There is another feature of glacial structure, intimately connected, by
similarity of position and aspect, with the stratification, which has
greatly perplexed the students of glacial phenomena. I allude to the
so-called blue bands, or bands of infiltration, also designated as
veined structure, ribboned or laminated structure, marginal structure,
and longitudinal structure. The difficulty lies, I believe, in the fact
that two very distinct structures, that of the stratification and the
blue bands, are frequently blended together in certain parts of the
glacier in such a manner as to seem identical, while elsewhere the one
is prominent and the other subordinate, and _vice versa_. According to
their various opportunities of investigation, observers have either
confounded the two, believing them to be the same, or some have
overlooked the one and insisted upon the other as the prevailing
feature, while that very feature has been absolutely denied again by
others who have seen its fellow only, and taken that to be the only
prominent and important fact in this peculiar structural character of
the ice.

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. It will be seen, on referring to the wood-cut on page
758, where the close, uniform, vertical lines represent the t