42 



THE EECESSION OF NIAGARA FALLS. 



Survey, Washington, D.C., discussed the 'History of Niagara River,' 

 in a long and very valuable paper, with maps, originally published 

 in the Sixth Annual Beport of the Commissioners of the State 

 Reservation at Niagara for the year 1889, pp. 61-84. This same 

 paper is republished in one of the Annual Reports of the Smithsonian 

 Institution for the year 1891. Mr. Gilbert finds the maximum rate 

 of retreat of the apex of the Horseshoe Fall (the re-entrant angle 

 where erosion is most rapid) to be ' between four feet and six feet 

 per annum.' [Mr. Bogart's figure may be taken as the average for 

 the whole line of the Horseshoe.] Arguing that * the rate of retreat 

 of the central portion of the Horseshoe is the rate at which the 

 gorge grows longer,' Mr. Gilbert concludes that probably ' 7,000 

 years were needed to excavate the six miles of gorge from Queens- 

 town Heights.' But various considerations qualify this estimate, 

 some of these tending to shorten and others to extend it. These 

 are discussed by him in the paper mentioned. 



" See also a report of Mr. Woodward's work and discussion by 

 Mr. Gilbert, in Proc, Am. Assoc. Adv. of Sci., vol. xxxv, for 1886, 

 pp. 222-3." 



