Vol. 59.] SEMNA CATARACT OF THE NILE. 77 



rate of enlargement. Then, by roughly counting the holes and 

 multiplying, a very fair idea of the total amount of rock removed per 

 j^ear by such action would be obtained. I trust later on to find an 

 opportunity of making such observations, time for which failed on 

 my last visit, and thus to confirm or otherwise the view which I 

 have expressed above, — that the lowering of the high-Nile level 

 observed between e.g. 2300 and the present day, is a simple conse- 

 quence of the operation, through the period of 4200 years, of that 

 erosive action which is going on in oar own times. 



EXPLANATION OF THE PLATES. 



Plate III. 



Map and section of the Nile Valley at the Semna Cataract on the scale of T oW 



[The distance to Sarras on the map is misprinted as '72' instead of 



12 kilometres, or 7* miles.] 



Plate IV. 

 Fig. 1. Augititefrom Semna, X 45. a, augite; h, hornblende; s, sphene; 

 q, interstitial quartz. (See p. 72.) 



2. Syenite-porphyry from Semna, normal form of the rock, X 40. 



_p=portion of a porphyritic ort^hoclase-crystal, with peripheral hasma- 

 tite-grains; o = orthoclase of groundmass; £= quartz; the dark masses 

 are haematite, perhaps an alteration-product of biotite. (See p. 71.) 



3. The same rock: outer crust, showing redistribution of iron-oxides, x40. 



The minerals bear the same letters as in fig. 2. 



Discuss roN. 



Sir Archibald Geikie remarked on the value of observations 

 which in any degree helped to furnish numerical measurements of 

 the rate of geological changes. The data supplied by the Author 

 appeared to be trustworthy, and to justify the inference which he 

 drew from them. They showed a comparatively rapid erosion at 

 that particular contracted part of the Nile. Though it might not 

 be safe to conclude that the conditions of this erosion had been for 

 a long time as they are now, it was at least an important point to 

 obtain an average rate of denudation during so long a period as 

 that which seemed to be indicated by the ancient Egyptian marks 

 of water-level. 



Prof. Sollas remarked that this was an interesting contribution 

 to the history of the Nile Valley, and that the measurements made 

 by the Author would afford useful data for subsequent observers. 

 While evidence was obtained of the total erosion produced during 

 a given period, no conclusions could be drawn as to a yearly rate, 

 unless it were assumed that both the volume of the Nile and the 

 slope of its channel had remained constant for the past 4000 years.. 

 It would be of interest to know whether the ancient records referred 

 to an average, or to an unusual, high Nile. • 



Prof. Hull thought that the Authors observations would have 

 been of more value if he had taken into consideration the changes 

 in the physical conditions of the Nile Valley which may have had 

 influence upon the rate of erosion at the Cataract. No problem of 

 this kind could be solved without recognizing the former much 



