As a result of the sharp folds the upper limestones were cracked, and their denudation by natural agencies followed, forming a slight hollow similar in shape to that which the oasis now exhibits; the agent of denudation cannot be stated with certainty, but whatever force came into operation it would find easy work in the cracked-up rocks, and still easier would be its task in partly removing the soft Cenomanian sandstones and clays after the harder limestones had disappeared. The primary excavation of the hollow was followed by the formation of a great lake, in which were laid down deposits of sandstone, quartzite, and iron-ore; this lake doubtless surrounded islands, represented to-day by those hills which still preserve their limestone-caps; it extended, or similar lakes existed, beyond the oasis-limits, forming the quartzites and ferruginous sandstones passed on the way from Maghagha to Baharia, and was perhaps continuous with the Oligocene and post-Oligocene sea which covered a large part of the country to the north. In later times the area finally became continental and denudation gradually sculptured the oasis to its present form; this sculpturing would no doubt proceed rapidly in the moist climate which is known to have existed in Egypt in Pliocene and early Pleistocene times, and is being continued to-day by the powerful agency of the desert wind-borne sand and changes of temperature. The water-supply of the oasis is probably derived from the tropical rains of the mountainous regions of Central Africa, the water from which penetrates the ground and flows northwards along permeable beds of sandstone, etc., in which it is confined by other impermeable strata, until tapped naturally or artificially in the great oases or depressions of the Libyan Desert. * * * * * [Footnote 35: _Voyage à Méroé_, etc., _op. cit._] [Footnote 36: _Geologie, u. Palæontologie der Libyschen Wüste_. Cassel, 1883.] [Footnote 37: Zittel had divided the Cretaceous of the Western Oases into the following main divisions:— _Top._ 1. Well-bedded limestone and earthy chalk. 2. Greenish and ash-grey shaly clays. 3. Beds with _Exogyra Overwegi_. ] [Footnote 38: _On the Stratigraphy and Physiography of the Libyan Desert of Egypt_, Quart. Jour. Geol. Soc., Nov. 1894, Vol. 50, p. 535.] [Footnote 39: Letter from Prof. Zittel to Capt. Lyons, dated May 19th, 1894. It was afterwards suggested (BEADNELL, _op. cit._ Int. Geol. Congress, p. 10) that these specimens were possibly not _E. Overwegi_ at all, and Mr. Bullen Newton, of the British Museum, who has undertaken the determination of the Baharia collection, confirms this, stating that the so-called specimens of _Exogyra Overwegi_, referred to in Capt. Lyons’ paper, are in reality examples of _E. mermeti_.]