Page:Africa by Élisée Reclus, Volume 2.djvu/136

 saline than that of the sea, and the fishes here captured in large quantities all belong to the marine fauna. The alternating current of its emissary, setting now towards the sea, now towards the lake, as already noticed by Pliny, is due to the changes of level caused by the rains, marine currents, and winds. After the heavy rains the channel is converted into a river discharging its overflow seawards; but when the evaporation exceeds the volume contributed by its affluents, the deficiency is supplied by the marine floods.The outflow usually coincides with the east winds, the inflow with those from the west.

The mouth of the Mejerda, the chief river in Tunis, is separated from Lake

Bizerta only by the range of hills terminating on the coast at the sharp headland of Sidi Ali-el-Mekki. The Mejerda, the Bagrada of the Romans and Makarath of the Carthaginians, rises in the same Algerian uplands that send northwards the waters of the Seybus. Following in all its thousand windings the normal direction of the Tunisian coast, it plunges south of the Suk-Ahras plateau into a meandering gorge, now traversed by railway, and by a road which crosses the torrent no less than twenty-seven times. At Ghardiman, within the Tunisian frontier, after receiving the contributions of numerous torrents, it enters an old lacustrine basin enclosed some 12 miles farther down by the projecting bluffs of two mountain ranges advancing in opposite directions. Through this gorge the river has