Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/352

 the west they are directed to Loch Laggan. The barrier itself gives no rise to waters, as it consists of a narrow ledge of rocks; nor does it appear at any time to have been liable to suffer from the course of rivers. This is a species of judgment which can be formed without perhaps any great risk of error, by inspecting the positions and shapes of rocks and mountains, but it is a judgment of which the grounds can scarcely be explained by drawings or descriptions. The observation itself is of importance in examining the general theories of the appearances in Glen Roy. Circumstances prevented me from ascertaining the actual height of this barrier above the sea, and the nature of the ground does not admit of any direct levelling to it from Glen Roy, without a series of most intricate and tedious operations, which would in fact be unnecessary in examining the question. But by comparing its level with that of the Spey at the point where the before mentioned sluggish stream joins that river, and by computing the elevation of that point from Garvamore of which the height was ascertained, its altitude is readily estimated to a sufficient degree of accuracy for the present purpose.

Comparing these heights therefore it will be found that the barrier of Loch Laggan to the east is depressed 369 feet below Loch Spey and 432 feet below the uppermost line of Glen Roy. Supposing therefore that the water stood at the highest elevation in Glen Roy in the present state of the earth, it would run into the Spey not only by the channel of Loch Spey, but by that of Loch Laggan also.

Such are the communications which Glen Roy has with the surrounding country, and through that with the sea. I have described them with all the minuteness in my power, as they are of the first importance in the investigation of the causes which are to