Vol. 59.J ^ SEMNA CATAEACT OP THE NILE. 79 



flattening the inclination of its channel, the situations of numerous 

 temples along its course are such as to prove the change to be a very 

 slow one. Moreover, the rapid of Semna lies between the Second 

 and Third Cataracts, where a similar, though perhaps slower, erosion 

 is going on ; and the average inclination of the river between 

 them would remain constant if, as is probable, the rates of erosion 

 at the upper and lower cataracts were approximately equal. Thus, 

 while it is not possible to assert that the volume and inclination of 

 the stream have remained constant for 4000 years, there is a strong- 

 probability that the change in these factors has been comparatively 

 small in the given time. 



The submersion of the lower part of the Nile Valley, as shown by 

 raised beaches above Cairo, is, as Prof. Hull remarks, of very great 

 importance in any consideration of the early history of the river. 

 But the fact of the existence of the city of Memphis (the ruins of 

 which are still to be seen near Cairo) for over a thousand years 

 before Amenemhat's time, is sufficient to prove that the deposits 

 referred to long antedate the historical period, and thus they indicate 

 nothing as bearing on the question of erosion within the last 

 4000 years. 



With respect to the question raised by Mr. Marr, I would only 

 add to what has been stated in the paper, that in the minor channels, 

 left dry at the time of my visit, I saw but little evidence of joints 

 cutting across the foliation-planes ; and thus, although it is quite 

 possible that the position of the main channel may have been 

 primarily determined by a joint or fissure, I was led to conclude 

 that it, like the others, was conditioned simply by the foliation- 

 planes, which themselves are planes of weakness. Joints and crush- 

 planes perpendicular to the foliation-planes are of great frequency 

 in the gneisses and schists immediately above Assuau, where they 

 account for most of the ' khors '" or side-valleys opening to the 

 river ; but they are less abundant near Semna, and, as I have said, 

 not evident at all in the rocky barrier itself. 



With regard to Prof. Groom's question concerning tectonic move- 

 ments, it is certain that large portions of the Nile Valley have been 

 subjected to such changes. But a great deal of work remains to be 

 done before the nature and extent of such movements can be even 

 approximately determined ; all the evidences that I have yet been 

 able to examine tend to show that the tectonic movements, like the 

 climatic changes, ceased long before the historical period with which 

 my paper is more immediately concerned. 



Thus I consider that the change which has undoubtedly taken 

 place at Semna within the last 4200 years cannot be explained as 

 due to any change in the discharge or general inclination of the 

 river, nor to fissures, nor to tectonic movements, nor, in fact, to 

 anything but simple erosion at a very rapid rate owing to the local 

 contraction of its channel. — J. B., December 9th, 1902.'] 



