410 



GEOLOGY. 



SPECIAL STRUCTURES. 



Flow structure. — Lavas that cool into glassy rocks frequently con- 

 tain 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 appHed to a 

 class of acidic rocks. The obsidians and pitchstones may be more or 



Fig. 3-12. — Porphyritic texture. Two-thirds natural size. (Photo, by Church.) 



less rhyoHtic 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 some- 

 times make up a notable part of the mass, the glassy portion being 

 scarcely more than a matrix in which the crystals, spheruHtes, and cavities 



