546 C. R. KEYES THE GEOGRAPHIC CYCLE IN AN ARID CLIMATE 



of through recent deformation, a very different explanation is made 

 possible and probable. 



If it be postulated that the high-lying surface of such a region as the 

 northern Mexican tableland, already referred to, was at the beginning 

 of the arid cycle a plain, an upraised peneplain possibly; that its major 

 folding and faulting were quite ancient, mainly prior to peneplanation, 

 and this seems highly probable;^'' that the present intermont plains rep- 

 resent the belts of weak rock undergoing vigorous deflation, as their 

 rock-floors indicate, and that in consequence the mountain belts of re- 

 sistant rock are now rapidly being brought into stronger and stronger 

 relief, as all observation goes to show, centripetal drainage must be ad- 

 vancing and expanding as the mountain ranges become relatively higher. 

 Such drainage systems are growing rather than withering. 



MIGRATION OF BASIN AL WASH 



General misconception has long prevailed concerning the derivation 

 and composition of the basin soils of arid regions. It is frequently 

 stated that the smooth intermont plains are formed by the wash from 

 their highland rims. The valleys of the Great Basin are notable ex- 

 amples. Concerning this region this view has been expressed by nearly 

 every one who has written on the geology of this district during the past 

 30 years. The best statement of this impression is that by Eussell.^^ 

 Even so late as the past year an eminent geographer^^ has seriously em- 

 phasized this old notion. Enormous depths are attributed to the wash 

 in the central parts of these intermont basins. Estimates of 3,000 to 

 4,000 feet are not infrequent. The contiguous mountain ranges are con- 

 sidered as "buried up to their shoulders." This conclusion is the direct 

 result of applying the normal humid-climate principles to such regions. 

 In accordance with the same principles the sides of each basin often to 

 the mountain crests are regarded as initial slopes of local deformation, 

 which lead the wash of the local sporadic rains toward the central de- 

 pression, whose lowest point serves as the baselevel for the basin. With 

 this interpretation the facts do not seem very well to agree. Expected 

 verification of hypothesis in the field is not only not realized, but there 

 is complete surprise at its manifest invalidity. 



The arroyos, or drainage channels of the desert ranges, do not appear 

 to be the notable wash carriers that they are sometimes thought to be. 

 Plain with beveled rock-floor and mountain with bare rocky sides sharply 



3'Proc. Iowa Acad. Scl., vol. xiii, 1908, p. 221, 



^ Geological Magazine, decade iii, vol. vi, 18S9, p. 242. 



8» Harper's Magazine, vol. cxxiil, 1911, p. 54. 



