WORK OF RUNNING WATER 



which preceded. The later rapids are steepest at the beginning of 

 their history, the earlier at their end. Stated in other terms, rapids 



are steepest when nearest falls 

 in time. Slight differences in 

 the resistance of successive 

 layers may occasion successive 

 falls or rapids (Fig. 77). 



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 

 Such falls 



Fig. 76. Diagram illustrating the con- 

 ditions at Niagara. (Gilbert.) 



falls, will develop, 

 would not recede. 



The inequality of resistance 

 in the rock which occasions a fall may be original or secondary. In 

 the case of Niagara Falls (Fig. 76) relatively resistant limestone 

 overlies relatively weak shale. 

 At the Falls of St. Anthony 

 (Minneapolis) limestone over- 

 lies friable sandstone. The 

 falls of the Yellowstone are in 

 igneous rock. In this case the 

 unequal resistance is caused by 

 unequal decay of the rock, due 

 perhaps to the rise of hot va- 

 pors which have decomposed 

 and weakened the rock in the 

 areas through which they have 

 ascended. Such action is com- 

 mon in volcanic regions. 



One waterfall may breed 

 others. Thus where a fall 

 recedes beyond the mouth of a 

 tributary stream, the tributary 

 falls. The Fall of Minnehaha 

 creek tributary to the Missis- 



Fig. 77. Bridal Veil Fall, Kamloops, 

 British Columbia. 



sippi near Minneapolis, is an 



illustration. Once in existence, the fall of a tributary follows the 



same history as that of a main stream. 



