538 C. R. KEYES THE GEOGRAPHIC CYCLE IN AN ARID CLIMATE 



Introductory 



Since as ordinarily developed the scheme of the geographic cycle postu- 

 lates an upraised land surface exposed to stream action and other erosive 

 influences peculiar to a humid climate, its designation "normal scheme" 

 is perhaps as fitting as any other term possibly could be. The normal 

 plan, however, has had to be modified to meet two special conditions. 

 On the one hand it has been adapted to a glacial climate, and on the 

 other hand to an arid climate. A number of writers have recently de- 

 scribed these variations, but always with the essentials of the normal 

 scheme conspicuously before them. 



In all of these considerations stream action or water alone is regarded 

 as the prime agency of regional leveling and lowering. That there 

 should be need of distinctive treatment to meet the requirements of 

 those special conditions of climate where the snows of winter do not all 

 vanish in summer, and for those conditions of aridity where all basins 

 do not overflow and the surface drainage never reaches the sea, gives 

 rise to the query whether this solution of the problem actually obviates 

 the difficulties presented. There is at once invited a comparison of the 

 relative efficiencies of the several erosive processes under diverse climatic 

 conditions. 



One of the immediate results of such comparison is the suggestion 

 whether instead of attempting longer to fit closely the two so-called 

 special cases to a humid-climate standard we could not with great ad- 

 vantage recognize a different standard for each of the three sets of cli- 

 matic conditions. Would it not be indeed more logical to develop sepa- 

 rate cyclic phases along lines indicated by the effects of the dominant 

 erosional process for each particular climate? 



There is now no question but that in the past we have been prone to 

 attach far too much general importance to the special products of the 

 geologic processes as they operate in a humid climate and with which 

 we are most familiar. A direct consequence has been to overlook, often 

 almost completely, the workings of other geologic processes the effects of 

 which are really quite extensive. Perfect familiarity with provincial 

 facts and provincial conditions readily leads to too broad generalization. 

 That relative to which we are pleased to denominate normal climate 

 appears to be no exception to the rule. 



The deflative scheme of an arid geographic cycle here outlined appears 

 to be abundantly supported by data recorded elsewhere. These facts are 

 set forth mainly in three recently published papers. Without reiterating 



