LAND WATERS STREAMS 149 



sluggish. The solvent power of water is not influenced by the 

 presence of sediment, though the presence of sediment offers the 

 I water a greater surface on which to work. 



CONDITIONS AFFECTING THE RATE OF EROSION 

 Declivity 



In general the greater the slope the more rapid the rate of 

 erosion by running water, whether in the stream's channel or on 

 the slopes above it. But high declivity does not favor every 

 element of erosion. The effect of declivity on weathering is far 

 from simple. Steep slopes favor some phases of weathering and 

 hinder others, but they favor both transportation and corrasion. 



Both corrasive power and transportive power increase rapidly 

 with increase of velocity, and under these circumstances, corrasion 

 will also be increased if the water has tools to work with, and trans- 

 portation if there is material which can be carried. Since high 

 declivity greatly increases both the transporting and the corrading 

 power of running water, and favors certain elements of weathering, 

 it is clear that the aggregate effect of high declivity is to favor 

 erosion. 



Rock 



The physical constitution, the chemical composition, and the 

 structure of a rock formation, influence the rate at which it may 

 be broken up and carried away. 



Physical constitution. Clastic rocks may be firmly cemented, 

 or their constituents may be bound together loosely. The less 

 the coherence the more ready the disintegration, and the finer the 

 particles the more easily they are carried away. If the materials 

 carried are harder than the bed over which they pass, corrasion of 

 the latter is favored. 



Chemical composition. Something also depends on the chemical 

 composition of the rock, since this affects its solubility and its rate 

 of decomposition. The more soluble the rock, the larger the pro- 

 portion of it which will be taken away in solution; but it does not 

 follow that the most soluble rock will be most rapidly eroded, since 

 the rate of erosion depends on abrasion as well as solution, and 



