This expedition, though limited to the south-western border of the massif of Erdi, revealed some interesting facts about the configuration of the country towards the 18th degree of latitude north and the 23rd degree of longitude east of Greenwich; the altitudes increased from west to east, and it seemed likely that the massif of Erdi was connected in one direction with the mountains of Tibesti by the plateau of Jef-Jef, and in another with the still unknown massif of El Aouinat, situated approximately between the 22nd and 23rd degrees of latitude north and the 24th and 25th degrees of longitude east. Later information gave me a few further indications about western Erdi, where two water-points were found; one Bini-Erdi, about 80 miles north- east of Dourdouro, and the other, Erdi-Fouchini, some 60 miles north of Dourdouro, at the foot of a line of tall cliffs. The deduction may be allowed, for the time being, that the central tableland of Erdi offers altitudes presumably superior to 4000 feet, and that it slopes gently down on the east to the great sandy plain, without vegetation or water, across which passes the route from El Aouinat to Merga, a route that establishes direct but very difficult communication between Koufra and Dar Four, to the east of the 24th degree of longitude. _Between Erdi and Ennedi._—In leaving Dourdouro to march southwards I was going into the unknown. I could, no doubt, see in front of me, 40 miles away, the crests of northern Ennedi, at the foot of which I was to find the water-points of Aga and Diona; but to seek the said points without guide in the chaos of rocks was a risky undertaking, and might have been held unreasonable if the way our supplies were running short had not obliged me to go forward.