nes and ice upon the surface of the glacier. The mass
thus brought down the steep was estimated at about three thousand
cubic yards, of which probably the fiftieth part was rock material. An
avalanche of this volume is unusual, and the proportion of stony
matter borne down exceptionally great; but by these sudden motions of
the frozen water a large part of the snow deposited above the zone of
complete melting is taken to the lower valleys, where it may disappear
in the summer season, and much of the erosion accomplished in the
mountains is brought about by these falls.

In all Alpine regions avalanches are among the most dreaded accidents.
Their occurrence, however, being dependent upon the shape of the
surface, it is generally possible to determine in an accurate way the
liability of their happening in any particular field. The Swiss take
precaution to protect themselves from their ravages as other folk do
to procure immunity from floods. Thus the authorities of many of the
mountain hamlets maintain extensive forests on the sides of the
villages whence the downfall may be expected, experience having shown
that there is no other means so well calculated to break the blow
which these great snowfalls can deliver, as thick-set trees which,
though they are broken down for some distance, gradually arrest the
stream.

As long as the region occupied by permanent snow is limited to sharp
mountain peaks, relief by the precipitation of large masses to the
level below the snow line is easily accomplished, but manifestly this
kind of a discharge can only be effective from a very small field.
Where the relief is not brought about by these tumbles of snow,
another mode of gravitative action accomplishes the result, though in
a more roundabout way, through the mechanism of glaciers.

We have already noted the fact that the winter's snow upon our
hillsides undergoes a movement in the direction of the slope. What we
have now to describe in a rather long story concerning glaciers rests
upon movements of the same nature, though they are in certain features
peculiarly dependent on the continuity of the action from year to
year. It is desirable, however, that the student should see that there
is at the foundation no more mystery in glacial motion than there is
in the gradual descent of the snow after it has lain a week on a
hillside. It is only in the scale and continuity of the action that
the greatest glacial envelope exceeds those of