612 DESCItlPTIVE GEOLOGY. 



Mount Neva to its very summit, and evidently cover most of tlic nortliern 

 slopes toward the Owyliee Valley, though these were not examined by 

 our parties. Doubtless, other outcrops of the older volcanic rocks might 

 have been found by a more detailed search. Sufficient data were obtained, 

 however, to show that this mountain mass consists mainly of a series of 

 steeply upturned quartzites and slates, along whose eastern base there has 

 been an outflow of propylite and andesite, the wliole having been covered 

 by thin flows of rhyolite, which have been sufficiently eroded in the 

 deeper-cut ravines to expose tlie underlying sedimentary rocks. 



In the deep ravine on the western face of Mount Neva, a considerable 

 thickness of quartzites and dark siliceous argillites, with cherty seams dipping 

 steeply to the westward, was observed. Their general character resembles 

 that of the quartzite body of the Seetoya Mountains on the east side of 

 Independence Valley, being that of rather saccharoidal rocks, of a bluish- 

 white color. One bed was noticeable for a wavy, vein-like, blue shading 

 through the white mass, occasioned by accumulations of graphite in fine 

 streaks through the rock. The lower end of the ravine is narrow and canon- 

 like, its walls being formed of a dark-gray pearlitic rhyolite, having an 

 unusually massive structure, and a tendency to columnar weathering. It 

 is very rich in sanidin and mica, and contains also some plagioclase and a 

 considerable amount of free quartz. The glassy groundmass is of a gray 

 color, as seen under the microscope, and full of microlites in a great variety 

 of forms, but contains neither hornblende nor augite. 



Squaw Valley Region. — The region to the west of the northern por- 

 tion of the Cortez Range consists of a series of low rolling hills and east 

 and west valleys, formed by flows of rhyolite and basalt, in which, with the 

 exception of the isolated outcrop near White's Ranch, no sedimentary rocks 

 were found. The valleys which occupy the lines of depression between 

 these ridges are mostly filled with Quaternary debris, but in Squaw Valley 

 and Rock Creek Valley there is a considerable development of stratified 

 deposits composed largely of volcanic ash, which have been referred to the 

 Humboldt Pliocene. 



The more northern line of elevation, that of Owyhee Bluffs, presents a 

 rather more abrupt face to the south and long gentle slopes to the north, 



