838 DESCKIPTIVE GEOLOGY. 



in points one-sixteenth of an inch in height, and each point is crowned 

 with a transparent globule of hyalite. A distinct horizontal bedding is 

 frequently noticed in these trachytes. Their limit to the westward is not 

 accurately known, but they are supposed to extend down to the valley west 

 of Ormsby Peak. 



The gray, glassy augite-trachytes which break out along the foot-hills 

 just south of Sheep Corral Canon are supposed to be later than the sanidin- 

 trachytes ; a decision, however, which does not depend upon any super- - 

 position, for they have not been seen in any immediate contact, but rather 

 upon field impressions, and because the gray trachytes rise in bold hills 

 with a much fresher and less weathered appearance. 



Dr. Anger, of Leipsic, who made an analysis of this augite-trachyte, 

 found the following constituents : 



Silica - 68.81 



Alumina 13.62 



Ferrous oxide 3.91 



Lime 4.30 



Magnesia 2.74 



Soda 2.68 



Potassa 2.56 



Loss b}'- ignition 2.30 



100.92 



This rock stands over 18 per cent, richer in silica than the augite-tra- 

 chyte from the hills back of the Truckee Ferry, and is unusually acidic for 

 a rock so full of augite, having an amount of silica equal to that found in 

 many varieties of rhyolite, while the Truckee Ferry Hills approach more 

 nearly in chemical composition to basalt. 



The propylite at the entrance of Sheep Corral Canon is a greenish- 

 gray rock, closely resembling the typical body of Washoe, except that it 

 carries a few grains of quartz. It is a dense, tough rock, with well-devel- 

 oped feldspars and occasional hornblendes. The microscope reveals the 

 presence of apatite and magnetite. 



