The Fayûm, one of the largest depressions of the Libyan Desert, is situated some 50 miles south-west of Cairo. It is cut out in rocks of Eocene and Oligocene age, while still younger deposits of Pliocene and Post-Pliocene date are found within the hollow. The depression owes its origin to the action of the ordinary subærial denuding agents, which I have shown in previous papers were capable of producing the oases-depressions of Baharia, Farafra, Dakhla, etc. Faulting, which has played so important a part in the formation of the Nile Valley, appears to have had little or nothing to do with the production of the Fayûm and other depressions of the Libyan Desert.