Being convinced that I had at last arrived at the true origin of the artesian water, I next began to entertain the idea of attempting to make a map which would show the contours of the underground water-sheet, and from which, in conjunction with the contour-map of the surface which I had already prepared, I might be able to predict the depth of boring required to tap the underground water at any point in the desert. But a little consideration showed that this idea was an impracticable one, by reason of our ignorance of the underground geological structure over the greater part of the desert. The underground water would naturally pass along permeable sandstone beds, often confined between impermeable clays above and below. And although the general structure of the Libyan Desert is doubtless one of simplicity as compared with that of other parts of Egypt, yet we know, from observations in the oases and in the Owenat region, that the beds are in some places folded and faulted, and that in others they have been uplifted and entirely removed by denudation, with the exposure of large areas of the underlying ancient crystalline rocks. The only parts of the desert for which the boring-depths could safely be predicted would be certain small areas within the oases, where wells have been sunk in sufficient numbers to give us definite information as to the local underground structure; and underground-water maps of these small areas, though they might usefully systematize our knowledge concerning them, would not be of any use for predictions at points situated elsewhere in the deserts.