The total thickness of the exposed strata is about 435 metres, a figure obtained by actual measurement. Numerous borings show the thickness of the unexposed underlying Impermeable Grey Shales to be 75 metres, and the deepest borings yet made have pierced the still lower Artesian-water Sandstone to a depth of 120 metres, making a grand total of known deposits of 630 metres, or 2,067 feet. The depth to which the water-bearing sandstone extends is at present a matter of speculation; the point is of great importance in connection with the water-supply, though up to the present no borings of sufficient depth have been made to determine its thickness, nature, and relation to the underlying igneous rocks. With the exception of a few isolated bosses of eruptive rock in the desert to the south of the oasis—indications of the granitic foundation which probably underlies the entire area—the geological deposits of the oasis-depression, and of the surrounding escarpments and plateaux, are entirely of sedimentary origin, that is to say, they were laid down on the shores and beds of pre-existing seas and inland lakes. The sand-dunes are, of course, an exception, having been deposited by the wind on the surface of the land. Although, geologically speaking, the oldest group of sediments with which we have to deal belongs to the later chapters of the earth’s history, many hundreds of thousands of years have elapsed since the sandstones and shales, now forming and underlying the floor of the oasis, were accumulated on the bed of a vast inland lake. This sheet of comparatively fresh water was then invaded by the sea, which held sway in the region while the whole of the series of sediments, now exposed in the cliffs of the oasis and some 350 metres in thickness, were being laid down. In Middle Eocene times the sea commenced to retreat to the north, and the area under description became dry land with a continually receding shore-line. Since that time the forces of denudation have constantly been at work lowering the general surface of the plateau and excavating those depressions in which alone at the present day man is able to exist.