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



direct result of normally wet climatic conditions. In cold or arid re- 

 gions the rocks are scarcely at all decayed/' The production of rock- 

 waste in desert lands through the process known as insolation is especially 

 considered in another place. '^ At the surface of the ground it is shown 

 to be very largely mechanical, hardly at all chemical in nature. 



EFFECTIVENESS OF WATER ACTION IN MOIST CLIMATE 



Those climatic conditions which tend to make of water action a most 

 effective erosive agent, that in a humid land make it appear to be the 

 sole process of general degredation, and that make it seem the only 

 universal erosional activity are the very conditions which tend to obscure 

 the effects of the erosional agencies which are most pronounced in a 

 glacial climate and in an arid climate. Attempt to ascribe all erosion 

 to stream action militates not so much against fact as it marks a dis- 

 tinctive period in the history of erosive thought. The composite effects 

 in humid as well as in both arid and glacial climates remain to be prop- 

 erly analyzed. The factor of relative efficiencies of each has to be deter- 

 mined in every region. 



If water be the dominant erosional power in a moist climate it does 

 not necessarily follow that less water is the sole erosive force in either 

 dry or glacial climates. 



DOMINANCE OF WIND-SCOUR IN A DRY CLIMATE 



N"otwithstanding the fact that during the past decade the Avind in the 

 capacity of a potent agent of general erosion has come to be recognized 

 more and more universally, tliere still lingers a certain reluctance to 

 admit its effectiveness in specific cases or its high relative rank among 

 the degradational processes. Even in instances in which the climatic 

 conditions are such as almost to preclude water action, where the annual 

 rainfall is so small as to be almost negligible, stream work is still given 

 first place and wind work a very subordinate place. 



That wind-scour in an arid land should be considered not only the 

 dominant erosional activity, but under the peculiarly favorable condi- 

 tions for its operations more potent and rapid in its effects than is 

 water action in a normal, wet country is a quite recent deduction, but 

 one which seems to be amply supported by many observations. More 

 fully to appreciate the enormous extent of eolation, it is necessary only 

 to peruse the later publications of certain astute observers who have 

 actually lived in desert lands, although in this country this subject has 

 not received the attention that it seems to deserve. Of these mention 



^Bull. Geol. Soc. America, vol. 21, 1910, p. 569. 



