part 2] PKOCEEDiNGS or the geological society. xcix 



the other end of the Red Sea, to accept the view that on the Red 

 Sea only fold-effects have been observed. He agreed that the 

 Lower Nile Valley is not a Rift Valley, its structure being the 

 antithesis to that of the Red-Sea trough. 



With regard to the length of the Rift Valley, he referred to the 

 explanation in his forthcoming book ' The Rift- Valleys & Geology 

 of East Africa ' (briefly stated in Geogr. Journ. vol. Ivi, p. 38), 

 representing it as due to worldwide mid-Kainozoic earth-movements, 

 and to its position between the mountain-forming movements 

 in Europe and Africa which were northward, and the simultaneous 

 Asiatic movements which were southward. Its great length is 

 due to the continental scale of the accompanying movements, and 

 seems no more inconsistent with its formation by tension than the 

 equal length of the contemporary fold-mountain system is incon- 

 sistent with their formation by compression. 



Dr. J. W. Evans thought that the structure of the Akaba and 

 Clysmic gulfs would prove very different, the latter being probablj" 

 the same as that of the Dead- Sea depression. He enquired whether 

 the faults shown in Dr. Blanckenhorn's map near Suakim, parallel 

 to the coast in that neighbourhood, and approximately parallel to 

 the Gulf of Akaba (though not in the same line), were authentic, 

 and, if so, whether they had a downthrow towards the sea. The 

 deep depression in the north of the Red Sea was sharply defined, 

 both on the north and on the south, and suggested a subsidence. 

 The existence of a ' graben ' seemed to point to a state of tension, 

 when it was found, but did not imply the existence of a rift as 

 wide as the sunken area. The speaker looked forward to the pro- 

 duction by the Egyptian Survey of further evidence on this most 

 important question. 



Mr. G. W. Lamplugh remarked that the ' rift- valley ' hypo- 

 thesis raised the wider question as to the supposed prevalence in 

 many parts of the world of large-scale surface-features produced 

 directly by comparatively recent faulting. The geological record 

 showed that the local development of troughs of dei^ression had 

 been frequent throughout the accumulation of the stratified rocks ; 

 and the resultant synclines were often faulted longitudinally at 

 the margins, as well as within. But the field-evidence generally 

 implied that the subsidences had been gradual, and the faults of 

 slow growth. Secondary 'fault-controlled' features, due to selective 

 denudation, were common enough both in valleys and on high 

 ground, but new original fault-scarps were difficult to find : he 

 had not yet himself seen a single convincing example, though 

 he had seen several to which this origin was ascribed. He knew 

 no case of the trunk-drainage of a land-area having been revolu- 

 tionized by the uprise of a fault-block athwart it ; and this seemed 

 to imply that the surface-effects of faults for a long time past had 

 never been rapid enough to overcome the ordinary course of 

 weathering and erosion. The conception of the ' rift-valley ' had 

 always been attractively simple, and there may be features on 

 the Earth to which the conception will apply absolutely ; but the 



