SURFACE GEOLOGY. 79 



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, perhaps 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' — i. 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 



