Each of these basins is again subdivided, the one by Crete, the other by the two islands of Sardinia and Corsica. These various subdivisions of the Mediterranean differ in area, and still more in depth. The Sea of Azof almost deserves the name of “Swamp,” which was bestowed upon it by the ancients, for if a ship sinks in it the masts remain visible above the water. The Black Sea has a maximum depth of over 1,000 fathoms, but the narrow strait which joins it to the Sea of Marmara is shallower than many a European river. The cavity filled by the Sea of Marmara is far inferior to that of many an inland lake; and the Dardanelles, like the Bosphorus, are hardly wider than a river. In the Archipelago and the eastern basin of the Mediterranean proper the depth corresponds with the protuberance of the land. Abyssal depths and “pits” of 260 and even of 540 fathoms are to be found in close proximity to the scarped mountain islands of the Cyclades, whilst on the low coasts of Egypt the water deepens only gradually, until in the centre of the Levantine Sea it attains a depth of 1,750 fathoms. The maximum depth—2,170 fathoms—is attained between Crete and Malta. If the whole of the waters of the Mediterranean were to be collected into an aqueous sphere, the latter would have a diameter of 90 miles; if it fell down upon the earth, it would not even wholly cover a country like Switzerland. The Ionian Sea is separated from the Adriatic by a submarine ridge rising in the Strait of Otranto, and bounded on the west by a shoal or submarine isthmus, already referred to by Strabo, which joins Sicily to Tunis. This isthmus forms the true geological boundary between the western and eastern basins of the Mediterranean, which are connected here by a narrow breach only, the depth of which hardly exceeds 100 fathoms. The western of these basins is the smaller and shallower of the two, but nevertheless it attains a depth of 1,100 fathoms in the Tyrrhenian, and of 1,360 fathoms and even 1,640 in the Balearic Sea, and is separated from the waters of the Atlantic by a submarine ridge lying outside the Strait of Gibraltar, and joining Europe to Africa.[7]