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



segregates into masses of comparatively rich manganese-ore, as a rule 

 either psilomelane or pyrolusite. 1 



Typical Laterite. — We may accept, then, that true laterite (or laterite 

 proper) is a hydrated mixture of oxides of aluminium, iron, and 

 titanium, of extremely variable composition, and showing every 

 gradation from ferruginous laterite almost free from alumina to 

 aluminous laterite almost free from iron. Such laterites are often 

 sufficiently pure to he termed respectively iron-ore and aluminium- 

 ore, which seem, at least in some cases, to be the result of segregative 

 tendencies acting on a more homogeneous form of laterite, doubtless 

 under the influence of percolating water. When pure enough to be 

 used as aluminium-ore the laterite is usually known as bauxite. 2 



As examples of typical laterites, reference should be made to the 

 analyses given in two papers by Sir Thomas Holland 3 and Dr. W. 

 Dunstan 4 respectively, although it should be noted that the samples 

 represent the paler and more aluminous varieties, as they were taken 

 with a view of testing the Indian laterites as a source of aluminium : 

 in order to give a thoroughly sound idea of the composition of Indian 

 laterites, it would be necessary to select for analysis another series of 

 samples illustrating the ferruginous varieties. The consequence is 

 that the analyses referred to all show high percentages of alumina, 

 ranging from 31-37 per cent to 6788 per cent, and it may be the 

 consideration of this series of analyses that has led the mineralogists 

 at the Imperial Institute to lay such stress on the essential presence 

 of alumina in laterite. The amount of ferric oxide present in these 

 samples ranges from 0-44 per cent to 40-18 per cent, but samples 

 could easily be selected running much higher in this constituent. 5 

 The titania ranges from 004 per cent to 13 - 76 per cent, the combined 

 water from 16*8 1 per cent to 33-74 per cent, and the silica from 

 - 05 per cent to 19-32 per cent. Apart from three samples showing 

 10*75 per cent, 14-58 per cent, and 19-32 per cent Si0 2 , however, the 

 amount of silica is never more than 4'68 per cent, and in all but five 

 cases does not even reach 3 per cent. 



Quartzose Laterite. — Accepting the definition of pure laterite given 

 earlier, it is evident that if a lateritic rock be markedly siliceous, 

 it will need special scrutiny and consideration before it can be 

 admitted as a laterite. The silica may be present in two principal 

 forms, namely, as quartz, either residual from the original rock, or 

 secondary, and as litlwmarge or kaolinite. If present as quartz, then 



1 The Indian manganiferous laterites have been discussed elsewhere (Mem. 

 Geol. Surv. Ind., xxxvii, pp. 380-9), and they need not enter further into the 

 discussion here, although they are probably of greater importance and wider 

 distribution than is generally recognized. 



2 I use this word in a petrographical sense, concerning which see Section IV 

 of this paper, infra. 



;i "The Occurrence of Bauxite in India": Eec. Geol. Surv. Ind., xxxii, 

 pp. 178-80, 1905. 



4 " Eeport on Laterites from the Central Provinces" : ibid., xxxvii, pp. 215, 

 216, 1909. 



5 The third group of analyses by H. and F. J. Warth shows, however, three 

 true laterites containing 47 per cent to 56 per cent of Fe 2 O s , with 33 per cent 

 to 26 per cent of Ah Oa. See infra. 



