56 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vox. VI. 
a differential elevation towards the north-northeast, but the rate is not 
uniform and there may also be local warping of the crust. 
Around the lakes of the Winnipeg basin, a similar phenomenon has 
been observed. On the western side of Lakes Manitoba and Winnipego- 
sis, which were one sheet when the water was slightly higher, the ancient 
beaches are very well marked, and Mr. J. B. Tyrrell, formerly of our 
Geological Survey, has shown that in going northward they rise at the 
rate of about one foot per mile or 300 feet in the length of the two lakes. 
Now lines drawn at right angles to the bearing of the maximum rise 
would represent isobases or axes along which there would be no change ; 
of level although the land might be rising to the northward or sinking to 
the southward. 
The country is tolerably low and level about the outlets of all our 
lakes and there is no evidence of the former existence of any kind of 
obstruction to the outflow of the waters—not even of “ice-dams,” 
which used to be convenient suppositions for getting over difficulties of 
this kind. Before the discovery of the earth movements, which have © 
been referred to, there was thus great difficulty in accounting for the 
former extensions, higher levels, shifting of outlets and other changes to 
which we knew the lakes had been subject. 
Around the northern shore of Lake Superior, Dr. A. C. Lawson has 
ascertained, by the spirit-level, the elevations at a considerable number 
of different localities of thirty-three of the most conspicuous of the 
terraces. These elevations range from near the present level up to 
more than 600 feet above it. In 1846, Sir William Logan described the 
remarkable set of distinct terraces at Les Ecrits or Terrace Bay near 
the mouth of Steel River or Schreiber, on the Canadian Pacific Railway, 
and Agassiz gives a picture of them in his ‘‘ Lake Superior” published 
in 1851. Dr. A. C. Lawson ascertained that the highest water-mark in 
this vicinity was 418 feet above the present surface of the lake, but 
further west near the mouth of Arrow River he found terraces marking 
old beach lines up to 607 feet above the lake. If the water stood at any 
of these upper levels at the present day, there is no ground to the 
southward high enough to prevent it extending to the Gulf of Mexico. 
But if the sand and gravel in which these terraces are cut had been. 
deposited by the sea, we would be pretty certain to find in them the 
remains of marine organisms, as we do in the post-glacial deposits of the 
i 
OO a ee a oe 
province of Quebec. Fresh water, especially in’a cold climate, produces , 
few or no mollusks, and even if it did, their shells do not last as long as 
