[Footnote 124: Wadi Abu Rahal is a small feeder of Wadi Abad, joining the latter from the south in latitude 25° 0′, longitude 33° 30′. At the point of junction of the two wadis, which lies on the usual camel road from Edfu to the Baramia mine, a well was sunk by the Mines Department in 1906, in the hope of obtaining a water supply. I visited the well in May 1906, when it had attained a depth of fifty-four metres. The strata passed through were ten metres alluvium, then thirty-seven metres of sandstones and clays, followed by a thin band of bituminous shale, and seven metres of dark grey clays. From near the bottom of the well I collected specimens of _Lingula_ and a mytiloid shell which Mr. Bullen Newton compared to _Septifer linearis_; the latter shell differs but little from specimens obtained from the English Gault, and thus tends to show that the Abu Rahal beds are of Cretaceous age. (_See_ HUME, _Preliminary Report on the Geology of the Eastern Desert of Egypt_. Cairo, 1907. p. 29). Since my visit, the well has been deepened to seventy-three metres in sandstones with a bituminous seam, but water was not reached, and the well has been abandoned.] CHAPTER IX. * * * * * =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.