THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 387 



transportation, a more continuous area of bed-rock in the higher 

 areas, and a more general concentration of waste in the lowest basins 

 will be established. The higher local baselevels are thus, by a 

 process of slow, inorganic natural selection, replaced by a smaller and 

 smaller number of lower and lower baselevels; and with all this go 

 a headward extension of graded piedmont slopes, a deeper dissection 

 of the highlands, and a better development of their subsequent and 

 adjusted drainage. The processes of drainage adjustment are, how- 

 ever, at the best, of less importance here than in the normal cycle, 

 because of the absence of main valleys, deep-cut by trunk rivers, and 

 the resulting deficient development of deep-set subsequent streams, 

 as has already been suggested. 



Some changes of this kind have probably taken place in the Basin 

 Range province of Utah and Nevada, but more field work will be 

 needed before they can be safely pointed out. Indeed, it seems to 

 be the case that certain changes of an opposite kind have taken place; 

 the long intermont troughs appear to be here and there subdivided 

 into separate basins by the undue growth of certain detrital fans 

 where large valleys have been opened in the neighboring ranges; but 

 this condition of things will pass when the mountains are worn lower 

 and the waste is discharged from them less actively. 



As the coalescence of basins and the integration of stream systems 

 progress, the changes of local baselevels will be fewer and slower, 

 and the obliteration of the uplands, the development of graded pied- 

 mont slopes, and the aggradation of the chief basins will be more and 

 more extensive. The higher parts of the piedmont slopes may be 

 rock-floors, thinly and irregularly veneered with waste, as has been 

 described by Keyes for certain basins (bolsons) in New Mexico; 

 here, as well as upon the aggraded slopes and plains, sheet-flood 

 action will prevail, as explained by McGee. The area occupied during 

 early maturity by the three different kinds of surface — dissected high- 

 lands or mountains, graded piedmont slopes of rock or waste, and 

 aggraded central plains with playas, salinas, or lakes — will depend on 

 the initial relief, on the rock structure and its relation to desert weath- 

 ering, on the percentage of material exported by the winds, and on the 

 climate itself. 



It is worth noting that, although the activity of streams and floods 



