THE DATE OP THE GLACIAL PERIOD. 337 



that he favoured extending the time to one hundred thou- 

 sand years. Before this the eminent French geologist 

 Desor had estimated that the recession could not have 

 been more than a foot in a century, which would throw 

 the beginning of the gorge back more than three mill- 

 ion years. But these were mere guesses of eminent men, 

 based on no well-ascertained facts ; while Mr. Bakewell, 

 an eminent English geologist, trusting to the data fur- 

 nished him by the guides and the old residents of 

 Niagara, had, even then, estimated that the rate of re- 

 cession was as much as three feet a year, which would 

 reduce the whole time required to about ten thousand 

 years. 



But the visit of Lyell and Hall in 1841 led to the be- 

 ginning of more accurate calculations. Professor Hall 

 soon after had a trigonometrical survey of the falls made, 

 from which a map was published in the State geological 

 report. From this and from the monuments erected, we 

 have had since that time a basis of comparison in which 

 we could place absolute confidence. 



In recent years three surveys have been made : the 

 first by the New York State Geologists, in 1875 ; and the 

 third by Mr. R. S. Woodward, the mathematician of the 

 United States Geological Survey, in 1886. The accom- 

 panying map shows the outlines of the falls at the time of 

 these three measurements, from 1842 to 1886. According 

 to Mr. Woodward, " the length of the front of the Horse- 

 shoe Fall is twenty- three hundred feet. Between 1842 

 and 1875 four and a quarter acres of rock were worn away 

 by the recession of the falls. Between 1875 and 1886 a 

 little over one acre and a third disappeared in a similar 

 manner, making in all, from 1842 to 1886, about five and 

 a half acres removed, and giving an annual rate of reces- 

 sion of about two feet and a half per year for the last 

 forty-five years. But in the central parts of the curve, 

 where the water is deepest, the Horseshoe Fall retreated 



