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 stream 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