ARID CYCLE IN A PLAINS REGION 593 



under conditions of moist climate. Assuming for the moment wind 

 scour to be the chief erosive factor, instead of water action, the broader 

 relief features need not be so very unlike the general topographic effects 

 produced by stream systems. In fancy, the immediate valleys of the 

 rivers only need to be filled up. As eolation progresses the belts of hard 

 and soft rocks would be perhaps brought into somewhat stronger contrast 

 than they commonly are at the corresponding stage of a humid cycle. 

 The geologic structure would be more sharply accentuated. The rock 

 floor would be cleaner swept. The areas of weak rocks would be removed 

 faster. At all times the plains aspect would be more strikingly dominant. 



If, after the main epeirogenic movement, local orogenic activity remain 

 quiescent, the general plains surface would continue indefinitely to per- 

 sist without very marked change in original expression, excepting the 

 accentuation of the more resistant rock belts into highlands. This ap- 

 pears to be the case of the South African tableland. To mark the stages 

 of the arid cycle there would not be necessarily a succession of distinct 

 features comparable to those of the normal cycle. Epeirogenic move- 

 ment, followed by frequent and widespread orogenic disturbances and 

 also by vigorous volcanic activity, would in a measure tend to greatly 

 disguise most evidences of the initial aspect of a region. The same 

 would be true of an uplifted peneplain the substructure of which before 

 planation was more or less complicated, faulted, and folded, as appears 

 to be the case in western United States. 



Considered alone, without reference to the neighboring districts, the 

 Great Basin presents many difficulties to a clear interpretation of some of 

 its most characteristic features. Farther south, in the desert region at 

 the northern end of the vast Mexican tableland, there are displayed cer- 

 tain phenomena which seem to offer critical testimony relative to the orig- 

 inal aspects of the country at the beginning of the present dry cycle. A 

 few years ago I incidentally referred^ ^ to the probable significance of cer- 

 tain remnantal plains surfaces in New Mexico as indicating an old pene- 

 plain upraised. 



The general plains level is not only the dominant relief feature of the 

 desert region, but occupies about four-fifths of the entire area of the arid 

 country. Above it rise the numerous and often lofty mountain ranges. 

 In the Great Basin area — which is perhaps the most familiar portion of 

 our desert land — the two most conspicuous topographic features are not 

 always so sharply contrasted as they are elsewhere. Moreover, in the 

 Great Basin region the salient aspects of the country are quite different 

 from the larger relief features of other parts of the western desert land. 



" American Geologist, vol. xxxiii, 1904, p. 22. 



