128 



THE GEOLOGIST. 



tions." He then tries the lower end of Loch Lochy, which terminates 

 in a wide alluvial plain ; and urges that if any probable place can be 

 selected here for the barrier, it is " at the narrowest part of the open- 

 ing which lies at Fort "William, between the skirts of the range below 

 Ben Nevis and the opposite heights of Ard Gowar." He then refers 

 necessarily to the other free opening which exists for the supposed 

 lake through Loch Shiel and Loch Moidart. Another barrier must 

 therefore be interposed in this direction ; and thus there will be formed 

 a large lake occupying Glen Boy to some point beyond the present 

 course of the Spey and Glen Spean, with the whole of Loch Laggan 

 and Glen Gloy, the great Caledonian valley, from a point of which 

 he does not pretend to define the northern limit, to Fort "William ; 

 Loch Arkeig and a part of the valley which includes it, and finally 

 the western valley of Loch Eil to some undefinable point lying to- 

 wards Loch Moidart and the western sea. The whole of this limit, 

 he admits, is not demonstrable, but he considers the similarity, if not 

 the actual community, of the "lines" of Glen Roy and Glen Gloy 

 demonstrates that a portion or the whole of Loch Lochy was included 

 in it. 



Here, therefore, a serious difficulty arises. This is the total ab- 

 sence of all corresponding watermarks on the borders of Loch Lochy, 

 as well as on the principal extent of the borders of Loch Laggan and 

 the valley of the Spean. There is a set of common features through 

 the whole tract, — the same rocks, the same slopes, the same causes of 

 waste ; yet the watermarks are strongly defined through a portion of 

 this wide space, while they are totally wanting in others. The 

 complete and sudden transition from the uppermost line of Glen 

 Hoy to the next succeeding one, and finally to the present bottom of 

 the valley, shows a perfect draining of the whole. Macculloch thinks 

 that the lake which occupied these valleys subsided at three different 

 intervals ; and further, that the more probable supposition is, that 

 these three drainings took place at the same point ; and he assumes 

 it most convenient to take the present and lowest communication, — 

 the exit of the water of the Spean and Eoy as that point. " Here 

 then," he says, " we must imagine a dam has existed, not gradually 

 worn down by the slow corroding action of the river issuing from the 

 lake, but by three successive failures occurring suddenly, or at least 

 within short intervals of time. Had much time elapsed between 

 these intervals, the several lines must have been more obscurely 

 marked, or intermediate ones of smaller dimensions must have been 



