namely the non-extension of the three shelves beyond Glen Roy; but I abide
by what I have written on that point, and repeat that if in Glen Roy, where
circumstances have been so favourable for the preservation or formation of
the terraces, a terrace could be formed quite plain for three-quarters of a
mile with hardly a trace elsewhere, we cannot argue, from the non-existence
of shelves, that water did not stand at the same levels in other valleys.
Feeling absolutely convinced that there was no barrier of detritus at the
mouth of Glen Roy, and pretty well convinced that there was none of ice,
the manner in which the terraces die out when entering Glen Spean, which
must have been a tideway, shows on what small circumstances the formation
of these shelves depended. With respect to the non-existence of shelves in
other parts of Scotland, Mr. Milne shows that many others do exist, and
their heights above the sea have not yet been carefully measured, nor have
even those of Glen Roy, which I suspect are all 100 feet too high.
Moreover, according to Bravais (522/8. "On the Lines of Ancient Level of
the Sea in Finmark." By A. Bravais, Member of the Scientific Commission of
the North. "Quart. Journ. Geol. Soc." Volume I., page 534, 1845 (a
translation).), we must not feel sure that either the absolute height or
the intermediate heights between the terraces would be at all the same at
distant points. In levelling the terraces in Lochaber, all, I believe,
have been taken in Glen Roy, nearly N. and S. There should be levels taken
at right angles to this line and to the Great Glen of Scotland or chief
line of elevation.

Thirdly, the nature of the outlets from the supposed lakes. This appears
to me the best and newest part of the paper. If Sir James Clark would like
to attend to any particular points, direct his attention to this:
especially to follow Glen Glaster from Glen Roy to L. Laggan. Mr. Milne
describes this as an old and great river-course with a fall of 212 feet.
He states that the rocks are smooth on upper face and rough on lower, but