The Egyptian oases are deep and broad hollows or depressions in the Libyan Desert plateau. In position they appear to coincide with areas where rocks of comparative softness became exposed at the surface during the gradual lowering of the country by denudation. At such points the general rate of weathering must have become greatly accelerated, with the result that those vast depressions, which form such conspicuous features in the configuration of the country at the present day, were eventually cut out. Underlying the greater part of the Libyan Desert are porous sandstones, and these, when pierced by deep borings put down from the lower-lying parts of the floors of the depressions, yield abundant supplies of water of remarkable purity. As these sandstones, as well as the shales with which they are associated, have a general dip or inclination from south to north, we are led to infer that they outcrop or come to the surface to the south, so that in all probability the water with which they are so highly charged has its origin in that direction. Whether the water obtains access to the sandstones by direct infiltration of the rains of Abyssinia or the Sudan, from the swamps of the sudd region of the Upper Nile, or from the Nile itself in the Nubian reaches, has not yet been decided with certainty. Recent observations, however, show that far more water is lost in some reaches of the Nile than can be accounted for by irrigation and evaporation, and it seems probable, therefore, that the excess disappears by infiltration into these sandstones. Little is known of the early history of the oases, though the remains of ancient towns and cemeteries are abundant, and only await systematic excavation by Egyptologists to bring our knowledge of this part of Egypt into line with that of the Nile Valley. That the oases were inhabited in prehistoric times is evident from the occurrence of flint implements of Palæolithic types, both on the margins of the surrounding plateaux and within the depressions, though there is not at present sufficient evidence to enable us to affirm that the makers and users of these flints were contemporaneous with Palæolithic man in Europe. Implements of Neolithic type, often of finished workmanship, are, moreover, common in places on the floors of the depressions, but it is probable that these were in use well into the historic period.