366 Arthur Holmes — Petrology of North- Western Angola. 



It is interesting to compare the relative amounts of the felsic * and 

 mafic minerals, as shown in Tables II and III. 



Bekinkinite (Table II, col. VI) . 

 Bekinkinite (Table III, col. I) 

 Bekinkinite (Table III, col. II) . 

 Lugarite, var. 1 (Table II, col. I) 

 Lugarite, Tar. 2 (Table II, col. II) 

 Lugarite, var. 2 (Table II, col. Ill) 

 Lugarite, var. 3 (Table II, col. IV) 



Felsic. 



30-7 

 49-0 

 49-9 

 53-9 

 51-9 

 55-0 

 80-6 



Mafic. 



69-3 

 52-0 

 50-7 

 46-1 

 48-1 

 45-0 

 19-4 



M 



ratio. 



•44 

 •94 

 •98 

 1-17 

 1-08 

 1-22 

 4-15 



Domafic 

 Mafelsic 

 Mafelsic 

 Mafelsic 

 Mafelsic 

 Mafelsic 

 Dofelsic 



VII. A CONTRIBUTION TO THE PETROLOGY OF KoRTH- WESTERN ANGOLA. 



By Arthur Holmes, B.Sc, A.E.C.S., F.G.S., F.K.G.S. 

 {Concluded from the July Number, p. 328.) 

 Olivine tjlrichite. — Megascopically this rock differs from the 

 foregoing nepheline monchiquite in having a well-marked porphyritic 

 structure, the phenocrysts including orthoclase and nepheline in 

 crystals having a maximum dimension of nearly a centimetre. Under 

 the microscope the minerals present are : — 



Phenocrysts. Groundmass. 



Nepheline. Titaniferous augite. Anorthoclase. 



Sanidine. iEgirine-augite. Amphiboles. 



Anorthoclase. iEgirine. Pyroxenes. 



Barkevikite. Olivine and serpentine. Calcite. 



Arfvedsonite. Analcime. Apatite. 



Augite. Iron-ores. 



Nepheline and sanidine call for no special description. Anorthoclase 

 is similar to that in the phonolites. The anorthoclase of the ground- 

 mass, in small prisms arranged at various angles, was determined by 

 its optical characters and specific gravity. 



1 The terms felsic and mafic have been proposed by Cross, Iddings, Pirsson, 

 and Washington (Joum. Geol., xx, p. 560, 1912) as names for two main groups 

 of rock-forming minerals, the one including quartz, felspars, and felspathoids ; 

 the other, pyroxenes, amphiboles, micas, olivine, iron-ores, etc. Variation in 

 the relative proportions of these two groups constitutes the important mode of 

 variation in igneous rocks which gives rise to leucocratic, mesocratic, and 

 melanocratic types. A quantitative expression may be given to this variation 

 by attaching the prefixes per- and do- to the terms felsic and mafic, with 

 a similar connotation in respect to the mode as the analogous terms persalic, 

 dosalic, etc., have in respect to the norm of the American Quantitative 

 Classification. 



perfelsic 

 dofelsic 

 mafelsic 

 domafic 

 permafic 



Heucocratic types. 



1 



<?>- 5 

 1 3 



5 3 



< — > - mesocratic types. 

 o 5 



^melanocratic types. 



