Vol. 4] Lawson. — The Robinson Mining District. 



349 



a mile in greatest length along the line of the fault. On the 

 south side the purple rhyolite reposes partly upon the porphyry, 

 partly upon the Ely limestone and partly upon the Arcturus 

 shaly limestone. The thickness of the lava is estimated to be 

 here not less than 300 feet. At its base where it reposes upon 

 the older formations, no exposures were found of either the 

 black pitchstone or the white tuff and it is probable that they 

 are here absent from the section. The purple rhyolite of White 

 Hill differs somewhat in detail from that described at Quarry 

 Hill. It is of a more compact texture and a lighter color and 

 shows a more pronounced flow structure. It breaks down readily 

 into thin slabs or shaly fragments, a tendency which was not 

 observed at Quarry Hill. 



Petrographical Characters. — Hand specimens of the dark 

 pitchstone variety of the rhyolite present the characters of a 

 back glass thickly studded with sharply defined phenocrysts of 

 vitreous to dull white feldspar, from 1 to 4 mm. in length, small 

 dark quartzes and a few flakes of brown biotite. Under the 

 microscope the ground mass is a stippled gray glass, the stippled 

 appearance being due to the sporadic distribution of clusters of 

 polygonal microlites and stellate trichites. The glass has a lower 

 refractive power than balsam. In this glass are imbedded nu- 

 merous phenocrysts of cracked sanidine, more or less corroded 

 quartz, a little plagioclase and a few crystals of greenish brown 

 biotite. The plagioclase is in some cases enclosed in the sanidine 

 which also holds occasionally inclusions of glass. The sanidine 

 has a small optic angle giving in isotropic sections a nearly un- 

 iaxial figure and a negative sign. 



The more abundant variety of the rhyolite of purplish tint 

 is composed of a compact ground mass in which are imbedded 

 numerous vitreous phenocrysts of feldspar ranging in size up 

 to 5 mm., and dark quartz rarely exceeding 3 mm. in diameter. 



In thin section the ground mass is a turbid glass with nu- 

 merous small vaguely defined doubly refracting areas. The 

 feldspar is mostly sanidine with a little plagioclase. The quartz 

 has well defined boundaries of the usual dihexahedral habit, but 

 is occasionally somewhat resorbed. The microscope reveals a few 

 foils of biotite not apparent in the hand specimens. 



