5G4 Dr. L. Leigh Fermor — What is Laterite ? 



apparent, judging from the work of Harrison and Du Bois, that the 

 term has been too widely used in the Guianas. 1 



Hardening properties of laterite — Professor Harrison's paper bristles 

 with other controversial points, but this communication has already 

 grown to an inordinate length, and consequently I will refer to one 

 other point only. It is a mistake to think that all late-rites will 

 behave in the way that the laterite first described by Buchanan 

 behaved, namely, that they will be found to be soft when first 

 quarried and will harden on exposure to the air, except to the 

 small extent that most rocks harden owing to evaporation of con- 

 tained moisture. In my experience certain varieties of laterite 

 (e.g. high-level laterites of Balaghat) that both in chemical composition 

 and in physical appearance are true laterites, and to which Buchanan 

 would undoubtedly have applied the term, are not found to be 

 markedly softer when quarried than after exposure to the air. 

 Further, Harrison on p. 493 of. his paper describes a clay-like mass 

 that hardens on exposure, and which analysis shows to contain 

 50-3 per cent of felspar and 22- 1 per cent of kaolinite, with only 

 24 - 5 per cent of lateritic constituents. 2 



Consequently, to appeal to this setting property of a rock, in 



1 The same remark probably applies to many another tropical area in which 

 lateritic rocks occur. Thus, I have quoted with approval the work of 

 M. Arsandaux on the French bauxites (see note, p. 561). But I do not agree with 

 his two later notes (pp. 682-5 and 1082-4 respectively, of C.E., vol. cxlix, 1909), 

 in which he gives and discusses analyses of ' laterites ' from the French Congo 

 and the Soudan. As before, he extracts the Fe 2 3 , and in this case the free 

 Ah 3 as well, with concentrated H CI, and decomposes the insoluble residue 

 by means of concentrated H2 S O4. To the former fraction (my lateritic 

 constituents) he pays no attention, but shows that the portion insoluble in H CI, 

 which amounts to 68-99 per cent of the whole rock, varies in composition from 

 micaceous alumino-potassic silicates allied to muscovite to kaolin practically 

 free from alkalies. Most of these rocks have been formed in situ by the 

 weathering of crystalline rocks (granites, schists, etc.), and it is evident from 

 the author's analyses and descriptions that his so-called laterites are really 

 clays containing a certain, usually small, proportion of lateritic material. The 

 very uppermost crust may be laterite, properly so called, but no analyses of this 

 are given. 



Reference may also be made to the analyses of ' laterites ' (derived from basic 

 rocks — diabase and ophite), from French Guinea, by J. Chateaud and 

 P. Lemoine (C.R., vol. cxlvi, pp. 239-42, 1908). The analyses show, in most 

 cases, a considerable amount of silica, but it is apparently (p. 241) almost 

 entirely in the free condition. On this assumption, No. 11, with only 

 5-52 per cent of Si0 2 , is a true laterite; Nos. 136, 179, 180, 181, and 185, 

 containing 12 to 28 per cent of Si02, are quartzose laterites, whilst No. 195 

 (derived from phyllite) with 62'30 per cent of SiCh cannot be regarded as 

 a laterite at all. That the authors do not regard these rocks as completely 

 formed laterites (my true laterites) is indicated by the following passage : 

 ' ' La silice est presque completement rendue libre ; les f environ ont 6te" 

 entraines ; le reste, qui aurait probablement disparu si la lateritisation avait 

 6te complete, ne joue qu'un role insignificant et est, en majeure partie, a l'etat 

 de silice libre." A diagram illustrates the chemical losses involved in the 

 formation of laterite No. 11 from diabase No. 2, on the assumption of constant 

 TiO-2. 



2 W. T. Blanford, in his account of the Laterite of Orissa (Mem. Geol. 

 Surv. India, i, p. 283, 1859), referring to the lithomarge underlying the laterite, 

 says that on exposure it becomes hard like laterite. 



