Dr. W. F. Eume — Petrography of Egypt. 503 



over wide areas leans to the view that these represent the earliest 

 igneous intrusions obsei'ved in Egypt, and that the magma was more 

 readily plastic than in the later intrusions. 



No such difficulty exists with regard to the Arabian Desert gneiss, 

 which in a large number of cases reveals its granitic or igneous 

 origin by the effects produced where it is in contact with the 

 ancient sedimentaries subsequently described, and is thus directly 

 connected with 



II. — The Pee- Carboniferous Plutonic Eocks. 



(c) The rocks of this series vary from the most highly acid granites 

 to ultra-basic varieties. Of these undoubtedly the most interesting is 

 the Red Granite of Aswan, which has recently been described by Ball 

 (" Aswan Cataract," p. 70) in some detail. This rock is characterized 

 by the large idiomorphic crystals of orthoclase, associated with 

 quartz, biotite, and hornblende. The latter is very variable in 

 quantity. 



Ball has further shown that the granite has undergone intense 

 crushing, the orthoclase having been completely changed to inicrocline,^ 

 while round the large crystals is a ' mortar ' of small granules resulting 

 from the attrition consequent on the sliding produced by the act of 

 crushing. 



During the course of the Survey much interesting material has been 

 collected bearing on the results of contact-metamorphism near the 

 junction of the plutonic rocks and the schistose (ancient sedimentary) 

 rocks into which they have been intruded. The plutonic rocks in 

 numerous instances have been found to undergo a change from acid to 

 more basic varieties. In Sinai, near Dahab, the gneissic fringes are 

 well marked, hornblende or mipa increase in quantity near the junction, 

 and sphene may become an important constituent. 



Syenites in general are rare, though well developed at Aswan and 

 Jebel Zeit, but diorites are of the widest extension throughout the 

 Arabian Desert. The dark rocks constituting the principal member of 

 the Second Cataract at Wady Haifa is also of this nature, the horn- 

 blende being present in large quantity. Quartz diorites are especially 

 present in the neighbourhood of the Imperial Porphyry region of 

 Dokhan. 



In some areas basic rocks attain unusual development, especially 

 in the neighbourhood of the Q,ena-Q,osseir Road, about lat. 26° N., 

 between the Nile and the Bed Sea. These usually are in the form 

 of coarse gahhros, which are composed of plagioclase and augite, but 

 the latter readily changes to hornblende. Locally the rock may be 

 almost purely composed of diallage, and under such conditions is 

 sometimes highly magnetic, while at El Banga, near the old Berenice, 

 haematite has been produced by concentration from a gabbro rich in 

 a ferruginous constituent. A very hard gabbro is also present in the 



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 forthcominsr. 



