SURFACE GEOLOGY. 719 
sheet, excavated the basins of Lake Michigan, Lake Huron, and Lake 
Ontario. The latter lake was apparently formed by the same glacier 
that made the Erie basin, but when much abbreviated. It flowed from 
the Laurentian hills and the north slope of the Adirondacks, and was 
deflected by the highlands south of the lake basin, so that its motion 
was nearly westward. This chapter in the history of our lakes was ap- 
parently a long one, for Lake Superior, Lake Michigan, Lake Huron, and 
Lake Ontario are all of great depth. 
7th. The melting of the glaciers was accompanied, he occasion- 
ed, by a sinking of the continent, which progressed until the waters 
of the Atlantic flowed up the valley of the St. Lawrence to Kingston, 
and up the Ottawa to Arnprior. (Dawson.) The valleys of the St. Law- 
rence and the Hudson were connected by way of Lake Champlain, and 
thus the highlands of New England were left as an island. It is also 
possible that the sea-water penetrated to the lake basin through the 
valley of the Mohawk and through that of the Mississippi, but of 
this we have no evidence in the presence of marine fossils in the sur- 
face deposits. The great area of excavation in which the lakes lie was 
probably at this time filled to the brim with ice-cold fresh water, and this 
flowing outward through all the channels open to it may have been suf- 
ficient. to prevent the entrance of the arctic marine mollusks, of which 
the remains are so abundant in the Champlain clays of the St. Lawrence 
valley and the Champlain basin. 
8th. When the continent was again elevated, and the water of the 
inland sea was drained away, the Mohawk channel was found dammed 
up with Drift, and a new line of drainage was established through the 
valley of the St. Lawrence. It is almost certain also that the elevation 
of the continent which took place after the Champlain epoch was not 
uniformly equal over all the country lying between the Atlantic and the 
Mississippi; for we find that the drainage of the lake system has been 
flowing in different directions at different times; now over barriers 
1000 feet above the level of the sea from Lake Erie into the Ohio, and 
again, through outlets much lower, from Lake Erie to the Wabash, and 
from Lake Michigan, by several channels, into the Illinois and Missis- 
sippi. These great changes may have been effected by warpings of the 
earth’s crust—+z. e., local elevation, or subsidence—or by the successive 
removal of ice-dams—glaciers—which occupied and obstructed different 
portions of the great interior basin. We may also find records here, as 
some geologists do in Europe, of great alternations of climate in the im- 
mensely long Quaternary age; and these alternations, building up and 
