Hbrshey. J 



Quaternary of Southern California . 



5 



slopes have filled the broad basin-like valleys to a depth greater 

 on the average than several hundred feet. Low knobs of 

 granite occur at many places near the center of these basins, 

 and where canons have been cut into the detrital material, as by 

 Mohave River, hard rock has been encountered at many points 

 at no great depth. There is an area thirty miles long by three 

 to four miles wide, and another thirty miles long by twelve to 

 fifteen miles wide, of undulating granite comparatively free of 

 detritus and characterized by long, broad, low, smoothridges in 

 which the granite (much weathered and softened,) is rarely more 

 than ten feet beneath the surface, and is often uncovered by 

 railway cuts at three to five feet. These ridges are surmounted 

 by knolls of broken pegmatite, and in places rise into short, 

 rugged hill ranges, but rarely reach the dignity of mountains. 



So strong was the impression that I was traveling over a 

 country that had been reduced nearly to a uniform level by 

 erosion, that it seemed natural to refer to it in my field notes as 

 the granite platform. I should hardly like to call it a peneplain, 

 as evidences of actual and completed base-leveling, if they ever 

 existed, are now obscured by the more recent detrital slopes and 

 alluvial deposits that floor the broad basins. It is a land whose 

 topographic forms have reached the stage of old age but not 

 that of senility. On the granite areas denudation effaced most 

 of the rugged mountains and left only a few standing widely 

 separated from each other. On a peneplain these would be 

 classed as monadnocks. Where the rocks were more resistant, 

 as on the gneiss, schist, quartzite, and limestone east of the 

 main granite area, residuals were more numerous, and that 

 region now has many rugged ranges. 



The relation between the granite and an overlying volcanic 

 series, apparently of Eocene age, indicates that the surface of 

 the former had been reduced nearly to its present level early in 

 the Tertiary era, and the country was probably even more 

 uniform in the Eocene period than at the present time. Many 

 of the rougher ranges in the desert are composed of rhyolite 

 resting on granite at a level not much above or below the 

 general level of the basius. Faults of small throw are common, 

 but do not appreciably affect the present topography. 



