AUGUSTA MOUNTAINS. 649 



To the west of the Desatoya Mountains is a broad, shallow Qua- 

 ternary valley, in whose lowest portion, to the west of the centre, is 

 a small alkali flat or deposit of Lower Quaternary silt. This flat is from 

 500 to 600 feet lower than the corresponding portion of Smith's Valley. 

 It was evidently occupied at one time by an enclosed lake, whose drainage 

 was to the southwest. This valley occupies a lower level than the valleys 

 to the eastward, being but little over 5,000 feet above the sea. It is suc- 

 ceeded to the westward by a still deeper valley, the Osobb Salt Valley, 

 whoso lowest portions are from 3,300 to 3,500 feet only above the level of 

 the sea. Between these two valleys rises one of the highest ranges of this 

 portion of Nevada. 



Augusta Mountains. — This range south of the Shoshone Pass con- 

 .sists of a series of lofty peaks, whose summits attain an elevation of nearly 

 10,000 feet, and whose sides are deeply scored by sharp, narrow canons. 

 Its mass is made up almost entirely of eruptive rocks, ranging in age from 

 the oldest granites to the most recent rliyolites and basalts, with a small 

 development of sedimentary rocks of Triassic and Jurassic ages. The south- 

 ern portion of the range, near the limits of the map, as far as Clan Alpine 

 Canon, is made up of an original elevation of diabase, which is exposed at the 

 highest summit, and in isolated outcrops along the eastern foot-hills, which 

 has been overwhelmed by successive flows of andesite, rhyolite, and basalt. 

 The diabase is a compact, light-greenish to purplish-gray rock, made up of 

 plagioclase and augite, with a very little quartz. 



Clan Alpine Caiion, which heads in a high valley to the west of the main 

 crest of the range, affords a section of an immense thickness of rhyolite, 

 which has evidently come to the surface in mass, and shows no evidences 

 of forming thin flows. The main crest of the range, at the head of the 

 canon, shows protruding points of andesite, which were not covered by the 

 rhyolite flows. The andesite has a somewhat columnar structure, is of a 

 dark color, containing a large development of slender, prismatic crystals of 

 black hornblende. It is made up of plagioclase-feldspar and hornblende, 

 in a groundmass which is seen microscopically to be an aggregation of 

 minute colorless feldspars and brown hornblende-microlites. These long 

 hornblende prisms have a general parallel arrangement through the rock, 



