First that the rate of advance
between the axis and the margins of a glacier differs in the ratio of
about ten to one and even less; that is to say; when the centre is
advancing at a rate of two hundred and fifty feet a year, the motion
toward the sides may be gradually diminished to two hundred, one hundred
and fifty, one hundred, fifty feet, and so on, till nearest the margin
it becomes almost inappreciable. Secondly, the rate of motion is not the
same throughout the length of the glacier, the advance being greatest
about half-way down in the region of the _neve_, and diminishing in
rapidity both above and below; thus the onward motion in the higher
portion of a glacier may not exceed twenty to fifty feet a year, while
it reaches its maximum of some two hundred and fifty feet annually in
the _neve_ region, and is retarded again toward the lower extremity,
where it is reduced to about one-fourth of its maximum rate. Thirdly,
the glacier moves at different rates throughout the thickness of its
mass; toward the lower extremity of the glacier the bottom is retarded,
and the surface portion moves faster, while in the upper region the
bottom seems to advance more rapidly.
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