So far, then, we have learned that the rocks exposed to view on the surface of the earth may be divided into two classes; that is, aqueous and igneous. There is yet a third class, which, though of aqueous origin, has in course of time suffered considerable from the internal heat of the earth and the enormous pressure due to the weight of overlying rocks. Such rocks have been greatly changed from their original condition, both in appearance and in mineral composition, and are said to be "metamorphic," a word which implies change. Thus chalk, or other limestone rock, has been metamorphosed into marble; shales and slates into various kinds of "schists,"[22] such as mica-schist, and even into gneiss, which closely resembles granite. And it is quite possible that even granite may in some cases be the result of the melting and consolidation under great pressure of certain familiar stratified rocks. It is quite conceivable that slate might be converted into granite, for their chemical composition is similar, only the minerals of which it is composed would require to be rearranged and grouped into new compounds. This would seem quite possible; but at present we have no direct proof of such a change having taken place. Even igneous rocks are found in some places to have suffered very considerable change. [22] Schists are so named from their property of splitting into thin layers. Their structure is crystalline; and the layers, or folia, consist usually of two or more minerals, but sometimes of only one. Thus mica-schist consists of quartz and mica, each arranged in many folia, but it splits along the layers of mica. In some inland seas, like the Caspian Sea, deposits of rock salt and gypsum may be formed by chemical precipitation, owing to evaporation from the surface. The various kinds of rock known to geologists may be conveniently arranged as follows: { { Clay, shale, slate, etc. { I. Sedimentary. { Sandstones. { { Conglomerates. { Rocks of { { Limestones. aqueous { II. Organic. { Flint. origin. { { Coal. { { III. Chemical. { Rock salt. { { Gypsum, etc. { I. Volcanic. { Lavas. Rocks of { { Volcanic ashes, etc. igneous origin. { { II. Plutonic. { Basalt. { { Granite. Metamorphic rocks { Marbles. of aqueous and { Various kinds of schists. igneous origin. { Gneiss, etc.