Great Lake Basins of the St. Lawrence. 275 
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ing the blufis on the northern side of the island, and with 
the strata dipping southward similarly to those of the 
Niagara age there. Again, the clitts of Green Bay face to the 
westward, and the dip is easterly towards and under Lake 
Michigan. 
This Niagara escarpment, in its course easterly from the 
western end of Lake Ontario, lies parallel to the axis of that 
lake, whilst in the other direction, it conforms in a general 
way to the course that more or less characterizes the out- 
crops of all the formations which, as it were concentrically, 
surround and underlie the coal measures of Michigan. The 
contours of Lakes Michigan and Huron and the Georgian 
Bay, and the subaqueous Corniferous escarpment crossing 
Lake Huron, also conform to this arrangement. 
At the western end of Lake Ontario, the Niagara lime- 
stones in their outcrop suddenly change from an east and 
west course to one which is north-west and south-east. When 
these limestones were elevated into an escarpment, two 
separate lines of force appear to have operated—the one 
taking an easterly direction and causing the strata on the 
southerly side of the lake to dip in a southerly direction— 
the other taking asomewhat north-westerly course resulting 
in the strata thence to the Georgian Bay dipping more to 
the westward. These two forces appear to have, at the 
point of meeting, created a vast fracture in the escarpment 
near Hamilton, forming what ultimately became, chiefly 
through the eroding force of water, the Dundas valley. 
Again, between the Bruce Peninsula and the Manitoulin 
[slands, another change in the direction of the outcrop of 
both the limestones and underlying shales, caused, when 
the escarpment was elevated there, a series of great fractures 
which, by the action of the waves and currents and of 
atmospheric forces, and possibly of glaciers and icebergs as 
well, became, ultimately, the interrupted subaqueous ridge 
there. To similar fractures were no doubt originally due 
the narrow straits which divide the Manitoulin Islands from 
each other and the most westerly of them, Drummond 
Island, from the State of Michigan. Such fractures may 
