8 THE ROYAL SOCIETY OF CANADA 
the eastern end of this dam the lake overflowed and gave origin 
to the Nelson river, which at this point is still cutting into the clay 
dam and rapidly enlarging its channel. 
North and northeast of Lake Winnipeg the country is underlain 
by Archaean rocks, with characteristic surface contour, the low 
rocky hills being separated by lakes and mossy swamps. 
Such country extends northwards for several hundred miles, 
when rather suddenly the rocks begin to be covered with clay, and not 
long afterwards the rocky hills and knolls disappear entirely, and a vast 
level plain extends to the limit of vision. This plain, which reaches to 
the shore of Hudson Bay, and has an area of something like 100,000 
square miles, is one continuous swamp covered with a thick water- 
soaked blanket of bog mosses with their usual association of northern 
swamp-loving plants. In order to define this swamp more clearly, 
and as it now covers an area which once formed part of the bottom 
of the ancient extended basin of Hudson Bay, I have elsewhere re- 
ferred to it as the Archudsonian Swamp. Unlike the country to 
the south of it, lakes are almost entirely absent, and the rivers flow 
in narrow channels cut into the clay-covered plain. Fish and wild 
animals are scarce, and consequently Indians who must depend on 
these for a supply of food, seldom attempt to make their homes in this 
swamp. ; 
GEOLOGY 
Pre-Cambrian é 
Rocks of Pre-Cambrian age underlie the whole country on the 
upper portions of both the Nelson and Hayes rivers, the lowest out- 
crop on the Nelson being 92 miles in a straight line from Beacon 
Point, while the lowest outcrops on the west and east branches of 
Hayes river, the latter known as God’s river, are respectively 104 
and 94 miles in a straight line from the same point. These rocks 
are chiefly granites and granitoid gneisses such as are usually included 
in the Laurentian System. Somewhere north or northeast of the 
mouths of these two rivers quartzites and other similar rocks, doubtless 
also of Pre-Cambrian age, must occur immediately beneath the drift, 
for the till transported from that direction contains large numbers 
of pebbles and boulders of such rocks. 
Ordovician 
Towards the north and east the Pre-Cambrian granites, etc., 
are overlain by flat-lying Ordovician limestones and dolomites. The 

1 Forests of the District of Patricia. By J. B. Tyrrell, Can. Forestry Journal, 
Feb., 1916, pp. 375-380. 
