230 Arthur Holmes — Petrology of North- Western Angola. 



To the south of the Congo, again in Portuguese territory, mica- 

 schists and quartzites occur along the road from. Noqui to San Salvador. 

 The schist series differs in this locality from its northern continuation 

 in the comparative rarity of epidote-bearing rocks, and by the 

 appearance of numerous bands of crystalline limestone. Near the 

 River Mavumi gneissose granites are exposed. Granulitic granites 

 are found just south of the Congo, similar in structure to the Boma 

 granites, and remarkable for their high content of albite. In many 

 places they are injected in sheets between crystalline limestones and 

 other members of the scbist series, and in the former, interesting 

 contact minerals, such as chondrodite and diopside, have been 

 developed. Syenites occur between Tombo and the River Lalu. At 

 kilometre 16 on the Noqui-San Salvador road the limit of the albite 

 granites is reached, and at this point Colonel Andrade found a variety 

 containing a relatively high percentage of riebeckite, accompanied by 

 segirine. This rock, a splendid example of an interesting type of 

 which very few occurrences are known, is described in detail below. 

 It illustrates Harker's view * that alkaline rocks, and especially 

 unusual types, tend to occur on the margins of igneous provinces. 



Between Bembe and the coast at Ambriz, the schists and gneisses 

 are not seen until Kiballa is passed, after which they continue to 

 the coastal belt, where they disappear under a thin narrow band of 

 Tertiary formations. Near the mission station of San JoSo intrusions 

 of diabase are found, the only examples of this rock met with in 

 Portuguese territory south of the Congo. At kilomeh'e 19 between 

 the coast and Bembe the western peripheral phase of the granulitic 

 granites is represented by a fine-grained l'eddish rock which is 

 described by M. de Sousa as a nordmar&ite. 2 It therefore seems to 

 differ from the segirine-riebeckite granite already mentioned only in 

 the possession of arfvedsonite in place of riebeckite. 



Palceozoic Rocks. — Lying unconformably on the basal complex is 

 a system of old sedimentary rocks which includes two distinct series 

 of formations. In the enclave of the Belgian Congo Cornet has 

 described a lower series which he calls the Berabizi Beds, including 

 conglomerates, calcareous sandstones, shales, and slates, and an upper 

 series, the ' schisto-calcareous ' beds, made up of dolomites with chert 

 bands. Their prevailing dip is away from the coast towards the 

 central part of the Congo Basin. Passing inland, the lower beds, 

 which are exposed in narrow strips around the schist boundaries, are 

 everywhere followed by the overlying dolomite series. 



Around Mayembe the two series are represented by calcareous 

 sandstones and cherty limestones respectively. Further south, 

 between San Salvador and Bembe and from there to Kiballa, thick 

 beds of arkose, quartzite, and calcareous sandstones are surmounted 

 by dolomites and limestones. 



The age of this system is doubtful. Micro-organisms have been 

 found in the limestone of Mayembe, but they are so ill-preserved as 

 to be indeterminable in sufficient detail to serve as an index of age. 

 Further south, however, the system continues through Benguela and 



1 Pres. Ad. Brit. Assoc. Kep. for 1911, p. 370. 



2 Brogger, Zeit. Kryst., xvi, p. 55, 1890. 



