300 Proceedings of Philosophical Societies. [April, 



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 fonnerly read to the Society, Mr. Lauder Dick 

 stated it as 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 12 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 con- 

 clusion, 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 Jirma, 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, 



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