidal plates, placed edge to edge in oblique rows; or they have the form of large-sized bony plates, which are commonly united in the region of the head to form a regular buckler. (3) The Placoid Fishes, or Elasmobranchii, comprising the Sharks, Rays, and Chimœrœ of the present day, in which the skeleton is cartilaginous; the tail is unsymmetrically lobed; and the scales have the form of detached bony plates of variable size, scattered in the integument.

It is to the two last of these groups that the Devonian fishes belong, and they are more specially referable to the Ganoids. The order of the Ganoid fishes at the present day comprises but some seven or eight genera, the species of which principally or exclusively inhabit fresh waters, and all of which are confined to the northern hemisphere. As compared, therefore, with the Bony fishes, which constitute the great majority of existing forms, the Ganoids form but an extremely small and limited group. It was far otherwise, however, in Devonian times. At this period, the bony fishes are not known to have come into existence at all, and the Ganoids held almost undisputed possession of the waters. To what extent the Devonian Ganoids were confined to fresh waters remains yet to be proved; and that many of them lived in the sea is certain. It was formerly supposed that the Old Red Sandstone of Scotland and Ireland, with its abundant fish-remains, might perhaps be a fresh-water deposit, since the habitat of its fishes is Page 151 uncertain, and it contains no indubitable marine fossils. It has been now shown, however, that the marine Devonian strata of Devonshire and the continent of Europe contain some of the most characteristic of the Old Red Sandstone fishes of Scotland; whilst the undoubted marine deposit of the Corniferous limestone of North America contains numerous shark-like and Ganoid fishes, including such a characteristic Old Red genus as Coccosleus. There can be little doubt, therefore, but that the majority of the Devonian fishes were truly marine in their habits, though it is probable that many of them lived in shallow water, in the immediate neighbourhood of the shore, or in estuaries.

The Devonian Galloids belong to a number of groups; and it is

Fig. 102.—Fishes of the Devonian rocks of America. a, Diagram of the jaws and teeth of Dinichthys Hertzeri, viewed from the front, and greatly reduced; b, Diagram of the skull of Macropetalichthys Sullivanti, reduced in size; c, A portion of the enamelled surface of the skull of the same, magnified; d, One of the scales of Onychodus sigmoides, of the natural size; e, One of the front teeth of the lower jaw of the same, of the natural size: f, Fin-spine of Machœracanthus major, a shark-like fish, reduced in size. (After Newberry.)] only possible to notice a few of the most important forms here. The modern group of the Sturgeons is represented, Page 152 more or less remotely, by a few Devonian fishes—such as Asterosteus; and the great Macropetalichthys of the Corniferous limestone of North America is believed by Newberry to belong to this group. In this fish (fig. 102, b) the skull was of large size, its outer surface being covered with a tuberculated enamel; and, as in the existing Sturgeons, the mouth seems to have been wholly destitute of teeth. Somewhat allied, also, to the Sturgeons, is a singular group of armoured fishes, which is highly characteristic of the Devonian of Britain and Europe, and less so of that of America. In these curious forms the head and front extremity of the body were protected by a buckler composed of large enamelled plates, more or less firmly united to one another; whilst the hinder end of the body was naked, or was protected with small scales. Some forms of this group—such as Pteraspis and Coccosteus—date from the Upper Silurian; but they attain their maximum in the Devonian, and none of them are known to pass upwards into the overlying Carboniferous rocks. Amongst the most characteristic forms of this group may be mentioned Cephalaspis (fig. 103) and Pterichthys (fig. 104). In the former of these the head-shield is of a

Fig. 103.—Cephalaspis Lyellii. Old Red Sandstone, Scotland. (After Page.) crescentic shape, having its hinder angles produced backwards into long "horns," giving it the shape of a "saddler's knife." No teeth have been discovered; but the body was covered with small ganoid scales, and there was an unsymmetrical tail-fin. In Pterichthys—which, like the preceding, was first brought to light by the labours of Hugh Miller—the whole of the head and the front part of the body were defended by a buckler of firmly-united enamelled plates, whilst the rest of the body was covered with small scales. The form of the "pectoral fins" was quite unique—these having the shape of two long, curved spines, somewhat like wings, covered by finely-tuberculated ganoid plates. All the preceding forms Page 153 of this group are of small size; but few fishes, living or extinct, could rival the proportions of the great Dinichthys, referred

Fig. 104.—Pterichthys cornutus. Old Red Sandstone, Scotland. (After Agassiz.) to this family by Newberry. In this huge fish (fig. 102, a) the head alone is over three feet in length, and the body is supposed to have been twenty-five or thirty feet long. The head was protected by a massive cuirass of bony plates firmly articulated together, but the hinder end of the body seems to have been simply enveloped in a leathery skin. The teeth are of the most formidable description, consisting in both jaws of serrated dental plates behind, and in front of enormous conical tusks (fig. 102, a). Though immensely larger, the teeth of Dinichthys present a curious resemblance to those of the existing Mud-fishes (Lepidosiren).

In another great group of Devonian Ganoids, we meet with fishes more or less closely allied to the living Polypteri (fig. 105) of the Nile and Senegal. In this group (fig. 106) the pectoral fins consist of a central scaly lobe carrying the fin-rays on both sides, the scales being sometimes rounded and overlapping (fig. 106), or more commonly rhomboidal and placed edge to edge (fig. 105, A). Numerous forms of these "Fringe-finned" Ganoids occur in the Devonian strata, such as Holoptychius, Glyotolœmus, Osteolepis, Phaneropleuron, &c. To this group is also to be ascribed the huge Onychodus (fig. 102, d and e), with its large, rounded, overlapping scales, an inch in diameter, and its powerful pointed teeth. It is to be remembered, however, that some of these "Fringe-finned" Ganoids are probably referable to the small but singular group of the "Mud-fishes" (Dipnoi), represented at the present day by the singular Lepidosiren of South America and Africa, and the Ceratodus of the rivers of Queensland.

Leaving the Ganoid fishes, it still remains to be noticed that the Devonian deposits have yielded the remains of a number of fishes more or less closely allied to the existing Sharks, Page 154 Rays, and Chimœrœ (the Elasmobranchii). The majority of the forms here alluded to are allied not to the true Sharks and Dog-fishes, but to the more peaceable "Port Jackson

Fig. 105.—A, Polypterus, a recent Ganoid fish; B, Osteolepis, a Devonian Ganoid; a a, Pectoral fins, showing the fin-rays arranged round a central lobe. Sharks," with their blunt teeth, adapted for crushing the shells of Molluscs. The collective name of "Cestracionts" is applied to these; and we have evidence of their past existence in the Devonian seas

Fig. 106.—Holoptychius nobilissimus, restored. Old Red Sandstone, Scotland. A, Scale of the same. both by their teeth, and by the defensive spines which were implanted in front of a greater or less number of the fins. These are bony spines, often variously grooved, serrated, or ornamented, with hollow bases, implanted in the integument, and capable of being erected or depressed at will. Page 155 Many of these "fin-spines" have been preserved to us in the fossil condition, and the Devonian rocks have yielded examples belonging to many genera. As some of the true Sharks and Dog-fishes, some of the Ganoids, and even some Bony Fishes, possess similar defences, it is often a matter of some uncertainty to what group a given spine is to be referred. One of these spines, belonging to the genus Machœracanthus, from the Devonian rocks of America, has been figured in a previous illustration (fig. 102, f).

In conclusion, a very few words may be said as to the validity of the Devonian series as an independent system of rocks, preserving in its successive strata the record of an independent system of life. Some high authorities have been inclined to the view that the Devonian formation has in nature no actual existence, but that it is made up partly of beds which should be referred to the summit of the Upper Silurian, and partly of beds which properly belong to the base of the Carboniferous. This view seems to have been arrived at in consequence of a too exclusive study of the Devonian series of the British Isles, where the physical succession is not wholly clear, and where there is a striking discrepancy between the organic remains of those two members of the series which are known as the "Old Red Sandstone" and the "Devonian" rocks proper. This discrepancy, however, is not complete; and, as we have seen, can be readily explained on the supposition that the one group of rocks presents us with the shallow water and littoral deposits of the period, while in the other we are introduced to the deep-sea accumulations of the same period. Nor can the problem at issue be solved by an appeal to the phenomena of the British area alone, be the testimony of these what it may. As a matter of fact, there is at present no sufficient ground for believing that there is any irreconcilable discordance between the succession of rocks and of life in Britain during the period which elapsed between the deposition of the Upper Ludlow and the formation of the Carboniferous Limestone, and the order of the same phenomena during the same period in other regions. Some of the Devonian types of life, as is the case with all great formations, have descended unchanged from older types; others pass upwards unchanged to the succeeding period: but the fauna and flora of the Devonian period are, as a whole, quite distinct from those of the preceding Silurian or the succeeding Carboniferous; and they correspond to an equally distinct rock-system, which in point of time holds an intermediate position between the two great groups just mentioned. As Page 156 before remarked, this conclusion may be regarded as sufficiently proved even by the phenomena of the British area; but it maybe said to be rendered a certainty by the study of the Devonian deposits of the continent of Europe—or, still more, by the investigation of the vast, for the most part uninterrupted and continuous series of sediments which commenced to be laid down in North America at the beginning of the Upper Silurian, and did not cease till, at any rate, the close of the Carboniferous.

LITERATURE.

The following list comprises the more important works and memoirs to which the student of Devonian rocks and fossils may refer:—

CHAPTER XII.

Overlying the Devonian formation is the great and important series of the Carboniferous Rocks, so called because workable beds of coal are more commonly and more largely developed in this formation than in any other. Workable coal-seams, however, occur in various other formations (Jurassic, Cretaceous, Tertiary), so that coal is not an exclusively Carboniferous product; whilst even in the Coal-measures themselves the coal bears but a very small proportion to the total thickness of strata, occurring only in comparatively thin beds intercalated in a great series of sandstones, shales, and other genuine aqueous sediments.

Page 158 Stratigraphically, the Carboniferous rocks usually repose conformably upon the highest Devonian beds, so that the line of demarcation between the Carboniferous and Devonian formations is principally a palæontological one, founded on the observed differences in the fossils of the two groups. On the other hand, the close of the Carboniferous period seems to have been generally, though not universally, signalised by movements of the crust of the earth, so that the succeeding Permian beds often lie unconformably upon the Carboniferous sediments.

Strata of Carboniferous age have been discovered in almost every large land-area which has been sufficiently investigated; but they are especially largely developed in Britain, in various parts of the continent of Europe, and in North America. Their general composition, however, is, comparatively speaking, so uniform, that it will suffice to take a comprehensive view of the formation without considering any one area in detail, though in each region the subdivisions of the formation are known by distinctive local names. Taking such a comprehensive view, it is found that the Carboniferous series is generally divisible into a Lower and essentially calcareous group (the "Sub-Carboniferous" or "Carboniferous Limestone"); a Middle and principally arenaceous group (the "Millstone Grit"); and an Upper group, of alternating shales and sandstones, with workable seams of coal (the "Coal-measures").

I. The Carboniferous, Sub-Carboniferous, or Mountain Limestone Series constitutes the general base of the Carboniferous system. As typically developed in Britain, the Carboniferous Limestone is essentially a calcareous formation, sometimes consisting of a mass of nearly pure limestone from 1000 to 2000 feet in thickness, or at other times of successive great beds of limestone with subordinate sandstones and shales. In the north of England the base of the series consists of pebbly conglomerates and coarse sandstones; and in Scotland generally, the group is composed of massive sandstones with a comparatively feeble development of the calcareous element. In Ireland, again, the base of the Carboniferous Limestone is usually considered to be formed by a locally-developed group of grits and shales (the "Coomhola Grits" and "Carboniferous Slate"), which attain the thickness of about 5000 feet, and contain an intermixture of Devonian with Carboniferous types of fossils. Seeing that the Devonian formation is generally conformable to the Carboniferous, we need feel no surprise at this intermixture of forms; nor does it Page 159 appear to be of great moment whether these strata be referred to the former or to the latter series. Perhaps the most satisfactory course is to regard the Coomhola Grits and Carboniferous Slates as "passage-beds" between the Devonian and Carboniferous; but any view that may be taken as to the position of these beds, really leaves unaffected the integrity of the Devonian series as a distinct life-system, which, on the whole, is more closely allied to the Silurian than to the Carboniferous. In North America, lastly, the Sub-Carboniferous series is never purely calcareous, though in the interior of the continent it becomes mainly so. In other regions, however, it consists principally of shales and sandstones, with subordinate beds of limestone, and sometimes with this beds of coal or deposits of clay-ironstone.

