OASIS, is the appellation given to those fertile spots, watered by springs and covered with verdure, which are scattered about the great sandy deserts of Africa. In Arabic they arc called wadys. The Arabic and the Greek name seem to contain the same root, and possibly the word may be originally a native African term. The most noted are in the Liriy an desert. namely, Anglia. Siwah, the great oasis west of Thebes, or El Khargeh ; the little oasis, or Waft el Bah rveh, and several smaller, ones, which are noticed under Egypt. Fezzan also may be considered as a great oasis of the Sahara. Hornemann has described Fezzan ; Brown has given an account of the oasis of El Khargeh, and Cailliaud of the her oases west of Egypt.
The oases appear to be depressions in the table-land of Libya. On going from the Nile westward, the traveller gra duail) ascends till he arrives at the summit of an elevated plam, which continues nearly level, or with slight undula tions, for a considerable distance, and rises higher on advan cing towards the south. The oases are valleys sunk in this
plain, and when you descend to one of them you find the level space or plain of the oasis similar to a portion of the valley of' Egypt, surrounded by steep hills of limestone at sonic distance from the cultivated land. The low plain of the oasis is sand-stone or clay, and from this last the water rises to the surfitee and fertilizes the country ; and as the table-land is higher in the latitude of Thebes than in that of Lower Egypt, we e may readily imagine that the water of the oases is conveyed from some elevated point to the south, and being retained by the bed of clay, rises to the surface wher ever the limestone superstratum is removed.