Geological Society. 385 



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 within 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. Sapid 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 

 4200 years 7*9 metres, the depth of the present river below the 

 lowest group of inscriptions dating from the time of Amenemhafc 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 River 

 Sirneto in Sicily. At Assuan and Silsilla the river has suffered 

 considerable lowering within geologically 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 River 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 the 

 foothills.' These have hardly been explored, though roughly mapped 

 on geological maps of India as belonging to a ' Transition Series.' 

 The whole area is rich in minerals. The author gives a brief 

 description of various rocks, met with mainly in this third belt. 

 They include slates with vein-quartz ; mica- and graphite-schists ; 



