ast--Estuary of the Thames--Goodwin
Sands--Coast of Kent--Formation of the Straits of Dover--South coast
of England--Sussex--Hants--Dorset--Portland--Origin of the Chesil
Bank--Cornwall--Coast of Brittany.

Although the movements of great bodies of water, termed tides and
currents, are in general due to very distinct causes, their effects
cannot be studied separately; for they produce, by their joint action,
aided by that of the waves, those changes which are objects of
geological interest. These forces may be viewed in the same manner as we
before considered rivers, first, as employed in destroying portions of
the solid crust of the earth and removing them to other places;
secondly, as reproductive of new strata.

_Tides._--It would be superfluous at the present day to offer any
remarks on the cause of the tides. They are not perceptible in lakes or
in most inland seas; in the Mediterranean even, deep and extensive as is
that sea, they are scarcely sensible to ordinary observation, their
effects being quite subordinate to those of the winds and currents. In
some places, however, as in the Straits of Messina, there is an ebb and
flow to the amount of two feet and upwards; at Naples and at the
Euripus, of twelve or thirteen inches; and at Venice, according to
Rennell, of five feet.[378] In the Syrtes, also, of the ancients, two
wide shallow gulfs, which penetrate very far within the northern coast
of Africa, between Carthage and Cyrene, the rise is said to exceed five
feet.[379]

In islands remote from any continent, the ebb and flow of the ocean is
very slight, as at St. Helena, for example, where it is rarely above
three feet.[380] In any given line of coast, the tides are greatest in
narrow channels, bays, and estuaries, and least in the intervening
tracts where the land is prominent. Thus, at the entrance of the estuary
of the Thames and Medway, the rise of the spring tides is eighteen feet;
but when we follow our eastern coast from thence northward, towards
Lowestoff and Yarmouth, we find a gradual diminution, until at the
places last mentioned, the highest rise is only seven or eight feet.
From this point there begins again to be an increase, so that at Comer,
where the coast again retires towards the west, the rise is sixteen
feet; and towards the extremity of the gulf called "the Wash," as at
Lynn and in Boston Deeps, it is from twenty-two to twenty-four feet, and
in some extraordinary cases twen