616 THE FRESH-WATER LOCHS OF SCOTLAND 
Native tradition appears to indicate that the valley of the Lukuga 
was originally formed by an affluent river, and that subsequently a 
river of the Congo basin rising on the other side of the divide worked 
its way gradually backwards, cutting through the ridge, and suc- 
cessively capturing the various tributaries of the other river, and 
finally the whole river- system. A connection having been thus 
established, it was an easy matter for the waters of the lake, on reach- 
ing the high level after the addition of the drainage from Lake Kivu, 
to drain away naturally westward to the Congo. Stanley ^ takes this 
view, but Moore - considers the bed of the Lukuga a continuation of 
the cross-valley in which Lake Rukwa lies. 
A good many readings of the water-temperature of Lake 
Tanganyika were made by Cunnington,^ and he concludes that 
the temperature in general must be very high, as the lowest reading 
obtained on the lake was 73°-3 Fahr. (22°-9 C), while the highest 
was 81°-0 Fahr. (27°-2 C). At a depth of 456 feet (the length of 
the sounding-line), readings taken on different occasions and at differ- 
ent spots only varied between T^'^'l and 74" '8 Fahr. 
Attempts were also made to observe the seiches by means of an 
improvised apparatus. The principal series of observations taken 
lasted for eight consecutive hours, during which readings were made 
at minute intervals. From the curve obtained there appear to be 
oscillations with a period of about 60 minutes or a little under, 
which occur with some degree of regularity, and probably a seiche of 
longer period : 4J hours or a little over. The greatest amplitude 
noticed is only 2 J inches (6*5 cm.). Unfortunately, sufficient details 
as to the depth and contour of the lake are lacking, so that the 
theoretical periods of the seiches cannot be worked out. 
The aquatic plants of Tanganyika are in no respect unique, and 
in many cases the same species occur in Nyasa or Victoria Nyanza, 
or both. The fauna is remarkable not only as including forms of 
unusual character for a fresh-water lake, and possibly distinct in 
origin from the general fresh-water fauna of Africa, but as containing 
a much larger number of species than the other big African lakes. 
This seems to indicate that Tanganyika was long isolated, and at 
some former time had some connection with the sea. Moore believes 
this to have taken place in Jurassic times.* From the configuration 
of the continent he considers the only possible connection of Tanganyika 
1 Through the Dark Continent^ vol. ii. p. 47, London, 1878, 
2 " Tanganyika and the Countries North of it," Geogr. Journ., vol. xvii. p. 10, 
1901. 
3 " The Third Tanganyika Expedition," Nature, vol. Ixxiii. p. 310, 1906. 
" On the Hypothesis tliat Lake Tanganyika represents an Old Jurassic Sea," 
Quart. Journ. Micr. Sci., N.S., vol. xli. p. 303, 1898. 
