From Miski I meant to make a rapid plunge into the valley of Yebbi, in the heart of central Tibesti, firstly to try to get into connection with a detachment of the garrison of Bardai, and then to make an attempt to reach the plateaux of Goumeur. Lastly, I thought I might be able to get over on to the western slope of the massif, explore its chief valleys, and effect a junction with the Zouar camel corps before returning to Borkou. I succeeded in carrying out this programme in its main lines, except for the operation in the direction of Goumeur, which had to be replaced at the last minute by a reconnaissance pushed as far as the post of Bardai. I was away, in all, for seventy-two days, or barely a fortnight in excess of my estimate. _From the Plains of Borkou to the Foot of Emi Koussi._—The name of Borkou is given by geographers to the group of low-lying stretches of country separating the mountain mass of Tibesti from that of Ennedi; it was confined at first to the depression, some 10 kilometres wide by 100 in length, that extends from east to west, from Faya to Ain Galakka. This hollow was long filled by a lake, of which numerous and conclusive traces are still found: beds of lake shells, whole skeletons of fishes up to a yard and half long, calcareous crust covering long streaks of rock, platforms of white clay marking the line of flats where the last pools left by the waters of the former lake have held out longest before drying up, and so forth. This lake was fed by mighty watercourses, coming down from the mountains of Tibesti and Ennedi; it poured its overflow through the valley of the Jurab into the Kirri, the deepest, largest, and most recently dried up among the ancient lakes and lowlands of the Chad. From Borkou to Emi Koussi there is a large choice of routes. The best, owing to the number of points at which water and pasturage may be found, is that which passes by way of Yarda to Yono. Hereabouts we leave behind the region of the oases characterized by numerous depressions in which water is found close to the soil in practically unlimited quantities, in wells less than a yard deep and in salt pools. From that point one enters the rocky zone where there is no more water underground, but only natural cisterns forming reservoirs with the water that streams down into them, and dries up a longer or shorter time after the passage of the accidental rains that filled them.