SILICIFIED TREES OF FLUVIO-MARINE SERIES, 4½ KILOMETRES NORTH OF QASR EL SAGHA.
In some localities pebbly bands occur in the sandstone-grits, especially in some of the beds above the basalt: the pebbles are mostly quartz or flint, subangular or rounded, the layer averaging perhaps two cm. in diameter, although occasional specimens three or four times that size are met with. Silicified trees of two distinct types[75] occur, and they are met with chiefly on two horizons; usually large numbers of trees occur together, completely covering the surface in places; they lie as a rule scattered about in every direction, although occasionally a large proportion may show considerable parallelism of deposition, as if arranged by the direction of the current which floated them to the spot. They always occur in a horizontal position or parallel to the dip of the bed, and it seems quite certain that none of them ever grew near where they are now found. The trees never bear attached branches, the latter having always been broken off at or near the point of junction with the trunk, where the scars are often plainly seen; this points to the trees having been drifted a considerable distance. Many trees over 25 metres[76] in length have been met with, but this by no means represents the original height, as the trunks have lost considerably in length during transport to their present localities. Although, as a rule, found completely weathered-out and exposed on the surface, in numerous localities these silicified trees are to be observed firmly embedded in the sandstones in which they were deposited, many being met with in our excavations for bones.
As the Fluvio-marine series is followed westwards from the central part of the area, the different divisions become more and more attenuated and the outcrops more and more obscured by superficial gravel. North-west of the Zeuglodon Valley an escarpment capped by a conspicuous bed of white calcareous grit occurs and perhaps represents the lower beds of the series. The higher are lost on the gravelly undulating plateau above.
F.—Age of the “Fluvio-Marine Series”.
The beds in question being as a whole remarkably unfossiliferous, a determination of their exact age on palaeontological grounds is an undertaking of some difficulty. The series, however, in certain beds is very rich in vertebrate remains; a considerable number of new and important forms have already been obtained and further additions are probable. Until the survey of the area in 1898 it appears that the only fossils obtained from these rocks were a few casts and badly-preserved specimens of mollusca from the highest beds above the basalt, collected by one or two observers from localities between the summit of the Fayûm escarpments and the Pyramids of Giza.
The Rohlfs Expedition did not visit this part of Egypt, and Zittel[77] tabulated the beds, which he called the “Schichten von Birket el Qurûn” as doubtfully Oligocene; probably the beds referred to are those of the island Geziret el Qorn, which, as already mentioned, belong to the lower division of the Birket el Qurûn series, and are therefore of Middle Eocene age. Mayer-Eymar[78] states that he was able to subdivide the series under discussion into Upper and Lower Ligurian and Lower Tongrian. Schweinfurth[79] considered the series as Miocene, comparing them with the lithologically similar Scutella beds of Der el Beda to the east of Cairo. Blanckenhorn, on the evidence of the writer’s fossil collections, states, as already mentioned, that the upper part is certainly to be regarded as Lower Oligocene and the lower part as Upper Eocene.
First as to the stratigraphical position of the series. There is no doubt that the lowest beds of the group were deposited (at any rate in the central part of the area) in practical continuity with the Qasr el Sagha series, which, as shown, is certainly of Middle Eocene age. A great change in the lithology of the beds, however, makes the junction a perfectly natural one. We pass from a truly marine series into an estuarine or fluvio-marine set of beds, and such a change near the summit of the Eocene is not an uncommon one in some parts of Europe. The stratigraphical position in the field, therefore, favours an Upper Eocene age for the lower beds. The dip being northwards, newer and newer beds are met with from south to north on the great undulating, but more or less level, desert north of the escarpment summit. The occurrence of Lower Miocene beds at Mogara, some 100 kilometres north or north-west, also points to a somewhat younger, or Oligocene, age for the underlying beds, (i.e., those between the Fayûm escarpment and Mogara). The actual relations, however, of the beds in the two localities have not yet been determined, but it is probable younger beds are continually met with from south to north.
Until the entire collection of fossils has been examined and determined, it is somewhat premature to attempt to fix the age of the series on palaeontological grounds. Up to the present the foregoing lists show the species which have been provisionally or finally determined. Some of these appear to be identical with species which have been recorded from Upper Eocene deposits of Europe, such as Potamides scalaroides, P. tiarella, while others, such as Melania cf. Nysti, Natica crassatina (found below the basalt in the so-called Sandberger Hills north-east of the Fayûm escarpment), are typically Lower Oligocene. Other forms, such as Turritella angulata, are common to both Eocene and Oligocene elsewhere.
If Blanckenhorn’s determinations of these forms are confirmed, we may regard the upper beds, i.e., those immediately above the basalt, as undoubtedly of Lower Oligocene age. The beds below the basalt mark the transition from the Eocene to Oligocene, while the base of the series, so far unfossiliferous as far as molluscan remains are concerned, must be regarded as of Upper Eocene (Bartonian) age.
We may hope that when the important vertebrate fauna occurring chiefly in the basal part of the series has been thoroughly exploited, and the remains systematically determined, confirmatory evidence will be obtained. At present the only forms described and determined, beyond pointing to a pre-Miocene age, do not indicate any definite horizon. Probably most of the animals will prove to be new, and although on that account more interesting from one point of view, will probably not assist us greatly in the exact determination of the age of the beds in question.