LAKE EANGE. 815 



consist mainly of triclinic forms, associated with a little orthoclase. Quartz 

 and hornblende are also present in considerable amounts. In thin sections, 

 under the microscope, the presence of large numbers of apatite crystals are 

 distinctly revealed, and the quartz-grains are shown to be very poor in 

 liquid-inclusions ; two characteristic features of the later eruptive granites 

 of Nevada. The rock is essentially a mica-hornblende-plagioclase-gneiss, 

 a composition that suggests the name of dioritic gneiss. Inasmuch as the 

 rock so closely resembles the surrounding granites, it may be well to men- 

 tion that neither macroscopically nor microscopically was the presence of 

 any amber-colored sphene detected. All the mineral constituents seem to 

 have a fresh, unaltered appearance. 



The gray slate beds have been referred to the Jurassic solely upon 

 theoretical grounds and from their resemblance to strata overlying well- 

 recognized Jurassic limestone in other localities. As they form smooth, 

 rounded hills and domes, contrasting sharply with the more rugged granite 

 and black basalts, the lines of contact are easily seen. Tliey strike approxi- 

 mately northeast and southwest. 



North of Winnemucca Lake, the range bifurcates, and there is a second 

 ridge to the eastward, made up of a basaltic eruption, which inclines t® the 

 eastward with gentle slopes, being for the most part abrupt along the 

 western face. Similar massive eruptions skirt the base of the slates, and 

 break out along the granite flanks in isolated bodies, which, so far as 

 observed, are all fine-grained compact rocks. At the northwest corner of 

 Winnemucca Lake, there occurs a basalt differing somewhat from the 

 others by the prominence of brilliant feldspar crystals in an almost homo- 

 geneous groundmass, and bright, fresh olivine grains. Under the micro- 

 scope, the groundmass develops the characteristic glass-base, and the 

 feldspars are shown to be exceedingly rich in foreign inclusions. 



South of Pah-rennen Peak, the interior of the range has only been vis- 

 ited by topographical parties, the geological notes being obtained along the 

 lake shores, where, for considerable distances, at least, acidic volcanic rocks 

 appear from underneath massive eruptions of basalt. For this reason, the 

 higher portions of the range have been provisionally colored on the 

 geological map as basalt, and it is by no means improbable that on the ridges 



