Southwest of the margin of what has long been known as the Archean continental nucleus lies a great drift-covered area, including in it most of the plains and prairies of northwestern Canada. It extends on the international boundary line from the western side of the Lake of the Woods to near the eastern base of the Rocky Mountains, through between sixteen and seventeen degrees of longitude, or a distance of more than 750 miles. Towards the northwest it stretches along the face of the Archean area to beyond the arctic circle in the valley of the Mackenzie river.
Lying on an irregular floor of old gneisses and schists, rocks of Silurian and Devonian age are known to occur over the whole eastern and northeastern portion of this district, while further westward these disappear under others of upper Mesozoic age; and thence westward to . . .