310 CONCLUSION. Chap. YIL 



climate is humid so that much dust cannot be 

 blown away, it appears at first sight im- 

 possible that there should be any appreciable 

 amount of sub-aerial denudation ; but worm- 

 castings are blown, especially whilst moist 

 and viscid, in one uniform direction by the 

 prevalent winds which are accompanied by 

 rain. By these several means the superficial 

 mould is prevented from accumulating to a 

 great thickness ; and a thick bed of mould 

 checks in many ways the disintegration of 

 the underlying rocks and fragments of rock. 



The removal of worm-castings by the above 

 means leads to results which are far from 

 insignificant. It has been shown that a 

 layer of earth, *2 of an inch in thickness, is in 

 many places annually brought to the surface 

 per acre ; and if a small part of this amount 

 flows, or rolls, or is washed, even for a short 

 distance down every inclined surface, or is 

 repeatedly blown in one direction, a great 

 effect will be produced in the course of ages. 

 It was found by measurements and calculations 

 that on a surface with a mean inclination of 

 9° 26', 2*4 cubic inches of earth which had 

 been ejected by worms crossed, in the course 



