270 Arthur Holmes — Petrology of North-Western Angola. 



which is a characteristic associate of riebeckite, though a very rare 

 mineral, was looked for, but could not be identified with certainty. 

 Tiny grains of ilmenite peripherally altered to leucoxene are present, 

 included in or near to riebeckite. 



Ferruginous stains occupy the borders between many of the crystals, 

 and vermicular aggregates and fibrous coatings of the same material have 

 already been mentioned. Both the colour, and the excess of combined 

 water in the rock over that required for riebeckite, point to limonite 

 as the identity of this mineral. The rock shows no trace of weathering 

 alteration, and it is possible, as Murgoci has suggested, 1 that the 

 limonite is primary, and that it now directs attention to areas which 

 at the moment of consolidation were still more or less impregnated 

 with iron compounds and water vapours. 



Chemical Characters and Classification. — A chemical analysis of 

 the rock was made by the methods advocated by "Washington. The 

 results are given below, together with the norm and the position of 

 the rock in the Quantitative Classification. The rock falls into the 

 subrang Varingose (II, 3, 1, 3) on the side of Grorudose (II, 4, 1, 3), 

 which differs from the former only in the inferior ratio of quartz to 

 felspar. 



A number of analyses of other riebeckite and segirine granites is 

 listed below for comparison with the Angola example. It will be 

 noticed that with the incoming of riebeckite the proportion of ferrous 

 iron relative to ferric is increased, and moreover that a high proportion 

 of ferrous iron is consistently accompanied by a superiority of potash 

 over soda. Analysis III, that of the Angola rock, falls into the same 

 subrang as that of the Grorudite from Varingskollen. The two 

 analyses are strikingly similar, and the total iron contents are 

 practically identical. There is, however, a sharp difference in the 

 relative proportions of ferrous and ferric iron, a difference which 



1 Murgoci, p. 145. 



2 Practically all Fe Si 3 . 



