532 CAPT. H. G. LYONS ON THE STRATIGRAPHY AND [Nov. 1 894, 



routes known as the Arbain road and the Terfau road. Between 

 these two roads the intervening desert was crossed in two directions, 

 and was found to he a sandstone plateau, falling slightly towards 

 the north as it neared the Cretaceo-Eocene limestone-escarpment of 

 the oases. 



Nowhere were any hills of a greater height than 200 to 250 feet 

 above the plain to be seen, though at a distance, when exag- 

 gerated by mirage, and especially when on the sky-line, small 

 hillocks appeared to be high hills several miles distant, and 

 Dr. Schweinfurth, in his map of the Kharga Oasis, records such high 

 hills in the desert to the south-west. 



Speaking generally, the Eocene beds stretch away northwards 

 from the line of the southern oases, where they end in an escarp- 

 ment facing south, until they pass under the Jebel Ahmar Sandstone 

 to the north of the Baharia Oasis, and under the Miocene beds to the 

 H7W. in the neighbourhood of the Siwa Oasis. These Eocene rocks 

 are underlain by Upper Cretaceous rocks, which form the floors of 

 the oases of Kharga, Dakhla, and Farafra, and the bases of the cliffs 

 which hem them in on their eastern, northern, and north-western 

 sides ; while to the south and south-west the ground rises gently 

 to the Desert plateau, consisting of the Nubian Sandstone, which 

 forms an immense tableland, rising and falling with gentle slopes, 

 hardly ever forming hills of any considerable height, but weathering 

 into flat-topped masses and truncated pyramids, which stand out as 

 witnesses of the amount of erosion which has taken place. These 

 isolated hills x are a special feature of the Libyan Desert, and 

 contrast strikingly with the sharp peaks occurring in the crystalline 

 areas, and with the rounded granite hills formed sometimes of 

 enormous boulders split off by the variations of temperature to 

 which they have been subject, and now crumbling away through 

 the breaking-up of their constituent minerals by the same agency. 



The earliest rocks occurring in this area are the crystalline rocks 

 exposed at the First and Second Cataracts and at Kalabsha (lat. 23° 

 30' N.), which have been frequently described 2 and have been classed 

 as Archaean. The only exposures that I know of in this part of the 

 western desert are, one at the small hill of Jebel Abu Bayan, 10 miles 

 south of the Kharga Oasis, and the other south of the Dungul 

 springs, and between them and the village of Tomas, on the Nile, a 

 few miles above Korosko. This latter spot I have not visited, but 

 at Jebel Abu Bayan the rock is a coarse-grained hornblendic granite, 

 with large crystals of pink orthoclase, and is apparently identical 

 with that described by Prof. Bonney. 3 The hill also contains dykes 

 of a fine-grained granitic rock, and some of a diorite, as well as one 

 of a fine-grained basalt. 



1 Zittel, ' Palseontographica,' vol. xxx. (1883) p. 38 ; Walther, ' Die Denu- 

 dation in der Wiiste,' p. 64, Leipzig, 1891, sep. cop., & Abh. d. k. sachs. Gesellsch. 

 d. Wissensch. vol. xvi. pp. 407 et seqq. 



2 Sir J. W. Dawson, Geol. Mag. 1884, pp. 289, 385, etc.; T. G. Bonney, 

 Geol. Mag. 1886, p. 103 ; C. A. Raisin, Geol. Mag. 1893, p. 436. 



3 Geol. Mag. 1886, p. 105. 



