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



Wherever these springs were occasioned by an east-and-west fold 

 they would occur along a considerable length — so as to form a recess 

 such as the Dakhla Oasis, wbile, where the north-and-south folds 

 came in, the erosive action would be most active in that direction ; 

 it is to this that I would attribute the width of Dakhla and the 

 southern end of Kharga from east to west, while the north-and- 

 south folds have determined the longitudinal shape of Kharga. 



Evidence of this erosion was found at a spot (lat. 24° 20' N. and 

 long. 29° 50' E.J where a small patch of limestone occurred, composed 

 of blocks containing Alveolina ovoidea, Schwag. (Lower Eocene), 

 bound together by a calcareous cement. In parts it was a true 

 limestone gravel. This mass lay directly on the Nubian Sandstone. 



The beds of calcareous tufa under the cliffs of the Kharga Oasis 

 containing Quercus ilex, etc., as described by Zittel, show that 

 during this earlier (Pliocene) period of erosion the oases attained 

 approximately their present dimensions, while I would refer the 

 tufa deposit to the later post-Pliocene time. 



The floors of these oases are now rather below the level of the 

 Nile at Assuan, Kharga being 140 feet, as compared with Assuan, 

 280 feet above sea-level ; but though wind-action may have helped 

 to deepen them, the depressions and elevations of the Nile Valley 

 and neighbouring deserts at various times render it impossible to 

 say to what level the river-action of the past could or could not 

 have worked. 



Mr. E. A. Ployer ' considers that no more rain than falls to-day 

 is required for the districts bordering the Nile, and cites Schwein- 

 furth as inclining to the same opinion. Besides the erosion already 

 described as having taken place in early Pliocene times, evidence in 

 the Libyan Desert tends to show that there was a time, probably 

 post-Pliocene, when the desert was finally being carved and moulded 

 into what is practically its present form, when there was a consider- 

 able rainfall over the area, though not necessarily an excessive one. 

 Three hours west of the cliff's overlooking the town of Girga in 

 Upper Egypt there is 8 to 12 feet of flints covering the limestone 

 surface of the desert ; these are closely packed together, with a 

 small amount of iron-stained, earthy material between them. While 

 the topmost layer is markedly fractured and blackened by exposure, 

 the lower one* are usually whole and show no signs of blackening. 

 This seems to show that the limestone was quietly dissolved away 

 during a period of rainfall, till the flints accumulated to a con- 

 siderable thickness. Then the physical conditions changed, and a 

 period of rainfall passed into one of true desert conditions, with 

 fracturing of the flints by variations of temperature, and blackening 

 of the surface -bayer by exposure. 



All along the foot of the limestone escarpment between Kharga 

 and Dakhla, and under the cliffs on the east side of the former oasis, 

 there are beds of streams with rolled limestone-pebbles and boulders. 

 Some of this is probably due to the rare rain-storms of our own 



1 Quart. Journ. Geol. Soc. vol. xlviii. (1892) p. 580. 



