Notes on some Analyses of Egyptian Igneous and Metamorphic Rocks

Extract

1. The gneiss of the Sikait-Zabara district is in its mineral composition an aplitic granite, which has been subjected to intense pressure. It formed part of a magma of granitic composition which invaded a series of serpentines, themselves altered peridotites. The biotite-schist in many places immediately overlying the gneiss is the direct result of the assimilation of parts of the ultrabasic Tocks by the intrusive magma, the quartz, beryl, fluorite, tourmaline rock, and talc-schists associated with the biotite-schist bearing itness to the accompanying pneumatolytic action and silicification.

2. The schist from Kosha in the North Sudan most nearlyresembles the acid gneiss of Wadi Shalul in the Eastern Desert of Egypt. If the latter were originally a granite, as has been suggested from the analysis, then the Kosha schist would itself be an intensely crushed granite containing potash and soda in approximately equal amount.

3. The schist from opposite Khor Abdel Rahman in the North Sudan has been classified as fine-grained, epidote-albite-gneiss of uncertain origin.

4. The schist between Khors Abu Dom and Sila in the North Sudan has been compared with the rock from the Lewisian formations of Coll in Scotland. It may be termed a hornblende-granulite,representing an extreme case of metamorphism with crystallization under great stresses of an originally sedimentary rock.

5. The green schist from the Wadi Um Selman ravine, near the Dm Garaiat gold mine in South-East Egypt, is also regarded as an epidote-albite-gneiss, the bands being alternately quartzose and epidotic. The lime-containing minerals originally present have been altered to epidote and calcite.

6. The rock from Da'aba Island, in the Second Cataract, is an oligoclase-mica-diorite-porphyry, the felspar having been proved to be more acid than originally concluded. This name has therefore to be adopted in place of labradorite-dolerite, the term applied to it in pi. lxxx of Hume's Geology of Egypt, ii, p. i, 1934.

7. The rock of Arkasa Island, also from the Second Cataract, is, judging from the analysis, probably a more acid form of the oligoclase-mica-diorite-porphyrite series.

8. The specimens Nos. 8 to 11 represent a group of rocks in which magnesite plays an interesting part in the composition.

While No. 8 from Wadi Haimur is in the main a serpentine with but little magnesite, the remaining three rocks contain a notable percentage of this mineral. Pending further investigation, it is suggested that these changes may have taken place during periods when the ancient peridotites were submerged beneath the waters of the sea at various times in the geological history of Egypt.

Dr. Hume desires to thank Mr. 0. H. Little, Director of the Geological Survey of Egypt, very warmly for having left Mr. A. I. Awad free to carry out the petrographical studies for this paper whenever circumstances allowed of his doing so. This paper is published with the approval of H.E. the Surveyor-General of Egypt.