42 LEOPOLD REIN EC KE 



liner with old age and its increasing depth of soil. But suspended 

 matter added to a stream retards its velocity, and the rate of 

 retardation becomes greater as the debris becomes finer. 1 Hence 

 as the slopes became lower, both the factors of decreased discharge 

 and increased fineness of debris would help to a further and more 

 rapid rate of decrease of maximum load carried. 



Obviously also the amount of creep and wash of debris down 

 hillsides into the stream beds is smaller on gentle than on high 

 slopes. The rate at which a land form is worn down by mechanical 

 erosion must therefore diminish very rapidly as the slopes decrease. 

 On the other hand, lower slopes may aid chemical erosion, in that 

 more rain water is absorbed and the solution of the rock materials 

 near the surface is increased. Chemical erosion must, however, 

 be a very small factor in the wearing down of a land surface, for the 

 matter dissolved in river waters is derived from surface rocks in the 

 process of weathering, and the greater bulk of the rocks at the sur- 

 face lose on an average less than one-third of their original weight 

 by the process of solution when weathering is complete. Moreover, 

 part or all of this loss is compensated for, both in weight and in 

 bulk, by gains in the form of oxygen, water, and carbon dioxide 

 obtained from the atmosphere, and recombined as insoluble mineral 

 products in the residual soil. 2 



If the average slopes of a land surface, therefore, be reduced in 

 the progress of the geographic cycle from, say, 4 per cent to 2, the 

 rate of reduction of the land surface by erosion will be less than 

 one-half what it was before, and as the slopes decrease, the process 

 becomes slower and slower. 



The final stages of old age in which the surface is reduced to 

 slopes as low as one-tenth of 1 per cent must therefore represent 

 a very much longer period of time than the stage of maturity or of 

 early old age. Chronologically, therefore, " beveled hills" are 

 probably closer to land forms in early maturity than to "pene- 

 plains," and for that reason alone the subdivision proposed in this 

 paper should be justified. 



1 Gilbert, op. cit., p. 228. 



2 F. W. Clarke, "The Data of Geochemistry," Bull. 491, U.S.G.S., pp. 462 

 and 465. 



