ggested to different observers. The
theory of the formation of lakes by barriers, presented by McCulloch and
Sir T. Lauder-Dick, that of continental upheavals and subsidences,
advocated by Sir Charles Lyell and Charles Darwin, that of inundations
by great floods, maintained by Professor H. D. Rogers and Sir George
Mackenzie, that of glacial action, brought forward by myself, have been
duly discussed with reference to this difficult case; all have found
their advocates, all have met with warm opposition, and the matter still
remains a mooted point; but the one of all these theories which shall
stand the test of time and repeated examination and be eventually
accepted will explain many a problem besides the one it was meant to
solve, and lead to farther progress in other directions.

I propose here to reconsider the facts of the case, and to present anew
my own explanation of them, now more than twenty years old, but which I
have never had an opportunity of publishing in detail under a popular
form, though it appeared in the scientific journals of the day.

Before considering, however, the phenomena of Glen Roy, or the special
glacial areas scattered over Scotland and the other British Isles, let
us see what general evidence we have that glaciers ever existed at all
in that realm. The reader will pardon me, if, at the risk of repetition,
I sum up here the indications which, from our knowledge of glaciers as
they at present exist, must be admitted, wherever they are found, as
proof of their former existence. Such a summary may serve also as a
guide to those who would look for glacial traces where they have not
hitherto been sought.

In the first place, we have to consider the singular abrasion of the
surfaces over which the glacier has moved, quite unlike that produced by
the action of water. We have seen that such surfaces, wherever the
glacier-marks have not been erased by some subsequent action, have
several unfailing characteristics: they are highly polished, and they
are also marked with scratches or fine _striae_, with grooves and deeper
furrows. Where best preserved, the smooth surfaces are shining; they
have a lustre like stone or marble artificially polished by the combined
friction and pressure of some harder material than itself until all its
inequalities have been completely levelled and its surface has become
glossy. Any marble mantel-piece may serve as an example of this kind of
glacier-worn surface.

The