Page:The American Cyclopædia (1879) Volume VII.djvu/702

690 vestiges of which were buried and remained with it as records of the condition of this portion of the earth during this time. The piles of strata of various kinds indicated changes in the character of the deposits introduced, sandstones formed from sand, alternating with shales formed from muddy and clayey deposits, and with calcareous strata, whose origin may have been in marl beds or the remains of calcareous organisms. The long succession of these strata, in connection with the evidences of their slow accumulation, observed in the undisturbed condition of the fossil remains which they contained, bore witness to long periods occupied in the production of a single group of strata constituting but a minor division of one of the formations. The lapse of long periods was also indicated by the fossils found in beds of older date becoming constantly more and more unlike existing species. The same localities, too, presented in their successive beds some that were filled with marine vestiges alone, corallines and sea shells, in layers of such thickness that ages must have passed while they were quietly accumulating; and above or below these were found other strata indicating that the surface at another period was covered with fresh water, the organic remains which they contained being only of the character of those belonging to ponds and rivers; and yet again these localities became dry land, and were covered with the forests of tropical climes, and peopled with numerous strange species of animals, whose nearest living analogues are met with only in hot countries. Such changes as these also plainly marked slowly progressing revolutions, the period of which no one could compute by years. It was apparent that the sediments had collected as beds of sand and clay now collect in seas and lakes, and especially about the mouths of large rivers; but it was only in such as were evidently the product of the streams of the present day that the organic vestiges were recognized as belonging entirely to familiar species. In these alone were discovered any relics of man or any indications of his existence; and here they were not wanting, for in the calcareous strata in process of formation and filled with recent species of shells human remains have been found. But with the first step backward the bones of extinct gigantic mammalia introduce us to strange groups of animals, and no satisfactory evidence is afforded, either in the strata or in tradition, that man was their contemporary. Thus in the closest connection, geologically speaking, are we presented with the most striking examples of other great principles developed by geological research, viz., the extinction of old and the introduction of new species.—In consequence of the system of observation and close investigation now established, geology lost its highly speculative character, and rapid progress continued to be made in acquiring correct information of the arrangement of the strata of different countries. While the defects of Werner's classification were exposed, the general plan of it was seen to be founded in nature, and attention was directed to collecting everywhere the materials for filling out the vertical column of the rocks, as well as mapping them throughout their horizontal range. In every country some formations could be recognized, from which as a base a local classification might proceed to contiguous groups, and thus at last the whole be included in one system of classification. So the work of descriptive geology has ever since been going on, new discoveries continually adding to its completeness and helping to the compilation of a perfect system, which in this case should present a full chart of the rocks from the lowest or oldest to the uppermost or newest. Strata lying in juxtaposition in one region, when identified in another, are found to be separated by the interpolation of a new series; and again, in tracing out over broad areas a group of sedimentary strata, they are found gradually to assume new features, and even to undergo an entire change of chemical composition. The deposits over different parts of the ocean's bed are found to be here sands and gravels brought by currents, and there soft calcareous muds, the remains of minute animal organisms accumulated in still waters. The organic remains as well as the mineral character of these contemporary deposits present wide differences. From the mode of their formation it is evident that all stratified formations must be of limited area, and must thin away at their edges, presenting the shape of lenticular sheets lapping upon each other.—In 1819 the geological society of London, through the labors of Mr. Greenough and his friends, published a map of England which was a great improvement upon that of Smith. About the same time Leopold von Buch prepared a similar map of a large part of Germany. A geological survey of France was ordered in 1822 by the French government, by which a complete geological map of France was finally constructed in 1841. M. Bronchant de Villiers, professor in the school of mines, was appointed to take charge of the work, and with him were associated Élie de Beaumont and Dufrénoy. The attention of these geologists was first given to an examination of the strata above the coal formation of England, where they had been most carefully studied and particularly described by Conybeare and Phillips in their treatise on “The Geology of England and Wales” (1821). The secondary strata of Germany also were familiar to geologists; and both countries consequently furnished important points of reference for the arrangement of the groups of France. The chalk formation of Paris, the upper member of the secondary, served as the starting point, and proceeding from this they examined in detail the lower strata as they appeared successively emerging from beneath it, and identified them,