Great Lake Basins of the St. Lawrence. 265 
the depression which crosses the State of Michigan along 
the Grand Valley and which, Rominger points out, seldom 
presents surfaces exceeding 100 feet above the lake. It 
does not average 30 feet in depth and it is suggestive 
whether it is not really a very shallow synclinal trough in 
the Carboniferous and Devonian rocks. 
Now, all these facts, with others, have their bearing on 
the origin of Lake Huron. The abrupt, subaqueous Corni- 
ferous ridge diagonally crossing the lake ; the different lines 
of direction of the Bruce Peninsula, its subaqueous exten- 
sion and the Manitoulin Islands, and of their deep bays and 
inlets ; the abrupt cliffs, both above and under water, show- 
ing rather the effects of undermining by waves and currents ; 
the directions of the lines of deepest depression ; and the 
varying and often sudden depths of the lake, showing that 
there has not been any general filling up of the hollows 
and depressions in the lake bottom, all militate against the 
idea that a great glacier from the north or north-east, 
gradually, in the course of ages, formed the depths and ont- 
lines of Lake Huron, nor do the directions of the ice grooves 
suggest what were evidently the travelling lines of the 
forces which gave rise to the above described and other 
physicial features of the lake. A reasonable conclusion, 
quite compatible with the existence of a fault, and with the 
elevation of the Niagara escarpment and of the land to the 
east of the Georgian Bay, would appear to be that the de- 
pression fronting this escarpment is in part the result of 
river excavation, and that through it flowed across Ontario, 
the drainage of the country to the northward and north- 
westward, until the waters joined the preglacial river which, 
as Spencer and Claypole point out, occupied the bed of 
Lake Ontario, This—supplemented by subsequent lake 
action—would account for much of the disintegration of the 
escarpment. The course of the river through Lake Huron 
was then, a8 shown by the line of depression, first to the 
south of eastward for some distance, then south towards the 
corniferous escarpment parallel to which it flowed, until, by 
a diversion to the north, it reached Cape Hurd and turning 
