Pirsson— Microscopical Characters of Volcanic Tuffs. 207 



of the rock in the section with the remains of the vesicular 

 character of many of the particles, often made clear by the fill- 

 ing of the vesicles with secondary substances, may greatly aid 

 in determining its original tuff-like nature. 



Devitrification. — In course of time, and aided perhaps by 

 heat and pressure, vitric tuffs like other glassy_ rocks may 

 become devitrified and pass into a crystalline condition. Con- 

 sidering first the tuffs formed from felsic magmas, the more 

 common kinds, this process consists essentially in a change of 

 the glass into feldspars and quartz ; it is quite similar to the 



Fiq. 6. 



Fig. 6. Devitrified and partly silicified rhyolite tuff from the Oelberg, 

 Schriesheim, Baden. Actual diameter 2" 



Jmm 



devitrification of glassy rhyolites which has been mentioned by 

 many writers and fully described by Professor Bascom.* The 

 result between crossed nicols is that the whole rock appears 

 composed of a mosaic of feebly polarizing particles, producing 

 a so-called "pepper and salt" appearance, which becomes so 

 minute in texture in places as to be almost micro-cryptocrystal- 

 line. Along with this change there is liable to be some 

 chemical alteration and the introduction of new material ; thus 

 secondary silica in the form of chalcedony ; shreds of sericite, 

 patches of carbonates, and limonitic material in greater or 

 lesser amounts may be found here and there. Between crossed 

 nicols little or nothing as to the origin of such rocks may be 

 gathered from the section ; they resemble glassy lavas which 



*Ancient Volcanic Kocks of South Mountain, Penna.. U. S. G. S., Bull., 

 136, 1896. 



