21.

Mesopotamia has its name from an accidental circumstance. We have said that it is situated between the Euphrates and the Tigris, that the Tigris washes its eastern side only, and the Euphrates its western and southern sides. To the north is the Taurus, which separates Armenia from Mesopotamia. The greatest distance by which they are separated from each other is that towards the mountains. This distance may be the same which Eratosthenes mentions, and is reckoned from Thapsacus,523 where there was the (Zeugma) old bridge of the Euphrates, to the (Zeugma) passage over the Tigris, where Alexander crossed it, a distance, that is, of 2400 stadia. The least distance between them is somewhere about Seleuceia and Babylon, and is a little more than 200 stadia.

The Tigris flows through the middle of the lake called Thopitis524 in the direction of its breadth, and after traversing it to the opposite bank, sinks under ground with a loud noise and rushing of air. Its course is for a long space invisible, but it rises again to the surface not far from Gordyæa. According to Eratosthenes, it traverses the lake with such rapidity, that although the lake is saline and without fish,525 yet in this part it is fresh, has a current, and abounds with fish.