PLATE XXI. 



Fig. a. 



(Sp. No. 32136. Without analyzer, x 90. ) 



Rhyolite-porphyry with aureoled phenocrysts. The finest-grained type of mieropoikilitic 

 "texture is hero represented. The groundiuass of this porphyry consists of rounded areas of material 

 ("quartz epongeuse"), corresponding to that forming the zones around the phenocrysts. Between 

 these areas there may be found in places small feldspars. These photomicrographs, represented in 

 iigs. A and Ji, PI. XX, and in this figure, show every gradation in the micropoiliilitic texture, from 

 that which is with difificulty distinguishable as such to the coarser-grained unmistakable variety. 

 (Described, p. 85.) 



Fig. B. 



(Sp. No. 32136. With analyzer, x 90.) 



Rhyolite-porphyry with aureoled phenocrysts. This is the same section as is represented above 

 when viewed between crossed nichols. The texture of the groundmass is brought out somewhat 

 better. The feldspars especially become more noticeable. For Instance, one Carlsbad twin may be 

 seen at the lower right-hand corner of the phenocryst partly indenting the aureole. Other feldspars 

 may lie noticed through the groundmass. In other portions of the .section from which this photo- 

 micrograph is taken the quartz phenocrysts have no aureoles and the groundmass possesses an imper- 

 fect micrograuitic texture. This figure brings out clearly- the gradation toward that texture. 

 (Described, p. 85.) 



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