146 Canadian Record of Science. 
having a maximum depth of about seventy feet, whilst 
Lake St. Clair has a maximum depth of only twelve feet. 
These lakes appear rather to be shallow overflows caused 
by the restricted passage now of the waters over the Nia- 
gara escarpment in the one case, and through the Detroit 
River in the other, than to be due to physical forces which, 
operating in past ages, excavated preparatory basins for 
them. There can be no doubt that, as Dr. Hunt suggests, 
the post-tertiary clays of south-western Ontario now occupy 
the basin of what may have in earlier times been a much 
larger lake or inland sea. 
Regarding the operation generally of glacial forces in 
contributing in some respects to the features of our Great 
Lakes, we can conclude that our whole Laurentian and 
Huronian country north of these lakes and of the St. Law- 
rence was elevated into great mountain chains, that, with 
the colder climate, enormous glaciers everywhere flowed 
down the mountain sides and over the country beyond, 
and that contemporaneously, probably towards the close of 
the age, there were, as has been shown by Sir William 
Dawson, extensive depressions in the eastern parts of 
Canada, and of the northern United States, which admitted 
tha Arctic current laden with huge icebergs up the St. 
Lawrence and across the basin of the Great Lakes; or, 
what is more probable, that the Great Lakes formed an 
inland sea which extended over parts of the Northern 
States as well. Across this inland sea and towards the 
Mississippi River, which was probably then its outlet, 
floated numberless icebergs, the offshoots of the Laurentian 
glaciers to the northward, freighted with their loads of 
boulders and debris, which were dropped on the sea bottom 
as the bergs melted, or were broken by contact with other 
bergs or with rocks. Our North-West, as far as the Rocky 
Mountains, was at this time, or subsequently, the floor of 
an even vaster sea, with the prevailing winds or currents 
carrying, in the direction of these mountains, fleets of ice- 
bergs from the great glaciers on the eastern borders of the 
sea, which were then on a line with the present Lake 
