Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/147

Rh Britain, Canada, parts of Europe and the United States, the geology of the accessible portion, even, of the earth is still very little known, large parts of Africa, Asia, and South America being as yet, comparatively speaking, unexplored. Civilization, through its railroad-building, tunneling, canal-making, and mining operations, furnishes a large amount of the material on which the Geologist bases his science. Through agencies of this kind, rocks have been exposed which otherwise would have perhaps remained forever concealed from view. Through the excavating incidental to mining and tunneling, there have been discovered the remains of plants and animals long since extinct, the relics of an indefinitely remote past, the existence of which had not been previously even dreamed of The detritus brought down by rivers, and the consequent filling up of their mouths, as seen in the deltas of the Mississippi and the Nile, with the preservation in the mud, etc. of the coral stones, shells, skeletons of fish, etc. which lived and died in the vicinity, give one a good idea of the manner in which petrified organic remains or fossils may have been preserved in the rocks. While in certain rocks of this kind the fossils are found in great profusion and in a very excellent state of preservation, in others very few occur, or only a fragment may have escaped destruction. This is often, however, so characteristic that the comparative anatomist can reconstruct the whole skeleton from a single bone, a knowledge of the correlation of forms enabling the osteologist to infer from the structure of the foot the nature of the jaws, teeth, etc., of the extinct animal. Many such inferences might be mentioned, all of which, while commanding praise as illustrating the osteological knowledge of the anatomist, scarcely merit the astonishment which they invariably excite. While many rocks seem to have experienced but little disturbance since their original deposition, the different layers or strata of which they