1875.] Niagara. 143 
As the ice melted back, it deposited the unstratified till 
under its receding foot, leaving a continuous mantle of it 
behind. Lying on the top of the till are seen scattered 
rounded boulders (£ in section), often of great size, of gra- 
nite, gneiss, and other crystalline rocks, that must have tra- 
velled from the Lawrentian hills in the far north. Amongst 
these, rocks of local origin are as scarce as in the till below 
those of distant derivation are rare. These foreign boulders 
are scattered over the surface, as if dropped by some agent 
that has left no other record of its movements. The rounded 
far-travelled blocks lie on soft unconsolidated beds that have 
not been disturbed. In some places, as on the top of a low 
hill on the Canadian side of the falls, I found great numbers 
of these blocks, and in some parts of northern New England 
and New York, great trains are found in lines along the 
sides of hills, as if stranded on a beach. They are found on 
the western prairies, according to Professor Hall, in long 
trains, ‘‘ where, for many miles, the difference in elevation 
is not more than 50 feet; and here we observe long lines of 
boulders stretching away for miles beyond the reach of 
vision, as if once forrnerly a line of coast.”* Speaking of 
the valley of the Hudson, Professor Hall says:—‘‘In the 
vicinity of Albany and Troy, I have searched in vain for a 
boulder or pebble of granite, or of any rock older than the 
Potsdam sandstones in the deposits below the clay, while, in 
a period subsequent to the deposition of the clays and sands, 
boulders of granite are by no means rare.’’t 
Only one satisfactory explanation has been given of the 
presence of these far-travelled blocks on the surface of the 
undisturbed loose beds of sand and clay, namely, that they 
have been dropped from floating ice, and most writers on the 
subject have concluded that they are proofs of the submer- 
gence of the land below the sea. There is certainly an area 
of land running from Lake Champlain northwards that has 
been elevated from below the level of the ocean since the 
glacial period, but there is no evidence whatever that the 
sea extended over the plateau of Lake Erie, and the entire 
absence of marine remains renders the supposition un- 
tenable. And if we follow the natural sequence of events 
that must have ensued during the retreat of the ice, we 
shall see that there is no occasion to call in the agency of 
the sea. For just as, during its advance, the ice from 
the north-east had blocked up the great valley of the St. 
* Natural History of New York, part iv., p. 321. 
+ Ibid., p. 320. 
VOL. V. (N.S.) 7 
