THE PAST AND FUTURE OF NIAGARA. 



565 



proposition. It is to be lamented, for another survey would give data 

 by which we could translate into time nearly half a mile of the chan- 

 nel. Until the Falls shall be examined again by instrumentation, in 

 estimating the rate of recession we must depend on the eye alone. 



In 1840 old citizens told Lyell that the Falls recede about a yard 

 in a year. I hear the same estimate from citizens now. They see a 

 notch in the Horseshoe which was not there thirty years ago, and they 

 see it growing deeper year by year ; they see the American Fall more 

 indented than it was when they used to observe it, and from such 

 changes they construct a scale and apply it to the entire periphery. 

 They deceive themselves. A careful study of the Falls from the 

 trigonometrical points, even without instruments, and a comparison 

 of what you see, with the map of 1842, would convince you that the 

 recession during the past 30 years would fall inside of 15 feet. 

 Let us take six inches a year as an approximation to the rate at which 

 the Falls are eating back through the ledges of shale and limestone. 

 The scale which answers to the last 30 years will apply to the 

 channel from the Horseshoe to Ferry Landing, nearly half a mile. 

 Through this part of the channel the Falls have cut through the same 

 rocks they are cutting now. "When they were at the site of Ferry 

 Landing, a hard limestone, a member of the Clinton group, No. 4 of our 

 section (Fig. 1), lay at their base, and the recession must have been ar- 

 rested. Again, when they were at the site of the Whirlpool, a very 

 hard, quartzose sandstone, marked 2 in the section, a member of the 

 Medina system, lay at their base and checked their recession. Here 

 the great cataract must have stood for ages almost stationary. With 

 these two exceptions, the Falls, in every stage of their retreat, have 

 cut through shale below, and the Niagara limestone above. 



Fig. 1. 



6 



tc wis ton 3 



I. ONTARIO 



Section of Strata axono the Eiveb feom Lake to Lake. 



Numbers 1, 2, 3, belong to the Medina group ; 4, to the Clinton group ; 5, 6, 7, to the Niagara group ; 

 and S and 9 to the Onondaga group. Numbers 1 and 3 are red shaly sandstone ; 2 is quartzose lime- 

 stone; 4 green shale and limestone; 5 dark shale; and 6 gray limestone. W. site of the "Whirlpool, 

 where the Falls were 120 feet higher than now, and where their recession was checked by 

 the quartzose sandstone, No. 2. The dotted line t, t, represents the highest terrace (or oldest river- 

 bank) from the Whirlpool to the head of the "Rapids, II. F represents the present site of the Falls, 

 and B, E, the surface of the river from lake talake. 



Another element in the problem of Niagara's age is the flow of 

 water. To construct a scale from the present and apply it to the past, 

 we should know that the amount of water in past ages has been essen- 

 tially the same as now. 



About 9,800 cubic miles of water nearly half the fresh water on 

 the globe are in the upper lakes, and 18,000,000 cubic feet of this 



