THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 389 



dependent only on the original form and altitude of the region, and 

 on the amount of dust that it has lost through wind transportation. 



The most perfect maturity would be reached when the drainage 

 of all the arid region becomes integrated with respect to a single 

 aggraded basin-baselevel, so that the slopes should lead from all 

 parts of the surface to a single area for the deposition of the waste. 

 The lowest basin area which thus comes to have a monopoly of depo- 

 sition may receive so heavy a body of waste that some of its ridges 

 may be nearly or quite buried. Strong relief might still remain in 

 certain peripheral districts, but large plain areas would by this time 

 necessarily have been developed. In so far as the plains are rock- 

 floored, they would truncate the rocks without regard to their struc- 

 ture. 



There is no novelty in the idea that a mountainous region of interior 

 drainage may be reduced to a plain by the double process of wearing 

 down the ranges and filling up the basins, and that the plain thus 

 formed, consisting partly of worn-down rock and partly of built-up 

 waste, will not stand in any definite relation to the general baselevel 

 of the ocean surface; yet the idea has seldom been applied in the 

 interpretation of uplifts by the physiographic method. In the case 

 of the plateaus that are now trenched by the Colorado river in north- 

 ern Arizona, for example, it has usually been tacitly postulated that 

 the baselevel with respect to which they were widely denuded in the 

 pre-canyon cycle was the normal baselevel of the ocean, and from 

 this postulate it has been argued that the cycle of canyon erosion was 

 introduced by a strong uplift. My own opinion has agreed with that 

 of Dutton and others in this respect. Yet it is not today easily 

 demonstrated that the Arizona plateaus had exterior drainage at the 

 time of their wide denudation; and until exterior drainage is shown 

 to have obtained, the altitude of the plateau region during its denu- 

 dation must remain uncertain. There are, however, several facts 

 which point to the correctness of the generally accepted view: the 

 course of the Colorado river through the Kaibab cannot easily be 

 explained as having originated in the present cycle; it appears to 

 have been established earlier; and it is doubtful whether there are 

 late Tertiary basin deposits within the desert area, or wind-carried 

 sand and loess deposits in the area to the eastward (leeward) of 



