Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/385

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As I have proved in another part of this paper, that the level of the upper line of Glen Roy is higher than those of many vallies which would at present afford passage to the supposed waters of Glen Roy into the sea, it follows that water could not now stand at that level, unless these apertures were obstructed to at least an higher elevation. The determination of the position of these imaginary barriers, is consequently the next point to be considered; as well as that of their number, since possibly two of these openings might be closed by a single obstruction. I must therefore proceed to examine more particularly into the conditions required for the formation of such a lake as that which I have supposed to be the cause of the lines in Glen Roy.

I have shown that the uppermost one is of such a height, that water standing at that level would now flow out by Loch Spey and Loch Laggan, through the valley of the Spey into the eastern, and by Loch Eil, Loch Shiel, and Loch Ness, into the western sea. The two lower lines lying below the barriers of Loch Spey and Loch Laggan, it would, under similar circumstances, find its way through the three latter openings only. The condition of the surrounding land must therefore have so far differed at that time from its present state, that various dams or barriers must have existed in the course of these openings.

In attempting to investigate their places it is proper to commence by assuming the least difficulties, assigning no more causes than are strictly necessary to the production of the desired effect. I have shown elsewhere, that the conditions of the present barriers existing at the source of Loch Spey, and to the east of Loch Laggan, are such as to give no reason to imagine that they have once been higher. I have also shown, that by the removal of the supposed barriers to a point below Dalchully, one obstruction would have answered