THE GEOLOGY OF THE N-QETHEEIf ETBAI. 577 



almost wholly of felspar and mica, tlie latter mineral causing it to dis- 

 integrate rapidly. It shows often dark spots and patches which the 

 Arabs call " leprous patches," and which are caused by a large pro- 

 portion of hornblende. This rock forms a large part of the surface 

 east of the watershed to the south, where it has been covered only 

 by sandstone. . It is worn into caverns and potholes and into large 

 balls ; it peels and blisters like a lump of dough. All authors who 

 have written about this country have noticed it, from Strabo down- 

 wards, and it looks so much as if it had been subjected to the action 

 of a rushing stream that Colston writes, " on seeing it one would 

 say there had been a cataract here." ^ 



The lapis psaronius quarried at Jebel Patirah is probably the 

 same rock that has become brighter and more sparkling by a larger 

 proportion of quartz and hornblende : it rises up into great pinnacles 

 and bosses called ' Mudarghag,' some of them 1600 feet in relative 

 height, and standing in a mass of their own scalings ; grey, like 

 those of Hamrat Mukbud, buff like Abu Diab, pink like Kodaboro, 

 which is composed almost entirely of pink felspar, and down whose 

 sides streams a gravel recalling in appearance pink snow. Where 

 this ' cataract ' rock touches the sandstone, the latter is metamor- 

 phosed into a brilliant green crystalline rock. This is a very hand- 

 some stone at the mines of Um Eleagher, near Jebel Abu Dhaher. 



Below the ' cataract ' rock is a very compact, hard granite, of ap- 

 parently great thickness. This does not often come to the surface, 

 but it forms the principal mountain-masses, such as Abu Erghiib, 

 Aybarun, Hamata, Abu Gurdi, and the twin peaks of Hamamid. 

 It is close by the huge mass of Hamata that the porphyry of Hullus 

 appears ; the rock is of various colours, but has chiefly a dark cho- 

 colate matrix with small quartz-grains. There are other colours : 

 perhaps the most noticeable being a clear sea-green which, when 

 weather-worn, has a purplish surface with yellow lights something 

 like a starling's breast. 



§ 2. ASSUAN TO THE EeD SeA. 



On leaving Assuan by the Bab el 'Ajjaj, the Pass of the Sand- 

 driving Wind, one first crosses a mass of crystalline and schistose 

 rock consisting of diorite, dolerite, and syenite. It rises gradually 

 to a height of 900 feet, and then, at the Wadi Allowi (which drains 

 into the Khareit), it falls a little to 700 feet, the level of the sandstone- 

 plain. 



Beyond this mass of rock the flat desert rises regularly and almost 

 imperceptibly to the east. The surface is furrowed by long ridges 

 of sandstone with horizontal bedding running from N.W. to S.E. 

 and ranging from 200 to 600 feet of relative height. Here is petri- 

 fied wood similar to that found outside Cairo. 



Up the broad Khareit valley, at the foot of the Jebel Mikbia, a 

 cluster of ' cataract' boulders will be readily recognized by the name 



. ^ ' Beconnaissance from Berenice to Berber/ Bull. Soc. Ehed. Geogr. 1878. 



