CENTEAL AI^D EASTERN NEVADA. 345 



is met : first, the small granite bodies ; then the dark dioritic rocks, forming 

 jagged, pinDacled cliffs on either side, and the high, sharp peak between the 

 two caiions, and having a horizontal width of about two miles. A small extent 

 of slates, in which some quartz veins have been prospected in the North Twin 

 River Canon, separates these rocks from the propylites, which stretch across 

 these two canons in a northwest and southeasterly direction. This would 

 seem to be the direction of the propylite fissure from the fact that a some- 

 what similar succession of varieties of propylite, having the same general 

 strike, is crossed in either caiion. The propylite forms only the entrance to 

 the narrow gorges of the lower portions of these canons, which open out above 

 into large interior basins, with comparatively smooth debris-covered slopes; 

 in the bottoms of these basins is found the propylite, extending up to near 

 their heads, while, except along the line of contact with the slates on the 

 northeastern limit of the body, the tops of the higher ridges are formed of 

 rhyolite, which extends in a semicircle around these basins, forming the 

 summits of the high points above the Hot Spring, and of the Mount Poston 

 ridge. 



The propylites are of colors varying from brown and green to mauve and 

 white; they consist in general of a homogeneous, feldspathic paste, inclosing 

 crystals of white, oligoclase feldspar and light-green hornblende, and, as acces- 

 sory constituents, rounded grains of quartz and crystals of bronze-colored 

 magnesian mica; only in the green varieties does the hornblende seem to form 

 any considerable portion of the rock, and these are also generally free from 

 quartz; the quartz is more abundant in the lighter-colored rocks, all of which, 

 however, contain a certain amount of hornblende. 



In the north fork of the North Twin River Canon the most quartzose 

 variety of propylite found, of a whitish green color, was overlaid by a rhyolite 

 of white, compact matrix, inclosing large grains of free quartz, which would 

 seem to form a transition between the two rocks; the rhyolite next west of 

 the Ophir Canon granite has also some resemblance to a propylite, being a 

 compact, reddish matrix, inclosing crystals of white feldspar with its quartz; 

 but neither of these rhyolites contains hornblende or mica, one of which is 

 always found in the propylites. 

 44 



