A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 543 



nockite series is unquestionably intrusive into the gneisses and schists of the 

 basement. 



It is interesting to record that Lacroix has described a very similar series of char- 

 nockites from the Ivory Coast. # These rocks cover a great area in a region on the 

 frontiers of French Guinea, the Ivory Coast, and Liberia. They form a series rang- 

 ing from hypersthene-granite very poor in ferro-magnesian minerals, through quartz- 

 norites, to norites witli 50 per cent, of hypersthene. These rocks are identical with 

 those described above with the single exception that hornblende does not occur to 

 any extent in the more basic types. The country rock is stated to be granitic. 



III. HORNFELSED PORPHYRIES AND OTHER HORNFELS. 



In this section is described a curious and puzzling group of rocks, which, though 

 unmistakably igneous rocks of an acid character, appear to have suffered consider- 

 able thermal metamorphism. The evidence for this lies in their perfect freshness, 

 the recrystallisation of the groundmass into an evenly granular, water-clear mosaic, 

 and the presence of abundant new-formed biotite, characters superposed upon their 

 original igneous characters. The rock described as charnockite-porphyry, although 

 of somewhat different mineral composition, has exactly the same characters as these 

 rocks, and must be included in their discussion. It might also be remarked that the 

 rocks of the charnockite series themselves present features, such as granularity, ideal 

 freshness, and presence of biotite, which might be interpreted as affording evidence 

 of thermal metamorphism. On the other hand, the mineral composition of these 

 porphyries, especially the common occurrence of magnetite-cassiterite intergrowths, 

 links them to the granites described later. Finally, associated with these porphyries, 

 and indubitably of thermometamorphic origin, are rocks determined as scapolite- 

 epidote-hornfels and cordierite-biotite-hornfels. 



In hand specimens these rocks may be described as " dark porphyries." They 

 generally show a fine-grained, but obviously crystalline groundmass of a dark-bluish 

 colour, with the peculiar saccharoidal texture of hornfels and granulites, and contain 

 dark phenocrysts of felspar. One or two examples are light-coloured, and are aplitic 

 or halleflintas-like in appearance. 



The most typical example of these rocks is a specimen from N.E. of Bailundo 

 (159). In thin section this shows phenocrysts of felspar together with peculiar clots 

 or felts of hornblende, biotite, magnetite, and cassiterite, embedded in a microcrystal- 

 line, granulitic mosaic consisting mostly of quartz, and necked with anhedral shreds 

 of biotite. A plagioclase felspar forms by far the most abundant set of phenocrysts. 

 It is extremely zonal and is characterised by minute, indefinite, bent, and discon- 

 tinuous twin-lamellse, which renders the exact determination of its composition 



* A. La.croix, "Sur l'existence a la Cote d'lvorie d'un series petrog. comparable a celle de la charnockite," 

 Gomptes Renins, cl (1910), pp. 18-22. 



