only occur near the shore, while farther from land, and in deeper
water, finer sand and broken shells are spread out over the bottom.
Still farther out, the finest mud and ooze are alone met with. Mr.
Austen observes that this rule holds good in every part of the English
Channel examined by him. He also informs us, that where the tidal
current runs rapidly in what are called "races," where surface
undulations are perceived in the calmest weather, over deep banks, the
discoloration of the water does not arise from the power of such a
current to disturb the bottom at a depth of 40 or 80 fathoms, as some
have supposed. In these cases, a column of water sometimes 500 feet in
height, is moving onwards with the tide clear and transparent above,
while the lower portion holds fine sediment in suspension (a fact
ascertained by soundings), when suddenly it impinges upon a bank, and
its height is reduced to 300 feet. It is thus made to boil up and flow
off at the surface, a process which forces up the lower strata of water
charged with fine particles of mud, which in their passage from the
coast had gradually sunk to a depth of 300 feet or more.[464]

One important character in the formations produced by currents is, the
immense extent over which they may be the means of diffusing homogeneous
mixtures, for these are often coextensive with a great line of coast;
and, by comparison with their deposits, the deltas of rivers must shrink
into significance. In the Mediterranean, the same current which is
rapidly destroying many parts of the African coast, between the Straits
of Gibraltar and the Nile, checks also the growth of the delta of the
Nile, and drifts the sediment of that great river to the eastward. To
this source may be attributed the rapid accretions of land on parts of
the Syrian shores where rivers do not enter.

Among the greatest deposits now in progress, and of which the
distribution is chiefly determined by currents, we may class those
between the mouths of the Amazon and the southern coast of North
America. Captain Sabine found that the equatorial current before
mentioned (p. 292) was running with the rapidity of four miles an hour
where it crosses the stream of the Amazon, which river preserves part of
its original impulse, and has its waters not wholly mingled with those
of the ocean at the distance of 300 miles from its mouth.[465] The
sediment of the Amazon is thus constantly carried to the northwest as
far as to