II. The Millstone Grit.—The highest beds of the Carboniferous Limestone series are succeeded, generally with perfect conformity, by a series of arenaceous beds, usually known as the Millstone Grit. As typically developed in Britain, this group consists of hard quartzose sandstones, often so large-grained and coarse in texture as to properly constitute fine conglomerates. In other cases there are regular conglomerates, sometimes with shales, limestones, and thin beds of coal—the thickness of the whole series, when well developed, varying from 1000 to 5000 feet. In North America, the Millstone Grit rarely reaches 1000 feet in thickness; and, like its British equivalent, consists of coarse sandstones and grits, sometimes with regular conglomerates. Whilst the Carboniferous Limestone was undoubtedly deposited in a tranquil ocean of considerable depth, the coarse mechanical sediments of the Millstone Grit indicate the progressive shallowing of the Carboniferous seas, and the consequent supervention of shore-conditions.

III. The Coal-measures.—The Coal-measures properly so called rest conformably upon the Millstone Grit, and usually consist of a vast series of sandstones, shales, grits, and coals, sometimes with beds of limestone, attaining in some regions a total thickness of from 7000 to nearly 14,000 feet. Beds of workable coal are by no means unknown in some areas in the inferior group of the Sub-Carboniferous; but the general statement is true, that coal is mostly obtained from the true Coal-measures—the largest known, and at present most productive coal-fields of the world being in Great Britain, North America, and Belgium. Wherever they are found, with limited exceptions, the Coal-measures present a singular general uniformity of mineral composition. They Page 160 consist, namely, of an indefinite alternation of beds of sandstone, shale, and coal, sometimes with bands of clay-ironstone or beds of limestone, repeated in no constant order, but sometimes attaining the enormous aggregate thickness of 14,000 feet, or little short of 3 miles. The beds of coal differ in number and thickness in different areas, but they seldom or never exceed one-fiftieth part of the total bulk of the formation in thickness. The characters of the coal itself, and the way in which the coal-beds were deposited, will be briefly alluded to in speaking of the vegetable life of the period. In Britain, and in the Old World generally, the Coal-measures are composed partly of genuine terrestrial deposits—such as the coal—and partly of sediments accumulated in the fresh or brackish waters of vast lagoons, estuaries, and marshes. The fossils of the Coal-measures in these regions are therefore necessarily the remains either of terrestrial plants and animals, or of such forms of life as inhabit fresh or brackish waters, the occurrence of strata with marine fossils being quite a local and occasional phenomenon. In various parts of North America, on the other hand, the Coal-measures, in addition to sandstones, shales, coal-seams, and bands of clay-ironstone, commonly include beds of limestone, charged with marine remains, and indicating marine conditions. The subjoined section (fig. 107) gives, in a generalised form, the succession of the Carboniferous strata in such a British area as the north of England, where the series is developed in a typical form.

As regards the life of the Carboniferous period, we naturally find, as has been previously noticed, great differences in different parts of the entire series, corresponding to the different mode of origin of the beds. Speaking generally, the Lower Carboniferous (or the Sub-Carboniferous) is characterised by the remains of marine animals; whilst the Upper Carboniferous (or Coal-measures) is characterised by the remains of plants and terrestrial animals. In all those cases, however, in which marine beds are found in the series of the Coal-measures, as is common in America, then we find that the fossils agree in their general characters with those of the older marine deposits of the period.

Owing to the fact that coal is simply compressed and otherwise altered vegetable matter, and that it is of the highest economic value to man, the Coal-measures have been more thoroughly explored than any other group of strata of equivalent thickness in the entire geological series. Hence we have already a very extensive acquaintance with the plants of the Carboniferous period; and our knowledge on this subject is Page 161 daily undergoing increase. It is not to be supposed, however, that the remains of plants are found solely in Coal-measures; GENERALIZED SECTION OF THE CARBONIFEROUS STRATA OF THE NORTH OF ENGLAND.
Fig. 107.

for though most abundant towards the summit, they are found in less numbers in all parts of the series. Wherever found, they belong to the same great types of Page 162 vegetation; but, before reviewing these, a few words must be said as to the origin and mode of formation of coal.

The coal-beds, as before mentioned, occur interstratified with shales, sandstones, and sometimes limestones; and there may, within the limits of a single coal-field, be as many as 80 or 100 of such beds, placed one above the other at different levels, and varying in thickness from a few inches up to 20 or 30 feet. As a general rule, each bed of coal rests upon a bed of shale or clay, which is termed the "under-clay," and in which are found numerous roots of plants; whilst the strata immediately on the top of the coal may be shaly or sandy, but in either case are generally charged with the leaves and stems of plants, and often have upright trunks passing vertically through them. When we add to this that the coal itself is, chemically, nearly wholly composed of carbon, and that its microscopic structure shows it to be composed almost entirely of fragments of stems, leaves, bark, seeds, and vegetable débris derived from land-plants, we are readily enabled to understand how the coal was formed. The "under-clay" immediately beneath the coal-bed represents an old land-surface—sometimes, perhaps, the bottom of a swamp or marsh, covered with a luxuriant vegetation; the coal bed itself represents the slow accumulation, through long periods, of the leaves, seeds, fruits, stems, and fallen trunks of this vegetation, now hardened and compressed into a fraction of its original bulk by the pressure of the superincumbent rocks; and the strata of sand or shale above the coal-bed—the so-called "roof" of the coal—represent sediments quietly deposited as the land, after a long period of repose, commenced to sink beneath the sea. On this view, the rank and long-continued vegetation which gave rise to each coal-bed was ultimately terminated by a slow depression of the surface on which the plants grew. The land-surface then became covered by the water, and aqueous sediments were accumulated to a greater or less thickness upon the dense mass of decaying vegetation below, enveloping any trunks of trees which might still be in an erect position, and preserving between their layers the leaves and branches of plants brought down from the neighbouring land by streams, or blown into the wafer by the wind. Finally, there set in a slow movement of elevation,—the old land again reappeared above the water; a new and equally luxuriant vegetation flourished upon the new land-surface; and another coal-bed was accumulated, to be preserved ultimately in a similar fashion. Some few beds of coal may have been formed by drifted vegetable matter brought down into the ocean by rivers, Page 163 and deposited directly on the bottom of the sea; but in the majority of cases the coal is undeniably the result of the slow growth and decay of plants in situ: and as the plants of the coal are not marine plants, it is necessary to adopt some such theory as the above to account for the formation of coal-seams. By this theory, as is obvious, we are compelled to suppose that the vast alluvial and marshy flats upon which the coal-plants grew were liable to constantly-recurring oscillations of level, the successive land-surfaces represented by the successive coal-beds of any coal-field being thus successively buried beneath accumulations of mud or sand. We have no need, however, to suppose that these oscillations affected large areas at the same time; and geology teaches us that local elevations and depressions of the land have been matters of constant occurrence throughout the whole of past time.

All the varieties of coal (bituminous coal, anthracite; cannel-coal, &c.) show a more or less distinct "lamination"—that is to say, they are more or less obviously composed of successive thin layers, differing slightly in colour and texture. All the varieties of coal, also, consist chemically of carbon, with varying proportions of certain gaseous constituents and a small amount of incombustible mineral or "ash." By cutting thin and transparent slices of coal, we are further enabled, by means of the microscope, to ascertain precisely not only that the carbon of the coal is derived from vegetables, but also, in many cases, what kinds of plants, and what parts of these, enter into the formation of coal. When examined in this way, all coals are found to consist more or less entirely of vegetable matter; but there is considerable difference in different coals as to the exact nature of this. By Professor Huxley it has been shown that many of the English coals consist largely of accumulations of rounded discoidal sacs or bags, which are unquestionably the seed-vessels or "spore-cases" of certain of the commoner coal-plants (such as the Lepidodendra). The best bituminous coals seem to be most largely composed of these spore-cases; whilst inferior kinds possess a progressively increasing amount of the dull carbonaceous substance which is known as "mineral charcoal," and which is undoubtedly composed of "the stems and leaves of plants reduced to little more than their carbon." On the other hand, Principal Dawson finds that the American coals only occasionally exhibit spore-cases to any extent, but consist principally of the cells, vessels, and fibres of the bark, integumentary coverings, and woody portions of the Carboniferous plants.

The number of plants already known to have existed Page 164 during the Carboniferous period is so great, that nothing more can be done here than to notice briefly the typical and characteristic groups of these—such as the Ferns, the Calamites, the Lepidodendroids, the Sigillarioids, and the Conifers.

In accordance with M. Brongniart's generalisation, that the Palæozoic period is, botanically speaking, the "Age of Acrogens," we find the Carboniferous plants to be still mainly referable to the Flowerless or "Cryptogamous" division of the vegetable kingdom. The flowering or "Phanerogamous" plants, which form the bulk of our existing vegetation, are hardly known, with certainty, to have existed at all in the Carboniferous era, except as represented by trees related to the existing Pines and

Fig. 108.—Odontopteris Schlotheimii. Carboniferous, Europe and North America. Firs, and possibly by the Cycads or "false palms."[18] Amongst the "Cryptogams," there is no more striking or beautiful group of Carboniferous plants than the Ferns. Remains of these are found all through the Carboniferous, but in exceptional numbers in the Coal-measures, and include both herbaceous forms like the majority of existing species, and arborescent forms resembling the living Tree-ferns of New Zealand. Amongst the latter, together with some new types, are examples of the genera Psaronius and Caulopteris, both of Page 165 which date from the Devonian. The simply herbaceous ferns are extremely numerous, and belong to such widely-distributed and

Fig. 109.—Calamites cannœformis. Carboniferous Rocks, Europe and North America. largely-represented genera as Neuropteris, Odontopteris (fig. 108), Alethopteris, Pecopteris, Sphenopteris, Hymenophyllites, &c.

[Footnote 18: Whilst the vegetation of the Coal-period was mainly a terrestrial one, aquatic plants are not unknown. Sea-weeds (such as the Spirophyton cauda-Galli) are common in some of the marine strata; whilst coal, according to the researches of the Abbé Castracane, is asserted commonly to contain the siliceous envelopes of Diatoms.]

The fossils known as Calamites (fig. 109) are very common Page 166 in the Carboniferous deposits, and have given occasion to an abundance of research and speculation. They present themselves as prostrate and flattened striated stems, or as similar uncompressed stems growing in an erect position, and sometimes attaining a length of twenty feet or more. Externally, the stems are longitudinally ribbed, with transverse joints at regular intervals, these joints giving origin to a whorl or branchlets, which mayor may not give origin to similar whorls of smaller branchlets still. The stems, further, were hollow, with transverse partitions at the joints, and having neither true wood nor bark, but only a thin external fibrous shell. There can be little doubt but that the Calamites are properly regarded as colossal representatives of the little Horse-tails (Equisetaceœ) of the present day. They agree with these not only in the general details of their organisation, but also in the fact that the fruit was a species of cone, bearing "spore-cases" under scales. According to Principal Dawson, the Calamites "grew in dense brakes on the sandy and muddy flats, subject to inundation, or perhaps even in water; and they had the power of budding out from the base of the stem, so as to form clumps of plants, and also of securing their foothold by numerous cord-like roots proceeding from various heights on the lower part of the stem."

