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Page:Popular Science Monthly Volume 44.djvu/704

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range for a distance of one hundred miles, and up to a height of two thousand and fifteen feet above the Lake of Neufchâtel. The first important point to notice is that this highest elevation is attained at a spot exactly opposite, and in the same direction as, the Rhone Valley, between Martigny and the head of the Lake of Geneva, while north or south of this point they gradually decline in elevation to about five hundred feet above the lake. The blocks at the highest elevation and central point can be traced to the eastern shoulder of Mont Blanc. All those to the southwest come from the left-hand side of the lower Rhone Valley, while those to the northeast are all from the left side of the upper Rhone Valley and its tributaries. Other rocks coming from the right-hand side of the upper Rhone Valley are found on the right hand or Bernese side of the great valley between the Jura and the Bernese Alps.[1]

Now, this peculiar and definite distribution, which has been worked out with the greatest care by numerous Swiss geologists, is a necessary consequence of well-known laws of glacier motion. The débris from the two sides of the main valley form lateral moraines which, however much the glacier may afterward be contracted or spread out, keep their relative position unchanged. Each important tributary glacier brings in other lateral moraines, and thus when the combined glacier ultimately spreads out in a great lowland valley the several moraines will also spread out, while keeping their relative position, and never crossing over to mingle with each other. So soon as this definite position of the erratics was worked out it became evident that the first explanation—of a great submergence during which the lower Swiss valleys were arms of the sea and the Rhone glacier broke off in icebergs which carried the erratics across to the Jura—was altogether untenable, and that the original explanation of Venetz and Charpentier was the true one. Sir Charles Lyell, who had first adopted the iceberg theory, gave it up on examining the country in 1857 and ascertaining that the facts were correctly stated by the Swiss geologists; and there is at the present day no writer of the least importance who denies this. Sir Henry Howorth, who is one of the strongest opponents of what he considers the extreme views of modern glacialists, gives a full summary of the facts as to the old Rhone glacier from Charpentier. He states that between Martigny and St. Maurice the moraine débris on each side of the valley shows the glacier to have reached a height of three thousand feet above the river; farther on, where the valley widens over the Lake of Geneva, it sank to

  1. A map showing the lines of dispersal of these erratics is given in Lyell's Antiquity of Man, p. 144, and is reproduced in my Island Life, p. 111.