GEOGRAPHIC CYCLE IN AlSf ARID CLIMATE 89 



climate are highly specialized initial conditions in place of generalized, 

 typical, or "no special" conditions. 



Under ordinary conditions of climatic aridity the initial relief may be 

 regarded as being either one of two extreme types. One is the rugged 

 mountainous type of topograph}', and the other is the general plains type, 

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

 mentioned type could serve for the foundation of the distinctly marked 

 scheme of an arid cycle, as recently so astutely developed by Davis ; the 

 second would result in a scheme of general desert-leveling, with no dis- 

 tinct stages, as formulated by Passarge. As will be noted later on, the 

 occurrence of instances of the first named scheme, corresponding in all of 

 its phases to those of the normal C3'cle, must be very rare indeed. The 

 prevailing mode of leveling and lowering of an arid country must be 

 very largely after the fashion described for the South African region. 

 The latter region would thus represent the general course of desert-level- 

 ing, rather than only the senile stage. 



With an upraised peneplain, the degradational processes may start to 

 reduce the country toward ultimate baselevel just the same imder con- 

 ditions of an arid climate as under conditions of a moist one. When the 

 wind is the chief erosive agency the resulting topographic effects are not 

 so very unlike the general relief effects produced by the streams. The 

 belts of hard rock are brought into somewhat stronger contrast, perhaps, 

 than in moist climates. The geologic structures are more sharply accen- 

 tuated. The rock-floor is cleaner swept. The belts of weak rocks are 

 faster removed. At all times the plain is more strikingly the dominant 

 relief .feature. If after the main epeirogenic movement orogenic activity 

 remains quiescent, the general plain contini^es indefinitely to persist 

 without marked change of expression, as is clearly the case of the South 

 African tableland, where there would be no distinct stages marking the 

 evolution of the arid cycle. Epeirogenic movement followed by frequent 

 and widespread orogenic disturbances, and also by vigorous volcanic 

 activities, as in western United States, would soon obliterate most evi- 

 dences of a former peneplain, if such ever existed over the region. There 

 are strong reasons for believing that such a peneplain actually did exist 

 there at the beginning of the period of aridity. 



"\AnhLen critically examined the scheme of an arid cycle, distinctly staged 

 and beginning with a rugged mountainous relief, is found to be in reality 

 the evolution of the normal wet cycle with somewhat less water. If the 

 periods of infancy, youth, and maturity could exist, they must be so brief, 

 compared with the duration of the characteristics of old age, that they 

 become stages altogether negligible, even when starting with a moun- 



