122 
THE TANGANYIKA PROBLEM. 
valley, the surface of the lake and the floor of the valley 
in which it is contained being far below the general level of 
the surrounding country. In consequence of this, whether 
the lake shores are flat and sandy or rocky and steep, they 
are always sooner or later backed up by precipitous flanking 
ranges which constitute the steep sides of the gigantic 
trough in which the lake lies. The surface of Nyassa is now 
1,500 feet above the level of the sea, and it extends for 
about 350 miles north and south with an average breadth of 
40 miles. In most parts of the valley the flanking ranges 
rise to heights varying between 7,000 and 10,000 feet, 
but at the extreme south there is a narrow valley lying 
between part of the Kirk range and the Mngochi moun- 
tains which serves to carry away the surplus waters of 
Nyassa over the Murchison Falls in the shape of the Shiri 
River which eventually opens into the Zambesi itself. 
Throughout the lake there is abundant evidence that its 
waters have fallen greatly, and also that they have fallen in 
a succession of drops ; for almost everywhere there are to 
be seen at least three old water-marks, old beach terraces 
in fact. The most conspicuous of these stands about 
14 feet above the water level, and upon the well-marked 
flat top of the terrace which it makes all round the lake 
there are always numbers of immense baobab trees. 
Baobab trees are known to be extremely slow growing ; 
they will not grow, and they die, if they are in any way 
submerged, and consequently it is extremely probable that 
many centuries have elapsed since the water of Nyassa 
first sank from this old beach line, and there are at least 
two more clearly discernible terraces above and behind it. 
These general falls in the water level have in all probability 
been due to the successive wearing away of definite 
obstacles in the floor of the Murchison Falls. There are 
