716 DESCRIPTIVE GEOLOGY. 



are either vertical or dip 80° to the southwest, while 'the northeast planes 

 are quite vertical. The schists themselves are chiefly made up of quartz 

 and muscovite, with a great quantity of infiltrated brown iron oxide. Oc- 

 casional crystals of monoclinic feldspar and grains of crystalline quartz as 

 large as a pea occur, around which the muscovite is bent, forming brilliant 

 little knobs. A number of narrow dikes penetrate the granites and schists, 

 including both diabase and Tertiary intrusive rocks. The former, so far as 

 observed, follow the northeast lines of contact between the two older 

 bodies, apparently agreeing roughly with the strike of the great Archaean 

 mass underlying the Pah-Ute and Havallah Ranges to the eastward; the 

 latter follow the northwest fissure-lines of the granite, and are evidently 

 more recent than the northeast dikes. These Tertiary dikes, which play 

 a very inconspicuous part, consist of a brown decomposed earthy material, 

 too fine to tell by the unaided eye their true composition. In thin sections 

 under the microscope, however, they are found to be made up of decom- 

 posed plagioclase and characteristic hornblende, with a groundmass having 

 the structure of andesite. 



Although, as has been already mentioned, the trend of the northern 

 half of the range lies nearly north and south, the Triassic strata, which 

 make up the greater part of the mountains, geologically form an anticlinnl 

 fold whose direction is diagonally across the topographical uplift, hav- 

 ing a strike about north 30° east. The depression of Spring Valley Pass 

 and Sacramento Canon cuts across the anticlinal, axis, which is here formed 

 by a series of porphyroids, closely resembling erupted felsitic porphyries^ 

 but which are considered to be metamorphic products of the mixed quartz 

 and feldspar rocks of the series of beds underlying the limestone of the 

 Star Peak Triassic, and have been designated, after the Indian name of the 

 West Humboldt Eange, as the Koipato beds. They are regarded as of the 

 same geological horizon as the lower members of the Eed Beds of the Eocky 

 Monntain region, although differing widely from them in their petrograph- 

 ical habit. These porphyroids are overlaid in some places, and in others 

 underlaid, by normal quartzites and argillaceous schists, which are more or 

 less metamorphosed. Directly and conformably overlying the . quartzites 

 are a series of intercalated formations, consisting in general of limestone, 



