THE WORK OF RUNNING WATER 



77 



';, 



^ 



stream, with the result shown in Fig. 73. The continued wear of 

 the water in such a case would cause the rapids at c (Fig. 72) to 

 become steeper, and in time the descending water would become 

 a fall d' (Fig. 73). In this case, the rapids and falls depend on in- 

 equalities of hardness in the bed of the stream. This is, perhaps, the 

 commonest way in which falls and rapids originate. Fig. 74 shows 

 the structure of the rock at 

 Niagara, where the falls are 

 due to a harder layer of rock 

 above others which are not so 

 hard. A landslide or a lava 

 flow may form a dam, over 

 which the water falls or flows 

 in rapids. Rapids and falls 

 arise in other ways also. 



Falls and rapids are under- 

 going constant change, although 

 the change is usually very slow. 

 The falls of the Niagara are 



moving slowly up-stream, because the falling water undermines the 

 hard layer of rock over which it drops (Fig. 74) . As a fall recedes, it 

 becomes lower in many cases (Fig. 73) . In such cases, it is clear that 

 the fall will disappear if it recedes far enough. If the hard rock 

 over which the water drops is in the position shown in Fig. 75, the 



Fig. 74. Diagram illustrating the 

 conditions at Niagara. (Gilbert.) 



Fig. 75. Diagram illustrating a condition where a fall will not recede. 



fall will not recede, though it will become lower, and will disappear 

 when the stream cuts down to base-level where the fall is. Rapids 

 and falls are temporary features of streams, and like canyons, are 

 marks of youth. In time, all rapids and waterfalls will disappear, 

 for they cannot exist after rivers have reached base-level. 



Natural bridges. If a stream with a waterfall flows over rock 

 in which there are deep, open cracks, as is sometimes the case, a 



