40 TWELFTH REPORT. 



of tlie outlet of Lake Erie may be sufficient to bring tliat body of water 

 up to the level of Lakes Michigan and Huron, for it now lacks only 7 

 or 8 feet of being as high as this higher Avater body. The continuation 

 of the uplift would bring the water at Chicago to a level high enough to 

 produce a reoccupancy of the Chicago Outlet as a line of discharge. 

 There might then be a tAvo outlet stage such as occurred in the case of 

 Lake Nipissing, one outlet being as at present over Niagara Falls, and 

 the other southAvestward along the old Chicago Outlet. A further con- 

 tinuation of the uplift might result in the abandonment of the Niagara 

 Outlet. 



The estimated rate of uplift, as determined by Gilbert, is only .42 

 of a foot in 100 miles in a century, or someAvhat less than 2 feet in 100 

 years at the opposite extremities of the Georgian Bay-Lake Michigan 

 body of water. The contingencies above mentioned, if uplift continues 

 at the present sIoav rate, lie many centuries in the future, and there 

 is a possibility that the uplift may die out entirely before the lake 

 reaches a stage high enough to transfer the discharge from Niagara 



to Chicago. 



HISTORY SHOAVN BY NIAGARA FALLS. 



The scA'eral changes of volume in the discharge from the Lake Erie 

 basin over the falls of Niagara are found to be clearly represented in 

 the bed of the gorge beloAV the falls. Dnring«the time when only Lake 

 Erie AA^as discharging over the falls, and Lake Algonquin had its dis- 

 charge through the Trent Outlet, the bed of the gorge Avas excavated to 

 a shalloAV depth, but Avhen the large discharge of Lake Algonquin was 

 united to that of Lake Erie a deeper excavation occurred in the vicinity 

 of the whirlpool. Then Avhen the Nipissing Great Lakes had eastward 

 discharge and left only Lake Erie to discharge over the falls a shallow- 

 portion of the bed of the gorge betAveen the A\hirlpool and the suspen- 

 sion bridge marks the Avork done by the small Lake Erie Outlet. With 

 the return of the Avaters that came about in the change from the 

 Nipissing Great Lakes to the modern Great Lakes a deep excavation of 

 the bed of the gorge set in Avhich continues to the base of the present 

 Horseshoe Fall. 



The rate of recession of Niagara Falls has been a subject of careful 

 iuA^estigatiou in order to form a basis for an estimate of the length of 

 time inA'Oh-ed in the entire recession of the cataract. The American 

 Fall, which carries only a small amount of Avater probably illustrates 

 conditions Avhich obtained during the times of small discharge, Avhile 

 the Horseshoe Fall shows a recession by Avhich the measurement of the 

 time invoh-ed in the discharge of the gi-eat volume of water can be 

 made. By means of these studies of the recession of the present falls 

 it is estimated that the time can not be less than 15,000 years and it 

 may be as great as 30,000 years since the cataract came into operation, 

 that is, when Lake Warren had given place to Lake Erie. From this it 

 appears that the time involved in the entire lake history from the be- 

 ginning of Lake Chicago and Lake Maumee doAvn to the pi-esent can 

 not well be less than 20.000 to 25,000 years on the basis of the lesser 

 estimate of 15,000 years for the recession of Niagara, and it would con- 

 siderably exceed 30,000 years on the basis of the larger estimate. This 



