332 W. H. HUDLESTON, ESQ.^ M.A., F.R.S., ON THE ORIGIN 



at a comparatively recent period. Oscillation of the floor and 

 containing walls of both these great Graben lakes is noticeable 

 in places. 



In attempting to construct a geological map of Tanganyika I 

 must be guided by Mr. Moore to a certain extent, not forgetting, 

 however, to consult the works of Cornet, Bornhardt, Kohls- 

 chiitter and other distinguished scientists. If there is obscurity 

 in the geology of Equatorial Africa, still there is a certain 

 degree of simplicity as far as the composition of the several 

 formations with which we have to deal. Around Tanganyika, 

 though not to the same extent as around Nyassa, the basement 

 granitoid rocks (Archaean) are strongly in evidence. Upon 

 these at the south and south-east end of the lake, in complete 

 unconformability, reposes the great sandstone and shale 

 formation, which, we have seen, Cornet in his numerous 

 writings on the Congo basin calls the Bed Felspathic Grits, or 

 " couches de Kundelungu " of the Lualaba district, and which 

 constitute the lower division of his " formations post-primaires." 

 Beds of this character also extend to the east as well as the 

 west of the lake, and this part of the area now occupied by 

 Tanganyika must have been within the limits of the original 

 basin of deposition (see ante, p. 327). These Eed Felspathic 

 Grits, so horizontal for the most part throughout the basin of 

 the Congo, are tilted in portions of the western wall and notably 

 at Mount M'rubi, where they are said to have an inclination to 

 the eastward. As previously observed, this shows that Lake 

 Tanganyika is within the influence of the movements connected 

 with the East African Plateau Eange, whereas the bulk of the 

 Congo basin is without the sphere of those influences. At the 

 south end of the lake the Eed Felspathic Grits are shown for 

 the most part as horizontal, althougli, according to Mr. Moore's 

 mapping, much cut up by subsidiary Graben, which carry on 

 the principal Graben of Tanganyika in a southerly direction. 

 In one of these subsidiary Graben, towards the south-west, is 

 situated both the true and the salt lake Mwero of the higher 

 Congo, and that perhaps is about as far west as the Graben- 

 systeni can be traced. In the neighbourhood of Cameron Bay 

 there are considerable indications of volcanic eruptive matter, 

 and, according to Cornet, much of the Eed Felspathic Grits 

 have been transformed into quartzites with intercalation of this 

 eruptive material. These most probably are the " metamorphic " 

 beds of Mr. Moore, which seem to occupy both sides of the 

 southern third of the lake. 



Towards the northern termination of the series which has 



