IBENPAH MOUNTAINS. 475 



.acid was clearly shown. It is probably combined with the excess of soda, 

 as borax. Somewhat similar incrustations are found in the Nevada Basin, 

 and usually in close relation with regions of rhyolitic outflows. It would 

 seem of interest that the occurrence here in Deep Creek Valley has the 

 same geological surroundings. 



To the west of the valley are long flat Tertiary ridges and narrow 

 stream-beds cut through white beds of fine sands and marls, with some 

 gravelly conglomerates. These beds have a general lithological resem- 

 blance to those of the Humboldt Pliocene, as developed in the valleys of the 

 Upper Humboldt, containing, in particular, a bed of white, very fine- 

 grained material, having a rough feel like a volcanic ash, which is almost 

 identical with some beds found to the north of Toano, and have, therefore, 

 been referred to that horizon, though, as far as our observations extended, 

 no palseontological evidence of their age was obtained. 



The divide between Deep Creek and Gosi-Ute Valley is a low, rather 

 flat-topped ridge, in which but few outcrops of underlying rocks were found. 

 The evidence of fragments contained in the float shows that it is largely 

 composed of eruptive rocks. These are mostly rhyolites. At one point, 

 however, was obtained a rather singular rock, which has been colored as 

 an andesite. On its weathered surface it has a light mauve color, and resem- 

 bles a rhyolite, but in fresh fracture shows a dark-brown, compact, felsitic 

 groundmass, in which are enclosed small white crystals of plagioclase feld- 

 spar and frequent brown micas, together with hornblende and occasional 

 rounded grains of cracked quartz. The dark-green crystals of hornblende 

 frequently show a zonal decomposition on the outer surfaces. The ground- 

 mass is extremely homogeneous and almost half-glassy. The feldspars, 

 as far as can be seen by the unaided eye, are mostly plagioclase ; while the 

 quartz seems to be an entirely accessory constituent, and the rock has, 

 therefore, been considered to hold the same relation to the normal andesite 

 as the quartziferous trachytes do to the normal trachyte. 



In the low hills to the north of Deep Creek Valley, and between it and 

 the Desert, are occasional obscure outcrops of limestones, very much broken 

 and dipping irregularly, associated with isolated flows of rhyolite. Theaccu- 



