EAFT ElVER MOUNTAINS. 429 



recent Quaternary deposits. The rock is a nearly structureless, medium- 

 grained granite-mass, characterized by uniform texture and a pearl-gray 

 color, and composed of the normal granitic constituents, quartz, feldspar, 

 and mica. It decomposes readily; its surface being generally covered with 

 detrital material, while the spurs and ridges everywhere present smooth, 

 rounded outlines, with many fanciful forms of erosion. 



The range to the north of the wagon-road was but cursorily examined. 

 Next to the granite, to the north, are hard, compact, steel-gray slates, which 

 pass into dark-bluish limestones, intercalated with cherty bands, in which no 

 fossils were found. The main ridge to the north seems to be mainly com- 

 posed of similar blue limestones, generally thinly bedded, with a large devel- 

 opment of dark-colored argillaceous and calcareous shales, splitting into very 

 thin laminae, having a north and south strike, and dipping to the eastward. 

 A microscopical examination shows that the dark color of these slates is due 

 to a mixture of opaque black particles, probably of carbonaceous material, 

 A few fossiliferous beds were observed, from which were obtained an Avi- 

 culopecten, whose species could not be determined. The western slopes of 

 the range are covered high up on the flanks by heavy white beds, sloping 

 gently toward the centre of the valley, composed of fine, white, pumiceous 

 sands, loose sandstones, and fine conglomerates, which have been referred to 

 the horizon of the Humboldt Pliocene, from their general resemblance to 

 these beds as developed in the valley of the Upper Humboldt. 



At the southernmost extremity of the mountains, and just below the 

 granite body, occurs a somewhat isolated group of hills, which, in their geo- 

 logical character, differ somewhat from the formation surrounding the granite 

 to the west, but which have, nevertheless, been referred to the same horizon. 

 Their structure is evidently somewhat complicated, and was not made out 

 very clearly. The same dark, steel-gray slates are seen as in the pass, and 

 would appear to be the underlying rock. Dark-brown quartzites carrying 

 small amounts of carbonate of lime and dark bluish-gray limestones make up 

 the series of the beds. These hills are traversed by dikes of fine-grained 

 intrusive rocks, which, under the microscope, present an exceedingly fine 

 crypto-crystalline base, filled with countless numbers of minute microlitic 

 forms, fragments of triclinic feldspar, and a few grains of magnetite. 



Along the base of the higher ranges, and bordering the desert at irreg- 



