228 Arthur Holmes — Petrology of North- Western Angola. 



Note. — The possibility that these embryonic crystals (Figs. 1 and 2) 

 may be scapolitic is not overlooked. Scapolite is occasionally developed 

 as a contact mineral in argillites around basic intrusives, being 

 restricted to a narrow zone at the contact. The optical features of 

 such scapolite are often obscured by carbonaceous mutter present as 

 inclusions, while the mineral itself is often decomposed to a matted 

 aggregate of mica-like flakes (colourless chlorite — ' leuchtenbergite '). 

 So far, therefore, the parallel between such occurrences of scapolite 

 and the mineral now in question is complete; but the shape of the 

 sections and the distribution of the carbonaceous matter give the 

 latter a strikingly chiastolitic appearance, while the content of lime 

 and alkali in the shales is much too low to favour the development of 

 scapolite. 



VII. — A Contribution to the Petrology of North- Western Angola. 

 By Arthur Holmes, B.Sc, A.B.C.S., F.G.S., F.B.G.S. 



(WITH A MAP, PLATE IX.) 



1. Introduction. 



2. Bibliography. 



3. Notes on the Geology of North- Western Angola. 



4. iEgirine-Riebeckite Granite from the Lower Congo. 



5. Alkaline Igneous Bocks between Senza do Itombe and Bango 



(Loanda). 



1. Introduction. 



LAST year I received from Colonel Freire d'Andrade a number of 

 interesting alkaline igneous rocks, from a valuable collection 

 made by him during an extensive exploration in North- Western 

 Angola undertaken some years ago. Until recently our knowledge 

 of the geology of that part of Africa was very scanty. Already in 

 1897 Cornet had described the western tongue of the Belgian Congo 

 which separates the Portuguese provinces of Cabinda and Congo. 

 Moreover, the occurrence of copper ores at Bembe and Senza do 

 Itombe had attracted some geological attention, but it was not until 

 Colonel Andrade's journey that any systematic geological observations 

 were made over a large area of the territory. Since then a number 

 of prospecting expeditions have been conducted by J. J. MacHugh, 

 the results of which serve to extend our knowledge to the south of 

 the area described by Colonel Andrade. In view of the inaccessibility 

 of much of the literature dealing with Angola, it may be interesting, 

 before giving petrological details of some of the rocks, to present in 

 English a short sketch of the geology of part of Portuguese Congo 

 and Loanda, embodying the observations made by Andrade and 

 MacHugh. 



2. Bibliography. 



1. Andrade, Freire d'. "As minas de cobre da provincia d ' Angola " : 



Bevista de Obras publieas e minas, vol. xxxvii, No. 436, pp. 275-315, 1906. 



2. BARRADAS, J. F. " Diario relatorio sobre a formagao geologica encontrada 



durante a recente expedigao atraves da Quissama e parte da regiao do 

 Amboim e Libolo por J. J. MacHugh, 1909" : Bevista portuguesa 

 colonial e maratima, No. 149, pp. 219-26, 1910. 