The Lepidodendroids, represented mainly by the genus Lepidodendron itself (fig. 110), were large tree-like plants, which attain their maximum in the Carboniferous period, but which appear to commence in the Upper Silurian, are well represented in the Devonian, and survive in a diminished form into the Permian. The trunks of the larger species of Lepidodendron at times reach a length of fifty feet and upwards, giving off branches in a regular bifurcating manner. The bark is marked with numerous rhombic or oval scars, arranged in quincunx order, and indicating the points where the long, needle-shaped leaves were formerly attached. The fruit consisted of cones or spikes, carried at the ends of the branches, and consisting of a central axis surrounded by overlapping scales, each of which supports a "spore-case" or seed-vessel. These cones have commonly been described under the name of Lepidostrobi. In the structure of the trunk there is nothing comparable to what is found in existing trees, there being a thick bark surrounding a zone principally composed of "scalariform" vessels, this in turn enclosing a large central pith. In their general appearance the Lepidodendra bring to mind the existing Araucarian Pines; but they are true "Cryptogams," and are to be regarded as a gigantic extinct type of the Page 167 modern Club-mosses (Lycopodiaceœ). They are amongst the commonest and most characteristic of the Carboniferous

Fig. 110.—Lepidodendron Sternbergii, Carboniferous, Europe. The central figure represents a portion of the trunk with its branches, much reduced in size. The right-hand figure is a portion of a branch with the leaves partially attached to it; and the left-hand figure represents the end of a branch bearing a cone of fructification. plants; and the majority of the "spore-cases" so commonly found in the coal appear to have been derived from the cones of Lepidodendroids.

Page 168 The so-called Sigillanoids, represented mainly by Sigillaria itself (fig. 111), were no less abundant and characteristic of the Carboniferous forests than the Lepidodendra. They commence their existence, so far as known, in the Devonian period, but they attain their maximum in the Carboniferous; and—unlike the Lepidodendroids—they are not known to occur in the Permian period. They are comparatively gigantic in size, often attaining a height of from thirty to fifty feet or more; but though abundant and well preserved, great divergence of opinion prevails as to their true affinities. The name of Sigillarioids (Lat. sigilla, little seals or images) is derived from the fact that the bark is marked with seal-like impressions or leaf-scars (fig. 111).

Externally, the trunks of Sigillaria present strong longitudinal ridges, with vertical alternating rows of oval leaf-scars indicating the points where the leaves were originally

Fig. 111.—Fragment of the external surface of Sigillaria Grœseri, showing the ribs and leaf-scars. The left-hand figure represents a small portion enlarged. Carboniferous, Europe. attached. The trunk was furnished with a large central pith, a thick outer bark, and an intermediate woody zone,—composed, according to Dawson, partly of the disc-bearing fibres so characteristic of Conifers; but, according to Carruthers, entirely made up of the "scalariform" vessels characteristic of Cryptogams. The size of the pith was very great, and the bark seems to have been the most durable portion of the trunk. Thus we have evidence that in many cases the stumps and "stools" of Sigillariœ, standing Page 169 upright in the old Carboniferous swamps, were completely hollowed out by internal decay, till nothing but an exterior shell of bark was left. Often these hollow stumps became ultimately filled up with sediment, sometimes enclosing the remains of galley-worms, land-snails, or Amphibians, which formerly found in the cavity of the trunk a congenial home; and from the sandstone or shale now filling such trunks some of the most interesting fossils of the Coal-period have been obtained. There is little certainty as to either the leaves or fruits of Sigillaria, and there is equally little certainty as to the true botanical position of these plants. By Principal Dawson they are regarded as being probably flowering plants allied to the existing "false palms" or "Cycads," but the high authority of Mr Carruthers is to be quoted in support of the belief that they are Cryptogamic, and most nearly allied to the Club-mosses.

Leaving the botanical position of Sigillaria thus undecided, we find that it is now almost universally conceded that the fossils originally described under the name of Stigmaria are the roots of Sigillaria, the actual connection between the two having been in numerous instances demonstrated in an unmistakable manner. The Stigmariœ (fig. 112) ordinarily present themselves in the form of long, compressed or rounded

Fig. 112.—Stigmaria ficoides. Quarter natural size. Carboniferous. fragments, the external surface of which is covered with rounded pits or shallow tubercles, each of which has a little pit or depression in its centre. From each of these pits there proceeds, in perfect examples, a long cylindrical rootlet; but in many cases these have altogether disappeared. In their internal structure, Stigmaria exhibits a central pith surrounded by a sheath of scalariform vessels, the whole enclosed in a cellular envelope. The Stigmariœ are generally found ramifying in Page 170 the "under-clay," which forms the floor of a bed of coal, and which represents the ancient soil upon which the Sigillariœ grew.

The Lepidodendroids and Sigillaroids, though the first were certainly, and the second possibly, Cryptogamic or flowerless plants, must have constituted the main mass of the forests of the Coal period; but we are not without evidence of the existence at the same time of genuine "trees," in the technical sense of this term—namely, flowering plants with large woody stems. So far as is certainly known, all the true trees of the Carboniferous formation were Conifers, allied to the existing Pines and Firs. They are recognised by the great size and concentric woody rings of their prostrate, rarely erect trunks, and by the presence of disc-bearing fibres in their wood, as demonstrated by the microscope; and the principal genera which have been recognised are Dadoxylon, Palœoxylon, Araucarioxylon, and Pinites. Their fruit is not known with absolute certainty, unless it be represented, as often conjectured, by Trigonocarpon (fig. 113). The fruits known under this name are nut-like, often of

Fig. 113.—Trigonocarpon ovatum. Coal-measures, Britain. (After Liudley and Hutton.) considerable size, and commonly three- or six-angled. They probably originally possessed a fleshy envelope; and if truly referable to the Conifers, they would indicate that these ancient evergreens produced berries instead of cones, and thus resembled the modern Yews rather than Pines. It seems, further, that the great group of the Cycads, which are nearly allied to the Conifers, and which attained such a striking prominence in the Secondary period, probably commenced its existence during the Coal period; but these anticipatory forms are comparatively few in number, and for the most part of somewhat dubious affinities.

CHAPTER XIII.

We have seen that there exists a great difference as to the mode of origin of the Carboniferous sediments, some being purely marine, whilst others are terrestrial; and others, again, Page 171 have been formed in inland swamps and morasses, or in brackish-water lagoons, creeks, or estuaries. A corresponding difference exists necessarily in the animal remains of these deposits, and in many regions this difference is extremely well marked and striking. The great marine limestones which characterise the lower portion of the Carboniferous series in Britain, Europe, and the eastern portion of America, and the calcareous beds which are found high up in the Carboniferous in the western States of America, may, and do, often contain the remains of drifted plants; but they are essentially characterised by marine fossils; and, moreover, they can be demonstrated by the microscope to be almost wholly composed of the remains of animals which formerly inhabited the ocean. On the other hand, the animal remains of the beds accompanying the coal are typically the remains of air-breathing, terrestrial, amphibious, or aerial animals, together with those which inhabit fresh or brackish waters. Marine fossils may be found in the Coal-measures, but they are invariably confined to special horizons in the strata, and they indicate temporary depressions of the land beneath the sea. Whilst the distinction here mentioned is one which cannot fail to strike the observer, it is convenient to consider the animal life of the Carboniferous as a whole: and it is simply necessary, in so doing, to remember that the marine fossils are in general derived from the inferior portion of the system; whilst the air-breathing, fresh-water, and brackish-water forms are almost exclusively derived from the superior portion of the same.

The Carboniferous Protozoans consist mainly of Foraminifera and Sponges. The latter are still very insufficiently known, but the former are very abundant, and belong to very varied types. Thin slices of the limestones of the period, when examined by the microscope, very commonly exhibit the shells of Foraminifera in greater or less plenty. Some limestones, indeed, are made up of little else than these minute and elegant shells, often belonging to types, such as the Textularians and Rotalians, differing little or not at all from those now in existence. This is the case, for example, with the Carboniferous Limestone of Spergen Hill in Indiana (fig. 114), which is almost wholly made up of the spiral shells of a species of Endothyra. In the same way, though to a less extent, the black Carboniferous marbles of Ireland, and the similar marbles of Yorkshire, the limestones of the west of England and of Derbyshire, and the great "Scar Limestones" of the north of England, contain great numbers of Foraminiferous shells; whilst similar organisms commonly occur in the shale-beds associated Page 172 with the limestones throughout the Lower Carboniferous series. One of the most interesting of the British Carboniferous forms

Fig. 114.—Transparent slice of Carboniferous Limestone, from Spergen Hill, Indiana, U.S., showing numerous shells of Endothyra (Rotalia), Baiteyi slightly enlarged. (Original.) is the Saccammina of Mr Henry Brady, which is sometimes present in considerable numbers in the limestones of Northumberland, Cumberland, and the west of Scotland, and which is conspicuous for the comparatively large size of its spheroidal or pear-shaped shell (reaching from an eighth to a fifth of an inch in size). More widely distributed are the generally spindle-shaped shells of Fusulina (fig. 115), which occur in vast numbers in the Carboniferous Limestone of Russia, Armenia, the Southern Alps, and Spain, similar forms occurring in equal profusion in the higher limestones which are found in the Coal-measures of the United States, in Ohio, Illinois, Indiana, Missouri, &c. Mr Henry Brady, lastly, has shown that we have in the Nummulina Pristina of the Carboniferous Limestone of Namur a genuine

Fig. 115.—Fusulina cylindrica, Carboniferous Limestone, Russia. Nummulite, precursor of the great and important family of the Tertiary Nummulites.

The sub-kingdom of the Cœlenterates, so far as certainly known, is represented only by Corals;[19] but the remains of these are so abundant in many of the limestones of the Carboniferous formation as to constitute a feature little or not at all less conspicuous than that afforded by the Crinoids. As is the case in the preceding period, the Corals belong, almost exclusively, to the groups of the Rugosa and Tabulata; and there is a general and striking resemblance and relationship between the coral-fauna of the Devonian as a whole, and that Page 173 of the Carboniferous. Nevertheless, there is an equally decided and striking amount of difference between these successive faunas, due to the fact that the great majority of the Carboniferous species are new; whilst some of the most characteristic Devonian genera have nearly or quite disappeared, and several new genera now make their appearance for the first time. Thus, the characteristic Devonian types Heliophyllum, Pachyphyllum, Chonophyllum, Acervularia, Spongophyllum, Smithia, Endophyllum, and Cystiphyllum, have now disappeared; and the great masses of Favosites which are such a striking feature in the Devonian limestones, are represented but by one or two degenerate and puny successors. On the other hand, we meet in the Carboniferous rocks not only with entirely new genera—such as Axophyllum, Lophophyllum, and Londsdaleia—but we have an enormous expansion of certain types which had just begun to exist in the preceding period. This is especially well seen in the Case of the genus Lithostrotion (fig. 116, b), which more than any other may be considered as the predominant Carboniferous group of Corals. All the species of Lithostrotion are compound, consisting either of bundles of loosely-approximated cylindrical stems, or of similar "coral-lites" closely aggregated together into astræiform colonies, and rendered polygonal by mutual pressure. This genus has a historical interest, as having been noticed as early as in the year 1699 by Edward Lhwyd; and it is geologically important from its wide distribution in the Carboniferous rocks of both the Old and New Worlds. Many species are known, and whole beds of limestone are often found to be composed of little else than the skeletons of these ancient corals, still standing upright as they grew. Hardly less characteristic of the Carboniferous than the above is the great group of simple "cup-corals," of which Clisiophyllum is the central type. Amongst types which commenced in the Silurian and Devonian, but which are still well represented here, may be mentioned Syringopora (fig. 116, e), with its colonies of delicate cylindrical tubes united at intervals by cross-bars; Zaphrentis (fig. 116, d), with its cup-shaped skeleton and the well-marked depression (or "fossula") on one side of the calice; Amplexus (fig. 116, c), with its cylindrical, often irregularly swollen coral and short septa; Cyathophyllum (fig. 116, a), sometimes simple, sometimes forming great masses of star-like corallites; and Chœtetes, with its branched stems, and its minute, "tabulate" tubes (fig. 116, f). The above, together with other and hardly less characteristic forms, combine to constitute a coral-fauna which is not only in itself perfectly distinctive, but which is of especial interest, Page 174 from the fact that almost all the varied types of which it is composed disappeared utterly before the close of the Carboniferous

Fig. 116—Corals of the Carboniferous Limestone. a. Cyathophyllum paracida, showing young corallites budded forth from the disc of the old one; a', One of the corallites of the same, seen in cross-section; b, Fragment of a mass of Lithostrotion irregulare; b', One of the corallites of the same, divided transversely; c, Portion of the simple cylindrical coral of Amplexus coralloides; c', Transverse section of the same species; d, Zaphrentis vermicularis, showing the depression or "fossula" on one side of the cup; e, Fragrent of a mass of Syringopora ramulosa; f, Fragment of Cœtetes tumidus; f', Portion of the same of the same, enlarged. From the Carboniferous Limestone of Britain and Belgium. (After Thomson, De Koninck, Milne-Edwards and Haime, and the Author.) period. In the first marine sediments of a calcareous nature which succeeded to the Coal-measures (the magnesian limestones of the Permian), the great group of the Rugose corals, which flourished so largely throughout the Silurian, Devonian, and Carboniferous periods, is found to have all but Page 175 disappeared, and it is never again represented save sporadically and by isolated forms.

