534 Arthur Holmes — Lateritic Be-posits, Mozambique. 



a hill near Nakavala, known as Mhala, a care has been formed by the 

 breaking away of a large mass of gneiss, clearly due to slipping 

 along a moistened joint plane parallel to the foliation, which here dips 

 steeply towards the west. Here also, three or four hundred feet 

 above the plateau, a slow trickle of water percolated through the hill, 

 but only a very thin coating of limonite had been deposited on the 

 roof, and practically none on the steeply sloping floor. In this case 

 the gneiss was free from impregnations of ii'on-ore. Near the 

 Portuguese fort at Chinga, a spring of iron-bearing water strongly 

 charged with carbon dioxide rises through the gneiss and forms the 

 source of a stream which flows during the dry season to the dried-up 

 channel of the l^amieta. On each side of the stream and around 

 Chinga itself are considerable spreads of laterite and lateritic earths. 



It is important to notice that the steep slopes of the inselberg peaks 

 and mountain blocks are always free from deposits of the lateritic 

 constituents. Solutions must certainly exist which on evaporation 

 would give rise to such deposits, but the mountain rains and night 

 dews suffice to carry them down to the debris-covered plateau surface 

 below, where they contribute to the formation of the lateritic earths 

 which so frequently sweep round the base of the hills. 



The close association of bands of laterite with watercourses has 

 already been mentioned. It is therefore significant that the lower 

 parts of large river beds (e.g. the Monapo), which in the dry season 

 shrink to a succession of stagnant pools, often contain a considerable 

 number of tiny ferruginous nodules distributed among the gneissic 

 and granitic debris of the river floor. In small tributary streams 

 which dry up completely, nodules which reach the size of a small 

 bean are frequently very plentiful, and in some cases the beds are 

 locally paved with sandy laterite, formed by the cementation of large 

 numbers of the nodules with gravel and sand. One of the best 

 examples occurs in a gully which drains into the Nrassi River from the 

 high peak of Ericola. Here many of the nodules were found to have 

 a nucleus of sand. This district was carefully studied by Mr. Wayland, 

 and independently by myself, and we found that the gully did not 

 pass through any lateritic deposits, so that the nodules were not detrital 

 but were clearly formed ah initio in the dried-up bed. Mr. Wray 

 found a lateritic conglomerate on the floor of a dry stream near 

 Ribawe, between the Sawa and Ntumba Valleys, where it rested 

 directly on a coarse gneiss. In the dry streams of the Nrassi district 

 every gradation can be found between tiny nodules of iron-ore and 

 massive nodular conglomerate which often is developed on the up- 

 stream side of large boulders and ridges, owing to the accumulation 

 of large numbers of nodules and their subsequent cementation. In 

 the beds of streams which do not dry up, such as the Sawa, the 

 Bwibwi, and other streams in the mountainous areas, no nodules 

 could be found amongst the gneissic gravel. This clearly proves that 

 the nodules are formed only where there is an alternation of saturation 

 and solution, with evaporation and precipitation, corresponding to the 

 'wet' and 'dry' seasons respectively. The ground water below 

 the thalweg dissolves the lateritic constituents from the underlying 

 rocks, and as the stream dries up some of this water rises to the surface 



