shingle brought down during the flood season. A
portion of the pebbles are seen in some localities, as near Nice, to
form beds of shingle along the shore, but the greater part are swept
into a deep sea. The small progress made by the deltas of minor rivers
on this coast need not surprise us, when we recollect that there is
sometimes a depth of two thousand feet at a few hundred yards from the
beach, as near Nice. Similar observations might be made respecting a
large proportion of the rivers in Sicily, and among others, respecting
that which, immediately north of the port of Messina, hurries annually
vast masses of granitic pebbles into the sea.

_Constant interchange of land and sea._--I may here conclude my remarks
on deltas, observing that, imperfect as is our information of the
changes which they have undergone within the last three thousand years,
they are sufficient to show how constant an interchange of sea and land
is taking place on the face of our globe. In the Mediterranean alone,
many flourishing inland towns, and a still greater number of ports, now
stand where the sea rolled its waves since the era of the early
civilization of Europe. If we could compare with equal accuracy the
ancient and actual state of all the islands and continents, we should
probably discover that millions of our race are now supported by lands
situated where deep seas prevailed in earlier ages. In many districts
not yet occupied by man, land animals and forests now abound where ships
once sailed; and, on the other hand, we shall find, on inquiry, that
inroads of the ocean have been no less considerable. When to these
revolutions, produced by aqueous causes, we add analogous changes
wrought by igneous agency, we shall, perhaps, acknowledge the justice of
the conclusion of Aristotle, who declared that the whole land and sea on
our globe periodically changed places.[377]

CHAPTER XIX.

DESTROYING AND TRANSPORTING EFFECTS OF TIDES AND CURRENTS.

Difference in the rise of tides--Lagullas and Gulf
currents--Velocity of currents--Causes of currents--Action of the
sea on the British coast--Shetland Islands--Large blocks
removed--Isles reduced to clusters of rocks--Orkney isles--Waste of
East coast of Scotland--and East coast of England--Waste of the
cliffs of Holderness, Norfolk, and Suffolk--Sand-dunes, how far
chronometers--Silting up of estuaries--Yarmouth estuary--Suffolk
coast--Dunwich--Essex co