Review of History of the Great Lakes. — Spencer. 291 
491 feet below sea level; of lake Michigan, 262 feet: of lake 
Huron, 168 feet ; and of lake Superior, more than 400 feet. 
Consequently, if these were erosion valleys, the}' must have 
been formed at such an altitude that the drainage of the 
region could have descended to the sea. As collateral evi- 
dence of the high elevation, we find that the lower St. Law- 
rence river and the Gulf are only a deeply submerged river 
valley, with tributary canyons, having a general depth increas- 
ing from 1,200 feet to 1,800 feet, but much deeper at the edge 
of the continental plateau. Hudson strait, the Gulf of Maine, 
New York harbor, and other points along the continental mar- 
gin, reveal great submerged canyons that were once river 
valleys. Indeed, portions of the continent were once very 
much higher than now, especially in the south, where the 
coast and the Antillean region appear to have sunk from one 
and a half miles to two and a half miles during the Pleisto- 
cene period. These changes of level have been in undulations, 
with the greatest subsidence along the coastal regions, and 
more particularly in the south than in the north. But this 
forms a separate and partly written chapter, in which much 
progress has recently been made. It is sufficient to know that 
the lake region has stood at a high elevation during most of 
the time from the Carboniferous to the Pleistocene days, 
which were followed by changes of level resulting in the 
present altitude of the land.* 
■ ',. Character of the Lake Basins. The valley-like charac- 
ter of the lake basins appears to be challenged when the casual 
observer finds that some of the outlets are mostly obstructed 
by rocky barriers. This condition gives rise to the hypothe- 
sis of the glacial origin of the basins, for the theorist did not 
stop to compare the course of the basins and the escarpments 
with the direction of the glacial stria-. However one might 
doubt the correctness of the flu via tile hypothesis, the futility 
of the glacial origin could only be confirmed when the causes 
of the barriers closing the lakes were discovered, which will 
*Previous papers on this subject by the prcseul writer are: ''High 
Continental Elevation preceding the Pleistooene period," Bulletin Geol. 
Soc. Am., vol. i. 1889, pp. 65-70; "Post-Pleistocene Subsidence versus 
Glacial Dams." Id., vol. n. 1890, pp. 165-476; "Terrestrial Subsidence 
southeast of the American Continent," Id., vol. v. 1893, pp. L9-22. Each 
of these papers is accompanied with a map. 
