Whereas Dr. Oudney, in the course of the long journey from Tripoli to Lake Tschad, estimated the elevation of the southern Sahara at 1637 English feet, to which German geographers have even ventured to add an additional thousand feet, the Ingenieur Fournel has, by careful barometric measurements based on corresponding observations, made it tolerably probable that a part of the northern desert is below the level of the sea. That portion of the desert which is now called “le Zahara d’Algérie” advances to the chains of hills of Metlili and el-Gaous, where the northernmost of all the Oases,--that of el-Kantara, fruitful in dates,--is situated. This low basin, which touches the parallel of 34° lat., receives the radiant heat of a stratum of chalk, (full of the shells of Inoceramus), inclined at an angle of 65° towards the south (Fournel sur les Gisemens de Muriate de Soude en Algérie, p. 6 in the Annales des Mines, 4me Série, T. ix., 1846, p. 546). “Arrivés a Biscara,” (Biskra), says Fournel, “un horizon indéfini comme celui de la mer se déroulait devant nous.” Between Biscara and Sidi Ocba the ground is only 228 (243 Eng.) feet above the level of the sea. The inclination increases considerably towards the south. In another work, (Asie Centrale, T. ii. p. 320), where I have brought together everything relating to the depression of some portions of continents below the level of the sea, I have already noticed that according to Le Père the “bitter lakes” on the isthmus of Suez, when they have a little water,--and, according to General Andréossy, the Natron lakes of Fayoum,--are also lower than the level of the Mediterranean.