ition,
chap. vi: Forms of Water, sections 55 and 56.] raise a quantity of
cast iron five times the weight of the glacier not only to a white
heat, but to its point of fusion. If, as I have already urged,
instead of being filled with ice, the valleys of the Alps were filled
with white-hot metal, of quintuple the mass of the present glaciers,
it is the heat, and not the cold, that would arrest our attention and
solicit our explanation. The process of glacier making is obviously
one of distillation, in which the fire of the sun, which generates the
vapour, plays as essential a part as the cold of the mountains which
condenses it. [Footnote: In Lyell's excellent 'Principles of Geology,'
the remark occurs that 'several writers have fallen into the strange
error of supposing that the glacial period must have been one of
higher mean temperature than usual.' The really strange error was the
forgetfulness of the fact that without the heat the substance
necessary to the production of glaciers would be wanting.]

It was their ascription to glacier action that first gave the parallel
roads of Glen Roy an interest in my eyes; and in 1867, with a view to
self-instruction, I made a solitary pilgrimage to the place, and
explored pretty thoroughly the roads of the principal glen. I traced
the highest road to the col dividing Glen Roy from Glen Spey, and,
thanks to the civility of an Ordnance surveyor, I was enabled to
inspect some of the roads with a theodolite, and to satisfy myself
regarding the common level of the shelves at opposite sides of the
valley. As stated by Pennant, the width of the roads amounts
sometimes to more than twenty yards; but near the head of Glen Roy the
highest road ceases to have any width, for it runs along the face of a
rock, the effect of the lapping of the water on the more friable
portions of the rock being perfectly distinct to this hour. My
knowledge of the region was, however, far from complete, and nine
years had dimmed the memory even of the portion which had been
thoroughly examined. Hence my desire to see the roads once more
before venturing to talk to you about them. The Easter holidays of
1876 were to be devoted to this purpose; but at the last moment a
telegram from Roy Bridge informed me that the roads were snowed up.
Finding books and memories poor substitutes for the flavour of facts,
I resolved subsequently to make another effort to see the roads.
Accordingly last Thursday fortnigh