264 Canadian Record of Science. 
While thus this ridge approaches in some places within 60 
feet of the present level of the lake, the profound depths on 
the immediate north-easterly side vary from 360 to 588 
feet. 
The deepest point in the lake is 750 feet, or 172 feet below 
ocean level, and is found in this central basin about thirty 
miles south-west of Cape Hurd. It is a sudden depression, 
as the depths a short distance on either side are 426 and 366 
feet, and it does not occur in the general line of deepest de- 
pression. This line, starting from near the Canadian shore, 
takes a direction irregularly parallel with the Corniferous 
limestone escarpment to a point somewhat more than half- 
way across the lake, when its direction is diverted north- 
ward towards Grand Manitoulin Island. A branch of this 
line of deepest depression runs from off Kincardine almost 
due north in an irregular line towards Cape Hurd. Lake 
Huron is thus somewhat deeper in its Canadian half, and the 
central basin gradually shallows to about 180 feet near the 
Straits of Mackinac. 
The southern basin comprises all that part of the lake 
south of the subaqueous Corniferous escarpment, and is much 
shallower than the central basin. The summit of the 
escarpment has an average breadth of about four miles, 
after which, on the south-western side, the slope becomes 
more distinctly to the south-west or west, andis somewhat 
gradual, though the greatest depth in this southern basin 
is reached at 330 feet in an abrupt depression at one point, 
at the beginning of this slope, about midway across the 
lake. The depth over the greater portion of this southern 
basin is very moderate, and about its centre is a large area, 
lying somewhat north-west and south-east, where, though 
almost surrounded by deeper water, the depth does not ex- 
ceed 180 feet, and is generally less. 
Whilst the bottom of the central basin is chiefly clay, with 
gravel in places, that of the southern basin is largely sand, 
especially in its lower third towards the outlet at the St. 
Clair River, and in Saginaw Bay. 
Saginaw Bay appears to be a subaqueous continuation of 
