Oasis, a name given by the ancients to the fertile spots in the Libyan desert, and now become a general term for those situated in any desert, it is derived from an Egyptian word preserved in the Coptic uali, and signifying an inhabited place, as there the caravans halted in their journeys between eastern and western Africa. Anciently they were supposed to be islands, rising from an ocean of sand; but generally they are depressions in the midst of a table land resting on a bed of limestone, whose precipitous sides encircle the hollow plain, in the centre of which is a stratum of sand and clay, retaining the water flowing from the surrounding cliffs. On the cultivated portions date palms, rice, barley, wheat, and millet are cultivated. The Libyan oases were never permanently occupied until after the conquest of Egypt by the Persians. Under the Ptolemies and the Ca3sars they were occupied by Greeks and Romans, and were places of banishment for state criminals; later they were places of refuge from persecution. In the Sahara desert upward of 30 oases are enumerated, of which about 20 are inhabited.

The most celebrated are the following, all in the Libyan desert. 1. Ammonium, the modern Siwah, the most remote from the Nile, in lat. 29°, N lon. 26° E., contains the ruins of the temple of Amnion, and the supposed " Fountain of the Sun," whose waters were warm in the morning and evening and cold at midday. This oasis is remarkable for the productiveness of the soil, which is strongly impregnated with salt. It has several towns, the principal of which is Siwah el-Kebir, and its inhabitants are subjects of Egypt. (See Siwah.) 2. Oasis Minor, the modern Bahryeh, is S. E. of Siwah, in lat. 28° 30' N., and contains temples and tombs belonging to the era of the Ptolemies. It was also under the government of the Romans, and was then distinguished for its wheat; but now it produces principally fruits. 3. Oasis Trinvtheos, the modern Dakhel, in lat. 25° 30', W. of ancient Thebes. The earliest monuments are those of the Romans, and there are artesian wells. 4. Oasis Magna, the modern Khargeh, S. E. of the preceding, and S. W. of Thebes, is about 90 m. W. of the Nile, with which it is parallel.

It is about 80 m. long and 10 m. broad, stretching from lat. 25° to 26° K It is sometimes called the oasis of Thebes; by Josephus it is denominated "the Oasis," and by Herodotus " the city Oasis" and the " island of the blessed." It had a temple 468 ft. long, dedicated to Amun-ra, and after the Christian era abounded in churches and monasteries. There are in the Libyan desert several other oases of considerable importance, among them Augila, S. of Barca, and Farafrah, between Siwah and Dakhel, in lat. 27°. Farafrah was visited by Rohlfs in December, 1873, and Dakhel in the following January. Many oases contain stagnant lakes, from which feverish exhalations arise.