46. =Igneous rocks.= The most ancient rocks in Egypt are found in the central igneous ranges of the Red Sea Hills and in the crystalline floor underlying the sandstones in the southern part of the country.
In Nubia the crystalline rocks consist largely of granite and gneiss, with associated diorites and schists, traversed by basaltic and felsitic dykes. Cataracts have been formed at those points where the river crosses the hard igneous belts, which may be regarded as the summits of the higher ridges of an old eroded continental land surface.
In the Red Sea Hills the most ancient rocks are the gneisses, schists, and slates, constituting the metamorphic series of Jebel Meeteq. Next in succession is a volcanic group, consisting of dolerite and sheared diabases in the south and of dolerites, andesites, tuffs and agglomerates in the north. These volcanic rocks are underlaid and intruded by still younger quartz-diorites and grey granites, and like them are pierced by masses of red granite and dykes of quartz felsite and dolerite. The red granite is itself traversed by dykes of diabase, which are thus the youngest of all, except for the still more recent andesitic intrusions into the Eocene limestones (occasional occurrences of which are met with on the plateaux on both sides of the Nile valley), and the basaltic sheets which commonly mark the base of the Oligocene sandstones in the north of the country.
The whole of the Red Sea Hills igneous complex has been planed down by marine erosion, the oldest sedimentary deposits being laid on to the smoothed denuded surfaces.
47. =Sedimentary rocks.= Geologically the sedimentary deposits of Egypt are not of great age. Broadly they consist of a great development of Upper Cretaceous and Eocene strata, followed by more restricted deposits of Oligocene and Miocene age, the still younger formations being represented only by comparatively local though important, accumulations. As a general rule the different members of the Cretaceous and Tertiary succeed each other in regular order from south to north, the strata being undisturbed and dipping northwards at a very low angle.
48. =Upper Cretaceous.= The Cretaceous system in Egypt is divisible into three main groups, (1) a great thickness of freshwater arenaceous sediments known as the Nubian Sandstone, of Senonian age in the south (Dakhla, Nile valley, and southern part of Eastern Desert), and Cenomanian age in the north (Baharia, Abu Roash(?), and Wadi Araba); (2) 300 metres of argillaceous deposits with bone-beds near the base, of Senonian age; (3) a deep water foraminiferal white chalk (Danian) 60 to 100 metres thick, especially developed in the region of the oases to the west of the Nile.