Prof. T. G. Bonnet/ — Structure of the Rocks of Assouan. 103 



rocks to be expected in ascending order would be the Upper Lauren- 

 tian or Norian and the Huronian. To the former the second Biggeh 

 series bears no resemblance, but there are known to be in the Arabian 

 chain, and probably associated with the equivalents of the Assouan 

 rocks, Norian rocks of the character of anorthosite gneiss, a rock 

 which was used by the ancient Egyptians for statuary, 1 but is gene- 

 rally called diorite by antiquaries, though it differs very much from 

 the true diorites of the country. 



Dr. Schweinfurth has sent me from the districts of the Arabian 

 chain north of Assouan, a rock similar to the more compact variety 

 of the dark Biggeh rock, which he states forms ridges parallel 

 to the main chain of crystalline rocks. Newbold refers to greenish 

 and chocolate-coloured schists and quartzite, as bordering the older 

 schists and granites, and Lartet notices talcose and chloritic slates 

 with granulite in a similar relation, crowned by the celebrated green 

 conglomerate and breccia of Kosseir and Gebel Doukhan. Further, 

 Eussegger connects the red porphyry and petrosilex porphyry with 

 large felspar crystals of Gebel Doukhan with this second series, and 

 Lartet has described the quartziferous porphyry of Mount Hor as 

 lapping around the granite nucleus of that mountain. 



It would thus appear that the old Laurentian gneisses and schists 

 of Upper Egypt and its eastern mountain chain are succeeded by 

 formations which may be held to represent the Norian and Huronian 

 series at least, and I would regard the Biggeh formation or Second 

 Assouan series as consisting mainly of bedded volcanic material 

 representing some portion of the Huronian, a formation which would 

 seem to have been more largely developed or to be better preserved 

 in some parts of the Arabian chain to the north and east, where it is 

 also overlain by slaty rocks, and by the green conglomerate which 

 either constitute an Upper Huronian series, or may represent the 

 Ammike and Kewenian formations of America. There would 6eem 

 in this district to be a great geological hiatus between these old rocks 

 and the Nubian Sandstones. 



III. — Note on the Microscopic Structure of some Eocks from 

 the Neighbourhood of Assouan, collected by Sir J. W. Dawson. 

 By Prof. T. G. Bonney, D.Sc, LL.D., F.B.S. 



I HAVE not deemed it necessary to examine microscopically the 

 whole of the collection made by Sir J. W. Dawson, but have 

 selected a series of fairly representative specimens, sufficient, I think, 

 to give a general idea of the principal varieties. For purposes of 

 identification I leave the reference numbers borne by the specimens, 

 though their sequence is accidental. 



(A.) Older Gneissic Series, Assouan. 



(3.) A moderately fine-grained holocrystalline pink and white 

 rock, speckled with black, of slightly gneissic aspect. 



Microsc. — Quartz, felspar, orchoclase with some microcline and 

 plagioclase, biotite, occasionally showing signs of alteration to the 



1 The fine statue of Kaphra or Cephren in the Boulak Museum is of this stone. 



