Arthur Holmes — Petrology of North- Western Angola. 231 



Mossamedes to Damaraland, where a thin series of conglomerates and 

 arkose beds is followed by the famous Otavi dolomites. Orthoceras 

 has recently been found in the latter formation, and it is therefore 

 certain that the dolomites are not older than the Silurian. 1 Barrat 

 considered the Angola and French Congo equivalents to be of 

 Devonian age, but the basis of his correlation is of doubtful value as 

 it depends only on lithological comparison with the rocks of far 

 distant areas. Subject to the same criticism is Studt's correlation of 

 the Otavi dolomites with the Dolomite series of the Transvaal system 

 and the dolomitic beds of the Cango Series. 



Important copper deposits are found in the Bembi dolomites and 

 limestones, occurring in the same way as those of the Otavi dolomites, 

 and of the still more valuable Katanga (Kambove) dolomites. All 

 these dolomites are probably of Silurian age, but the copper ores are 

 not altogether confined to this horizon, as they occur also in the 

 schists of Angola and in the Cenomanian calcareous grits and 

 conglomerates of Senza do Itombe. 



Karroo (?) Formation. — In the basin of the Quanza Biver, around 

 Dondo, a coal-bearing formation has been known for many years. It 

 rests unconformably on the Palaeozoic limestone and on the Archaean 

 complex, and dips gently towards the south-west. The lower beds 

 contain oil-shales and a thick seam of coal, and are followed by grey 

 grits and conglomerates, above which lies a much coarser conglomerate 

 which is characterized by a chocolate or reddish tint. Similar grey 

 and red grits, but without the coal-bearing formation, are found on 

 the Palaeozoic rocks of the French Congo. It seems probable, from 

 their lithological characters and position in the sequence, that the 

 •carbonaceous rocks of Angola are of Lower Karroo age. They are 

 certainly post-Silurian and pre-Cretaceous. 



Cretaceous and Tertiary. — The Cretaceous beds of Senza do Itombo 

 lie unconformably on the Karroo and the Archaean, and dip to the 

 south-west. Elsewhere Cretaceous rocks unconformably underlie the 

 Tertiary deposits of the coastal belt, but they are recorded only in 

 vague terms, and their distribution cannot yet be laid down on a map. 

 The Tertiary limestones and sandstones extend along the coastal belt 

 uninterruptedly, except for a doubtful break south of Ambriz, where 

 their outcrop narrows down to a mile or two. Both Eocene and 

 .Miocene fossils have been collected, but the distribution of the two 

 types, and the possibility of an unconformity between them remain to 

 be worked out. Alluvial deposits and lateritic sands and gravels form 

 a widespread blanket along the coastal belt, adding a further obstacle 

 to the inherent difficulty of mapping in a tropical country. 



Tertiary Igneous Rocks. — Between Senza do Itombo and Bango an 

 interesting series of alkaline igneous rocks was discovered by Colonel 

 Andrade. They are apparently intrusive -into both Karroo and 

 Cretaceous formations, and in common with the similar igneous rocks 

 of the Kamerun and Adamaua, they are most probably of Tertiary 

 age. Lava-flows, associated with plutonic and minor intrusions, are all 

 represented in this igneous suite, which includes nepheline phonolites 



1 Studt, p. 91, 1912. 



