32 



PHYSICAL GEOLOGY 



generally black in color. Its glassy condition signifies rapid 

 cooling, while its compact texture means that gas bubbles 



FIG. 12. Scoriacebus texture. About % natural size. 

 (Photograph by Baker.) 



were not forming as it solidified. Obsidian usually has a com- 

 position much like that of granite. Pitchstone is a glassy rock 



FIG. 13. Amygdules in lava. About natural size. The material of the 

 amygdules was deposited from solution in ground water in the cavities of a 

 seoriaceous lava. (Photograph by Baker.) 



