A CLOCK OF RECENT GEOLOGICAL TIME 



357 



maps made in 1842, 1875, 1886, 1890, and 1905 of the brink of 



the Canadian Fall (Fig. 383) indicates that for the period covered 



the rate of recession has been about five feet per year, and similar 



studies made of the 



American Fall show that 



it has been receding at 



the rate of only three 



inches per year, or one 



twentieth the rate of the 



recession of the Canadian 



Fall. 



Future extinction of the 

 American Fall. It is 

 because of this many 

 times more rapid reces- 

 sion of the Canadian 

 Fall that the Niagara 

 cataract, instead of lying 

 athwart the gorge, enters 

 it from its side. The 

 Canadian Fall is thus in 

 reality swinging about 

 the American, and the 

 time can already be 



roughly estimated when FIG. 383. Map to show the recession of the brink 



of the Canadian Fall, based upon maps of differ- 

 ent dates (after Gilbert). 



O E 



this more effective drill- 



ing tool will have brought 

 about a capture, so to speak, of the American Fall through the 

 cutting off of its water supply. It will then be drained and left 

 literally " high and dry," an enduring witness to the geological 

 effect of an island in making an unequal division of the waters for 

 the work of two cataracts. 



As already pointed out, the inefficiency of the American Fall 

 as an eroding agent is amply attested by the wall of blocks 

 already appearing above the water below it. The tourist who a 

 thousand years hence pays a visit to the Niagara cataract, pro- 

 vided the water flow is allowed to remain as it has been, will find 

 above this rampart of blocks a bare cliff in part undermined, and 

 surmounted by a nearly flat table surface which is cut of? from the 



