L. Leigh Fermor — Laterites of French Guinea. 35 



the rock, still evident in the zone of leaching, ceases to be perceptible. 

 More exactly the lower zone would be spoken of as the zone of 

 maximum departure or leaching. 



In the preceding chapters Professor Lacroix has discussed the 

 products of alteration of three distinct groups of rocks, there being 

 distinct types of laterite corresponding to each. These three groups 

 are: (1) nepheline-syenites, gabbros, diabases,' and peridotites ; 

 (2) micaceous schists and granites ; (3) alluvium composed of debris 

 of non-lateritic origin. In this final chapter the author compares 

 the three. 



IV («). Zone of Leaching [depart). 



This zone is characterized by the elimination of the greater part of 

 the constituents, the disappearance of which characterizes the 

 phenomenon of lateritization ; three cases can be distinguished, 

 marked by the formation of gibbsitic laterites, ferruginous laterites, 

 and bauxitic laterites, respectively. 



Gibbsitic Laterites. — In the first case, namely that predominating 

 in the alteration of gabbros, diabases, and nepheline-syenites, the 

 transformation is abrupt without transition, the same thin slice 

 showing the contact of the absolutely fresh rock with the portion in 

 which there is no longer any unaltered mineral, and where the 

 leaching of constituents destined to elimination is often almost complete. 



In such rocks, the ratios of Ti 2 , Fe 2 3 , and Al 2 3 are at first 

 sensibly the same as in the fresh rock, but as one rises in the section 

 the proportion of iron diminishes very rapidly in consequence of its 

 removal to the higher levels, and later the amount of Al 2 03 increases 

 with reference to that of the Ti 2 . 



From the mineralogical point of view the essential feature is the 

 transformation of the felspars into gibbsite, which occupies the body 

 of the destroyed mineral, the composition of the original felspar 

 having no influence on the result; at the same time colloidal products 

 take the place of the other minerals. The transformed rock, with 

 the original structure still visible, is porous and light in weight, with 

 a very characteristic variegated aspect, which to Professor Lacroix 

 suggests pain d'epices. This laterite is more or less rich in aluminium 

 silicate, concentrated in the skeletons of the ferro-magnesian minerals, 

 and in the examples represented by analyses Nos. 2, 6, 9, given in 

 Part II, the quantity of silica ranges from 2 - 21 to 12-67 per cent, 

 corresponding to 5 to 30 per cent of silicate of composition corre- 

 sponding to the formula 2 II 2 . Al 2 O3 . n Si 2 . 2 



In Los Archipelago where the nepheline-syenites occur, the 

 alteration normally corresponds with that above described, but at 



1 I have retained the term diabase throughout this article, although 

 personally I think it an unnecessary term, the .word dolerite being the more 

 desirable. 



2 In a footnote Lacroix remarks that the essentially kaolinic transformation 

 of a diabase is an exceptional case that he has not himself observed. Such 

 cases occur, however, in India : cf. Mem. Geol. Surv. Ind., xxxvii, p. 376, 

 1909, where is described the occurrence at Yeruli of a zone of variegated 

 argillaceous material intervening between the underlying basaltic rock and 

 overlying laterite. 



