THE TANGANYIKA PROBLEM. 
35 
The water of Lake Kivu is very deep, often more than 
100 fathoms, within a few hundred yards of the shore. But, 
owing to the fact that we had no other boats besides native 
canoes, and to the persistent stormy, squally character of 
the weather which we encountered, we were unable to carry 
out any sounding operations far from the land. 
The east, west and south shores of Kivu are, as I have 
said, composed entirely of schist and gneiss ; but the northern 
shore of the lake is quite different. It is formed wholly of 
the modern volcanic materials, the layers of lava and ash, 
which have been ejected by the still active Mfumbiro 
Mountains. As we approached the northern limit of the 
lake, the trough-like sides of the great valley were seen to 
continue northwards, after the lake had come to an end, 
just as the sides of the same great valley were seen to 
run on beyond the north shore of Lake Tanganyika. But 
here, instead of the floor of the trough rising gently, in 
a series of raised alluvial flats, the whole space, between 
the lateral ramparts of the depression, is filled up and 
blocked to the north by a group of great and modern 
volcanic cones. These cones extend in a string or chain, 
not parallel with the valley, but from east to west ; form- 
ing, in fact, a vast transverse dam, which stretches in this 
place completely across the floor of the depression. The 
numerous peaks of which the chain is composed are very 
lofty ; one of them, Karisimbi, being often snow-capped, and 
reaching an altitude of about 14,000 feet. 
An examination of the individual volcanic cones shows 
that those to the east are, unquestionably, the oldest. They 
are, in the first place, quite extinct, while their old crater- 
walls are much denuded, and partially enclose secondary 
cones, and the irregular products of the last efforts of their 
activity. The slopes of the two great cones to the west of 
