124 L. Leigh Fermor — Laterites of French Guinea. 



It should be noted here, although it is not so stated by Lacroix, that 

 this conglomerate is also covered by my definition of lateritite. 



Lateritized Alluvium (Laterites (P alluvions). 



This type of laterite is perhaps the most widely developed in 

 Guinea, existing as a continuous ferruginous crust to ordinary non- 

 lateritic alluvium in the coastal regions of Guinea between the sea 

 and the foot-hills of Futa-Jallon and in the plain lying between the 

 eastern limits of these mountains and the Niger. In this type of 

 laterite the iron is not derived from the material undergoing 

 lateritization, but is derived from the rock underlying the alluvium, 

 as I gathered from a personal discussion with Professor Lacroix. 

 This iron rising in solution from the underlying rocks becomes 

 oxidized and precipitated so as to be superposed on the clastic 

 elements of the alluvium, which are themselves more or less- 

 unaltered. The iron is therefore of extraneous origin. For this 

 form of laterite Lacroix proposes to use my term lateritoid as 

 follows (p. 323) : — 



" Peut-etre serait-il bon de distinguer par un norn special ses laterites 

 d'alluvions de celles formees aux depens de roches en place, et, dans ce cas, on 

 pourrait les appeler latdrito'ides, en dormant de l'extension au terme propose 

 par M. Fermor pour designer des roches lateritiques de PInde, resultant de 

 l'envanissement de gres, de schistes, par des oxydes de fer et de manganese et 

 constituant souvent des minerals de ces metaux ! ' ' 



My term lateritoid was proposed for the case of lateritic rocks 

 formed by the metasomatic replacement of non-lateritic rocks by 

 lateritic constituents presumed to be of extraneous origin ; and in all 

 the cases described by me the rock undergoing replacement was an 

 old, properly consolidated rock. Professor Lacroix' ' lateritoid ' was 

 formed at the expense of a recent alluvium, by the introduction of 

 extraneous iron oxide, and should be regarded as a lateritoid only 

 if the iron oxide has introduced itself by metasomatic replacement 

 into the alluvium and has not merely superposed itself on the latter 

 by filling up interstices. 



Professor Lacroix assures me that the introduction of iron has been 

 effected by a replacement of the cement between the grains of quartz 

 in the alluvium, the alumina in the laterite d 'alluvions being derived 

 from the decomposition of the aluminous silicates in the cement. 

 In India the quartz of a quartzite may suffer complete replacement 

 by iron or manganese oxides with formation of lateritoid, but in the 

 Guinea laterite d'alluvions the quartz itself does not appear to have 

 suffered replacement, and it is evident that the two cases are not 

 strictly comparable as regards mode of formation. In Table II 

 of this article are shown two analyses (Nos. 14 and 15) of laterites 

 d'alluvions (lateritized alluvium) from Guinea. Professor Lacroix 

 excludes 24-51 per cent of quartz from No. 14 and 67'39 per cent 

 of the same constituent from No. 15, so that ISo. 14 appears to 

 contain 63 per cent of lateritic constituents and 35 per cent of 

 clay, and No. 15 to contain 79 per cent of lateritic constituents and 

 21 per cent of clay. This gives a misleading impression, and in 

 calculating the mineralogical compositions (shown in the same table) 



