§ 1. This group of rocks, the last we have to examine, is, as far as respects geographical extent and usefulness to the human race, more important than any of the preceding ones. It forms the greater part of all low hills and uplands throughout the world, and supplies the most valuable materials for building and sculpture, being distinguished from the group of the slaty coherents by its incapability of being separated into thin sheets. All the rocks belonging to the group break irregularly, like loaf sugar or dried clay. Some of them are composed of hardened calcareous matter, and are known as limestone; others are merely hardened sand, and are called freestone or sandstone; and others, appearing to consist of dried mud or clay, are of less general importance, and receive different names in different localities.

§ 2. Among these rocks, the foremost position is, of course, occupied by the great group of the marbles, of which the substance appears to have been prepared expressly in order to afford to human art a perfect means of carrying out its purposes. They are of exactly the necessary hardness,—neither so soft as to be incapable of maintaining themselves in delicate forms, nor so hard as always to require a blow to give effect to the sculptor's touch; the mere pressure of his chisel produces a certain effect upon them.