At last, after many of these swallow-like retreats and advances, the warmer climate at length came to stay, and the ice retreated farther and farther to the north. It still remained among the mountains, so that we might describe the glaciers of the Alps and of the Canadian Rockies as the last relics existing to-day of the great Ice Age of the past. The retreat to the Arctic Circle left many other relics behind it, the great lakes, for example, like Winnipeg and Manitoba, and the Great Salt Lake of Utah. All were once mightier sheets, because during the Ice Age their waters were held back. Other smaller lakes formed by the dumping-heaps or terminal moraines of the glaciers still exist, and are especially noticeable in Finland. During the later stages of the Ice Age the level of the land was lower than it is now in Western Europe. When the ground began to rise--in slow upheavals with long pauses for rest--it left its impress in raised beaches, which can be seen on both sides of Scotland and on the Norwegian coast. The climate grew gradually milder, the animals and the plants followed in the train of the retreating ice, and even the traces of man's existence began to appear. The change was not sudden; it was so gradual that the Ice Age slipped as imperceptibly as its own glaciers into the age in which Man's activities in Europe began. CHAPTER VII THE FIRE-HARDENED ROCKS So far we have been considering the deposits laid down, for the most part, in a leisurely and orderly manner, by the action of air and water; by floods, rivers, lakes, the sea, or by the slow movements of ice. If these, however, had been the only agents by which the earth's strata were accumulated, then it is clear that for the most part these deposits and these strata would lie evenly, one on top of the other, like the lines of print on this page. But as a matter of observation the earth's strata do not lie like that. If we were to tear this page out and crumple it up in a ball, first having torn it in half and shredded a few irregular pieces out of it, we should get a truer picture of the way in which many of the earth's strata are contorted, crumpled, and displaced. They have not been so distorted by the action of the sea, violent as are some of the sea's assaults on the land; nor would the heat of the sun at its greatest ever produce such effects. They must have taken place from some causes which arise in the earth itself. These causes can be summed up in one word--fire. Some of the strata of which we have spoken, and which are called sedimentary strata, although they were composed of soft materials to begin with, have become very hard since, in some cases owing to the enormous pressure of the accumulated deposits above them, in other cases because of chemical action. In a few cases they have become hardened not so much by losing their water, as by direct heat. But the hardest of them is not so hard as another class