EROSION AND ITS SIGNIFICANCE. 129 



The canon will recede along this shallow mature valley 

 and in the divide it will assume a fairly broad amphitbe- 

 atrical shape, the depth and breadth of the form so pro- 

 duced being strongly influenced by the height of the plateau, 

 and both by the rainfall and the distance of the divide from 

 the sea or from the nearest heavy fault or flex scarp. At 

 the actual divide the curve of the surface should be convex 

 to the sky, because in such places weathering runs ahead 

 of corrasion — because of lack of catchment area — and the 

 material tends to sink down hill under both its own weight 

 and the influence of rains, thus tending to the parabolic 

 curve convex to the sky. 1 This feature needs attention to 

 be directed to it, inasmuch as it illustrates the influence of 

 weathering, even in this early stage in plateau dissection. 2 



Subsequent stages of reduction, — The incapacity of the 

 stream to directly scour its fresh rock structures at a cer- 

 tain definite critical stage (varying in time from point to 

 point in its history) is not so much because of the absolute 

 efficiency of weathering processes, as that the streams 

 have reduced the slopes of their thalwegs to such an extent 

 that their velocities in turn have been much reduced. This 

 again implies an almost incredible decrease in power of 

 transportation, and this it is which furnishes the real check 

 to the initial rapid corrasion of the uplifted peneplain. 



The influence of such a factor on the rate of sedimenta- 

 tion will be dealt with later. 



This stage when weathering is so powerful a factor, and 

 when lateral wear is in excess of vertical wear, is probably 

 that which Davis describes as "the balance between 

 erosion and deposition," 3 and that to which Gilbert 4 refers 

 in his statement " that downward wear ceases when the 

 load equals the capacity for transportation." 



1 Corrasion, p. 216. 



2 Convexity of Hill Tops, G. K. Gilbert, Journ. Geol. 1909, pp. 340 - 350. 



3 Journ. Geol,. 1902, pp. 86-87. * Henry Mountains, 1877, pp. 126-127. 



I— August 2, 1911. 



