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lower extremity proportionate to their advance. But in considering the past history of glaciers, we must think of their changes as retrograde, not progressive movements; since, if the glacial theory be true, a great mass of ice, of which the present glaciers are but the remnants, formerly spread over the whole Northern hemisphere, and has gradually disappeared, until now no traces of it are to be found, except in the Arctic regions and in lofty mountain-ranges. Every terminal moraine, such as I described in the last article, is the retreating footprint of some glacier, as it slowly yielded its possession of the plain, and betook itself to the mountains; wherever we find one of these ancient semicircular walls of unusual size, there we may be sure the glacier resolutely set its icy foot, disputing the ground inch by inch, while heat and cold strove for the mastery. There may have been a succession of cold summers, or, if now and then a warmer summer intervened, a colder one followed, so that the glacier regained the next year the ground it had lost during the preceding one, thus continuing to oscillate for a number of years along the same line, and adding constantly to the _debris_ collected at its extremity. Wherever such oscillations and pauses in the retreat of the glacier occurred, all the materials annually brought down to its terminus were collected; and when it finally disappeared from that point, it left a wall to mark its temporary resting-place. By these semicircular concentric walls we can trace the retreat of the ice as it withdrew from the plain of Switzerland to the fastnesses of the Alps. It paused at Berne, and laid the foundation of the present city, which is built on an ancient moraine; it made a stand again at the Lake of Thun, and barred its northern outlet by a wall which holds its waters back to this day. Other moraines, though less distinct, are visible nearer the base of the Bernese Alps, and, above Meyringen, the valley is spanned by one of very large dimensions. Again, on the other side of the first chain of high peaks, the glacier of the Rhone, descending the valley toward the Lake of Geneva, has everywhere left traces of its ancient extension. We find the valley crossed at various distances by concentric moraines, until we reach the lake. There are no less than thirteen concentric moraines immediately below the present termination of the glacier of the Rhone, the one nearest to the ice, and t
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