Arthur Holmes — Lateritic Deposits, Mozambique. 535 



by capillarity and evaporation. The solutions are strengthened both by 

 adsorption iu the interstices of the sand and by contact with a previous 

 precipitation. For this reason a nodule once begun tends to continue 

 growing, for at its surface the concentration of the solutions is 

 strongest and precipitation by oxidation is therefore more favoured 

 than elsewhere. In this connexion it is interesting to notice that the 

 sands and gravels of dry streams are not strongly iron-stained, except 

 where a hard floor of laterite has formed, the iron being almost 

 entirely concentrated in the nodules. The smooth, rounded, and 

 frequently spherical forms of the latter are due to the fact that 

 deposition is most favoured where the radius of curvature is greatest. 



Having now demonstrated the existence of waters, bearing in 

 solution the lateritic constituents, and draining ah>ng the foliation 

 planes of the gneiss, and having also shown that these constituents 

 are deposited where evapoi'ation and oxidation can occur, it is possible 

 to turn to the laterite proper and attempt an explanation of its 

 formation. IN^ear the eastern banks of streams where the gradient 

 has cut across the foliation of the gneiss, ground water from the 

 thaliveg is drawn up by capillary attraction and evaporation, and at 

 or near the surface, where the solutions become oxidized, deposition 

 takes place. It is possible that this action is strongest immediately 

 after the wet season, when the laterite itself is soaked, and deposition is 

 most vigorous at the upper surface. In this way the formation gradually 

 grows upwards as a mechanical replacement, incorporating rock debris 

 (generally quartz and sand) which has fallen upon its surface. Later 

 in the dry season, as the level of the ground water sinks, deposition will 

 occur at lower and lower levels, a possibility which is in keeping with 

 the fact that the Mozambique laterite becomes less and less cavernous 

 with depth. On the other hand, the lateritic earths which envelop 

 the base of the mountains are always damp, fed daily by the heavy 

 dews which are precipitated on the steep rocky slopes, and here a thin 

 hard capping of laterite is found only at the surface, the deposition 

 falling off rapidly in depth owing to the prevalence of permanent 

 moisture. The explanation offered above for the formation of laterite 

 on the sides of streams serves equally well for its occurrence on the 

 sides of ridges where the foliation is truncated by the slope of the 

 land, for fissures are more abundant parallel to the foliation than 

 across it and the upward drainage takes place more easily along the 

 foliation than aci'oss it. 



The simple cases so far considered do not, of course, include all 

 the occurrences of laterite in Mozambique. Bands of laterite are 

 occasionally found running across the strike of the gneissic foliation, 

 and in so far as such cases are rare it is possible that the circumstances 

 attending their formation are also somewhat special. Pegmatite 

 dykes and joints may also be found running transversely to the main 

 structural lines, and it is obvious for these and other reasons that the 

 underground drainage is not solelj' conditioned by the foliation. If, 

 however, the more general statement be made that the formation of 

 laterite depends on the drainage and evaporation of the ground 

 waters, I believe most of the Mozambique examples would be 

 included. 



