GEANITE MOUNTAIN. (391 



lah and the Carboniferous of the Battle Mountain group, or whether the Tri- 

 assic actually passed conformably over the Carboniferous-, and has subse- 

 quently suffered complete destruction by erosion, is a problem which cannot 

 at present be definitely solved, although the former proposition seems most 

 in accordance with observed facts. 



The crystalline rock-masses of Granite Mountain may, with scarcely 

 ainy doubt, be referred to the Archaean series. The rocks which form the 

 mountain are not, as the name would indicate, typical granite, but are allied 

 to the granitic family, being made up chiefly of quartz and feldspar, with 

 scarcely any mica or hornblende, a binary compound, which may perhaps 

 be designated as aplitlc granite. They are distinctly bedded, but without 

 any observable regularity in the arrangement of the individual constituent 

 minerals. The rock-masses vary somewhat in texture, but may be classed 

 as medium-grained. Light flesh-red is the prevailing color ; occasional 

 beds are more or less stained with red oxide of iron, deepening the tint, but in 

 general the feldspars present quite a fresh, undecomposed appearance. Ortho- 

 clase is the prevailing feldspar, usually dull and opaque ; the quartz occurs 

 in colorless translucent grains ; mica, when present, is found as dark biotite, 

 and seems much more prevalent in defined zones, and is usually segregated 

 into small bunches. In the interstices between the larger crystals are thin 

 coatings of ferruginous material. Thin sections under the microscope show 

 characteristic inclusions of carbonic acid, which disappear at a temperature 

 of 31° centigrade. 



On the northern slopes of the mountain, in a ravine near the summit, 

 and about one mile northeast from the geodetic station, are found minute 

 brilliant-black crystals of tourmahne. Still further to the eastward, and in 

 the region of the diabase dikes indicated on the map, occur small brown 

 iron garnets, not much larger than a pin-head. 



On the ridge east of the highest point occurs a narrow dike of typical 

 feldspar-porphyry. It has a nearly east and west strike ; where observed, 

 it was well defined, but was not followed for any distance. In texture, this 

 porphyry has an exceedingly fine-grained crypto-crystalline groundmass, 

 breaking with a sharp angular fracture. The color is grayish-white, and 

 but for the secretions of iron stains would be a beautiful rock. Brilliant 



