376 The American Geologist. j un e, i89i 
Uphara as sediments laid down in glacial lakes or on land areas across 
which the floods discharged from the melting ice-sheet flowed away on 
their course to the sea. Glacial lakes in the basins of the Peace and 
Athabasca rivers, and of the Saskatchewan and Souris rivers, are shown 
to have been tributary to lake Agassiz, which covered the valley of the 
Red river of the North and the low area of Manitoba, being still repre- 
sented there by the large lakes Winnipeg, Manitoba, and Winnipegosis, 
besides others of smaller size. The area of lake Agassiz is stated to 
have been about 110,000 square miles, or more than the combined areas 
of the five great lakes which outflow by the St. Lawrence. Its highest 
shore, marked by beach ridges and rarely by low eroded cliffs, Mr. 
Upham has traced with levelling along an extent of about 600 miles- 
across the southern prairie portion of its area ; and Mr. J. B. Tyrrell 
has extended this examination about 100 miles farther north along the 
escarpments of Riding and Duck mountains. The great Laurentian 
lakes were also held at much higher levels than now by the barrier of the 
waning ice-sheet, and the Canadian shores of these enlarged lakes are 
very distinct in many localities north of lake Superior, about lake 
Huron, and in the vicinity of Toronto. 
From surveys by Gilbert, Spencer, and others, the elevations of the 
highest glacial shore lines about the Laurentian lakes, especially lake 
Ontario, have been ascertained continuously through long distances. It 
is thus found that since the departure of the ice-sheet portions of this 
lake area have undergone a differential uplifting, of increasing amount 
from south to north and northeast. The maxium rate of northward 
ascent of the old beaches is adjacent to the eastern end of lake Ontario, 
being about 5 feet per mile through a distance of more than fifty miles. 
On the area of lake Agassiz, however, where northward uplifting had 
been previously discovered by Mr. Upham, the ascent, which is con- 
tinuous along a distance of at least 400 miles from south to north, varies 
from a minimum of about six inches to a maximum of only about one and 
a half feet per mile. The levels of the Laurentian lakes in the Cham- 
plain epoch, or time of recession of the latest ice-sheet, and the contem- 
poraneous sea level which reached along the St. Lawrence valley to 
Ogdensburgh and Brockville, near the mouth of lake Ontario, with the 
changes produced by the uplifting of the land shown to have been then 
in progress by successive beach lines, lead Mr. Upham to the conclusion 
that the altitude of the glacial outlet from lake Michigan to the Des 
Plaines river at Chicago has remained nearly the same from the Cham- 
plain epoch to the present time, while the northern and northeastern 
part of the area of the Laurentian lakes has been elevated 300 to 500 
feet during and since that epoch. 
The latest North American ice-sheet, by which these glacial lakes 
were formed at the time of its recession, is shown by Mr. Upham to 
have probably been during its culmination of greatest extent and depth 
a continuous mer de glace from the Atlantic to the Pacific, overtopping 
the Rocky mountains in the Peace river region, and outflowing from the 
interior portion of its area both southward and northward. The thick- 
