heavier grains the motion would have to
be quicker to keep them supported in the water.

Now it is just the same thing with our rivers and streams. Running
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
slackening speed makes even the sand too heavy a weight, and that in
turn falls to line the river-bed, while the now broad and placid
stream carries only the finer particles of mud suspended in its
waters. Soon it reaches the ocean, and the flow being there checked by
the incoming ocean-tide, even the mud can no longer be held up, and it
also sinks slowly in the shallows near the shore, forming sometimes
broad mud-banks dangerous to the mariner.

This is the case only with smaller rivers. Where the stream is
stronger, the mud-banks are often formed much farther out at sea; and
more often still the river-detritus is carried away and shed over the
ocean