126 L. Leigh Fermor — Laterites of French Guinea. 



Should examples be found in which the replacement has progressed 

 further with production of rocks carrying over 50 per cent of 

 lateritic constituents when quartz is taken into account, then the 

 term later itoid might be applied, especially if the replacement has 

 affected the quartz itself. At present all that can be admitted on 

 the evidence is that these laterites d" alluvions from Guinea tend 

 towards conversion into a particular variety of lateritoid. They are 

 perhaps best designated lateritized alluvium. The consideration of this 

 case shows the undesirability of disregarding quartz in classifying 

 laterites as proposed by Professor Lacroix. 



V. Conditions of Formation or Lateeite. 



On approaching the problem of the origin of laterite, the first 

 thought that strikes one is that there is an apparently fundamental 

 difference between the modes of decomposition of aluminous silicates 

 in temperate and in tropical regions. For in temperate regions the 

 end product is a clay, in which the original aluminous silicates 

 are represented by hydrated aluminium silicates approximating in 

 composition to kaolinite, whilst in the tropics the end product is 

 laterite, in which the original aluminous silicate is represented by 

 a mixture of hydrated aluminium oxides. The two processes may be 

 expressed by the following chemical equations: — 



(1) K 2 O.Al 2 Os.6Si0 2 +C0 2 +2H 2 = Al 2 3 .2Si0 2 .2H 2 0+K 2 C0 3 + 4Si0 2 . 



Orthoclase. Kaolinite. 



(2) Al 2 3 .2Si0 2 .2H 2 + H 2 = A1 2 3 . (2 ±1)H 2 + 2Si0 2 . 



Kaolinite. Bauxite. 



Reaction No. 1 belongs to the group of changes characteristic of 

 Van Hise's zone of katamorphism, and is doubtless of exothermic 

 character, so that it should take place in both temperate and tropical 

 climates, as is the case, lleaction No. 2 is a further stage in the 

 process of decomposition, namely, a hydrolysis of the hydrated 

 aluminium silicate with formation of the hydrated aluminium oxides 

 characteristic of laterite. The thermal character of this second 

 reaction is of great importance. If it be exothermic, then it is 

 possible to regard it as representing a further stage in the changes 

 expressed in equation No. 1, to be promoted possibly by the same 

 agents ; whilst if it be endothermic it will require for its promotion 

 an external supply of heat. There are apparently no experimental 

 data beating on this question, but from inspection of the equation 

 there seems to be no obvious reason why its thermal character should 

 be different from that of equation No. 1, as each of them involve the 

 liberation of silica from a silicate. 



Sir Thomas Holland, however, evidently regards the second 

 reaction as endothermic, for he speaks of a reversal of the 

 chemical reaction. 1 The higher temperatures of the tropics as 

 compared with the temperate regions would properly not be sufficient 

 to produce this reversal, and therefore Holland, seeking for the 

 operation of some other form of energy that could produce exothermic 

 results, suggested the vital influence of life in the form of bacteria, 2 



1 Trans. Inst. Min. Met., xix, p. 4£4, 1910. 



2 Geol. Mag., 1903, pp. 61-4. 



