rainfall taken together. The total rise caused by all these contributions is estimated by Rohlfs at about 80 feet, the area of the flooded depression increasing during the inundations by many thousand square miles, and exceeding in extent the lake of Geneva ten or even twenty times.
Unlike all other large closed basins, Tsad is a freshwater lake, a phenomenon all the more surprising that wells sunk in Kanem yield a brackish fluid, while several islands in the eastern archipelago contain saltpetre. Doubtless its main influent, the Shari, flows through a region extremely poor in salt; but if the lake
were of great geological age, the saline particles, however small in quantity, must necessarily have accumulated by the effect of concentration and evaporation, whence the inference that this reservoir is of comparatively recent formation. At present it is the scene of incessant change, due mainly to the action of the Shari, whose alluvial delta advancing on the south side causes the liquid domain to encroach on the other sides, and especially on the west coast, where the route between Bornu and Kanem is constantly receding farther and farther inland. Here the district of Kuka is exposed to frequent inundations, which laid the city under water in 1878, when the Sheikh proposed to remove his residence much
