PLATE XIX. 



Fig. a. 



(Sp. No. 32756. Without analyzer, x90.) 



Photomicrograpli of fractured quartz phenocryst from a rhyolite-porpliyry. It includes num- 

 berless liquid inclusions, which diminish in quantity as the distance from the plane of fracture is 

 increased, thus indicating their close connection with the fracturing of the cjuartz. The fracture in 

 the quartz phenocryst continued into the groundmass, as may be seen on the left-hand side of the 

 figure. It has been healed with secondary quartz. (Described, p. 82.) 



Fig. B. 



(Sp. No. 329U. With analyzer, X 47. ) 



Photomicrograph of a section of rhyolite-porphyry, designed to show the vhombohedral part- 

 ing, which is very common in many of the quartz phenocrysts. (Described, p. 82.) 



