362 



University of California Publications. 



[Geology 



complete effacement of the original valleys appears to have taken place 

 in the region known as the Mohave Desert. Here the detrital slopes 

 have risen to near the tops of the ranges." I entered the region with 

 that idea in mind and came out convinced that it requires a radical 

 modification. There are, indeed, thick accumulations of detrital material 

 that have been built up close to the foot of such prominent ranges as 

 the Tehaehapi and the Sierra Madre, and may have buried the foothills, 

 but in the central and by far the larger portion of the desert region I 

 do not believe the detrital 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 

 Kiver, 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 an- 

 other thirty miles long by twelve to fifteen miles wide, of undulating 

 granite comparatively free of detritus and characterized by long, broad, 

 low, smooth ridges 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 obscurred 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 limestones east of the main granite area, residuals were more numer- 

 ous, and that region now has many rugged ranges.iB 



Iii the center of the basin between Black Mountain and the 

 outcrop of the Rosamond Series in the locality north of Barstow 

 strata resembling lithologically those of the Rosamond out- 

 cropped in the bank of a shallow arroyo. Low knobs of granitic 

 rock and lava outcrop in the basins on all sides of the Barstow 

 syncline. The Mojave River in the vicinity of Barstow is at 

 present in the process of uncovering residual knobs of lava once 

 buried beneath coarse alluvium. This river is excavating a nar- 

 rows at the town of Victorville through a residual knob of 

 granite, although it flows through an alluvium-covered area of 



13 The Quaternary of Southern California, Univ. Calif. Publ., Bull. 

 Dept. Geol., vol. 3, pp. 4 and 5, 1902. 



