548 C. E. KEYES THE GEOGEAPHIC CYCLE IN AN AEID CLIMATE 



claimed for the deep well at Albuquerque, New Mexico, that its 2,000 

 feet of depth were entirely in wash materials, whereas later, more care- 

 ful and discriminating examination of the data clearly showed that 

 scarcely 200 feet of the entire depth could be so considered. Similarly, 

 certain deep drill-holes in southern Arizona are now known to have pene- 

 trated mainly tilted Tertiary beds instead of enormously thick wash 

 deposits of quite recent date. 



INITIAL RELIEF FEATURES UNDER ARIDITY 



As already mentioned, there are two relief extremes on which an arid 

 climate may be considered as imposed. They are a plain and a moder- 

 ately mountainous surface. For obvious reasons a region of lofty moun- 

 tains seems to be precluded. In the first instance the region is essen- 

 tially a peneplain, although deprived of its streams. In the second case 

 there is considerable diversity of relief, but no sharp, local contrasts 

 such as are presented in a moist country in the beginning of a new cycle 

 of erosion. 



Contrasted Features of arid and moist Topographic Juvenility 



CONDITIONS GENERALLY POSTULATED 



As the early stage of the normal geographic cycle is commonly re- 

 garded, the relief is ordinarily and rapidly increased by the incision of 

 consequent valleys from the trunk rivers that flow to the sea. In the 

 early stage of the arid cycle the relief is considered, on the hypothesis 

 of water action, to be slowly diminished by the removal of waste from 

 the highlands and its deposition on the lower gentler slopes and on the 

 basin beds of all of the separate centripetal drainage systems. In con- 

 sequence all the local baselevels are thought to rise, and the areas of 

 deposition are given a nearly level central floor of fine waste. Streams, 

 floods, and lakes, then, are made the chief agencies in giving form to the 

 aggraded basin floors, as well as to the dissected basin margins in the 

 early stage of the cycle. In illustration the Great Basin is commonly 

 pointed out. The winds are sometimes conceded to have some impor- 

 tance, but in the youthful stage wind-blown hollows are claimed to be 

 Qot likely to be formed. For the normal headward growth of many sub- 

 sequent streams it is explained that in the arid cycle such streams have 

 smaller opportunity for development because all the belts of weak 

 structure under the basin deposits are buried out of reach. 



Clearly these statements are the result of deduction based on the con- 

 sideration of conditions as they prevail in a moist climate rather than 

 of generalization supported by long continued observations in a dry cli- 



