A large part of the materials ejected from volcanoes is in the form of dust, cinders and angular fragments of rock. These soon become solidified, forming volcanic tuff, or volcanic breccia. In submarine eruptions these fragments are spread out by the water into strata, upon which other materials, not volcanic, are afterwards deposited. These interposed strata are called volcanic grits.
The trappean rocks are composed of felspar, mingled intimately and in small particles with augite or hornblende. They also contain iron and potassa. They are often porphyritic. When they contain spherical cavities, filled with some other mineral, such as chlorite, carbonate of lime or agate, they are called amygdaloidal trap.
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.
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.