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three provinces are: a rugged, mountainous highland on the west, known as
the Cordilleran region, which is a continuation of the mountainous region that
forms the backbone of the North American continent; a relatively more sub-
dued, but rocky, and partly treeless plateau in the east is part of the Laurentian
plateau that almost encircles Hudson bay; and between these two a broad, almost
level, forested plain through which the trunk stream flows northwestward to the
Arctic ocean, fed on the one hand by swift-flowing streams from the Cordilleran
region and on the other hand from the numberless lakes of the plateau to the
east. The last is the northward continuation of the Great Central plain of
_ North America, and in this the Mackenzie river occupies the same position and
performs the same functions that the Mississippi river does in the south as it flows
towards the gulf of Mexico.

The drainage area of the Mackenzie river is a great basin, its western side
dipping somewhat steeply northeastward and its eastern side sloping more gently
westward to a central depression; and the whole tilted with a long easy slope
northwestward to the Arctic ocean. The degree of slope of the central depression
towards the northwest from the divide between the Saskatchewan river:and the
Mackenzie to the Arctic ocean is about 2 feet to the mile. This slope, however,
is not uniform throughout, for besides the irregular hills and mountain ranges
which rise out of the lowland there are north facing escarpments which interrupt
and break the grades of the streams, and large local areas of depression in which
the water accumulates to form the great lakes of this northern region. The
largest of these lakes are Great Bear, Great Slave, and Athabaska, all of which
rank among the large freshwater lakes of the world. The Laurentian Plateau
region also holds innumerable lakes of smaller size on its uneven pitted surface.

Laurentian Plateau

The great physiographic province known as,the Laurentian plateau (Plate
II A), which covers such a great part of northeastern and northern Canada on
both sides of Hudson bay, extends for some distance into the basin of Mackenzie
river and occupies a strip along its eastern edge about 80 to 280 miles wide

extending over a length of 800 miles from the height of land at the south to the ~

northern shores of Great Bear lake.

The western border of this province, where it abuts against the central plain,
is a fairly well-defined line marked by the contact between the Precambrian
crystalline or metamorphic rocks and the flat-lying Paleozoic sedimentary rocks.
This line of contact enters the Mackenzie basin from the south at Methy portage

on Clearwater river in longitude 110 degrees west. Running northwesterly from —

there it passes the east end of Athabaska lake and follows the valley of Slave river
to Great Slave lake. Crossing Great Slave lake in a northwesterly direction it
runs from the northern end of the north arm of the lake to the southern point of
(McTavish bay on Great Bear lake and continues across the lake in the same direc-
tion until it passes outside the limits of the basin north of Smith bay. East of
this line and extending far beyond the limits of the Mackenzie basin towards
Hudson bay is the Laurentian Plateau province.

South of lake Athabaska there is apparently no topographic break to mark
the change from the Laurentian plateau to the Great Central plain, the one