Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/391

 affecting the notions which have, with otherwise much appearance of reason, been entertained relating to the ancient state and posterior changes of the great Caledonian valley. It is conceived by many persons that Scotland was once entirely or partially divided in this place by the sea, the highest elevation of the present land being, as we have already seen, ninety feet. By the constant descent and accumulation of alluvium from the mountains, it is supposed that the dams have been formed which now separate Loch Oich both from Loch Ness and Loch Lochy, while these lakes have been disjoined from the sea by the large alluvial plains that now extend from them at each end along the courses of the Lochy and the Ness. The operations required in constructing the Caledonian canal have ascertained the reality and extent of these alluvia, while daily observation shows that they are, in many places at least, receiving an augmentation which has a tendency at some far distant period to obliterate the lakes, and convert the whole into one prolonged strath, of which the future summit will be Loch Oich, or some point in its vicinity. If indeed we examine the changes which the lakes of Scotland are now undergoing, we shall find that they are receiving accumulations of alluvial matter at all the points where they are fed by the surrounding streams, while a comparatively small quantity of this alluvium is carried from their exits towards the sea. The result of this operation is to obliterate them, and to convert them into alluvial valleys or straths. Instances of this revolution more or less perfected are numerous, while no case of the obliteration of a lake by drainage, similar to that of Glen Roy, can be pointed out. A different series of operations must have been required for this effect, and we have to reconcile the opposite processes which at different times have been carried on in the same place; in the present case for example, in the course of the Caledonian valley. It is