teadily and continually; the main fact
with which we have to do is the gradual transport, by the Po and its
great collateral rivers, of vast masses of the finer sediment to the
sea. The character of the Lombardic plains is most strikingly expressed
by the ancient walls of its cities, composed for the most part of large
rounded Alpine pebbles alternating with narrow courses of brick; and
was curiously illustrated in 1848, by the ramparts of these same
pebbles thrown up four or five feet high round every field, to check
the Austrian cavalry in the battle under the walls of Verona.[142] The
finer dust among which these pebbles are dispersed is taken up by the
rivers, fed into continual strength by the Alpine snow, so that,
however pure their waters may be when they issue from the lakes at the
foot of the great chain, they become of the colour and opacity of clay
before they reach the Adriatic; the sediment which they bear is at once
thrown down as they enter the sea, forming a vast belt of low land
along the eastern coast of Italy. The powerful stream of the Po of
course builds forward the fastest; on each side of it, north and south,
there is a tract of marsh, fed by more feeble streams, and less liable
to rapid change than the delta of the central river. In one of these
tracts is built RAVENNA, and in the other VENICE.

What circumstances directed the peculiar arrangement of this great belt
of sediment in the earliest times, it is not here the place to inquire.
It is enough for us to know that from the mouths of the Adige to those
of the Piave there stretches, at a variable distance of from three to
five miles from the actual shore, a bank of sand, divided into long
islands by narrow channels of sea. The space between this bank and the
true shore consists of the sedimentary deposits from these and other
rivers, a great plain of calcareous mud, covered, in the neighbourhood
of Venice, by the sea at high water, to the depth in most places of a
foot or a foot and a half, and nearly everywhere exposed at low tide,
but divided by an intricate network of narrow and winding channels,
from which the sea never retires. In some places, according to the run
of the currents, the land has risen into marshy islets, consolidated,
some by art, and some by time, into ground firm enough to be built
upon, or fruitful enough to be cultivated: in others, on the contrary,
it has not reached the sea level; so that, at the average low water,
sh