FAUNA OF TUE AFIUCAN LAKES. 513 



From the very nature of the troughs in which the lakes are 

 situated, the latter are likely to be deep, with shores quickly 

 shelving, and this is precisely what investigation reveals. Both 

 Nyasa and Tanganyika have now been pretty adequately sounded, 

 and the figures obtained are very remarkable for fresh-water 

 basins. Nyasa shows a maximum depth of about 786 metres 

 (430 fathoms), while Tanganyika reaches no less than 1435 

 metres (about 784 fathoms)*. Tanganyika is thus the second 

 deepest lake in the world, Lake Baikal coming first, while Nyasa 

 takes fourtli place, following the Caspian Sea. Where sufficient 

 soundings have been taken, tlie contours constructed show in a 

 number of instances the remarkably steep shores which were only 

 to be expected in such cases. 



Victoria Nyanza, as already indicated, is a lake of a very 

 difierent nature from those of the Great Rift Yalley. With, a 

 length of some 200 miles and a breadtli of 130 miles, it has 

 roughly the form of a parallelogram. While it has the largest 

 area of any of the African lakes, it occupies only a shallow depres- 

 sion, and instead of being bounded by steeply rising shores two 

 or three thousand feet in height, the surrounding hills reach a 

 maximum of 300 feet or so. The greatest depth is a mere 

 73 metres (40 fathoms). 



In most other respects, the lakes included in this survey do not 

 dift'er greatly so far as physical conditions are concerned. Thej 

 are all large sheets of water, where consequently almost oceanic 

 conditions prevail, while the three biggest lakes ai-e sul)ject to 

 storms of a violent description. In dealing with the evidence 

 afforded by the plants and animals inhabiting them, this fact 

 must not bo overlooked, as physical surroundings may have 

 induced some of the features which are noted. 



Although extending considerable distances both north and 

 south of the equator, these lakes all exist under practically the 

 same climatic conditions. They are tropical lakes, and differences 

 in fauna and flora are clearly not to be cori'elated with the very 

 slight climatic variations which may possibly occur. 



The temperature of the water is natui'a,lly high. I took 

 sufficient temperature readings during my visit to Tanganyika to 

 afford some idea of the general conditions, although an exhaustive 

 survey was impossible. Only a few figures need be mentioned 

 here. The lowest reading I ever obtained in Tanganyika was 

 73*3'^ Fahr. and the highest 81 "0^. Surface readings at different 

 periods of the year varied only between 74*7° and 81 "O"^, and 

 an average of 51 observations gave a mean of 78"0°. At con- 

 siderable depths the water temperature remains relatively 

 constant, for readings taken at a depth of about 139 metres 

 (7(3 fathoms) — the length of the sounding line — on various 



* For furtlier details coiicuriiiug Nyasa, consult Moore (184, p. 7) ami Rlioades 

 (148) — coiiccrniug Tanganyika, sue Stappers (174 & 175) and Marquardseu 



(111). 



