of a glacier entrap a good deal of the morainal _debris_,
which falls through them to the bottom of the glacier. Smaller bits
are washed into the _moulin_, by the streams arising from the melting
ice, which is brought about by the warm sun of the summer, and
particularly by the warm rains of that season. On those glaciers
where, owing to the irregularity of the bottom over which the ice
flows, these fractures are very numerous, it may happen that all the
detritus brought upon the surface of the glacier by avalanches finds
its way to the floor of the ice.

Although it is difficult to learn what is going on at the under
surface of the glacier, it is possible directly and indirectly to
ascertain much concerning the peculiar and important work which is
there done. The intrepid explorer may work his way in through the
lateral fissures, and even with care safely descend some of the
fissures which penetrate the central parts of a shallow ice stream.
There, it may be at the depth of a hundred feet or more, he will find
a quantity of stones, some of which may be in size like to a small
house held in the body of the ice, but with one side resting upon the
bed rock. He may be so fortunate as to see the stone actually in
process of cutting a groove in the bed rock as it is urged forward by
the motion of the glacier. The cutting is not altogether in the fixed
material, for the boulder itself is also worn and scored in the work.
Smaller pebbles are caught in the space between the erratic and the
motionless rock and ground to bits. If in his explorations the student
finds his way to the part of the floor on which the waters of a
_moulin_ fall, he may have a chance to observe how the stones set in
motion serve to cut the bed rock, forming elongated potholes much as
in the case of ordinary waterfalls, or at the base of those shafts
which afford the beginnings of limestone caverns.

The best way to penetrate beneath the glacier is through the arch of
the stream which always flows from the terminal face of the ice river.
Even in winter time every large glacier discharges at its end a
considerable brook, the waters of which have been melted from the ice
in small part by the outflow of the earth's heat; mainly, however, by
the warmth produced in the friction of the ice on itself and on its
bottom--in other words, by the conversion of that energy of position,
of which we have often to speak, into heat. In the summer time this
subglacial s