E.—Fluvio-Marine Series (Jebel el Qatrani Beds).
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.
In the Fayûm, over a length of 80 to 90 kilometres, the basal beds of the Fluvio-marine series, at a height of only a few metres above the top of the Qasr el Sagha series, are frequently found to contain the remains of land-animals, often in sufficient quantities to form in places a true “bone-bed.” Besides land-mammals, remains of large tortoises, turtles and crocodiles, are very common, some of the latter being identical with those of the Qasr el Sagha series below. Chelonian and crocodilian remains are to be found on various horizons, but so far none but fragmentary mammal remains have been observed in the higher parts of the series. It is interesting to note that the bones in these beds appear to be most common near the accumulations of fossil trees, thus suggesting that they were floated out from the land at the same time and by the same river-currents. The porous character of the sands and sandstones of this group has resulted in the remains not being in nearly so hard or durable a condition, except when coated with ferruginous sand, as those in the series below, although the actual state of preservation is even more perfect.
Analysis shows that these bones, with the exception of the loss of all organic matter, have undergone very little change. A sample examined by Mr. Lucas was taken from a typically preserved pelvis of Arsinoitherium and gave the following result:—
| Determined. | Calculated. | ||
|---|---|---|---|
| Silica | 0·57 | Silica | 0·57 |
| Oxide of Iron | 1·98 | Oxide of Iron | 1·98 |
| Lime | 51·40 | Calcium Phosphate | 76·11 |
| Magnesia | trace | Magnesium Phosphate | trace |
| Phosphoric Acid | 34·86 | Calcium Sulphate | 4·64 |
| Sulphuric Acid | 2·74 | Calcium Carbonate | 14·75 |
| Loss on ignition, being Carbon dioxide | 6·13 | Organic Matter | nil |
| Chlorine | trace | Sodium Chloride | trace |
| Not determined | 2·32 | Not determined | 1·95 |
| 100·00 | 100·00 |
The following composition of the bones of an ox, from an analysis by Berzelius, is appended for comparison:—
| % | |
|---|---|
| Phosphate and Fluoride of Calcium | 57·35 |
| Carbonate of Calcium | 3·85 |
| Phosphate of Magnesium | 2·05 |
| Soda and a little Sodium Chloride | 3·45 |
| Organic Matter | 33·30 |
| 100·00 |
It is curious that these Eocene bones should have so completely preserved their original composition considering the almost universal silicification of the trees deposited in the same beds.
Most frequently the vertebrate remains are found in an unconsolidated false-bedded clean quartz sand, the grains of which are semi-rounded or angular; in some layers the sand is very coarse and polished, and mixed with fine gravel. These deposits of sand, apparently brought down by river floods, are not continuous along any particular horizon, but are intercalated here and there in the ordinary sandstones, clays and marls of the series; they occur chiefly, however, as local lenticular masses along a more or less constant horizon near the base of the series. The bone-remains are not absolutely confined to these deposits of river-sand, but like the silicified trees are far more common in them than elsewhere. Scattered mammal bones occur in the lower clays, marls, and hard concretionary sandstones, while the remains of aquatic animals, such as turtles and crocodiles, may be found almost anywhere.