104 THE BOOK OF NATURE STUDY 



sufficient to allow of downward movement under the influence of 

 gravitation. 



In countries where denudation more generally escapes observa- 

 tion the slopes are gentle and the bare rock is covered with subsoil 

 and soil. The surface may be cloaked with forest or affected by 

 cultivation, but until the slopes become so gentle that gravitation 

 and its agents are rendered inoperative through friction, denuda- 

 tion will still be carried on under the covering of forest or of field 

 vegetation. 



In order to appreciate this, the soil and subsoil should be 

 studied. The latter consists of fragments and products derived 

 from the disintegration of the rock below by frost and rain armed 

 with carbonic acid. The latter contains the same substances in 

 a finer state of division mingled with the results of the life of 

 animals or plants. A section through from the surface of the soil 

 down to the rock shows that there is a gradual and imperceptible 

 transition from one to the other. At first the rock has its joints 

 widened somewhat, then large pieces become detached, and these, in 

 turn, are broken again and again. This is mainly frost work aided by 

 carbonic acid. The latter agent partly dissolves and disintegrates 

 the outer parts of the fragments, rounding them a little, enabling 

 the outer crusts to scale off, and increasing the proportion of finer 

 material. The roots of vegetation help in the process, partly 

 by splitting rocks open, partly by introducing carbonic and 

 organic acids as they decay. Burrowing animals give new 

 channels for action, bring material to the surface, and help to mix 

 the constituents. Worms are very important at this stage. 

 They pass the fine material through their bodies, extract organic 

 matter from it, and deposit it in a still more finely divided state at 

 the entrance to their burrows. 



If this all occurred on a flat plain it would afford no aid to 

 denudation, but practically slopes are everywhere ; and as gravi- 

 tation will always tend to pull everything downward on a slope, 

 its power will be increased by whatever brings material into an 

 easier condition to move. Worm castings and the heaps of soil 

 thrown out by rats, moles, and rabbits, will be washed flat by rain, 

 but if on a slope a larger amount of the debris will go in the down- 

 hill than in the uphill side of the heap. Again, when disused 



