posit which at first clung to the surface on which it lay becomes so
heavy that it tends to slide down the slope; a trifling action, the
slipping, indeed, of a single flake, may begin the movement, which at
first is gradual and only involves a little of the snow. Gathering
velocity, and with the materials heaped together from the junction of
that already in motion with that about to be moved, the avalanche in
sliding a few hundred feet down the slope may become a deep stream of
snow-ice, moving with great celerity. At this stage it begins to break
off masses of ice from the glaciers over which it may flow, or even to
move large stones. Armed with these, it rends the underlying earth.
After it has flowed a mile it may have taken up so much earth and
material that it appears like a river of mud. Owing to the fact that
the energy which bears it downward is through friction converted into
heat, a partial melting of the mass may take place, which converts it
into what we call slush, or a mixture of snow and water. Finally, the
torrent is precipitated into the bottom of a valley, where in time the
frozen water melts away, leaving only the stony matter which it bore
as a monument to show the termination of its flow.

It was the good fortune of the writer to see in the Swiss Oberland one
very great avalanche, which came from the high country through a
descent of several thousand feet to the surface of the Upper
Grindelwald Glacier. The first sign of the action was a vague tremor
of the air, like that of a great organ pipe when it begins to vibrate,
but before the pulsations come swiftly enough to make an audible note.
It was impossible to tell when this tremor came, but the wary guide,
noting it before his charge could perceive anything unusual, made
haste for the middle of the glacier. The vibration swelled to a roar,
but the seat of the sound amid the echoing cliffs was indeterminable.
Finally, from a valley high up on the southern face of the glacier,
there leaped forth first a great stone, which sprang with successive
rebounds to the floor of ice. Then in succession other stones and
masses of ice which had outrun the flood came thicker and thicker,
until at the end of about thirty seconds the steep front of the
avalanche appeared like a swift-moving wall. Attaining the cliffs, it
shot forth as a great cataract, which during the continuance of the
flow--which lasted for several minutes--heaped a great mound of
commingled sto