Reports and Proceedings — Geological Society of London. 45 



Inscriptions placed on the rocks at Serana, between the second and 

 third cataracts, under the Twelfth and Thirteenth Dynasties, serve 

 as a means of gauging the local changes due to river-erosion during 

 a period of about 4,200 years. Horner, in 1850, came to the conclusion 

 that " the only hypotheses which could meet the requirements of the 

 facts observed would be either the wearing away of a reef orl)arrier 

 at the place in question — a process requiring too long a period — or 

 the existence at some distant period of a dam or barrier, formed 

 perhaps by a landslip of the banks, at some narrow gorge in the 

 river's track below Semna." The author is in favour of the former 

 explanation. The river, above and below the Kumna and Semna 

 temples, has a width of 400 metres, but between the two temples 

 a narrow band (200 metres wide) of hard red and grey gneiss 

 contracts the river at low Nile w^ithin a central channel about 

 40 metres wide. Through this deep channel not less than 400 cubic 

 metres of water pass per second. The gneiss itself, dykes of syenite- 

 porphyry, hornblende-schists, and augitite are described ; and it is 

 shown that the foliation of the gneiss is parallel to the channel, and 

 probably accounts for the direction of the latter. Eapid erosion with 

 the formation of pot-holes is observed to be now taking place ; and the 

 author calculates that if 200 cubic metres (approximately 500 tons) 

 of rock per year has been removed from the barrier, the lowering 

 of it would amount to 2 millimetres a year, or in 4,200 years 

 7-9 metres, the depth of the present river below the lowest group 

 of inscriptions dating from the time of Amenemhat III. The 

 yearly discharge of the Nile past Semna is nearly 100,000 million 

 tons of water ; and the author considers that the removal of 500 tons 

 of rock under existing conditions in a year is not only not impossible, 

 but highly probable, as all this erosion only amounts to 5 milligrams 

 of rock per ton of silt-laden water. This erosion is compared with 

 the classic instance of the Elver Simeto in Sicily. At Assuan and 

 Silsilla the river has suffered considerable lowering within geo- 

 logically recent times, probably brought about by the removal of long 

 pre-existent hard barriers. The sluices of the new dam at Assuan 

 may in the future give a quantitative determination of silt erosion in 

 granite, and it would appear to be not difficult to ascertain at Semna 

 the rate of pot-holing. The formation of new pot-holes 1^ feet 

 deep, in an artificial channel in rock in Sweden, has been observed 

 to take place in 8 or 9 years, and the author hopes in future to 

 attempt some measurements of this kind at Semna. 



2. " Geological Notes on the North- West Provinces (Himalayan) 

 of India." By Francis J. Stephens, Esq., F.G.S., A.I.M.M. 



The country examined extends in a north-westerly direction 

 across the line of strike, from the borders of Nepal and South- 

 Eastern Kumaon to north of the Alakmunda Eiver in the vicinity of 

 Badrinath and the Marra Pass. The foothills consist of Tertiary 

 clays and sandstones, the snowy ranges of gneissose, granitic, and 

 metamorphic rocks of various descriptions. " Between the snowy 

 ranges, or, rather, the most southerly range of the Himalaya chain, 

 a band of hills extends, for nearly 50 miles on an average, to th& 



