A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 545 



Scapolite-epidote-homfels. — This rock, which occurs E. of Chingwari (199), is a 

 dark porphyry of appearance similar to that of the rocks described above. It consists 

 mainly of a mosaic of quartz, biotite, and epidote, in which the first-named is perhaps 

 the most abundant constituent. Magnetite and blue tourmaline form subordinate 

 members of the mosaic. Embedded in this groundmass are occasional phenocrysts 

 of andesine, witnessing to the original igneous character of the rock. These crystals 

 frequently enclose the biotite and epidote of the groundmass. There are also 

 numerous irregular areas of scapolite which are crowded with grains of the mosaic 

 groundmass. The mineral is easily recognised by its bright second-order polarisation 

 tints, its cleavage, straight extinction, and uniaxial character. Whilst the poikilitic 

 areas frequently retain optical continuity, they are occasionally broken up into 

 grains with different orientations. The abundant biotite presents no unusual optical 

 features, and is very closely intergrown with epidote. This rock is clearly the pro- 

 duct of combined contact-metamorphism and pneumatolysis. 



Cordierite-biotite-liornfels. — This rock occurs seven miles E. of Candumbo (230) 

 and in hand specimen is a dark-bluish, finely crystalline granulite. In thin section it 

 is seen to consist of a thoroughly granulitic groundmass of cordierite, quartz, together 

 with abundant biotite, muscovite, and magnetite. The cordierite forms rounded 

 sections, occasionally showing lamellar twinning, and the frequent development of 

 a strong yellow pleochroism around minute inclusions. It is undergoing an incipient 

 alteration to small flakes of white mica. The biotite is intergrown with muscovite. 

 The magnetite is evenly distributed in minute subhedral grains throughout the rock. 

 Occasional areas of turbid material with traces of multiple twinning suggest the 

 presence of lime-soda felspar. The original nature of this rock is not so evident as 

 in the foregoing. It was probably an impure sandstone or greywacke, and was 

 almost certainly not of igneous origin, as it is devoid of the felspar phenocrysts 

 characteristic of the types above described. 



The rocks described in this section appear to form part of the ancient basement 

 of the Benguella plateau, into which the batholiths of tin-bearing granite described 

 later were injected. They are interstratified with and injected into gneiss, and into 

 the quartzites, quartz-schists, and sillimanite-schists of the Bailundo series. They 

 formed a series of acid igneous rocks associated with various sediments. The presence 

 of minerals such as cassiterite, tourmaline, and scapolite, suggest that the alteration 

 they have undergone was due to pneumatolysis as much as to contact-metamorphism. 

 The cassiterite-magnetite intergrowths of the Bailundo rock are identical with those 

 occurring in the Benguella granites (p. 547), suggesting that this mineral association, 

 together with scapolite and tourmaline, were pneumatolytic introductions derived 

 from the Benguella granite batholiths, and that the peculiar modifications these 

 rocks have undergone were consequent upon the intrusion of these great masses 

 of molten material. 



The characters of this group of rocks suggest comparison with the " leptites " 



TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 14). 79 



