The general look of the country is fairly uniform. It is a vast sandstone plateau sloping from north to south, ravined with narrow gullies running in a general direction from north-east to south-west, and which are real rivers of sand in which the shifting dunes pile themselves up and overlap to the point of being impassable at times to laden beasts of burden. This direction, from north-east to south-west, being that of the prevailing wind in Borkou, the parallelism of these gullies and the general appearance of the landscape give colour to the supposition that they were hollowed out of the sandstone by the erosive action of the dunes driven before the wind. The rocky plateau is commanded at intervals by a few blackish peaks of low relief, among which the most noticeable are those of Kazzar, near Yarda, 75 metres above the surrounding country; Olochi, near Dourkou, 130 metres; Ehi Kourri, near Kouroudi, 350 metres in relief. From the height of these natural observatories nothing is to be seen, in whatever direction one turns, but vast dark-tinted expanses strewn with stones, where no sort of topographical order can be discerned. So confused and scattered are the rocky masses that the impression they leave is less that of a sequence of alternating plateaux and valleys than of a chaos of disconnected reefs rising above a sea of sand, amid breakers of billowy dunes. Much going and coming was needed before I could form an exact notion of the physiognomy of these regions, for the fact is that their valleys are more or less blocked, at longish intervals, by heaps of rock debris and sand, and so divided into a succession of elongated hollows communicating only by subterranean infiltration. In these hollows may be found, here and there, layers of shells that enable us to fix the period when they were still underwater at a comparatively recent and no doubt Quaternary epoch. From place to place there still exist permanent salt pools, of greater or less depth, and usually at the foot of the cliffs that shut in some of these valleys on the east. One supposes that the strong back draughts of the north-east wind have mainly concentrated their action on those points of the surface where the sandstone was softest; in the excavations thus produced the sheet of subterranean water has been able to make its appearance in the open air, and under the influence of a persistent evaporation, due to the extreme dryness of the air and the intensity of the solar heat, the salts in solution in the water have undergone a progressive concentration, sometimes to the point of floating on the surface of the pool with the appearance of translucent blocks of ice.