REVIEW OF GEOLOGICAL STRUCTUEE. 37 



continuous sheet of ice several thousand feet in thickness* This was 

 growing at the surface by continual accessions of snow, while it wore 

 away and melted below. Even at its edge, no considerable accumulation 

 of clay could take place, for this would be continually washed by the 

 water supplied from its melting, and we have no warrant for supposing 

 that earth and stones would work up from below far into it. The present 

 glaciers of the Alps, Himalayas, Greenland, Terra del Puego, and Alaska 

 are all witnesses against this theory. The testimony of the ancient 

 glaciers is still stronger in the same direction. The only recent glaciers 

 which in their breadth and thickness are to be compared with those of 

 the ice period, are those of Greenland and the Antarctic continent, and 

 all observers agree that they are composed mainly of pure crystalline 

 ice, and that their surfaces are entirely free from earth or stones. The 

 reason why no Bowlder clay is found in the terminal moraines of modern 

 Alpine glaciers, is that these moraines are thoroughly washed and the 

 fine flour ground by the glacier is carried away in the draining streams 

 and forms the milkiness which is a marked characteristic of these 

 streams and the lakes into which they flow. The highlands of Canada 

 and New England show thousands of glacial deposits of the character of 

 the Alpine moraines, and the Champlain clay along the Atlantic coast 

 represents the finer material ground up by the glaciers which covered 

 the highlands at the time of its deposition. In Ohio, the moraine 

 material was never levigated except on the divide between the basin of 

 Lake Erie and the Ohio River — where it now forms Karnes — and along 

 certain drainage lines where the water from the melting glaciers flowed 

 away. 



Hence, the Bowlder clay of Ohio, Indiana, Illinois, etc., may be said to 

 be the entire grist ground by the glacier, which never having been 

 screened or sorted, contains both the bran and the flour — and since most 

 of the rock ground up was limestone or shale, most of the grist is flour 

 or clay. In Canada and New England where the surface was higher and 

 better drained, the flour was, for the most part, washed out, and re- 

 deposited where the flow of the draining streams was stopped, as Cham- 

 plain clay ; the bran was left behind as sand, gravel, and bowlders. 



The answer to the inquiry made by Professor Geikie in regard to the 



• Prof. Dana says, on page 537 of his Manual : "The surface of the glacier in North 

 America must have been of unblemished whiteness, for from New England to the Rocky 

 Mountains there was not a peak above the surface except the White Mountains, and 

 these probably had their cap of snow." This might have been made still stronger by 

 omitting the exception, for Prof. C. H. Hitchcock reports having found glacial mark 

 and transported Drift on the summit of Mt. Washington. 



