RATE OF EROSION 81 



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 

 a rock which is readily soluble, as rocks go, may be less easily 

 al traded than one which is made of discrete and insoluble par- 

 ticles bound together by a soluble cement. In such rocks, for 

 example a conglomerate in which the pebbles are cemented together 

 by lime carbonate, the solution of the cement sets free a considerable 

 quantity of gravel, so that a small amount of solution prepares 

 a large amount of sediment for removal. A stream might cut its 

 valley much more rapidly in such rock than in a compact lime- 

 stone, though the latter is, as a whole, the more soluble. 



Structure. The structure of rock has much to do with the rate 

 of its erosion. Other things equal, stratified rock is more readily 

 eroded than massive rock, since stratification planes are planes of 

 cleavage, and therefore of weakness. Taking advantage of these 

 planes, the water has less breaking to perform to reduce the material 

 to a transportable condition. For the same reason, a thin-bedded 

 formation is eroded more easily than a thick-bedded one. 



The beds of stratified rock may be horizontal, vertical, or in- 

 clined, and inclined strata may stand at any angle between hori- 

 zontality and vertically. In indurated formations the rate of 

 erosion is influenced both by the position of the. strata and by the 

 relation of the direction of the flowing water to their dip and strike 

 (Chapter X). In general, strata which are horizontal, or but slightly 

 inclined, are probably less favorably situated for rapid erosion than 

 those which are vertical or inclined at considerable angles. Joints 

 have somewhat the effect of bedding planes, so far as erosion is con- 

 cerned. 



Influence of climate. Climate has both a direct effect on erosion, 

 chiefly through precipitation, changes of temperature, and wind; 

 and an indirect effect, chiefly through vegetation. Like declivity 

 and rock structure, climate does not affect all elements of erosion 

 equally. 



Direct effects. The effects of variations of temperature on rock 

 weathering have been discussed in Chapter II. Since high tem- 

 perature favors chemical action, the weathering of rock by decom- 

 position is at its best where the temperaturg is uniformly high, and 



VWV*W*4 J 



