The two shores of the Adriatic are totally different in their natural characteristics; the western being almost islandless and destitute of harbours, while the eastern is fringed by an almost continuous chain of islands and possesses several magnificent harbours which communicate with the open sea by narrow channels easily fortified, the rocks rising precipitously from the water along the greater part of the coast, whereas on the Italian side there is an equally continuous strip of alluvial plain between the foothills and the sea. The Adriatic was once bounded by a kind of ridge stretching from Monte Gargano to Albania. North of this line the depth is much less than in the Ionian Sea. When the surface of the earth sank, the Dalmatian islands were formed by the letting in of the sea. The depth near Parenzo is about 120 ft.; in the Quarnero, near Fiume, 195 ft.; between Cherso and Arbe, 335 ft.; and south-west of the island Zuri (some 24 miles from the mainland), about 700 ft. Depths as great as 335 ft. to 490 ft. are, however, not very common within nine miles of the mainland. In the Bocche di Cattaro the depth near the mouth is 165 ft., but half a mile west of the Punta d'Ostro, 335 ft. North of the line from Monte Gargano to Pelagosa, Cazza, and Curzola it is never as much as 780 ft.; south-east of this line the bottom sinks so much that between Cattaro and Brindisi it reaches a depth of over 5,000 ft. The tide is scarcely perceptible, and the currents are very slight. The land is still sinking, as is proved by the Roman sarcophagi found beneath the water at Vranjic and the submerged roads between Aquileia and Grado; while there are records of the destruction of ancient towns from sudden subsidences, as that of Cissa, near Rovigno. The subsidence has been calculated as about a yard in 1,000 years. Cluverius proves from Ptolemy that in antiquity the name Adriatic only applied to that part of the gulf which lay to the north of a line between Monte Gargano and Durazzo. A passage of Strabo, describing the people of Epirus, runs: "The Adriatic being ended, the Ionian commences, the first shore of which is in the neighbourhood of Epidamnus and Apollonia." When Venice conquered Durazzo the limits of the Adriatic were extended, and it was thenceforth called the Gulf of Venice. In 1859 the almost incredible fact is recorded that it was frozen for several days!