will depend on the size and weight of the pieces how long they
will be in falling through the water. If you take a handful of
gravel and throw it into a glass full of water, you will notice
that the stones in it will fall to the bottom at once, the grit
and coarse sand will take longer in sinking, and lastly, the fine
sand will be an hour or two in settling down, so that the water
becomes clear. Now, suppose that this gravel were sinking in the
water of a river. The stones would be buoyed up as long as the
river was very full and flowed very quickly, but they would drop
through sooner than the coarse sand. The coarse sand in its turn
would begin to sink as the river flowed more slowly, and would
reach the bottom while the fine sand was still borne on. Lastly,
the fine sand would sink through very, very slowly, and only
settle in comparatively still water.

From this it will happen that stones will generally lie near to
the bottom of torrents at the foot of the banks from which they
fall, while the gravel will be carried on by the stream after it
leaves the mountains. This too, however, will be laid down when
the river comes into a more level country and runs more slowly.
Or it may be left together with the finer mud in a lake, as in
the lake of Geneva, into which the Rhone flows laden with mud
and comes out at the other end clear and pure. But if no lake
lies in the way the finer earth will still travel on, and the
river will take up more and more as it flows, till at last it
will leave this too on the plains across which it moves
sluggishly along, or will deposit it at its mouth when it joins
the sea.