Limestone is a very abundant rock, and occurs in many different forms. In transparent crystals it is Iceland spar. When white and crystalline, it is primary limestone, saccharine limestone, or statuary marble. When sub-crystalline it is generally more or less colored. It is often clouded with bands or patches of white in a ground of some dark color. When its texture is close, and the crystallization scarcely apparent, it is compact limestone. The white, earthy variety is chalk. A variety of limestone composed of small spheres is called oölite. Lias is the name given to an impure argillaceous variety of a brown or blue color. Any rock which contains a considerable proportion of carbonate of lime, and which rapidly disintegrates on exposure to the atmosphere, is called marl. Limestone sometimes contains carbonate of magnesia. It is then magnesian limestone, or dolomite.
Clay consists of a mixture of siliceous and aluminous earth. It is tough, highly plastic, and generally of a lead blue color. It is always stratified, and often divided into very thin laminæ, which are separated by sprinklings of sand only sufficient to keep them distinct.
Clay slate, or argillaceous schist, is composed of the same materials as clay, and differs from it only in having become solidified. Its color is gray, dark brown or black. In some beds it is purple. Shale is the same material in a state of partial solidification. On exposure to the weather, it soon disintegrates, and is finally reconverted into clay. All the varieties of argillaceous rock are easily distinguished by a peculiar odor which they emit when breathed upon.
Argillaceous slate sometimes takes into its composition portions of some other mineral, such as talc, mica, or hornblende. When any of these minerals becomes so abundant as to constitute a considerable part of the mass, the rock becomes talcose, micaceous, or hornblende slate. Sometimes this last variety loses all appearance of a fissile structure, and is composed almost wholly of hornblende. It is then called hornblende rock.
Diluvium is the name applied to masses of sand, gravel, and large rocks, called boulders, heaped confusedly together on the surface of the earth. It is also called drift.
CHAPTER II.
OF THE ARRANGEMENT OF THE MATERIALS WHICH COMPOSE
THE CRUST OF THE EARTH.