346 MDflNG INDUSTRY. 



On the high ridge which separates South Twin River from the next canon 

 south,! a green breccia, of slightly foliated structure, inclosing purple-colored 

 siliceous concretions, with quartz crystals disseminated through the mass, 

 marks the line of separation of the mauve-colored propylite and the rhyolite, 

 which forms the somewhat flat-topped peaks between the head of Twin River 

 Canon and the Hot Springs. This rhyolite has a reddish-gray, microcrystal- 

 line, feldspathic matrix, inclosing quartz grains, many of which are quite black, 

 so that, at the first glance, they might be taken for mica, but no mica is found 

 in the rock; the mass seems vesicular, but the larger cavities are shown to be 

 the molds of feldspar crystals, and the same may be true of those which are 

 so small that their form cannot be determined ; how far this rock extends to 

 the south is not known — probably for a considerable distance; it is, however, 

 essentially the same rock as the Mount Poston rhyolite, and may be considered 

 to form part of the same eruption. The main crest of the range, from Ophir 

 Caiion south to Mount Poston, seems to form an independent line of fissure, 

 having a direction nearly due north, this ridge being formed of rhyolites which 

 have a remarkable uniformity in composition and general appearance ; the rhy- 

 olite of Mount Beseler only differs from that of Mount Poston in that the latter 

 has a more compact matrix and a conchoidal fracture, while the former is more 

 vesicular and inclined to crumble on the weathered surfaces ; it has a purplish- 

 brown color, a rough, trachyte-like paste, carrying crystals of glassy feldspar 

 and smoky quartz, with apparently no mica. 



Mount Beseler is a conical-shaped peak, with debris-covered slopes, but 

 the ridge is generally rather flat, being formed of rhyolite beds, having a general 

 dip to the westward; just north of Mount Poston it spreads out into a wide, 

 table-topped hill, on whose edge semicircular, crater-like depressions, sur- 

 rounded by steep walls of columnar rhyolite, form the head of some of the 

 streams which run together to form the Reese River. 



Mount Poston rises a thousand feet above this ridge in a sharp peak, hav- 

 ing on the west a slope of nearly 40°, covered by lava blocks, and on the east 



^The topograpTiy of the map is liere slightly at fault; the waters of this cafion 

 take their rise at the northernmost of the peaks above the Hot Springs, and, flowing 

 north, break through the eastern ridge in a gorge similar to those of the Twin Eiver 

 Canons, indicated on the map by the short stream next south from the South Twin 

 River. 



