664 DESCRIPTIVE GEOLOGY. 



sanidins occur associated with some plagioclase. A specimen in the collec- 

 tion from the summit of Mount Moses may be regarded as a typical one, 

 not only of the Fish Creek Mountains,* but of this portion of Central Ne- 

 vada. A careful study of the rock, together with its microscopical analysis, 

 oifers little in addition to what has already been written, though it may be 

 well to state that the groundmass shows a tendency to develop a micro- 

 felsitic structure, that the sanidins are particularly well developed, while 

 hornblende and mica appear to be wanting. 



Mr. R. W. Woodward submitted the rock to chemical analysis, and 

 obtained the following constituents : 



Silica- '. 75.44 75.55 



Alumina '. 13.98 18.67 



Ferric oxide 0.54 0.56 



Lime 0.50 0.51 



Magnesia trace trace 



Soda 3.48 3.50 



Potassa 5.36 5.29 



Water 0.77 0.85 



100.07 99.93 

 Specific gravity,2.48, 2.5. 



Although the rock is rich in large quartz-grains, it does not show a higher 

 percentage of silica than is found in many other rhyolites, in which the 

 quartz is less prominent, but finely disseminated through the groundmass. 



In the region of Dacie Canon and the northern end of the mountains, 

 the rhyolites present some features not observed at Mount Moses. Here 

 they possess much less of a crystalline granular groundmass, and instead 

 a microfelsitic one, with a more compact texture, showing a great variety of 

 color. In crystalline secretions, they are comparatively poor, although 

 the same black quartz and vitreous sanidin are easily recognized. In some 

 of the reddish earthy varieties, dark magnesian biotite is also present. 



Apparently underlying these more compact varieties may frequently 

 be seen, exposed in the ravines which penetrate the edges of the table, alter- 

 nating bands of gray, chocolate, red, orange, and purple rocks composed of 



