The principal varieties of trappean rock are basalt, green stone, and trachyte. In _basalt_, augite, or, in some cases, hornblende, is the predominant mineral. It is a heavy, close-grained rock, of a black or dark brown color. _Greenstone_ differs from basalt in containing a much larger proportion of felspar. Its structure is more granular, and frequently it assumes so much of the crystalline form as to pass insensibly into syenite or granite. It is a dark colored rock, with a slight tinge of green. Both green stone and basalt are disposed to assume the columnar form, the columns being arranged at right angles to the faces of the fissure into which the trap is injected. When it is spread out into broad horizontal masses, the columns are vertical. (Fig. 1.) _Trachyte_ is composed principally of felspar, is of a grayish color, and rough to the touch. [Illustration: Fig. 1.] Of the _stratified_ rocks the following are the most important: _Gneiss_ is a rock closely resembling granite. It is an aggregate of the same minerals, but the proportion of mica is somewhat greater. The only distinction between them is that the gneiss is stratified, but the stratification is often so indistinct that it passes insensibly into granite. Generally, however, the stratification is so distinct as to present a marked difference. _Mica slate_ is such a modification of gneiss that the mica becomes the predominant mineral, with a small intermixture of quartz and felspar. Consequently the stratification becomes very distinct, so as sometimes to render the mass divisible into thin sheets. The stratification is often wavy, and sometimes much contorted. _Sandstone_ consists of grains or fragments of any other rock, but more frequently of siliceous rocks. The fragments are consolidated, sometimes without any visible cement, but often by a paste of argillaceous or calcareous substance. The color varies with that of the rock from which it was derived. Generally, however, it is either drab or is colored red by oxide of iron. The fragments are sometimes so minute as scarcely to give the rock the appearance of sandstone. When they are of considerable size and rounded, the rock is called _conglomerate_. When they are angular, it is called _breccia_. _Greensand_ is a friable mixture of siliceous and calcareous particles, colored by a slight intermixture of green earth or chlorite.