Arthur Holmes — Petrology of North-Western Angola. 369 



Among the phenocrysts, those of nepheline differ from those in the 

 grey phonolite only in. size, and in having numerous inclusions of tiny 

 laths of segirine, arranged in rows, just within and parallel to their 

 borders. Anorthoelase, as before, is present in long, narrow, vaguely 

 outlined laths. The sanidines are free from inclusions of a sodalite- 

 like mineral, but irregular patches and streaks of caleite and of the 

 brownish glassy base are common. 



JEgirine-augite occurs in dark-green, spongy crystals, in ragged 

 shreds and patches, and to a less extent in small needles and tufts. 

 The ground-mass is yellow to brown in colour, and is frequently 

 broken by tiny laths, often radiating from a common centre, of 

 anorthoelase and nepheline, and by granular masses and blebs of 

 ealcite. The latter does not appear to be the result of alteration by 

 weathering, and its presence with glass in the felspar phenocrysts 

 suggests the view that it is an original product of the magma from 

 which the rock was formed. It is noteworthy that cancrinite is 

 present in the rocks described in the July number, and the calcium 

 carbonate of that mineral may be here represented as free caleite. 



The greater part of the base is isotropic, but there are a few 

 pleochroic (yellow to brown) areas of indefinite form. They may 

 represent a phase in the magmatic decay of hornblende under 

 superficial conditions, for otherwise this mineral is not represented 

 in the volcanic rocks. 



Summary and General Considerations. — In the following table 

 the mineralogical constituents of the five rocks described are 

 summarized and their specific gravities are added : — 



DECADE VI. — VOL. II. — NO. VIII. 



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