The mountainous country to be crossed set an extremely awkward problem: many points where water would have to be found were often hard for the camels to reach. Pasture-grounds were rare and scanty. The tracks, inexistent or deceptive, would now stretch away across successive heaps of sharp-edged pebbles, and now twist and turn endlessly along winding torrent beds, deep sunk between sheer banks. To cross from one valley to the next one had to climb a succession of cliff ledges, rising tier on tier to several hundred metres by the merest suggestion of paths winding along the sides of spurs formed by the rolling down of _débris_ from above; when the slopes grew too steep, the baggage had to be carried up from one shelf to the next on men’s heads. Our camels, used to the easy going of the great sandy plains, were discouraged by the asperities of the sharp-angled rocks, by the narrow ledges, the steep and slippery steps, the loose pebbles, the excessively sharp turns; and so only short distances could be covered in spite of long hours under way and intense fatigue. It goes without saying that we had no sort of map of these unknown regions, and that we were utterly at the mercy of the guides whom by good or evil fortune the patrols put at our disposition. Accordingly, the choice of our routes was dictated to us at once by the necessity of reducing to a minimum the efforts and privations of our camels and by that of keeping within the limits familiar to our ordinary and occasional guides. It may be added that the latter showed the utmost unwillingness to lead us into regions where the unsubdued tribes habitually take refuge; for these tribes are in the habit of holding them responsible, on their own heads and those of the members of their families, for all the harm and losses incurred when fights arise with our detachments. The general plan of this series of operations included, first of all, the reconnoitring of Emi Koussi, an extinct volcano 3400 metres high, followed by an inroad into the valley of Miski, the usual meeting-ground of the Tibestian freebooters threatening the roads to Kanem. The central position of the valley is strengthened by the natural shelter afforded by high mountains and almost impassable rocky foothills, through which lead only two defiles, both of them long and dangerous.