of the Rhone crept on down its valley past Martigny and St. Maurice till it reached the lake; it is then supposed not to have marched on with an ice-wall, say five hundred or more feet high, but to have at once spread out like so much soft pitch, and to have filled the lake to its present water-level or thereabouts. Then, over this great plain of ice, the sub-glacial torrent of the Rhone is supposed to have flowed, carrying with it and depositing at the end of the lake that ancient alluvium which, somehow, has got to be accounted for!1 Having thus filled the lake with ice instead of water, the main body of the glacier is supposed to start afresh and to travel over the ice, and thus obviate the imaginary difficulty of a glacier moving up hill, though every student of glaciers now admits that they did so, and though it is universally acknowledged that this very glacier of the Rhone moved over higher, steeper, and more irregular hills on its way to the Jura and to Soleure. Now this extraordinary theory involves two difficulties which are passed by in silence, but which seem to entirely contravene all that we know of the nature of glaciers, and to be entirely unsupported by facts. The first is the glacier ceasing to move onward as a glacier, but spreading out to fill up a lake basin, as if the lake were simply frozen to the bottom. Is this conceivable or possible? I think not. When glaciers come down to a fiord or to the sea they do not spread out laterally, but move on till the water is deep enough to buoy them up and break off icebergs, and no reason is given why anything different should have happened in the case of the great Swiss and Italian lakes, supposing they existed before the ice age came on. That the glacier should afterwards slide over this level plain of ice is equally inconceivable, in view of the property of regelation of ice under pressure. Owing to this property the glacier and the lake ice would become one mass, and would move on together under the law of decreasing velocity with depth. This, however, is of little importance, if, as I conceive the supposition of the formation of an ice-sheet at the water level for fifty miles 1 A. Falsan, La Période Glaciaire, p. 135, 137. The only in advance of the glacier is an impossible one. other theory is, that the lake was filled up by alluvium before the ice age, and that the glacier re-excavated it. I have, however, already given reasons why the glacier would not have done so, and the very existence of this ancient alluvium in the course of the ancient glacier is a proof that it did not do so. This theory seems now to have no supporters. Summary of the Evidence.-As the subject here discussed is very complex, and the argument essentially a cumulative one, it will be well briefly to summarize its main points. In the first place, it has been shown that the valleylakes of highly glaciated districts form a distinct class, which are highly characteristic if not altogether peculiar since in none of the mountain ranges of the tropics or of non-glaciated regions over the whole world are any similar lakes to be found. The special conditions favourable to the erosion of lake-basins and the mode of action of the ice-tool are then discussed, and it is shown that these conditions have been either overlooked or ignored by the opponents of the theory of ice-erosion. The objections of modern writers are then considered, and they are shown to be founded either on mistaken ideas as to the mode of erosion by glaciers, or on not taking into account results of glacier-action which they themselves either admit or have not attempted to disprove. The alternative theory that earth-movements of various kinds led to the production of lake-basins in all mountain ranges, and that those in glaciated regions were preserved by being filled with ice-is shown to be beset with numerous difficulties, physical, geological, and geo-. graphical, which its supporters have not attempted to overcome. It is also pointed out that this theory in no way explains the occurrence of the largest and deepest. lakes in the largest river valleys, or in those valleys where there was the greatest concentration of glaciers, a peculiarity of their distribution which points directly and unmistakably to ice-erosion. A crucial test of the two theories is then suggested and it is shown that both the sub-aqueous contours of the lake-basins, and the superficial outlines of the lakes, are exactly such as would be produced by ice-erosion, while they could not possibly have been caused by submergence due to any form of earth-movements. It is submitted that we have here a positive criterion, now adduced for the first time, which is absolutely fatal to any theory of submersion. Lastly, the special case of the Lake of Geneva is discussed, and it is shown that the explanation put forth by the anti-glacialists is wholly unsupported by facts, and is opposed to the known laws of glacier motion. The geologists who support it themselves furnish evidence. against their own theory in the ancient alluvium at Geneva on which the glacial deposits rest, and which is admitted to be mainly derived from the distant Alps. But as all alluvial matter is necessarily intercepted by large and deep lakes, the presence of this Alpine alluvium immediately beneath the glacial débris at the foot of the lake, indicates that the lake did not exist in pre-glacial times, but that the river Rhone flowed from the Alps to Geneva, carrying with it the old alluvium consisting of mud, sand, and gravel, which it had brought down from the mountains. Still more conclusive, however, is the fact that the three special features which have been shown to indicate erosion rather than submergence are present in this lake as fully as in all other Alpine valley-lakes and unmistakably point to the glacial origin of all of them. On the whole, I venture to claim that the facts and considerations set forth in this chapter show such a number of distinct lines of evidence, all converging to establish the theory of the ice-crosion of the valley-lakes of highly glaciated regions-a theory first advocated by the late Sir Andrew Ramsay-that that theory must be held to be established, at all events provisionally, as the only one by which the whole body of the facts can be explained and harmonized. EXPLANATION OF MAP OF THE ANCIENT RHONE Outline map of part of France and Switzerland, showing the course of the ancient Rhone glacier at the period of its greatest extension, to illustrate the account here given of the dispersal of Alpine erratics, as well as of the erosion of the basin of the Lake of Geneva by the glacier. EXPLANATION. 1. The parallel and diverging blue lines show the courses of the various streams of erratics carried by the glacier. 2. The arrows at end of these lines show the positions of the most distant Alpine erratics, or of drift containing fragments of Alpine rocks. 3. The Blue figures show the height in feet of the ice-surface above sea-level, as shown by ice-worn rocks or erratics in the vicinity. The actual elevation of the ice was perhaps much greater. 4. The Black figures show the heights of land or water surfaces above sea-level. 5. The tributary glaciers (except that of the Arve) are omitted for clearness; but the whole area between the Alps and the Jura, except a few high summits, was buried in ice. N.B. The lines showing the course of the erratics are taken from A. Falsan's large map published in 1887. CHAPTER VI THE GORGE OF THE AAR AND ITS TEACHINGS THERE is perhaps no valley in Switzerland that offers to the tourist so much variety and grandeur, and to the glacialist so much instruction, as the Haslithal or valley of the Aar. I visited it for the first time last summer, walking over the Grimsel Pass to the Hospice and the Aar glacier, and thence along the old mule-track and fine new carriage-road to Meiringen; staying there three days to visit the Reichenbach Falls, the Kirchet Hill, the gorge of the Aar, and other interesting localities. It seemed to me at the time that the phenomena presented by this valley afforded a striking example of the vast amount of glacial erosion, and that some of the conclusions to which they point had been overlooked by English writers. They give us, in fact, a fresh and very powerful argument in support of the power of the ancient glaciers both to deepen valleys and to grind out lake-basins; and I now propose to lay before my readers the facts which seem to me to prove the correctness of this view. The Grimsel Pass is a low one, only a little over 7,000 feet, but for this reason, and because it lies directly between extensive areas of perpetual snow, which give rise to some of the finest glaciers in Switzerland, it has been very largely ice-ground and presents a scene of savage grandeur which is often absent from higher passes. Everywhere the rocks are ground into huge domes or smooth slopes or rounded hollows, and these ice-ground VOL. I. K |