TRANSLATIONS AND NOTICES 



GEOLOGICAL MEMOIRS. 



On Altered Tertiary Rocks near Cairo. 



The following remarkable case of metamorphosis in rocks adjoining 

 the alluvial land of Lower Egypt is well deserving the attention of 

 geologists, from the difficulty of assigning the cause or causes to 

 which the change of structure in so modern a sedimentary deposit can 

 be ascribed. It appears from Russegger's geological map of Egypt, 

 that the nearest igneous rock is seventy miles distant, on the southern 

 ■flank of the mountain Dschebel Areidy, on the border of the Gulf of 

 Suez, in the parallel of the town of Benisuef on the Nile. 



[L. H.] 



[From Russegger's Reisen in Europa, Asien und Afrika, B. i. s. 272-277.] 



'* North-east from the Mokattam* the coarse limestone beds are 

 covered by a sandstone, consisting of a mixture of fragments of quartz, 

 agate, flint, chalcedony, hornstone and flinty slate, with a pretty uni- 

 form grain, and of considerable hardness. It contains a few marine 

 organic remains, and, in my opinion, is an ancient marine diluvium. 

 This sandstone forms in general a plateau, with low flat-topped hill- 

 ocks, but rises here and there into groups of hills, the height of which 

 however is inferior to that of the Mokattam. It is the prevailing rock 

 throughout the whole Isthmus of Suez, and on the coast it is overlaid 

 by a recent marine formation. This sandstone contains siliceous 

 concretions, and also fragments of fossil wood, which is converted 

 wholly into silica. 



" In several places this rock exhibits some remarkable alterations of 

 structure, which, at first sight, one is almost convinced must have been 

 occasioned by volcanic action. The grains of the sandstone appear 

 agglutinated, as if changed into the state of a frit ; the mass appears 

 penetrated throughout by pure siliceous matter, which gives it a 

 homogeneous structure, until at last it acquires the aspect of a horn- 

 stone passing into an obsidian, with a flat conchoidal fracture and a 

 ringing sound when struck, like phonolite. This altered rock is in 

 parts remarkably beautiful, exhibiting all manner of colours, with a 

 lustre between greasy and vitreous. The included nodules appear 



* A hill composed of nummulite limestone and other tertiary sedimentary rocks, 

 near Cairo, rising to a height of 430 feet above the Mediterranean. 



VOL. V. — PART II. B 



