Fossils.-Paleontological geology, then, deals with the fossils or organic remains preserved in the rocks, and endeavours to gather from them information as to the history of the globe and its inhabitants. The term "fossil," meaning literally anything "dug up," was formerly applied indiscriminately to any mineral substance taken out of the carth's crust, whether organized or not. Ordinary minerals and rocks were thus included as fossils. For many years, however, the meaning of the word has been restricted, so as to include only the remains or traces of plants and animals preserved in any natural formation whether hard rock or superficial deposit. The idea of antiquity or relative date is not necessarily involved in this conception of the term. Thus the bones of a sheep buried under gravel and silt by a modern flood, and the obscure crystalline traces of a coral in ancient masses of limestone, are equally fossils. Nor has the term fossil any limitation as to organic grade. It includes not merely the remains of organisms, but also whatever was directly connected with or produced by these organisms. Thus the resin which was exuded from trees of long-perished forests is as much a fossil as any portion of the stem, leaves, flowers, or fruit, and in some respects is even more valuable to the geologist than more determinable remains of its parent trees, because it has often preserved in admirable perfection the insects which flitted about in the woodlands The burrows and trails of a worm preserved in sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelide life among old geological formations. The droppings of fishes and reptiles, called coprolites, are excellent fossils, and tell their tale as to the presence of vertebrate life in ancient waters. The little agglutinated cases of the caddis-worm remain as fossils in formations from which perchance most other traces of life may have passed away. Nay, the very handiwork of man, when preserved in any natural manner, is entitled to rank among fossils; as where his flint-implements have been dropped into the prehistoric gravels of river-valleys, or where his canoes have been buried in the silt of lake-bottoms.

The term fossil, moreover, suffers no restriction as to the condition or state of preservation of any organism. In some rare instances the very flesh, skin, and hair of a mammal have been preserved for thousands of years, as in the case of the mammoths entombed within the frozen mud cliffs of Siberia. In most cases all or most of the original animal matter has disappeared, and the organism has been more or less completely mineralized or petrified. It often happens that the whole organism has decayed, and a mere cast in amorphous mineral matter, as sand, clay, ironstone, silica, or limestone remains; yet all these variations must be comprised in the comprehensive term fossil.

Conditions for the Preservation of Organic Remains. At the outset the question naturally suggests itself how the

I. Consider, in the first place, what chances exist for the country. The surface of the land may be densely clothed with forest, and abundantly peopled with animal life. But the trees die and moulder into soil. The animals, too, disappear, generation after generation, and leave no percep tible traces of their existence. If we were not aware from authentic records that central and northern Europe was covered with vast forests at the beginning of our era, how could we know this fact? What has become of the herds of wild oxen, the bears, wolves, and other denizens of primeval Europe? How could we prove from the examination of the surface soil of any country that those creatures had once abounded there? We might search in vain for any such superficial traces, and would learn by so doing that the law of nature is everywhere "dust to dust.”

The conditions for the preservation of any relics of the plant and animal life of a terrestrial surface must therefore be always exceptional. They are supplied only where the organic remains can be protected from the air and superficial decay. Hence they may be observed in

1. Lakes.-Over the floor of a lake deposits of silt, peat, marl, &c., are formed. Into these the stems, branches, leaves, flowers, fruits, or seeds of plants from the neigh bouring land may be carried, together with the bodies of land animals, insects, and birds. An occasional storm may blow the lighter debris of the woodlands into the water. Such portions of the wreck as did not float, and were not washed ashore again, might sink to the bottom. Of these the larger part would in most cases probably rot away, só that, in the end, only a very small fraction of the whole vegetable matter cast over the lake by the wind would be covered up and preserved at the bottom. In like manner the animal remains swept by winds of by river floods into the lake would run so many risks of dissolution that only a proportion of them, and probably merely a small propor tion, would be preserved. When we consider these chances against the conservation of the vegetable and animal life of the land, we must admit that, at the best, lake-bottoms can contain but a meagre and imperfect representation of the abundant life of the adjacent hills and plains,

But lakes have a distinct flora and fauna of theirown. Their aquatic plants may be entombed in the gathering deposits of the bottom. Their mollusks, of characteristic types, sometimes form, by the accumulation of their remains, sheets of soft calcareous marl, in which many of the undecayed shells are preserved. Their fishes, likewise distinctly lacustrine, no doubt must often be entombed in the silt or marl.

