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 



