THE GEOLOGY OP THE JflLE YALLET. 481 



33. Notes on the Geology of the Nile Yalley. By E. A. Johnson^ 

 Pasha, Chief Inspector of Egyptian Police, and H. Deoop 

 EicHMOND, Esq., E.I.C., late Second Chemist to the Egyptian 

 Government. (Communicated by A. Noeman Tate, Esq., 

 F.G.S. Eead June 8th, 1892.) 



These notes refer to some observations on the geology of the Nile, 

 during journeys from Cairo to Wady Haifa. 



The rocks on both sides of the Nile are, speaking generally. 

 Eocene (and ? Cretaceous) from Cairo to Esneh ; at Esneh, or about 

 live miles south, we enter a sandstone region, which continues to 

 near Assouan, where it meets the granite and basalt. A few miles 

 south of Assouan the sandstone recommences and continues to Wady 

 Haifa, broken only by enormous dykes of granite, which occur at 

 Bab-el-Kalabsha and one or two other localities. 



The following points are worth noticing as we follow the Nile 

 from Cairo to Wady Haifa : — 



(i) The saturation with silica of the old sand-and-gravel beds 

 overlying the Nummulitic Limestone near Cairo, forming sandstones 

 of all qualities from an extremely hard stone to one that can be 

 rubbed down to powder between the fingers. 



A similar saturation, apparently from water containing silica in 

 solution, is visible in the beds overlying the Nummulitic Limestone 

 in Biluchistan. 



(ii) The fact that these old sands and gravels are formed from 

 very old rocks, principally granitic. They contain great quantities 

 of the Assouan pebbles, brown and red, wholly or semi-transparent, 

 which were so largely used by the Eomans to cut cameos and 

 intaglios. Some minerals are found in these gravels, notably cobalt, 

 the source of which is unknown. 



(iii) The petrified forests of the same region. 



(iv) The occurrence in greater or less quantity of titaniferous 

 magnetic oxide of iron in the beds forming the Nile Yalley ; above 

 Assiout this forms roughly Jg- of the total bank section, and is mixed 

 with silicates of iron in a more or less advanced stage of oxidation. 



The clays containing it are darker than the rest, and have a ten- 

 dency to break away in perpendicular prisms which in appearance 

 recall those of basalt. These banks assume a red coloration when 

 exposed to the air. 



The ironsand is carried down by the Nile despite its weight, and 

 careful dredging of the bottom of the river has revealed its presence 

 (about 1 °/o) ^^ f^^ down as Cairo ; this sand probably plays a very 

 important part in the remarkable process by which the natural 

 purification of the Nile is effected. It occurs in greatest quantity 

 at Jebel Tarif, near Fashoda, and in the plain of Koos.^ 



^ [For a discussion of the purification of the Nile water, see 'Report to the 

 Egyptian Government on the River Nile,' which the writer hopes to publish in 

 a forthcoming number of the ' Journal of the Society of Chemical Industry.' — 

 H. D. R.J 



