Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/333

Rh v. rALmoNToLoG1cAL.] PART V.-—l’-lL-EONTOLOGICAL GEOLOGY. Palaeontology is the science which treats of the struc- ture, aflinities, classiﬁcation, and distribution in time of the forms of plant and animal life embedded in the rocks of the e.1rth’s crust. In one sense it may be regarded as a branch of zoology and of botany, its claim in this view to rank as a separate science resting almost solely on the fact that of the forms with which it deals but a small proportion belongs to the living world. In another aspect it may be looked upon as a branch of geology, seeing that its assist- ance is absolutely indispensable in many of the most fami- liar and fundamental problems of the latter science. It is under this last aspect that we have to regard it here. We shall consider merely those leading features of palaeon- tological inquiry without some knowledge of which progress in modern geology would be impossible. Fossils.—Palzeontological geology, then, deals with the /"zssils or organic remains preserved in the rocks, and endea- vours to gather from them information as to the history of the globe and its inhabitants. The term “fossil,” mean- ing literally anything “dug up,” was formerly applied in- discriminately 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 (late 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 11ot 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 forma- tions. The droppings of ﬁshes and reptiles, called copro- lites, are excellent fossils, and tell their tale as to the presence of vertebrate life in ancient waters. The little agglutinated cases of the caddis—wor1n 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 haveabeen 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 111ost 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. ('oncl-e'tio7zs for the Preservalion of Organic Remai1zs.—At the outset the question naturally suggests itself how the GEOLOGY 319 remains of plants and animals come to have been preserved in rocks at all. If we observe what takes place at the present day, and argue that it may fairly be taken as an indication of what has been the ordinary condition of things in the geological past, we see that there must have been so many chances against the conservation of either animal or plant remains that their occurrence among stratiﬁed forma- tions should be regarded as exceptional, and as the result of various fortunate accidents. I. Consider, in the ﬁrst place, what chances exist for the preservation of remains of the present fauna and flora of a country. The surface of the land may be densely clothed with forest, a11d abundantly peopled with animal life. But the trees die and moulder into soil. The animals, too, dis- appear, 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 ex- amination 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 super-- ﬁcial decay. Hence they may be observed in 1. I.al'es.—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, so 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 or 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. Butlakeshavea distinct ﬂora and faunaof theirown. Their aquatic plants may be entombed in the gathering deposits of the bottom. Their mollusks, of characteristic types, some- times 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 ]l[outIzs.—From what has been said in previous pages (mate, 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 down