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312 road of Glen Roy in the same manner, he assumed that at a certain point—the level of this road—the barrier which had been wasting away held its ground for a sufficiently long time to form the road. But, on the same principle, there would naturally have been a greater number of roads in this glen, and additional roads in the other glens. A weakness was thus admitted into the theory which was immediately attacked by Mr. Darwin. He believed that the whole region had once been covered by the sea, and that, in the upheaval of the earth, there were pauses during which these roads were formed. But this would not account for the sea being higher in one of the glens than in another, nor for the unequal number of terraces by which the mountains are belted. As soon as Mr. Darwin detected these fallible points, he abandoned his theory.

In 1847 the Dick-Lander hypothesis received new strength from a discovery made by Mr. Milne-Home. There is a lateral glen, called Glen Glaster, running eastward from Glen Roy, which had escaped the notice of Sir Thomas Dick-Lander. Mr. Milne-Home entered this glen, pursued a branch of it extending to the southeast, and came upon a water-shed exactly level with the second Glen Roy road. On the same theory as before, when the barrier should be properly removed, the water in Glen Roy would sink to the second road, and the surplus water would escape over the Glen Glaster water-shed into Glen Spean. But this mode of explanation could not yet be accepted, for there is scarcely a trace left of the immense quantity of detritus that would have been necessary to form the barriers. Nor could the detritus have been swept away by glaciers, for there have been no glaciers in these valleys since the retreat of the lakes.

At the time when Sir Thomas Dick-Lander was making his investigations, the action of ancient glaciers was not understood. The subject had been pursued in Switzerland, but it was not till 1840 that unmistakable marks of glacier-action were pointed out in Great Britain by Agassiz. He visited Glen Roy, and, having detected the traces of glaciers, pronounced these to have been the barriers blocking up the glens. This theory was afterward examined and confirmed by Mr. Jamieson. "It was their ascription to glacier-action," says Prof. Tyndall, "that first gave the parallel roads of Glen Roy an interest in my eyes; and in 1867, with a view to self-instruction, I made a solitary pilgrimage to the place, and explored pretty thoroughly the roads of the principal glen." At different places he found that the effects of the lapping of the water on the more friable portions of the rock are still perfectly distinct. Several months ago he again visited the place, prior to delivering a lecture upon the subject. The entire ground was thoroughly explored, and the principal hills were found to be intensely glaciated. The collecting-ground of these glaciers, which blocked up the valleys, were the mountains south and west of Glen Spean—among others, Ben Nevis. These lofty mountains