Vol. 50.] PHYSIOGRAPHY OF THE LIBYAN DESERT. 533 



II. The Nubian Sandstone. 



Wherever the junction can be seen these rocks are overlain by 

 the Nubian Sandstone, which becomes coarser at its base, usually- 

 forming a quartz-conglomerate. In no case have I ever observed any 

 sign of the metamorphism of the sandstone by the granite as 

 recorded by Mr. E. A. Eloyer 1 and Johnson Pasha. 2 I have not seen 

 the localities referred to by them, but such metamorphism is 

 totally at variance with the mode of occurrence of these rocks in 

 the area that I have examined. The only case of an alteration of the 

 sandstone which I have met with is at Jebel Burka, 20 miles 

 W.N.W. of Wadi Haifa, where a mass of olivine-dolerite has 

 forced its way through the sandstone, porcellanizing and altering 

 it for a few inches at the junction. 



The Nubian Sandstone varies much in colour and durability, 

 according to the amount of staining by oxides of iron and 

 manganese and the amount of cementing silica. Lenticular beds 

 of clay occur which sometimes extend for several miles, and these 

 are considerably developed at the base of the hills east of Wadi 

 Haifa (lat. 21° 55'), while on the western bank of the river at this 

 point a bed of stiff blue clay may be seen in some ancient Egyptian 

 tombs. A similar clay has been recorded as occurring at the Shebb 

 wells, 100 miles W.N.W. , and I saw a sample of stiff blue clay 

 brought up from a depth of about 160 feet from the surface at the 

 village of Mushia in the Dakhla Oasis, where it underlay the water- 

 bearing sandstone. I am not, however, in a position to bring 

 forward evidence of a continuous stratum of this blue clay extending 

 over a large area, and serving to hold up the water in the overlying 

 beds, though Mr. E. A. Eloyer 3 describes it as continuous over a 

 large tract, and overlying the granite floor on the east side of the 

 Nile between lat. 23° and 25° N. 



These beds seem to be lenticular deposits rather than continuous 

 beds, and the appearance of the Nubian Sandstone, wherever I 

 have seen it, is strongly suggestive of an estuarine deposit. The 

 strata are often strongly false-bedded, and fine clay-partings occur 

 from time to time. Nodules 4 of peroxides of iron and manganese 

 are very widely distributed, especially in the portion containing the 

 fossil trees (Araucarioxylon and Nicolia), and owing to their black 

 colour, metallic ring, and fantastic shapes these nodules have been 

 constantly described by travellers as lava, volcanic bombs, etc. A 

 small hill, Jebel Karan, at the southern end of the Kharga Oasis 

 has been mapped as a ' black basalt hill,' whereas it is nothing but 

 a small sandstone knoll that has its sides covered with these nodules, 

 and with fragments of dark-red sandstone from the bed forming the 

 top of the hill. 



The sandstone itself varies from a dark purple-red mass of 

 quartz-grains cemented by silica, coloured by the oxides of iron and 

 manganese, hard and refractory, breaking through the quartz- 



i Quart. Joum. Geol. Soc. vol. xlviii. (1892) p. 577. 2 Ibid. p. 483. 

 3 Ibid. p. 576. * Zittel, op. jam cit. p. 58. 



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