SPECIAL STRUCTURES.
Flow structure.—Lavas that cool into glassy rocks frequently contain gas cavities, colored spots and variations of texture which, together with the hair-like embryo crystals, are drawn out into lines, streaks, and parallel belts by the flow of the viscous mass, giving rise to rhyolitic or flow structure (Figs. [338] and [339]). Rocks in which this is the most pronounced feature were formerly known as rhyolites, though the term has drifted away from this original meaning and has been applied to a class of acidic rocks. The obsidians and pitchstones may be more or less rhyolitic under the microscope, though to the naked eye they may appear only as a glassy or resinous mass. The rhyolites generally have but an imperfect glassy texture, since the crystals and the cavities sometimes make up a notable part of the mass, the glassy portion being scarcely more than a matrix in which the crystals, spherulites, and cavities are carried. By an increase of the crystals in number and size, the rock passes by gradations into porphyry or phanerite.
Fig. 342.—Porphyritic texture. Two-thirds natural size. (Photo. by Church.)
Amygdaloids.—In lava-flows the included steam often collects in bubbles near the surface as the lava cools and forms a vesicular portion with a scoriaceous texture ([Fig. 341]). In its upper part, the vapor bubbles may be numerous, while below they become more and more scattered until they disappear. Similar bubbles are also often found near the bottom of a sheet of lava. This is perhaps due to the rolling under of the frontal surface of the lava-stream as it flows. Later, these cavities often become filled with minerals deposited from solution and the rock then becomes an amygdaloid, but this filling is a secondary action.
Fig. 343.—Porphyritic texture. Natural size. (Photo. by Church.)