79 
streams to cut their valleys. to a much greater depth than 
they now exhibit. 
The mouth of the Hudson was then eighty miles south 
and east of its present position and six-hundred feet lower. 
The Mississippi discharged itself into the Gulf through a 
valley of its own making, bordered by bluffs as bold and 
picturesque as those between which it flows in the vicinity 
of St. Paul. In process of time the continent sank at least 
a thousand feet. The glaciers retreated northward leaving 
behind them water basins of great extent in which were 
deposited the older Drift strata, the Hrie clays in the interior, 
the Champlain clays on the coast, both fine stratified deposits 
the clay flour ground by the glaciers. In these clays, so far 
as yet known, no remains of Elephant and Mastodon have 
been found. In Canada and New England the clays contain 
large numbers of arctic marine shells, in the interior some 
fresh-water shells have been reported in them, but it is not 
certain that they have not: been derived from overlying and 
more recent beds. In the Western States the lower drift 
clays are covered by beds of sand and gravel of considerable 
thickness, and these in turn are strewed over with innumera- 
ble bowlders, many of large size, for the most part granites, 
greenstones, &c., transported from the Canadian and Lake 
Superior highlands. Among these bowlders or in the super- 
ficial gravel some hundreds of masses of native copper have 
been found, brought, without question, from the Kewanaw 
copper range. 
Still more recent than the latest Drift deposits, there are. 
found running through the Drift-covered area, terraces and, 
beach-lines which prove that the water-level of the great, 
Lakes was once more than. three hundred feet higher than, 
now. 
The superficial bowlders and gravels of the Drift, are 
clearly the result of iceberg action. It is proven by the 
undisturbed condition of the clays below, that they must 
have been floated to their present resting places just as boul- 
ders, sand and gravel are floated from Greenland to the banks 
