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]).
Fig. 222. Polished Section of an Agate
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.
| Fig. 223. Microsection showing the Beginnings of Crystal Growth in Glassy Lava | Fig. 224. Perlitic Structure and Spherulites, a, a |