Fig. 338.—Flow structure in rhyolite. Nearly natural size. (Photo. by Church.)
The obsidians.—Of the compact glassy rocks, obsidian is the best type. It is essentially a natural glass, formed usually of acid silicates. It has the close texture, conchoidal fracture, and other qualities of glass. It is usually black, but sometimes red, brown, purple, bluish, or gray. While chiefly of glass, it usually contains more or less of the incipient crystals above described, showing that even here the first step in the gradation to the next or the crystalline stage has been taken. These incipient crystals sometimes become so abundant as to change the texture from the vitreous to the stony order. In some cases, the stony texture seems to have been developed in the obsidian after it was formed, the change being a part of a subsequent process of devitrification, but in other cases the crystals seem to be original. Besides these, there are often small globular bodies known as spherulites.
Varieties of glassy rock in which the embryo crystals are more numerous and the glassy texture less perfect, are known as pitchstones. The fresh surfaces of these have rather the aspect of pitch or resin than that of true glass; hence their name. Like the obsidians, they are usually dark, but they take on greenish, brownish, yellowish, and light-colored hues as well. Sometimes glassy rock fractures in small spheroidal forms like pearls, and is known as perlite. Basic glasses are relatively rare, and while usually included under the term obsidian, are sometimes given special names.
Fig. 339.—Flow structure in volcanic glass. About half natural size. (Photo. by Church.)
Fig. 340.—Flow structure in porphyry, shown by the position of the large crystals. About two-thirds natural size. (Photo. by Church.)
Fig. 341.—Scoriaceous texture. About four-fifths natural size. (Photo. by Church.)