ABSTRACT
The Chad basin, occupying some three-quarters of a million square miles of west-central Africa, stretches from the central Sahara to the Congo watershed. At the rim of the main, saucer-shaped depression rise several mountain massifs: Air, the Hoggar, Tibesti, and Ennedi in the Sahara; Jebel Marra in the Sudan; the Jos Plateau in Nigeria; and Adamawa in the Cameroun Republic. Diatomaceous earth and diatomite occur at various levels; the main species involved is said to be a form of Melosira not older than the Pleistocene. Systems of dunes probably of differing ages give some indication of the extension of arid conditions south of the present limits of the Sahara at different stages in the Pleistocene. The Chad basin when compared with East Africa seems to have been free from violent tectonic activity during the Pleistocene period and may have been scarcely affected at all by earth movements in the upper Pleistocene.
