7 o THE TANGANYIKA PROBLEM. 



certainly be made to deal in an exhaustive manner with 

 the geology of different districts. It is, however, even 

 now possible, and it is certainly interesting, to attempt 

 to form some definite conception of the relative ages of 

 the great lakes and the valleys in which they are contained. 



If we consider the Nyassa region, there is, as we have 

 seen, little or no evidence in its southern districts of any 

 disturbance of the country since it acquired its present 

 characters. Although Nyassa, Pamalombi, and Shirwa were 

 probably all at one time connected together as a great 

 lake, their present separation has been brought about 

 by nothing more than the gradual silting up of the 

 valleys, the drift of sand, and the wearing away of the rock- 

 ridges underlying the Murchison cataracts. By this latter 

 process the general water-level fell, Nyassa, Pamalombi, 

 and Shirwa became distinct lakes, Shirwa ceased to over- 

 flow, and all three contracted in their beds, leaving wide 

 alluvial plains and terraces exposed about their ancient 

 margins. 



One general fall of Nyassa was marked by the baobab 

 beach terrace, to which I have referred, and which can be 

 traced more or less distinctly all round the lake, but it is 

 obliterated and more or less indistinct in certain districts 

 towards the north, owing to the fact that the great structural 

 changes which have taken place in the northern portion of 

 Nyassa have continued down to the present time, and are 

 still going on. During the progress of these changes it is 

 almost certain that the character and shape of the lake has 

 been greatly altered. The sides of the valley, as we have 

 seen, have been bodily raised, carrying the lake deposits 

 with them, while, as a concomitant phenomenon of the 

 folding, the floor of the present lake has been depressed, and 

 is still being depressed, in the north. In consequence of this 



