The water-supply is derived from the Cretaceous sandstones forming the floor of the depression, the water rising naturally to the surface of the lowest areas. In numerous cases long adits have been driven into the rock to obtain an increased supply; these tunnels communicate with the surface of the ground by a series of air shafts; they mostly date from early times. No deep wells appear to exist in the oasis and certainly no borings have been made in modern times. The fall of the water-level is probably due to the gradual choking of the passages; an unsatisfactory and laborious method of cleaning out wells is in vogue but little trouble is taken to prevent the deterioration of the water-supply generally. Practically all the available land in this oasis is under cultivation, although with the reduced output of the springs the supply is barely sufficient for efficient irrigation. 44. =Farafra oasis= occupies a large semicircular depression 300 kilometres west of Assiut. The floor is formed of the white chalk at the top of the Cretaceous, but at Ain-el-Wadi, a spring in the north part of the depression at 26 metres above sea level, the underlying beds are locally exposed. The solitary village of Qasr Farafra is situated on the western side at 76 metres above sea level, and contained 542 inhabitants at the last census. In the entire area there are some 20 springs, mostly grouped round the village, each irrigating a small patch of cultivated ground; the total area of the latter, including the few palmgroves, probably does not amount to 500 acres. Wheat, barley, durra, rice, onions and some fruit are grown, and small quantities of dates and olives are exported; formerly the olives of Farafra were celebrated for their quality, but of late years the trees have deteriorated. The water rises as springs from the white chalk and does not necessitate the use of lifting appliances, though the output appears to be decreasing through natural causes. There are a few examples of horizontal conducting channels of ancient date and two or three of the springs appear to have deep vertical shafts as in the ancient wells of Dakhla. Sweet and brackish water-holes occur in several outlying localities within the depression, as well as in the neighbouring little known oasis of Iddaila to the west. Owing to the absence of waste pools and marshy land the climate of Farafra is more healthy than that of the other oases. 45. =The Geology of Egypt=[8]. The north-east corner of Africa, lying between the Red Sea on the east and the sand merged portion of the Libyan Desert on the west, and stretching from the Mediterranean to the 22nd parallel of north latitude, both in its topographical and geological characters is distinctly tripartite, as follows:-- (1) A rugged broken undulating sandstone desert, forming the southern part of the country; (2) Elevated plateaux, for the most part of limestone, stretching from lat. 25° N. (approximately) to the Mediterranean;