834 DESCEIPTIVE GEOLOGY. 



Ferric oxide 6.12 6.05 



Ferrous oxide 3.84 3.86 



Manganous oxide 0.30 0.42 



Lime- 8.85 8.81 



Magnesia 3.02 2.98 



Soda 3.21 3.33 



Potassa 1.95 2.27 



Water and carbonic acid 5.35 5.26 



100.00 100.00 

 Specific gravity, 2.6, 2.7. 



Trachytes of a similar composition form the detached hills between 

 Wadsworth and Sheep Corral Cafion. Here the rock is dark brown to 

 nearly black; it overlies propylite and carries large crystals of both sanidin 

 and augite associated with a little hornblende. Although different in text- 

 ure from the heavy flows of augite-trachyte north of the Purple Hills, and 

 showing considerable mineralogical distinctions, such as carrying conspicu- 

 ous black hornblende crystals, the occasional occurrence of brown mica, 

 and a large proportion of glass, these rocks are doubtless derived from a 

 common source, and from their relations to the other trachytes are probably 

 also of the same age. 



In the open basin country just north of the Truckee Ferry are some 

 hills of light-colored rhyolite presenting various shades of brown, yellow, 

 reddish-yellow, light brick-red, and purple, the purple tints in general decid- 

 edly predominating. All these colors, although very brilliant, occupy but 

 small local patches in a prevailing white field of rhyolite. The purple and 

 green masses seem to belong to an earlier eruption than the white. Directly 

 over the white rhyolite lies a bed 150 feet thick, of dark trachytic breccia, 

 which, in passing upward, becomes more and more solid and compact, merg- 

 ing finally into a gray augite-trachyte. Indeed, this breccia is the first part 

 of the augite-trachyte eruption. In a microfelsitic groundmass appear 

 well-developed crystals of sanidin, biotite, and occasional large clear grains 

 of quartz. The latest flows of purple rhyolite are slightly brecciated, the 

 fragments which they contain being of the same material as the rock itself. 

 It is evident that these rhyolites pass under, and therefore antedate the 



