SECTION IV. I will not tax the reader's faith in modern science by insisting on the singular depression of the surface of Lombardy, which appears for many centuries to have taken place steadily 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. 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.