2. Peat-mosses.-Wild animals venturing on the more treacherous watery parts of a peat-bog are sometimes engulphed or "laired." The antiseptic qualities of the peat preserve such remains from decay. Hence from European peat-mosses numerous remains of deer and oxen have been exhumed. Evidently the larger beasts of the forest ought chiefly to be looked for in these localities.

3. Deltas at River Mouths.-From what has been said in previous pages (ante, pp. 276-8) regarding the geological operations of rivers, it is obvious that to some extent both the flora and the fauna of the land may be buried among the sand and silt of deltas. When we consider, however, that though occasional or frequent river-floods sweep dow,.

trees, herbage, and the bodies of land animals, the remains so transported run every risk of decaying or being otherwise destroyed while still afloat, and that even if they reach the bottom they will tend to dissolution there unless speedily covered up and protected by fresh sediment, we must perceive that delta formations can scarcely be expected to give us more than a meagre outline of the varied terrestrial flora and fauna.

4. Caverns.-These are eminently adapted for the preservation of the higher forms of terrestrial life. Most of our knowledge of the prehistoric mammalian fauna of Europe is derived from what has been disinterred from bone-caves. As these recesses lie for the most part in limestone or in calcareous rock, their floors are commonly coated with stalagmite from the drip of the roof; and as this deposit is of great closeness and durability it has effectually preserved whatever it has covered or enveloped. The caves have in many instances served predatory beasts, like the hyæna, cave-lion, and cave-bear, as dens in which they slept, and into which some of them dragged their prey. In other cases they have been merely holes into, which different animals crawled to die, or into which they fell or were swept by inundations. Under whatever circumstances the animals left their remains in these subterranean retreats, the result has been that the bones have been covered up and preserved. Still, we must admit that after all but a mere fraction even of the mammals of the time would enter the caves, and therefore that the evidence of the cavern-deposits, profoundly interesting and valuable as it is, presents us with merely a glimpse of one aspect of the life of the land.

II. In the next place, if we turn to the sea, we find certainly many more favourable conditions for the preservation of organic forms, but also many circumstances which operate against it. While the level of the land remains stationary, there can be but little effective entombment of marine organisms in littoral deposits; for only a limited accumulation of sediment will be formed until subsidence of the sea-floor takes place. In the trifling beds of sand or gravel thrown up on a stationary shore, only the harder and more durable forms of life, such as gasteropods and lamellibranchs, which can withstand the triturating effects of the beach waves, are likely to remain uneffaced.

Below tide-marks, along the margin of the land where sediment is gradually deposited, the conditions are favourable for the preservation of marine organisms. Sheets of sand and mud are there laid down. In those sediments the harder parts of many forms of life may be entombed and protected from decay. But only a small proportion of the total marine fauna may be expected to occur in such deposits. At the best, merely littoral and shallow-water forms will occur, and even under the most favourable conditions they will represent but a fraction of the whole assemblage of life in these juxta terrestrial parts of the ocean. As we recede from the land the rate of deposition of sediment on the sea-floor must become feebler, until in the remote central abysses it reaches a hardly appreciable minimum. Except, therefore, where organic deposits, such as voze, are forming in these more pelagic regions, the conditions must be on the whole unfavourable for the preservation of any adequate representation of the deep-sea fauna. Hard enduring objects, such as teeth and bones, may slowly accumulate and be protected by a coating of peroxide of manganese, or of some of the silicates above (p. 288) referred to as now forming here and there over the deepsea-bottom. But such a deposit, if raised into land, would supply but a meagre picture of the life of the sea.

We must conclude therefore that of the whole sea-floor the area best adapted for preserving a varied suite of marine organic exuviæ is that belt which, running along

the margin of the land, is ever receiving fresh layers of sediment transported by rivers and currents from the adjacent shores. The most favourable conditions for the accumulation of a thick mass of marine fossiliferous strata will arise when the area of deposit is undergoing a gradual subsidence. If the rate of depression and that of deposit were equal, or nearly so, the movement might proceed for a vast period without producing any great apparent change in marine geography, and even without seriously affecting the distribution of life over the sea-floor within the area of subsidence. Hundreds or thousands of feet of sedimentary strata might in this way be heaped up round the continents, containing a fragmentary series of organic re mains belonging to those forms of shallow-water lif which had hard parts capable of preservation.

