Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/386

 the purpose of confining the waters in this direction. If this were a mass of alluvial matter occupying the strath in which the junction of the Spey and Truim takes place, it is not difficult to conceive that it was gradually worn down by the action of the waters of the Spey, causing the drainage of the highest level in Glen Roy; or else after that drainage had taken place by the failure of some other barrier. The flow of the Spey and of the Roy would then follow the directions of the intermediate ground, and the present courses of these rivers, as far as they were then free, would be established. If we now turn our attention to the western side of Glen Roy, and examine the elevation and direction of the ground at its junction with the vale of the Lochy, we shall see that both Glen Roy and Glen Spean bear one common water mark or line, and unite into a common wide valley before they join the vale of the Lochy. The imaginary barrier must therefore be removed, at least to that part of this valley where the lines terminate; which is to a point beyond Teindrish. But I have shown that the form of this ground, and the gradual dilatation of the valley into that of the Lochy is such, that no barrier could have existed here without occupying the whole present valley of the Lochy. This barrier towards the sea, may therefore be removed with considerable probability to some more distant point, a probability increased by considering the circumstances which attend Glen Gloy. Although no direct continuation of level can be traced between it and Glen Roy, as there is between the latter and Glen Spean, yet I have shown that in both there are one or more lines at the same probable altitude, and that the condition of the upper junction of these glens is such, that the communication could not have taken place at that point. If also we suppose that Glen Gloy was dammed by a barrier of its own, independently of that which occupied the common opening of Glen Roy and Glen Spean, we multiply our