570 C. R. KEYES REIiATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



the rocks so very greatly predominates over their mechanical disintegra- 

 tion that often the latter in comparison is scarcely noticeable. In the 

 arid regions not only is the very reverse true, but it is so to a preeminent 

 degree. 



It is a well known fact that the chemical decay of rock-masses takes 

 place with greatest facility under conditions of heavy rainfall and warm 

 climate. Eock decay appears to be, as EnsselP^ has observed, the direct 

 result of normally wet climatic conditions. In cold or arid regions the 

 rocks are scarcely at all decayed. 



On the moist Atlantic slope, within the granite areas of the Piedmont 

 plateau of Maryland, for example, complete rock decay has been noted^^ 

 to extend to depths of 40 to 50 feet. Farther south, in the Coosa Valley 

 of Alabama, general decomposition of the rocks frequently reaches depths 

 of 200 to 300 feet. Derby^^ has shown that in Brazil chemical breaking 

 down of the rocks attains even greater depths than any of those men- 

 tioned. Under especially favorable structural conditions, as in the case 

 of fault-planes, mineral veins, and other local influences of like nature, 

 rock decay sometimes goes on to distances of more than 1,000 feet from 

 the surface of the ground. Almost everywhere in a moist climate rock 

 decay takes place faster than the decomposed materials are removed. 



In marked contrast to the breaking down of rock-masses in the moist 

 regions, the arid lands present extensive bedrock surfaces showing little 

 or no signs of real decay. Destruction of these rocks is almost entirely 

 mechanical in character ; in comparison the chemical effects are practically 

 nil. The slightest amount of chemical decomposition which rock mate- 

 rials undergo at the ground surface is well shown by the great talus slopes 

 and other accumulations of colluvial deposits that form veritable rubble 

 piles of ponderous size, and with materials so fresh to all appeai'ances 

 that they seem to ha^^e come direct from some gigantic rock crusher. 

 Even the adobe soils of the arid region, when examined under the micro- 

 scope, attest the strictly mechanical origin of the finer materials. 



In the desert region of the West, only in open mineral veins where 

 moisture can accumulate do normal signs of rock decay appear. The 

 presence of moisture in such situations is sometimes shown most strik- 

 ingly by fault-lines which are marked on the barren surface of the ground 

 by rows of green bushes and small trees as sharply defined as hedge-rows 

 set by the side of regularly surveyed country roads. Not only do the 

 rugged mountain ranges of the desert disclose little rock decay, but the 



18 Bulletin of the Geological Society of America, vol. 1, 1890, p. 134. 

 1'^ Keyes : U. S. Geological Survey, 1.5th Annual Report, 1895, p. 728. 

 18 American Journal of Science (3), vol. xxii, 1884, p. 138. 



