OF THE HALOLIMNIC FAUNA OP LAKE TANaANYIKA. 359 



Geological structure of the Congo hasin. — The above considera- 

 tions present to us only one phase of Tanganyika's history. If 

 we desire even to try to account for its peculiar i'auna we must 

 now turn to another factor in the case, viz., the geological 

 structure of the Congo basin, with v/hich it seems, almost by 

 accident, as it were, to be connected. This is a very large 

 subject, and the region under consideration is quite the .con- 

 verse of the one previously described ; for we are about to deal 

 with an immense circular area having only an elevation of 

 from 1,000 to 2,000 feet above sea level, and which, for the 

 most part, seems to have been free from tectonic disturbance. 

 It might be thought there would be immense variety of forma- 

 tions in this region, but if the Belgian and French geologists, 

 whom I shall presently quote, are correct, we have the old 

 story over again:— a rim of crystalline and, possibly, palaeozoic 

 rocks, with absolutely unfossiliferoas sedimentaries, largely 

 consisting of sandstones, dumped down in the centre. 



The best evidence we obtain of the general structure of the 

 Congo basin is derived from the writings of Professor Cornet, 

 of Mons, supplemented for tlie French Congo by those of Mons. 

 Barrat, a mining engineer, and inspector of public works.* 

 The first mentioned author is a geologist of great experience, 

 and his earliest work in this region (Katanga) relates co the 

 geology of the Uppermost Congo in the basin of the Lualaba, 

 which is almost m touch with Tanganyika itself. Before ven- 

 turing, however, to deal with this ground, I will bring to your 

 notice Professor Cornet's experiences on the Lower Congo. 

 The railway from Boma to Stanley Pool has materially helped 

 the engineer to obtain a fairly accurate idea of this piece of 

 country. It is true that this railway is only 350 kilometres 

 (216"35 miles) in length, and that the distance from the outlet 

 of the Lukuga on Lake Tanganyika, measured in a straight 

 line along the sixth parallel of south latitude, is nearly 1,300 

 miles, yet the section traversed by the railway and prolonged 

 to about the neighbourhood of Bolobo, appears to be the key to 



* Cornet. " Terrains anciens du Katanga (expedition de 1891-93)," Liege 

 (1897). 



Cornet. " Observations sur la geologie du Congo occidental." Bull. Soc. 

 Oeol. Belg., vol. x (1896). 



Cornet. "Etudes sur la geologie du Congo occidental." Op. cit., vol. xi 

 {1897). 



Cornet. " Les formations post-primaires du bassin du Congo," Ann. Soc. 

 Geol. Belg., vol. 21 (1893-4). 



Barrat. "La geologie du Congo Francais." Ann. rfes ifwjes, Livraison 

 d'avril (1895). 



