Great Lake Basins of the St. Lawrence. 251 
CENTRES OF DEPRESSION. 
When examining attentively the general geological fea- 
tures of the country surrounding the Great Lakes, the care- 
ful student will not fail to observe that three great centres, 
as it were, of depression existed in its bygone history. 
One occupies nearly the western half of Lake Superior, the 
floor of which here is overlaid by the Cambrian and upper 
division of the Keweenawan rocks. Beyond these, on the 
north-west and south-east sides of this part of the lake there 
occur, in successive descending order, the lower division of 
Keweenawan, the Animikie division of the Huronian, and 
what are supposed to be the Laurentian rocks. 
Eastward of Lake Superior, it will be observed that, as 
far onward as the Carboniferous period, there were, near 
the present lakes, two other great centres, as it were, of 
depression, the one in Northern Pennsylvania, the other in 
Michigan. In passing southward from the Laurentian 
region lying between the Georgian Bay and the Upper 
Ottawa, the formations are met with in a regular, almost 
unbrok en, ascending order, from the Laurentian of Canada, 
through the Lower and Upper Silurian and Devonian, until 
the Carboniferous rocks of Northern Pennsylvania appear. 
The strata representing these formations occur in this regu- 
lar succession, all within a distance from north to south of 
one hundred and seventy-five miles. ‘The outcrops of 
several of these formations are, on the south side of Lake 
Ontario, more or less parallel to the length of the lake and 
to each other, whilst the outcrop of the Trenton and Black 
tiver limestones to the north of the lake runs in a line dia- 
gonally from the east end of Lake Ontario to the Georgian 
Bay. 
That the area presently occupied by Lake Ontario was 
overlaid in part by Trenton limestones and Utica slates, 
but perhaps more by rocks of the Hudson River and Medina 
age, is apparent from the way in which these strata on the 
north-western side are again represented to the eastward 
and southward of the lake. Thus, the interesting questions 
