134 



GEOLOGY. 



reacts effectively against the less resistant material beneath (/) , sapping 

 ceases, and the point of greatest erosion may be shifted from the soft 



Fig. 112. — Diagram illustrating the development of a fall where the hard layer dips 



gently up-stream. 



material beneath the fall to the hard layer itself. The actual rate 

 of erosion at this point may be no greater than before, though the 

 relative rate is. Under these circumstances the vertical edge of the 

 hard layer will presently be converted into an incline (/), and a3 this 

 takes place the fall becomes rapids. The conversion of the falls into 

 the rapids begins about the time the lower edge of the hard stratum 

 in the channel reaches grade. By 

 continuation of the process which 



' :x\ 



Fig. 113. — Diagram illustrating, the 

 conditions which exist at Niagara 

 Falls. (Gilbert.) 



transformed the falls into rapids, g^g^f^-^g^g 

 the rapids become less rapid, and ^^^-=^-^^=-— '■ 

 when the upper edge of the hard layer ^ 

 has been brought to grade, the rapids 

 disappear {h, Fig. 112). The history 

 of rapids which succeed falls is the 

 reverse of that which preceded. The 

 later rapids are steepest at the be- 

 ginning of their history, the earlier 

 at their end. Stated in other terms, 

 rapids are steepest when nearest falls 



in time. Shght differences in hardness in successive layers often 

 occasion successive falls or rapids (Fig. 114). 



If the hard layer which occasions the falls be horizontal, instead 

 of dipping up-stream, the general result would be the same; but, other 

 things being equal, the duration of the falls developed under these 

 conditions would be greater, since they must recede farther before 

 becoming rapids. 



If the layers of unequal hardness in a stream's bed be vertical and 

 the course of the stream at right angles to the strike, rapids, and 

 perhaps falls, will develop (Fig. 115). The chances for falls are 

 greater, the greater the difference in hardness. Falls developed under 



