MONTEZUMA EANGE. 763 



liantly colored red, yellow, gray, pink, and white rliyolites, having- a 

 tendency to horizontal bedding-. The texture of most of these rocks is 

 somewhat earthy and porous, passing into lithoidal varieties. In their topo- 

 grtiphical features and field aspects, they resemble those from the Mopung 

 Hills on the ojDposite side of the Humboldt Valley, with the single excep- 

 tion that there are here many well-developed feldspars. Quartz-grains are 

 present, although not very abundant, other secreted minerals being some- 

 what rare. A bright orange-colored rhyolite having a comparatively 

 homogeneous groundmass, through which are scattered numerous small 

 feldspars, yielded to Mr. R. W. Woodward, upon chemical analysis- 



Silica 70.29 70.15 



Alumina 16.10 15.77 



Ferric oxide 1.17 1.18 



Lime 1.09 1.12 



Manganese 0.16 Ull 



Magnesia 0.26 0.27 



Soda - 3.66 3.79 



Potassa.". - ' 5.66 5.60 



Lithia. trace trace 



Water ." 1.36 1.38 



99.75 . 99.37 

 Specific gravity, 2.12, 2.20. 



Overlying these brilliantly colored rhyolites are broad masses of black 

 basalt, which not only by their marked contrast of color, but by their great 

 difference in topographical features, may be easily distinguished at long 

 distances from the underlying rock. 



In the low ravines eroded in the light rhyolite are several obscure 

 outcrops of dark crystalline rocks, some of them resembling the augitic 

 trachytes which break through the rhyolite at Truckee Ferry, Truckee 

 Canon, while others look more like propylite, and show hornblende in fine 

 green spiculse, scattered through the dark feldspathic groundmass. Although 

 they have been but little examined, and their true relations to the surround- 

 ing rocks not well understood, they afford some interest as suggesting the 



