The admirable
experiments of Dr. Tyndall have shown that water may be generated in ice
by pressure, and it is therefore possible that at a lower depth in the
glacier, where the incumbent weight of the mass above is sufficient to
produce water, the water thus accumulated may be frozen into ice-layers.
But this depends so much upon the internal temperature of the glacier,
about which we know little beyond a comparatively superficial depth,
that it cannot at present afford a sound basis even for conjecture.
There are, then, in the upper snow-fields three kinds of horizontal
deposits: the beds of snow, the sheets of dust, and the layers of ice,
alternating with each other. If, now, there were no modifying
circumstances to change the outline and surface of the glacier,--if it
moved on uninterruptedly through an open valley, the lower layers,
forming the mass, getting by degrees the advance of the upper ones, our
problem would be simple enough. We should then have a longitudinal mass
of snow, inclosed between rocky walls, its surface crossed by straight
transverse lines marking the annual additions to the glacier, as in the
adjoining figure.
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