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. These
observations of mine have the advantage over those of other observers,
that, while they embrace the whole extent of the glacier, transversely
as well as in its length, they cover a period of several successive
years, instead of being limited to summer campaigns and a few winter
observations. The consequence of this mode of progressing will be that
the straight lines drawn transversely across the surface of the glacier
above will be gradually changed to curved ones below. After a few years,
such a line will appear on the surface of the glacier like a crescent,
with the bow turned downward, within which, above, are other crescents,
less and less sharply arched up to the last year's line, which may be
again straight across the snow-field. (See the subjoined figure, which
represents a part of the glacier of the Lauter-Aar.)

[Illustration]

Thus the glacier records upon its surface its annual growth and
progress, and registers also the inequality in the rate of advance
between the axis and the sides.

But these are only surface-phenomena. Let us see what will be the effect
upon the internal structure. We must not forget, in considering the
changes taking place within glaciers, the shape of the valleys which
contain them. A glacier lies in a deep trough, and the tendency of the
mass will be to sink toward its deeper part, and to fold inward and
downward, if subjected to a strong lateral pressure,--that is, to dip
toward the centre and slope upward along the sides, following the scoop
of the trough. If, now, we examine the face of a transverse cut in the
glacier, we find it traversed by a number of lines, vertical in some
places, more or less oblique in others, and frequently these lin