AMERICAN GEOLOGY DECADE OF 1860-1869. 545 



argued that, synchronously with the glacial epoch of Europe, the 

 northern half of North America had a climate comparable with that of 

 Greenland, and that, as a result, glaciers were formed, 

 onThe Dnft V i'870 S the direction of flow of which corresponded in a gen- 

 eral way with the present drainage channels. The 

 Lower Mississippi he looked upon as a " half -drowned river " — that is, 

 one with its lower channel deeply submerged and silted up, an unmis- 

 takable proof that at that period the country stood at a considerably 

 greater elevation than at present. 



The trough of the Mississippi, to his mind, was simply a valley of 

 erosion which, since the close of the Carboniferous period, had been 

 traversed by a river which drained the area of the northern Mississippi, 

 the Ohio, and the Tennessee, and, since the Miocene period, the Mis- 

 souri, Arkansas, and the Red rivers as well. Through alternate eleva- 

 tion and depression the mouth of the stream had varied its position 

 from time to time to the amount of a thousand miles, the final long- 

 continued depression being the primary cause of the climatic amelio- 

 ration which brought the glacial period to an end. 



The fine blue-gray and highly plastic so-called Erie clays and their 

 accompanying sands and bowlders were to him due to floating icebergs 

 during a period of continental depression and great inland seas. 

 deposited, in fact, just as similar materials are conceived as now being 

 scattered over the sea bottoms about Newfoundland." 



If we restore in imagination this inland sea, which we have proved once filled the 

 basin of the lakes, gradually displacing the retreating glaciers, we are inevitably led 

 to a time in the history of this region when the southern shore of this sea was formed 

 by the highlands of Ohio, etc., the northern shore a wall of ice resting on the hills 

 of crystalline and trappean rocks about Lake Superior and Lake Huron. 



From this ice wall masses must from time to time have been detached, just as they 

 are now detached from the Humboldt glacier, and floated off southward with the 

 current, bearing in their grasp sand, gravel, and bowlders — whatever composed the 

 beach from which they sailed. Five hundred miles south they grounded upon the 

 southern shore — the highlands of now western New York, Pennsylvania, and Ohio, 

 or the shallows of the prairie region of Indiana, Illinois, and Iowa. There melting 

 away and depositing their entire loads, as I have sometimes seen them, a thousand 

 or more bowlders on a few acres, resting on the Erie clays and looking in the distance 

 like flocks of sheep, or dropping here and there a stone and floating on east or west 

 till wholly dissipated. 



The loess, as one would naturally expect from the foregoing, was 

 looked upon as the finer sediment deposited in the quiet waters of one 

 of these inland seas, to which the icebergs had no access. The lake 

 basins, with the exception of that of Lake Superior, which is a syn- 

 clinal trough, were regarded as excavated by glacial action and once 

 much deeper than now, having become partially filled with drift 

 deposits as the}^ gradually emerged. Temporary pauses in the period 

 of uplift would give time for wave action, and thus would be formed 

 the terraces and ancient beaches so commonly found in the lake regions. 

 NAT mus 1904 35 



