128 
THE GEOLOQTST. 
tions." He then tries the lower end of Loch Locliy, 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 Eoy to some point beyond the present 
course of the Spey and Glen Spean, with the w^hole of Loch Laggan 
and Glen Gloy, the great Caledonian valley, from a point of which 
he docs 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 Eoy 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 difi'erent 
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 
