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_ Great Lake Basins of the St. Lawrence. 281 
Valley, is considered by some to be debatable ground, but 
it is difficult to now predicate what the levels were in the 
land surrounding these ancient rivers and seas. There have 
since been general changes in elevation extending over large 
areas, and there have also been local warpings within re- 
stricted areas which have completely altered within these 
areas the former levels in their relations to each other. 
Prof. Spencer’s view of this ancient river was limited to 
a connection between the southern end of Lake Huron and 
the eastern end of Lake Ontario by way of Port Stanley, 
Long Point and the Dundas Valley. Itseems most probable, 
however, that the subaqueous escarpment which diagonally 
crosses Lake Huron from opposite Kincardine in the direc- 
tion of the Straits of Mackinac, and which parallels the 
deepest depression there, may have been the south-western 
boundary of an npper section or expansion of this pre-glacial 
river valley. The hard Corniferous rocks would form an 
effective protecting side for such a river valley. Allusion has 
already been made to the probably earlier northward direc- 
tion of this river in the line of depression toward Cape 
Hurd and over the subaqueous ridge there. The sub- 
aqueous river channels, already referred to, on each side of 
the Straits of Mackinac and in Whitefish Bay, in Lake 
Superior, also indicate higher sections of this preglacial 
river, and if the view be accepted that Lake Superior had 
its outlet in these older times across the upper peninsula of 
Michigan, it is most in consonance with facts that the 
waters of this great and ancient inland sea found their 
course to the ocean at, at least, one period of its history, by 
way of these broad rivers of Tertiary and antecedent 
times, though the St. Croix valley has, probably, at another 
time, also formed an outlet. 
At what time, however, was this Niagara escarpment 
elevated? ‘This is a question difficult of answer. And yet 
the facts already given would indicate that it was prior in 
time to the deposit of the clays, sands and gravels against 
the escarpment in the Dundas Valley, at the Bruce 
Peninsula and elsewhere; prior to the deposit of the 
