538 CAPT. H. G. LYONS ON THE STKATIGRAPHY AND [Nov. 1894, 



Arbain road leading to Dongola and the Sudan, where caravans 

 water their camels, the water being too salt for men to drink. 

 Geologically it lies in a small depression eroded out of the 

 Cretaceous (Exogyra Overwegi) beds, and is situated on the crest of 

 a sharp anticlinal fold. 



As this spot furnishes an important clue to the stratigraphy of the 

 desert, I will describe it somewhat in detail. On approaching the 

 spring from any direction, attention is at once arrested by a large 

 bed of white limestone forming a small plateau. The limestone 

 contains many Cretaceous fossils, and is underlain by a bed of 

 Exogyra Overwegi. Below this is a series of grey, green, and 

 yellow beds of sand and clay with two or three bands of a calcareous 

 sandstone which weathers to a curious moss-like form ; in fact the 

 beds seem to correspond exactly with those of the section * at Jebel 

 Ter, near Kharga, and that of Jebel Omm-el-Ghenaim quoted by 

 Zittel from Schweinfurth. The extent of the limestone is not large, 

 not more than about 8 miles from north to south and perhaps as 

 much as 15 to 20 miles from east to west. 



The spring itself is situated on the south side of a small fault 

 with a downthrow of perhaps 20 feet to the south, and its direction 

 is about 25° N. of E. for a distance of about a mile. This direction 

 is only maintained locally, for the dips usually observed along the 

 road to the north of these springs for 30 miles are N. N.E., and 

 S. S.W., the strike being some 20° S. of E. Farther north the dip 

 becomes too small to be readily determined, till we come to the 

 Kharga Oasis. 



In dealing with rocks such as the Nubian Sandstone, exposed 

 over large areas and not presenting any marked lithological 

 difference between the beds, it is extremely difficult to trace the 

 minor folds across a country where want of water compels the 

 observer to keep to certain definite tracks or to hurry from point to 

 point. This must be my excuse for attempting to' elucidate the 

 stratigraphy of the Libyan Desert from such slender data, but the 

 exceptionally uniform character of the beds facilitates the endeavour. 



Dr. Zittel has pointed out that the springs of the oases are 

 fed from an underground water-bearing bed which draws its 

 supplies from the rainy districts of Darfur, etc., to the south, and 

 not from the Nile. The water drains down the dip-slope of these 

 sandstone beds, till it can find its way to the surface through fissures 

 in the overlying beds, or by artificial borings, being forced up by 

 the pressure due to the elevation of the gathering-grounds to the 

 south. Thus we shall have the water brought nearest to the 

 surface at points on the axes of the anticlinal folds, and may expect 

 therefore to find that the oases and desert springs are so situated. 

 Should this prove to be the case, the whole question of an increased 

 water-supply for the oases — in other words, their improvement and 

 development — is intimately connected with the geological structure 

 of the area. 



To the west of the Arbain road from Assiut to the oases of 



1 Zittel, op. jam cit. p. 80. 



