Vol. 50.] PHYSIOGRAPHY OP THE LIBYAN DESERT. 537 



In general appearance, in mode of occurrence, and even, according 

 to Dr. Schenck, 1 in some of the genera, they are the same as those 

 occurring in the Nubian Sandstone south of the oases of Kharga 

 and Dakhla. 



Schweinfurth 2 has pointed out that at Mokattam this sandstone 

 (which may be called the sandstone of Jebel Ahmar, 3 where it is 

 typically developed, and whence it has been frequently described) 

 lies unconformably on the Eocene beds ; A. B. Orlebar 4 has 

 described it as resting on Miocene beds in the neighbourhood of the 

 Middle Station of the old Suez Railway ; I found it overlying the 

 Miocene beds on the south side of the Wadi Natrun, while at its 

 southern margin it overlies a thick bed of Ostrecu (0. Fraasi and 

 0. Cloti) which is of Upper Eocene (Upper Mokattam) age, thirty 

 miles north-east of the Baharia Oasis. 



A tongue of it, 25 miles broad, crosses the road from the village of 

 Mandisha (Baharia Oasis) to Bahnessa (lat. in the Nile Valley), 

 commencing 40 miles from Mandisha, and ending at a point rather 

 above the banks of Nummulites gizensis in the Mokattam series. 

 Thus it seems that this Jebel Ahmar Sandstone is later than the 

 marine Miocene beds, being perhaps of Upper Miocene age, and it 

 may be of a not very different age from the freshwater Miocene 

 beds in the neighbourhood of Siwa, as suggested by Zittel. 5 It is 

 this sandstone which furnishes the quartz-sand of the dunes of this 

 western part of the Sahara, till we come to the outcrop of the 

 Nubian Sandstone. 



Of later age than these are the beds south of the Giza pyramids, 

 containing Clypeaster cegyptiacus, and the sea-beaches of Cairo, to 

 which a late Pliocene age has been assigned. 



In the oases, and especially in the southern part of Kharga, there 

 is a considerable area covered by a fine sandy loam, slightly cal- 

 careous, unstratified, containing rootlet-tubes and occasionally land- 

 shells. This is evidently the result of fine sand and dust drifted by 

 the wind till it was retained and bound together by vegetation, 

 when the oasis was more cultivated than now. In recent times, 

 from want of cultivation, the loam has been deeply eroded and 

 rapidly removed by the wind. 6 



V. The Anticlinal Folds, and their Relation to the 

 "Water-supply. 



About 90 miles south of the Kharga Oasis (lat. 23° 20' N.) is the 

 spring Bir Murr (the bitter well), the first watering-place on the 



1 Op. tit. p. 15 : species given in columns 1, 3, 5, and 8 are from tbis 

 Jebel Abmar sandstone. 



2 Zeitscbr. deutscb. geol. G-esellscb. vol. xxxv. (1883) p. 718. 



3 Tbis term seems preferable to tbat of ' iVi'cofc-sandstone,' since tbat fossil 

 tree is described as occurring botb in tbese beds and in tbe Nubian Sandstone. 



4 Journ. Bombay Brancb Roy. Asiat. Soc. vol. ii. (1845) p. 232. 



5 Op. jam tit. pp. 132, 134. 



6 See Ricbtbofen, ' On tbe Mode of Origin of Loess.' Geol. Mag. 1882, p. 293. 



