CHAP. XI. THE OASES. 119 



southerly point. The mountains of the high plain 

 are limestone; the low plain of the Oases is sand- 

 stone on clay ; and it is from this last that the water 

 rises, and by this it is retained. The limestone 

 mountains of the Thebaid rest in like manner on 

 clay; and thus we may conclude that the water 

 is conveyed from some point to the South of, and 

 at a greater elevation than, the Oasis, its escape to 

 the surface taking place wherever the limestone 

 superstratum is removed ; and that a continuation 

 of the same bed of clay conducts it northward to 

 the Oasis Parva, — occasional opportunities being 

 afforded it for rising, as at Farafreh, and other 

 places on the way. 



Though I have represented the mountains, as if 

 the tableland of their summit were perfectly level, 

 in order to show the comparative depressions of 

 the Oases, it is not to be supposed that they are 

 perfectly horizontal : if so, those of Lower Egypt 

 would be more elevated than in the Thebaid, which 

 is not the case ; the mountains of Thebes being 

 1200 feet above the Nile, which is a much greater 

 elevation than any in the latitude of Cairo. 



From what has been said, it is evident that 

 the Oases are not fertile spots in the midst of 

 a sandy plain, but depressions in the lofty table 

 land of Africa, where, by the removal of the su- 

 perincumbent limestone strata, the water has the 

 power of rising to tlie surface ; nor is the desert 

 a dreary plain of sand, which has overwhelmed a 

 once fertile country, whose only traces are the 

 isolated gardens of the Oases ; where the traveller 



I 4 



