1. The widest and most important generalization of modern geology is that all the materials of the earth's crust, to the greatest depth that man can reach, either by actual excavation or inference from superficial arrangements, are of such a nature as to prove that they are not, in their present state, original portions of the earth's structure; but that they are the results of the operation, during long periods, of the causes of change--whether mechanical, chemical, or vital--now in operation, on the land, in the seas, and in the interior of the earth. For example, the most common rocks of our continents are conglomerates, sandstones, shales, and slates; all of which are made up of the debris of older rocks broken down into gravel, sand, or mud, and then re-cemented. To these we may add limestones, which have been made up by the accumulation of corals and shells, or by deposits from calcareous springs; coal, composed of vegetable matter; and granite, syenite, greenstone, and trap, which are molten rocks formed in the manner of modern lavas. So general has been this sorting, altering, and disturbance of the substance of the earth's crust, that, though we know its structure over large portions of our continents to the depth of several miles, the geologist can point to no instance of a truly primitive rock which can be affirmed to have remained unchanged and _in situ_ since the beginning. "All are aware that the solid parts of the earth consist of distinct substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but, previously to observation, it is commonly imagined that all had remained from the first in the state in which we now see them--that they were created in their present forms and in their present position. The geologist now comes to a different conclusion; discovering proofs that the external parts of the earth were not all produced in the beginning of things in the state in which we now behold them, nor in an instant of time. On the contrary, he can show that they have acquired their actual condition and configuration gradually and at successive periods, during each of which distinct races of living beings have flourished on the land and in the waters; the remains of these creatures lying buried in the crust of the earth."[140]