Throughout the north of the Fayûm depression the Qasr el Sagha beds, forming the uppermost Middle Eocene, are followed by an unique series[63] of variegated[64] sands and sandstones, with alternating beds of clay and clayey marl. The ever-recurring bands of limestone, so common to the underlying marine beds, have now almost completely disappeared, being represented by only an occasional bed of calcareous grit, marl, or thin band of limestone. In the upper part of the series occurs a horizontal sheet of basalt,[65] in all probability contemporaneously interbedded; this forms a convenient datum line and may perhaps be provisionally taken as an arbitrary junction between the Eocene and Oligocene. Although as a rule remarkably barren of organic remains, certain bands, especially in the upper part, yield numerous individuals of a few species of mollusca, including _Lucina_, _Arca_, _Mutela_, _Spatha_, _Unio_, _Lanistes_, _Turritella_, _Melania_, _Potamides_, _Cerithium_ and _Pleurotoma_. From such an assemblage we may without doubt conclude that the conditions under which the series was deposited were estuarine or fluvio-marine, and this is further proved by the non-marine lithological character of the beds. The enormous quantities of silicified wood which occur in certain beds, in the shape of hundreds of trees of great length and girth, together with the numerous remains of land-animals, crocodiles, tortoises and turtles, indicate that rivers of considerable size emerged from the land to the south, the coast-line of which was probably not far distant. In fact the retreat of the sea, which as already mentioned, had probably already begun in Middle Eocene times, was now still further continued, although the cause of this was apparently not so much due to elevation of the land as to the continued deposition of sediment from south to north beyond the land-shore. We may in fact regard the series as a huge delta deposit in an area of local depression, in which the great accumulation of sediment brought down from the land continually caused the gradual retreat of the sea to the north. The same conditions would even appear to have continued on to Pliocene times, as from the Fayûm northwards stretches an immense plain of lithologically similar rocks, evidently accumulated under similar conditions, and which appear to contain newer and newer faunas from south to north. Thus, while in the Fayûm the remains are of Middle and Upper Eocene and Lower Oligocene age, when the latitude of Mogara is reached, some 70 kilometres further north, a fauna distinctly Lower Miocene in aspect occurs; further north again, as at Wadi Natrûn, Pliocene remains are abundant. We may hope therefore that this otherwise barren desert, when carefully and systematically explored, will yield us a continuous record of the vertebrate life of the northern part of the African continent from Eocene to Pleistocene times.