=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. [Illustration: Fig. 221. Amygdules in Lava] =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 (Fig. 222). [Illustration: Fig. 222. Polished Section of an Agate] [Illustration: Fig. 223. Microsection showing the Beginnings of Crystal Growth in Glassy Lava] =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. Spherulites, which also mark the first changes of glassy lavas toward a stony condition, are little balls within the rock, varying from microscopic size to several inches in diameter, and made up of radiating fibers. Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to contraction. [Illustration: Fig. 224. Perlitic Structure and Spherulites, _a_, _a_] [Illustration: Fig. 225. Flow Lines in Lava]