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rizontal. One very remarkable circumstance attending them is, that in one or two instances, they can be traced in a perfect circle, around little isolated hills, on a level with the corresponding line on the sides of the valley.

In his former visit to Glen Roy, he traced the shelves in that valley only; on the late occasion, however, he discovered that they are also to be found in Glen Spean and Glen Gluoy. This last valley contains one range, at an elevation twelve feet higher than that of any of those in the other glens. The two shelves next in altitude are to be found in Glen Roy alone. The uppermost runs through both lower and upper Glen Roy, and loses itself in the flat mossy ground forming the summit level of the country near the Loch of Spey. Besides these two shelves, which are the particular property of Glen Roy, there is another at a lower level, common to Glen Roy and Glen Spean. Its two extremities are to be traced, one on the mountain of Ben-y-vaan, near Highbridge, and the other on the side of Aonachmore, one of the Ben Nevis group, nearly opposite. This shelf may be followed almost every where, in its progress through both glens. It runs up the whole extent of Glen Spean, Loch Laggan, and the river Pattaig, as far as the Pass of Muckull, where it sweeps round on what is the summit level of the country there, and returns back. It is also distinctly traced running into the valley of Loch Treig.

In the paper formerly read to the society, Mr Lauder Dick stated his opinion, that such appearances in general were to be attributed to the operation of the waters of a lake. His last inspection of those in Lochaber has not only confirmed his conviction in the truth of this theory with respect to them, but has led him to imagine that he has discovered the boundaries, extent, and shape of the ancient lakes, as well as the cause which produced their evacuation. He conceives that he is warranted to conclude, from the observations he has made, that Glen Gluoy was at one time an independent lake, having its level twelve feet above the lake of Roy when at its highest, into which it discharged a stream from its N. E. extremity. Glen Roy must have contained an independent lake in two different states, as indicated by its uppermost and second shelves. Whilst in the first state, its level must have been such, that it discharged its waters, and those tributary to it from Loch Gluoy, in the direction of the Loch of Spey, and by it towards the eastern sea. When this was the case, a barrier must have existed at the mouth of Glen Roy, separating its lake from one at that time occupying the whole valley of the Spean, at the level of the lowest shelf of all, and which has such a relation to the summit level at the Pass of Muckull, as to warrant the conclusion, that it must have sent its stream through it towards the eastern sea, by the course of the river Spey. Two different ruptures took place in the barrier of division between Loch Roy and Spean. The first diminished the surface of Loch Roy so much as to render it tributary to Loch Spean. The second breach reduced it to the level of Loch Spean, of which it now formed a portion. Whilst the lakes were in this state, Mr Lauder Dick supposes that the whole ground at their south western end was an unbroken mass, and that the Great Glen of Scotland had then no existence, and, consequently, that what are now the mouths of Glen Gluoy and Glen Spean, were shut in by a terra firma, and that the united waters of the whole lakes formed a river, running through the Pass of Muckull, towards the eastern sea.

An examination of the Glen-morna-albin, or Great Glen of Scotland, stretching in a diagonal line across the island from Inverness to Fort William, has convinced Mr Lauder Dick that it has owed its origin to some convulsion of nature, and that the opening of this vast chasm was the cause of the discharge of the water of the lakes, and of the change of the direction of the current of the rivers, which now run to the western instead of the eastern sea, as they seem to have done formerly. He conceives also, that the horizontal shelves of Lochaber, and this vast crack across the island, reflect a mutual light on each other, elucidating the history of both.

March 16th.—Mr Leslie read an account of a new instrument, called an Othrioscope; but as a full description of it has been already published, it is unnecessary to give any further account of it here.

At the same meeting, Dr Brewster communicated a paper on a new theory of the phenomena of Double Refraction.