46 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. VI. 
and present such a conspicuous figure on the maps, they are quite shal- 
low relatively to their area, and as the surface of this part of the conti- 
nent is so generally level, a very small amount of denudation and a very 
slight undulation in the land would suffice to produce the shallow 
depressions which they fill. 
Before the advent of the glacial period a great part of North America 
stood at an elevation of 3,000 feet or more above its present level, and 
the disintegration and removal of the rocks due to rainfall alone, which 
was facilitated by the greater elevation, went on for a vast length of 
time. This is proved by the existence of long and deep river valleys, 
some of them running down from the central parts of the continent to 
the present sea coasts, and beyond them into the depths of the ocean. 
The old valley of the St. Lawrence can be traced for 800 miles. The 
ancient bed of the Hudson has been followed by soundings, from its 
present mouth, down the slope of the bottom of the ocean far out to sea, 
The cafion of the Saguenay, and those of the Grand or Hamilton and 
other rivers of Labrador, as well as the channels of many of the long 
straight lakes and river stretches in the Archzan country to the north 
and northwest of us, are due partly to atmospheric and aqueous erosion 
during a long geological period while the land stood at higher levels. 
It can be shown that the grade of the Mississippi from its source to the 
sea was much steeper formerly than now, the present modification or 
lowering of the slope amounting to some 3,000 feet. The valleys thus 
excavated guided, to a considerable extent, the movements of the great 
glaciers which ploughed the surface of hill and dale, excavating and 
carrying forward vast quantities of the softened surface rock, together 
with harder portions in the shape of fragments, boulders and pebbles, 
which together constitute our till or hard-pan and other deposits, 
collectively called drift. The general tendency of the glacial action 
would be to enlarge the main valleys of the preglacial surface and to 
fill up or modify the smaller ones. 
The geological structure and the relative resisting powers of the rocks 
were the primary or fundamental causes which predetermined the loca- 
tion, direction, extent, etc., of the valleys and basins thus formed by the 
combined action of aqueous denudation and glacial action. 
The theory of preglacial river-erosion as the main factor in originating 
the lake basins may apply to all of them with certain differences in each 
case. The soundings show the existence in the bottom of each, of deep 
channels resembling river valleys on the land traversing their beds and 
leading to former outlets now closed up by drift materials, but which 
