Page:Africa by Élisée Reclus, Volume 3.djvu/169

 surprisingly regular line of sands, the so-called "Langue de Barbarie," which has a mean breadth of from 400 to 450 yards, and which is strewn with little dunes from 15 to 20 feet high.

Being constantly exposed on one side to the fury of the surf, on the other to the pressure of the fluvial overflow, this sandy dyke yields now at one point now at another, again repairing the breach with the alluvial matter here arrested by the opposing fluvial and marine forces. The curve of the shore-line bears witness to the ceaseless encroachment on the sea which has been going on for ages. The sedimentary matter already deposited beyond the normal coastline, and rising above the surface, covers an area of at least 1,000 square miles, and the range of dunes which at one time developed a regular concave curve between Capes Mirik and Verd now bulges out some 12 miles seawards. Off the delta the waters are also much shallower than elsewhere along the coast, so that the 50-fathom line, running within 2 miles of Cape Verd, is deflected to 18 or 20 miles off the mouth of the Senegal.

During the present period the bar at the entrance of the river has constantly changed its form and position. For a stretch of 13 miles below Saint-Louis, the breach in the sandy dyke has continually shifted up and down, according to the abundance of the fluvial discharge, the force and direction of the fluvial and marine currents, and of the winds and surf. Usually the bar is slowly displaced southwards, owing to the gradual extension of the sandy dyke formed by the combined action of the parallel marine and fluvial currents, both trending in the same direction. But as it gains in length, this narrow tongue becomes more exposed to the pressure of the river, yielding sooner or later at some weak point. It happens at times that the sill breaks into eight or ten distinct channels; but these openings are soon filled up by the action of the two conflicting currents, leaving only a single passage, through which the lighter fresh water spreads over the marine surface, while the heavier salt water penetrates up the river-bed. In 1825 the bar nearly faced Gandiole, 8 miles south of Saint -Louis; in 1851 it had shifted still farther south, almost to the southern extremity