790 DESCEIPTIYE GEOLOGY. 



other crystals than the feldspars, which are seen to be sanidin, but in the 

 groundmass are black opacites, which by their form should be magnetite. 



In the canon on the western slopes of this ridge are seen a considerable 

 variety of rhyolites, and some Tertiary beds upturned at 20°, which, from 

 their composition and angle, have been assigned to the Truckee Miocene. 

 They consist of finely-laminated siliceous shales, of a whitish- drab color, 

 resembling some of those of the Truckee Valley beds, but containing no 

 diatoms in the specimens brought in. Over these are fine-grained con- 

 glomerates. The rhyolites exposed have evidently broken through these 

 Tertiary beds, and walled them in on the side toward the desert. They 

 are generally of white, porous, earthy varieties, showing only a few broken 

 crystals of feldspar and quartz. One variety is a fine-grained purplish rock, 

 containing no crystalline ingredients, but speckled all through the mass by 

 fine white dots, which may be decomposed feldspars. 



Other rhyolites and rhyolitic breccias presenting no specific differences 

 from those already described occur in the low hills which dot the open 

 region but little above the desert level, in the vicinity of Indian Spring. 

 In general, the slopes toward the desert on this eastern side are so gentle, • 

 and are composed of such soft material, that no distinct terrace-lines can' be 

 traced, although, as will be seen later, the ancient lakes probably extended 

 well up on the slopes of the hills, and covered most of the interior valleys. 

 The first distinct tufa deposits or outcrops ^re found about 8 or 4 miles west 

 of the foot-hills on the Kamma Mountains, where they mark rudely the limit 

 of the sagebrush-covered Upper Quaternary slopes, the desert beyond being 

 occupied by the fine muds and silts of the Lower Quaternary, which in 

 dry seasons present a hard mud flat as level as a floor, and which are 

 practically without vegetation, supporting only here and there a sparse 

 growth of the desert shrubs locally known as "grease- wood" {Ralo- 

 stacJiys occidentalism Sarcobaius vermiculatus). 



Black Rock Mountains. — The great Lower Quaternary plain of the 

 larger Mud Lake, or Black Eock Desert, which occupies, in the region 

 included in the map, an extent of about 60 miles in a northeast and south- 

 west direction, extending still some distance to the north beyond its northern 

 boundary, forms the sink of Quinn's River, a considerable stream, which 



