_Between Ennedi and Borkou._—I next set out northwards from Ennedi in the direction of Madadi and Wadi-Doum, which had been adopted for the time being as their headquarters by some rebel bands from Tibesti, which attacked indifferently the caravans from Wadaï going to Arouelli for salt and our unescorted convoys of supplies circulating between the posts of Faya, Fada, and Ounianga. At the moment when I arrived in the neighbourhood they had just carried out successfully several of these surprise attacks, and were making off to their mountains to get their booty into a safe place. Unable to go after them, for my camels, exhausted by three months’ reconnoitring and hard fare, could not challenge those of the rebels for speed, I decided to return without delay to Faya to organize reprisals. On the way I passed through a low-lying zone of country once occupied by lakes and marshes of considerable extent and of about 1000 feet in altitude, or 250 or 300 feet higher than the region of the ancient lakes of Borkou and Djourab, with which it is connected by a continuous valley, the bed of which, very clearly visible in places, is often buried in sand. This lake-zone seems to be the end of the great depression I had crossed two months earlier, between the massifs of Erdi and Ennedi. Except in the immediate neighbourhood of the springs of Madadi and around the permanent pool of the Wadi Doum (or Touhou) the soil is absolutely barren, consisting either of very pure siliceous sand or of soft friable earth, whitish in colour and as fine as flour, into which we sank to the ankles at every step, raising thick clouds of stifling dust. Towards the south stretched chains of shifting sand- dunes, separating that depression from the last foothills of Ennedi, while to the north extended endless rocky terraces, in which were hollowed here and there basins of 1 or 2 square miles, wells of water impregnated with soda.