LAND WATERS STREAMS 



155 



time. Slight differences in the resistance of successive layers often 

 occasion successive falls or rapids (Fig. 114). 



If the layers of unequal hardness in a stream's bed are vertical, 

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

 rapids, and perhaps falls, will 

 develop. Falls developed un- 

 der these conditions would not 

 recede. 



The inequality of resistance 

 in the rock which occasions a 

 fall may be original or second- 

 ary. In the case of Niagara 

 Falls (Fig. 113) relatively 



resistant limestone overlies 



relatively weak shale. At the Fig ii3._ Diagram illustrating the 



Falls of St. Anthony (Min- conditions at Niagara. (Gilbert.) 



neapolis) limestone overlies 



friable sandstone. The falls of the Yellowstone, and the Sho- 



shone Falls of the Snake River (Idaho), are in igneous rock. 



In the former case the unequal resistance is occasioned by unequal 



decay of the rock, due perhaps to the rise of hot vapors which 



have decomposed the rock along the lines of their ascent; in 



the latter, a more resistant sort of igneous rock overlies a less 



resistant. 



One waterfall often breeds others. Thus where a fall recedes 

 beyond the mouth of a tributary stream, the tributary falls. The 

 Fall of Minnehaha creek tributary to the Mississippi near Minne- 

 apolis, is an illustration. Once in existence, the fall of a tributary 

 follows the same history as that of a main stream. 



The fall of the Niagara 1 (PL IX) is one of the most remarkable 

 known both because of its large volume of water and its great 

 descent, between 160 and 170 feet. This fall is divided into two 

 parts, separated by Goat Island, the Horseshoe fall (Fig. 115) on 

 the west, and the American fall on the east. Between 1842 and 

 1905, the Horseshoe fall receded about five feet a year, while be- 



1 For a brief account of this fall see Gilbert in Physiography of the United 



States. 



