These layers of ice become numerous and are
parallel to each other, like the layers of ice formed from slosh. Such
crusts of ice I have myself observed again and again upon the glacier.
This stratified snowy ice is now the bottom on which the first autumnal
snow-falls accumulate. These sheets of ice may be formed not only
annually before the winter snows set in, but may recur at intervals
whenever water accumulating upon an extensive snow-surface, either in
consequence of melting or of rain, is frozen under a sharp frost before
another deposit of snow takes place. Or suppose a fresh layer of light
porous snow to have accumulated above one the surface of which has
already been slightly glazed with frost; rain or dew, falling upon the
upper one, will easily penetrate it; but when it reaches the lower one,
it will be stopped by the film of ice already formed, and under a
sufficiently low temperature, it will be frozen between the two. This
result may be frequently noticed in winter, on the plains, where sudden
changes of temperature take place.
There is still a third cause, to which the same result may possibly be
due, and to which I shall refer at greater length hereafter; but as it
has not, like the preceding ones, been the subject of direct
observation, it must be considered as hypothetical.
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