FISH CEEEK MOUNTAINS. 663 



Lime 3.87 3.78 



Magnesia . 2.65 2.76 



Manganese trace trace 



Soda .,... 3.39 3.30 



Potassa 4.46 4.46 



Lithia trace trace 



Carbonic acid trace trace 



Water 2.23 2.25 



100.00 100.14 

 Specific gravity, 2.6, 2.66. 



The percentage of silica agrees closely with the type-specimens from 

 the Washoe district ; but, at the expense of the lime, the yield of alkalies 

 seems very high, with a somewhat surprising quantity of potassa for a pro- 

 pylite, indicating a relatively large amount of orthoclase-feldspar. It is 

 interesting to note the detection of carbonic acid in the chemical analysis, 

 the microscopical analysis revealing the presence of minute calcite crystals 

 in the hornblende, as already described. 



The rhyolite of Mount Moses and the hills to the north and east pre- 

 sents but little variety in its general aspect, and resembles the main mass 

 of the Augusta and Desatoya Mountains. It extends westward in a low 

 ridge to the Havallah Range, eastward to Reese River Valley, and may 

 be traced to the Shoshone Range, being covered only by a thin deposit of 

 Quaternary material in the valley of the river. The greater portion of the 

 rhyolite is characterized by a crystalline granular groundmass, with pre- 

 vailing gray and reddish-gray colors, more or less discolored by iron, which 

 appears to be derived from minute grains of magnetite. In texture, the 

 rock has a rough, porous habit, disintegrating readily. The most marked 

 feature is the great number of large grains of black quartz scattered through 

 the groundmass, whose color Professor Zirkel has shown, under the microscope, 

 is due to the presence of oxide of iron filling microscopical fissures in the 

 quartz. In general, the sanidin crystals are much shattered, and, where 

 well preserved, appear quite small. As in the propylite, the triclinic forms 

 are accompanied hy some monoclinic ones ; so here, in the rhyolite, the 



