548 MK G. W. TYRRELL. 



green hornblende. Cassiterite occurs ratEer abundantly in spongy anliedral crystals 

 which enclose quartz and felspar. It is yellowish-brown in colour, distinctly pleochroic, 

 and shows a fine lamellar twinning. It is commonly associated with small grains of 

 magnetite. No other mineral of the tin association is found, except, perhaps, one 

 faint purplish section doubtfully referred to axinite. 



The only specimen which may be regarded as the hypabyssal equivalent of the 

 granites is a coarsely porphyritic rock from E. of Lepi (224), which shows numerous 

 large white felspars in a dense greyish groundmass. In thin section the felspar 

 phenocrysts are seen to form at least half the rock. They are much altered, with 

 the formation of white mica and epidote. Hence they belong to an acid soda-lime 

 felspar (probably oligoclase), a conclusion strengthened by the occasional retention of 

 multiple twinning. These phenocrysts frequently have a ragged external zone of 

 orthoclase which interpenetrates with the groundmass. The only other phenocrysts 

 are large, partially chloritised crystals of hornblende. This mineral also appears in 

 the dense groundmass, which, in the main, is a fine micrographic intergrowth of quartz 

 and alkali felspar. Magnetite and large apatite crystals form rather abundant acces- 

 sories. This rock is a granodiorite-porphyry, and is clearly to be associated with the 

 Benguella granodiorites. It differs entirely in appearance from the hornfelsed and 

 granulitic porphyries described in a previous section. 



V. Rhyolite (Dellenite). 



An extensive series of lava-flows belonging to this group occurs in the neigh- 

 bourhood of Lepi and Quingeng*. In hand specimens they are dense flinty rocks, 

 generally light-grey in colour, occasionally dark-grey to black, with numerous, small, 

 inconspicuous phenocrysts of quartz and felspar. 



In thin section they show numerous small phenocrysts of quartz, orthoclase, 

 oligoclase, and biotite, in a dense, cryptocrystalline groundmass. The quartz ranges 

 from 1 to 3 mms. in diameter, and is subhedral, rounded, and embayed by corrosion. 

 The felspars are euhedral, averaging 2 mms. in diameter, and mostly belong to ortho- 

 clase. In some of the rocks, however, oligoclase becomes a prominent constituent, 

 and nearly equals the orthoclase in quantity. Its composition ranges from oligoclase 

 (AbaAni) to oligoclase-andesine (Ah 5 An 3 ). The biotite is bleached and decomposed, 

 and is often represented by chloritic decomposition-products and epidote (257, 260, 

 261, Quingenge). The groundmass is dense, uniform, cryptocrystalline to glassy, 

 and is besprinkled with minute unidentifiable microlites. A fiow-orientation is indi- 

 cated by faint differences of colour and texture in different parts of the groundmass. 

 The proportion of phenocrysts to groundmass is estimated at one to two, and quartz 

 is about equal to the felspars amongst the phenocrysts. In some rocks (245, Quin- 

 genge) the groundmass has apparently undergone silicification, which has also 

 involved most of the felspars, leaving "ghosts" of the latter and much diffused 

 sericitic matter. 



