128 L. Leigh Fermor — Later •ites of French Guinea. 



not, however, go further into this matter as he thinks the solution of 

 the problem of the origin of laterite should come from the experi- 

 mental side, and that it would be profitless to discuss this question 

 again without the support of laboratory experiments. 1 Instead, the 

 author prefers to state the conditions under which lateritization takes 

 place and those under which it does not. 



In the first place it is evident. Professor Lacroix thinks, that 

 lateritization is not the result of a direct attach by the atmosphere, nor 

 by running rain-water, as is shown by the freshness of basic and 

 syenitic rocks in cliffs and denuded surfaces. 2 On the contrary, 

 lateritization is everywhere intense where the slope of the ground is low 

 enough to permit the infiltration of ivater and allow it to remain for 

 a long time in contact tvith the rocks. One must also give great weight, 

 even according to Professor Lacroix, to the action of vegetation, which 

 develops with extreme rapidity under such conditions, so that one 

 cannot accept the idea recently advanced by Vageler 3 that the clayey 

 decomposition of temperate climes is due to the action of humic acids, 

 whilst in tropical countries the silicates undergo hydrolysis, due to 

 the rarity or absence of humus. 



For the development of a ferruginous cuirass the most favourable 

 topographical forms are certainly the tabular plateaux provided by 

 diabase flows, and valley-bottoms of very low gradient. In proportion 

 as superficial concretion progresses the conditions become more and 

 more unfavourable for the existence of vegetation, which finally to 

 a large extent disappears. If this view be correct, then the sterility 

 of the bowals is not the cause, but the consequence of lateritization 

 ■commenced under a cover of vegetation. It thus becomes evident that 

 Holland and Lacroix are really in agreement in so far as they regard 

 the action of vegetation as a factor in the formation of laterite. 



The climatic conditions already referred to are those, as the author 

 notes, to which Dr. Maclaren has attached so great an importance for 

 •explaining the laterite of India, namely the alternation of very humid 

 and very dry seasons^ At the beginning of the wet season the water 



1 See Meigen, Geol. Rundschau, ii, pp. 197-207, 1911, for a recent though 

 incomplete resume of theories of lateritization. 



2 This first proposition is, however, not self-evident. 



3 Fiihling's Landw. Zeitung, lix, p. 873, 1910 (Lacroix). 



4 Geol. Mag., p. 546, 1906. In his contribution to the discussion of 

 J. M. Campbell's paper, Trans. Inst. Min. Met., xix, p. 414, 1911, Maclaren 

 states that while he regards an alternation of wet and dry seasons as essential 

 in humid regions, nevertheless a truer statement may be given, as follows : 

 ' ' Laterization may take place in intra-tropical regions in a desiccated zone 

 overlying a zone charged with oxidized waters, capillary attraction translating 

 the waters from the lower to the upper zone." " The upper zone, in humid 

 regions with heavy monsoon rainfall, where the ground water level is near the 

 surface, may be at the surface ; in more arid regions it may be as much as 

 three feet below." This modification is to account for the laterite in arid parts 

 of Western Australia, believed by Maclaren still to be growing. This condition, 

 namely, alternation of "wet and dry seasons, does not, however, fit all known 

 cases. Thus, as shown by Scrivenor, there is no alternation of seasons at 

 Malacca ; but it has yet to be shown that the laterite of Malacca is still in 

 process of formation, or at least that its formation was not initiated under 

 different climatic conditions. For the present, however, it is necessary to 

 admit the formation of laterite in regions of constant rainfall. 