[Footnote 19: A singular fossil has been described by Professor Martin Duncan and Mr Jenkins from the Carboniferous rocks under the name of Palœocoryne, and has been referred to the Hydroid Zoophytes (Corynida). Doubt, however, has been thrown by other observers on the correctness of this reference.]

Amongst the Echinoderms, by far the most important forms are the Sea-lilies and the Sea-urchins—the former from their great abundance, and the latter from their singular structure; but the little group of the "Pentremites" also requires to be noticed. The Sea-lilies are so abundant in the Carboniferous rocks, that it has been proposed to call the earlier portion of the period the "Age of Crinoids." Vast masses of the limestones of the period are "crinoidal," being more or less extensively composed of the broken columns, and detached plates and joints of Sea-lilies, whilst perfect "heads" may be exceedingly rare and difficult to procure. In North America the remains of Crinoids are even more abundant at this horizon than in Britain, and the specimens found seem to be commonly more perfect. The commonest of the Carboniferous Crinoids belong to the genera Cyathocrinus, Actinocrinus, Platycrinus, (fig. 117), Poteriocrinus, Zeacrinus,

Fig. 117.—Platycrinus tricontadactylus, Lower Carboniferous. The left-hand figure shows the calyx, arms, and upper part of the stem; and the figure next this shows the surface of one of the joints of the column. The right-hand figure shows the proboscis. (After M'Coy.) and Forbesiocrinus. Closely allied to the Crinoids, or forming a kind of transition Page 176 between these and the Cystideans, is the little group of the "Pentremites," or Blastoids (fig. 118). This group is first known to have commenced its existence in

Fig. 118.—A, Pentremites pyriformis, side-view of the body ("calyx"); B, The same viewed from below, showing the arrangement of the plates; C, Body of Pentremites conoideus, viewed from above. Carboniferous. the Upper Silurian, and it increased considerably in numbers in the Devonian; but it was in the seas of the Carboniferous period that it attained its maximum, and no certain representative of the family has been detected in any later deposits. The "Pentremites" resemble the Crinoids in having a cup-shaped body (fig. 118, A) enclosed by closely-fitting calcareous plates, and supported on a short stem or "column," composed of numerous calcareous pieces flexibly articulated together. They differ from the Crinoids, however, in the fact that the upper surface of the body does not support the crown of branched feathery "arms," which are so characteristic of the latter. On the contrary, the summit of the cup is closed up in the fashion of a flower-bud, whence the technical name of Blastoidea applied to the group (Gr. blastos, a bud; eidos, form). From the top of the cup radiate five broad, transversely-striated areas (fig. 118, C), each with a longitudinal groove down its middle; and along each side of each of Page 177 these grooves there seems to have been attached a row of short jointed calcareous filaments or "pinnules."

A few Star-fishes and Brittle-stars are known to occur in the Carboniferous rocks; but the only other Echinodemls of this period which need be noticed are the Sea-urchins (Echinoids). Detached plates and spines of these are far from rare in the Carboniferous deposits; but anything like perfect specimens are exceedingly scarce. The Carboniferous Sea-urchins agree with those of the present day in having the body enclosed in a shell formed by an enormous number of calcareous plates articulated together. The shell may be regarded as, typically, nearly spherical in shape, with the mouth in the centre of the base, and the excretory opening or vent at its summit. In both the ancient forms and the recent ones, the plates of the shell are arranged in ten zones

Fig. 119.—Palœchinus ellipticus, one of the Carboniferous Sea-urchins. The left-hand figure shows one of the "ambulacral areas" enlarged, exhibiting the perforated plates. The right-land figure exhibits a single plate from one of the "inter-ambulacral areas." (After M'Coy.) which generally radiate from the summit to the centre of the base. In five of these zones—termed the "ambulacral areas"—the plates are perforated by minute apertures or "pores," through which the animal can protrude the little water-tubes ("tube-feet") by which its locomotion is carried on. In the other five zones—the so-called "inter-ambulacral areas"—the plates are of larger size, and are not perforated by any apertures. In all the modern Sea-urchins each of these ten zones, whether perforate or imperforate, is composed of two rows of plates; and there are thus twenty rows of plates in all. In the Palæozoic Sea-urchins, on the other hand, the "ambulacral areas" are often like those of recent forms, in consisting of two rows of perforated plates (fig. 119); but the "inter-ambulacral areas" are always quite Page 178 peculiar in consisting each of three, four, five, or more rows of large imperforate plates, whilst there are sometimes four or ten rows of plates in the "ambulacral areas" also: so that there are many more than twenty rows of plates in the entire shell. Some of the Palæozoic Sea-urchins, also, exhibit a very peculiar singularity of structure which is only known to exist in a very few recently-discovered modern forms (viz., Calveria and Phormosoma). The plates of the inter-ambulacral areas, namely, overlap one another in an imbricating manner, so as to communicate a certain amount of flexibility to the shell; whereas in the ordinary living forms these plates are firmly articulated together by their edges, and the shell forms a rigid immovable box. The Carboniferous Sea-urchins which exhibit this extraordinary peculiarity belong to the genera Lepidechinus and Lepidesthes, and it seems tolerably certain that a similar flexibility of the shell existed to a less degree in the much more abundant genus Archœocidaris. The Carboniferous Sea-urchins, like the modern ones, possessed movable spines of greater or less length, articulated to the exterior of the shell; and these structures are of very common occurrence in a detached condition. The most abundant genera are Archœocidaris and Palœchinus; but the characteristic American forms belong principally to Melonites, Oligoporus, and Lepidechinus.

Amongst the Annelides it is only necessary to notice the little spiral tubes of Spirorbis Carbonarius (fig. 120),

Fig. 120.—Spirorbis (Microconchus) Carbonarius, of the natural size, attached to a fossil plant, and magnified. Carboniferous Britain and North America. (After Dawson.) which are commonly found attached to the leaves or stems of the Coal-plants. This fact shows that though the modern species of Spirorbis are inhabitants of the sea, these old representatives of the genus must have been capable of living in the brackish waters of lagoons and estuaries.

The Crustaceans of the Carboniferous rocks are numerous, Page 179 and belong partly to structural types with which we are already familiar, and partly to higher groups which come into existence here for the first time. The gigantic Eurypterids of the Upper Silurian and Devonian are but feebly represented, and make their final exit here from the scene of life. Their place, however, is taken by peculiar forms belonging to the allied group of the Xiphosura, represented at the present day by the King-crabs or "Horse-shoe Crabs" (Limulus). Characteristic forms of this group appear in the Coal-measures both of Europe and America; and though constituting three distinct genera (Prestwichia, Belinurus, and Euproöps), they are all nearly related to one another. The best known of them, perhaps, is the Prestwichia rotundala of Coalbrookdale, here figured (fig. 121). The ancient

Fig. 121.—Prestwichia rotundata, a Limuloid Crustacean. Coal-measures, Britain. (After Henry Woodward.) and formerly powerful order of the Trilobites also undergoes its final extinction here, not surviving the deposition of the Carboniferous Limestone series in Europe, but extending its range in America into the Coal-measures. All the known Carboniferous forms are small in size and degraded in point of structure, and they are referable to but three genera (Phillipsia, Griffithides, and Brachymetopus), belonging to a single family. The Phillipsia seminifera here figured (fig. 122, a) is a characteristic species in the Old World. The Water-fleas (Ostracoaa) are extremely abundant in the Carboniferous rocks, whole strata being often made up of little else than the little bivalved shells of these Crustaceans. Many of them are extremely small, averaging about the size of a millet-seed; but a few forms, such as Entomoconchus Scouleni (fig. 122, c), may attain a length of from one to three quarters of an inch. The old group of the Phyllopods is is likewise still represented in some abundance, partly by tailed forms of a shrimp-like appearance, such as Dithyrocaris (fig. 122, d), and partly by the curious striated Estheriœ and their allies, which present a curious Page 180 resemblance to the true Bivalve Molluscs (fig. 122, b). Lastly, we meet for the first time in the Carboniferous rocks with the remains of the highest of all the groups of Crustaceans—namely, the so-called "Decapods," in which there are five pairs of walking-limbs, and the hinder end of the body ("abdomen") is composed of separate rings, whilst the anterior end is covered by a head-shield or "carapace." All the Carboniferous Decapods hitherto discovered resemble the existing Lobsters, Prawns, and

Fig. 122.—Crustaceans of the Carboniferous Rocks. a, Phillipsia seminifera, of the natural size—Mountain Limestone, Europe; b, One valve of the shell of Estheria tenella, of the natural size and enlarged—Coal-measures, Europe; c, Bivalved shell of Entomoconchus Scouleri, of the natural size—Mountain Limestone, Europe; d, Dithyrocaris Scouleri, reduced in size—Mountain Limestone, Ireland; e, Palœocaris typus, slightly enlarged—Coal-measures, North America; f, Anthrapalœmon gracilis, of the natural size—Coal-measures, North America. (After De Koninck, M'Coy, Rupert Jones, and Meek and Worthen.) Shrimps (the Macrura), in having a long and well-developed abdomen terminated by an expanded tail-fin. The Palœocaris typus (fig. 122, e) and the Anthrapalœmon gracilis (fig. 122, f), from the Coal-measures of Illinois, are two of the best understood and most perfectly preserved of the few known representatives of the "Long-tailed" Decapods in the Carboniferous series. The group of the Crabs or "Short-tailed" Page 181 Decapods (Brachyura), in which the abdomen is short, not terminated by a tail-fin, and tucked away out of sight beneath the body, is at present not known to be represented at all in the Carboniferous deposits.

