The Artesian-water Sandstone has been proved to reach a thickness of 122 metres, and probably its total thickness is considerably more. Assuming, however, a vertical extent of only 122 metres, the water-bearing beds under 1 square kilometre would, if fully saturated, hold 4,965,000,000 gallons, which is the equivalent of the water which would be discharged in ninety-four years by a well flowing at the rate of 100 gallons a minute. When we consider that the area of the floor of the depression is some thousands of square kilometres, and that the water-sandstones, except in the immediate neighbourhood of the existing wells, are probably fully saturated, we realize the vast amount of water which is stored under the depression alone, irrespective of the still greater quantities which underlie the surrounding plateaux. The movement of the water through the sandstones, except along fissures and through particularly porous beds, is, however, very slow, so that the amount of water which can economically be made available at the surface is more or less limited, self-flowing wells being only obtainable over that portion of the area lying below the general static head. CHAPTER XI THE ORIGIN OF THE ARTESIAN WATERS Flow of Water through Porous Rocks — Importance of Pressure, Porosity, and Temperature — Intermittent Flows — Abundance of Extinct Wells — Former Prosperity — Possibility of increasing Present Total Discharge — Local Traditions regarding Origin of Artesian Waters — Possible Sources of Origin — Seepage from Nile into Nubian Sandstone — Sandstones as Storage Reservoirs — The Oasis Waters of Meteoric Origin — Fissures — Rate of Flow — Strongly-flowing Wells not necessarily dependent on Fissures — Local Pressure from Variation in Level of Water-Table — Rise of Water due to Hydraulic Pressure — Points requiring Investigation — High-Level Springs: Ain Amûr; on Escarpment near Beris; at Nakhail.