THE GEOLOGY OF THE K'OETHERN ETBAI. 579 



detritus in floods that at its mouth a considerable island has formed. 

 For the Wadi Jemal forms a depression within a depression — a fold, 

 as it were, in the hollow of the camel-saddle. 



In the latitude of the Emerald Mines the ridge is very low. Con- 

 sequently the westward dip of the sandstone is very slight. It is 

 here that the sandstone lies over blue clay, and the slight angle 

 formed with the horizon makes what is apparently a bed of great 

 thickness spread out over some miles of surface from which the 

 sandstone has been denuded. On this Kne may be seen every kind 

 of change in the blue clay : from the blue clay full of ferruginous 

 nodules which is seen in the cliffs of Natash and Shaid, it changes 

 eastward into mounds of laminated clay or slate. Then follows 

 the pistachio-breccia (described as a volcanic breccia) where the 

 topazes are found, and then the mica-schist, mica-slate, and talcose 

 blue clay of the mass of Zabara where the emeralds are found, 

 while farther north it merges into the hard slate-breccia of the road 

 between "Kin a and Kosair. 



§ 4. The Wadis Aebad, Shaid, and Natash. 



Between the 24th and 25th parallels the edge of the sandstone- 

 plateau lies near the 34th meridian. On the east it stands 200-300 

 feet above the blue clay, and it slopes down rather rapidly to the west. 

 The two Wadis, Shaid and JN'atash, which have cut through this edge, 

 have clearly percolated through the blue clay which shows in the cliffs 

 from 50 to 100 feet above the bed, generally obscured by great boulders 

 of sandstone which have fallen from the cliffs on either side. The 

 thickness of the blue clay, if it be not folded at Zabara, may be esti- 

 mated by the height of that mountain, which seems to be formed of 

 it from top to bottom, at about 1200 feet. Between the sandstone- 

 edge on the 34th meridian and the mass of Zabara are two north- 

 west and south-east ranges converging to the north, and called 

 Nugrus and Hafafit. These seem to be the sandstone metamorphosed, 

 and may indicate the thickness of that formation at 1000 feet. 



These ranges at their northern end nearly join the metamorphic 

 mass of Mijif in lat. 24° 50'. Hence northwards the crest is 

 ' cataract ' rock, and the characteristic feature of it is the great 

 bosses, principally of felspar, which rise along it, as Abu Diab, Ko- 

 daboro, Um Nagad, Sabai, and Abu Tiur. 



To the east the ' cataract ' granite sinks under the blue clay, here 

 found in every kind of metamorphosis from a dark grey or black 

 schist to laminated clay and slate fuU of quartz-veins, which at Um 

 Rus have been extensively worked for gold. 



The Wadi Abbad was not followed to its junction with the Nile at 

 Edfu, so the line where the sandstone sinks under or meets the lime- 

 stone was not accurately determined. But at the Eoman station of 

 Abu Gerayeh (lat. 25° 18' Is^., long. 34° 8' W.) pockets or isolated out- 

 crops of limestone occur in the sandstone, and have been used by the 

 builders to cement the large burnt-brick tanks which collected water 

 for the station. North of Abu Tiur the ' cataract ' rock sinks below 



