758 DESCRIPTIVE GEOLOGY. 



Mountain, rises 1,000 to 1,200 feet in precipitous walls above the bottom 

 of tlie valley, and trends approximately northwest and southeast. 



North of Black Canon, bordering the Humboldt Valley, is an irregu- 

 lar grou]) of rhyolitic hills, but a few hundred feet in height, through which 

 protrude small bodies of granite, already mentioned, and obscure patches 

 of Jurassic slates, the latter only being observed on the western side. This 

 rhyolite is a gray, microfelsitic rock, in general poor in mineral secretions, 

 but traversed by glassy hyaline varieties of obsidian, pitchstone and pearl- 

 ite. One variety of rhyolite, in some of the hand-specimens, looks remark- 

 ably like trachyte, and yet in the field, and even under the microscope, 

 shows the banded structure with the broken, undulating lines so character- 

 istic of rhyolite. Macroscopically, the only mineral observed is sanidin. 

 Still another variety is characterized by a lithoidal base, through which are 

 scattered only fragments of sanidin and quartz. It has an appearance and 

 fracture closely resembling porcelain, and a mottled surface like many varie- 

 gated marbles. ' Although presenting marked physical differences from the 

 microfelsitic and granular varieties rich in secreted minerals, a chemical 

 analysis reveals much the same ultimate composition as the rhyolites from 

 the West Humboldt and Pah-Ute Ranges, except that it may be a little 

 richer in lime. In this case the silica seems to be dissolved in the magma, 

 producing a porcelainous base instead of a microfelsitic groundmass rich in 

 secreted quartz. 



Mr. R. W. Woodward, who analyzed this rock, reports as ingredients : 



Silica 74.95 74.87 



Alumina 13.61 . . . . 



Ferric oxide 0.54 



Lime 2.02 2.18 



Soda 3.72 3.83 



Potassa 4.85 4.90 



Water ' 0.64 0.66 " 



100.33 

 Specific gravity, 2.44, 2.55. 



