tream is swollen by the surface waters descending through
the crevices and the _moulins_ which come from them, so that the
outflow often forms a considerable river, and thus excavates in the
ice a large or at least a long cavern, the base of which is the bed
rock. In the autumn, when the superficial melting ceases, this gallery
can often be penetrated for a considerable distance, and affords an
excellent way to the secrets of the under ice. The observer may here
see quantities of the rock material held in the grip of the ice, and
forced to a rude journey over the bare foundation stones. Now and then
he may find the glacial mass in large measure made up of stones, the
admixture extending many feet above the bottom of the cavern, perhaps
to the very top of the arch. He may perchance find that these stones
are crushing each other where they are in contact. The result will be
brought about by the difference in the rate of advance of the ice,
which moves the faster the higher it is above the surface over which
it drags, and thus forces the stones on one level over those below.
Where the waters of the subglacial stream have swept the bed rock
clean of _debris_ its surface is scored, grooved, and here and there
polished in a manner which is accomplished only by ice action, though
some likeness to it is afforded where stones have been swept over for
ages by blowing sand. Here and there, often in a way which interrupts
the cavern journey, the shrunken stream, unable to carry forward the
_debris_, deposits the material in the chamber, sometimes filling the
arch so completely that the waters are forced to make a detour. This
action is particularly interesting, for the reason that in regions
whence glaciers have disappeared the deposits formed in the old ice
arches often afford singularly perfect moulds of those caverns which
were produced by the ancient subglacial streams. These moulds are
termed _eskers_.

If the observer be attentive, he will note the fact that the waters
emerging from beneath the considerable glacier are very much charged
with mud. If he will take a glass of the water at the point of escape,
he will often find, on permitting it to settle, that the sediment
amounts to as much as one twentieth of the volume. While the greater
part of this detritus will descend to the bottom of the vessel in the
course of a day, a portion of it does not thus fall. He may also note
that this mud is not of the yellowish hue which he