When laboriously traversing the hot and arid plateaux of the Libyan Desert, our thoughts divided between the fertile plains of the Nile we have left behind and the still far distant oasis, it is difficult to realize the presence, within a distance of a few hundred yards, of an abundant supply of the purest water. Yet there is little doubt that the water-bearing beds underlie practically the whole of the Libyan Desert, though it is only on the floors of the depressions that they lie within accessible distance of the surface. On the high tablelands the cost of sinking bores to reach the sandstones would be prohibitive, and without the aid of powerful and costly pumps the water would not rise to the level of the ground. But although the subterranean waters of the great desert plateaux cannot economically be made available at the surface, it must not be forgotten that the deeply buried sandstones of these regions must act to a great extent as storage reservoirs capable of replenishing the beds underlying the oases-depressions, from which large volumes of water are continuously drawn by the numerous artesian wells. The essential conditions required to produce an artesian basin, which, when tapped by borings, will produce self-flowing wells, are the presence of strata of sufficient porosity to carry water, enclosed above and below by beds of sufficient impermeability to prevent the escape of that water; the outcrop of the porous beds in some higher and distant region, with an adequate exposure to an abundant source of water, whether rain, river, or lake; and, finally, the absence of an easy escape at a lower level, unless at a considerable distance from the sites of the wells. These requirements are well fulfilled in the Libyan Desert, where the Nubian Series at the base of the Cretaceous consists of highly porous sandstones, subdivided by impervious beds of shale, overlain by a great thickness of absolutely water-tight beds, and underlain by probably almost equally impermeable crystalline rocks. These porous sandstones rise gradually to the south, and presumably outcrop in Nubia and the Sudan, though whether, as we shall see later, they derive their water-supplies from the Nubian reaches of the Nile, from the great swamp regions of the Sudan, or from the rains of Abyssinia or Darfur, is still, to some extent, an open question. [Illustration: AN ARTESIAN WELL (BORE NO. 39).] In Northern Kharga we find two similar but distinct sandstones, separated by a 75-metre band of Impermeable Grey Shale. The upper bed, which we have for convenience designated the ‘Surface-water Sandstone,’ is exposed at the surface, and contains standing or sub-artesian water; while the lower, from which the flowing wells derive their supplies, never approaches within 80 metres of the surface, and forms the true ‘Artesian-water Sandstone.’ It will be convenient to give these sandstones separate consideration. THE SURFACE-WATER SANDSTONE.