hich cannot be answered in the
present state of science.

The Straits of Gibraltar are said to become gradually wider by the
wearing down of the cliffs on each side at many points; and the current
sets along the coast of Africa, so as to cause considerable inroads in
various parts, particularly near Carthage. Near the Canopic mouth of the
Nile, at Aboukir, the coast was greatly devastated in the year 1784,
when a small island was nearly consumed. By a series of similar
operations, the old site of the cities of Nicropolis, Taposiris, Parva
and Canopus, have become a sand-bank.[460]

CHAPTER XXI.

REPRODUCTIVE EFFECTS OF TIDES AND CURRENTS.

Estuaries, how formed--Silting up of estuaries does not compensate
the loss of land on the borders of the ocean--Bed of the German
Ocean--Composition and extent of its sand-banks--Strata deposited by
currents in the English channel--On the shores of the
Mediterranean--At the mouths of the Amazon, Orinoco, and
Mississippi--Wide area over which strata may be formed by this
cause.

From the facts enumerated in the last chapter, it appears that on the
borders of the ocean, currents and tides co-operating with the waves of
the sea are most powerful instruments in the destruction and
transportation of rocks; and as numerous tributaries discharge their
alluvial burden into the channel of one great river, so we find that
many rivers deliver their earthy contents to one marine current, to be
borne by it to a distance, and deposited in some deep receptacle of the
ocean. The current, besides receiving this tribute of sedimentary matter
from streams draining the land, acts also itself on the coast, as does a
river on the cliffs which bound a valley. Yet the waste of cliffs by
marine currents constitutes on the whole a very insignificant portion of
the denudation annually effected by aqueous causes, as I shall point out
in the sequel of this chapter (p. 339).

In inland seas, where the tides are insensible, or on those parts of the
borders of the ocean where they are feeble, it is scarcely possible to
prevent a harbor at a river's mouth from silting up; for a bar of sand
or mud is formed at points where the velocity of the turbid river is
checked by the sea, or where the river and a marine current neutralize
each other's force. For the current, as we have seen, may, like the
river, hold in suspension a large quantity of sediment, or, co-operating
with th