New Hampshire, in a cleft known as Tuckerman's Ravine, where the
deposit accumulates to a great depth, the snow-ice remains until
midsummer. It is, indeed, evident that a very slight change in the
climatal conditions of this locality would establish a permanent
accumulation of frozen water upon the summit of the mountain. If the
crest were lifted a thousand feet higher, without any general change
in the heat or rainfall of the district, this effect would be
produced. If with the same amount of rainfall as now comes to the
earth in that region more of it fell as snow, a like condition would
be established. Furthermore, with an increase of rainfall to something
like double that which now descends the snow bore the same proportion
to the precipitation which it does at present, we should almost
certainly have the peak above the permanent snow line, that level
below which all the winter's fall melts away. These propositions are
stated with some care, for the reason that the student should perceive
how delicate may be--indeed, commonly is--the balance of forces which
make the difference between a seasonal and a perennial snow covering.

As soon as the snow outlasts the summer, the region which it occupies
is sterilized to life. From the time the snow begins to hold over the
warm period until it finally disappears, that field has to be reckoned
out of the habitable earth, not only to man, but to the lowliest
organisms.[6]

[Footnote 6: In certain fields of permanent snow, particularly near their
boundaries, some very lowly forms of vegetable life may develop on a
frozen surface, drawing their sustenance from the air, and supplied with
water by the melting which takes place during the summertime. These
forms include the rare phenomenon termed red snow.]

If the snow in a glaciated region lay where it fell, the result would
be a constant elevation of the deposit year by year in proportion to
the annual excess of deposition over the melting or evaporation of the
material. But no sooner does the deposit attain any considerable
thickness than it begins to move in the directions of least
resistance, in accordance with laws which the students of glaciers are
just beginning to discern. In small part this motion is accomplished
by avalanches or snow slides, phenomena which are in a way important,
and therefore merit description. Immediately after a heavy snowfall,
in regions where the slopes are steep, it often happens that the
de