282 Canadian Record of Science. 
Artemesia gravels, which for long distances crown the sum- 
mit of the escarpment parallel to its face, and are largely 
derived from its debris; prior to the elevation of the ridge 
or anticlinal which lies between Lake Huron and the Trent 
Valley, and gives to the escarpment its highest elevations 
above the lakes; prior to the Niagara Falls; and prior to 
the erosion which widened the fractures in the escarpment 
at the Dundas Valley and at the points of meeting of the 
waters of the Georgian Bay with those of Lake Huron 
proper, as well as the waters of Green Bay with those of 
Lake Michigan. On the other hand, this period of eleva- 
tion of the escarpment was contemporaneous with the ap- 
pearance in their present outlines of Grand Manitoulin, 
Cockburn and Drummond Islands in Lake Huron, and 
viewing all the facts was undoubtedly pre-glacial. Whilst the 
elevation of the escarpment gave in general terms the 
outlines of the basin of the three lakes, it is not to be in- 
ferred that these basins were at once filled with water to 
present levels. The country surrounding the lakes must 
have been higher than now to enable the pre-glacial river 
to cut the deep channels in Lakes Ontario and Huron 
_ which now exist. 
Lakes Erie Anp St. Crate. 
These two lakes have undoubtedly been within a very 
recent period more intimately united than now, and are 
probably the most recent in origin of the St. Lawrence 
Great Lakes. They lie in a Devonian basin with the 
Silurian rocks forming the portion of the rim of Lake Hrie 
between Sandusky and Toledo. This basin is, however, over- 
laid with superficial deposits to such an extent that both 
lakes really fill shallow depressions on the surface of these 
deposits, and appear rather to be overflows caused by the 
restricted passage now of the waters over the Niagara 
escarpment in the one case, and through the Detroit River 
in the other, than to be due to physical forces which 
operating in past ages excavated preparatory basins. 
