A CONTRIBUTION TO THE PETROGRAPHY OF BENGUELLA. 539 



group of gneisses with very pronounced cataclastic textures, showing all gradations 

 to mylonite. In the first stage the pseudo-porphyritic " eyed " felspar becomes highly 

 rounded, and the quartz is broken into coarse-grained aggregates of angular frag- 

 ments. Both minerals are set in a minutely crystalline paste of crushed quartz and 

 felspar (190, N. of Saccanjimba). In succeeding stages all the quartz is reduced to 

 mylonite, and the crushed groundmass is thus increased at the expense of the intact 

 crystals of felspar. The latter are much reduced in size, show angular outlines, and 

 are traversed by lines of mylonised material (173, W. of Ochilesa). The ferro- 

 magnesian constituents, such as biotite, have been crushed out of recognisable 

 existence, and appear now as aggregates of chlorite and limonite. A few large 

 crystals of anthophyllite, now in process of alteration to talc, have been formed in 

 the Ochilesa rock. 



The three hornblendic gneisses in the collection differ considerably from those 

 described above. They occur towards the lower end of the Lengwe Gorge (1056), 

 N. of the Saccanjimba Mission (189), and in the river W. of Kambengi (197). In 

 thin section they consist of a coarse foliated aggregate of hornblende and plagioclase 

 felspar, with subordinate quartz, orthoclase, and biotite. The ferro-magnesian 

 minerals bulk more largely in these rocks than in the biotite-gneisses. The horn- 

 blende is a strongly pleochroic variety, of strong absorption, and with colour extremes 

 of bluish-green and yellowish-brown. Small felts of biotite and epidote cluster with 

 the larger hornblende crystals. The felspars mostly belong to andesine, and contain 

 colourless needles similar to those described above in the biotite-gneisses of the 

 Lengwe Gorge. Cataclastic structures are also frequent. These rocks are typical 

 diorite-gneisses. 



Only one basic hornblende rock occurs in the collection. This comes from the 

 river W. of Kambengi (195). It is a fine-grained, dark-green, non-foliated rock, con- 

 sisting, in thin section, of an evenly granular mass of hornblende crystals of the usual 

 bluish-green colour. The small and sparse interspaces are filled with quartz and 

 decomposed felspathic material. The rock is a granulitic amphibolite. 



II. Rocks of the Charnockite Series. 



These are dark-greenish or greyish fine-grained rocks, in which none of the 

 minerals, save an occasional flake of biotite, are macroscopically identifiable. The 

 outstanding microscopical characters of these rocks are their thoroughly granulitic 

 texture, their pyroxenic composition, and their ideal freshness. The mineral con- 

 stituents, occurring in varying proportions throughout the series, are a pale-green 

 monoclinic pyroxene, hypersthene, plagioclase, orthoclase, quartz, hornblende, biotite, 

 and magnetite. This assemblage is identical with that of the charnockite series of 

 Southern India.* The invariable presence of hypersthene, the granulitic texture, 



* Holland, "The Charnockite Series," Mem. Geol. Surv. India, vol. xxviii, part ii, p. 124, 1900. 



