Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/394

 not then in existence. We have therefore two distinct alternation: of a formation, with one intermediate removal, of alluvia; within a period subsequent to that at which the present general distribution of hill and valley were made, and therefore in times comparatively recent.

In considering the conditions requisite to the maintenance of a lake in Glen Roy, I have already been obliged to anticipate some of the observations which would more properly have been introduced in this place. They are those which relate to the position of the supposed barriers, and which necessarily therefore involve the magnitude of the original lake. But the details relating to the eastern boundary including Loch Spey and Loch Laggan require to be more particularized. It has been seen that the boundary of the lake of Glen Roy towards the east could not have been further westward than Loch Spey. In inquiring next where it is likely to have been situated, we have seen that there is a communication between the mouths of Glen Roy and Glen Spean, and that a free level would exist in the present state of things between the lowest line of Glen Roy and Loch Laggan, since it can be traced within a few miles of the foot of this lake. It appears therefore probable, that no barrier existed at the western end of Loch Laggan so as to dam Glen Spean immediately above the present marked line and between this point and the foot of that lake. This probability amounts almost to certainty, when we compare the elevation of the supposed barrier necessary for the purpose of retaining the water of the common lake of Glen Roy within the limits of the highest line, with the actual barrier which now bounds Loch Laggan to the east. It is evident that if the waters of Loch Laggan were now raised suddenly by any power to the height of a barrier so supposed, they would flow over their eastern