In addition to the water-inhabiting group of the Crustaceans, we find the articulate animals to be represented by members belonging to the air-breathing classes of the Arachnida, Myriapoda, and Insecta. The remains of these, as might have been expected, are not known to occur in the marine limestones of the Carboniferous series, but are exclusively found in beds associated with the Coal, which have been deposited in lagoons, estuaries, or marshes, in the immediate vicinity of the land, and which actually represent an old land-surface. The Arachnids are at present the oldest known of their class, and are represented both by true Spiders and Scorpions. Remains of the latter (fig. 123) have been found both in the Old and New Worlds, and indicate the existence

Fig. 123.—Cyclophthalmus senior. A fossil Scorpion from the Coal-measures of Bohemia. in the Carboniferous period of Scorpions differing but very little from existing forms. The group of the Myriapoda, including the recent Centipedes and Galley-worms, is likewise represented in the Carboniferous strata, Page 182 but by forms in many respects very unlike any that are known to exist at the present day. The most interesting of these were obtained by Principal Dawson, along with the bones of Amphibians and the shells of Land-snails, in the sediment filling the hollow trunks of Sigillaria, and they belong to the genera Xylobius (fig. 124) and Archiulus. Lastly, the true insects are represented by

Fig. 124.—Xylobius Sigillariœ, a Carboniferous Myriapod. a, A specimen, of the natural size; b, Anterior portion of the same, enlarged; c, Posterior portion, enlarged. From the Coal-measures of Nova Scotia. (After Dawson.) various forms of Beetles (Coleoptera), Orthoptera (such as Cockroaches), and Neuropterous insects resembling those which we have seen to have existed towards the close of

Fig. 125—Haplophlebium Barnesi, a Carboniferous insect, from the Coal-meastures of Nova Scotia. (After Dawson.) the Devonian period. One of the most remarkable of the latter is a huge May-fly (Haplophlebium Barnesi, fig. 125), with Page 183 netted wings attaining an expanse of fully seven inches, and therefore much exceeding any existing Ephemerid in point of size.

The lower groups of the Mollusca are abundantly represented in the marine strata of the Carboniferous series by Polyzoans

Fig. 126.—Carboniferous Polyzoa. a, Fragment of Polypora dendroides, of the natural size, Ireland; a' Small portion of the same, enlarged to show the cells; b, Glauconome pulcherrima, a fragment, of the natural size, Ireland; b', Portion of the same, enlarged; c, The central screw-like axis of Archimedes Wortheni, of the natural size—Carboniferous, America; c', Portion of the exterior of the frond of the same, enlarged; c'', Portion of the interior of the frond of the same showing the mouths of the cells, enlarged. (After M'Coy and Hall.)] and Brachiopods. Amongst the former, although a variety of other types are known, the majority still belong to the old group of the "Lace-corals" (Fenestellidœ), some of the characteristic forms of which are here figured (fig. 126). The graceful Page 184 netted fronds of Fenestella, Retepora, and Polypora (fig. 126, a) are highly characteristic, as are the slender toothed branches of Glauconome (fig. 126, b). A more singular form, however, is the curious Archimedes (fig. 126, c), which is so characteristic of the Carboniferous formation of North America. In this remarkable type, the colony consists of a succession of funnel-shaped fronds, essentially similar to Fenestella in their structure, springing in a continuous spiral from a strong screw-like vertical axis. The outside of the fronds is simply striated; but the branches exhibit on the interior the mouths of the little cells in which the semi-independent beings composing the colony originally lived.

The Brachiopods are extremely abundant, and for the most part belong to types which are exclusively or principally Palæozoic in their range. The old genera Strophomena, Orthis (fig. 127, c), Athyris (fig. 127, e), Rhynchonella (fig. 127, g), and Spirifera (fig. 127, h), are still well represented—the latter, in particular, existing under numerous specific forms, conspicuous by their abundance and sometimes by their size. Along with these ancient groups, we have representatives—for the first time in any plenty—of the great genus Terebratula (fig. 127, d), which underwent a great expansion during later periods, and still exists at the present day. The most characteristic Carboniferous Brachiopods, however, belong to the family of the Productidœ, of which the principal genus is Producta itself. This family commenced its existence in the Upper Silurian with the genus Chonetes, distinguished by its spinose hinge-margin. This genus lived through the Devonian, and flourished in the Carboniferous (fig. 127, f). The genus Producta itself, represented in the Devonian by the nearly allied Productella, appeared first in the Carboniferous, at any rate, in force, and survived into the Permian; but no member of this extensive family has yet been shown to have over-lived the Palæozoic period. The Productœ of the Carboniferous are not only exceedingly abundant, but they have in many instances a most extensive geographical range, and some species attain what may fairly be considered-gigantic dimensions. The shell (fig. 127, a and b) is generally more or less semicircular, with a straight hinge-margin, and having its lateral angles produced into larger or smaller ears (hence its generic name—"cochlea producta"). One valve (the ventral) is usually strongly convex, whilst the other (the dorsal) is flat or concave, the surface of both being adorned with radiating ribs, and with hollow tubular spines, often of great length. The valves are not locked together by teeth, and there is no sign in the Page 185 fully-grown shell of an opening in or between the valves for the emission of a muscular stalk for the attachment of the shell to foreign objects. It is probable, therefore, that the Productœ, unlike the ordinary Lamp-shells, lived an independent existence, their long spines apparently serving to anchor them firmly in the mud or ooze of the sea-bottom; but Mr Robert Etheridge, jun.; has recently shown that in one species

Fig. 127.—Carboniferous Braciopoda. a, Producta semireticulata, showing the slightly concave dorsal valve; a' Side view of the same, showing the convex ventral valve; b, Producta longispina; c, Orthis resupinata; d, Terebratula hastata; e, Athyris subtilita; f, Chonetes Hardrensis; g, Rhynchonella pleurodon; h, Spirifera trigonalis. Most of these forms are widely distributed in the Carboniferous Limestone of Britain, Europe, America, &c. All the figures are of the natural size. (After Davidson, De Koninck, and Meek.) the spines were actually employed as organs of adhesion, whereby the shell was permanently attached to some extraneous object, such as the stem of a Crinoid. The two species here figured are interesting for their extraordinarily extensive geographical range—Producta semireticulata (fig. 127, a) being found in the Carboniferous rocks of Britain, the continent of Europe, Central Asia, China, India, Australia, Spitzbergen, and North Page 186 and South America; whilst P. Longispina (fig. 127, b) has a distribution little if at all less wide.

The higher Mollusca are abundantly represented in the Carboniferous rocks by Bivalves (Lamellibranchs), Univalves (Gasteropoda), Winged-snails (Pteropoda), and Cephalopods. Amongst the Bivalves we may note the great abundance of Scallops (Aviculopecten and other allied forms), together with numerous other types—some of ancient origin, others represented here for the first time. Amongst the Gasteropods, we find the characteristically Palæozoic genera Macrocheilus and Loxonema, the almost exclusively Palæozoic Euomphalus, and the persistent, genus Pleurotomaria; whilst the free-swimming Univalves (Heteropoda)are represented by Bellerophon and Porcellia, and the Pteropoda by the old genus Conularia. With regard to the Carboniferous Univalves, it is also of interest to note here the first appearance of true air-breathing or terrestrial Molluscs, as discovered by Dawson and Bradley in the Coal-measures of Nova Scotia and Illinois. Some of these (Conulus priscus) are true Land-snails, resembling the existing Zonites; whilst others (Pupa vetusta, fig. 128) appear to be generically inseparable from

Fig. 128.—Pupa (Dendropupa) vetusta, a Carboniferous Land-snail from the Coal-measures of Nova Scotia. a, The shell, of the natural size; b, The same, magnified; c, Apex of the shell, enlarged; d, Portion of the surface, enlarged. (After Dawson.) the "Chrysalis-shells" (Pupa) of the present day. All the known forms—three in number—are of small size, and appear to have been local in their distribution or in their preservation. More important, however, than any of the preceding, are the Cephalopoda, represented, as before, exclusively by the chambered shells of the Tetrabranchiates. The older and simpler type of these, with simple plain septa, and mostly a central siphuncle, is represented by the straight conical shells of the ancient genus Orthoceras, and the bow-shaped shells of the equally ancient Cyrtoceras—some of the former attaining a great size. The spirally-curved discoidal shells of the persistent genus Nautilus are also not unknown, and some of these likewise exhibit very considerable dimensions. Lastly, the more complex family of the Ammonitidœ, Page 187 with lobed or angulated septa, and a dorsally-placed siphuncle (situated on the convex side of the curved shells), now for the first time commences to acquire a considerable prominence. The principal representative of this group is the genus Goniatites (fig. 129), which commenced its existence in the Upper Silurian, is well represented in the Devonian, and attains its maximum here. In this genus, the shell is spirally curved, the septa are strongly lobed or angulated, though not elaborately frilled as in the Ammonites, and the siphuncle is dorsal. In addition to Goniatites, the shells of true Ammonites, so characteristic of the Secondary period, have been described by Dr Waagen as occurring in the Carboniferous rocks of India.

Fig. 129.—Goniatites (Aganides) Fossœ. Carboniferous Limestone.

Coming finally to the Vertebrata, we have in the first place to very briefly consider the Carboniferous fishes. These are numerous; but, with the exception of the still dubious "Conodonts," belong wholly to the groups of the Ganoids and the Placoids (including under the former head remains which perhaps are truly referable to the group of the Dipnoi or Mud-fishes). Amongst the Ganoids, the singular buckler-headed fishes of the Upper Silurian and Devonian (Cephalaspidœ) Page 188 have apparently disappeared; and the principal types of the Carboniferous belong to the groups respectively represented at the present day by the Gar pike (Lepidosteus) of the North American lakes, and the Polypterus of the rivers of Africa. Of the former, the genera Palœoniscus and Amblypterus (fig. 130), with their small rhomboidal and

Fig. 130.—Amblypterus macropterus. enamelled scales, and their strongly unsymmetrical tails, are perhaps the most abundant. Of the latter, the most important are species belonging to the genera Megalichthys and Rhizodus, comprising large fishes, with rhomboidal scales, unsymmetrical ("heterocercal") tails, and powerful conical teeth. These fishes are sometimes said to be "sauroid," from their presenting some Reptilian features in their organisation, and they must have been the scourges of the Carboniferous seas. The remains of Placoid fishes in the Carboniferous strata are very numerous, but consist wholly of teeth and fin-spines, referable to forms more or less closely allied to our existing Port Jackson Sharks, Dog-fishes, and Rays. The teeth are of very various shapes and sizes,—some with sharp, cutting edges (Petalodus, Cladodus, &c.); others in the form of broad crushing plates, adapted, like the teeth of the existing Port Jackson Shark (Cestracion Philippi), for breaking down the hard shells of Molluscs and Crustaceans. Amongst the many kinds of these latter, the teeth of Psammodus and Cochliodus (fig. 131) may be mentioned as specially characteristic. The fin-spines are mostly similar to those so common in the Devonian deposits, consisting of hollow defensive spines implanted in front of the pectoral or other fins, usually slightly curved, often superficially ribbed or sculptured, and not uncommonly serrated or toothed. The genera Ctenacanthus, Gyracanthus, Homacanthus, &c., have been founded for the reception of these defensive weapons, some of which indicate fishes of great size and predaceous habits.

