The general appearance of the veined structure may be thus briefly
described:--The ice of glaciers, especially midway between their
mountain-sources and their inferior extremities, is of a whitish hue,
caused by the number of small air-bubbles which it contains, and which,
no doubt, constitute the residue of the air originally entrapped in the
interstices of the snow from which it has been derived. Through the
general whitish mass, at some places, innumerable parallel veins of
clearer ice are drawn, which usually present a beautiful blue colour,
and give the ice a laminated appearance. The cause of the blueness is,
that the air-bubbles, distributed so plentifully through the general
mass, do not exist in the veins, or only in comparatively small numbers.