REVIEW OF GEOLOGICAL STRUCTURE. 37 
continuous sheet of ice several thousand feet in thickness.* This was 
erowing at the surface by continual accessions of snow, while it wore 
away and.melted below. Hvenat 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 intoit. The present 
glaciers of the Alps, Himalayas, Greenland, Terra del Fuego, 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 Hngland 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 Kames—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 Canadaand 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. 
