OP THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 375 



for the most part by lofty enclosing walls. Was this outflow 

 caused by a cross-fissure (Graben) such as ipight be produced by 

 the prolongation of the great Rukwa-Graben in the way indi- 

 cated by Mr. Moore ? At any rate these drainage facilities may not 

 always have existed, and in that case Tanganyika during part 

 of its history would be a closed water, and consequently more 

 or less saline. Whether such conditions as these had anything 

 to do either with the origin or conservation of the halolimnic 

 gasteropods I do not venture to say. My endeavour has been to 

 tind any geological evidence in favour of the view that they 

 were derived either primarily or secondarily from a Jurassic 

 stock of Inferior Oolite age. It must be confessed that thus 

 far my efforts have been without success. At the same time 

 mere negative evidence must not be accepted as final. 



In conclusion, then, since neither the zoological, the 

 palseontological nor the geological evidence affords much 

 support to Mr. Moore's theory, we must regard the Tanganyika 

 problem in its main features as unsolved. In the present state 

 of our knowledge we are not bound to submit an alternative 

 hypothesis. Yet, if we still cling to the notion of a specially 

 marine origin for the halolimnic gasteropods, the most promising 

 quarter for a solution of the riddle is to be sought along the 

 northern margin of the Congo basin, where it adjoins that of 

 the Shari. This opens up the notion of a possible communication 

 through the depression in which Lake Tchad is situated with 

 the undoubted marine deposits of the second geological division 

 of Africa. That the so-called "post-primary" deposits of 

 Equatorial Africa, like their equivalents at the Cape, are, with 

 the exception of coastal strips, mainly of terrestrial and fresh- 

 water origin, I entertain no doubt. The only exception to 

 this rule appears to be a Jurassic formation in Abyssinia known 

 as the Antalo limestone. 



It should be distinctly understood that I have not taken up 

 this investigation in a controversial spirit ; nor indeed, in the 

 first instance, with a view to controverting the theory of a 

 Jurassic origin for the Tanganyika gasteropods. If, during the 

 course of the inquiry, I have been unable to find evidence in 

 favour of that hypothesis, it has at least been a source of 

 gratification to follow Mr. Moore's lead in his character of 

 explorer and naturalist. In this way both myself and those 

 members of the Victoria Institute who have taken the trouble 

 to follow me must feel indebted to him for having awakened a 

 more than passing interest in one of the many problems of 

 Equatorial Africa. 



