TRANSACTIONS OF SfiCTION E; 769 



Mediterranean to Lake Victoria, a distance of 3,500 miles.* This flewly acquired 

 information enables us to discuss the longitudinal section of the Nile more com- 

 pletely than has hitherto been possible. A discussion of the material now 

 available shows that the probable discrepancy where the two portions of the line 

 join is not greater than about 25 or 30 feet. 



The section shows clearly the great crustal blocks of which the equatorial 

 plateau is composed, while the lakes occupy the level surfaces of the blocks — as 

 Lake Victoria, Lake Ohoga, Lake Albert Edward, and Lake Albert, This portion of 

 the basin exhibits the characteristic features of recent development. At the foot of 

 the equatorial plateau the level plains of the Sudan commence. Through these the 

 rivers of the region drain — viz., the Bahr el Jebel, the Bahr el Zaraf, and others — 

 but now their slope is known not to be very low, being about 1 in 20,000, or a 

 little more than three inches to one mile. The great development of marshes in 

 this region is not so much due to the flatness of the plains, but to the water-level 

 being so near to that of the flood plains that vegetation grows luxuriantly 

 throughout the year, and all the river silt is arrested in the upper reach, so that 

 the flood plains are not being built up. 



Between the Sobat River and Khartoum is the flattest portion of the whole 

 course of the river ; the slope varies from one-half to one-third of an inch per 

 mile, and when the Blue Nile is in flood the White Nile is ponded up and presents 

 an absolutely horizontal surface for 300 miles. 



Below Khartoum the character of the river-bed changes ; reaches of rapids 

 where the river is eroding the crystalline rocks alternate with reaches of lower 

 slope where the rock is sandstone. These crystalline rocks therefore form natural 

 obstructions which can easily be converted into dams for storing up water in the 

 valley above. When the question of increasing the volume of water stored in the 

 reservoir at Aswan was under consideration, three of these reaches were contoured 

 carefully to determine what volume of water could be stored in them. 



In these rapids the slope is at times steep, but never for any considerable 

 distance, and the mean inclination rarely exceeds 1 in 1,000 for any length of 

 river. At no point can the river be said to descend a vertical fall, as the term 

 cataract might be thought to imply; but the cataracts of the Nile are in fact 

 reaches of the river in which it has cut its way down to portions of the ancient 

 and uneven land-surface formed of the crystalline rocks on which the sandstones 

 of Cretaceous age have been laid down. The position of the ridges of this ancient 

 land have determined the places where cataracts occur, but the water-channels in 

 them follow the lines of weakness due to bands of softer rock or to lines of fracture 

 caused by the movement of the crust during long ages. The last of these barriers 

 is at Aswan, and below this the river flows in the wide alluvial plains which it 

 has formed by deposition, and which it is building up at the average rate of about 

 four inches per century. The equatorial plateau and the region of the cataracts 

 are regions in which the river is eroding at the present time, while in the Sudan 

 plains and in Egypt deposition is taking place. But little accurate levelling has 

 as yet taken place along the Abyssinian tributaries, and from Khartoum to Fazogli 

 along the Blue Nile only are the levels reliable. Here in one reach near Singa 

 the slope is markedly less than either upstream or downstream of it, and this 

 may be due to a warping of the crust at this part ; but more evidence is needed. 



TUESDAY, SEPTEMBER 8. 

 The following Papers and Reports were read :— 



1. A Journey into the Primeval Forests of Tropical Peru, 

 By L. C. Bernacchi. 



This paper dealt nlainly with the region of the Rio Inambari, which the author 

 visited, and where a preliminary survey was' carried out in 1 907, at the expense 

 of an English rubber company, in a district practically unexplored. The author 



1908. 3 D 



