Great Lakes, and t he Valley of the Mississippi. 225 



But I think we have evidence that the continent did not 

 sink uniformly in all its parts, but m >st at the North. Not to 

 cite any other proof of this, — northern coast fiords, &c- the 

 altitude of the loess-like deposits of the upper Mississippi and 

 Missouri (the lacustrine non-glacial sediments of this period of 

 submergence), the upward reach of the Drift clays of the lake 

 basin, the filling of the valleys of the streams flowing into the 

 Ohio and Lake Erie, the old lake beaches marking the former 

 water-level in the lake basin — all indicate that the continental 

 subsidence was greatest toward- the north. To this subsidence 

 we must, as I think, attribute the accumulation of water in the 

 lake basin and Mississippi valley to form the great inland sea 

 of fresh water, of which traces everywhere abound. It seems 

 to me scarcely necessary to suppose any other barriers by 

 which this sea was enclosed than the highlands that encircle 

 it — such as are roughly outlined by the light tint on Prof. 

 Guyot's map of North America — and the sea-water which 

 filled the mouths of the two* straits by which it communica- 

 ted with the ocean. 



Yellow Sands and Surface Bould 

 I have mentioned that on the Erie clays are beds of gravel, 

 sand, and clay, and over these again great number- of trans- 

 ported boulders, often of large size and of northern and remote 

 origin. 



These surface deposits have been frequently referred to 

 the direct and normal product of glacial action, the materials 

 torn up and scraped off by the greal ice ploughs in their ! 

 journeys from the North; in fact, as some sort of huge termi- 



* If there were two. That there was one ii I now, 



and that so long that, though Ball al one end, it must have : 



The eastern outlet of the lak may Dot ha^ • it Lawn 



but as likely through the gap between the Adir lUegbanies. The 



Bhallow channels betv '1 Islands and I 



to indicate that the St Lawrence is a comparatively new line of drainage for the 



lakes. 



