XCviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [vol. Ixxvii, 



Col. H. Gr. Ltois's expressed his great interest in the informa- 

 tion that Dr. Hume had laid before the Society, which made -the 

 structm-e of this part of North-Eastern Africa much clearer. He 

 agreed that former assumptions of the rift character of the Nile 

 Valley were not tenable, and that the Grulf of Suez, too, could not 

 be strictlj" described as a rift-valley. For the Ked Sea, he asked 

 Avhether more information had been collected of late as to the 

 structure of the eastern shore, to show whether it was as free 

 from fracture-lines as the western was, according to Dr. Hume's 

 account. 



Prof. J. W. Gregoet. in a letter sent as a contribution to the 

 discussion, remarked that the agreements between Dr. Hume's 

 views and his own were more essential than the differences. The 

 sequence of events stated by Dr. Hume for the Clysmic Gulf is 

 similar to that which he had adopted for the Rift Valley as a 

 Avhole — including Jurassic subsidences due to the Mesozoic 

 deformation of the crust, an Eocene land over the Red Sea, 

 subsidence of the Rift- Valley trough in the Oligocene, renewed 

 and extended especially in the late Pliocene Period. The Gulf 

 of Suez is not typical of the Rift Valley, as its lines there inter- 

 sect those of the Levant, and as the rocks traversed are mostly 

 young stratihed deposits. 



The main difference between the waiter's interpretation and 

 that stated by Dr. Hume is the relative importance of fold and 

 fault. The importance of the faults is indicated by Dr. Hume's 

 remark that 



' the (largely) fault-bounded ranges dominate this depressed area,' 



and by his statements (Geol. Mag. 1910) regarding the dome over 

 the Clysmic Gulf : — 



' I can conceive of no erosive agent which would break across this great earth- 

 feature without the intervention of fractiu'e,' [and that] ' faulting, and fault- 

 ing alone, can explain the phenomena.' 



These conclusions are supported by the recent statements in 

 bulletins by the Geological Survey of Egypt that faulting was 



' the controlling factor in the formation of the shore line of the Gulf ' [and in] 

 ' determining the present position of the Gulf of Suez.' 



The attribution of the Red- Sea section of the Rift Valley to 

 folds appears to be due to the use of the term ' fold ' for movements 

 which the writer resrards as faults. The Geoloo-ical Survey of 

 Egypt has recently'' explained its use of the word 'fold' by a diagram ^ 

 which identifies a steep, plane, slickensided surface as a fold. The 

 writer reo'ards the movement shown by this dias:ram as a fault, so 

 that the difference is a question of terms. He found it impossible, 

 in view of the post-Eocene faults with fault-breccias beside the 

 Gulf of Aden and the maps of the Egyptian Geological Sm'vey at 



^ PetroL Ees. Bull. Xo. 6, 1920, sketch before p. 1. 



