216 B. N. A. BOUNDARY COMMISSION. 
water. It may thus be safely assumed that the general fall of the surface 
of the rock ; from the northern to the southern part of the region covered 
by this portion of the lake, is pretty nearly 100 feet. The distance by 
the direct course of the glacial striation is about thirty miles, and the 
resulting general slope of the country would be somewhat less than 3°5 
feet to the mile. The general height of the Laurentian watershed, where- 
ever it has been ascertained, is extremely uniform, and the slope so gradual 
that it does not merit the designation of a mountain region, but rather 
may be called a rocky plateau. The average elevation above the sea, 
according to Sir William Logan, may be estimated at 1,500 to 1,600 
feet.* where crossed by Prof. Bell, between Lake Superior and Hudson’s 
Bay, it is only 1,200 feet. The southern extension of the watershed, 
passing between Lake Superior and Lake of the Woods, is estimated by 
Palliser at 1,500 feet. North-east of the Lake of the Woods, the water- 
shed region is but little known, but near Lac Seul, which closely corres- 
ponds with the direction required by glaciation, according to Mr. 
Selwyn’s measurements it cannot be over 1,400 feet. The summit of 
this great watershed, lies at from 200 to 250 miles north-eastward of the 
lake, and the glaciation is pretty nearly at right angles to its general 
direction. Estimating the maximum height of the watershed at 1,600, 
and the elevation of the lake at 1,000 feet, a general slope of about three 
feet per mile results, and nearly coincides with that ascertained on a 
smaller scale in the Lake of the Woods district itself. 
492. This rate of descent is scarcely sufficient to account for the sponta- 
neous descent of an ordinary glacier, over a country which Sir W. Logan has 
well characterized as “ mammillated,” yet the surface is heavily marked by 
glacier action, and the supposition of the existence of many local glaciers, 
is negatived by the extreme uniformity of the striation. If icebergs and 
floating ice, be called in to account for the facts, the difficulties appear 
even more insuperable. To give rise to such heavy parallel marking, it 
would be necessary to suppose ice moving under the influence of a 
determinate current, which must have crossed the watershed to the 
north-east. Icebergs having passed over this ridge, would in their 
subsequent course float gradually into deeper water, acting on the rocks 
very feebly if at all; the more so, as in their southern journey they 
would no doubt be continually decreasing in size. Two suppositions 
only, remain ; either the glaciation was accomplished by the irresistable 

movement of a great polar ice-cap,—such as has been considered a neces- 
UE 
* Geology of Canada, p. 5. 

