EROSION AND ITS SIGNIFICANCE. 131 



Whenever two peneplain or [old age] surfaces are found 

 associated in resistant rock structures, such as granites, 

 crystalline schists, and dense quartzites or sandstones, and 

 the two such surfaces are situated the one above the other, 

 and the two are separated by a youthful or mature topo- 

 graphy, it may be considered that they were formerly 

 continuous, but are now discontinuous, owing to earth 

 processes other than those due to erosive activities. Such 

 earth processes may be either warping or faulting. This 

 is absolutely irrespective of any irregularity of plan 

 possessed by the youthful topography separating the two 

 peneplains. 



Applications. 



Several important corollaries may now be deduced from 

 the main principles of stream erosion : — 



(1) Height of peneplain above sea level. The profile of 

 corrasion in the ideal case is a function of stream volume. 

 At each point corrasion varies both as the stream volume 

 and as the cube of the velocity thus derived. The thalweg 

 may be considered as receiving equal increments of volume 

 at points equidistant from each other. These produce a 

 definite increase in velocity, but this in turn implies a much 

 higher increase in kinetic energy. Therefore the profile 

 of corrasion should steepen with relative rapidity as it is 

 traced headwards. 



After the limiting profile of corrasion of fresh rock struc- 

 tures has been formed, weathering ensues and the work is 

 carried on by transportation of the rock material thus 

 broken down. There is practically no limit to this action. 

 The final stages of corrasion, however, will apparently be a 

 low plain generally convex to the sky and not much raised 

 above sea level. The convexity of profile to the sky will 

 be assumed long before the reduction of the surface to very 

 low levels, and a peneplain may well be conceived as having 



