Arthur Holmes — Petrology of North- Western Angola. 267 



IV. A CONTRIBUTION TO THE PETROLOGY OF NoRTH-WESTERN ANGOLA. 



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



(Continued from the May Number, p. 232.) 



4. ^GIRINE-RlEBECKITE GRANITE FROM THE LOWER CoKGO. 



(PI. XI, Fig. 1.) 



THIS remarkable rock occurs as a marginal phase of the granulitic 

 albite granites which are found between Noqui and kilometre 16 

 on the road to San Salvador. In the hand-specimen it shows a fine- 

 grained slightly foliated structure with a characteristic ' pepper and 

 salt ' appearance, due to the sprinkling of lustrous black plates of 

 riebeckite among iron-stained granular crystals of quartz and felspar. 

 A few small porphyritic crystals of orthoclase are embedded in the 

 main mass of the rock, making up approximately 15 per cent of the 

 whole. 



The minerals present, and their relative proportions by weight, 

 measured by the Posiwal method, are as follows : — 



Quartz ..... 32 

 (Albite \ 23 



Alkalic J Microcline/ 



alkalic j Microcline j 



Felspars J Orthoclase microperthite . . 15 

 ^•Microcline microperthite . . 10 

 Biebeckite . . . - . . 16 

 iEgirine ..... 4 



I Zircon \ 



Ilmenite f , 



Limonite | 



Sfvrifif.A nr Vnnlin (91/ 



^■Sericite or kaolin 



Total . . .100 



A separation was effected by means of Klein's solution, which was 

 gradually diluted with water. Specific gravity measurements were 

 made with a Westphal balance at each critical point. 



Specific Gravity. Minerals. 



3-441 ^girine (3-5) sank. 



3-381 Biebeckite sank. 



2-645 Quartz sank. 



2-599 Albite sank. 



2-545 Orthoclase and microcline sank. 



Special care was taken to examine the grains which fell at 2 645 for 

 oligoclase, but none could be detected. The absence of oligoclase was 

 further verified by refractive index measurements. Using clove-oil 

 (fi — 1-544) only quartz among the colourless essential minerals give 

 a higher value. Using monochlor benzine (fi = 1*527), albite gave 

 a. higher value, orthoclase and microcline a lower one. 



The felspars, with the exception of the larger microperthite crystals, 

 form with quartz a fine-grained glassy mosaic, the constituents of 

 which give sharp extinctions between crossed nicols with no trace of 

 strain shadows. The porphyritic crystals, on the contraiy, do show 

 strain phenomena. They have ill-defined borders, owing to interlocking 



