542 THE ICE AGE IN NORTH AMERICA. 



stream, composed of the waters of the Tonawanda and Chip- 

 pewa Creeks, was of considerable extent, and by its action 

 had doubtless predetermined the course of the present river 

 above the Whirlpool, and may actually have worn a consid- 

 erable part of the present gorge above the Whirlpool. 



Another element of uncertainty, which has led Mr. Gil- 

 bert and others to retract their former views, or at least to 

 hold them in suspense, relates to the variations of the water 

 supply since the beginning of the erosion. 



It should, however, be noted that the erosion at Niagara 

 began long before the close of the Glacial period, namely 

 when the ice had melted off from the Mohawk Valley so as 

 to permit the drainage to take that course to the Hudson, and 

 lower the level of the existing glacial lake to that of the col 

 at Rome, N. Y. This would permit erosion to begin at the 

 mouth of the Niagara gorge, long before the ice had retreated 

 from the lower St. Lawrence Valley and from Canada in 

 general. 



A most interesting state of things respecting the varia- 

 tions of the water supply at Niagara comes to light in connec- 

 tion with the differential northerly depression of land during 

 the Glacial period, and its re-elevation after the disappearance 

 of the ice. From the fact that there was a northerly depres- 

 sion of 600 feet at Montreal, and presumably as much in the 

 northern basin of the Great Lakes, it follows that upon the 

 melting off of the ice from the Ottawa Valley, and from the 

 water parting between it and Lake Huron, the drainage might 

 for the most part be diverted in that direction, leaving the 

 Niagara with only that supply of water which would be fur- 

 nished by the local basin about the east end of Lake Erie. In 

 1892, 1 was so fortunate as to find clear evidence of this outlet 

 leading across from Lake Nippissing, past the town of North 

 Bay, into the Mattawa River and thence into Ottawa at the 

 town of Mattawan. The col at North Bay is less than 100 

 feet above the present level of Lake Huron, while the evi- 



