_The Subarctic Forest Plains._--The Prairie plains merge at the north with a great tract of forest-covered lowlands, which extend from the Laurentian hills on the east to near the base of the Rocky Mountains on the west. The change as one travels northward from the grassy prairies to the country of equally mild relief, but clothed with trees adapted to a rigorous climate, is gradual. Along the irregular and in part indefinite junction of these two vast plains, the alignment of the forest is broken in many places, and its margin fringed by a picket-line of groves and of isolated trees, which has advanced southward and invaded the grass-lands. Between these outposts the prairie with its wealth of summer bloom reaches well into the realm of perennial shadow. The southward extensions of the forest are mainly in the valleys and adjacent to the streams, while the drier steppes between are open grass-lands. No conspicuous change in the topography of the land or of the rocks or the soil coincides with the change from grass to forest. The differences in vegetation must therefore be sought in climatic conditions, and mainly in the influence of atmospheric changes on the water contained in the soil. Throughout practically the whole of the region occupied by the subarctic forest, between Hudson Bay and the foot-hills of the Rocky Mountains, the land is low and the valleys monotonous. Many lakes are present, several of them of large size, and the rivers are remarkable for their lengths, low gradients, and large volumes. The subdued topography of the region here considered, and the presence of vast numbers of lakes and swamps, is due in general to the influence of the ice-sheets which formerly covered it. In a minor way the presence of the innumerable small lakes and swamps is owing to the obstructions formed by growing vegetation, the damming of streams by driftwood, the work of beavers, and possibly the influence of subsoil ice. To the north of the Subarctic Forest plain, as already described, occur the desolate tracts known in Canada as the Barren Grounds, which form a part or merge into the tundras bordering the Arctic Ocean.