A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 551 



broad, simply-twinned anhedrons of soda-orthoclase, which are irregularly mottled 

 in polarised light, together with subordinate nepheline and sodalite. The nepheline 

 is distinguished by its lower interference tints, and total absence of the mottling 

 which characterises the orthoclase, as well as by refractive index and interference 

 figure tests. The sodalite has occasionally a faint bluish tinge, and is crowded with 

 minute bubbles and other inclusions. It is identical with a mineral which occurs in 

 the shonkinite described later, and which has been determined by chemical means. 

 The ferro-magnesian constituents consist of ragged anhedrons of a pleochroic 

 greenish-brown hornblende, which envelops small tablets of biotite, and occasionally 

 contains a small core of almost colourless pyroxene. Irregular grains of eegirine are 

 frequently inserted around the margins of the crystals. Apatite and magnetite are 

 also abundant as inclusions in the amphibole sections. The pleochroism of the 

 mineral is from pale yellowish-green to a light brown-red which has a tinge of violet. 

 There is always an irregular marginal zone in the crystals in which the colour 

 becomes bluish-green. The extinction in most sections is in the neighbourhood of 

 17°, but may range up to 31°. The mineral is probably a soda-amphibole belonging 

 to the katoforite-arfvedsonite series, or possibly to ksersutite. iEgirine also occurs 

 as small anhedrons embedded in the felspars. An estimate of the relative propor- 

 tions of the constituents is : soda-orthoclase, 80 per cent. ; nepheline and sodalite, 

 10 per cent. ; hornblende, augite, biotite, etc., 10 per cent. 



B. Sodalite- syenite. — A second variety of syenite occurs in the volcanic vent of 

 Chieuca (163). It differs from the above in its much finer grain, in the substitution 

 of a pale-purplish titanaugite for hornblende as its principal mafic constituent, and 

 in a smaller proportion of nepheline and sodalite. The titanaugite encloses biotite 

 tablets, apatite, and magnetite ; and is in all stages of alteration to granular eegirine- 

 augite. The mass of the rock, however, is soda-orthoclase, with a very subordinate 

 amount of euhedral nepheline and sodalite. The latter mineral penetrates the felspars 

 in channels directed by the cleavages, as though it had made its way by corrosion. 

 Sphene is an important accessory constituent. 



Decomposed specimens of this type, in which the pyroxene has completely passed 

 over to aggregates of fibrous amphibole, and the felspar completely " kaolinised," occur 

 in the Ochilesa district (208a, 209e, 219/). 



C. Aherite. — An alkali-syenite containing neither nepheline nor sodalite occurs 

 to the W. of Ochilesa (1726). It consists mainly of anhedrons of mottled soda- 

 orthoclase, with oligoclase (AbaAni) in little inferior quantity. Biotite forms the 

 principal ferro-magnesian constituent, but is accompanied by a little brown hornblende 

 similar to that of the nepheline-sodalite-syenite of Ochilesa described above. These 

 crystals may contain a core of nearly colourless pyroxene. Crystals of apatite and 

 magnetite are common enclosures within the ferro-magnesian minerals. This rock 

 has a close affinity with the akerite of Norway, although the principal ferro-magnesian 

 constituent of the latter is augite. 



