414 A. Holmes 8f D. A. Wraij— 



approaehing tlie mouth of tlie Lurio or Luli River, chlorite- and 

 mica-schists are recorded, with thin bands of hseraatite- and graphite- 

 schists. They appear to be isolated remnants of a formation now 

 mostly removed by denudation. Dr. Voit has remarked that a 

 schistose series is preserved more typically in the south of the 

 continent, while towards the Equator gneisses are found to be more 

 important. In each case the metamorphic rocks are accompanied 

 bv intrusive granites. Our investigations fully bear out Dr. Voit's 

 observations.^ 



The dominant rock of the country is a grey biotite gneiss, often 

 containing liornblende, which varies from coarsely banded varieties 

 to finely foliated types. Tlie former contain granular bands of quartz 

 and felspar, and it is often impossible to distinguisli them from 

 pegmatite intrusions. With the development of patches of felspar 

 in place of bands the rock passes into an augen-gneiss, and this in 

 turn may sometimes be traced into a slightly porphyritic granite, 

 with little sign of any directive structure. The fine-grained type on 

 the other hand merges into mica-schist, though it is but rarely 

 that the latter is found. Associated with these rocks are lenticular 

 masses of hornblende-gneiss and, to a less extent, of garnet and 

 epidote-bearing rocks. Quartzites and crystalline limestones are 

 nowhere common in this territory, but several bands intercalated 

 with the gneiss have been noted, sometimes of considerable thickness. 

 The gneisses are the oldest rocks of the counti-y, and into them 

 granite intrusions have penetrated at two distinct periods, and 

 perhaps also at others. The granite rises in great domes, often with 

 steep sides, and from them the overlying gneiss dips away in every 

 direction. By exfoliation thin flakes of gneiss are split off parallel to 

 the foliation planes, and in this way the hills tend to retain the out- 

 line of the granite core, a favourable condition for the production of 

 the 'inselberge' landscape. The granites are notably poor in ferro- 

 magnesian minerals, and present a contrast to the gneisses with their 

 much higher proportion of biotite and hornblende. Otherwise the 

 mineralogical constituents are very similar in the two cases. Felspar 

 is the most abundant mineral, raicrocline ^ being constantly present, 

 as is characteristic of all the old granites of Central and South Africa. 

 Associated with the granites are enormous numbers of pegmatite 

 dykes, sometimes as much as 10 feet across, but more usualh^ not as 

 many inches. Iron-ores are very commonly present, htematite and 

 magnetite being everywhere very plentiful. Easic dj-kes also enter 

 into the architecture of the complex, but they are sparingly distributed, 

 and of their genetic relations and age notliing is yet known. 



The coastal strip of Cretaceous rocks has an average width of 

 10 to 12 miles, but the strata do not form a continuous outci'op, and 

 at Memba Bav the coarse Archaean gneisses reach the sea in rusged 

 sea-cliifs fringed with recent coral-reefs. The Cretaceous beds have 

 a verv gentle dip towards the south-east, and seem to have been 

 subjected to little movement since their deposition. Three main 



1 F. W. Voit, Trans. Geol. Soc. South Africa, vol. x, p. 92, 1907. 



- C. B. Horwood & A. Wade, Geol. Mag., Dec. V, Vol. VI, p. 459, 1909. 



