Arthur Holmes — Petrology of North-Western Angola. 327 



Olivine-free. 

 Nepheline monchiquite. 

 Leucite monchiquite. 

 Biotite monchiquite (including 



ouchitite) . 

 Monchiquite. 

 Pyroxene monchiquite (including 



fourchite). 

 Amphibole monchiquite. 



OUvine-bearing . 

 Nepheline olivine monchiquite. 

 Leucite olivine monchiquite. 

 Biotite olivine monchiquite 



(including alno'ite). 

 Olivine monchiquite. 

 Pyroxene olivine monchiquite. 



Amphibole olivine monchiquite. 



According to this nomenclature the rock under discussion would be 

 called a nepheline monchiquite. 



Chemical characters. — A chemical analysis of the work and a calcu- 

 lation of the norm gave the following results : — 



It is significant that most olivine monchiquites have the magmatic 

 symbol III, 6, 2, 4 (analyses IV and Y in the table below), and the 

 nepheline monchiquite of Angola falls into Group II, as might be 

 anticipated, on account of its lower proportion of femic to salic 

 constituents. This difference is at once evident in thin section, and 

 is also indicated chemically by the comparatively high percentages of 

 alkalies and alumina and the low percentages of iron and magnesium. 

 The analyses cited below also bring out the-relation of the rock to the 

 tinguaites (I) and the augitites (VI). The magmatic symbols clearly 

 show that it lies midway between these somewhat extreme types. 

 The olivine monchiquite of Cabo Frio, Brazil (V) is that which agrees 

 most closely in chemical composition with the Angola variety, and 

 the differences, especially between the contents of magnesium, iron, 

 and aluminium oxides, are of the kind to be expected in comparing 

 an olivine-bearing rock with one which is free from that mineral. 



