A MODERN LAKE 191 



remarkable and the most extensive of these Rift 

 Valleys extends from Tanganyika in the south to 

 the outlet of the Nile from Lake Albert in the 

 north. It is a long wide trough or depression 

 in the surrounding country, and in it lie the four 

 lakes — Tanganyika, Kivu, Albert Edward, and 

 Albert. It is bounded on either side by a ridge 

 of hills, which are steep and mountainous in some 

 places, such as the two sides of Lake Kivu, and 

 low and insignificant in others, as the east side 

 of Lake Albert Edward and the west side of the 

 Semliki Valley. In former times the uplifting of 

 the floor of the trough at the northern end of 

 Tanganyika formed the watershed between that 

 lake and Lake Kivu, whose waters flowed out 

 northwards into Lake Albert Edward, and so into 

 the Nile. 



This state of things was upset by the disturbances 

 of the earth's interior, which caused the formation 

 of the Mfumbiro volcanoes. Starting on the eastern 

 side of the trough, these mountains burst up, one 

 after another, until they formed a complete dam 

 across the trough between Lake Kivu and the 

 Albert Edward Nyanza. The immediate effect of 

 this was to cut off Lake Kivu from the Nile basin, 

 and to lower the level of the water in the Albert 

 Edward. Old lake levels can be seen now in many 

 places along the shores of the latter lake. Another 

 and equally important result of the formation of the 

 volcanic dam was the raising of the level of the 



