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PRINCIPLES OF PALAEONTOLOGY.

INTRODUCTION.

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 Palaeontology--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 Palaeontological evidence in stratigraphical Geology--General sequence of the great formations.

The breaks in the palaeontological 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 palaeontological 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.

HISTORICAL PALAEONTOLOGY.

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--Palaeontological 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 Palaeozoic 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 Palaeontology on the general doctrine of Evolution.

GLOSSARY.

PRINCIPLES OF PALAEONTOLOGY.

INTRODUCTION.

THE LAWS OF GEOLOGICAL ACTION.

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 primaeval 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 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 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 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 SCOPE AND MATERIALS OF PALAEONTOLOGY.

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 palaeontology; and though it is possible to proceed to a certain length in geology and zoology without much palaeontological 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 palaeontology without some acquaintance with both geology and natural history.

Palaeontology, 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 palaeontological record is extremely imperfect, and our knowledge is interrupted 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.

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 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.

It only remains to add that there is sometimes a further complication. If the rock be very porous and permeable by 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 . 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 . The whole, however, instead of being composed of the original carbonaceous matter of the wood, is now converted into flint. The only explanation that can be given 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.

THE FOSSILIFEROUS ROCKS.

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 palaeontological 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 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 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 laminae; 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 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.

Lastly, the sea itself--irrespective of the materials delivered into it by rivers--is constantly preparing fresh stratified deposits 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" ; so that we may speak of stratified and unstratified formations, aqueous or igneous formations, fresh-water or marine formations, and so on.

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