This view requires not only the supposition of great refrigeration, but of a great subsidence of the land in the temperate latitudes, with large residual islands and hills in the Arctic regions. That such subsidence actually took place is proved, not only by the frequent occurrence of marine shells in the boulder clay itself, but also by the occurrence of stratified marine clays filled with shells, often of deep-water species, immediately over that deposit. Further, the shells, and also occasional land plants found in these beds, indicate a cold climate and much cold fresh water pouring into the sea from melting ice and snow. In Canada these marine clays have been traced up to elevations of 600 feet, and in Great Britain deposits of this kind occur on one of the mountains of Wales at the height of 1300 feet above the level of the sea. Nor is it to be supposed that this level marks the extreme height of the Post-pliocene waters, for drift material not explicable by glaciers, and evidences of marine erosion, occur at still higher levels, and it is natural that on high and exposed points fewer remains of fossiliferous beds should be left then in plains and valleys. At the present day the coasts of Britain and other parts of Western Europe enjoy an exceptionally warm temperature, owing to the warm currents of the Atlantic being thrown on them, and the warm and moist Atlantic air flowing over them, under the influence of the prevailing westerly winds. These advantages are not possessed by the eastern coast of North America, nor by some deep channels in the sea, along which the cold northern currents flow under the warmer water. Hence these last-mentioned localities are inhabited by boreal shells much farther south then such species extend on the coasts and banks of Great Britain. In the Glacial period this exceptional advantage was lost, and while the American seas, as judged by their marine animals, were somewhat colder then at present, the British seas were proportionally much more cooled down. No doubt, however, there were warmer and colder areas, determined by depth and prevailing currents, and as these changed their position in elevation and subsidence of the land, alternations and even mixtures of the inhabitants of cold and warm water resulted, which have often been very puzzling to geologists.