CEFTEAL AND EASTERN NEVADA. 333 



by several quartz veins, and some dikes of a light-colored breccia mat- 

 ter, having a northwest direction ; the latter may have the same origin 

 as the rhyolite, which forms the saddle connecting these hills with the 

 Western or Austin ridge. On the eastern slopes the granite forms more 

 jagged spurs, which are covered with a considerable growth of juniper, while 

 the extreme eastern foot-hills are so covered with debris that the underlying 

 rocks are not apparent. Geneva, or Birch Creek, Cafion presents a fine sec- 

 tion of this body of granite ; it is a narrow gorge, worn by the action of the 

 water, in a generally straight course, though zigzagging in detail owing to the 

 unequal resistance offered by different parts of the granite to the action of 

 erosion; the walls rise very steeply on either side from 1,000 to 2,000 feet. 

 The western part of the body is a very interesting variety of the so-called 

 giant granite, composed of a mass of large crystals of orthoclase from one to 

 two inches long, and rounded grains of quartz filling up the interstices of, and 

 frequently inclosed within, these crystals ; the mica, which is of the white potash 

 variety, occurs in thin sheets between the crystals of feldspar. The general 

 body of the granite is a white mass of quartz and feldspar, in which large crystals 

 of orthoclase are porphyritically imbedded ; small crystals of green magnesia 

 mica and a little white potash mica are generally distributed throughout the 

 rock. As exposed in the Commercial Company's tunnel, however, the mica 

 is replaced by a light green mineral resembling chlorite. This tunnel has 

 been cut over 800 feet into the granite spur to the south from near the mouth 

 of the canon, for the purpose of reaching the Big Smoky vein, which crops 

 out about two hundred feet above it on the ridge of the spur, of which it has 

 the general direction. The workings on this vein disclose a large irregular 

 body of quartz and calcspar from 15 to 20 feet thick, stained by graphite ; it 

 would seem to be at the contact of the slates and granite, since these rocks are 

 cut in alternate bands by the tunnel which reaches it from the south. 



A noticeable feature of the granite exposed in this gorge is the regular 

 bedding of the mass, amounting almost to a stratification, having a gentle dip 

 to the westward ; it may be estimated that a thickness of over 3,000 feet of 

 granite is exposed here. 



About half-way up the canon the granite is traversed by a dike about 25 

 feet thick of close-grained greenstone, having a northeast and southwest strike, 



