We have already seen how the stratification of the glacier arises,
accompanied by layers of dust and other material foreign to the glacier,
and how blue bands of compact ice may be formed parallel to the surface
of these strata. We have also seen how the horizontality of these strata
may be modified by pressure till they assume a position within the mass
of the glacier, varying from a slightly oblique inclination to a
vertical one. Now, while the position of the strata becomes thus altered
under pressure, other changes take place in the constitution of the ice
itself.
Before attempting to explain how these changes take place, let us
consider the facts themselves. The mass of the glacial ice is traversed
by thin bands of compact blue ice, these bands being very numerous along
the margins of the glacier, where they constitute what Dr. Tyndall calls
marginal structure, and still more crowded along the line upon which two
glaciers unite, where he has called it longitudinal structure. In the
latter case, where the extreme pressure resulting from the junction of
two glaciers has rendered the strata nearly vertical, these blue bands
follow their trend so closely that it is difficult to distinguish one
from the other.
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