
GENERAL PHYSICAL GEOGRAPHY. 5 
of its details, shows evidence in the closely corresponding heights of the 
portions now remaining of its original surface, of previous uniformity. 
The apparent indefiniteness of the three prairie levels and other features 
of the country to the north, no doubt, arises in great part, from the 
deficiency of our knowledge of that portion of the region, and it is highly 
probable that subsequent investigation will show the second and third 
steppes to be well marked in the elevated district between the 
Saskatchewan and Mackenzie Rivers. 
12. The central portion of British North America, may therefore be 
. regarded as a great shallow trough, of which the western edge is formed 
by the Rocky Mountain watershed, the eastern by that of the Laurentian 
axis, but of which the western portion of the floor is now more elevated 
than its easternrim. The age of the former of these bounding watersheds, 
with that of the greater part of the mountains themselves, probably does 
not go much further back than early Tertiary times. The latter is a 
part of the oldest watershed and axis of the continent. There are, how- 
ever, two other and transverse watersheds in the area in question, whieh, 
though apparently not marked by any great geological breaks, are of 
great importance geographically, Of these the southern, with a general 
course of east and west, follows a sinuous line, and several times crosses 
the Boundary on the forty-ninth parallel. It separates the waters of the 
Red River and Saskatchewan, which find their way through Winnipeg 
Lake to Hudson’s Bay, from those of the Mississippi and Missouri and 
their various tributaries. Commencing with an elevation of about 1,600 
feet, in that region of swamp and lake in northern Minnesota which 
feeds the variously destined head-waters of the Winnipeg, St. Lawrence, 
Mississippi and Red Rivers ; it dips southward between the tributaries of 
the latter two streams, and passes between Lake Traverse and Big Stone 
Lake, about two hundred miles south of the Line, with an elevation of 
only 960 feet. Thence, pursuing a general north-westward course along 
the high-lands formed by the southern extensions of Pembina escarpment 
and the Missouri Coteau, it finally becomes identified with the latter, and | 
crosses the Boundary-line near its intersection with the 104th meridian, 
three hundred miles west of Red River, and continues with the same 
course toward the Elbow of the South Saskatchewan. The tributaries of 
the Souris River here lie along the north-eastern base of the Coteau, while 
those of the Missouri cut deeply into the soft Lignite formation of its 
south-western side. Between these river systems it lies as a belt of 
country without determinate direction of drainage or systems of river 
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