Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/335

Rh USE or FOSSI Ls.] small chance 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 maybe 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 com- parison of (lifferent 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 funtlamental law is based on what is termed the “order of superposition.” This lav 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 steadilyin 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 super- position 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 o11, 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 I-t’/tg/m:/zonellzt pleuroclon is a shell of the Carboniferous Lime- stone, and It’/z_z/nchonella tetrukeclra 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 palzeontologist 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 compari- son between the faunas and ﬂoras of different periods, it is absolutely essential ﬁrst of all to have the order of super- position of strata rigorously determined. Unless this is done the most fatal mistakes may be made in palaeontolo- gical chronology. But when it l1as 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 forma- tions cannot be satisfactorily determined. Uses ofFossils in G'eolo_qy.—Tl1ere are two 111ain purposes to which fossils may be put in geological research :—(I ) 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 to furnish a guide in geological chronology whereby rocks may be classiﬁed according to relative date, and the facts of geo- logical history may be arranged and interpreted as a con- nected record of the earth’s progress. GEOLOGY 321 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—stu1nps in their positions of growth, with their roots branching freely in the underlying stratum, which, represent- ing the ancient soil, often contains leaves, fruits, and other sylvan remains, together with traces of the bones of land ani- mals, 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 co111e the numerous coal—growths of the Carboniferous period, pointing to wide jungles of ter- restrial or aquatic plants, like the modern mangrove swamps, which were submerged and covered with sand and silt. ((2.) 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 water- worn 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 con- tinuous growth and decay of generation after generation of organisms i11 clear sea-water. ((1.) Variations in the nature of the water or of the sea- bottom 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 favouiable 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 terres- trial 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 have 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 Englanycil and of France. . — 4r