WACHOE MOUNTAINS. 479 



sometimes having a semi-translucent character. Under the microscope, 

 the quartz is seen to contain great numbers of hquid-inckisions, within 

 which are often remarkably perfect cubes of salt, some of which seem to 

 be rounded on the edges. The groundmass is a crystalline aggregation of 

 feldspar and quartz, isolated particles of which are also found included 

 within the quartz. 



The rhyolites which surround the limestone body of the Wachoe 

 Mountains present considerable variety in appearance and structure. The 

 most prominent rhyolite in Spring Caiion is a compact porphyritic variety, 

 having somewhat the appearance of a hornstone. It contains, in a greenish- 

 drab groundmass, crystals of dark, smoky quartz and occasional feldspars, 

 but no mica. Through the mass are inclusions of jaspery-looking frag- 

 ments, which seem to be pebbles entangled in the groundmass. Besides 

 these are frequent green, earthy spots, apparently resulting from a partial 

 decomposition of the felsitic groundmass. In the centre of some of the 

 larger of such parts is found a greenish, granular substance, which efferves- 

 ces with acids, around which the color of the groundmass shades off from 

 a zone of green into the reddish hue of the unaltered rock. It would seem, 

 therefore, that these greenish colors are due to the decomposition of included 

 particles of limestone, which have reacted upon the groundmass of the rock. 

 Under the microscope, the groundmass is seen to be made up of axial and 

 central iibrations, and both quartz and feldspars to be rich in glaBS-inclu- 

 sions. 



A second variety of rhyolite from Spring Canon is also a porphyritic 

 rock of a brick-red color, in which, however, the groundmass is less com- 

 pact and homogeneous. It contains, porphyritically-imbedded, white crys- 

 tals of sanidin-feldspar and prisms of hornblende, but likewise no mica. 

 Another variety from near the mouth of the canon is a gray rock of a 

 somewhat granitoid structure, rich in hornblende and feldspar, but showing 

 no free quartz. The groundmass, which is in relatively small proportion 

 in the rock, is of a leaden-gray color and compact felsitic texture. Under 

 the microscope, this groundmass is seen to be a dense aggregation of almost 

 colorless microlites, full of transverse cracks, along which it has become 



