THE GEOLOGY OF EGYPT. 407 



but in the Wadi Natrun it is stated by Lyons to rest upon 

 Miocene beds. The palaeontological evidence, however, 

 is not given, and these "Miocene" beds may well be 

 Lower Tongrian. 



The silicification of the Jebel Ah mar Sandstone and 

 of the trees which it contains, is one of the most interest- 

 ing problems of Egyptian geology, and has been frequently 

 discussed. Schweinfurth was of opinion that it was due to 

 the former presence of geysers or springs of heated waters, 

 and the discovery of siliceous tubes penetrating the sand- 

 stone is strong evidence in favour of this view. Walther 

 has given a figure (33, p. 476) showing these tubes stand- 

 ing up above the weathered surface of the rock. A 

 similar kind of silicification on a large scale is taking place 

 at the present day at the hot wells called Hammam- 

 Meskhutin in the Algerian province of Constantine. 



About a dozen miles west of the great pyramids Schwein- 

 furth has found deposits which he referred to the Miocene 

 (28). But they have since been examined by Mayer-Eymar 

 (19), and according to him they belong to the Lower Ton- 

 grian. The fossiliferous beds are sometimes covered by a 

 cap of dolerite. 



Similarly the supposed Miocene beds on the high ground 

 surrounding the depression of Fayum are believed by Mayer- 

 Eymar to be of Ligurian and Tongrian age. But no satis- 

 factory palaeontological data have yet been published con- 

 cerning these deposits. 



Miocene- -The observations of Mayer-Eymar have 

 thrown so much doubt on the supposed occurrences of 

 Miocene rocks in the neighbourhood of Cairo that it has 

 become necessary to exercise some caution in attributing a 

 Miocene age to any of the Egyptian deposits. The diffi- 

 culty is not lessened by the fact that geologists have used 

 the term in very various senses. 



We may, however, without much hesitation, accept as 

 Miocene the rocks which form the walls and most of the 

 floor of the oasis of Siwa (34), and which probably occupy 

 most of the Barka plateau. In the oasis itself they consist 

 of limestones and marls containing numerous echinoderms, 



