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. 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 color 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.

Sec. V. 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 neighborhood 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, shallow
lakelets glitter among its irregularly exposed fields of seaweed. In the
midst of the largest of these, increased in importance by the confluence
of several large river channels t