THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 



177 



Ruwanzori in the north, to the little group of volcanic cones 

 near the coast of Lake Nyassa. " The first part of Murchison's 

 theory, however, which affirms that Central Africa has never 

 heen below the level of the sea, is still in harmony with the 

 known geological facts, for no deposits of a certainly marine 

 origin have as yet been discovered in the interior." The sedi- 

 mentary rocks described by Burton and Speke to the west of 

 the Victoria Nyanza have yielded no fossils to indicate the 

 conditions under which they were formed. The triassic 

 Ganoids and Gastropods unearthed by Drummoud ^ at the 

 north end of Lake Nyassa have been generally regarded as 

 fresh-water forms.'^ The great red sandstones and shales 

 which stretch from the north of Nyassa far up the coasts of 

 Tanganyika, which were examined by Joseph Thomson, and 

 more recently by myself, have not yet been found to contain 

 any animal forms ; the only indication which might lead to a 

 belief that fossiliferous rocks occur in these regions being the 

 fact that the natives of the west coast of Tanganyika are said 

 to wear necklaces of beads which they dig out of limestone 

 rocks, and which, if this statement is true, are probably the 

 disarticulated segments of crinoid stems. 



Marine, Jurassic, triassic, and probably carboniferous 

 deposits have been found along the coast at many points from 

 Mombasa to the Cape, but these have never been shown to 

 extend any distance inland, and they seem to have no connec- 

 tion with the great sedimentary deposits of the interior, such 

 as those north of Lake Nyassa, which underlie Drummond's 

 fresh-water triassic (?) beds. There is thus at present no 

 geological evidence of the sea, or of even an arm of the sea, 

 ever having been in the region of Tanganyika within reason- 

 able geological times. 



There has, however, been steadily accumulating a mass of 

 observations relating to the formation of the so-called rift 

 valleys, the general tenor of which has been to reveal a 



* Drummond, ' Tropical Africa.' 



2 It appears, however, that these fossils have been by uo means satisfac- 

 torily described. 



VOL. 41, PART 1. NEW tjERIKS. M 



