In petrographical characters the Nubian sandstone is remarkably uniform, consisting of medium-sized silica grains set in a more or less ferruginous cement; the colour varies from nearly white, through various shades of brown, to nearly black, according to the amount of iron present. It is generally well bedded, and frequently much jointed. There is usually a marked absence of the clay beds which are associated with the Nubian sandstone in other parts of Egypt. The basal beds are generally pebbly, forming conglomerates in some places. Concretions are found in the rock at some points, notably to the north of Gebel Um Harba. Some of the concretions are hollow, the shell being of hard dark ferruginous sandstone while the interior is filled with a powder of snow-white chalky matter containing sand grains. The only fossils observed in the sandstone within the area described were collected by Mr. Charteris Stewart in the plain of Um Harba; they consist of crocodile scales and various shells. From these and the fossil shells found further west in sinking a well in Wadi Abu Rahal,[124] it may be concluded that the Nubian sandstone here, as in the Nile Valley, is of Cretaceous age. Intrusions of igneous rock into the sandstone have only been observed at two points in the area, _viz._, near the western foot of Gebel Awamtib, where a basic dyke runs through the sandstone, and on the Red Sea coast, near Bir el Ranga, where Dr. Hume has found andesite interbedded in the sandstone. A bed of diabase, now much altered, underlies the sandstone near Wadi Muelih, and is possibly of contemporaneous origin. In thickness, the Nubian sandstone attains a maximum of about 350 metres in several places round about Bir Abraq and the Wadi Hodein. Its thickness must at one time have exceeded this, for the upper surfaces have suffered much denudation, and are not capped by younger rocks. Some interest attaches to the maximum altitude reached by the Nubian sandstone beds. The highest point at which it has been observed is at Gebel Seiga, whose sandstone cap is 905 metres above sea-level. The highest point reached by the rock on the great plateau round about Wadi Hodein is Gebel Kala, 846 metres above sea. In contradistinction to the same beds which form the plateau between longitude 34° and the Nile, the Nubian sandstones within the area here described show considerable disturbances from their original horizontal bedding, the dips being as a rule greater and more variable the nearer one approaches to the watershed mountain-ranges. These tectonic disturbances, which are important in connexion with the geological history of the region, will be considered in Chapter XI. * * * * * [Footnote 122: _Geol. Mag._, Decade V, Vol. VI (1909), p. 271.] [Footnote 123: The numbers in square brackets are the specimen-numbers in the Cairo Geological Museum.]