There can be little doubt that such has in fact been the history of the rain mass of stratified formations in the earth's crust. These piles of marine strata have unquestionably been laid down in comparatively shallow water within the area of deposit of terrestrial sediment. Their great depth seems only explicable by prolonged and repeated movements of subsidence, interrupted, however, as we know, by other movements of a contrary kind. These geographical changes affected at once the deposition of inorganic materials and the succession of organic forms. One series of strata is sometimes abruptly succeeded by another of a very different character, and we generally find a corresponding contrast between their respective organic contents.

It follows from these conclusions that representatives of the abyssal deposits of the central oceans are not likely to be met with among the geological formations of past times. Thanks to the great work done by the "Challenger" expedition, we now know what are the leading characters of these abyssal deposits of the present day. They have absolutely no analogy among the formations of the earth's crust. They differ, indeed, so entirely from any formation which geologists considered to be of deep-water origin as to indicate that, from early geological times, the present great areas of land and sea have remained on the whole where they are, and that the land consists mainly of strata formed at successive epochs of terrestrial debris laid down in the surrounding shallow sea.

Relative Value of Organic Remains as Fossils.-As the conditions for the preservation of organic remains exist more favourably under the sea than on land, marine organisms must be far more abundantly conserved than those of the land. This is true to-day, and has been true in all past geological time. Hence for the purposes of the geologist the fossil remains of marine forms of life far surpass all others in value. Among them there will necessarily be a gradation of importance regulated chiefly by their relative abundance. Now, of all the marine tribes which live within the juxta-terrestrial belt of sedimentation, unquestionably the Mollusca stand in the place of pre-eminence as regards their aptitude for becoming fossils. In the first place they almost all possess a hard durable shell, capable of resisting considerable abrasion, and readily passing into a mineralized condition. In the next place they are extremely abundant both as to individuals and genera. They occur on the shore within tide mark, and range thence down into the abysses. Moreover, they appear to have possessed those qualifications from early geological times. In the marine Mollusca, therefore, we have a common ground of comparison between the stratified formations of different periods. They have been styled the alphabet of paleontological inquiry. It will be seen, as we proceed, how much in the interpretation of geological history depends upon the testimony of sea-shells.

Looking at the organisms of the land, we perceive that as a rule the abundant terrestrial flora has a comparatively

small cliance of being well represented in a fossil state, that indeed, as a rule, only that portion of it of which the leaves, twigs, flowers, and fruits are blown into lakes is likely to be partially preserved. Terrestrial plants, therefore, occur in comparative rarity among stratified rocks, and furnish in consequence only limited means of comparison between the formations of different ages and countries. Of land animals the vast majority perish and leave no permanent trace of their existence. Predatory and other forms whose remains may be looked for in caverns or peat-mosses, must occur more numerously in the fossil state than birds, and are correspondingly more valuable to the geologist for the comparison of different strata.

Relative Age of Fossils.--Although absolute dates cannot be fixed in geological chronology, it is not difficult to deter mine the relative age of different strata, and consequently of their enclosed organic remains. For this purpose the fundamental law is based on what is termed the "order of superposition." This law may be thus defined:-in a series of stratified formations the older must underlie the younger. It is not needful that we should actually see the one lying below the other. If a continuous conformable succession of strata dips steadily in one direction we know that the beds at the one end must underlie those at the other, because we can trace the whole succession of beds between them. Rare instances occur where strata have been so folded by great terrestrial disturbance that the younger are made to underlie the older. But this inversion can usually be made quite clear from other evidence. The true order of superposition is decisive of the relative ages of stratified rocks.

