water can support and carry along sand and earth, which in still water
would quickly sink to the bottom; and the more rapid the movement of
the water, the greater is the weight it is able to bear.

This is plainly to be seen in the case of a mountain torrent. As it
foams fiercely through its rocky bed it bears along, not only mud and
sand and gravel, but stones and even small rocks, grinding the latter
roughly together till they are gradually worn away, first to rounded
pebbles, then to sand, and finally to mud. The material thus swept
away by a stream, ground fine, and carried out to sea--part being
dropped by the way on the river-bed--is called _detritus_, which
simply means _worn-out_ material.

[Illustration: A MOUNTAIN TORRENT.]

The tremendous carrying-power of a mountain torrent can scarcely be
realized by those who have not observed it for themselves. I have seen
a little mountain-stream swell in the course of a heavy thunderstorm
to such a torrent, brown and turbid with earth torn from the
mountainside, and sweeping resistlessly along in its career a shower
of stones and rock-fragments. That which happens thus occasionally
with many streams is more or less the work all the year round of many
more.

As the torrent grows less rapid, lower down in its course, it ceases
to carry rocks and stones, though the grinding and wearing away of
stones upon the rocky bed continues, and coarse gravel is borne still
upon its waters. Presently the widening stream, flowing yet more
calmly, drops upon its bed all such coarser gravel as is not worn away
to fine earth, but still bears on the lighter grains of sand. Next the