QUATERNARY DEPOSITS 247 



is composed of comparatively small pebbles of a granitic rock (quartz- 

 monzonite) and of a siliceous argillite like that of the Ordovician. It is 

 capped by basalt and at one point lies on a bed of pumice with a second 

 pumice layer, about 3 feet thick, intercalated in the gravels near the top 

 of the series. The entire thickness of the formation is here over 200 

 feet and the beds are disturbed, dipping at some points 10 degrees or 

 more easterly; at some places they are faulted. Exactly similar gravel 

 is found underlying the pumice at the west end of the Piper Peak ridge. 

 mMzm^ ,,,, 



Quaternary Deposits. 



DESERT DETRITUS 



There is nothing so striking in the Great Basin region as the numerous 

 detrital slopes, which spread out from all the canyons and fill consider- 

 able portions of the valleys. In view of the very small precipitation in 

 this region, the formation of these numerous fans would seem to involve 

 a very long period of time. They are composed chiefly of coarse material, 

 often containing boulders tons in weight. When the older detrital 

 material is cut by the present watercourses or "washes," the stratified 

 arrangement of these materials is clearly evident. There can be no doubt 

 that their distribution is due to the action of water. A consideration of 

 the manner in which rain falls in all this desert country suffices to ex- 

 plain the formation of these detrital slopes, for although the precipitation 

 is very small when the region as a whole is considered, it is often very 

 great within the space of a few hours over a limited number of square 

 miles. The action of the sun and the frost on the rocks of this dry region 

 results in the surface rocks being everywhere extensively cracked, and 

 the fragments, although about in their original position, are easily dis- 

 placed. When a cloudburst occurs the rain runs off in torrents and 

 sweeps before it large quantities of this loose material, and when the 

 cloudbursts are of sufficient size they will carry boulders many tons in 

 weight far out on the plains. There is therefore no difficulty in account- 

 ing for the formation of the alluvial fans, but the time that must be 

 allotted to their formation, if we suppose the precipitation to have been 

 no more in the early Pleistocene than at present, would be enormous. 

 It is quite certain, however, that in earlier Pleistocene time the precipi- 

 tation was much greater than at present. It is probable, therefore, that 

 the larger part of these detrital slopes was formed during the first half 

 of the Pleistocene. This would harmonize with the record in the Sierra 

 Nevada. The larger part of Pleistocene time was required for the exca- 

 vation of the canyons. This early Pleistocene period of erosion has been 



