In a passage I have already quoted from Mr. Petrie’s writings, he states that in prehistoric times the Nile was a vastly greater river than it is now, due to an enormous rainfall. Let us then assume the Pliocene sea-level at R.L. 60 to 70, according to Dr. Schweinfurth, and an enormous volume of water coming down from the Upper Nile Valley according to Mr. Petrie. The sea which then invaded the Nile Valley would have been in communication with the Red Sea, and may have had a tide of 5 metres range, which would have complicated the currents, and added to the scouring action. Below Wâstah, the channel of the Nile Valley, contained between the Libyan and Arabian Hills, is much contracted. Under these conditions the floods from the Upper Nile would escape sideways through the depressions in the Libyan Hills into the Fayûm and the Wadi Raiân; into the latter by way of the Wadi Muellah, and possibly by other connections with the Nile Valley of a low enough level. A large volume of water would thus be forced westwards out of the Nile Valley, and would find its way towards the sea to the west of Alexandria. In its endeavours to dig out a channel for itself it would erode laterally, or scour down vertically according as the softer material was found in one direction or the other. The different points of delivery and volumes of the water contributing to the flow, and the nature of the rock met with in its path would determine the form the channel would take at the various stages of its development. Tremendous eddies would be produced by projections of hard rock and contractions of the irregular channels, which would lift material from the bed and produce deep holes. Had this action not been arrested by the further upheaval of the land, perhaps a second Nile Valley would have been formed branching from the main valley at Behnesa, passing through the Wadi Raiân and Fayûm depression, and continuing through the Wadis Fadhi and Faragh, west of Memphis and Cairo, to the Natron Lakes, and thence to the sea west of Alexandria; or returning to the Nile Valley, or side of what is now the Delta, but was then sea, at some point south of Alexandria. If this theory is a sound one, the remarkable depressions of the Fayûm and Wadi Raiân are paralleled on a small scale by the deep holes (_bayarât_) scoured out below the bridges or cuts in the Upper Egypt Basin embankments, or outside a breach in a Nile bank. The section, given on Plate XVIII., of 50 miles of the Nile Valley and desert opposite Cairo, is taken from Mr. Cope Whitehouse’s article, entitled “The Pyramid Hill of Gizeh,” which appeared in the ‘Quarterly’ some time ago. It shows (assuming it on Mr. Whitehouse’s authority to be a correct representation of the ground) the channels, that I have named, in the arrested state of development which they had reached, when the flow of water, which was digging them out, was cut off. [Illustration: PLATE XVIII. SECTION OF 50 MILES ON LAT. 30°.