512 Dr. L. Leigh Fermor — What is Laterite? 



Each of these three analyses shows more Kthoruarge than lateritic 

 constituents, and consequently, according to what precedes, the Ponri 

 rocks are most correctly described as lateritic lithomarges, whilst the 

 Afendil specimen is a lithomarge, without any qualifying adjective. 



Other examples. — for further examples amongst Indian laterites 

 reference may again he made to the analyses referred to on p. 460. 

 All the rocks represented by analyses in Dunstan's paper are correctly 

 described as laterites, and can also be termed aluminous laterites or 

 bauxites. The term laterite can also be applied without qualification 

 to all the rocks represented by analyses in Holland's paper, with the 

 exception of samples D, E, and G, containing respectively 1458, 

 19'32, and 10 - 75 per cent of Si 3 . These silica percentages, assuming 

 the whole of the silica to be combined silica, all lie above the 

 5 per cent limit given above, but below the 23 per cent limit, and 

 the rocks should therefore be described as lithomargic laterites, the 

 silica percentages corresponding to 31 -26, 41-43, and 23-05 per cent 

 of lithomarge; sample P>, containing 4'68 per cent of Si 2 , 

 corresponding to 10-03 per cent of lithomarge, is near the border- 

 line at which I would prefix the adjective 'lithomargic'. These 

 lithomargic laterites are all from the JVIadras Presidency, three of 

 them coming from the Palni Hills and one from the Vizagapatam 

 Hill Tracts. 



Pef'erence may also be made to three other analyses, namely, those 

 of three aluminous laterites or bauxites from the Yeruli plateau, 

 Satara district. 1 One of these contains 5 - 20 per cent of silica, and, 

 therefore, assuming it is combined silica, as is probably the case, 

 just comes within the limits of lithomargic laterites. Apart from 

 this constituent, however, the rock is a fairly high-grade bauxite. 

 The other two samples are true laterites, containing only 2-63 

 and 1*10 per cent of Si 2 respectively. Noteworthy is the presence 

 of 0'63 and - 13 per cent of Cr 2 3 in these two latter rocks; 

 chromium sesquioxide must, of course, be regarded as a true lateritic 

 constituent. 



Lateritic earths. — For many mixtures of clay and sand with iron 

 and aluminium oxides it is quite unnecessary to use the term laterite 

 at all, more suitable terms being ferruginous clay, loam, or sand ; 

 where the free oxides are high in amount, the terms lateritic clay, 

 lateritic soil, lateritic earth, etc., are available. 



Buchanan's original definition. — It will be noticed that I have so 

 far made no reference to Buchanan's original definition. This has 



laterites do not make their presence evident in any form in thin sections of the 

 corresponding rocks. Consequently I am inclined to think that the titania is 

 present as a collodial constituent, either as a hydrated oxide of titanium, or even 

 possibly as a hydrated aluminium titanate, of composition parallel to that of 

 kaolin and lithomarge, although this latter alternative is put forward as a mere 

 suggestion, for which there is at present no evidence. Whether the titania be 

 present as a hydroxide or as a collodial titanate does not matter for our 

 present purpose, as the above analyses are susceptible of arrangement to agree 

 with either alternative. I find that M. Arsandaux (note, infra) supposes the 

 titania of French bauxites to be present as metatitanic acid, Ti 2 , H2O. 

 1 Mem. Geol. Surv. India, xxxvii, p. 378. 



