THE TANGANYIKA PROBLEM. 
73 
these animals had entirely disappeared. The valley of 
Nyassa is, however, as we have seen, continued into that 
of Lake Rukwa, and this in turn has been shown by Dr. 
Kohlschtitter to run into the great trough of Tanganyika it- 
self. The Rukwa valley appears bodily to cross Tanganyika 
in the vicinity of Karema and the Luakuga gap, and to 
continue westward into the great Congo depression. Un- 
fortunately little or nothing is known of the deposits in the 
northern extension of the Nyassa valley in which Rukwa lies, 
but it appears from the observations of Dr. Ftilleborn 
that the fauna of Rukwa differs from that of Nyassa, 
or of any of the lakes to the south with which he was 
acquainted.* 
Thus it would appear that there was once a marine, 
possibly a triassic or Jurassic, fauna in the region of the 
north end of Nyassa, the existence of which entirely 
anteceded the northward extension of the present lake ; 
that there is now no trace of this fauna in Nyassa or to 
the south of it ; that the structural fold in which Nyassa 
lies can be traced to a junction with that of Rukwa; that 
the fauna of Rukwa differs from that of Nyassa or any 
of the other African lakes with which Ftilleborn was 
acquainted ; that the valley of Rukwa intersects that of 
Tanganyika and passes to the west, and lastly, that at 
the present time there is what appears to be the remains 
of a marine fauna containing jelly-fishes, the old ganoids, 
and what are apparently Jurassic gastropods, still living in 
Tanganyika. 
From these considerations it would appear that the 
old sea which was once in the vicinity of Nyassa retreated 
west (it may be shown eventually that it retreated east 
* I do not know whether any definite result of Fiilleborn J s investigations of Lake 
Rukwa have yet been published, and Dr. Kohlschtitter says he does not cither. 
