370 Arthur Holmes — Petrology of North- Western Angola. 



The whole suite of rocks is characterized by a common association 

 of minerals. The absence of analcime from the foyaite and of 

 cancrinite and amphiboles from the phonolites is entirely consistent 

 with the usual behaviour of these minerals under the varying 

 conditions which control the crystallization of magmas. Caleite is 

 present in the phonolites and in the olivine ulrichite in precisely the 

 same way as cancrinite occurs in the other rocks. Amphiboles, 

 recognizable as such, are absent from the phonolites, but decomposition 

 products which may well be their representatives, still remain. 

 There is, therefore, strong mineralogical evidence pointing to 

 consanguinity and differentiation from a common magma. The rocks 

 differ chiefly in the relative proportions of the minerals present, and 

 in the structures determined by those proportions and by the mode 

 of crystallization. The only exceptions to this are found in the 

 isolated occurrence of olivine in olivine ulrichite and in the absence 

 of sphene from that rock and from the phonolites. "With these 

 exceptions the foyaite and the nepheline monchiquite clearly 

 represent leucocratic and melanocratic phases respectively of an 

 original magma which may be closely approximated to by the 

 phonolites. In the absence of field evidence as to the relative 

 abundance and order of intrusion, it is not yet possible to go further 

 in the attempt to elucidate the origin of the various rock-types. 



No basaltic rocks were collected by Colonel Andrade, but near the 

 old fort of Duque de Braganqa (lat. 9° S., long. 16° E.), which is not 

 far from the road between Senza and Bango, M. John Monteiro 

 discovered in 1875 l a series of trachytes and of porphyritic basalts, 

 which still await description. 



With the exception of a brief note by M. Pereira de Sousa in 1913, 

 there has hitherto been no published record of nepheline-bearing 

 rocks in South-West Africa between Namaqualand and the Kamerun. 

 Nepheline syenites and phonolites, with associated intrusions of 

 a barkevikite-bearing monchiquite, occur near Pomona, just to the 

 north of the Orange River.* Nepheline syenite has also been found 

 to the west of Kamerun Mountain, and in the Benue Valley between 

 Yola and Garua in Adamaua, 3 associated in the latter locality with 

 nepheline basalts and phonolites. The suite of nepheline rocks 

 described in this paper falls midway between Pomona and Kamerun, 

 and the gap has now been further bridged by Professor J. W. Gregory's 

 discovery of a similar suite of rocks at Chiucca in the province of 

 Benguela, where alkaline volcanics occur in association with solvs- 

 bergite and shonkinite. 



1 P. Choffat, Mem. de la Soc. Phys. et d'Hist. Nat. de Geneve, xxx, No. 2, 

 1888. 



2 See also Kaiser, in Bibliographic Geol. du Portugal et de ses colonies, 

 ser. II, 1913, p. 21. 



3 Passarge, Adamaua, 1895, p. 384. 



