The glaciers have their periods of advance and of recession. A series of wet and cool summers causes them to advance and encroach on the plains, pushing before them their moraines, and even forests and human habitations. In dry and warm summers they shrink and recede. Such changes seem to have occurred in bygone times on a gigantic scale. All the valleys below the present glaciers present traces of former glacier action. Even the Jura mountains seem at one time to have had glaciers. Large blocks from the Alps have been carried across the intervening valley and lodged at great heights, on the slopes of the Jura, leading the majority of the Swiss and Italian geologists to believe that even this great valley and the basin of Lake Leman were once filled with glacier ice. But, unless we can suppose that the Alps were then vastly higher than at present, this seems scarcely to be physically possible, and it seems more likely that the conditions were just the reverse of those supposed, namely, that the low land was submerged, and that the valley of Lake Leman was a strait like Belle-Isle, traversed by powerful currents and receiving icebergs from both Jurassic and Alpine glaciers, and probably from farther north. One or other supposition is required to account for the appearances, which may be explained on either view. The European hills may have been higher and colder, and changes of level elsewhere may have combined with this to give a cold climate with moisture; or a great submergence may have left the hills as islands, and may have so reduced the temperature by the influx of arctic currents and ice, as to enable the Alpine glaciers to descend to the level of the sea. Now, we have evidence of such submergence in the beds of sea-shells and travelled boulders scattered over Europe, while we also have evidence of contemporaneous glaciers, in their traces on the hills of Wales and Scotland and elsewhere, where they do not now occur. I have long maintained that in America all the observed facts imply a climate no colder than that which would have resulted from the subsidence which we know to have occurred in the temperate latitudes in the Pleistocene period, and though I would not desire to speak so positively about Europe, I confess to a strong impression that the same is the case there, and that the casing of glacier ice imagined by many geologists, as well as the various hypotheses which have been devised to account for it, and to avoid the mechanical, meteorological, and astronomical difficulties attending it, are alike gratuitous and chimerical, as not being at all required to account for observed facts, and being contradictory, when carefully considered, to known physical laws as well as geological phenomena.[161] [161] _Canadian Naturalist_, vols. viii. and ix. _Geological Magazine_, December, 1865.