OP THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 371 



The next formation in order of time is the volcanic series to 

 which allusion has already been made towards the south-west 

 corner of the lake, and with this may be associated the metamor- 

 phosed sandstones, etc., which appear, in fact, to be portions of the 

 Eed Felspathic Grits and not " primary metamorphics," such as 

 those described by Cornet in Katanga. These volcanics most 

 probably belong to the graben- system, and must be approxi- 

 mately of the same date as similar volcanics towards the north 

 end of the great Nyassa-graben and elsewhere. 



The latest formations in point of time are deposits derived 

 from the lake itself, and these are of especial interest as 

 containing the remains of the existing halolimnic molluscs. 

 It is probable that they may be met with at many places alonj^ 

 the shore. Mr. Moore refers especially to the line of coast 

 between Ujiji and Usambora, where layers of modern lake- 

 deposit, somewhat shattered, are found dipping 20° to the east 

 conformably to the sheets of Old Sandstone on which they 

 repose. According to the same author the flat floor of the 

 Ruzizi valley (at the head of the lake) is composed chiefly of 

 modern sandstones and alluvium. Higher up the valley, to 

 about 200 feet above the present surface of the lake, his party 

 kept passing over older and older ground, and the plains thus 

 traversed were found to be intersected by water-courses in some 

 cases to a depth of 90 feet, so that the older stratified materials 

 were exposed. These strata were found to consist of brown and 

 yellow sandstones, having a slight dip to the south, and 

 contained many shell fragments' and also some fossilised shells 

 which could be identified as Neothauiiia, JVassopsis and Para- 

 melania. The age of the deposit is probably Pleistocene, and 

 not only has the water level of the lake fallen, but he thinks 

 that the valley-flat north of Tang;inyika has undergone elevation 

 also since those days. It should not be forgotten that Mr, 

 Moore {Tanganyika ProUem, p. 90) states that the water of the 

 lake is somewhat salt. He observes that it seems to be fresher 

 now than when Livingstone and Stanley examined it. More- 

 over, as both these explorers aver, there are traditions among 

 the Arabs that, within the recollection of living men, it was a 

 lake which never flowed out at aU. 



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