324 Arthur Holmes — Petrology of North- Western Angola. 



X, near to a . . . brown-green j 



Y, near to c . . . . blue-green \Z"> Y> X 



Z = b . . . . olive-green j 



The extinction angle Y A c reaches a maximum of 35°. The 

 birefringence is exceedingly low, being less than that of quartz, but 

 on account of the strong axial dispersion the interference figure is 

 coloured. The axial plane is at right angles to the plane of symmetry, 

 and the optic axial angle is unusually small for an amphibole. 

 Associated with the hastingsite are small patches of another horn- 

 blende, having a somewhat higher birefringence and a strong 

 pleochroism, from olive-green to a pale ruddy or yellowish brown. 

 It probably belongs to the arfvedsonite-katoforite series. 



Sphene occurs in good lozenge-shaped crystals, which are almost 

 invariably twinned. The pleochroism is from clove to grey. Sphene, 

 and ilmenite in small grains, occur as inclusions in segirine-augite 

 and hastingsite. Small zircons and apatites, though few in number, 

 are constant accessories, and occur as inclusions not only in the more 

 abundant minerals but also in ilmenite. Tiny shreds of biotite, 

 pleochroic from green to yellow, occur here and there as inclusions in 

 orthoclase or microperthite. 



The Mount Jombo (south-west of Mombasa) nepheline syenite 1 

 is strikingly similar to the Angola rock in structure and mineral 

 composition, and both belong to the foyaite type as restricted by 

 Brogger, 2 and bear a close resemblance to the type-rock of Foya. 



Nepheline monchiquite. 3 — In the hand-specimen this rock has the 

 characteristic aspect of a melanocratic rock, showing prismatic crystals 

 of dark-brown hornblende up to half an inch in length embedded in 

 a blue-grey aphanitic groundmass. 



The minerals present, as seen under the microscope, are as follows, 

 in order of abundance : — 



Phenocrysts. Groundmass. 



Pyroxenes. 



Titaniferous augite. 



iEgirine-augite. Nepbeline. 



.ZEgirine. Analcime. 



Augite. Cancrinite. 



Amphiboles. Pyroxenes. 



Barkevikite. Amphiboles. 



Arfvedsonite. Apatite. 



Hastingsite. Ilmenite. 



Sphene. 

 Nepheline. 

 Analcime. 



As indicated above, the pyroxenes form a connected series ranging 

 from augite and titaniferous augite to segirine-augite and pure aegirine. 

 The largest crystals are idiomorphic (1x2 mm. in section), and 

 exhibit a patchy colouring and zoning in various light tints of purple 

 and green. The borders are well defined by bright-green segirine, 

 which is strongly pleochroic from blue-green to yellow-green. The 



1 J. W. Gregory, Q.J.G.S., lvi, 1900, p. 223. 



2 Brogger, Zeit. KrisL, xvi, p. 30, 1890. 



3 PL XI, Fig. 3. 



