the mouths of the Orinoco, and an immense tract of swamp is
formed along the coast of Guiana, with a long range of muddy shoals
bordering the marshes, and becoming converted into land.[466] The
sediment of the Orinoco is partly detained, and settles near its mouth,
causing the shores of Trinidad to extend rapidly, and is partly swept
away into the Carribean Sea by the Guinea current. According to
Humboldt, much sediment is carried again out of the Carribean Sea into
the Gulf of Mexico.

It should not be overlooked that marine currents, even on coasts where
there are no large rivers, may still be the agents of spreading not only
sand and pebbles, but the finest mud, far and wide over the bottom of
the ocean. _For several thousand miles_ along the western coast of South
America, comprising the larger parts of Peru and Chili, there is a
perpetual rolling of shingle along the shore, part of which, as Mr.
Darwin has shown, are incessantly reduced to the finest mud by the
waves, and swept into the depths of the Pacific by the tides and
currents. The same author however has remarked that, notwithstanding the
great force of the waves on that shore, all rocks 60 feet under water
are covered by sea-weed, showing that the bed of the sea is not denuded
at that depth, the effects of the winds being comparatively superficial.

In regard to the distribution of sediment by currents it may be
observed, that the rate of subsidence of the finer mud carried down by
every great river into the ocean, or of that caused by the rolling of
the waves upon a shore, must be extremely slow; for the more minute the
separate particles of mud, the slower will they sink to the bottom, and
the sooner will they acquire what is called their terminal velocity. It
is well known that a solid body, descending through a resisting medium,
falls by the force of gravity, which is constant, but its motion is
resisted by the medium more and more as its velocity increases, until
the resistance becomes sufficient to counteract the farther increase of
velocity. For example, a leaden ball, one inch diameter, falling through
air of density as at the earth's surface, will never acquire greater
velocity than 260 feet per second, and, in water, its greatest velocity
will be 8 feet 6 inches per second. If the diameter of the ball were
1/100 of an inch, the terminal velocities in air would be 26 feet, and
in water .86 of a foot per second.

Now, every chemist is familiar w