Great Lake Basins of the St. Lawrence. 261 
the temperature of its water was maintained at a low point 
by cold inflowing streams, by currents, and by icebergs. 
These crustaceans thus aid in identifying the conditions 
under which the northern and maritime plants existed on 
the inland coasts. 
LAKE Huron. 
This lake presents a totally different set of circumstances 
from those of Lake Superior. Its floor is laid in the Arch- 
aean Silurian and Devonian formations, whilst the Niagara 
escarpment, continued across the Ontario peninsula, gives 
shape to the two great divisions into which the lake surface 
is separated in its northern half. 
In its profound depths the lake really forms three great 
basins—the Georgian Bay, the Central, and the Southern 
basins. 
The continuation of the great Niagara escarpment in an 
irregular, subaqueous ridge connecting Cape Hurd, the 
Grand Manitoulin Island, and the various islands between 
them, gives the Georgian Bay a distinctive character. This 
ridge appears to present, under water, bold, precipitous cliffs 
facing the Georgian Bay, similar to the heights from Cabot’s 
Head to Owen Sound, and with similar deep inlets, though 
penetrating the ridge in somewhat different directions. 
Whilst the cliffs on the islands form the real summit of the 
ridge, and its subaqueous portions rise to an average of 
within 30 to 40 feet of the lake surface, the depths on its 
immediate eastern sides often reach 250 feet. At Over- 
hanging Point, between Cabot’s Head and Cape Hurd, the 
depth at half a mile from the cliff reaches 540 feet, the deep- 
est point of the Georgian Bay. Through this subaqueous 
ridge there does not appear to be any break permitting 
direct access from the deeper waters of the bay to those of 
the central parts of the lake beyond. Further, the dip of 
the strata forming the ridge appears by the soundings to 
fall gradually to the westward and south-westward, just as. 
the same strata on the Bruce Peninsula slope to the west- 
