482 DESCRIPTIVE GEOLOGY. 



than the typical andesites from Washoe, and, unlike the latter, the potassa 

 is far in excess of the soda. All augitic andesites are apparently higher in 

 silica than the hornblendic group, but probably yield a much lower per- 

 centage than this rock ffom the Wachoe Mountains. A silica determination 

 of the Spring Cai3on augitic andesite gave 62,46 per cent. 



In general, the rhyolites from the northern ' end of the mountains 

 resemble those already described from the region of Spring Canon, Few 

 localities, however, within the same area, exhibit such an endless variety of 

 rhyolitic forms, at least so far as superficial appearances are concerned. 

 They mainly consist of rocks, with a fine microfelsitic groundmass, carry- 

 ing small crystals of sanidin and quartz-grains, with an occasional horn- 

 blende. They occur in all shades of gray, red, purple, black, white, and 

 cream-color, and varying from a fine-grained compact mass to a coarse, 

 rough, vesicular texture. This variety in color is rendered still more strik- 

 ing by the presence of pumice, glassy, and half-glassy bodies of brilliant 

 tints in a light-colored groundmass. One very characteristic rock is made 

 up of a light-red groundmass, through which occur blood-red bands of a 

 jasper-like substance, frequently enclosing both quartz and sanidin, which 

 suggests the lithoid variety of rhyolites from the Pah-Ute Range. These 

 bands are arranged in a roughly parallel manner, giving the rock a some- 

 what stratified appearance. 



Under the microscope, the quartz and feldspar are shown to be almost 

 free from microscopical impurities. But in one of the quartz-grains, Pro- 

 fessor Zirkel detected a number of liquid-inclusions, with mobile bubbles, 

 a very surprising observation, as in no other known rhyolite has such an 

 occurrence been noted. And it is prcfbably no exaggeration to state that the 

 number of such quartz-grains that have been subjected to microscopical 

 scrutiny must reach many thousands. Professor Zirkel also mentions that 

 close by this grain of quartz occurs a decomposed feldspar, which closely 

 resembles those found in granites, and suggests that both these minerals 

 are bodies foreign to the rhyolite. This suggestion seems all the more 

 plausible when we recall the close proximity of the later rock to the large 

 body of Wachoe Mountain granite. 



