Correspondence. 127 
Bui. Sci. Labor, of Denison Univ., Vol. VI, Part I. By W. G. Tight. 
Nebraska Flowering Plants, by G. D. Swezey. Doane College, May, 
1891. 
Twenty-third and Twenty-fourth Ann. Reps, of Trustees of Peabody 
Mus. of Amer. Arch, and Ethnol. Cambridge, 1891. 
Rep. of the Sci. Exped. into South. Maryland. Johns Hopkins Univ. 
Cir. Vol. X, No. 89. 
CORRESPO^^DENCE. 
Area and Duration of Lake Agassiz. Your July number contains 
a valuable article by J. B. Tyrrell, in which he criticizes the estimate of 
the area of lake Agassiz, about 110,000 square miles, or more than the 
conbined areas of the great Laurentian lakes, as given in my recent re- 
port published by the Canadian Geological Survey. In reply I wish to 
■explain that I have not attributed so great extent to lake Agassiz at any 
one stage of its existence, and to notice briefly how the beaches and 
terminal moraines indicate that this lake during both its earlier and 
later stages covered the greater part, probably three-fourths, of this area. 
The chief argument for this is the observed extent of the higher and 
earlier Herman and Norci'oss beaches, which have been mapped from 
near Red lake, Minnesota, southward to lake Traverse and thence north- 
ward through North Dakota to Riding and Duck mountains in Man- 
itoba, a distance of about 700 miles (Am. Geologist, vol. vii, pp. 194, 
222). Delta sand deposits, brought into lake Agassiz by the Saskatche- 
wan and referable to the Norcross and lower stages, reach from near 
Prince Albert, on the North Saskatchewan about forty miles west of the 
forks of the North and South branches, through a distance of more than 
a hundred miles eastward to the head of the Seepanock channel and the 
103d meridian (Canadian Pacific Railway Report, 1880, pp. 14, 19). The 
descent of the river in this distance is approximately from 1,250 or 1,300 
to 950 feet ; and the elevation of the west part of the delta is probably 
about 1,350 feet above the sea. As early as the time of the Norcross 
beaches, therefore, the recession of the ice-sheet had permitted the lake 
to extend along the whole front of the Manitoba escarpment, to the lat- 
itude of the north end of lake Winnipeg. The length of the Agassiz at 
that time was 550 miles or more, and I believe that its average width 
was not less than 150 miles, reaching east to the moraine wiiich Mr. 
Tyrrell describes as forming the eastern shores and islands of lake Win- 
nipeg, with a hight of 100 feet on Black island. This moraine would 
then have been deposited in water 600 to 700 feet deep, bordering the 
ice-margin ; its knolly and irregular accumulations of drift would not 
have been subjected to the levelling action of the lake waves until the 
farther melting of the ice opened avenues of outflow to the Hudson bay 
and reduced the glacial lake nearly to the level of lake Winnipeg ; and 
the latest change of the northward outlets may liave lowered the water 
surface so rapidly and to sucji vertical amount that it left no distinct 
marks of erosion or shore lines on the upper portion of tin' moraine. 
