Vol. 4 i Smith. — Upper Region of Main Walker River. 



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rocks. The extinction of orthoclase was obtained in the section 

 of the clinopinacoid, oo p oo , (010). It. was further recognized 

 by a relief Lower than quartz. The plagioclase is larger, having 

 dimensions of 0.5 mm. and 1.5 mm. It contains inclusions of 

 biotite, pyroxene partly altered to green hornblende, iron ore, 

 some of which shows crystal faces, and hexagonal and lath- 

 shaped sections of apatite. Some of the larger plagioclases 

 include smaller ones of a slightly different orientation. Mag- 

 netite occurs in all the essential minerals, and is mostly devoid 

 of crystal faces. 



The occurrence of pyroxene in these two rocks indicates that 

 they belong to, perhaps, a less frequent class of rocks, namely, 

 the augite granites. It should be noted that these rocks show- 

 effects of pressure, since many of the minerals show wavy extinc- 

 tion and small crystals are faulted. 



The next three rocks, Nos. 94-, 95 and 157, show remarkable 

 similarity. They are all samples of the granite-porphyry dyke. 

 Nos. 94 and 95 were obtained west of the river, in Sing-ats'-e 

 Ridge, below the middle of the ridge, and No. 157 was obtained 

 in Cemetery Hill. The rock possesses porphyritic structure. 

 Carlsbad phenocrysts 1 em. in length, in granular groundmass 

 of about 2 mm., is characteristic. The groundmass is phaneric, 

 and its mineral content is readily recognizable in the hand- 

 specimen. There is some difference in the ferro-magnesian con- 

 tent. There is less of it in 157 than in 94, but No. 161, which 

 will not be described, is from the same exposure as No. 94, and 

 it shows less biotite. The hornblende seems to be as abundant 

 in one case as the other. In 95 the light and dark colored min- 

 erals are about equally abundant. In the microscopic slides of 

 all three the phenocrysts are microcline, which is recognized by 

 the combination of albite and pericline twins, which are very 

 fine and have an equal and simultaneous extinction. The ground- 

 mass is made up of microcline, plagioclase, quartz, biotite and 

 hornblende. The grain of the groundmass varies from 1.5 mm. 

 to 2 mm. The biotite is brown, with some leaching to a green 

 color. The hornblende is original, as it shows in sections normal 

 to c the prism and clino-pinacoidal faces go P(110), and co Poo 

 (010), and the prism faces are at an angle to one another of 

 125°. This also is the angle of the cleavages. The color is green. 



