530 Arthur Hohnes — Lateritic Deposits, Mozambique. 



minerals and rocks, less commonly found, sometimes makes up a con- 

 siderable proportion of the formation. Many of the laterites are 

 richly ferruginous, and comparatively poor in aluminium hydroxides.^ 

 On the other hand, examples of nearly pure bauxite are known, and 

 between the richly ferruginous and richly aluminous varieties 

 intermediate types occur. Oxides of manganese and titanium are also 

 present, sometimes considerable quantities of the former constituent. 

 Only qualitative analyses of specimens examined in the field and of 

 those brought home have so far been made, so that it is not possible 

 to give more than the above general indication of their composition. 



The rocks over which laterite occurs in Mozambique are of various 

 kinds. Mr. Wray records its presence in numerous instances on 

 hornblende gneiss (e.g. north of Namkoko) and on biotite gneiss 

 (e.g. between Mkonta and Ntia). Mr. Way land found it on different 

 varieties of gneiss and on basalt. The sedimentary belt (Cretaceous 

 and Tertiary) of the coast is free from laterite, which, however, is of 

 frequent occurrence on the basalts of the Tertiary volcanic belt. 

 Although detrital quartz is here still the commonest inclusion, 

 occasionally fragments of agate or of basalt may be found. Indeed, the 

 former distribution of the basaltic flows may to some extent be inferred 

 from the presence of fragments of basalt in laterite where no basalt 

 now exists in situ. On the metamorphic plateau beyond the coastal 

 belt, it is worthy of notice that no ferruginous laterite was ever found 

 on granite. The Mozambique granites are uniformly poor in ferro- 

 magnesian minerals, whereas the gneisses, which are very abundant 

 and are the most usual hosts of the laterite, are generally rich in dark 

 bands of hornblende and biotite. On the Mluli River, just north of 

 the Mwipwi Mountains, laterite occurs on a gabbro rich in labradorite 

 and impregnated with pyrite. It occurs on a similar rock on the 

 eastern side of the Lulaua River near its confluence with the Ligonia. 

 Near the Erikola Peaks I found laterite on a hypersthene-plagioclase 

 dyke rock which also contained pyrite. The ubiquitous gneiss of 

 Mozambique is penetrated by numerous pegmatite veins which are 

 frequently rich in magnetite and haematite, especially in the western 

 parts of the territory. They are generally parallel to the strike of 

 the banding and foliation of the gneiss, although they may also be 

 transverse, sometimes forming a complicated network. Although the 

 average width of the veins amounts to only a few inches, crystals of 

 iron-ore as large as a walnut are not uncommon. South-west of the 

 Ribawe Mountains the superficial rock debris is rich in haematite, 

 a mineral greatly prized by the natives, who collect it for the manu- 

 facture of spear-heads and knives. The importance of such iron- 

 bearing rocks in the genesis of ferruginous laterite is obvious. 



In some cases the laterite was found to lie on unaltered fresh rock, 

 a conclusion which microscopic investigation has since supported. 

 However, the underlying rocks were always moist and frequently 

 they were soft and disintegrated, limonite having been deposited 

 between the mineral grains. Pits sunk in the lateritic earths showed 

 that these lay on a floor of soft moist gneiss, with large and small 



^ By many geologists some of these deposits would be referred to as lateritic 

 iron- ores. 



