_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. SECTION I.--THE CLASSIFICATION OF ROCKS. In the first place, we divide rocks into _stratified_ and _unstratified_. This division is one which will in general be easily recognized, even by the most inexperienced observer; and the distinction is important, because it separates the rocks of igneous origin from those which have been produced by deposition of sediment from water.