Summing up the dips and disturbances of the sandstone beds at the different places, it is clear that the strata are the more disturbed, the more closely they approach the main mountain ranges; this and the seaward dip of the beds near El Ranga are so far in favour of the view that the sandstone may once have extended right over the present mountains. But it will be seen that the disturbances of bedding are of an irregular character, especially between Gebels Zergat Naam and Um Reit, where the folding and faulting are often in directions quite distinct from that of the main mountain axes, and it may be that these disturbances are due to later movements rather than to the elevation of the main mountain ranges. So far as observations on the sandstone itself go, therefore, the question as to whether the Nubian sandstones of the Red Sea border ever directly joined those on the west of the mountains remains open; the observed disturbances of the beds give us clear proof that considerable earth movements have taken place since the sandstone was deposited, but they are inconclusive as to whether these same movements caused the elevation of the great mountain ranges.
The Igneous and Metamorphic Rocks.
Turning now to the main mountain-forming rocks, we find them composed of the two great classes, igneous and metamorphic. Though in places we have transition members, such as granites passing into gneiss, yet on the whole the two groups are quite distinct, and we find typical igneous masses rising in the midst of equally typical schists. Under these circumstances there can be no hesitation in considering the two groups to be of different ages, and that the igneous rocks must be the younger.
The igneous masses are divisible geologically into the three classes of (a) lavas, (b) dykes, and (c) plutonic rocks.
Lavas.—Amongst the igneous rocks, the lavas are those most poorly represented. Apart from the andesite of Gebel Sufra, and the diabase sheets in or under the Nubian sandstone, there are only a few occurrences of volcanic rocks in all the area, a circumstance which is easily explained by the enormous denudation which has gone on and the fact that lavas, being superficial out-pourings, will have been most exposed to denuding forces.
Dykes.—Dykes seam the schists and igneous rocks, frequently in such numbers and with such parallelism as to give to the land the form of a succession of ridges separated by long narrow depressions, forming “dyke country.” In nature, the dykes vary from extremely acid rocks like aplites and quartz-felsites, to very basic forms such as diabase and basalt. There is on the whole a preponderance of basic over acid types. As already remarked, only one instance of a dyke cutting the Nubian sandstone has been observed in the district. Another significant feature in the distribution of dykes is that while dykes of all kinds are found cutting schists and acid plutonic igneous rocks, the basic plutonic masses are as a rule free from dykes; this suggests that the basic plutonic rocks may be on the whole younger than the acid forms.
Plutonic Rocks.—The plutonic rocks include granites, syenites, diorites, gabbros, pyroxenites, amphibolites, and peridotites. But these are not all of equal importance, and a natural division from the field observations is a two-fold one into acid and basic groups, which are tolerably well marked off from each other and are most likely of different ages. The acid class on this view consists of the granites and syenites, while the basic class contains all the other rocks above-mentioned. The granites are on the whole of a very acid type; syenite is very scarce and nearly always occurs in close association with granite. Of the basic group, probably the most abundant and most typical rock is gabbro; diorites and the various ultra-basic rocks are almost always closely associated with gabbro. Where acid and basic rocks occur in proximity (as for instance near Bir Abraq, where an acid granite occurs side by side with a serpentine) there is a sharp change, without transitional forms.
The much greater abundance of dykes in the plutonic rocks of the acid group as compared with those of basic composition, as mentioned above, inclines us to regard the basic group as the younger—a view which is further supported by the greater frequency with which a tendency to gneissose structure is noticeable in the acid rocks. The fact that the basic members are frequently in a higher state of decomposition than the acid ones is not contrary to this view, being due to the greater ease with which their constituents undergo weathering; and as explained on [p. 315,] the fissured state of the altered peridotes is probably not due to the same earth forces which have sheared the granites, but to internal stresses set up by the expansion of the rocks on serpentinisation.
Though we may be fairly sure that the basic plutonic rocks are on the whole younger than the acid ones, we have no certain guide from which to estimate the geological age of either. We know from their relations to the sandstone that they are both older than the Upper Cretaceous, but we cannot say how much older; they may be anything from Archæan to Jurassic; the circumstance that similar rocks underlie Carboniferous strata in Sinai inclines us to place them at least as far back as Palæozoic times.
Gneisses and Schists.—The gneisses and schists which cover so much of the country are obviously older than the plutonic masses which are intruded in them, and for these metamorphic masses we need have small hesitation in speculating on an Archæan age. They exhibit a wonderful variety of composition. Most of the gneisses are doubtless sheared ancient igneous rocks, such as granite and diorite, and a similar origin may be assigned to many of the schists. Other rocks, as for instance the clay-schists, graphite-schists, and marble, have almost certainly originated from the metamorphism of ancient sedimentary rocks. But in a large number of cases we have at present no clue to the parent rock from which the schists have been formed.