Vol. 50.I PHYSIOGRAPHY OF THE LIBYAN DESERT. 517 



quakes atYVadi Haifa, etc., as seen in the demolition of the temple at 

 Karnak, and in vertical rifts on the face of perpendicular rocks, on 

 which figures were sculptured that were now split completely down : 

 Wadi Haifa being, as Capt. Lyons observed, in the direct line of the 

 anticlinals, which extended in a slightly N.W. and S.E. direction. 



Prof. Hull concurred with the view of the Author that the course 

 of the Nile above Cairo had been determined by the line of fault, 

 which follows the valley for many miles upward. As regards 

 the age of the Nile in Egypt, he considered it as referable to the 

 Miocene stage rather than to the Pliocene. The Miocene period in 

 that part of the world was one in which the main features of the 

 present land-areas received their general contours. Referring to an 

 observation by Mr. Hudleston regarding the absence of Carboniferous 

 beds in the Nile Valley, he reminded the Society that deposits of 

 this age had been discovered by Dr. Schweinfurth in the Wadi-el- 

 Arabah, between the Nile and the Gulf of Suez. 



Dr. Irving could not resist the temptation to say a word to con- 

 gratulate his old friend and former pupil on the excellent use he had 

 made of the opportunities which his service in Egypt had put in his 

 way, and on the interest of the results of his work now before the 

 Society. Remarking on the silicification of wood, he wished again to 

 emphasize the difference in the action of carbonic acid in penological 

 changes, according as it existed as a free acid or in combination with' 

 a base, as in sodium carbonate. The extent of the ' Natron ' deposits 

 pointed to the supply of alkaline waters over large areas in former 

 times, holding the mineral in solution. The reaction of such waters 

 upon the potash-felspar of the sands, furnished by the disintegration 

 of the crystalline rocks, would not lead to the' deposition of free 

 silica (as in the ordinary process of kaolinization), because, while the 

 potassium was taken up as a carbonate and carried away, the silica 

 was also removed in solution, through combination with the sodium, 

 to form sodium silicate. This last-named salt in solution would be 

 readily decomposed by the organic acids and the carbonic acid fur- 

 nished by decaying vegetable tissue, the silica being then deposited 

 as a colloid in situ, and thus retaining the structural forms of the 

 original tissue. 



The Author, in replying, agreed with Mr. Hudleston as to the 

 occurrence of Carboniferous beds underlying the Nubian Sandstone 

 east of the Nile, but he had been unable so far to detect them in 

 the Libyan Desert. The silicification of the fossil wood he believed 

 to occur separately in each period, and were Egypt of to-day a 

 wooded country, he would expect to find the same in progress in 

 the Delta. He agreed with Prof. Hull that the north-and-south 

 folds of Kharga were probably connected intimately with the Nile 

 Yalley fault. Dislocations fracturing inscriptions show movements 

 to have taken place in historic times, as suggested by Mr. Henslow ; 

 and the water-system of the Desert, as determined by the folds of 

 the strata, seems to indicate the position of oases other than those 

 that we at present are acquainted with. 



Q. J. G. S. No. 200. 2 q 



