3S GEOLOGY 



these crystalline rocks with the superincumbent stratified 

 formations, we find a great absence of all fissures, rents, and 

 faults, except such as have evidently been formed by atmo- 

 spheric causes ; and the stratified rocks throughout Egypt and 

 Nubia being remarkably horizontal, we are led to believe that 

 they rose slowly without any very marked local volcanic action. 



The atmospheric causes have had a far greater power in 

 decomposing the strata than one would expect to find in such 

 dry countries. Rain does occasionally fall, though rarely, but 

 at such times generally in tremendous downfalls, as testified 

 by the ravines and deep torrent-beds, which are by no means 

 uncommon in the limestone strata, as, for instance, the 

 valley by which we approach the " Tombs of the Kings " at 

 Thebes. 



The desert is constantly encroaching upon the cultivated 

 land ; and this is especially the case in Nubia, where the 

 inhabitants have become scarce by emigration to the busier 

 parts of Egypt ; for arable land on the confines of the desert, 

 if left alone for a few years, gets covered up by drift sand, 

 and becomes barren and useless, so that it cannot again be 

 cultivated without great labour. 



Besides the formations above enumerated, there are two 

 minor ones, parts of the great freshwater deposit of the Nile, 

 which deserve special notice. 



1. Natron or Kunkur. — A species of travertine, composed 

 of lime in hard concretionary masses, or perhaps more often 

 tufaceous and nodular. It is formed by the action of the air 

 upon the chemical constituents of the alluvium ; and its 

 presence, as already mentioned, on many parts of the desert 

 near the river indicates an alluvial subsoil, and testifies to the 

 encroachment of the desei't upon the cultivated land. 



