Arthur Holmes — Petrology of North-Western Angola. 323 



platform reappears. The phonolites lie on the Cretaceous beds, but 

 unfortunately their relations to the other members of the igneous 

 suite has not yet been determined, and no order of intrusion can be 

 stated. The whole suite of rocks is of post-Cretaceous and probably 

 of Tertiary age. They have not, however, been seen in contact with 

 Eocene or Miocene formations, which, although strongly developed 

 on the west, are not now represented to the north-east of Senza. 



Nepheline syenite ' (Fotaite type). — In the hand-specimen the 

 rock is seen to be made up chiefly of tabular felspars with conspicuous 

 Carlsbad twinning, and nepheline. The felspar is white and grey in 

 colour and the nepheline is similar, but sometimes exhibits a warmer 

 tint. Idiomorphic crystals of pyroxene and sphene are scattered 

 sparsely through the rock, making up less than 10 per cent of the 

 whole. 



The mineral composition as seen under the microscope is as follows. 

 On the left the order is that of relative abundance ; on the right it 

 is that of crystallization. 



Orthoclase and microperthite. Zircon and apatite. 



Nepheline and eegirine-augite. Sphene and ilmenite. 



Cancrinite. iEgirine-augite. 



Sphene and amphiboles Biotite. 



(including hastingsite). Amphiboles and nepheline. 



Apatite. Cancrinite. 



Ilmenite and biotite. Orthoclase and microperthite. 

 Zircon. 



The felspar is almost entirely orthoclase, the proportion of 

 perthitically intergrown albite being small. Carlsbad twinning is 

 common, and most of the felspars are turbid, especially along cracks 

 and cleavages, owing to the development of minute scales of sericite. 



Nepheline is fresh and clear, though along the more conspicuous 

 cracks micaceous alteration products are present. It is frequently 

 idiomorphic towards orthoclase, and sometimes towards amphibole. 

 Cancrinite is associated in small amount with the nepheline, occurring 

 in parallel growth with the latter, or in irregular veins which traverse 

 it. It can be easily distinguished from nepheline by its inferior 

 refractive index and superior birefringence. 



The coloured and accessory minerals have a strong tendency to 

 aggregate together in groups, the chief members of which are aegirine- 

 augite and sphene. JEgirine-augite occurs in well-shaped crystals, 

 the central parts of which approximate to augite with a grey-green 

 colour, or more rarely to titaniferous augite with a faint purple 

 tint and a slight pleochroism. The outer borders are, however, of 

 a bright green colour, and belong more nearly to aegirine proper than 

 the main mass of the interiors. 



Around these pyroxenes, and also around inclusions of magnetite 

 or ilmenite within them, there is usually an irregular border of 

 amphibole, the properties of which point to its being hasting site, 2 

 The pleochroism and optical orientation are as follows : — 



1 Plate XI, Fig. 2. 



2 Adams & Barlow, The Haliburton and Bancroft Areas (Geol. Surv. Canada), 

 1910, p. 247. Quensel, Bull. Geol. Inst. Upsala, xii, p. 145, 1914. 



