Vast amounts of steam are, as we have seen, emitted from volcanoes, and comparatively small quantities of other vapors, such as various acid and sulphurous gases.  The rocks erupted from volcanoes differ widely in chemical composition and in texture.

Acidic and basic lavas.  Two classes of volcanic rocks may be distinguished,—­those containing a large proportion of silica (silicic acid, SiO2) and therefore called acidic, and those containing less silica and a larger proportion of the bases (lime, magnesia, soda, etc.) and therefore called basic.  The acidic lavas, of which rhyolite and THRACHYTE are examples, are comparatively light in color and weight, and are difficult to melt.  The basic lavas, of which basalt is a type, are dark and heavy and melt at a lower temperature.

Scoria and pumice.  The texture of volcanic rocks depends in part on the degree to which they were distended by the steam which permeated them when in a molten state.  They harden into compact rock where the steam cannot expand.  Where the steam is released from pressure, as on the surface of a lava stream, it forms bubbles (steam blebs) of various sizes, which give the hardened rock a cellular structure (Fig. 220), In this way are formed the rough slags and clinkers called scoria, which are found on the surface of flows and which are also thrown out as clots of lava in explosive eruptions.

On the surface of the seething lava in the throat of the volcano there gathers a rock foam, which, when hurled into the air, is cooled and falls as pumice,—­a spongy gray rock so light that it floats on water.

Amygdules.  The steam blebs of lava flows are often drawn out from a spherical to an elliptical form resembling that of an almond, and after the rock has cooled these cavities are gradually filled with minerals deposited from solution by underground water.  From their shape such casts are called amygdules (Greek, amygdalon, an almond).  Amygdules are commonly composed of silica.  Lavas contain both silica and the alkalies, potash and soda, and after dissolving the alkalies, percolating water is able to take silica also into solution.  Most agates are banded amygdules in which the silica has been laid in varicolored, concentric layers.

Glassy and stony lavas.  Volcanic rocks differ in texture according also to the rate at which they have solidified.  When rapidly cooled, as on the surface of a lava flow, molten rock chills to a glass, because the minerals of which it is composed have not had time to separate themselves from the fused mixture and form crystals.  Under slow cooling, as in the interior of the flow, it becomes a stony mass composed of crystals set in a glassy paste.  In thin slices of volcanic glass one may see under the microscope the beginnings of crystal growth in filaments and needles and feathery forms, which are the rudiments of the crystals of various minerals.