e gneiss rock.--Other large
boulders intervened, and they had not observed whether it was gone.
They trembled to think of such a thing.

The hours passed; and still they dared not descend to the glacier.
Night came on, and they still stood upon their narrow perch. They
hungered, but it would have been of no use to go down to the cold icy
surface. That would not have satisfied their appetite.

All night long they remained standing upon the narrow ledge; now on one
foot, now on the other, now resting their backs against the granite
wall, but all night, without closing an eye in sleep. The dread of the
capricious ice kept them on their painful perch.

They could bear it no longer. With the first light of morning they
determined upon descending.

The ice had remained firm during the night. No farther noises had been
heard. They gradually recovered confidence; and as soon as the day
began to break, all three left the ledge, and betook themselves once
more to the glacier.

At first they kept close to the cliff; but, after a while, ventured out
far enough to get a view of the ravine below.

Caspar mounted upon a rocky boulder that lay upon the surface of the
glacier. From the top of this he could see over the others. _The
crevasse was many yards wide. The bridge-rock was gone_!

CHAPTER TWENTY THREE.

THE PASS.

The philosophy of the movement of glaciers is but ill understood, even
by the most accomplished geologists. It is supposed that the under
surface of these great icy masses is detached from the ground by the
thaw which continually takes place there, caused by the radiating heat
of the earth. Water is also an agent in loosening their hold; for it is
well-known that currents of water--sometimes large streams,--run under
the glaciers. The icy mass thus detached, and resting on an inclined
surface, is carried down by its own weight.

Sometimes only a very small portion of a glacier moves, causing a
fissure above the part that has given way; and at other times these
fissures are closed up, by the sliding of that portion next above them.
An unusually hot summer produces these effects upon the glacier ice,
combined with the falling of avalanches, or mountain slides, which, with
their weight, serve to impel the icy mass downwards.

The weight of our three hunters was but as a feather, and could have had
no effect in giving motion to the glacier; but it is possible that the
gneiss rock was just upon