IGNEOUS ROCKS.
Igneous rocks cover about one-third of the entire area of South-Eastern Egypt, forming irregularly distributed tracts, alternating with others occupied by metamorphic rocks, entering largely into the composition of the principal mountain masses and also underlying a large portion of the areas covered by sand on the coast-plain and elsewhere.
Regarded broadly, the igneous rocks of this portion of the Eastern Desert form two main divisions, namely, an acid division typified by granite rich in felspar, and a basic division typified by gabbro. Intermediate types occur, as well as ultra-acid and ultra-basic rocks, but these are less abundant, and are generally found in close association with one or other of the two main types. Rocks of the acid type predominate to the north of latitude 24°, while further south basic rocks form large portions of the main mountain tracts, with acid rocks at intervals on either side down to the Sudan frontier. Most of the igneous rocks are plutonic. There is a striking paucity of volcanic rocks, a circumstance doubtless to be ascribed largely to the enormous denudation which has taken place in the district. The plutonic masses, with the dykes seaming them, have been elevated and laid bare, while most of the volcanic outpourings have been denuded away and others have been altered or devitrified till their original volcanic nature has become less apparent. Metamorphism has also affected many of the plutonic masses to such a degree that it is often difficult to decide whether the rocks should now be placed in the igneous group, or classed as metamorphic rocks.
For purposes of classification, the igneous rocks of South-Eastern Egypt may be divided into five main groups, based on silica-percentage:—
1. [Ultra-acid rocks,] containing over 80 per cent of silica.
2. [Acid rocks,] containing from 65 to 80 per cent of silica.
3. Intermediate rocks:—
(a) [Sub-acid rocks,] with from 60 to 65 per cent of silica.
(b) [Sub-basic rocks,] with from 55 to 60 per cent of silica.