This change will take place sooner, if the snow be damp at first,--but
if dry, the action of the hand will presently produce moisture enough to
complete the process. In this case, mere pressure produces the same
effect which, in the cases we have been considering above, was brought
about by alternate thawing and freezing,--only that in the latter the
ice is distinctly granular, instead of being uniform throughout, as when
formed under pressure. In the glaciers we have the two processes
combined. But the investigators of glacial phenomena have considered too
exclusively one or the other: some of them attributing glacial motion
wholly to the dilatation produced by the freezing of infiltrated
moisture in the mass of snow; others accounting for it entirely by
weight and pressure. There is yet a third class, who, disregarding the
real properties of ice, would have us believe, that, because tar, for
instance, is viscid when it moves, therefore ice is viscid because it
moves. We shall see hereafter that the phenomena exhibited in the onward
movement of glaciers are far more diversified than has generally been
supposed.

There is no chain of mountains in which the shape of the valleys is more
favorable to the formation of glaciers than the Alps. Contracted at
their lower extremity, these valleys widen upward, spreading into deep,
broad, trough-like depressions. Take, for instance, the valley of
Hassli, which is not more than half a mile wide where you enter it above
Meyringen; it opens gradually upward, till, above the Grimsel, at the
foot of the Finster-Aarhorn, it measures several miles across. These
huge mountain-troughs form admirable cradles for the snow, which
collects in immense quantities within them, and, as it moves slowly down
from the upper ranges, is transformed into ice on its way, and compactly
crowded into the narrower space below. At the lower extremity of the
glacier the ice is pure, blue and transparent, but, as we ascend, it
appears less compact, more porous and granular, assuming gradually the
character of snow, till in the higher regions the snow is as light, as
shifting, and incoherent, as the sand of the desert. A snow-storm on a
mountain-summit is very different from a snow-storm on the plain, on
account of the different degrees of moisture in the atmosphere. At great
heights, there is never dampness enough to allow the fine snow-crystals
to coalesce and form what are called "snow-flakes." I have even s