When the eye falls casually on a map of Europe, there is no feature by which it is more likely to be arrested than the strange sweeping loop formed by the junction of the Alps and Apennines, and enclosing the great basin of Lombardy.  This return of the mountain chain upon itself causes a vast difference in the character of the distribution of its debris on its opposite sides.  The rock fragments and sediment which the torrents on the north side of the Alps bear into the plains are distributed over a vast extent of country, and, though here and there lodged in beds of enormous thickness, soon permit the firm substrata to appear from underneath them; but all the torrents which descend from the southern side of the High Alps and from the northern slope of the Apennines meet concentrically in the recess or mountain-bay which the two ridges enclose; every fragment which thunder breaks out of their battlements, and every grain of dust which the summer rain washes from their pastures, is at last laid at rest in the blue sweep of the Lombardic plain; and that plain must have risen within its rocky barriers as a cup fills with wine, but for two contrary influences which continually depress, or disperse from its surface, the accumulation of the ruins of ages.

I will not tax the reader’s faith in modern science by insisting on this 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.