The Great Lake Basins of Canada. 145 
these enormous masses of ice would have, and of the heat 
generated underneath which would probably prevent any 
excessive accumulation, the withdrawal of a depth of 600 
feet of water from the North Atlantic Ocean would have 
moved the whole United States coast line from Texas to 
Maine about seventy-five to one hundred miles seaward of 
its present position, would have rendered the Guif of St. 
Lawrence dry land, and brought to the surface the Great 
Banks of Newfoundland, would have obliterated the Ger- 
man Ocean, thus connecting Great Britain with the conti- 
nent of Europe, and would have almost formed an isthmus 
between Great Britain and Iceland. How far are we pre- 
pared to accept these results as occurring simultaneously 
at this time? Some of them actually did occur at other 
periods, but through the slow elevation of the land. 
The subject of the origin of the Great Lakes is still beset 
with some difficulties. Whitney, and more recently R. D. 
Itving, have shown that Juake Superior throughout its 
whole area is a synclinal trough or depression, and that the 
Keweenaw series of rocks in its upper and lower divisions 
probably underlies nearly the whole lake. This, then, 
largely dispels the idea of the glacial origin of this lake. 
When this depression took place is a more difficult ques- 
tion. Through its western half the axis of the depression 
lies in a southwesterly direction and, in a general sense, 
parallel to the trap overflows of the western shore, showing 
that they may both be due to the same force. 
Again, Lakes Erie and St. Clair, which without doubt 
have at one time been united more intimately than now, 
are probably the most recent in origin of the Great Lakes. 
The county of Essex, which now separates them, has quite 
the characteristics of the modern prairie, and its formation 
is undoubtedly due to similar causes. Centuries of growth 
and decay of rich grasses and sedges in the extensive 
marshes here bordering the lake, gradually contributed a 
loamy soil, which even now is not much above the level of 
Lake St. Clair. These two lakes lie in very shallow depres- 
sions in the Erie clays—Lake Erie in its southwestern half 
