Through countless thousands of years, perhaps, this gradual process went on. Nature, shifting its huge scenery, depressed the ocean beds and piled up the dry land of the continents. In place of the vast 'Continental Sea,' which once filled the interior of North America, there arose the great plateau or elevated plain that now runs from the Mackenzie basin to the Gulf of Mexico. Instead of the rushing waters of the inland sea, these waters have narrowed into great rivers--the Mackenzie, the Saskatchewan, the Mississippi--that swept the face of the plateau and wore down the surface of the rock and mountain slopes to spread their powdered fragments on the broad level soil of the prairies of the west. With each stage in the evolution of the land the forms of life appear to have reached a higher development. In place of the seaweed and the giant ferns of the dawn of time there arose the maples, the beeches, and other waving trees that we now see in the Canadian woods. The huge reptiles in the jungle of the Carboniferous era passed out of existence. In place of them came the birds, the mammals,--the varied types of animal life which we now know. Last in the scale of time and highest in point of evolution, there appeared man. We must not speak of the continents as having been made once and for all in their present form. No doubt in the countless centuries of geological evolution various parts of the earth were alternately raised and depressed. Great forests grew, and by some convulsion were buried beneath the ocean, covered deep as they lay there with a sediment of earth and rock, and at length raised again as the waters retreated. The coal-beds of Cape Breton are the remains of a forest buried beneath the sea. Below the soil of Alberta is a vast jungle of vegetation, a dense mass of giant fern trees. The Great Lakes were once part of a much vaster body of water, far greater in extent than they now are. The ancient shore-line of Lake Superior may be traced five hundred feet above its present level.