s
water of Glen Gluoy discharging itself over the col into Glen Roy. As
long as the barrier stopping the mouth of Glen Gluoy continued high
enough, we should have in that glen a lake at the precise level of its
shelf, which lake, acting upon the loose drift of the flanking
mountains, would form the shelf revealed by observation.

So much for Glen Gluoy. But suppose the mouth of Glen Roy also
stopped by a similar barrier. Behind it also the water from the
adjacent mountains would collect. The surface of the lake thus formed
would gradually rise, until it had reached the level of the col which
divides Glen Roy from Glen Spey. Here the rising of the lake would
cease; its superabundant water being poured over the col into the
valley of the Spey. This state of things would continue as long as a
sufficiently high barrier remained at the mouth of Glen Roy. The lake
thus dammed in, with its surface at the level of the highest parallel
road, would act, as in Glen Gluoy, upon the friable drift
overspreading the mountains, and would form the highest road or
terrace of Glen Roy.

And now let us suppose the barrier to be so far removed from the mouth
of Glen Roy as to establish a connection between it and the upper part
of Glen Spean, while the lower part of the latter glen still continued
to be blocked up. Upper Glen Spean and Glen Roy would then be
occupied by a continuous lake, the level of which would obviously be
determined by the col at the head of Loch Laggan. The water in Glen
Roy would sink from the level it had previously maintained, to the
level of its new place of escape. This new lake-surface would
correspond exactly with the lowest parallel road, and it would form
that road by its action upon the drift of the adjacent mountains.

In presence of the observed facts, this solution commends itself
strongly to the scientific mind. The question next occurs, What was
the character of the assumed barrier which stopped the glens? There
are at the present moment vast masses of detritus in certain portions
of Glen Spean, and of such detritus Sir Thomas Dick-Lauder imagined
his barriers to have been formed. By some unknown convulsion, this
detritus had been heaped up. But, once given, and once granted that
it was subsequently removed in the manner indicated, the single road
of Glen Gluoy and the highest and lowest roads of Glen Roy would be
explained in a satisfactory manner.

To account for the second or middle