100 



University of California Publications. 



[Geology 



Bhyolite. — The rhyolite is of constant characteristics. It is 

 a porphyritic rock, with well-developed phenocrysts of quartz, 

 glassy feldspar, and flakes of brown biotite, in a white ground- 

 mass weathering to pink. A tendency to vescicular texture is 

 frecpiently observed. The lower portion of this volcanic rock is 

 often a breccia, as seen capping the river gravels south of Frank- 

 town. No rhyolitie dikes or vents of any kind have been noted. 



Pitchstone. — An acid pitchstone occurs in a few small areas 

 associated with rhyolite. One of these is in the low hills south 

 of Franktown. Here the glassy rock occurs as a small oval spot 

 a few hundred feet in diameter on the flat top of the hill with 

 the longer axis east and west. A dike extends from the larger 

 mass several hundred yards northerly down the canon. Three 

 other oval patches of this rock occur in the low spur of the Vir- 

 ginia Range north of Carson. These are shown rather diagram- 

 matically on the map (pi. 28) due to their small size. The longer 

 axis of these small areas is always easterly and westerly, and 

 appear to make a line of movement in this direction. No de- 

 tailed investigation has been made of this rock, but because of 

 its association with only the rhyolite and the fact that it shows a 

 few phenocrysts of glassy feldspar in a light colored vitreous 

 ground-mass, it is regarded as genetically connected with the 

 rhyolite. It is probably but little later in age. 



Andesite. — The larger part of the andesite is a hornblende 

 bearing rock, but the grain varies from very fine to porphyritic, 

 with the phenocrysts of hornblende 1 cm. in length. The color 

 is a dark gray, weathering light gray. A slight tendency to 

 vescicular texture is at times noted, as in Little Valley. This 

 rock is found in a great number of dikes, in a few cones, and in 

 a number of small residual patches, or in the granite, represent- 

 ing once much larger flows. The dikes usually have a north- 

 south or north-northeast — south-southwest strike, but a few 

 show an east-west direction. They are exceedingly common 

 throughout the granite area, but on account of their small size 

 and complex displacement by faults only the largest and most 

 important one, that in Lakeview Hill, is mapped. A few feet 

 east of the rhyolite area northeast of Marlett Peak is a small 

 four-foot dike of andesitic breccia or agglomerate. Unfortu- 



