He
suggests that layers of water may be formed in the glacier at right
angles with the pressure, and pass into a state of solid ice upon the
removal of that pressure, the pressure being of course relieved in
proportion to the diminution in the body of the ice by compression. The
number of blue bands diminishes as we recede from the source of the
pressure,--few only being formed, usually at right angles with the
surfaces of stratification, in the middle of a glacier, half-way between
its sides. If they are caused by pressure, this diminution of their
number toward the middle of the glacier would be inevitable, since the
intensity of the pressure naturally fades as we recede from the motive
power.
Dr. Tyndall also alludes to another structure of the same kind, which he
calls transverse structure, where the blue bands extend in
crescent-shaped curves, more or less arched, across the surface of the
glacier. Where these do not coincide with the stratification, they are
probably formed by vertical pressure in connection with the unequal
movement of the mass.
With these facts before us, it seems to me plain that the primitive blue
bands arise with the stratification of the snow in the very first
formation of the glacier, while the secondary blue bands are formed
subsequently, in consequence of the onward progress of the glacier and
the pressure to which it is subjected.
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