76 ECONOMICS OF FORESTRY. 



with them, and as the Httle rivulets run together 

 and acquire force, gravel, stones, and even large 

 rocks and boulders are broken loose and moved to 

 lower levels by the torrent. This action, known 

 as erosion, takes place everywhere more or less 

 rapidly, according to the presence or absence and 

 character of the soil cover, and no better and more 

 efficient protection against it is to be found than a 

 dense forest cover. 



A grass cover may also protect the soil under- 

 neath against the erosive action of the waters, 

 whenever the declivity is not too steep, but since 

 the rains do not penetrate through the dense 

 greensward of the mountain meadows, and hence 

 are carried off superficially, they acquire a mo- 

 mentum which finally leads to the same gullying 

 and erosive action which a naked soil experiences. 



The forest alone is capable of obstructing the 

 mechanical effect of the rainfall upon the soil, and 

 retarding the rapid surface drainage which be- 

 comes the carrier of the debris. Here, again, the 

 condition of the forest floor, rather than the tree 

 growth, is the effective element. 



If it is considered that, in the United States, the 

 amount of erosion at present may be estimated at 

 200 square miles per year, rendering thereby large 

 areas of fertile soil unfertile and at least tempo- 

 rarily useless for human occupancy, the economic 

 importance of a conservative policy for the moun- 

 tain forests may be readily apparent. 



