Dr. L. Leigh Fermor — What is Laterite ? 



511 



Unfortunately the siliceous nature of these three rocks was not 

 suspected at the time the samples were despatched for analysis, and 

 consequently no instructions were issued to the analysts to determine 

 separately the combined and free silica. There can be no doubt, 

 however, judging from the microscopical examination, that practically 

 all the silica is in the combined condition, for no evidence was seen of 

 any free silica. These analyses have consequently been re-arranged in 

 terms of their mineralogical composition on the following assumptions, 

 which are fully justified in the light of the macroscopical and 

 microscopical examination of the specimens. The whole of the silica 

 has been calculated as lithomarge, 1 of the formula 2 H~ 2 . Al 2 3 . 2 Si 2 , 

 and any alumina left over has been combined with the amount of the 

 remaining water requisite for the formation of gibbsite. With the aid 

 of any remaining water the ferric oxide is divided into limonite and 

 haematite. The Ti 2 is returned as such, because little is at present 

 known as to the form in which this constituent exists in laterite. 2 

 The item entered up by the analysts as ' manganese oxide ' is assumed 

 to have been determined as Mn 3 4 , and as it was not visible as 

 a separate constituent in the samples, and is present in but very small 

 quantities, it is reported as such in the mineralogical re-arrangement. 

 The remaining constituents shown in the analyses are neglected, being 

 trivial in quantity, and necessary neither for laterite nor lithomarge. 

 The re-arranged analvses are as follows : — 



1 As seen under the microscope in thin sections, the aluminous silicate is 

 isotropic wherever it shows any translucency or transparency, and it cannot 

 therefore be designated kaolinite, which is a crystalline mineral. Substances 

 corresponding to Dana's description of lithomarge, which he gives as a variety 

 of kaolin, are invariably, in my experience, amorphous and non-crystalline, and 

 consequently I think that lithomarge is to be regarded as the colloidal variety of 

 kaolinite. 



2 Some authors assume its presence as ilmenite, e.g. Professor Harrison in 

 his paper on the lateritic earths of British Guiana ; but it cannot be present 

 in this form in many of the Indian laterites. Eeference to the analyses of 

 aluminous laterites given in Eec. Geol. Surv. India, xxxii, p. 179, and xxxvii, 

 p. 215, shows that in the majority of these laterites there is insufficient iron 

 present for all the titanium to be in the form of ilmenite. Further, I have seen 

 no evidence under the microscope of the existence of ilmenite in laterites, and, 

 in fact, as far as I have seen, the high amounts of titanic oxide in some 



