408 



GEOLOGY. 



crystals sometimes become so abundant as to change the texture from 

 the vitreous to the stony order. In some cases, the stony texture seems 

 to have been developed in the obsidian after it was formed, the change 

 being a part of a subsequent process of devitrification, but in other cases 

 the crystals seem to be original. Besides these, there are often small 

 globular bodies known as spherulites. 



Varieties of glassy rock in which the embryo crystals are more nu- 

 merous and the glassy texture less perfect, are known as pitchstones. The 



Fig. 339, — Flow structure in volcanic glass. About half natural size. 

 (Photo, by Church.) 



fresh surfaces of these have rather the aspect of pitch or resin than that 

 of true glass; hence their name. Like the obsidians, they are usually 

 dark, but they take on greenish, brownish, yellowish, and hght-colored 

 hues as well. Sometimes glassy rock fractures in small spheroidal 

 forms like pearls, and is known as perlite. Basic glasses are relatively 

 rare, and while usually included under the term obsidian, are some- 

 times given special names. 



