Arthur Holmes — Petrology of North- Western Angola. 367 



Analcime is seen in roughly circular spaces, surrounded by the 

 coloured minerals, some of which are arranged at sharp angles 

 suggesting the trapezohedron form. This mineral is more abundant 

 as phenocrysts than in the nepheline monchiquite, but seems to be 

 absent from the groundmass. 



The chief amphibole present is barkevikite, which differs from that 

 of the nepheline monchiquite only in having somewhat darker colours. 

 A little arfvedsonite is associated with it. The pyroxene series, again, 

 is very similar to that already described (p. 324), except that the 

 titaniferous augites are larger and more independent of the aegirine- 

 augites. Another difference lies in the absence of long wisp-like 

 growths, stout prisms of all sizes up to 1 mm. being the chief habit 

 developed. 



The rock differs sharply from the nepheline monchiquite in the 

 total absence of sphene and in the presence of phenocrysts of olivine. 

 These are about the same size as the larger titaniferous augites, 

 averaging 1 mm. in length. Idiomorphic forms are rare, and the 

 crystals have been altered to serpentine along numerous cracks and 

 around the borders. Many of the crystals are surrounded by small 

 prisms of barkevikite and aegirine-augite. 



Although the rock has not been analysed, it bears so close 

 a mineralogical resemblance to the ulrichite of Dunedin, New 

 Zealand, 1 that the writer has no hesitation in applying to it the same 

 name. Ulrichite seems to occupy the same position among the dyke 

 rocks as canadite, recently defined by Quensel, 2 holds among the 

 plutonic rocks. Canadite is a nepheline syenite, rich in mafic 

 minerals, and containing normative lime-soda felspar, which does not 

 necessarily appear in the mode of the rock. Ulrichite may be defined 

 in the same way with reference to the tinguaites, and as already 

 discussed in the case of monchiquites ; it would save much confusion 

 to retain the term ulrichite for olivine-free varieties, and describe 

 those containing olivine as olivine ulrichite. 



Nepheline phonolite (grey). — Megascopically the rock consists of 

 a green-grey aphanitic groundmass in which large tabular phenocrysts 

 of nepheline and sanidine are distributed. The former are of a yellow 

 colour with an occasional reddish hue, and may reach a length of 

 1 inch. The sanidines are thin and glassy, and exhibit Carlsbad 

 twinning ; they rarely exceed £ inch in length. On the weathered 

 surface long ridges of nepheline stand out above the less resistant 

 groundmass. Under the microscope, the following minerals are seen 

 to be present : — 



Phenocrysts. Groundmass. 



Nepheline. Anorthoclase and nepheline. 



Sanidine (with inclusions of albite 2Egirine and segirine-augite. 



and a mineral of the sodalite Caleite. 



group). Iron-ores. 



Anorthoclase. Brown decomposition products. 



■ZEgirine-augite. Isotropic base probably consisting of 

 Analcime. analcime. 



Almost all the sanidine phenocrysts contain rhomb - shaped 



1 P. Marshall, Q.J.G.S., Ixii, p. 397, 1906. 



2 P. Quensel, Bull. Geol. Inst. Upsala, xii, p. 130, 1914. 



