OF THE HALOLIMNIC FAUNA OF LAKK TANGANYIKA. 323 



the structure of nearly the whole basin of the Congo. I may 

 be pardoned, therefore, if I dwell upon this section on the 

 Lower Congo in some detail. 



The western Congo may, from a geological point of view, be 

 divided into four zones from west to east as follows (see 

 Fig. 3, page 324.) 



I. The Maritime Zone. 

 11. The Crystalline Zone. 



III. The Calcareo-schistose Zone. 



IV. Zone of the Sandstones. 



/. The Maritime Zone. — This consists of old estuarine 

 deposits, and more particularly of fragments of Tertiary beds. 

 Cretaceous beds, and of continental pre-Cretaceous sandstones. 

 It is interesting to note that the only fossiliferous beds whose 

 age may be known from their contents, constitute a narrow 

 and insignificant fringe on the borders of the Atlantic, just as 

 we have seen to be the case on the east coast of equatorial 

 Africa (see Fig. 2, p. 320). All the other zones are without 

 any definite traces of organisms. 



//. The Arckman and metamo^yhic heels. — The Archaean is 

 well represented on the Lower Congo from the granitoid 

 gneisses of Boma, in the west, to the chlorite and sericite 

 schists of the higher portions. The dip is generally towards 

 the west at variable angles, which are sometimes low. Both 

 north and south of the Congo this zone can be traced for some 

 distance. The so-called metamorphic beds are less crystalline, 

 and in some cases calcareous. 



///. The calcareous- scJiist system. — There is a massive con- 

 glomerate at the base, and this is succeeded by schistose argil- 

 laceous limestones. The middle member consists largely of 

 marbles, whilst the highest beds are calcareous schists with 

 silicious concretions. The beds of this system exist in a series 

 of synclinal basins indicative of a thrust towards the west, and 

 with a diminution of folding as one advances eastwards, until 

 the beds pass under the felspathic grits of the fourth zone 

 with a slight dip to the eastwards. The age of these beds is 

 uncertain, but it is thought that part of them may represent in 

 time the Devonian of other countries. 



Zones II and III represent the rim of the basin in which 

 the nearly horizontal sandstones of the fourth zone were 

 deposited. 



Zone IV. Zone of the Sandstones, or beds of the Congo basin 

 proper. These are the beds to which Professor Cornet more 



