At present any attempt to explain the origin of the artesian waters of the oases must be regarded as little better than speculation. More information is required concerning the geology of the country to the south of the oases, and as to the relative levels of the oases-depressions and the different parts of the Nile Valley and Libyan Desert, as far south as the more elevated regions of Kordofan, Darfur, and Tibesti. Little has been written on this subject, but the source generally assigned appears to have been Darfur. Possible sources of origin lie in the rainy districts of the Sudan, in the mountainous region of Abyssinia, in the great swamps of the Upper Nile, in the Nile River itself, and in past accumulations of water absorbed from the extensive lakes which covered parts of the country in the pluvial period which preceded the existing desert conditions. In the present state of our knowledge I am personally inclined to agree with those who regard the Nile River as a present source of supply. It is known to flow for a considerable part of its course through a valley cut out in the Nubian Sandstone, and it is believed to lose an appreciable volume of water into that sandstone, though the exact amount has not been determined. Mr. J. I. Craig, of the Egyptian Survey, has estimated that at low Nile as much as 6,000 cubic metres of water (1,320,000 gallons) per minute drains back into the river from the sandstones on either side of the reach between Khartum and Wadi Halfa, and, as Captain Lyons remarks, this indicates that there is considerable percolation into the sandstones from the river when in flood.[10] There is one point of the greatest importance to which we should like to draw attention, as it is generally entirely overlooked. The stores of water in the sandstones may represent the accumulations of hundreds and thousands of years, and the conditions to which the beds formerly owed their sources of supply may at the present time have become materially altered. It is quite conceivable that it may have required centuries or thousands of years to saturate the huge block of sandstone underlying the Libyan Desert, and even were the original sources of supply entirely cut off at any particular time, the effect on a few hundred bores, discharging only 50,000 cubic metres a day, would not necessarily be appreciable in one, or even five, centuries.