272 Arthur Holmes — Petrology of North-Western Angola. 



Dahamis, Socotra : riebeckite-acmite granite. 1 



Cevadaes, Portugal : segirine-riebeckite granulite. 2 



Narsarsuk, Greenland : Eegirine-riebeckite granite. 3 



Shira and Kila, Northern Nigeria : eegirine-riebeckite granites. 4 



Ampasibitika, Madagascar : segirine-riebeckite granite.^ 



Although the ages of these nine rocks are not yet known except 

 within broad limits, it is clear that they range from late Archaean to 

 early Tertiary, and it is interesting to notice that, rare as it is, 

 segirine-riebeckite granite is a type which has recurred at widely 

 different periods and in widely separated localities. 



Granites contain iug riebeckite or segirine are not rare in West Africa. 

 They occur with segirine rhyolites and alkaline trachytes in the Shari 

 basin (Mburao and Melfi) and at Hadj el Hamis on the south-east of 

 Lake Tchad. 6 At Zinder and Mounio, west of Tchad, riebeckite, 

 segirine, and segirine-riebeckite granites are associated with rhyolites 

 of corresponding composition, 7 and further to the north, in the Saharan 

 complexes of Ahaggar and Air, segirine rhyolites are found. 8 

 lliebeckite granites are known from Feta in Dahomey, and they are 

 well represented in Nigeria in the Gurkawa Hills, on the Bauchi 

 plateau, near the borders of Bauchi and Kano, and at Shira in 

 Katajum. 9 Unfortunately the ages of these rocks are not yet known 

 with certainty. The volcanic rocks appear to be early Tertiary, but 

 the granites may lie anywhere between late Pre-Cambrian and early 

 Tertiary. 



EXPLANATION OF PLATE XI. 



FlG. 1. — iEgirine- riebeckite Granite, near kilometre 16, on the road from Noqui 

 to San Salvador, Congo, Angola, showing riebeckite in longitudinal 

 sections, and allotriomorphic quartz and felspar with films of limonite 

 between adjacent individuals. 



FlG. 2. — Nepheline Syenite (Foyaite), near kilometre 24, on the road from 

 Senza do Itombe to Bango, Loanda, Angola ; showing nepheline (clear 

 except along cracks) and orthoclase (cloudy), with a characteristic group 

 of segirine augite bordered with hastingsite and sphene. 



FlG. 3. — Nepheline Monchiquite, near kilometre 20, on the Senza-Bango road, 

 showing phenocrysts of amphiboles and pyroxenes and wisp-like ferns 

 embedded in a clear groundmass of nepheline and of what is probably 

 analcime. 



FlG. 4. — Nepheline Phenolite, near kilometre 22, on the Senza-Bango road, 

 showing phenocrysts of nepheline in a dark groundmass which includes 

 segirine augite and a yellow-brown isotropic base. 

 Ordinary light and magnification 40 in each case. 



The photo-micrographs were made for me by Mr. G. S. Sweeting, of the 

 Geological Department of the Imperial College, to whom my best thanks 

 are due. 



1 Bosenbusch, Elemente d. Gesteinslehre, p. 86, 1910. Analysis by Lud wig. 



2 Osann, Neu. Jahre Min. GeoL, etc., 1907, p. 109 (2) ; de Sousa, C.B., 

 vol. clvii, p. 1451, 1913. 



3 Flink and Boggild, Meddelelser om Greenland, p. 24, 1899. 



4 Falconer, The Geology and Geography of Northern Nigeria, p. 133, 1911. 



5 Lacroix, Nouv. Arch. Mus. Hist. Nat., ser. iv, p. 164, 1902. 



6 Gentil & Freydenberg, Bull. Soc. Geol. France, ser. IV, viii, p. 35, 1908. 



7 Chudeau, Sah. Lond. 1909, p. 266. 



8 Chudeau, op. cit., p. 258. 



9 Falconer, The Geology of Northern Nigeria, pp. 133-4. See p. 135 for 

 other references. 



