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summer. This is called the snow-line. The height
of this line varies with the latitude, aspect,
proximity to the sea, length of summer, &c.
In order to understand what must occur at and
above this line, let us first of all imagine a high
mountain range, forming a single, long, smooth,
sloping, rounded back; or an angular ridge like a
house-roof, and perfectly unbroken at its sides.
The snow, in this case, would begin thawing in
the spring at the lower part, and a nearly even
line would be formed by its boundary. This line
would rise during the advance of summer until
it reached an elevation at which the whole heat
of the year would be just capable of melting
the whole snowfall of the year: and thus would
form the “snow-line.” Below this the summer
heat would overpower the snow and bring it
down as water, while above it the snow would
prevail, the whole year’s heat being less than
sufficient to melt the whole year’s snowfall. Thus
there must be an annual increase of snow in this
upper region. What must become of this
continuous accumulation? It is obvious that it must
increase in depth; and the greater the elevation,
within certain limits, the greater the rate of this
increase: thus it must top up, till it exceeds the
angle of repose and then slip down in avalanches.
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