Great Lakes, and the Valley of the Mississippi. 219 



lakes, having a thickness of over 200 feet, and containing a 

 few scattered stones. 



4th. Above the Erie clays are sands ol variable thickness 

 and less widely spread than the underlying clays. These 

 sands contain beds of gravel, and, near the surface, teeth of 

 elephant have been found, water-worn and rounded. 



5th. Upon the stratified clays, sands, and gravel of the Drift 

 deposits are scattered boulders and blocks of all sizc<, of granite, 

 greenstone (diorite and dolerite), silicious and mica dates, and 

 various other metaniorphic and eruptive rocks, generally 

 traceable to some locality in the Eozoic area north of the laki 9. 

 Among these boulders many balls of native copper have been 

 found, which could have come from nowhere else than the 

 copper district of Lake Superior. 



Most of these masses are rounded by attrition, but the large 

 blocks of Corniferous limestone which are scattered over the 

 southern margin of the lake basin in Ohio show little marks 

 of wear. These masses, which are often 1<» to 20 feet in diame- 

 ter, have been transported from 100 to 200 miles south-east- 

 ward from their places of origin, and deposited sometimes -'500 

 feet above the position they once occupied. 



6th. Above all these Drift deposits, and more recent than any 

 of them, are the "lake ridges," — embankment- of sand, gravel, 

 sticks, leaves, <kc, which run imperfectly parallel with the 

 present outlines of the lake margins, where highlands lie in the 

 rear of such margins. Of these, the lowest on the Si. nth shore 

 of Lake Erie is a little less than LOO feet above the present lake 

 level; the highest, some 250 feet. In New Fork, Canada, 

 Michigan, and on Lake Superior, a similar series of ridge- has 

 been discovered, and they have everywhere been accepted as 

 evidence that the waters of the hike- once reached the points 

 which they mark. That they are nothing else than ancient 

 . lake beaches we Bhall hope to prove farther on. 



In the southern half of the Mississippi valley the evidences of 

 glacial action are entirely wanting, and there is nothing cor- 



