640 DESCRIPTIVE GEOLOGY. 



microscope is seen to have a good deal of tlie character of a rhyolitic tufa, 

 being composed of grayish bodies of a groundmass rich in ferrite, between 

 which are bands and veins of colorless, granulated, hornstone-like material, 

 which has filled up the intervals between these other bodies. This is the 

 prevailing rock over the broad western slopes of Ravenswood Peak. 



On the northern slope of Ravenswood Peak, between the diorites and the 

 limestone body, the rhyolites are generally of dark color, and often so finely 

 laminated and have so much of an earthy appearance that they might be 

 mistaken for metamorphic slates. A careful examination, however, shows 

 the peculiar banded arrangement of the groundmass of rhyolite and a con- 

 siderable development of feldspar crystals. The rhyolites associated with 

 these are of a dark reddish-purple color, generally containing a little free 

 quartz, and feldspars which are much decomposed, while the groundmass 

 shows microscopically a sphserulitic structure. 



Along the eastern base of Ravenswood Peak is a great variety of flinty 

 hornstone-like rhyolites associated with reddish rhyolitic breccias. The 

 more compact of these rhyolites resemble very much the compact rhyolitic 

 tufas found on the River Range, near the Penn Canon coal-mines, having 

 something of the same bands of various colors, though less well defined, and 

 being entirely devoid of crystals. The mass seems to be an aggregation of 

 fine fragments of rhyolite, cemented by a chalcedony-like material which 

 surrounds them in agate-like bands. Among these rhyolites is the same dark 

 purple-gray, hornstone-like variety found in Reese River Canon, which 

 contains crystals of feldspar and quartz. Within the rhyolites is a very 

 considerable development of pure hornstone and flint of various colors, 

 and the summits of the hills are almost covered with flint chips made by the 

 Indians, to whom this has evidently been a source of supply for obtaining 

 their arrow-heads. High up on the spurs is an imperfectly bedded deposit 

 of coarse volcanic tufa, forming gravel-like beds made up of broken crystals 

 of feldspar, quartz, and mica, in a somewhat limy groundmass. Similar 

 reddish breccia-like rhyolites occur along the ridge between Ravenswood 

 Peak and the Archaean body, carrying only crystals of feldspar and quartz, 

 but as a general rule no mica or hornblende, in a somewhat earthy, felsitic 

 groundmass. 



