consequence which is performed. It is there that the underrunning
streams become charged with sediment, which, as we have noted, they
bear in surprising quantities, and it is therefore in this section of
the valley that the impress of the ice work is the strongest. Its
effect is not only to widen the valley and deepen it, but also to
advance the deep section farther up the stream and its tributaries.
The step in the stream beds which we find at the _seracs_ appears to
mark the point in the course of the glacier where, owing to the
falling of stones to its base, as well as to its swifter movements and
the firmer state of the ice, it does effective wearing.

There are many other features connected with glaciers which richly
repay the study of those who have a mind to explore in the manner of
the physicist interested in ice actions the difficult problems which
they afford; but as these matters are not important from the point of
view of this work, no mention of them will here be made. We will now
turn our attention to that other group of glaciers commonly termed
continental, which now exist about either pole, and which at various
times in the earth's history have extended far toward the equator,
mantling over vast extents of land and shallow sea. The difference
between the ice streams of the mountains and those which we term
continental depends solely on the areas of the fields and the depth of
the accumulation. In an ordinary Alpine region the _névé_ districts,
where the snow gathers, are relatively small. Owing to the rather
steep slopes, the frozen water is rapidly discharged into the lower
valleys, where it melts away. Both in the _névé_ and in the distinct
glacier of the lower grounds there are, particularly in the latter,
projecting peaks, from which quantities of stone are brought down by
avalanches or in ordinary rock falls, so that the ice is abundantly
supplied with cutting tools, which work from its surface down to its