CEDAR MOUNTAINS. 463 



owing to the absence of deep-cut canons or of extensive rock exposures, 

 renders their exploration peculiarly unfruitful to the geologist. 



The extreme southern point of the Cedar Mountains, as represented on 

 the map, is mostly covered by the broad, flat gravel-terraces of the ancient 

 lake, out of which rise a few isolated hills of red iron-stained quartzites and 

 limy shales, upturned at varying angles, but generally dipping to the north 

 and east. 



At White Rock Spring, the eastern foot-hills, forming a somewhat 

 broken line of secondary elevation, are formed of beds of a coarse cherty 

 limestone, abounding in cylindrical moulds of encrinites, like the calcareous 

 qud,rtzite found on Emma Hill in Little Cottonwood Canon. These beds 

 have a northwest strike, with a gentle dip to westward, and are succeeded 

 higher on the foot-hills by limestone beds which are almost horizontal. 

 They are partly covered by a flow of reddish breccia, which has occupied 

 the stream-bottom between the spurs. 



The White Rocks themselves are three very singular masses of grayish- 

 white quartziferous trachyte, the largest, about 300 feet high, having a 

 rudely conical shape, with perfectly smooth sides, without cleavage-planes 

 or cracks, and so steep that it is difficult to ascend to their summit. This 

 trachyte is a coarsely crystalline, almost granitoid rock, made up of large 

 crystals of sanidin, in a few instances an inch in length, wdth rounded, 

 cracked grains of quartz, occasional prisms of hornblende and flakes of black 

 mica in a crypto-crystalline groundmass. The groundmass is quite porous, 

 and shows many rounded cavities, from which the quartz-grains have fallen 

 out. Under the microscope, it is seen to be made up of a crystalline aggre- 

 gation of feldspar, quartz, and augite, the quartz being unusually rich in 

 glass-inclusions. This rock resembles in every respect the quartziferous 

 trachytes of the Elkhead Mountains, except that it contains microscopical 

 quartz in the groundmass. For this reason, it has been classed by Pro- 

 fessor Zirkel in his report as a rhyolite. 



The broad, flat-topped crest of the hills at this point is occupied by 

 a flow of andesite, which extends for several miles to the northward, and 

 forms an outlying hill to the west, which just rises above the accumulations 

 of desert sand. In external habit, this andesite cannot be distinguished 