Page 189 In the Devonian rocks we meet with no other remains of Vertebrated animals save fishes only; but the Carboniferous deposits have yielded remains of the higher group

Fig. 131.—Teeth of Cochliodus contortus. Carboniferous Limestone, Britain. of the Amphibians. This class, comprising our existing Frogs, Toads, and Newts, stands to some extent in a position midway between the class of the fishes and that of the true reptiles, being distinguished from the latter by the fact that its members invariably possess gills in their early condition, if not throughout life; whilst they are separated from the former by always possessing true lungs when adult, and by the fact that the limbs (when present at all) are never in the form of fins. The Amphibians, therefore, are all water-breathers when young, and have respiratory organs adapted for an aquatic mode of life; whereas, when grown up, they develop lungs, and with these the capacity for breathing air directly. Some of them, like the Frogs and Newts, lose their gills altogether on attaining the adult condition; but others, such as the living Proteus and Menobranchus, retain their gills even after acquiring their lungs, and are thus fitted indifferently for an aquatic or terrestrial existence. The name of "Amphibia," though applied to the whole class, is thus not precisely appropriate except to these last-mentioned forms (Gr. amphi, both; bios, life). The Amphibians also differ amongst themselves according as to whether they keep permanently the long tail which they all possess when young (as do the Newts and Salamanders), or lose this appendage when grown up (as do the Frogs and Toads). Most of them have naked skins, but a few living and many extinct forms have hard structures in the shape of scales developed in the integument. All of them have well-ossified skeletons, though some fossil types are partially deficient in this respect; and all of them which possess limbs at all have these appendages supported by bones essentially similar to those found in the limbs of the higher Vertebrates. All the Carboniferous Amphibians belong to a group which has now wholly passed away—namely, that of the Labyrinthodonts. In the marine strata which form the base of the Carboniferous series these creatures have only been recognised by their curious hand-shaped footprints, similar Page 190 in character to those which occur in the Triassic rocks, and which will be subsequently spoken of under the name of Cheirotherium. In the Coal-measures of Britain, the continent of Europe, and North America, however, many bones of these animals have been found, and we are now tolerably well acquainted with a considerable number of forms. All of them seem to have belonged to the division of Amphibians in which the long tail of the young is permanently retained; and there is evidence that some of them kept the gills also throughout life. The skull is of the characteristic Amphibian type (fig. 132, a), with two occipital condyles, and having its surface

Fig. 132.—a, Upper surface of the skull of Anthracosaurus Russelli, one-sixth of the natural size: b, Part of one of the teeth cut across, and highly magnified to show the characteristic labyrinthine structure; c, One of the integumentary shields or scales, one-half of the natural size. Coal-measures, Northumberland. (After Atthey.) singularly pitted and sculptured; and the vertebræ are hollowed out at both ends. The lower surface of the body was defended by an armour of singular integumentary shields or scales (fig. 132, c); and an extremely characteristic feature (from which the entire group derives its name) is, that the walls of the teeth are deeply folded, so as to give rise to an extraordinary "labyrinthine" pattern when they are cut across (fig. 132, b). Many of the Carboniferous Labyrinthodonts are of no great size, some of them Page 191 very small, but others attain comparatively gigantic dimensions, though all fall short in this respect of the huge examples of this group which occur in the Trias. One of the largest, and at the same time most characteristic, forms of the Carboniferous series, is the genus Anthracosaurus, the skull of which is here figured.

No remains of true Reptiles, Birds, or Quadrupeds have as yet been certainly detected in the Carboniferous deposits in any part of the world. It should, however, be mentioned, that Professor Marsh, one of the highest authorities on the subject, has described from the Coal-formation of Nova Scotia certain vertebræ which he believes to have belonged to a marine reptile (Eosaurus Acadianus), allied to the great Ichthyosauri of the Lias. Up to this time no confirmation of this determination has been obtained by the discovery of other and more unquestionable remains, and it therefore remains doubtful whether these bones of Eosaurus may not really belong to large Labyrinthodonts.

LITERATURE.

The following list contains some of the more important of the original sources of information to which the student of Carboniferous rocks and fossils may refer:—

Also numerous memoirs by Huxley, Davidson, Martin Duncan, Professor Young, John Young, R. Etheridge, jun., Baily, Carruthers, Dawson, Binney, Williamson, Hooker, Jukes, Geikie, Rupert Jones, Salter, and many other British and foreign observers.

CHAPTER XIV.

The Permian formation closes the long series of the Palæozoic deposits, and may in some respects be considered as a kind of appendix to the Carboniferous system, to which it cannot be compared in importance, either as regards the actual bulk of its sediments or the interest and variety of its life-record. Consisting, as it does, largely of red rocks—sandstones and marls—for the most part singularly destitute of organic remains, the Permian rocks have been regarded as a lacustrine or fluviatile deposit; but the presence of well-developed limestones with indubitable marine remains entirely negatives this view. It is, however, not improbable that we are presented in the Permian formation, as known to us at present, with a series of sediments laid down in inland seas of great extent, due to the subsidence over large areas of the vast land-surfaces of the Coal-measures. This view, at any rate, would explain some of the more puzzling physical characters of the formation, and would not be definitely negatived by any of its fossils.

A large portion of the Permian series, as already remarked, consists of sandstones and marls, deeply reddened by peroxide Page 193 of iron, and often accompanied by beds of gypsum or deposits of salt. In strata of this nature few or no fossils are found; but their shallow-water origin is sufficiently proved by the presence of the footprints of terrestrial animals, accompanied in some cases by well-defined "ripple-marks." Along with these are occasionally found massive breccias, holding larger or smaller blocks derived from the older formations; and these have been supposed to represent an old "boulder-clay," and thus to indicate the prevalence of an arctic climate. Beds of this nature must also have been deposited in shallow water. In all regions, however, where the Permian formation is well developed, one of its most characteristic members is a Magnesian limestone, often highly and fantastically concretionary, but containing numerous remains of genuine marine animals, and clearly indicating that it was deposited beneath a moderate depth of salt water.

It is not necessary to consider here whether this formation can be retained as a distinct division of the geological series. The name of Permian was given to it by Sir Roderick Murchison, from the province of Perm in Russia, where rocks of this age are extensively developed. Formerly these rocks were grouped with the succeeding formation of the Trias under the common name of "New Red Sandstone." This name was given them because they contain a good deal of red sandstone, and because they are superior to the Carboniferous rocks, while the Old Red Sandstone is inferior. Nowadays, however, the term "New Red Sandstone" is rarely employed, unless it be for red sandstones and associated rocks, which are seen to overlie the Coal-measures, but which contain no fossils by which their exact age may be made out. Under these circumstances, it is sometimes convenient to employ the term "New Red Sandstone." The New Red, however, of the older geologists, is now broken up into the two formations of the Permian and Triassic rocks—the former being usually considered as the top of the Palæozoic series, and the latter constituting the base of the Mesozoic.

In many instances, the Permian rocks are seen to repose unconformably upon the underlying Carboniferous, from which they can in addition be readily separated by their lithological characters. In other instances, however, the Coal-measures terminate upwards in red rocks, not distinguishable by their mineral characters from the Permian; and in other cases no physical discordance between the Carboniferous and Permian strata can be detected. As a general rule, also, the Permian rocks appear to pass upwards conformably into the Page 194 Trias. The division, therefore, between the Permian and Triassic rocks, and consequently between the Palæozoic and Mesozoic series, is not founded upon any conspicuous or universal physical break, but upon the difference in life which is observed in comparing the marine animals of the Carboniferous and Permian with those of the Trias. It is to be observed, however, that this difference can be solely due to the fact that the Magnesian Limestone of the Permian series presents us with only a small, and not a typical, portion of the marine deposits which must have been accumulated in some area at present unknown to us during the period which elapsed between the formation of the great marine limestones of the Lower Carboniferous and the open-sea and likewise calcareous sediments of the Middle Trias.

The Permian rocks exhibit their most typical features in Russia and Germany, though they are very well developed in parts of Britain, and they occur in North America. When well developed, they exhibit three main divisions: a lower set of sandstones, a middle group, generally calcareous, and an upper series of sandstones, constituting respectively the Lower, Middle, and Upper Permians.

In Russia, Germany, and Britain, the Permian rocks consist of the following members:—

1. The Lower Permians, consisting mainly of a great series of sandstones, of different colours, but usually red. The base of this series is often constituted by massive breccias with included fragments of the older rocks, upon which they may happen to repose; and similar breccias sometimes occur in the upper portion of the series as well. The thickness of this group varies a good deal, but may amount to 3000 or 4000 feet.

2. The Middle Permians, consisting, in their typical development, of laminated marls, or "marl-slate," surmounted by beds of magnesian limestone (the "Zechstein" of the German geologists). Sometimes the limestones are degenerate or wholly deficient, and the series may consist of sandy shales and gypsiferous clays. The magnesian limestone, however, of the Middle Permians is, as a rule, so well marked a feature that it was long spoken of as the Magnesian Limestone.

3. The Upper Permians, consisting of a series of sandstones and shales, or of red or mottled marls, often gypsiferous, and sometimes including beds of limestone.

In North America, the Permian rocks appear to be confined to the region west of the Mississippi, being especially well developed in Kansas. Their exact limits have not as yet been Page 195 made out, and their total thickness is not more than a few hundred feet. They consist of sandstones, conglomerates, limestones, marls, and beds of gypsum.

The following diagrammatic section shows the general sequence of the Permian deposits in the north of England, where the series is extensively developed (fig. 133):—

GENERALISED SECTION OF THE PERMIAN ROCKS IN THE NORTH OF ENGLAND.

Fig. 133.

The record of the life of the Permian period is but a scanty one, owing doubtless to the special peculiarities of such of the Page 196 deposits of this age with which we are as yet acquainted. Red rocks are, as a general rule, more or less completely unfossiliferous, and sediments of this nature are highly characteristic of the Permian. Similarly, magnesian limestones are rarely as highly charged with organic remains as is the case with normal calcareous deposits, especially when they have been subjected to concretionary action, as is observable to such a marked extent in the Permian limestones. Nevertheless, much interest is attached to the organic remains, as marking a kind of transition-period between the Palæozoic and Mesozoic epoch

(1)	'Report of Progress of the Geological Survey of Canada from its Commencement to 1863,' pp. 38-49, and pp. 50-66.
(2)	'Manual of Geology.' Dana. 2d Ed. 1875.
(3)	'The Dawn of Life.' J. W, Dawson. 1876.
(4)	"On the Occurrence of Organic Remains in the Laurentian Rocks of Canada." Sir W. E. Logan. 'Quart. Journ. Geol.' Soc.,' xxi. 45-50.'
(5)	"On the Structure of Certain Organic Remains in the Laurentian Limestones of Canada." J. W. Dawson. 'Quart. Journ. Geol. Soc.,' xxi. 51-59.
(6)	"Additional Note on the Structure and Affinities of Eozoön Canadense." W. B, Carpenter. 'Quart. Journ. Geol. Soc.,' xxi. 59-66.
(7)	"Supplemental Notes on the Structure and Affinities of Eozoön' Canadense," W. B. Carpenter, 'Quart. Journ. Geol. Soc.,' xxii. 219-228.
(8)	"On the So-Called Eozoönal Rocks." King & Rowney. 'Quart. Journ. Geol. Soc.,' xxii. 185-218.
(9)	'Chemical and Geological Essays.' Sterry Hunt.

(1)	'Siluria.' Sir Roderick Murchison. 5th ed., pp. 21-46.
(2)	'Synopsis of the Classification of the British Palæozoic Rocks.' Sedgwick. Introduction to the 3d Fasciculus of the 'Descriptions of British Palæozoic Fossils in the Woodwardian Museum,' by F. M'Coy, pp. i-xcviii, 1855.
(3)	'Catalogue of the Cambrian and Silurian Fossils in the Geological Museum of the University of Cambridge.' Salter. With a Preface by Prof. Sedgwick. 1873.
(4)	'Thesaurus Siluricus.' Bigsby. 1868.
(5)	"History of the Names Cambrian and Silurian." Sterry Hunt.—'Geological Magazine.' 1873.
(6)	'Système Silurien du Centre de la Bohême.' Barrande. Vol. I.
(7)	'Report of Progress of the Geological Survey of Canada, from its Commencement to 1863,' pp. 87-109.
(8) Page 90	'Acadian Geology.' Dawson. Pp. 641-657.
(9)	"Guide to the Geology of New York," Lincklaen; and "Contributions to the Palæontology of New York," James Hall.—'Fourteenth Report on the State Cabinet.' 1861.
(10)	'Palæozoic Fossils of Canada.' Billings. 1865.
(11)	'Manual of Geology.' Dana. Pp. 166-182. 2d ed. 1875.
(12)	"Geology of North Wales," Ramsay; with Appendix on the Fossils, Salter.—'Memoirs of the Geological Survey of Great Britain,' vol. iii. 1866.
(13)	"On the Ancient Rocks of the St David's Promontory, South Wales, and their Fossil Contents." Harkness and Hicks.—'Quart. Journ. Geol. Soc.,' xxvii. 384-402. 1871.
(14)	"On the Tremadoc Rocks in the Neighbourhood of St David's, South Wales, and their Fossil Contents." Hicks.—'Quart. Journ. Geol. Soc.,' xxix. 39-52. 1873.

