und_. Passage over it is
often one of the most difficult feats to accomplish which the Alpine
explorer has to undertake. In fact, the very appearance of the
surface, which is that of a river with continuous down slopes, is
sufficient evidence that the mass is slowly flowing toward the
valleys. Following it down, we almost always come to a place where it
passes from the upper valleys to the deeper gorges which pierce the
skirts of the mountain. In going over this projection the mass of
snow-ice breaks to pieces, forming a crowd of blocks which march down
the slope with much more speed than they journeyed when united in the
higher-lying fields. In this condition and in this part of the
movement the snow-ice forms what are called the _seracs_, or curds, as
the word means in the French-Swiss dialect. Slipping and tumbling
down the steep slope on which the _seracs_ develop, the ice becomes
broken into bits, often of small size. These fragments are quickly
reknit into the body of ice, which we shall hereafter term the
glacier, and in this process the expulsion of the air goes on more
rapidly than before, and the mass assumes a more transparent icelike
quality.

The action of the ice in the pressures and strains to which it is
subjected in joining the main glacier and in the further part of its
course demand for their understanding a revision of those notions as
to rigidity and plasticity which we derive from our common experience
with objects. It is hard to believe that ice can be moulded by
pressure into any shape without fracturing, provided the motion is
slowly effected, while at the same time it is as brittle as ice to a
sudden blow. We see, however, a similar instance of contrasted
properties in the confection known as molasses candy, a stick of which
may be indefinitely bent if the flexure is slowly made, but will fly
to pieces like glass if sharply struck. Ice differs from the sugary
substance in many ways; especially we should note that while it may be
squeezed into any form, it can not be drawn out, but fractures on the
application of a very slight tension. The conditions of its movement
we will inquire into further on, when we have seen more of its action.

Entering on the lower part of its course, that where it flows into the
region below the snow line, the ice stream is now confined between the
walls of the valley, a channel which in most cases has been shaped
before the ice time, by a mountain torrent, or perha