e
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. 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.

[Illustration]

But that mass of snow, before it reaches the outlet of the valley, is to
be compressed, contorted, folded, rent in a thousand directions. The
beds of snow, which in the upper ranges of the mountain were spread out
over broad, open surfaces, are to be crowded into comparatively
circumscribed valleys, to force and press themselves through narrow
passes, alternately melting and freezing, till they pass from the
condition of snow into that of ice, to undergo, in short, constant
transformations, by which