(1)	'Siluria.' Sir Roderick Murchison.
(2)	'Geology of Russia in Europe.' Murchison (with M. de Verneuil and Count von Keyserling).
(3)	'Bassin Silurien de Bohême Centrale.' Barrande.
(4)	'Introduction to the Catalogue of British Palæozoic Fossils in the Woodwardian Museum of Cambridge.' Sedgwick.
(5)	'Die Urwelt Russlands.' Eichwald.
(6)	'Report on the Geology of Londonderry, Tyrone,' &c. Portlock.
(7)	"Geology of North Wales"—'Mem. Geol. Survey of Great Britain,' vol. iii. Ramsay.
(8)	'Geology of Canada,' 1863. Sir W. E. Logan; and the 'Reports of Progress of the Geological Survey' since 1863.
(9)	'Memoirs of the Geological Survey of Great Britain.'
(10)	'Reports of the Geological Surveys of the States of New York, Illinois, Ohio, Iowa, Michigan, Vermont, Wisconsin, Minnesota,' &c. By Emmons, Hall, Worthen, Meek, Newberry, Orton, Winchell, Dale Owen, &c.
(11)	'Thesaurus Siluricus.' Bigsby.
(12)	'British Palæozoic Fossils.' M'Coy.
(13)	'Synopsis of the Silurian Fossils of Ireland,' M'Coy.
(14)	"Appendix to the Geology of North Wales"—'Mem. Geol. Survey,' vol. iii. Salter.
Page 132 (15)	'Catalogue of the Cambrian and Silurian Fossils in the Woodwardian Museum of Cambridge.' Salter.
(16)	'Characteristic British Fossils.' Baily.
(17)	'Catalogue of British Fossils.' Morris.
(18)	'Palæozoic Fossils of Canada.' Billings.
(19)	'Decades of the Geological Survey of Canada.' Billings, Salter, Rupert Jones.
(20)	'Decades of the Geological Survey of Great Britain.' Salter, Edward, Forbes.
(21)	'Palæontology of New York,' vols. i.-iii. Hall.
(22)	'Palæontology of Illinois.' Meek and Worthen.
(23)	'Palæontology of Ohio.' Meek, Hall, Whitfield, Nicholson.
(24)	'Silurian Fauna of West Tennessee' (Silurische Fauna des Westlichen Tennessee). Ferdinand Rœmer.
(25)	'Reports on the State Cabinet of New York.' Hall.
(26)	'Lethæa Geognostica.' Bronn.
(27)	'Index Palæontologicus.' Bronn.
(28)	'Lethæa Rossica.' Eichwald.
(29)	'Lethæa Suecica.' Hisinger.
(30)	'Palæontologica Suecica.' Angelin.
(31)	'Petrefacta Germaniæ.' Goldfuss.
(32)	'Versteinerungen der Grauwacken-Formation in Sachsen.' Geinitz.
(33)	'Organisation of Trilobites' (Ray Society). Burmeister.
(34)	'Monograph of the British Trilobites' (Palæontographical Society). Salter.
(35)	'Monograph of the British Merostomata' (Palæontographical Society). Henry Woodward.
(36)	'Monograph of British Brachiopoda' (Palæontographical Society). Thomas Davidson.
(37)	'Graptolites of the Quebec Group.' James Hall.
(38)	'Monograph of the British Graptolitidæ.' Nicholson.
(39)	'Monographs on the Trilobites. Pteropods, Cephalopods, Graptolites,' &c. Extracted from the 'Système Silurien du Centre de la Bohême.' Barrande.
(40)	'Polypiers Fossiles des Terrains Paleozoiques,' and 'Monograph of the British Corals' (Palæontographical Society). Milne Edwards and Jules Haime.

(1)	'Siluria.' Sir Roderick Murchison.
(2)	'Geology of Russia in Europe.' Murchison (together with De Verneuil and Count von Keyserling).
(3)	"Classification of the Older Rocks of Devon and Cornwall"—'Proc. Geol. Soc.,' vol. iii., 1839. Sedgwick and Murchison.
(4)	"On the Physical Structure of Devonshire;" and on the "Classification of the Older Stratified Rocks of Devonshire and Cornwall"—'Trans. Geol. Soc.,' vol. v., 1840. Sedgwick and Murchison.
(5)	"On the Distribution and Classification of the Older or Palæozoic Rocks of North Germany and Belgium"—'Geol. Trans.,' 2d ser., vol. vi., 1842. Sedgwick and Murchison.
(6)	'Report on the Geology of Cornwall, Devon, and West Somerset.' De la Beche.
(7)	'Memoirs of the Geological Survey of Ireland and Scotland.' Jukes and Geikie.
(8)	"On the Carboniferous Slate (or Devonian Rocks) and the Old Red Sandstone of South Ireland and North Devon"—'Quart. Journ. Geol. Soc.,' vol. xxii. Jukes.
(9)	"On the Physical Structure of West Somerset and North Devon;" and on the "Palæontological Value of Devonian Fossils"—'Quart. Journ. Geol. Soc.,' vol. iii. Etheridge.
(10)	"On the Connection of the Lower, Middle, and Upper Old Red Sandstone of Scotland"—'Trans. Edin. Geol. Soc.,' vol. i. part ii. Powrie.
(11)	'The Old Red Sandstone,' 'The Testimony of the Rocks,' and 'Footprints of the Creator.' Hugh Miller.
(12)	"Report on the 4th Geological District"—'Geology of New York,' vol. iv. James Hall.
(13)	'Geology of Canada,' 1863. Sir W. E. Logan.
(14)	'Acadian Geology.' Dawson.
(15)	'Manual of Geology.' Dana.
(16)	'Geological Survey of Ohio,' vol. i.
(17)	'Geological Survey of Illinois,' vol. i.
(18)	'Palæozoic Fossils of Cornwall, Devon, and West Somerset.' Phillips.
(19)	'Recherches sur les Poissons Fossiles.' Agassiz.
(20)	'Poissous de l'Old Red.' Agassiz.
(21)	"On the Classification of Devonian Fishes"—'Mem. Geol. Survey of Great Britain,' Decade X. Huxley.
Page 157 (22)	'Monograph of the Fishes of the Old Red Sandstone of Britain' (Palæontographical Society). Powrie and Lankester.
(23)	'Fishes of the Devonian System, Palæontology of Ohio.' Newberry.
(24)	'Monograph of British Trilobites' (Palæontographical Society); Salter.
(25)	'Monograph of British Merostomata' (Palæontographical Society). Henry Woodward.
(26)	'Monograph of British Brachiopoda' (Palæontographical Society). Davidson.
(27)	'Monograph of British Fossil Corals' (Palæontographical Society). Milne-Edwards and Haime.
(28)	'Polypiers Foss. des Terrains Paléozoiques.' Milne-Edwards and Jules Haime.
(29)	"Devonian Fossils of Canada West"—'Canadian Journal,' new ser., vols. iv.-vi. Billings.
(30)	'Palæontology of New York,' vol. iv. James Hall.
(31)	'Thirteenth, Fifteenth, and Twenty-third Annual Reports on the State Cabinet.' James Hall.
(32)	'Palæozoic Fossils of Canada,' vol. ii. Billings.
(33)	'Reports on the Palæontology of the Province of Ontario for 1874 and 1875.' Nicholson.
(34)	"The Fossil Plants of the Devonian and Upper Silurian Formations of Canada"—'Geol. Survey of Canada.' Dawson.
(35)	'Petrefacta Germaniæ.' Goldfuss.
(36)	'Versteinerungen der Grauwacken-formation.' &c. Geinitz.
(37)	'Beitrag zur Palæontologie des Thüringer-Waldes.' Richter and Unger.
(38)	'Ueber die Placodermen der Devonischen System.' Pander.
(39)	'Die Gattungen der Fossilen Pflanzen.' Gœppert.
(40)	'Genera et Species Plantarum Fossilium.' Unger.

(1)	'Geology of Yorkshire,' vol. ii.; 'The Mountain Limestone District.' John Phillips.
(2)	'Siluria.' Sir Roderick Murchison.
(3)	'Memoirs of the Geological Survey of Great Britain and Ireland.'
(4)	'Geological Report on Londonderry,' &c. Portlock.
(5)	'Acadian Geology.' Dawson.
(6)	'Geology of Iowa,' vol. i. James Hall.
(7)	'Reports of the Geological Survey of Illinois' (Geology and Palæontology). Meek, Worthen, &c.
(8)	'Reports of the Geological Survey of Ohio' (Geology and Palæontology). Newberry, Cope, Meek, Hall, &c.
(9)	'Description des Animaux fossiles qui se trouvent dans le Terrain Carbonifère de la Belgique,' 1843; with subsequent monographs on the genera Productus and Chonetes, on Crinoids, on Corals, &c. De Koninck.
(10)	'Synopsis of the Carboniferous Fossils of Ireland.' M'Coy.
(11)	'British Palæozoic Fossils.' M'Coy.
(12)	'Figures of Characteristic British Fossils.' Baily.
(13)	'Catalogue of British Fossils.' Morris.
(14)	'Monograph of the Carboniferous Brachiopoda of Britain' (Palæontographical Society). Davidson.
(15)	'Monograph of the British Carboniferous Corals' (Palæontographical Society). Milne-Edwards and Haime.
(16)	'Monograph of the Carboniferous Bivalve Entomostraca of Britain' (Palæontographical Society). Rupert Jones, Kirkby, and George S. Brady.
Page 192 (17)	'Monograph of the Carboniferous Foraminifera of Britain' (Palæontographical Society). H. B. Brady.
(18)	"On the Carboniferous Fossils of the West of Scotland"—'Trans. Geol. Soc.,' of Glasgow, vol. iii., Supplement. Young and Armstrong.
(19)	'Poissons Fossiles.' Agassiz.
(20)	"Report on the Labyrinthodonts of the Coal-measures"—'British Association Report,' 1873. L. C. Miall.
(21)	'Introduction to the Study of Palæontological Botany.' John Hutton Balfour.
(22)	'Traité de Paléontologie Végétale.' Schimper.
(23)	'Fossil Flora.' Lindley and Hutton.
(24)	'Histoire des Végétaux Fossiles.' Brongniart.
(25)	'On Calamites and Calamodendron' (Monographs of the Palæontographical Society). Binney.
(26)	'On the Structure of Fossil Plants found in the Carboniferous Strata' (Palæontographical Society). Binney.

