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University of California. 



[Vol. 2. 



matter how deep the soil or how dense the vegetation. In both 

 young and mature regions, some of this soil removal may be by 

 sliding of the land, particularly in seasons of great rainfall. Land- 

 slides are common in the California Coast Ranges, even on slopes 

 covered with vegetation. 



As commonly agreed, vegetation tends to decrease surface 

 erosion by binding the soil together, thus making its removal more 

 difficult, and checking sliding earth; by protecting the surface, in 

 many instances, with a more or less complete mat of leaves etc. • 

 by breaking the force of the falling raindrops and hindering their 

 free flow after reaching the ground, thus not only decreasing the 

 velocity of the flow at the surface and distributing its volume, but 

 also decreasing its volume, by allowing a longer time during which 

 the water may be absorbed by the soil ; by keeping the surface soil 

 comparatively porous, and thus facilitating its absorption of water. 

 The erosion on the slopes being thus decreased by vegetation, if at 

 the same time the cutting in the adjacent stream beds remains the 

 same, the angle of the slopes must ultimately be increased, since, 

 according to the principles already laid down, the erosion over the 

 surface must keep pace with that in the stream beds. Vegetation 

 may tend, therefore, to increase the angle of graded slopes, — a con- 

 clusion in direct opposition to the impression which seems gener- 

 ally to prevail, that bare slopes are always steeper than those 

 covered with vegetation (other things being equal). 



While vegetation tends to protect the soil from erosion, it tends 

 also to greater soil depth, both by its binding of the soil, which 

 prevents washing, and by the action of vegetable acids, conserved 

 water, root penetration etc., leading to more rapid rock decomposi- 

 tion. With the increase in the depth of the soil, the readier erosion 

 of soil as compared with rock will counterbalance, to a greater or 

 less extent, the protection against erosion furnished by the veg- 

 etation. The rate of rock decomposition (due to the causes men- 

 tioned above) will increase until a maximum is reached, after which 

 it will follow essentially the rate of wearing down of the 

 vegetation-covered slopes. 



Vegetation tends to a uniform distribution of the flow of water 

 over the entire surface of the slope, and the surface erosion is there- 



