150 GEOLOGY 



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

 abraded than a rock 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 the rock has much to do with the 

 rate of its erosion. Other things being equal, stratified rock is 

 more readily eroded than massive rock, since stratification planes 

 are planes of cleavage, and therefore of weakness. Taking advan- 

 tage 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 more easily eroded 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 verticality. 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. 

 On the whole, the strata which are horizontal, or but slightly in- 

 clined, are probably less favorably situated for rapid erosion than 

 those which are vertical or inclined at considerable angles. This 

 is at least true where the layers are of uniform hardness and the 

 joints infrequent. In general, joints have somewhat the effect 

 of bedding planes, so far as erosion is concerned. 



The Influence of Climate 



Climate has both a direct and an indirect effect on erosion. 

 Its direct influence is chiefly through precipitation, chants of 

 temperature, and wind; its indirect, through vegetation. Like 

 declivity and rock structure, climate does not affect all elements 

 of erosion equally. 



Direct effects. The effects of variations in temperature on 

 rock weathering have been discussed in Chapters II and III. 