FIG.
1.	Cast of Trigonia longa.
2.	Microscopic section of the wood of a fossil Conifer.
3.	Microscopic section of the wood of the Larch.
4.	Section of Carboniferous strata, Kinghorn, Fife.
5.	Diagram illustrating the formation of stratified deposits.
6.	Microscopic section of a calcareous breccia.
7.	Microscopic section of White Chalk.
8.	Organisms in Atlantic Ooze.
9.	Crinoidal marble.
10.	Piece of Nummulitic limestone, Pyramids.
11.	Microscopic section of Foraminiferal limestone—Carboniferous, America.
12.	Microscopic section of Lower Silurian limestone.
13.	Microscopic section of oolitic limestone, Jurassic.
14.	Microscopic section of oolitic limestone, Carboniferous.
15.	Organisms in Barbadoes earth.
15.	Organisms in Barbadoes earth.
16.	Organisms in Richmond earth.
17.	Ideal section of the crust of the earth.
18.	Unconformable junction of Chalk and Eocene rocks.
19.	Erect trunk of a Sigillaria.
20.	Diagrammatic section of the Laurentian rocks
21.	Microscopic section of Laurentian limestone.
22.	Fragment of a mass of Eozoön Canadense.
23.	Diagram illustrating the structure of Eozoön.
24.	Microscopic section of Eozoön Canadense.
25.	Nonionina and Gromia.
26.	Group of shells of living Foraminifera.
27.	Diagrammatic section of Cambrian strata.
28.	Eophyton Linneanum.
29.	Oldhamia antiqua.
30.	Scolithus Canadensis.
31.	Group of Cambrian Trilobites.
32.	Group of characteristic Cambrian fossils.
33.	Fragment of Dictyonema sociale.
34.	Generalised section of the Lower Silurian rocks of Wales.
35.	Generalised section of the Lower Silurian rocks of North America.
36.	Licrophycus Ottawaensis.
37.	Astylospongia prœmorsa.
38.	Stromatopora rugosa.
39.	Dichograptus octobrachiatus.
40.	Didymograptus divaricatus. Page xvi
41.	Diplograptus pristis.
42.	Phyllograptus typus.
43.	Zaphrentis Stokesi.
44.	Strombodes pentagonus.
45.	Columnaria alveolata.
46.	Group of Cystideans.
47.	Group of Lower Silurian Crustaceans.
48.	Ptilodictya falciformis.
49.	Ptilodictya Schafferi.
50.	Group of Lower Silurian Brachiopods.
51.	Group of Lower Silurian Brachiopods.
52.	Murchisonia gracilis.
53.	Bellerophon argo.
54.	Maclurea crenulata.
55.	Orthoceras crebriseptum.
56.	Restoration of Orthoceras.
57.	Generalised section of the Upper Silurian rocks.
58.	Monograptus priodon.
59.	Halysites catenularia and H. agglomerata.
60.	Group of Upper Silurian Star-fishes.
61.	Protaster Sedgwickii.
62.	Group of Upper Silurian Crinoids.
63.	Planolites vulgaris.
64.	Group of Upper Silurian Trilobites.
65.	Pterygotus Anglicus.
66.	Group of Upper Silurian Polyzoa.
67.	Spirifera hysterica.
68.	Group of Upper Silurian Brachiopods.
69.	Group of Upper Silurian Brachiopods.
70.	Pentamerus Knightii.
71.	Cardiola interrupta, C. fibrosa, and Pterinœa subfalcata.
72.	Group of Upper Silurian Univalves.
73.	Tentaculites ornatus.
74.	Pteraspis Banksii.
75.	Onchus tenuistriatus and Thelodus.
76.	Generalised section of the Devonian rocks of North America.
77.	Psilophyton princeps.
78.	Prototaxites Logani.
79.	Stromatopora tuberculata.
80.	Cystiphyllum vesiculosum.
81.	Zaphrentis cornicula.
82.	Heliophyllum exiguum.
83.	Crepidophyllum Archiaci.
84.	Favosites Gothlandica.
85.	Favosites hemisphœrica.
86.	Spirorbis omphalodes and S. Arkonensis.
87.	Spirorbis laxus and S. Spinulifera.
88.	Group of Devonian Trilobites.
89.	Wing of Platephemera antiqua.
90.	Clathropora intertexta.
91.	Ceriopora Hamiltonensis.
92.	Fenestella magnifica.
93.	Retepora Phillipsi.
94.	Fenestella cribrosa.
95.	Spirifera sculptilis.
96.	Spirifera mucronata.
97.	Atrypa reticularis.
98.	Strophomena rhomboidalis.
99.	Platyceras dumosum.
100.	Conularia ornata.
101.	Clymenia Sedgwickii.
102.	Group of Fishes from the Devonian rocks of North America.
103.	Cephalaspis Lyellii.
104.	Pterichthys cornutus.
105.	Polypterus and Osteolepis.
106.	Holoptychius nobilissimus.
107.	Generalised section of the Carboniferous rocks of the North of England.
108.	Odontopteris Schlotheimii.
109.	Calamites cannœformis.
110.	Lepidodendron Sternbergii.
111.	Sigillaria Grœseri.
112.	Stigmaria ficoides.
113.	Trigonocarpum ovatum.
114.	Microscopic section of Foraminiferal limestone—Carboniferous, North America.
115.	Fusulina cylindrica.
116.	Group of Carboniferous Corals.
117.	Platycrinus tricontadactylus.
118.	Pentremites pyriformis and P. conoideus. Page xvii
119.	Archœocidaris ellipticus.
120.	Spirorbis Carbonarius.
121.	Prestwichia rotundata.
122.	Group of Carboniferous Crustaceans.
123.	Cyclophthalmus senior.
124.	Xylobius Sigillariœ.
125.	Haplophlebium Barnesi.
126.	Group of Carboniferous Polyzoa.
127.	Group of Carboniferous Brachiopoda.
128.	Pupa vetusta.
129.	Goniatites Fossœ.
130.	Amblypterus macropterus.
131.	Cochliodus contortus.
132.	Anthracosaurus Russelli.
133.	Generalised section of the Permian rocks.
134.	Walchia piniformis.
135.	Group of Permian Brachiopods.
136.	Arca antiqua.
137.	Platysomus gibbosus.
138.	Protorosaurus Speneri.
139.	Generalised section of the Triassic rocks.
140.	Zamia spiralis.
141.	Triassic Conifers and Cycads.
142.	Encrinus liliiformis.
143.	Aspidura loricata.
144.	Group of Triassic Bivalves.
145.	Ceratites nodosus.
146.	Tooth of Ceratodus serratus and C. Altus.
147.	Ceratodus Fosteri.
148.	Footprints of Cheirotherium.
149.	Section of tooth of Labyrinthodont.
150.	Skull of Mastodonsaurus.
151.	Skull of Rhynchosaurus.
152.	Belodon, Nothosaurus, Palœosaurus, &c.
153.	Placodus gigas.
154.	Skulls of Dicynodon and Oudenodon.
155.	Supposed footprint of Bird, from the Trias of Connecticut.
156.	Lower jaw of Dromatherium sylvestre.
157.	Molar tooth of Microlestes antiquus.
158.	Myrmecobius fasciatus.
159.	Generalised section of the Jurassic rocks.
160.	Mantellia megalophylla.
161.	Thecosmilia annularis.
162.	Pentacrinus fasciculosus.
163.	Hemicidaris crenularis.
164.	Eryon arctiformis.
165.	Group of Jurassic Brachiopods.
166.	Ostrea Marshii.
167.	Gryphœa incurva
168.	Diceras arietina.
169.	Nerinœa Goodhallii.
170.	Ammonites Humphresianus.
171.	Ammonites bifrons.
172.	Beloteuthis subcostata.
173.	Belemnite restored; diagram of Belemnite; Belemnites canaliculata.
174.	Tetragonolepis.
175.	Acrodus nobilis.
176.	Ichthyosaurus communis.
177.	Plesiosaurus dolichodeirus.
178.	Pterodactylus crassirostris.
179.	Ramphorhynchus Bucklandi, restored.
180.	Skull of Megalosaurus.
181.	Archœopteryx macrura.
182.	Archœopteryx, restored.
183.	Jaw of Amphitherium Prevostii.
184.	Jaws of Oolitic Mammals.
185.	Generalised section of the Cretaceous rocks.
186.	Cretaceous Angiosperms.
187.	Rotalia Boueana.
188.	Siphonia ficus.
189.	Ventriculites simplex.
190.	Synhelia Sharpeana.
191.	Galerites albogalerus.
192.	Discoidea cylindrica.
193.	Escharina Oceani.
194.	Terebratella Astieriana.
195.	Crania Ignabergensis.
196.	Ostrea Couloni.
197.	Spondylus spinosus.
198.	Inoceramus sulcatus.
199.	Hippurites Toucasiana.
200.	Voluta elongata.
201.	Nautilus Danicus.
202.	Ancyloceras Matheronianus. Page xviii
203.	Turrilites catenatus
204.	Forms of Cretaceous Ammonitidœ.
205.	Belemnitella mucronata.
206.	Tooth of Hybodus.
207.	Fin-spine of Hybodus.
208.	Beryx Lewesiensis and Osmeroides Mantelli.
209.	Teeth of Iguanodon.
210.	Skull of Mosasaurus Camperi.
211.	Chelone Benstedi.
212.	Jaws and vertebræ of Odontornithes.
213.	Fruit of Nipadites.
214.	Nummulina lœvigata.
215.	Turbinolia sulcata.
216.	Cardita planicosta.
217.	Typhis tubifer.
218.	Cyprœa elegans.
219.	Cerithium hexagonum.
220.	Limnœa pyramidalis.
221.	Physa columnaris.
222.	Cyclostoma Arnoudii.
223.	Rhombus minimus.
224.	Otodus obliquus.
225.	Myliobatis Edwardsii.
226.	Upper jaw of Alligator.
227.	Skull of Odontopteryx toliapicus.
228.	Zeuglodon cetoides.
229.	Palœotherium magnum, restored.
230.	Feet of Equidœ.
231.	Anoplothelium commune.
232.	Skull of Dinoceras mirabilis.
233.	Vespertilio Parisiensis.
234.	Miocene Palms.
235.	Platanus aceroides.
236.	Cinnamomum polymorphum.
237.	Textularia Meyeriana.
238.	Scutella subrotunda.
239.	Hyalea Orbignyana.
240.	Tooth of Oxyrhina.
241.	Tooth of Carcharodon.
242.	Andrias Scheuchzeri.
243.	Skull of Brontotherium ingens.
244.	Hippopotamus Sivalensis.
245.	Skull of Sivatherium.
246.	Skull of Deinotherium.
247.	Tooth of Elephas planfrons and of Mastodon Sivalensis.
248.	Jaw of Pliopithecus.
249.	Rhinoceros Etruscus and R. megarhinus.
250.	Molar tooth of Mastodon Arvernensis.
251.	Molar tooth of Etephas meridionalis.
252.	Molar tooth of Elephas antiquus.
253.	Skull and tooth of Machairodus cultridens.
254.	Pecten Islandicus.
255.	Diagram of high-level and low-level gravels.
256.	Diagrammatic section of Cave.
257.	Dinornis elephantopus.
258.	Skull of Diprotodon.
259.	Skull of Thylacoleo.
260.	Skeleton of Megatherium.
261.	Skeleton of Mylodon.
262.	Glyptodon clavipes.
263.	Skull of Rhinoceros tichorhinus.
264.	Skeleton of Cervus megaceros.
265.	Skull of Bos primigenius.
266.	Skeleton of Mammoth.
267.	Molar tooth of Mammoth.
268.	Skull of Ursus spelœus.
269.	Skull of Hyœna spelœa.
270.	Lower jaw of Trogontherium Cuvieri.

THE ANCIENT LIFE-HISTORY OF THE EARTH

Page v PREFACE.

Page ix CONTENTS.

Page xv LIST OF ILLUSTRATIONS.

Page xix PART I.

Page 1 THE ANCIENT LIFE-HISTORY OF THE EARTH

INTRODUCTION.

CHAPTER I.

CHAPTER II.

CHIEF DIVISIONS OF THE AQUEOUS ROCKS.

Page 37 CHAPTER III.

CHAPTER IV.

CHAPTER V.

Page 57 CHAPTER VI.

Page 63 PART II.

Page 65 CHAPTER VII.

THE HURONIAN PERIOD.

LITERATURE.

Page 77 CHAPTER VIII.

LITERATURE.

CHAPTER IX.

Page 115 CHAPTER X.

LITERATURE.

CHAPTER XI.

LITERATURE.

CHAPTER XII.

CHAPTER XIII.

LITERATURE.

CHAPTER XIV.

THE
ANCIENT LIFE-HISTORY
OF
THE EARTH

Page 1 THE
ANCIENT LIFE-HISTORY
OF
THE EARTH