MATURE STAGE OF ARID RELIEF 555 



Of the larger relief features which distinguish maturity in a humid 

 climate none is more conspicuous than a notable rounding of the sharp 

 interstream tracts, hills, and divides and their rapid lowering. In an 

 arid climate this same tendency is even more pronounced. The land- 

 scape effect, I take it, is perhaps nowhere so t3^pically developed as in the 

 Great Basin. With the vast planation effects displayed in the intermont 

 areas of this region the sojourner at first is apt to get the erroneous im- 

 pression so often described, that mountains are there buried up to their 

 shoulders in their own debris. Tlie idea long held that a mountainous 

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

 of wearing down of the ranges and the filling up of the basins seems not 

 to be very well supported by the latest observations. 



The unmistakable deflative features already noted in connection with 

 the discussion of the relief of arid 3'outh are even more pronounced in 

 arid maturity. No known effects of rainfall and stream action can pos- 

 sibly produce the larger features of the relief expression which a region 

 as extensive as the western American dry tract presents ; the work accom- 

 plished is too prodigious, the time too infinitely long, the space affected 

 too vast. Only by means of the wind under especially favorable circum- 

 stances could effects such as we see today be reasonably accomplished. 

 Deflation seems the only explanation which is at all satisfactory. 



DEVELOPMENT OF ORIGINAL DRAINAGE LINES IN DESERT REGIONS 



The origin and growth of drainage lines, such as they are, in desert 

 regions under conditions of general aridity is an aspect of erosion which 

 has not, so far as I know, received the critical notice that it appears to 

 deserve. This want of special attention to this single point has done 

 more than anything else to mislead all who have traveled through the 

 mountainous arid tract of America regarding the real ineffectiveness of 

 stream action. Particularly deluding have been the impressions gained 

 in such lands as those of our western countn^ In many mountainous 

 belts of that region there is, indeed, an apparent approach to stream 

 effects as they are known in humid climates. Upon this really quite 

 restricted and peculiarly modified effect of normal water work has been 

 based the usual scheme of the arid cycle. 



In its broader relations stream action in the mountain belts of arid 

 regions admits of an interpretation of origin wholly different from that 

 commonly held. For example, in the arid region of the nortliem ^lexi- 

 can tableland it is perfectly conceivable — and T have already set forth the 

 data in support of the idea®^ — that between the initiation of the present 



^ Journal of Geology, vol. xvil, 1909, p. 31. 



