772 DESCEIPTIVE GEOLOGY. 



rock in the Kawsoh Mountains, which forms a group of low diorite hills at 

 their northern end. It is a fine-grained, dense rock, of a dark-gray color, 

 carrying a little quartz, and breaking with difficulty under the hammer. It 

 is penetrated by a number of quartz veins striking north and south. 



At the extreme northeastern corner of the mountains bordering the 

 Carson Desert are exposed some bluffs of fine-grained, dark-gray sanidin- 

 trachyte, rising not more than 200 feet above the plain, and inclined gently 

 to the northwest. The microscope reveals the presence of small secretions of 

 tridymite. All the rest of the range, extending in a north and south direc- 

 tion for 20 miles, consists of basalt capping Tertiary strata. The latter are 

 exposed along the base of the ridges and in the low interior valleys, one of 

 which cuts completely across the mountains, having a broad interior basin, 

 but offering no good section of the beds. These basaltic masses form long 

 narrow parallel ridges, inclined toward the east, and presenting their steeper 

 slopes westward. In lithological habit, they exhibit a great uniformity of 

 character, varying somewhat in texture, as in all other localities of equal 

 area, but in general are fine-grained, and in places quite porous. Macro- 

 scopically, few minerals can be distinguished, but microscopical analysis 

 shows the normal plagioclase-augite composition of basalt, with the half- 

 glassy globulitic base. The rock from the highest peak is quite fine-grained, 

 and has a reddish tinge, which appears to be produced by ferric oxide fill- 

 ing minute cavities and fissures. Mineralogically, the most interesting fea- 

 ture of these basalts is the finding, by Professor Zirkel,* of hexagonal 

 flakes of tridymite, arranged in a manner quite similar to those observed in 

 the acidic volcanic rocks. It is noticeable that tridymite, which is of rare 

 occurrence even in acidic rocks, has also been recognized in the sanidin- 

 trachyte from the extreme northern end of the Kawsoh Mountains. Olivine 

 appears to be pretty well disseminated through the basalt, and may be 

 easily recognized, even macroscopically. 



The denuding action of drifting sands upon hard crystalline rocks, 

 which is by no means an uncommon phenomenon in the arid regions of 

 Nevada, is beautifully shown in the ravines and basins at the southern end 

 of the Kawsoh Mountains, where the basaltic walls show polished, grooved, 



'Microscopical Petrography, vol. vi, 239. 



