This appears to be the result of the cooling and crystallisation of that molten mass which many circumstances (hereafter to be mentioned) point out as making up the great body of the earth. Granite differs, in various places, in colour and quality (the varieties are known as "sienite," "porphyry," "greenstone," &c.), but still retains its own distinctive characters; it is a hard, crystalline rock, consisting of "felspar," "mica," and "quartz," in separate crystals, but mechanically blended; its chemical composition as a whole is silica, alumina, and potassa, with small quantities of lime and oxide of iron. This granite is met with everywhere, if the outer crust of the earth be penetrated to a sufficient depth; it however frequently exists on the surface, having no strata below it, and in some places overlies other strata--this, and the fact that cracks and crevices of some of the lower strata are filled up with granite, which could only have taken place while it was in a liquid state, as in fig. 8, together with evidences of the effects of calcination, and other changes producible only by extreme heat, all around those parts and in the strata immediately overlaying the granite, point out that it was once in a state of fusion. Above the granite formation, in many places, especially in Norway and Sweden, there is a stratum of rock called _gneiss_, which consists of the same ingredients as granite in a slightly altered form. Granite will often pass into gneiss gradually, and this again into those slaty formations which rest upon it and come next in the series. These consist of mica-slate, hornblend-slate, and chlorite-slate or schist, a name given to all those rocks of a slaty structure which have a cleavage or capability of being cleft into thin laminæ or slices (hence their applicability to the purposes of building, forming excellent roofing materials, &c.). [Illustration: _Granite_ FIG. 8.]