PLATE XX. 



Fig. a. 



(Sp. No. 32119. With analyzer, x 90.) 



Micropoikilitic liiyolite-porphyry, showing the peculiar U^xture of the zones which invariably 

 surround the quartz phenocrysts in sections in which the texture occurs. The same texture prevails 

 in the groundmass. The irregular white areas which are continuous with the quartz phenocrysts and 

 are connected with each other represent quartz. Disconnected dark and light areas between the 

 quartz stringers are feldspar grains. These do not possess uniform orientation; hence the texture is 

 not micropegmatitic. (Described, p. 84.) 



Fig. B. 



(Sp. No. 32137. With analyzer, x 90.) 



Photomicrograph of micropoikilitic rhyolite-porphyry. In this rhyolite-porphyry the micropo- 

 ikilitic texture is much finer than that represented in Fig. A, and the quartz in the zones shows a 

 tendency toward spherulitic development. Owing to the extreme fineness of grain it is difficult to 

 distinguish the quartz and feldspar in many cases. The greater part of the light areas shown in the 

 photomicrograph are quartz. The dark areas between the quartz, and also some of the lighter areas, 

 represent irregular pieces of feldspar. (Described, p. 84.) 



