academy or BC..NC.8] NIAGARA AND THE GREAT LAKES 227 



The other observer's statement, in 1S94, was: 



If the late rate of terrestrial deformation shall continue into the future . . . the drainage of the upper 

 lakes will be diverted from the Niagara into the Mississippi in perhaps 5,000 or 6,000 years hence." 



The difference between the two statements is hardly more than a difference of auxiliary 

 verbs. One says, in effect, a further tilting would send a great river out from the lakes to the 

 Mississippi at Chicago; the other says a continued deformation will divert the lake discharge 

 to the Mississippi. Whether the crediting of the prediction to the other observer was prompted 

 by an indifferent forgetfulness or by an exaggerated generosity does not appear; but in either 

 case it is historically misleading to a reader who happens on Gilbert's later article without 

 knowing of his earlier one. 



THE PROFILE OF THE BED OF THE NIAGARA GORGE 



In the Toronto lecture on Niagara no special attention was paid to the influence of varia- 

 tions in the volume of Niagara River on the width and depth of its gorge; but this subject, 

 with others, was taken up in the summer of 1896, following the three summers in Colorado, and 

 quickly carried to a successful conclusion. The width of the gorge is seen to be reduced to its 

 least measure for a certain distance below and above the whirlpool, and to increase again near its 

 upper end. The depth of the mouth of the gorge by Lewiston had been sounded as 96 feet, 

 and in the pool near the great cataract as 189 feet. In the intermediate stretch the strength 

 of the current was such as to make sounding impracticable. Gilbert therefore determined the 

 missing data by an ingenious method, suggested by a fellow worker, in which the volume of 

 discharge being known the depth was calculated in terms of the surface velocity from the 

 formula : 



Central depth = Volume 



\ 5 / s central velocity X width. 



The central velocity was estimated by timing the passage of patches of foam over meas- 

 ured distances; the river width was taken from accurate surveys; the volume was taken from 

 the records of the Lake Survey. 10 The formula being supplied with these quantities, the depth 

 of the water below Wintergreen Flats was found to be 70 feet; opposite Wintergreen Flat, 35 

 feet; at the whirlpool outlet, 50 feet; in the whirlpool, 150 feet; and in the rapids above the 

 whirlpool, 35 feet. In the interpretation of these values the great depth at the whirlpool was 

 ascribed to the existence there of the drift-filled St. Davids Channel; the shallow stretches 

 near Wintergreen Flats and above the whirlpool were correlated with epochs when the short- 

 cut discharge of the upper lakes, first by the Trent and later by the Nipissing outlet, left the 

 Niagara River so small and weak that it could not excavate a deep gorge ; the somewhat deeper 

 stretch between Wintergreen Flats and the whirlpool was tentatively ascribed to a temporary 

 restoration of full river volume by a partial elevation of the northern land which raised the 

 Trent outlet higher than Detroit, and therefore turned the discharge of the upper lakes through 

 Lake Erie while the lingering ice sheet still obstructed the Nipissing outlet, but this interpreta- 

 tion has since been questioned; the deep pool between the whirlpool rapids and the present 

 falls was the work of the finally restored full volume of the river, when the rise of land in the 

 northeast finally turned the discharge of the upper lakes through the Detroit Channel, where 

 it had run before while the Trent and the Nipissing outlets were obstructed by ice. 



• J. W. Spencer. The geological survey of the Great Lakes. Proc. Amer. Assoc. Adv. Sci., iliii, 1894, 237-243; see p. 243. 

 "> Profile of the Niagara in its gorge. (Abstract) Ainer. Oeol., rviii, 1896, 232. 



