CONTROLLING EROSIONAL AGENTS 539 



here what they contain they may be pertinently considered in connection 

 with the present discussion. Hence frequent reference is made directly 

 to them. The present paper is the fourth of a closely connected series 

 bearing especially on the arid regions of this country and the different 

 phases of its eolative development. In the first account is described the 

 remarkable rock-floors of the intermont plains of southwestern United 

 States and northern Mexico.^ A second article, dealing with deflation 

 and the relative efficiencies of erosional processes under conditions of 

 aridity, describes the salient relief features of an area to all appearances 

 now undergoing rapid denudation.^ The third memoir,^ on mid-conti- 

 nental eolation, refers especially to a broad region where deposition of 

 wind-blown materials is believed to be taking place on a large scale. This 

 fourth statement is an argument for the development of the geographic 

 features in an arid region chiefly through means of wind-scour rather 

 than stream action. 



Controlling erosional Agents under Diverse climatic Conditions 

 preparation of rock materials for erosion 



Since the effects of erosion are made most conspicuous through the 

 removal of loose rock materials from the surface of the ground, the con- 

 dition in which the degradational agencies find this rock -waste becomes 

 a prime consideration. To the breaking down of rock-masses so largely 

 influenced by climate the somewhat vague term "rock weathering" is ap- 

 plied. However, this term, familiar as it is, does not fully express the 

 exact manner in which the transforaiation from the rock to rock-waste 

 takes place. 



The scheme of the normal geographic cycle premises moist-climate 

 conditions, whereby the breaking down of the rocks at the surface is 

 more largely chemical than mechanical. By implication at least the 

 effect is regarded as a universal one, and little notice is taken of possible 

 exceptions. 



With greatest facility does chemical decay of rock-masses take place 

 under conditions of heavy rainfall and warm climate. Yet long ago 

 Von Richthofen^ drew attention to the fact that in cold or in dry cli- 

 mates the rocks display few signs of chemical decay. EusselP further 

 emphasized this feature when he said "that rock decay appears to be the 



2 Bull. Geol. Soc. America, vol. 19, 1908, pp. 63-92. 



3 Ibid., vol. 21, 1910, pp. 5G5-598. 

 Mbid., vol. 22, 1911, pp. 687-714. 



^ Fiihrer fiir Forschungsreisende, p. 100. Berlin, 1886. 

 "Bull. Geol. Soc. America, vol. 1, 1890, p. 134. 



