78 DR. J. BALL OX THE [Feb. I903, 



larger volume of the river, and the submergence of the Nile Valley 

 to the extent of 200 feet below its present level, as shown by the 

 raised beach at Mokattam above Cairo. 



Mr. Marr asked whether there was any mention of joints in the 

 paper. In an ordinary flowing river, the rock on the sides of joints 

 passing from bank to bank could not be eroded more rapidly than 

 the rock down stream, but in the case of waterfalls this was changed. 

 Erosion was much more rapid along the joint-planes than along 

 the intervening solid rock, and thus waterfalls usually bifurcated 

 or split into even more numerous branches. As erosion proceeded 

 along the dominant joints, the minor fissures became channels in 

 wet weather only, and were at last deserted by the stream, so that 

 ultimately the stream was confined to a channel along a dominant 

 joint-plane, producing an effect such as that shown in the Author's 

 plan. It must, therefore, be borne in mind that other factors than 

 increased velocity of stream, as for instance the excessive erosion 

 along joint-planes, must be taken into account in comparing the rate 

 of erosion at falls with that in ordinary parts of a river's course. 



Prof. Groom asked whether there was any evidence that tectonic 

 movements had had an effect on the conformation of this part of 

 the Nile Valley, and whether some of the differences in the depth 

 of particular parts of the channel might not be due to this cause. 



Prof. Jtjdd (in the absence of the Author) thanked the Society 

 for the kiud manner in which they had received the paper. He 

 felt sure that it would be a great encouragement and incentive to 

 future work, both to the Author and to his colleagues upon the 

 Geological Survey of Egypt. In reply to Mr. Marr he stated that the 

 Author found in the foliation, rather than in the jointing of the rock, 

 those points of weakness that were favourable to excessive * pot- 

 holing" and erosion. The speaker recalled the papers on the subject 

 published by the Society during the early years of its history, and 

 stated that shortly before Sir Charles Ly ell's death, he had visited 

 w T ith the veteran geologist the places where mason's marks had 

 been made by Sir Charles w^hen quite young, in the hope of deter- 

 mining the amount of erosion by rivers. Considerable erosion had 

 taken place in the course of fifty years, but the mason's marks were 

 not altogether obliterated. 



Postscript to Discussion. 



[In reply to Prof. Sollas, the Author observes that there are a 

 large number of high-Nile marks dated in the different years of 

 Amenemhat's reign, and the inscription selected in the calculation 

 of the erosion is one of the lowest of them; so that it maybe 

 taken to represent fairly an average, and not an exceptional, high 

 Nile of the period. Although it is certain that the volume of the 

 Nile was formerly greater than at present, there is no evidence that 

 any very considerable change of volume has taken place within 

 historical times. The. changes at Assuan and Silsila are proved, 

 by the positions of numerous inscriptions, to have occurred before 

 the dynastic period ; and though the river is of course gradually 



