OF THE HALOLIMNIC FAUNA OF LAKE TANGANYIKA. 325 



particularly alludes in his paper on the " Formations post- 

 primaires." The lowest, or Bed Fdspathic Grits, is divided 

 into two sections by a slight unconformity. It reposes in 

 complete discordance upon the flanks of Zone III. There is a 

 great variety of detrital matter in this formation, including 

 conglomerates, fine grained sandstones and argillaceous schists, 

 but one of its characteristics consists of thick beds of grit 

 largely charged with big grains of altered felspar causing a 

 reddish or brownish tinge. These beds correspond to the 

 "couches de Kundalungu" of the highest Congo (Lualaba), and 

 form part of the margin of Lake Tanganyika, as we shall see 

 presently. On the Lower Congo the Eed Felspathic Grit series 

 extended to the westward of its present outcrop, as shown by 

 outliers, possibly as far as the crystalline zone. 



The upper portion of Zone IV, d of the section, which is 

 strongly in evidence near Stanley Pool, extends up the river 

 as far as Bolobo. It consists of white or yellowish silicious 

 sandstones, very pure, soft and friable under the fingers, 

 forming beds several hundred metres in thickness and having 

 a wavy and current-bedded stratification. Enormous sarsens 

 attest the former presence of these beds in areas where the 

 softer material has been removed by denudation. The beds of 

 this system, in this region, are nearly flat, or with a slight dip 

 to the eastward. They correspond to the " couches de 

 Lubilache " of the Lualaba district, and may be known as the 

 White Friahle Sandstones. 



It will not be necessary to carry the geological r4sum4 of the 

 Lower Congo any further, beyond pointing out one or two 

 matters which may have a bearing in future discussion relative 

 to the fauna of the Middle Congo, and, in consequence, of Lake 

 Tanganyika. In the first place it must be borne in mind that 

 from Stanley Pool to Boma the present river Congo has cut for 

 itself a passage through what may be regarded as the western 

 coastal range, in a series of falls and rapids which precludes 

 any present connection with marine conditions. We cannot 

 doubt that during the initiatory stages of this escape from the 

 interior, the waters of the Congo basin selected the most de- 

 pressed portion of the coastal range, which thus presents an 

 appearance, in section, of less importance than would be the 

 case either to the north or the south of the river's course ; also 

 denudation has been active in lowering the rim of the original 

 basin. It may be mentioned in this connection that the 

 coastal range in the north of the French Congo (province) 

 attains elevations of 1,500 metres in the " Monts de Cristal," 



