|Oxides, Sulphates, or Carbonates; | ...............: | |_i.e._, all eruptive rocks, so | : : : : : | |far as known, are principally | :Obsidian. : Tachylite.: | |composed of minerals belonging |............................ : | |to the class of Silicates. | : : : : : | | | Petrosilex. :Porphyrite.: | | |............................ : | | | : : : : : | | | : : Felsite. Melaphyr. | | | : : ....... .......: | | | : : : : : | +------------------------------------+-------------------------------------+ The geological agencies, as we have already learned, may be arranged in two great classes: first, the aqueous or superficial agencies originating in the solar heat, and producing the sedimentary or stratified rocks; and, second, the igneous or subterranean agencies originating in the central or interior heat, and producing the eruptive or unstratified rocks. Hence, we want to know first of any rock whether it is of aqueous or igneous origin. Then, if it is a sedimentary rock, whether it has been formed by the action chiefly of mechanical forces, or of chemical and organic forces. And, if it is an eruptive rock, whether it has cooled and solidified below the earth’s surface in a fissure, and is a dike or trappean rock, or has flowed out on the surface and cooled in contact with the air, and thus become an ordinary lava or volcanic rock. Here we have the outlines of our classification, and it will be observed that we have simply reached the conclusion, in a somewhat roundabout manner, that there should always be a general correspondence between the classification of rocks and the classification of the forces that produce them. The general plan of the preceding scheme of the classification must now be clear, and the details will be explained as we go along. _Descriptions of Rocks._ 1.—Sedimentary or Stratified Rocks.