THE WORK OF RUNNING WATER. 



125 



the solution of the cement sets free a considerable quantity of sand, 

 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 a sandstone than in a compact limestone, though the latter is, 

 as a whole, the more soluble. The constituent minerals of crystalhne 

 rocks resist solution and decay unequally, and when any one is dissolved 

 or decomposed the rock crumbles and the less soluble constituents are 

 ready for removal by mechanical means. So long as the material 

 loosened by disintegration is removed, chemical heterogeneity favors 

 erosion; but if the loosened debris is not removed erosion is not favored 

 by chemical heterogeneity. In such a case erosion would be most 

 rapid where the rock was most 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 advantage of these 



Or- 



Figs. 103 and 104. — Diagrams to illustrate the fact that a stream crosses many more 

 cleavage-planes when the beds of rock are inclined than when they are horizontal. 



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 inclined, 

 and inclined strata may stand at any angle between horizontality 

 and verticality. In indurated formations the rate of erosion is in- 

 fluenced 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 inclined, are probably 

 less favorable for rapid erosion than those which are vertical or in- 

 clined at considerable angles. This is at least true Avhere the layers 

 are of uniform hardness and the joints infrequent. 



Horizontal strata expose fewer cleavage planes to the water flow- 

 ing over them than strata in any other position. In Fig. 103 the stream 

 which has the profile ad crosses bedding-planes at h and c. In Fig. 104, 



