d these
glens to a height, it may be, of 1,500 feet above the bottom of Glen
Spean, have dissolved and left not a wreck behind; in presence of the
fact, insisted on by Professor Geikie, that barriers of detritus would
undoubtedly have been able to maintain themselves had they ever been
there; in presence of the fact that great glaciers once most certainly
filled these valleys--that the whole region, as proved by Mr.
Jamieson, is filled with the traces of their action; the theory which
ascribes the parallel roads to lakes dammed by barriers of ice has, in
my opinion, a degree of probability on its side which amounts to a
practical demonstration of its truth.

Into the details of the terrace formation I do not enter. Mr. Darwin
and Mr. Jamieson on the one side, and Sir John Lubbock on the other,
deal with true causes. The terraces, no doubt, are due in part to the
descending drift arrested by the water, and in part to the fretting of
the wavelets, and the rearrangement of the stirred detritus, along the
belts of contact of lake and bill. The descent of matter must have
been frequent when the drift was unbound by the rootlets which hold it
together now. In some cases, it may be remarked, the visibility of
the roads is materially augmented by differences of vegetation. The
grass upon the terraces is not always of the same character as that
above and below them, while on heather-covered hills the absence of
the dark shrub from the roads greatly enhances their conspicuousness.

The annexed sketch of a model will enable the reader to grasp the
essential features of the problem and its solution. Glen Gluoy and
Glen Roy are lateral valleys which open into Glen Spean. Let us
suppose Glen Spean filled from v to w with ice of a uniform elevation
of 1,500 feet above the sea, the ice not filling the upper part of
that glen. The ice would thrust itself for some distance up the
lateral valleys, closing all their mouths. The streams from the
mountains right and left of Glen Gluoy would pour their waters into
that glen, forming a lake, the level of which would be determined by
the height of the col at A, 1170 feet above the sea. Over this col
the water would flow into Glen Roy. But in Glen Roy it could not rise
higher than 1150 feet, the height of the col at B, over which it would
flow into Glen Spey.

The water halting at these levels for a sufficient time, would form
the single road in Glen Gluoy and the highest road in G