HYDROGRAPHIC SYSTEM.
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excavated a deep channel to the Dakhla plain, an old lacustrine basin at least 300 square miles in extent, which has been filled in by alluvia of the Mejerda, Melleg, and other affluents. To a height of 70 feet above the present level of the plain, traces occur of the sedimentary deposits formed before the emissary from the
Fig. 32. — Old Beds of the Lower Mejerda.
lake had cut through the rocky sill above the upper bed, which dammed up the lacustrine waters.
In the Dakhla plain the Mejerda is joined by its largest tributary, the Melleg, which is at least 60 miles longer than the main stream. Rising near Tebessa, in Algeria, it flows mainly north-west and south-east, but loses much of its volume by
