536 Arthur Holmes — Lateritic Deposits, Mozamhiqiie. 



It has already been stated that ferruginous laterite was not found 

 on granite. The occurrence of aluminous laterite or bauxite on 

 granite has, however, been observed. Mr. Wayland found a reddish 

 iron-stained bauxite on pyrite bearing pegmatite veins running 

 through the gneiss near the Nrassi River due east of Erikola. Later 

 I found similar material over a small granite intrusion which was 

 plainly visible in a gully draining the southern slopes of Erikola to 

 the Nrassi. Mr. Barton, tbe leader of the expedition of which we 

 were members, found at Nakota. north of the Lijionia River, large 

 pegmatites mineralized with sulphides. Some of these were capped 

 with bauxite, and, equally interesting, the gneiss in the neighbour- 

 hood was for a considerable depth raetasomatically replaced by silica. 

 This association would appear to be genetic, but as I was ill with 

 fever at the time and camping at Akwari, several miles away, I was 

 unable to study the deposits myself and cannot therefore discuss 

 them further. 



It only remains to place on record the occurrence of kaolin as an 

 alteration product of granite. This was found by Mr. Wayland near 

 the Chika Range, and by myself on the side of a deep gully north of the 

 Mwipwi Mountains and west of Arunto. In the latter case the granite 

 is reduced to a china clay rock in all respects similar to that of the 

 Cornish granites. It was well exposed in section, as the Portuguese 

 had made a small quarry in order to pave with tlie soft china clay 

 a wooden bridge which carried the militarj^ road over the gully. 

 Traced to the north and east, the kaolinized granite gradually gives 

 place to fresh unaltered granite as seen where it outcrops. Where 

 the kaolin occurs, however, there was no natural outcrop, for above 

 the rock a thick deposit of black damp soil, rich in organic material, 

 hid it from view. The vegetation on each side of the gully was 

 unusually thick. It is possible that the kaolin may have been 

 derived from felspars by the action of organic acids or of carbonated 

 waters produced by them. Similar cases have previously been 

 described.^ 



There are, then, in Mozambique, precisely similar granites, in some 

 of which the felspars have broken down into kaolin, while in others 

 aluminium hydroxides have been formed, combined silica having been 

 liberated and removed, or redeposited metasomatically. In looking 

 for different conditions to correlate with these two processes, all that 

 can be said is that the formation of kaolin seems to be conditioned by 

 the presence of a heavy growth of vegetation, and that of bauxite by 

 the absence of heavy vegetation and humus, and the presence in the 

 parent rock of sulphides. What chemical significance these contrasts 

 may have, I do not feel at present justified in discussing. Laboratory 

 experiments of a kind not yet attempted are evidently necessary 

 before the central problem of lateritization can be satisfactorily 

 solved. It is possible, however, that the secret processes now in 

 operation for the manufacture of aluminium from felspar might throw 

 an important light on the origin of laterite. In the absence of 



^ Stremme, Zeit. Prakt. Geol., 1908, pp. 122, 443; AViist, Zeit. Prakt. 

 GeoL, 1907, p. 19. 



