266 Canadian Record of Science. 
eastward, joined this river channel in a great fall over the sub- 
aqueous ridge now worn back to a line between Cape Hurd 
and Grand Manitoulin Island. Another stream from the 
north joined it at this point. These great preglacial rivers 
would continue their flow until the elevation of the anticlinal 
between the Georgian Bay and Lake Ontario blocked their 
course, and filling the Georgian Bay with water, created a 
new outlet, not by the St. Clair River, but to the south- 
eastward of Lake Huron as hereafter referred to. 
Though the eastern coasts, between the Bruce Peninsula 
and the County of Lambton, present bold clay cliffs of con- 
siderable height, the general dip of the strata from the 
Niagara escarpment which crosses Lake Ontario to the 
Georgian Bay, is towards and under the main body of Lake 
Huron. As already mentioned, this is also the case on the 
Manitoulin Islands, and south-eastward across the suba- 
queous escarpment to the Bruce Peninsula. Again, thestrata 
on the Canadian side of Lake Huron proper appear on the 
Michigan side in the same relative positions. These facts 
tend to prove that the lake is in part now a synclinal 
trough which has been further depressed, in common with 
the surrounding country, at the time when the superficial 
deposits were formed, but which, in its rise to its present 
levels, has left behind the great clay cliffs now lining its 
eastern sides, which have been gradualy worn backwards 
by the action of waves and atmospheric causes. 
The subject will be further referred to when discussing 
Lakes Michigan and Ontario, for the final shaping of the 
contour of these three lakes was in part due to one com- 
mon cause. 
LAKE MIcHIGAN. 
This lake rests, to a limited degree, on the Lower Carboni- 
ferous rocks, but chiefly on those of Upper Silurian and 
Devonian age. Its depth has been said to reach even 1,800 
feet;' but the soundings made under the direction of the 
1 Encyclop. Britann. 9th ed. vol. 21, p. 178. 
an 
