588 C. K. KEYES RELATIVE EFFICIENCIES OF EROSIONAIi PROCESSES 



author mentioned has outlined. The scheme is particularly adaptable to 

 climatic conditions characterizing a normally moist country ; but the pecu- 

 liarities of a dry climate are such as to indicate clearly that under the 

 latter conditions it is not, if we are to argue for a distinctly staged arid 

 cycle comparable to the more familiar normal cycle. Whether we have 

 or have not such a cycle is dependent on the valuations given the several 

 factors. Moreover, under conditions of aridity the dominant erosive 

 agency must be of a kind radically different from what it is in the wet 

 climate, as has been already urged. 



In an arid climate, according to the author just quoted, it is assumed 

 that the typical initial condition of the earth's surface is that of a more 

 or less rugged and mountainous country, very much, I should judge, as 

 we see today in the Cordilleran region.*^ In other words, the American 

 desert country is regarded practically as having passed into its arid state 

 very recently. In the normally moist climate, in which there is a succes- 

 sion of completed cycles, the most typical initial condition of the surface 

 relief is that of a peneplain. However, instead of postulating similar 

 topographic types for the beginnings of the normal and the special cycles, 

 the latter is regarded as starting out under conditions of the most an- 

 tithetical sort. This leads at once into difficulties many of which are 

 unsurmountable. The selection of an antithetical type of relief instead 

 of a normal type for the initiation of the arid cycle appears to be due 

 largely to deductions resting upon conditions thought to obtain in the 

 desert regions of western United States and central Asia. So far as 

 America is concerned, there seems to be as yet very little specific exempli- 

 fication brought out in support of the contention. In reality, what 

 actually has been postulated in the Davis scheme of an arid cycle is a 

 highly specialized initial condition in place of generalized, typical, or 

 "no special" conditions. 



Under conditions of climatic aridity the initial relief may be, for sim- 

 plicity's sake, regarded as belonging to either one of two extreme types. 

 One is a mountainous type of topography and the other the general plains 

 type, as in the normal cycle — the peneplain, if you please. Only the 

 first mentioned type could serve as the foundation for the distinctly 

 marked scheme of an arid cycle, such as that recently developed by 

 Davis ;*^ the second would result in a scheme of general desert-leveling, 

 with no distinct stages dependent on water action, as in reality is urged 

 by Passarge.*^ As will be noted later on, illustration of the first named 



*«Ibid., p. 380. 



*'' Journal of Geology, vol. xiii, 1905, p. 385. 



*«Zeitscli. d. deut. geol. Gesellschaft, LVI Bd., Protokol. 1904, p. 193. 



