part 1] THE PRE-CAMBKIAX HOCKS OF MOZAMBIQUE. 33 



form of a gently undulating plateau that gradually rises towards 

 the west. As one proceeds inland its surface becomes increas- 

 ingly diversified by inselberge and clusters of abrupt hills until, 

 west of Ribawe, where the plateau reaches an elevation of nearly 

 2000 feet, the scenery becomes more typically of a highland 

 character and the stretches of unbroken plateau less extensive. 

 Throughout the whole area traversed by the staff of the Memba 

 Minerals Ltd. (after the Cretaceous and Tertiary formations had 

 been left behind) no rocks were found that were not of igneous 

 or metamorphic origin, or that, like laterite, 1 could not be traced 

 immediately back to an igneous or metamorphic parent-rock 

 or to solutions percolating through such rocks. (See index-map, 

 fig. 1, p. 32.) The dominant rock of the country, persistent to 

 a degree that often becomes monotonous, is a grey biotite-gneiss. 

 Interfoliated with, the gneiss are occasional lenticular masses of 

 hornblende -gneiss and amphibolite, and with these, smaller bands 

 and lenticlesof crystalline limestone are sometimes intimately asso- 

 ciated. Elsewhere the gneisses become coarse and garnetiferous, 

 and in some places eclogites and basic granulites occur. 



Schists were found very sparingly, and the known exposures are 

 limited to three areas : the north-east of the territory behind the 

 broadened coastal plain in the neighbourhood of the mouth of the 

 Lurio, the district around Mernba, and the central coastal region 

 between the Mitikiti River and Sokoto Hill. In the latter district, 

 the schists a^e probably older than the gneisses that enclose them : 

 for the gneisses are mainly, though perhaps not wholly, foliated 

 granites, and therefore of igneous origin. This important but 

 difficult question will be discussed in detail in §V, pp. 63-65. 

 Into the gneisses, later granites of at least two different periods 

 have penetrated, riddling them with enormous numbers of small 

 intrusions, lit-par-lit injections, tongues, apophyses, and dykes of 

 pegmatite and aplite. 



As a general rule, the foliation or banding of the schists and 

 gneisses is well defined in parallel uncontorted planes, which thus 

 make possible observations of dip and strike. The strike is most 

 commonly along, or a little north of, a north-east to south-west 

 direction ; though, in the Ribawe district and near the coastal 

 belt, it swings round to a nearly north-and-south direction. Near 

 the coast the predominant direction of dip is away from the sea. 

 Farther west this statement ceases to hold; but, except locally 

 where the rocks are contorted by later intrusions or faulted, or in 

 the neighbourhood of certain inselberge, the dips show no tendency 

 to rapid variation, the same direction being maintained over large 

 areas, and the angles, usually low, changing but slowly. Some- 

 times, however, the structures revealed on the wind-swept slopes of 

 the inselberge indicate a striking divergence from the comparative 

 uniformity displayed across the plateau. In certain cases, it is 



1 A. Holmes, ' The Lateritic Deposits of Mozambique ' Geol. Mag-, dec. 6, 

 vol. i (1914) p. 529. 



Q.J.G.S. No. 293. d 



