88 



C. E. KEYES INTERMONT PLAINS OF THE ARID REGION 



fuKS 



posed at the foots of the longer slopes, eroded from their summits. 

 These blocks are not simple residual elevations, since they have been 

 partly reared through recent and profound faulting. How much of such 

 mountains should be ascribed to strictly residual effects and how much to 

 elevation above the adjoining plains would have to be determined in each 

 particular instance. Ordinary desert-leveling has been greatly compli- 

 cated by extensive extravasation as well as by profound erogenic move- 

 ments. The specific effects of epeirogenic movement have never been 

 considered. 



The observation is made by Davis^^ that "no special conditions need be 

 postulated as to the initiation of an arid C3^cle. The passive earth's crust 

 may be (relatively) uplifted and offered to the sculpturing agencies with 



any structure, any form, any altitude, 

 in dry as well as in moist regions." 

 While this statement is eminently 

 true under conditions of a wet cli- 

 mate, a little reflection must make it 

 clear that it is not necessarily so with 

 respect to a dry country, where the 

 dominent erosive agency is entirely 

 distinct. In an arid climate, accord- 

 ing to the writer just quoted, the 

 typical initial condition of the earth's 

 surface is that of a more or less 

 rugged mountainous region.^^ In a 

 moist climate, in which there is a suc- 

 cession of completed cycles, the most 

 typical initial condition of surface 

 relief is that of a peneplain. Instead of postulating similar topographic 

 types, the beginnings of the normal and the special cycles are considered 

 as taking place under the most antithetical of relief effects possible. This 

 surely is not necessarily so. 



For the initiation of an arid cycle the selection of an antithetical type 

 of relief instead of a normal type, as would naturally be expected, ap- 

 pears to be due largely to deductions resting upon present conditions 

 obtaining in the desert regions of western United States and central 

 Asia. So far as this country is concerned, there seems to be as yet very 

 little specific exemplification brought out in support of the contention. 

 In reality, what is actually postulated in the Davis scheme of an arid 



FiGDEE 7. — Profile of Crest of Plomas 

 Range, Arizona 



Vertical distance about 1,000 feet 



==> Journal of Geology, vol. xlii, 1905, p. 382. 

 " Ibid., p. 380. 



