7 ° 
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 
