368 Arthur Holmes — Petrology of North- Western Angola. 



inclusions of albite, some of which show lamellar twinning. There 

 are also inclusions of a clear and colourless mineral having a much 

 lower refractive index than orthoclase. The inclusions are idio- 

 morphic and present four- or six-sided sections, some of the latter 

 being elongated in one direction. These features point to their 

 belonging to the sodalite group, but further identification is not 

 possible on account of the minuteness of the individuals. 



The phenocrysts of anorthoclase differ from those of sanidine in 

 being smaller in their dimensions, in having a slightly higher refractive 

 index, more obscure boundaries, minute twin striations, and a mottled 

 extinction. 



Nepheline is not abundant except as large crystals, and very tiny 

 ones which enter into the make-up of the groundmass. It is note- 

 worthy, in view of the presence of cancrinite in the associated dyke 

 rocks and nepheline syenite, to notice its absence both from this and 

 the brown phonolite. A small phenocryst of analcime was seen in 

 one section attached to a nepheline. 



JEgirine-augite is rare as phenocrysts, but in short needles and 

 grains in the groundmass it is abundant. Needles and flakes of 

 segirine are also present, the extinction being nearly straight, and 

 the pleochroism strong in tints of yellow-brown, yellow-green, and 

 blue-green. Amphiboles are entirely absent, though there are 

 numerous ragged shreds and aggregates of a brown decomposition 

 product, which may represent some variety of hornblende. Laths of 

 nepheline and anorthoclase can be distinguished in the groundmass, 

 and interspersed between them and the pyroxenes is a clear glassy 

 isotropic base, which may be glass or analcime. 



The rock was crushed, and the material which floated in brorno- 

 form diluted with benzine to give a liquid of specific gravity 2 - 28, 

 was collected. Most of the grains were completely isotropic, and 

 after washing and drying, their refractive index was found to lie 

 between those of castor-oil (1*48) and xylol (1'495). These details, 

 together with the fact that the rock contains over 2 per cent of 

 combined water, point to the identity of the isotropic base with 

 analcime, or with a glass of similar composition. While no signs of 

 crystalline structures on the one hand, or of devitrification or flow- 

 structures on the other, can be discerned, the exact nature of the 

 base must remain in doubt, though its correspondence in all essential 

 respects with the analcime phenocrysts of the associated ulrichite 

 throws the balance of evidence in favour of its being analcime. 



Nepheline phonolite (brown). 1 — In this rock the phenocrysts 

 are smaller than those of the preceding, but nepheline in good idio- 

 morphic crystals is much more abundant, while orthoclase is 

 subordinate. The minerals present are as follows: — 



Phenocrysts. Groundmass. 



Nepheline. Anorthoclase and nepheline. 



Sanidine (with inclusions of albite iEgirine and segirine-augite. 



and calcite). Iron-ores. 



Anorthoclase. Isotropic base with calcite and 



iEgirine-augite. indeterminate brown pleochroic 



patches. 

 1 PL XI, Fig. 4. 



