L. Leigh Fermor — Laterites of French Guinea. 127 



by which he meant, according to a later explanation, not bacteria 

 of any specific kind, but vegetable life of all sorts, of which the 

 lowest forms would be the most potent, because the most abundant. 

 Vegetable life is vastly more active in the tropics than in temperate 

 regions, and, as pointed out by Holland, owing to the absence of a true 

 winter in the tropics, it is not compelled to pass a part of the year 

 in enforced inactivity, as in temperate regions. Holland's suggestion 

 is undoubtedly a valuable contribution to the problem, and cannot be 

 rejected as long as the thermal character of the second equation remains 

 doubtful; or if the equation be truly endothermic, until some other 

 agent be found more capable of providing the energy required. 



Although, roughly speaking, clays and laterites may be regarded 

 as characteristic respectively of temperate and tropical climes, yet 

 they ai'e not confined to their respective regions. True clays are 

 found very commonly in the tropics ; e.g. in India we have them 

 in abundance in the Nilgiri Hills as weathering products of the 

 charnockite series, whilst they are associated with laterite in many 

 parts of India. Lacroix in this memoir has, as already noted, shown 

 that the aluminous silicates of the original rocks sometimes pass 

 directly into gibbsite, and sometimes change first into a hydrated 

 aluminium silicate, which passes afterwards into hydrated oxides. 

 Furthermore, bauxitic oxides are known to occur in variable amounts 

 in the clays of temperate regions, and on investigation will probably 

 be found to be more widely and commonly distributed therein than 

 was suspected until recently. In a paper published about the same 

 time as Professor Lacroix' monograph, Mr. M. Gr. Edwards 1 gives the 

 results of the examination of a large series of analyses of clays of the 

 United States. Out of 244 analyses in which the free and combined 

 silica had been separated, 108, or 44 per cent, showed an excess of 

 alumina above that required for the kaolinite ratio of Si 2 to Al 2 Os 

 of 2 : 1. The percentage of bauxite thus detected in these clays 

 ranges from 0*54 to 43-5 ; and that the distribution of these bauxitic 

 clays is not a function of latitude is shown by the fact that two 

 analyses from Florida, the most southern state, show an average of 

 only 0'8 per cent of bauxite, thirty-six analyses from Georgia show 

 an average of 1 1 - 8 per cent of bauxite, and twelve analyses from Iowa, 

 a northern state, an average of 31*7 per cent of bauxite. There are 

 also southern states with a high percentage of bauxite in their clays, 

 and northern states with a low percentage. 2 



It is evident, therefore, that neither form of decomposition is 

 confined to its characteristic region, as would be expected were both 

 equations of the same thermal character. Professor Lacroix refers to 

 Holland's hypothesis, but without referring to the thermal aspects of 

 the problem he reasons from the continuity he has established in 

 certain cases between the formation of clay and of laterite, and thinks 

 it more natural to regard both modes of alteration as due to reactions 

 of the same order, manifesting themselves with greater intensity in 

 tropical countries because of special climatic conditions. He does 



1 " The Occurrence of Aluminium Hydrates in Clays " : Economic Geology, 

 ix, pp. 112-21, 1914. 



2 See also H. Ries, op. cit. , p. 402, 1914, for criticisms of Edwards' paper. 



