len Roy. This
state of things would continue as long as the ice dam was sufficiently
high to dominate the cols at A and B; but when through change of
climate the gradually sinking dam reached, in succession, the levels
of these cols, the water would then begin to flow over the dam instead
of over the cols. Let us suppose the wasting of the ice to continue
until a connection was established between Glen Roy and Glen Glaster,
a common lake would then fill both these glens, the level of which
would be determined by that of the col c, over which the water would
pour for an indefinite period into Glen Spean. During this period the
second Glen Roy road and the highest road of Glen Glaster would be
formed. The ice subsiding still further, a connection would
eventually be established between Glen Roy, Glen Glaster, and the
upper part of Glen Spean. A common lake would fill all three glens,
the level of which would be that of the col D, over which for an
indefinite period the lake would pour its water. During this period
the lowest Glen Roy road, which is common also to Glen Glaster and
Glen Spean, would be formed. Finally, on the disappearance of the ice
from the lower part of Glen Spean the waters would flow down their
respective valleys as they do to-day.

Fig. 7

Reviewing our work, we find three considerable steps to have marked
the solution of the problem of the Parallel Roads of Glen Roy. The
first of these was taken by Sir Thomas Dick-Lauder, the second was the
pregnant conception of Agassiz regarding glacier action, and the third
was the testing and verification of this conception by the very
thorough researches of Mr. Jamieson. No circumstance or incident
connected with this discourse gives me greater pleasure than the
recognition of the value of these researches. They are marked
throughout by unflagging industry, by novelty and acuteness of
observation, and by reasoning power of a high and varied kind. These
pages had been returned 'for press' when I learned that the relation
of Ben Nevis and his colleagues to the vapour-laden winds of the
Atlantic had not escaped Mr. Jamieson. To him obviously the
exploration of Lochaber, and the development of the theory of the
Parallel Roads, has been a labour of love.

Thus ends our rapid survey of this brief episode in the physical
history of the Scottish hills,--brief, that is to say, in comparison
with the immeasurable lapses of time through which, to produce i