FIELD GEOLOGY. 225 



where the downward progress of rain-wash on a hill- 

 side has been arrested by a fence, its result is after a 

 few years very evident, in the ground being unduly 

 higher on the upper than on the lower side. 2. The 

 annual growth and decay of vegetation, which is the 

 most important of the modifying causes. The plants 

 that have lived and died upon the soil, from the time 

 when the first thin layer was formed on the exposed 

 surface of the subsoil, have all (minute as they are in 

 many instances) performed a useful part, by absorbing 

 from the air carbonic-acid, water, and ammonia. They 

 yielded to the soil, by their own decay, these substances 

 which form its organic constituents. The proportion of 

 decaying vegetable matter in soils is usually small 

 about one-fiftieth but sometimes it is excessive as in 

 peat and the soil rendered thereby almost worthless 

 for purposes of cultivation. 3. The matter derived 

 from animals living on, above, and beneath the surface, 

 and from their ultimate decomposition. All wild 

 creatures birds included thus naturally contribute to 

 the fertility of soil, to an extent perhaps much larger 

 than is generally appreciated. 



