WATERFALLS. 13 



that the position of the fall is not stationary, but slowly recedes at a 

 rate which has been variously estimated from one to three feet per an- 

 num. The process of recession has been carefully observed, and the 

 reason why it maintains its perpendicularity is very clear. The surface- 

 rock of Erie plateau is a firm limestone (a) . Beneath this is a softer 

 shale (b). This softer rock is rapidly eroded by the force of the falling 

 water, and leaves the harder limestone projecting as table-rocks. From 

 time to time these projecting tables are loosened and fall into the chasm 

 below. This process is facilitated by the joint structure spoken of on 

 page 5. 



Recession of the Falls. — Now, there is every reason to believe that 

 the fall was originally situated at Queenstown, the river falling over 

 the escarpment at that place, and that it has worked its way backward 

 seven miles to its present position by the process we have just described. 

 These reasons are as follows : 1. The general configuration of the country 

 as already described forcibly suggests such an explanation to the most 

 casual observer. 2. A closer examination confirms it by showing that 

 the gorge is truly a valley of erosion, since the strata on the two sides 

 correspond accurately (see Fig. 7). 3. As already seen, the falls have 

 receded in historic times at a rate of from one foot to three feet a year. 

 The portion of the gorge thus formed under our eyes does not differ in 

 any essential respect from other portions farther down the stream. The 

 evidence thus far is not perfectly conclusive that the gorge was formed 

 by the present river during the present geologic epoch, since the gorge 

 may have been eroded during a previous epoch, and the present river 

 found it, appropriated it as its channel, and continued to extend it. 

 But (4) certain stratified deposits have been found by Mr. Lyell and 



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Fig. 7.— Ideal Section across Chasm below the Falls. 



others on the upper margin of the ravine, containing shells, all of which 

 are identical with the shells now living in Niagara River. On the mar- 

 gins of all rivers we find stratified deposits of mud and sand containing 

 dead shell. The stratified deposits found by Mr. Lyell were such mud- 

 banks of the Niagara River before the falls had receded so far, and 

 therefore when the river still ran on the Erie plateau at this point. 



