546 CAPT. H. G. LYONS ON THE STKATIGBAPHY AND [Nov. 1 894, 



of the Selima Oasis, are very extensive natron-deposits in the midst 

 of the Nubian Sandstone area. 



Dr. A. H. Hooker, Director of the Salt Department of Egypt, 

 informs me that he has seen silicification of woody structure in 

 progress to-day in the Wadi Natrun. 



PLATE XXI. 



Geological Map of the Libyan Desert of Egypt, at the scale of - 



= '0211 inch to the mile, or about 50 miles to 1 inch. ' u ' 



Discussion. 



The President congratulated the Author on the good use that he 

 had made of his opportunities in carrying on geological observations 

 in so very difficult a country as the Libyan Desert — a country so 

 destitute of escarpments. The Author had made use of lines of wells 

 when no other feature was available to guide him in predicting the 

 underground lie of the rocks. From the evidences everywhere of 

 aeolian action, and the abundance of silicified and highly weathered 

 tree-trunks, it appeared that this area must have been exposed to 

 subaerial conditions through a vast period of time as an old land- 

 surface. He hoped that Capt. Lyons would make further good use 

 of his military travels in the Desert. 



Mr. Hudleston congratulated the Society on at last receiving an 

 interesting paper on the geology of the Egyptian Desert, where the 

 general conclusions of well-known authors were confirmed and 

 supplemented. He would like, amongst other matters, to have a 

 possible explanation of a statement made here two years ago, that 

 the Nubian Sandstone rested on basalt, but was invaded and meta- 

 morphosed by granite. The absence of the sandstone of Car- 

 boniferous age was not surprising in the level area, but when 

 Capt. Lyons carried out his plan of investigating the Eastern 

 Desert, it was not improbable that he would find this formation on 

 the margin of the crystalline mountain-chain flanking the Red Sea. 

 The silicification of large masses of wood was a feature characteristic 

 of all the sandstone-beds of many ages in Egypt. Was this feature 

 a contemporaneous one, or had there been a period of general silici- 

 fication ? The methods by which this had been effected were the 

 result of replacement due to decomposition of woody tissue, and were 

 observed in all parts of the world. The hydrographic questions 

 raised by Capt. Lyons were of extreme interest and economic 

 importance. He had received a good training in a sandy, thirsty 

 district nearer home, namely, the Bagshots. 



The Rev. G. Henslow remarked upon the great practical import- 

 ance of Capt. Lyons 's observations as to the Nubian Sandstone 

 being a water-bearing stratum : thereby correcting the old view that 

 oases were low-lying localities in which the water of the Nile, by 

 penetrating soft strata, was accessible by wells. He observed that, 

 by following the anticlinals to north-western localities, water might 

 probably be found in the Western Desert, in places where it is at 

 present unknown. He also drew attention to the evidence of earth- 



