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.