G44 DESCRIPTIVE GEOLOGY. 



Mountains, and near the base of the Desatoya Mountains slope upward with 

 an easterly dip. The flows consist generally of reddish porphyritic rhyolites, 

 among which are some earthy varieties comparatively poor in crystals. 

 Alternating with these are prominent beds of black pearlitic rhyolite, pre- 

 senting all the gradations from a half-glassy rock containing crystals of sani- 

 din to an almost pure obsidian, with only a few imperfect casts of feldspar 

 crystals. Under the microscope, the groundmass of the latter is seen to con- 

 sist of deep-brown glass, containing no product of devitrification, while the 

 crystals of quartz, sanidin, and plagioclase-feldspars which occur in the 

 mass contain inclusions of the same dark-brown glass. These darker bands 

 are generally from 10 to 20 feet in thickness. 



Desatoya Mountains. — Only the northern portion of this range is 

 included within the map, whose most elevated portion is generally known 

 as the New Pass Mountains, from the mining district which was once estab- 

 lished at its southern base. This mountain mass, which is some six miles in 

 length, and rises about 4,000 feet above the surrounding valleys, consists 

 of a portion of an anticlinal uplift of Triassic limestones and sandstones 

 representing the Star Peak and Koipato groups. The main mass of the hills 

 is made up of heavy beds of green and purple quartzites and conglomerates, 

 striking north 20° east and dipping from 30° to 35° to the westward. Along 

 the western flanks are exposed in the ravines heavy blue limestones resting 

 conformably on these, which, toward the southern end of the range, bend 

 in strike sharply to the east, with a dip of 70° to the south, and soon dis- 

 appear under the flows of rhyolite. Owing to the smooth, rounded slopes 

 and comparatively few outcrops presented by the main mass of the hills, it 

 was impossible to obtain a continuous section of the siliceous beds, which 

 may correspond in Western Nevada to the lower red sandstones of the Tri- 

 assic formation, as observed in the Eocky Mountain region. 



In a section taken across the range from Gilbert's Creek to Ammonite 

 Canon, shown in section C — Dat the bottom of the map, about 6, 000 feet of the 

 Koipato beds are crossed. The lowest exposures show strata of a green- 

 ish, somewhat cherty quartzite. Above these, forming the summit of the 

 ridge, is a breccia-like conglomerate made up of greenish and purple cherty 

 fragments, with a red cement, overlaid by a thickness of about 1,000 feet 



