TRANSACTIONS OF SECTION C, 473 
_ In its earliest. and highest stage, Lake Agassiz was nearly 200 feet deep 
above Moorhead and Fargo; a little more than 300 feet deep above Grand 
Forks and Crookston; about 450 feet above Pembina, St. Vincent, and 
Emerson ; more than 500 feet above the site of the city of Winnipeg; and 
about 500 and 600 feet respectively above Lakes Manitoba and Winnipeg. 
The length of Lake Agassiz is estimated to have been nearly 700 miles, and 
its greatest width more than 200 miles. 
Reports on the explorations of this ancient lake have been published by 
the Geological Surveys of Minnesota, the United States, and Canada. In 
the present paper the latest explanations are reviewed to account for the 
northward ascent of its beaches. 
The highest and earliest beach or shore-line has an ascent of about a 
foot per mile toward the north-north-east ; the lower and later shores ascend 
less ; and the lowest shore, marked by beaches only 60 to 70 feet above Lake 
Winnipeg, are almost perfectly horizontal. It is thus known that the land 
was being uplifted differentially while Lake Agassiz existed, and that the 
uplift was nearly completed before the ice-sheet was wholly melted away. 
The chief cause of the uplift is thought to be the unburdening of the 
land by the removal of the vast weight of the ice-sheet, this part of the 
earth-crust being restored to equilibrium or isostasy by an inflow of the 
plastic magma at a great depth within the earth, which took place during 
the time of departure of the ice. 
Measures of the shore erosion and beach accumulation indicate that the 
duration of Lake Agassiz was only about 1,000 years ; and from the rate of 
recession of the Falls of St. Anthony, forming the gorge of the Mississippi 
River between Fort Snelling and Minneapolis, the length of the Postglacial 
period is estimated to be between 6,000 and 10,000 years. 
4. The Rainfall Run-off Ratio in the Prairies of Central North America. 
By Professor E. F. CHANDLER. 
A brief definition of terms was given in connection with the division 
of rainfall, into ‘evaporation’ and ‘run-off.’ The river-flow, or run-off, 
is the residual remaining from the total precipitation after the demands 
of evaporation and plant-growth are supplied. 
A summary was furnished of results secured from the systematic 
measurements of the rivers of the northern United States that have been 
maintained continuously for the past six years. In the Red River valley 
prairies the mean annual rainfall is from 15 to 25 inches. Kvaporation 
from the soil and from vegetation consumes here nearly the whole of the 
rainfall; the remainder that enters the streams is but a small fraction— 
3 to 20 per cent. of the whole—its amount depending on the quantity 
and distribution of the rainfall. This condition was contrasted with 
other regions where the rainfall is greater so that the run-off is half 
or three-fourths or more of the rainfall, or where the precipitation is less 
fortunately distributed through the seasons so that vegetation is unable 
to secure so great a portion. 
FRIDAY, AUGUST 27, 
The following Papers and Report were read :— 
1. The Bearing of Pre-Cambrian Geology on Uniformitarianism, 
By Dr. A. P. CoLeman. 
Pre-Cambrian rocks cover an enormous area in Canada and furnish pro- 
bably as complete a series as any part of the world. Except for the lack of 
