10 B. N. A. BOUNDARY COMMISSION. 
ing into the Winnipeg group of lakes, including the area of the lakes 
themselves, cover 52,800 square miles. The head waters of the Missouri, 
and especially of its tributary the Milk River, drain a considerable area 
to the south, embracing about 22,800 square miles; while the Athabasca 
and North Pembina Rivers, flowing into the Mackenzie, drain an area of 
only about 10,000 square miles in the north-western corner of the region. 
18. The total area of prairie country within the same limits, including 
that of all three prairie levels, may be estimated at 192,900 square miles. 
Though much of this vast region is not absolutely treeless like its south- 
western part, its aggregate tree-clad area is quite insignificant as com- 
pared with that of its open plains. North of the fifty-fourth parallel, 
the area of the prairie region is inconsiderable, and has not yet been so 
well defined as to render any estimate possible. 
General Outline of the Geology. 
19. After the Huronian period, the geography of which cannot yet 
be defined, the succeeding formations of the western prairie region seem 
to have accumulated on a great submarine plateau, stretching westward 
from the base of the Laurentian and Huronian range, and probably con- 
tinuous with an ocean occupying the present position of the Pacific. The 
region now occupied by the western mountain ranges, was at different 
times outlined by areas of shoals and sand-banks, but true marine lime- 
stones were formed over it at several epochs. Atleast as early as the close 
of the Cretaceous period, the elevation of mountain ranges appears to 
have begun; and from this time, the formations accumulating between 
them and the Laurentian and palaeozoic barriers to the east, were those 
of a great interior continental basin. These waters, at first in free 
communication with the sea to the south and north, were soon, by the 
gradually increasing elevation of the continent, cut off from it more or 
less completely ; and, after a period of transition, became a great fresh- 
water lake or series of lakes. At the close of the Lignite Tertiary 
formation—generally attributed to the Eocene period—the elevation of 
the Rocky Mountain ranges was resumed with renewed vigour, and in 
lakes of constantly decreasing size, the sands and clays of the later 
Tertiaries were deposited. These include, in great abundance, the 
remains of the numerous land animals which inhabited the then extensive 
area of the continent; but are not yet known to occur north of the forty- 
ninth parallel, The Cretaceous and Tertiary rocks lying along the base 

