THE WORK OF RUNNING WATER. 133 



falls on the land and runs off over it, wear will be less rapid where the 

 hard la^^er comes to the surface than at the higher or lower levels. 

 As a result, the slope will become steeper at and below the outcrop 

 of the hard layer, and less steep immediately above it, as shown by 

 ah in Fig. 111. Under these conditions the water passing over the 

 hard ledge constitutes rapids. The increased erosion which ac- 

 companies the increased velocity makes the rapids more rapid. The 

 process may continue until the water falls, rather than floivs over the 

 hard layer (cd, Fig. 111). With continued rainfall the edges of the 

 hard layer, together with the slopes above and below, would continue 

 to recede toward the center of the island. Under conditions of abso- 

 lute homogeneity of material, save for the hard layer specified, no 

 valley would be developed, and therefore no stream. 



If the surface was so changed as to allow of the development of a 

 valley (p. 63) the same principles would be apphcable. As an active 

 stream passes from a hard layer to one less resistant, the greater wear 

 on the latter gives origin to rapids. At first the rapids would be 

 slight (a, Fig. 112), but would become more considerable (h) as time and 

 erosion go on. When the bed of the rapids becomes sufficiently steep, 

 the rapids become falls ^ (cd). When the water falls rather than 

 flows over the rock surface below the hard layer, erosion assumes a new 



Fig. 111. — Diagram representing a horizontal layer of hard rock in an island, and 



its effects on erosion. 



phase. The hard layer is then undermined, and the undermining 

 causes the falls to recede. This phase of erosion is sometimes called 

 sapping. 



If the hard layer which occasions a fall dips up-stream (Fig. 112), 

 its outcrop in the stream's bed becomes lower as the fall recedes (e). 

 When it has become so low that the water passing over it no longer 



^ The terms rapids, falls, and cataracts are rather loosely used. Many moderate 

 rapids are incorrectly called falls. The "Falls of the Ohio" is an example. The 

 term cataract is often applied to very steep rapids or falls. 



