Dr. L. Leigh Fermor — What is Laterite ? 459 



to which materials should, and which should not, be designated by 

 the term laterite. 



Difference between Laterite and Clay. — In the discussion summarized 

 above three points are taken into consideration. Mr. Crook lays 

 stress on the presence of free aluminium hydroxide, Mr. Scrivenor on 

 the suitability of the rock for building purposes, whilst Dr. Evans 

 lays stress on the small amount of combined silica in proportion to the 

 amount of alumina present. In Dr. Evans' view lies the key to the 

 question, I think. One should not decide whether a given rock is 

 laterite on the basis of the presence or absence of alumina in quantity, 

 but on the presence or absence of any considerable proportion of 

 combined silica. Combined silica means the presence of kaolin or 

 lithomarge, and the larger the amount of such material the closer 

 does the rock approach a clay in composition. jSTow clays 1 are to be 

 regarded as the end products resulting from one mode of superficial 

 decomposition of rocks, and laterites as the end products of another 

 totally distinct mode of decomposition. When a rock breaks down 

 into a clay hydrated aluminium silicate is to be regarded as the pure 

 end product, all oxides being removed in solution. When a rock is 

 converted into laterite, on the other hand, the reverse holds ; 

 aluminium and other silicates are decomposed, and the silica is 

 removed in solution, presumably in the colloidal form, whilst the 

 oxides of iron, aluminium, titanium, and manganese, which were 

 relatively soluble under the clay-forming conditions, are relatively 

 insoluble under laterite-forming conditions. The oxides of calcium, 

 magnesium, sodium, and potassium are apparently soluble under both 

 sets of conditions. I do not propose to advance here any reasons to 

 account for these two diverse modes of surface alteration of rocks, 

 nor to say anything about the conditions, whether climatic or organic, 

 that bring them about, but to deal only with the results of such 

 changes. Pure clay, then, is hydrated aluminium silicate, whilst 

 pure laterite 2 is a mixture of one or more, or all, of the oxides of 

 iron, aluminium, titanium, and manganese, more or less hydrated, 

 which I refer to in this paper as the lateritic constituents. 3 Manganese 

 oxide is a somewhat exceptional constituent, and when present usually 



1 i.e. clays formed in situ, as distinguished from those deposited as aqueous 

 sediments. 



2 F. W. Clarke holds similar views, as may be judged from the following- 

 quotation from his Data of Geochemistry, pp. 417-18 (1908) : "Whatever its 

 derivation may be, wbether from rocks in place or as a transported sediment, 

 true laterite [rny italics] is essentially a mixture of ferric hydroxide, aluminium 

 hydroxide, and free silica in varying proportions. To laterite in situ this 

 statement applies very closely ; detrital laterite is usually contaminated by 

 admixtures of clay. Just as in the formation of kaolin, the process of 

 laterization may be complete or partial, the typical product appears only when 

 the alteration of the parent rock has gone to the end. Then the silicates seem 

 to be completely broken down, whereas in kaolinization a stable, hydrous 

 silicate remains." Mr. J. M. Campbell's definition (loc. cit., p. 437) is also 

 similar ; he mentions manganese, titanium, and phosphorus, in addition to the 

 constituents named by Clarke. 



3 Other related oxides, such as Ci'2 O3, have been found in laterite, and are to 

 be regarded as true lateritic constituents. See Mem. Geol. Surv. India, xxxvii, 

 p. 378, and infra of this paper. 



