[Footnote 4: ANDREWS and BEADNELL, _A Preliminary Note on Some New Mammals from the Upper Eocene of Egypt_. Survey Dept. P.W.M., Cairo, 1902.] [Footnote 5: ANDREWS, _Notes on an Expedition to the Fayûm, Egypt, with Description of some New Mammals_. Geol. Mag. N.S. Dec. IV, Vol. X. Aug. 1903, pp. 337-343. Also _Further Notes on the Mammals of the Eocene of Egypt_ (Parts I, II, III). Geol. Mag. N.S. Dec. V., Vol. I. March, April, May 1904.] [Illustration: PLATE II. BAHR YUSUF AT LAHUN BEFORE ENTERING THE FAYUM.] PART I. =TOPOGRAPHY AND STRUCTURAL GEOLOGY.= * * * * * The Fayûm, a large circular depression in the Libyan Desert, is situated immediately west of that part of the Nile Valley lying between Kafr el Ayat and Feshn (Plate XVII.) The depression, which has an area, roughly speaking, of 12,000 square kilometres, is primarily divisible into three distinct parts—cultivated land, lake, and desert. SECTION I.—CULTIVATED LAND. The cultivated land has an area of about 1,800 square kilometres and, with the exception of the lake and part of the Wadi Rayan, occupies the lowest part of the depression. Cultivation is necessarily strictly limited to the area covered with alluvial soil. The latter, for the most part identical in origin and composition with the river-alluvium of the Nile Valley, covers a leaf-shaped tract between the bounding desert on the east side and the lake (the Birket el Qurûn) on the north-west. The easterly and central part of the cultivated area forms a more or less level table-land, from which the ground slopes gently away, especially on the north side, where the slope is towards the lake and very marked. The cultivated land of the Fayûm is directly connected with that of the Nile Valley by a narrow strip of low ground, a natural passage through the desert separating the Nile Valley and the depression of the Fayûm. Through this gap runs the natural canal known as the Bahr Yusef, which is practically the sole source of water in the Fayûm and irrigates the entire district.