Below the confluence the Niger again develops an intricate system of channels and backwaters penetrating 90 miles southwards under the meridian of Timbuktu. The riverain populations capture large quantities of fish in this labyrinth of waters, which rise and fall with the seasons; they also grow rice in the moist depressions, harvesting the crops before the periodical return of the floods, thus alternately using the same tracts for fishing and husbandry. Fig. 125. — Backwaters South of Timbuktu.
Some 15 miles farther down the main stream, arrested in its northerly course by the southern escarpments of the Sahara, is abruptly deflected for about 240 miles eastwards to the gorges in the Burum district, immediately below which it sweeps round to the south, retaining that direction for the rest of its course to the Gulf of Guinea. But before opening this passage seawards it is probable that the Niger converted into a vast inland sea all the low-lying region which is now intersected by the network of backwaters flooded during the inundations. One of these channels still runs northwards in the direction of Timbuktu, beyond which, according to the information collected by Pouyanne and Sabatier, it appears to be continued through a series of depressions probably marking the course of the Niger at a geological epoch anterior to the piercing of the Burum gorges. But the suggestion that the Wed Messaura of Southern Mauritania and the Twat oasis now occupies the same depression with its sandy bed, seems to be contradicted by the provisional measurements taken by de Soleillet and Lenz in Twat and Timbuktu, the latter point being apparently some 430 feet higher than the former.
About 60 miles below the Burum defiles, where at Tossai the fluvial bed is contracted to less than 300 feet, the Niger passes from the zone of the Sahara to that of Sudan. Here two branches of the stream at the foot of the sandstone Ausongo hills enclose an island 18 miles long and strewn with rocks in the form
