CONTRASTED CHARACTERISTiCS OF ARID AND HUMID STAGES 647 



meet, and the great, thick, alluvial fans which one expects to encounter 

 on every hand as the intermittent streams leave the highlands, are gener- 

 ally found to be singularly inconsequential. In place of huge fans miles 

 in areal extent, hundreds of feet in thickness, and cubic miles in volume, 

 which one is led to anticipate, their usual size and importance are almost 

 ridiculously insignificant. Instead of vast extent and gj-eat bulk, exami- 

 nation shows that in a few days a steam-shovel and a train of cars could 

 often remove every vestige down to bedrock. 



The fact of the absence of thick wash accumulations in the central 

 parts of many, if not of the majority, of intermont basins is strangely 

 at variance with the assumption that the highland rims of bolson plains 

 are eventually carried down by the rains and permanantly deposited in 

 the lowest depressions. It appears that not only are many of these inter- 

 mont plains not deeply covered by washed-in rock-waste, but that they 

 are only veneered with soil.*" The exceptionally dry Mojave Basin, in 

 southeast California, seems to be a good example, if there be one, of an 

 intermont plain so situated as to receive centrally the wash from a lofty 

 rim because it is bounded on one side by the high wooded Sierra Nevada 

 and on another side by the Sierra Madre. Moreover, the various low 

 desert ranges within its boundaries are among the best instances known 

 of "lost'^ or ^l3uried" mountains. Yet nowhere in all of this desert is 

 it more clearly shown that the basin floor is not deeply covered by rock- 

 waste. Not only do the mountains and hills display the beveled edges 

 of the strata beneath, but many square miles of its plains surface, even 

 in its central part, are so thinly covered by soil that the underlying rocks 

 are everywhere well exposed. The bedrock surface is, as I have else- 

 where shown,*^ itself an even plain. This fact was long ago brought out 

 in the geological descriptions of the region before its true significance 

 was understood. Hershey*^ is especially explicit on this point, and more 

 recently Baker*^ gives additional data of the same sort. 



The records of deep drill-wells put down in various portions of arid 

 America are often interpreted in support of the hypothesis that the 

 intermont basins are deeply filled with rock-waste recently brought in 

 by the rains. When critically examined, these drill-logs are found to be 

 very misleading. In the majority of cases the great part of a drill-sec- 

 tion is discovered to be in but slightly indurated Tertiary or Cretaceous 

 or even Carboniferous deposits. Citing a specific instance: It was 



"Bull. Geol. Soc. America, vol. 10, 1908, p. 63. 

 *i Trans. American Inst. Mining Eng., vol. xl, 1909, p. 697. 

 "Univ. California Pub., Bull. Dept. Geol., vol. iii, 1902, p. 4. 

 «Ibid., vol. vl, 1911, p. 333. 



