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



Limiting Percentages. — The difficulty in attaching names to rocks 

 intermediate between pure laterite and pure clay or lithomarge is 

 to fix on limiting percentages, which must be chosen more or less 

 arbitrarily. Considering the case of a rock that had been analysed, 

 I should .say that not more than about 10 per cent of non-lateritic 

 material (i.e. quartz or lithomarge, in ordinary cases) or 5 per cent 

 of combined silica (10'72 per cent of lithomarge is equivalent to 

 5 per cent of combined silica) should be allowed to pass without 

 prefixing to the term laterite a qualifying adjective, such as quartzose 

 and lithomargic respectively. Material containing more than half its 

 weight of either lithomarge or of quartz could no longer be called 

 laterite, but might he called lateritic lithomarge, sand, soil, or earth, 

 if there were a sufficient quantity of free oxides of iron and aluminium 

 present to justify the use of the qualifying adjective. Fifty per cent 

 of lithomarge of formula 2 Ho . Al 3 3 . 2 Si 3 corresponds to 

 23-25 per cent of combined silica. Considering that, in India, as 

 a rule, impure laterites are more commonly predominantly lithomargic 

 than quartzose, I should not allow any rock, the analysis of which 

 showed over 23 - 25 per cent of Si 3 of undetermined character, to be 

 classed as laterite, assuming, in the absence of evidence to the 

 contrary, that the silica was present in the combined condition. 

 For laieritite or detrital laterite I consider that greater freedom should 

 be allowed, as already explained, and that these terms might be 

 allowed to include detrital lateritic rocks containing as much as 

 60 per cent of non-lateritic constituents. 1 



Analyses by IL. and F. J. Warth. — To exemplify the application of 

 this nomenclature, we may refer to the most complete series of 

 analyses of laterites yet published, namely that made by Dr. H. Warth, 

 formerly of the Geological Survey of India, and his son, Mr. F. J. 

 Warth. 2 This paper gives twenty-three analyses, divided into four 

 groups — 



I. " Pure Gibbsite " (No. 1). 

 II. Bauxites rich in AL 3 and poor in Fe2 :! (Nos. 2 to 5). 



III. " Laterites in situ which are Bauxites " (Nos. 6 to 13). 



IV. "Detrital Laterites " (Nos. 14 to 23). 



Groups I to III are high-level laterites. Sample 1 is practically pure 

 gibbsite. Samples 2 to 5 show 67*88 to 57 - 5() per cent of Al 3 3 , 

 409 to 6-53 per cent of Fe 3 3 , and only 0-93 to 2-35 per cent of Si 3 . 

 They are, therefore, correctly designated bauxites, and can also be 

 termed aluminous laterites. 



Samples 6 to 13 show 54-80 to 26-27 per cent of A1 3 3 , 13-75 to 

 56 01 per cent of Fe 3 3 , and 0-37 to 4-20 per cent of Si 6 3 . They all, 

 therefore, like the samples of Groups I and II, are true laterites, 

 except jS"o. 10, which contains 10 - 52 per cent of quartz, and should 

 therefore be designated quartzose laterite, although it is only just 

 above the limit for true laterite. The authors call the rocks of this 

 group " Laterites . . . which are Bauxites " ; in my opinion, however, 



1 It is interesting to note that as long ago as 1838 Dr. J. Clark divided lateritic 

 rocks into the three following classes : (1) lithomargic, (2) quartzy, (3) detrital. 

 See Madras Journ. Lit. and Sci., viii, pp. 338, 344. 



2 Geol. Mag., 1903, pp. 154-9. 