If therefore formations lie regularly above each other, B upon A, C upon B, D upon C, and so on, it is evident that the organic remains found in A must have lived and died before those in B were entombed; the latter must have been covered up before those in C, and these again before those in D. The chronological sequence of fossils must be determined first of all by the order of superposition of their enclosing strata. There is nothing in the fossils themselves, apart from experience, to fix their date. Unless, for example, we knew from observation or testimony that Rhynchonella pleurodon is a shell of the Carboniferous Limestone, and Rhynchonella tetrahedra is a shell of the Lias, we could not, from mere inspection of the fossils themselves, pronounce as to their real geological position. It is quite true that by practice a paleontologist has his eye so trained that he can make shrewd approximations to the actual horizon of fossils which he may never have seen before; but he can only do this by availing himself of a wide experience based upon the ascertained order of appearance of fossils as determined by the law of superposition. For geological purposes therefore, and indeed for all purposes of comparison between the faunas and floras of different periods, it is absolutely essential first of all to have the order of superposition of strata rigorously determined. Unless this is done the most fatal mistakes may be made in paleontological chronology. But when it has once been done in one typical district, the order thus established may be held as proved for a wide region where, from paucity of sections, or from geological disturbance, the true succession of formations cannot be satisfactorily determined.

Uses of Fossils in Geology.-There are two main purposes to which fossils may be put in geological research :-(1) to throw light upon former conditions of physical geography, such as the presence of land, rivers, lakes, and seas, in places where they do not now exist, changes of climate, and the former distribution of plants and animals; and (2) to furnish a guide in geological chronology whereby rocks may be classified according to relative date, and the facts of geological history may be arranged and interpreted as a consected record of the earth's progress.

1. A few examples will suffice to show the manifold assistance which fossils furnish to the geologist in the elucidation of ancient geography.

(a.) Former land-surfaces are revealed by the presence of tree-stumps in their positions of growth, with their roots. branching freely in the underlying stratum, which, representing the ancient soil, often contains leaves, fruits, and other sylvan remains, together with traces of the bones of land animals, remains of insects, land-shells, &c. Ancient woodland surfaces of this kind are found between tide-marks, and even below low-water line, round different parts of the British coast. They unequivocally prove a subsidence of the land. Of more ancient date are the "dirt-beds" of Portland, which, by their layers of soil and tree-stumps, show that woodlands of cycads sprang up over an upraised sea-bottom and were buried beneath the silt of a river or lake. Still further back in geological history come the numerous coal-growths of the Carboniferous period, pointing to wide jungles of terrestrial or aquatic plants, like the modern mangrove swamps, which were submerged and covered with sand and silt.

(b.) The former existence of lakes can be satisfactorily proved from beds of marl or lacustrine limestone full of fresh-water shells, or from fine silt with leaves, fruits, and insect remains. Such deposits are abundantly forming at the present day, and they occur at various horizons among the geological formations of past times. The well-known nagelflue of Switzerland-a mass of conglomerate attaining a thickness of fully 6000 feet-can be shown from its fossil contents to be essentially a lacustrine formation.

(c.) Old sea-bottoms are vividly brought before us by beds of marine shells and other organisms. Layers of waterworn gravel and sand, with 'rolled shells of littoral and infra-littoral species, unmistakably mark the position of a former shore line. Deeper water is indicated by finer muddy sediment, with relics of the fauna which prevails beneath the reach of waves and ground-swell. Limestones full of corals, or made up of crinoids, point to the slow continuous growth and decay of generation after generation of organisms in clear sea-water.

(d.) Variations in the nature of the water or of the seabottom may sometimes be shown by changes in the size or shape of the organic remains. If, for example, the fossils in the central and lower parts of a limestone are large and well-formed, but in the upper layers become dwarfed and distorted, we may reasonably infer that the conditions for their continued existence at that locality must have been gradually impaired. The final complete cessation of these favourable conditions is shown by the replacement of the limestone by shale, indicative of the water having become muddy, and by the disappearance of the fossils, which had shown their sensitiveness to the change.

(e.) That the sea-floor represented by a fossiliferous. stratum was not far from land is sufficiently proved by mere lithological characters, as has been already explained; but the conclusion may be further strengthened by the occurrence of leaves, stems, and other fragments of terrestrial vegetation which, if found in some numbers among marine organisms, would make it improbable that they had been drifted far from land.

(f) The existence of different conditions of climate in former geological periods is satisfactorily demonstrated from the testimony of fossils. Thus an assemblage of the remains of palms, gourds, and melons, with bones of crocodiles, turtles, and sea-snakes, proves a sub-tropical climate to bave prevailed over the south of England in the time of the older Tertiary formations. On the other hand, the presence of an intensely cold or arctic climate far south in Europe during post-Tertiary time can be shown from different kinds of evidence, such as the existence of the remains of arctic animals even as far as the south of England and of France