JACOB'S PROMONTORY' 641 



- The rliyolites along the western slopes of the Archaean body belong 

 to the typical red porphyritic variety, already so frequently described, here 

 containing considerable free quartz. They seem to overlie extensive 

 deposits of white volcanic ash, which form the flanks of the range, and 

 which can with difficulty be distinguished from the upturned Tertiary beds. 

 These rhyolites all incline to the westward with the slopes of the range. 

 The same prevailing red porphyritic rhyolite forms the summit of the range 

 south of the Archaean body, extending to the low gap west of Jacob's 

 Promontory. 



In the valley of Job's Creek, to the west of the range, and in the lowj 

 bench-like foot-hills,, at the head of Lone Hill Valley, are exposures of 

 Miocene beds, consisting of brown and buff sandstones, dipping at an angle 

 of 20° to 25° to the w^estward. As indicated by the fragments on the 

 surface and in the stream-beds, these Tertiaries are underlaid by quartz- 

 ites. Of the quartzites, a small, obscure outcrop is found at the head 

 of Job's Creek, dipping to the west, which has been referred to the AVeber 

 group. The bluffs facing the low gap west of Jacob's Promontory are 

 formed of beds of a dark-gray trachytic-looking rhyolite, which dip about 

 ] 0° to the westward. The rock has the rough, porous texture of a trachyte, 

 and contains sanidin, hornblende, and mica in a dark-gray groundmass. 

 The groundmass, however, has the fluidal structure of the rhyolite, and is 

 seen to be full of very minute grains of free quartz. 



Jacob's Promontory is a group of low hills extending eastward, out into 

 the Reese River Valley, from the Shoshone Range, made up of highly-meta- 

 morphosed dark-blue quartzites with cherty seams, which are exposed in 

 the ravines, having a general, north and south strike, but whose structure is 

 very obscure.. The quartzites are so blackened on the surface that they are 

 easily confounded at a little distance with the volcanic rocks, which cover 

 the greater part of the surface of the hills. They are much stained with 

 iron, and apparently mineral-bearing. 



The northern point of the hills, and the little outlying hill through 

 which Reese River has cut its bed in a caiion-like gorge, as well as the 

 southern foot-hills, are composed of a dark augite-andesite, having the 

 resinous lustre peculiar to this rock.- It has a columnar structure, and is 



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