[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 the primitive stratification will be
extensively modified. In the first place, the more rapid motion of the
centre of the glacier, as compared with the margins, will draw the lines
of stratification downward toward the middle faster than at the sides.
Accurate measurements have shown that the axis of a glacier may move
ten- or twenty-fold more rapidly than its margins. This is not the place
to introduce a detailed account of the experiments made to ascertain
this result; but I would refer those who are interested in the matter to
the measurements given in my "Systeme Glaciaire," where it will be seen
that the middle may move at a rate of two hundred feet a year, while the
margins may not advance more than ten or fifteen or twenty feet.
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