The molten magma, which on solidification gives rise to the rocks of this group, is very rich in silica (65 to 80%) and has from 10 to 15% of alumina; the quantity of alkalies (Na and K) is relatively large (6 to 8 %), and there are small amounts of iron oxides (2 to 4%), magnesia (1 to 2%), and lime (1 to 4%). In the process of consolidation the principal minerals formed are orthoclase and quartz, with smaller amounts of oligoclase, iron oxide, and of the ferro-magnesian minerals, biotite or hornblende. Differences of texture, produced in the manner already described, give rise to rocks of totally different appearance, which it is difficult to imagine are of similar or identical composition.

Obsidian is a volcanic glass, which is usually black or dark brown or green (but sometimes blue, red, or yellow). It breaks with a shell-like fracture, and in very thin pieces is translucent.

The microscope shows " crystallites," the incipient stages of crystals, which are present in great numbers. The name obsidian is used for the various kinds of volcanic glass in which the percentage of water is small, and so for exact description a prefix is necessary, such as rhyolite obsidian, andesite obsidian. Though the glasses are of varying composition, by far the greater number of them belong to the granite family. When the glass is divided by concentric cracks, due to shrinkage on cooling, so as to form onion-like spherules, it is called Perlite.

Pitchstone has much the same appearance as obsidian, but contains from 5 to 10 % of water.

Pumice is a glass blown up by the bubbles of escaping steam and other vapours into a rock froth, so light that it will float upon water. A very similar substance is produced when a jet of steam is blown through the melted slag from an iron furnace.

It not infrequently happens that, in course of time, the volcanic rocks become devitrified, losing their glassy texture and assuming a stony one. The homogeneous rock becomes converted into a mass of extremely minute crystals of quartz and felspar, and the original glassy texture is then shown only by the lines of flow, or by the perlitic character, which are not affected by the change. Devitrification has also been observed in artificial glasses, especially when the glass, owing to insufficient annealing, has been subject to internal stress.

Rhyolite ordinarily occurs as the lava outflow of a granitic magma, cooled rapidly, but yet more slowly than obsidian. The texture is porphyritic, the phenocrysts being chiefly quartz and the glassy form of orthoclase known as sanidine, while the ferro-mag-nesian minerals are present in very much smaller quantities, and of these the commonest is biotite. The phenocrysts are embedded in a ground mass of minute felspar crystals and a varying proportion of glass. Other names used for rhyolite are liparite and quartz trachyte. The rhyolites are exceedingly common in the western part of the United States. The Felsites are very dense, fine-grained, and light-coloured rocks, in which phenocrysts are absent or scanty; they are rocks which have been formed in different ways, by the devitrification of obsidians and rhyolites, by the recrystallization of tuffs, and by original cooling from fusion.

Quartz Porphyry shades imperceptibly into rhyolite or felsite on the one hand, and into granite on the other; it is made up of phenocrysts of quartz, or of quartz and orthoclase, in a crystalline ground mass of the same minerals. If the phenocrysts are very abundant and the ground mass rather coarse grained, the rock is called granite porphyry. Syenite porphyry, Diorite porphyry, etc., bear similar relations to the other members of their respective families and need no further description.