Thus we are provided with a point well westward of any hitherto known on the northern limits of the Nubian Sandstone; in other words, the evidence suggests the continuation westward by some 600 km. of the Nubian boundary shown on the existing 1:1,000,000 colored geological map of Egypt (1910).
Mention may here be made of the apparent absence of specimens definitely representing the younger Cretaceous rocks which are shown colored green, on the map just referred to, to the east; but this may be readily accounted for by the presence of the expansive plain passed over between Jalo and Zieghen, the “sand and alluvium” covering of which is quite sufficient to hide all evidence of the younger rocks of that age.
Another question rendered obscure by the presence of this large flat expanse of “sand and alluvium” is the exact position of the southern limits of Miocene formations. If we assume (which seems to be the case) that the point, 180 km. to the south of Jalo, where the last Miocene specimens were collected, be the southern boundary of the Miocene, then we find that the distribution of that formation as now suggested is of particular interest as (1) indicating a western extension of the general outline of the ancient Mediterranean Ocean in Miocene time; and (2) adding extra weight to our conceptions as regard the period (in geological chronology) of the Egyptian-Sinai uplift which caused the elevation of land involving a considerable portion of Egypt in pre-Miocene times, and ultimately defining the shores of the Miocene Sea more or less as we now follow it from this newly discovered spot between El Harrash (Zieghen) and Jalo to a short distance east of Siwa from where it runs northeastward to the thirtieth degree of latitude, along which it continues with little variation to Suez.
It would appear that all Egypt lying between the Miocene Gulf of Suez in the east and the Siwa-Zieghen Miocene shore-line in the west and southward was dry land in Miocene times, and therefore exposed to denudation extending over a vast geological period during which the Nubian Sandstone and younger Cretaceous rocks were laid bare and in a position to have Miocene strata laid down upon or against them.
The Nubian Sandstone, as indicated by the Specimens 5-10, shows identical characteristics to those exhibited wherever it occurs in Egypt or Sinai. It is a sandstone consisting in the main of more or less fine rounded grains of pure quartz, through which are distributed, in greater or lesser proportions, larger grains or pebbles. In cases where the latter predominate, a pudding-stone or conglomerate results; where the larger grains are sparsely distributed, a porphyritic appearance is suggested.
The cementing material, which may be either calcareous, silicious, or ferruginous, is the color-imparting medium, the depth of color depending upon the amount and composition of iron oxides present in it, and when this material is weathered, washed out, and accumulated into pockets it becomes, when finely ground, suitably adapted for the manufacture of paints.
The Nubian Sandstone continues south of the Miocene-Nubian boundary above referred to, to a point some 15 km. north of the Arkenu Mountains.
Approaching this point, still another change in the general aspect of the country met the eye of the explorer: the brighter coloring of the sandstones gave place to the duller browns, grays, and blacks of an abrupt range of igneous hills, the position on the ground where the igneous rocks come up through the Nubian Sandstones being approximately 25 km. north of Arkenu.
The changes of scenery in passing from one formation to another are strikingly seen in the excellent collection of photographs procured by Hassanein Bey, who is to be highly complimented on the success he obtained under great difficulties and inconveniences.
Judging from the Specimens 11-12 submitted for examination, the igneous rocks, of which the Arkenu-Ouenat Hills are composed, consist mainly of coarsely crystalline granites and syenites varying in texture and appearance, and through them run intrusive veins of finer-grained dike rocks.