The remaining nine tenths of the desert consist of stony, wind-swept plateaus, known as _hammada_, vast tracts strewn with water-worn pebbles, called _serir_, and scattered highland regions. Of the latter the most extensive is the Ahaggar, or Hoggar Plateau, which lies in the center of the Sahara, nine hundred miles due south from Algiers. Covering an area considerably greater than that of the Swiss, French, Italian, and Austrian Alps put together, this mass of mountains is dominated by two great peaks, Hikena and Watellen, whose summits during five months of the year are covered with snow. To the northeast of the Ahaggar is the Tasili Plateau, and directly to the east, forming the southwestern boundary of Italian Libya, stretch the Tummo or War Mountains. In the extreme east, near the frontier of the Anglo-Egyptian Sudan, is the great table-land of Tibesti, with an average height of nearly a mile and a half above sea-level, the volcanic cone of Tussid rising eighty-eight hundred feet into the African blue. On the east and south the highlands of Tibesti run down to connect with the lower ranges of Ennedi and Borku, which merge, in turn, into the rich plains of Darfur and Wadai. In addition to the plateaus and ranges mentioned, there are several disconnected mountain masses, such as the rugged region in Mauretania known as the Adrar of the Iforas, and, to the north of the Nigerian frontier, the Hills of Air. The country of the sand-dunes, which has provided so many fiction writers and motion-picture directors with highly romantic settings, consists of a broad belt which, starting on the Atlantic seaboard, in the vicinity of Cape Blanco, sweeps in a great arc northward and eastward, completely masking the southern frontiers of Morocco, Algeria, and Tunisia, to the shores of the Gulf of Gabés. That portion of this sand-belt which stretches from the Senegal River to the Moroccan Atlas is generally referred to as the Igidi (the Berber word for sand-dunes); then, to the south of Algeria, comes the region known as the Grand Erg Occidental, separated by a narrow valley at El Golea from the Grand Erg Oriental, which sweeps right across the southern end of Tunisia. The Igidi and the Ergs can best be described as sandy oceans—the simile has been used many times before but I cannot think of a better—with waves of sand instead of water. The dunes of the Eastern Erg, which range in height from sixty to three hundred feet or more, lie in long parallel rows, with a gradual slope to windward and an abrupt descent to leeward, so that, particularly in the gray mists of early morning or at nightfall, they present a striking resemblance to a stormy sea. Generally speaking, the dunes maintain a state of comparative permanency, certain of the larger ones having been given names of their own, though it is by no means unusual for their contours to change almost beyond recognition in a single night under the influence of the wind. The oft-told tales of caravans