204 R. R. Walls — The Geology of Portuguese Nyasaland. 



coast extend much further inland, forming what has been called the- 

 Ma via or Makonde Plateaa. The exact area of the Tertiary rocks 

 on this Makonde Plateau has never been mapped out, but the 

 deposits are known to be of no great depth and to rest uncon- 

 formably on metamorphic rocks of much greater age. The two north- 

 south ridges first mentioned are conspicuous features to anyone 

 crossing the country in the south from the Indian Ocean, and the 

 folding — except where it has been complicated by intrusive granite — • 

 into two anticlines with a syncline between is very distinct. 



To the west of these two north-south ridges gneiss and associated 

 foliated rocks cover almost the whole country. Careful plotting of 

 these metamorphic rocks is almost impossible for lack of accurate 

 maps on a sufficiently large scale, but some general observations 

 may be made. The most important feature to be noticed is that the 

 country rock noAV shows folding in a direction at right angles to 

 that already mentioned. Over the centre and south of the country 

 the ridges and lines of foliation run in an east-west direction, 

 and the compression thrust which produced these ridges has been 

 just as intense as that which produced the north-south ridges over 

 the rest of the country. The folding of the rocks in this country will 

 be treated more fully later on. The gneiss is predominant every- 

 ■^here, but varies greatly in its texture. Sometimes it is extremely 

 coarse with conspicuous felspar crystals. More usually it is crushed 

 considerably until in hand specimens it scarcely appears like gneiss 

 at all, and it is only in a large scale on the field that the real nature 

 of the rock can be ascertained, but all gradations between the two 

 types can be seen in traversing a few miles of country. Often 

 felspars are conspicuous by their absence, and the rocks appear as 

 banded quartzites with considerable quantities of mica. Yet, taking 

 the whole outcrop together, it would not be wrong to state that the 

 rock was gneiss. Iron ores occur very frequently in bands in the 

 gneisses, and is the only mineral of economic importance found 

 in the country. 



Bands of schist are not uncommon interfoliated with the gneisses, 

 but they are, generally speaking, remarkable for their thinness and 

 variety of form. Garnet schist and mica schist are most often met, 

 but there are also considerable seams of graphite schist. Attempts 

 have been made in the past to work this graphite, especially at 

 Mualia, but owing to the fact that it is so interlaminated with gneiss^ 

 and grit it has proved of no commercial value. Associated with 

 these graphite schists are peculiar black bands often found in the 

 gneiss. These bands are likewise very thin— 1 or 2 inches at 

 most — and usually dull, though sometimes showing an adamantine 

 lustre. The dull variety is really black flint, but the lustre shown 

 on other varieties sometimes suggests anthracite, and a bigger 

 proportion of carbon. It cannot be made to burn by itself, however, 

 and if burned in the blowpipe flame it leaves a huge quantity of ash- 

 There is evidently some carbon about it, but quartz predominates.. 



