Great Lake Basins of the‘St. Lawrence. | 25% 
third, in a direction parallel with the eastern coast line, the 
White Fish Bay subaquecus cliffs and depression, and the ap- 
parent ridge between Caribou Island and Michipicoten 
Island. Other less important forces acted in other direc- 
tions in forming Thunder Bay, Black Bay, with its deeply- 
channelled entrance, and the eastern and deeper side of 
Nepigon Bay. These forces probably operated at different 
times, each inits turn contributing to the further enlarge- 
ment of the lake, which originally was no doubt of modest 
dimensions compared with the present area. 
It is just probable that the operation of the second force 
in the order given above was more recent than that of the 
first, as a very marked subaqueous anticlinical in a line with 
and forming a continuation under the lake of the Keweenaw 
Peninsula, crosses to the centre of the lake, somewhat ab- 
ruptly severing in two the deep, lake depression which runs 
‘from Michipicoten Island westward. There is a presump- 
tion that this anticlinal was formed subsequently to the de- 
pression, and, considering also the sandstones on the 
south-west coast, that the central part of the lake may thus 
be older than the south-western. Again, the Cariboo Island 
anticlinal apparently likewise crosses the deep, lake depres- 
sion, and thus the central parts of the lake may also be older 
than the eastern. The White Fish Bayriver channel being 
cut through the Potsdam sandstones, will also be more 
recent, 
If we regard these earlier forces as having a common 
source with some of those which resulted in the eruptive 
rocks, forming so prominent a feature in, and so conspicu- 
ously interstratified with, the Huronian and Keweenawan 
Series, then we may date the origin of Lake Superior as far 
back as it may be Huronian and Keweenawan times. And 
this is by no means improbable. Foster and Whitney, and 
especially and more recently, R. D. Irving, have shown that 
the lake is, in both its eastern and western halves, a great 
synclinal trough or depression. This conclusion has been 
arrived at from—particularly in the western half—the gen- 
erally constant dip of the Keweenawan rocks towards and 
