is accustomed to
behold in the materials laid down by ordinary rivers, but has a
whitish colour. Further study will reveal the fact that the difference
is due to the lack of oxidation in the case of the glacial detritus.
River muds forming slowly and during long-continued exposure to the
action of the air have their contained iron much oxidized, which gives
them a part of their darkened appearance. Moreover, they are somewhat
coloured with decayed vegetable matter. The waste from beneath the
glacier has been quickly separated from the bed rock, all the faces of
the grains are freshly fractured, and there is no admixture of organic
matter. The faces of the particles thus reflect light in substantially
the same way as powdered glass or pulverized ice, and consequently
appear white.

A little observation will show the student that this very muddy
character of waters emerging from beneath the glacier is essentially
peculiar to such streams as we have described. Ascending any of the
principal valleys of Switzerland, he may note that some of the streams
flow waters which carry little sediment even in times when they are
much swollen, while others at all seasons have the whitish colour. A
little further exploration, or the use of a good map, will show him
that the pellucid streams receive no contributions of glacial water,
while those which look as if they were charged with milk come, in part
at least, from the ice arches. From some studies which the writer has
made in Swiss valleys, it appears that the amount of erosion
accomplished on equal areas of similar rock by the descent of the
waters in the form of a glacier or in that of ordinary torrents
differs greatly. Moving in the form of ice, or in the state of
ice-confined streams, the mass of water applies very many times as
much of its energy of position to grinding and bearing away the rocks
as is accomplished where the water descends in its fluid state.

The effect of the intense ice action above noted is rapidly to wear
away the rocks of the valley in which the glacier is situated. This
work is done not only in a larger measure but in a different way from
that accomplished by torrents. In the case of the latter, the stream
bed is embarrassed by the rubbish which comes into it; only here and
there can it attack the bed rock by forcing the stones over its
surface. Only in a few days of heavy rain each year is its work at all
effective; the greater part of the energy of p