The lagoons along the Adriatic have decreased in extent in the course of centuries, and whilst new lagoons are being formed, the old ones are gradually being converted into dry land. The old maps of the Venetian littoral differ essentially from our modern ones, and yet all the vast changes they indicate have been wrought in the course of a few centuries. The swamps of Caorle, between the Piave and the Gulf of Trieste, have changed to an extent which prevents us from restoring the ancient topography of the country; and if the lagoons of Venice and Chioggia exhibit a certain permanence of contour, this is only on account of the incessant interference of man. The ancient lagoon of Brondolo has been dry land since the middle of the sixteenth century. The large lagoon of Comacchio, to the south of the Po, has been cut up into separate portions by alluvial embankments formed by the agency of rivers and torrents. For the most part it consists now of valli, or alluvial deposits, but there still remain a few profound cavities, or chiari, which the rivers have not yet succeeded in filling up. Formerly these {203} lagoons extended far to the south in the direction of Ravenna, and, according to Strabo and other ancient writers, that ancient city once occupied a site very much like that of Venice or Chioggia in our own days.

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Fig. 64.—BEECH AND PINE WOODS OF RAVENNA.

Scale 1 : 2,470,000.

There can be no doubt that these lagoons were anciently separated from the Adriatic by a narrow strip of land over 120 miles in length, and similar to what we still meet with on the coasts of Carolina and of the Brazils. This ancient barrier still exists in the lidi of Venice and Comacchio, which are pierced at intervals, admitting the vivifying floods of the open sea. Elsewhere the traces of this ancient beach must be looked for on the mainland. The low delta of the Po is traversed from north to south by a range of dunes constituting the continuation of the lidi of Venice, and extending into the swamps of Comacchio, where they form a natural embankment running parallel with the coast. These dunes, between the Adige and Cervia, are covered with sombre pine woods, replaced here and there by oaks. The underwood mainly consists of hawthorns and juniper-trees, and wild boars still haunt it.

No sooner have the lagoons protected by these barriers been converted into dry land than the sea seizes upon the sand, and forms it into new curvilinear barriers similar to the former ones. The principal range of dunes to the east of Ravenna, which is about 20 miles in length, and varies in width between 50 and 3,300 yards, has thus two other ranges of dunes running parallel with it, one of them being still in course of formation. Signor {204} Pareto has estimated the annual advance of the land at 7½ feet, and at much more near the mouths of rivers.

The sea thus marks by a series of barriers its successive recoils. Sometimes, however, the sea gains upon the land in consequence of a gradual subsidence of the Venetian shore, the cause of which has not yet been elucidated. Thus the gravel bank of Cortellazzo, opposite the swamps of Caorle, appears to have anciently been a lido which has sunk nearly 70 feet below the level of the sea. The islands which fringed the littoral of Aquileja during the Middle Ages have almost wholly disappeared. In the time of the Romans these islands were populous; there were forests and fields upon them, and the inhabitants built ships. The chronicles of the Middle Ages tell us that the Doge of Venice and the Patriarch of Aquileja hunted stags and wild boars upon them, much to the scandal of the inhabitants. At the present day the dunes which of yore protected these islands have almost wholly disappeared, the forests have been supplanted by reeds, and Grado is the only place on the littoral which may still boast of a certain number of inhabitants. Piers, walls, mosaic pavements, and even stones bearing inscriptions, which are found occasionally at the bottom of the sea or of swamps, prove that the mainland was formerly more extensive there. Farther to the west the littoral of Venice bears evidence of a similar subsidence. Artesian wells sunk in the city of the lagoons have led to the discovery of four beds of turf, the deepest no less than 420 feet below the level of the sea. The subterranean church of St. Mark has within historical times been converted into a submarine church, and streets and buildings are gradually sinking beneath the waters of the lagoons. If it were not for the alluvium brought down by the rivers, the sea would continually encroach upon the land. Ravenna, too, participates in this subsidence, which Signor Pareto estimates to amount to 0·60 inch in the course of a century.

Amongst the geological agents constantly at work to modify the surface of the earth, the rivers and torrents irrigating the plain lying at the foot of the Alps are the most active, and no other country of Europe, Holland alone excepted, can compare in this respect with Northern Italy.