If we proceed a little distance to the mountains of the Jura, now wholly devoid of Glaciers, we shall find that the same glacial phenomena with which we have become so familiar in the Alps, are still everywhere presented to the eye. And we feel instinctively impelled to pursue the same line of inductive reasoning. Moving ice, we know from abundant observation, is capable of producing these effects: nor have we ever seen effects of this kind produced by any other cause: nay, there is no other natural agent known that is capable of producing such effects: it is therefore reasonable to infer that moving ice was the cause of these effects; and that, in some bygone age, great masses of ice moved slowly over the valleys of the Jura as they now move slowly over the valleys of the Alps.
Another circumstance may here be noticed which is well worthy of consideration. The Alps are composed of granite, gneiss, and such like crystalline rocks: the Jura, of limestone and various other formations, altogether different from those of the Alps. Now, scattered loosely over the valleys of the Jura, and perched upon its lofty crests, we find immense angular blocks--some of them as large as cottages--of the Alpine rocks. The question naturally arises, how have they been transported to their present site. Certainly not by the action of water; for in that case the projecting angles would have been rounded off, and the sharp edges worn away. But the work might have been easily accomplished by the power of moving ice, and could not have been accomplished by any other natural agency with which we are acquainted. Thus we are led to conclude that the Glaciers of the Alps must, by some means or another, have once made their way northward across the great valley of Switzerland, fifty miles wide, and deposited their ponderous burdens of gravel, sand, and erratic blocks on the mountains of the Jura.
It would carry us too far from our present purpose to draw out this theory in all its details. But we cannot for-bear briefly to touch upon some of the bold and startling conclusions to which it has led. The Geologist having, by patient and varied exercise, in the regions of existing Glaciers, trained his eye and his judgment in the observation of those phenomena that mark the action of moving ice, soon begins to discover that they are not wanting in other countries. They are not to be found, indeed, beneath the burning sun of Africa, nor on the borders of the Mediterranean Sea. But as he travels northward they begin by degrees to appear; and when at length he reaches the shores of the Baltic, they are spread out profusely before him as they were in the bosom of the Alps. All this had puzzled Geologists for years; but the clue has been found at last. What is going on to-day in Switzerland, and in Greenland, and on the shores of Labrador, must have been going on, ages ago, in Germany, and in Denmark, and on the shores of the Baltic. We may argue from the effect to the cause. Here are the moraines, the erratics, the perched blocks, and the surfaces of rock furrowed and scratched with ice: at some past time there must have been the moving Glaciers and the floating Icebergs.