826 W. H. HUDLESTON, ESQ., M.A., P.R.S.^ ON THE ORIGIN 



which are of granite. Secondly, it must be remembered, that, 

 as far as what we may term the solid geology is concerned, the 

 "White Friable Sandstone series is the highest in the sequence 

 of the beds which form the vast interior. These are often 

 concealed by horizontal beds of clayey and sandy alluvium 

 (silt), dating from a period when the mean level of the river 

 was higher ; also by spreads of what the French geologists 

 call "Laterite." Even these alluvial beds seem devoid of 

 organic remains, except that in one case shells of JStheria are 

 mentioned.* 



Having thus briefly considered in some detail the material of 

 which the Congo basin, in a geological sense, is constituted, we 

 are now in a position to glance at the structure and physical 

 history of that immense area, including some attempt to fix the 

 chronology and parallelism of the two great sandstone systems, 

 which probably cover more ground than any other sedimentary 

 beds throughout Africa. Cornet, in speaking of the physio- 

 graphy of the Congo basin, describes it as an immense " vat," 

 whose peripheral margins are always higher than the central 

 region. 



The periphery of the Congo hasin (Plate I). — The western 

 portion we have already studied in the traverse from Boma to 

 Stanley Pool. Although the topography varies throughout this 

 immense circle, the geological sequence is pretty much what we 

 have seen. Thus, on the southern margin, the watershed between 

 the Congo and the Zambesi, towards the sources of the Lualaba, 

 runs from elevations of 4,000 feet to 5,000 feet. On the south- 

 east the headwaters of the Congo-Luapula proceed from a 

 region of gneiss, mica schists and argillaceous schists with 

 granitic massifs, which extend between lakes i^yassa and 

 Tanganyika. The " ancient rocks " of Katanga, so well described 

 by Cornet, of course form a part of the general periphery in 

 these regions. It would be well to mention here that, although 

 such ancient rocks are, in the flatter parts of the basin, covered 



* There is an article by Stainier {Trans. Inst. Mining Engineers, 

 voL 15 (3 898) p. 491), in which the author, besides summarising the 

 results of Cornet and others on the solid geology of the Congo basin, 

 gives a very useful abstract of the superficial formations of this immense 

 area. These include (1) Products of the alteration in situ of subsoil 

 rocks ; (2) Products of decomposition on slopes under the influence of 

 rainfall ; (3) Alluvial deposits in watercourses ; and (4) Ancient alluvial 

 deposits. It can readily be understood that the solid geology of the 

 Congo basin is largely masked by some one or other of the above 

 conditions, to say nothing of vast districts under water and swamps. 



