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IV.—Notes on the Petrography of Egypt1
Published online by Cambridge University Press: 01 May 2009
Extract
1. The ancient core of the North-East African Continent consists of the Cataract and Sudan banded gneisses, which may represent a very ancient igneous magma. They are usually much veined by granitic dykes.
2. In certain places in the Arabian Desert, Cataracts, etc., these underlie highly metamorphosed schists (the mica-schists of Sikait, the calcareous schists of Um Garaiart and Haimar and of the Amara Cataracts, also the dolomites of the latter region) which are sharply separated from the banded gneisses and are possibly the oldest sedimentary representatives in Egypt.
3. The greater part of the mountainous regions of the Eastern Desert and Sinai are occupied by two types of rocks, a schistose constituent overlying or being surrounded by the acid member. (a) The first-named, the Dokhan volcanic rocks and schists, are partly volcanic in origin and partly sedimentary, the former being represented by lavas of various types, while the latter are clearly altered sedimentary strata (grits, conglomerates, etc.). No fossils have yet been found, but they have their nearest lithological analogues in the latest pre-Cambrian and Cambrian series. Here are included some of the most interesting rocks of Egypt, such as the Imperial Porphyry and the Breccia Verde Antico. (b) The igneous member intruded into these ancient sediments, etc., includes a great diversity of igneous rocks, varying from highly basic to acid types.
Contact-phenomena of complex nature occur at the junctions of (a) and (b).
4. Red granite and dyke rocks, whose parallelism and extent of distribution present one of the most conspicuous features of the Eastern Desert of Egypt, mark the final eruptive action before Carboniferous times.
5. Three periods of volcanic activity have been subsequently noted—
(a) In Western Sinai in late Carboniferous times.
(b) An undated series of eruptions interbedded with the base of the Nubian Sandstone or intrusive into it with marked contact alterations.
(c) The basic intrusions near Cairo and the Fayum, etc., which are intimately associated with the Oligocena Continental Period in Egypt.
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Footnotes
1
A brief abstract of this paper appeared in the Geological Magazine for October, p. 465. Read at British Association, September 4, 1908. Given by permission of Director-General, Survey Department.
References
page 500 note 2 “On the Geology of Egypt”: Q.J.G.S., 1848, vol. iv, pp. 324–49.Google Scholar
page 500 note 3 “On the Rose-Coloured Syenite of Egypt”: Q.J.G.S., 1851, vol. vii, pp. 9–13.Google Scholar
page 500 note 4 “Geology of Egypt”: Geol. Mag., 1884, pp. 289–92, 385–92, 439–42.
page 500 note 5 “Note on the Microscopic Structure of some Rocks from the neighbourhood of Aswan”: Geol. Mag., 1886, pp. 103–7.
page 500 note 6 Geol. Mag., 1893, pp. 436–40, and Q.J.G.S., 1897, vol. liii, pp. 364–73Google Scholar (on rocks collected by Captain Lyons).
page 500 note 7 There are also some notes on the rocks from the Sinai Peninsula by Professor Bonney in Palmer's “Desert of the Exodus” (1871, p. 556), and by Mr. Rudler in Appendix B of Professor Hull's Memoir on the Geology, etc., of Arabia Petræa.
page 503 note 1 The formation of microcline appears to be associated with the crushing in this case, but that it always originates in this way is not implied in the above remarks. Professor Bonney (in a letter) mentions the ‘Laurentian gneisses’ as a case where no such evidence is forthcoming.
page 505 note 1 The identification is based on the predominance of simple twinning.
page 506 note 1 The writer has stated the opinion to which he was led by the field relations in dogmatic form, but fully recognizes that the conditions are exceptional. The ordinary explanations of the origin of these schists failed to satisfy the conditions observed.
page 507 note 1 Mr. Wells had previously brought the writer an instructive photograph of Jebel Sufra, and had remarked on the marked evidence of volcanic action at that locality.