250 A JRIFT-VALLET IN WESTEBN PEESIA. [vol. lxXV, 



of the slickensided surfaces. Such surfaces were often siliciform 

 and highly resistant to weathering, on account of their composition 

 and physical state. 



Mr. R. D. Oldham said that he agreed with the remarks of 

 Sir Jethro Teall regarding the title of the paper. The valley was 

 certainly not a rift-valley in the sense given to the term by its 

 introducer, Prof . J. W. Gregory, but a valley of erosion determined 

 by the letting-down of soft rock into more resistant rock by trough- 

 faulting. The paper gave no indication of the age of the faults, 

 except that they certainly were older than the excavation of the 

 valley ; nor was there any note of special mineralization of the 

 slickensided surfaces. The freshness of appearance of those on 

 the scarp-face was attributed to the rapidity of erosion, and in this 

 he considered that the Author had adopted the true explanation. 

 The climatic conditions of the region were peculiar : although arid 

 it was by no means rainless, and at intervals heavy rains occurred, 

 which, aided by the absence of soil and vegetation, gave rise 

 to rapid erosion, while the dryness of the intervening periods 

 checked weathering. Consequently, the effect of stream-erosion in 

 modelling the outward form of the country was more obvious than 

 in countries of greater rainfall. 



Postsceipt to Discussion. 



[If I interpret Prof. Gregory's writings correctly, his intention, 

 in introducing the term ' rift- valley ' in 1894, was that it should 

 take the place, in English, of the German term ' Graben ' ; and it 

 is in this sense that I have used it. But both ' Graben ' and the 

 French : fosse ' are defined purely as geological structures, without 

 any reference to physiographic development, and 1 cannot find any 

 expression of Prof. Gregory's that shows that he intended ' rift- 

 valley ' to be used in a more restricted sense. In his ' Geography ' 

 he simply states that 



' rift-valleys have been formed by earth-movements ; they are found where 

 long and comparatively narrow strips of the Earth's crust have sunk beneath 

 parallel fractures.' 



Furthermore, all the authors (including Prof. Gregory) who have 

 written of the 'rift- valley of the Rhine ' and the ' rift-valley of 

 Central Scotland ' have used the term in the same sense as I have, 

 for I believe that it is not maintained that these valleys are simple 

 fault- troughs owing nothing to erosion. I think, then, that my 

 use of the term in question is not contrary to current practice ; 

 whether it is contrary to Prof. Gregory's intention is another 

 question, and I should be glad to have a ruling from Prof. Gregory 

 in the matter.— & J. S. April, 1920.] 



