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, 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 towards one of the openings in the sea bank, the city of Venice itself is built, on a crowded cluster of islands; the various plots of higher ground which appear to the north and south of this central cluster, have at different periods been also thickly inhabited, and now bear, according to their size, the remains of cities, villages, or isolated convents and churches, scattered among spaces of open ground, partly waste and encumbered by ruins, partly under cultivation for the supply of the metropolis.

The average rise and fall of the tide is about three feet (varying considerably with the seasons); but this fall, on so flat a shore, is enough to cause continual movement in the waters, and in the main canals to produce a reflux which frequently runs like a mill stream.  At high water no land is visible for many miles to the north or south of Venice, except in the form of small islands crowned with towers or gleaming with villages:  there is a channel, some three miles wide, between the city and the mainland, and some mile and a half wide between it and the sandy breakwater called the Lido, which divides the lagoon