THE DATE OF TEE GLACIAL PERIOD. 539 



pool and not over half as long, is still at its mouth by St. 

 Davids several times as wide as that of the present Niagara 

 gorge at Queenston. 



Coming to the main question, and taking the whole of 

 the gorge from Queenston to the present cataract as the 

 work done by the Niagara River since the ice-barrier in the 

 valley of the Mohawk gave way, the problem is to find the 

 rate of recession. Until very recently the estimates of this 

 rate have been scarcely more than mere guesses. The emi- 

 nent French glacialist Desor thought it could not have been 

 greater than one foot in a century, which would place the 

 beginning back 3,500,000 years. In 1841 Sir Charles Lyell 

 and Professor James Hall examined the gorge together ; and 

 Sir Charles, in his lectures in Boston before the Lowell Insti- 

 tute soon after, estimated that the maximum rate of reces- 

 sion could not be greater than one foot a year, which would 

 fix the minimum date of its beginning at about thirty-five 

 thousand years ago. On the contrary, all the guides of that 

 period who had observed the falls for many years, were con- 

 fident that the rate of recession was as much as two feet a 

 year ; * while Mr. Bakewell, an eminent English geologist, who 

 had given much personal study to the question, estimated 

 that, for the forty years previous to 1830, the rate of recession 

 had been about three feet a year. Mr. Bakewell's son care- 

 fully reviewed the phenomena again in 1846, in 1851, and in 

 1856, and found no occasion to revise his father's estimate.f 



To furnish the basis for more accurate calculations, Pro- 

 fessor James Hall had a map of the falls made in 1842, from 

 a trigonometrical survey, so that there should be a fixed 

 standard for future comparison. Within the past few years, 

 accurate surveys have again been made, both by the geolo- 

 gists of the State of New York, and by members of the 

 I'nited States Coast Survey. In 1886 the American Associa- 

 tion for the Advancement of Science held its annual meeting 



* Lyell's "Travels in America " (first series), vol. i, p. 27. 



f "American Journal of Science," vol. lxxiii, 1857, pp. 87, 93. 



