36 GEOLOGY 



river. The sandstone varies considerably in itself, its lower 

 strata often forming a coarse conglomerate, and varies in 

 colour from deep red to green and yellow, and is generally 

 darker than the sand upon the desert, this being probably 

 due to the colouring-matter having been more easily de- 

 composed and washed away than the pink and white grains 

 of silica. This formation appears chiefly, if not entirely, to 

 belong to the Miocene period ; and the deserts on each side of 

 the Nile are, no doubt, due solely to its decomposition. I 

 believe it to be erroneous to suppose that the sea left the 

 sand of the desert as we now behold it, but that it was for- 

 merly a sandstone, its particles being cemented together by 

 the same materials as that which we now see forming the 

 " Red Mountain," near Cairo, and the thick formation at 

 Silsilis, and that the loose sand of the desert has been formed 

 by its disintegration by atmospheric action ; for we cannot 

 study these strata without frequently meeting with examples 

 of recent degradation going on upon a large scale ; and when 

 we consider the vast time during which the atmosphere has 

 been acting upon these strata, we need not be surprised at 

 the extent of its ravages. 



3. Limestone. — This is throughout of an extremely pure 

 whiteness, and is first met with in the mountains of Nummu- 

 litic limestone near Cairo, and extends throughout the whole 

 of Egypt in remarkably flat-top})ed ranges, forming at places 

 steep perpendicular cliffs down to the water's edge, as at 

 Gebel e' Tayr and Gebel Aboofayda, but is driven back 

 inland at Silsilis by the sandstone strata, while in Nubia, 

 near the river, it is only met with in detached masses forming 

 outlying hills. The desert on each side of the Nile is gene- 

 rally bounded by steep cliffs ; for the cultivated land of Egypt 



