L. Leigh Fermor — Later ites of French Guinea. 77 



IV. — The Work of Professor Lacroix on the Laterites of 

 French Guinea. 



By L. Leigh Fermor, D.Sc, A.E.S.M., F.G.S., Geological Survey of India. 



(Continued from January Number, p. 37.) 



IV {}). Zone of Concretion. 



IN this zone the phenomenon of depart or leaching is carried to 

 a finish, and though of little significance in the case of the gibbsitic 

 laterites of the diabases, gabbros, and syenites, since these from the 

 beginning of alteration in the zone of leaching have been deprived of 

 the greater part of tbe elements to be removed, yet in the case of the 

 mica-schists it is of great importance. But in every case there is an 

 accentuation of a phenomenon already evident in the preceding zone, 

 namely the emigration of iron towards the surface, where it becomes 

 concentrated. 1 At the same time concretionary phenomena, often 

 resulting in the separation of the hydrates of iron and aluminium 

 one from the other, become increasingly important and reach their 

 maximum development quite close to the surface, where they lead 

 to the formation of a resistant crust (cuirasse), in which hydroxide of 

 iron acts as cement; this hydroxide may even become sufficiently 

 abundant to constitute an ore of iron, particularly in the case of 

 certain diabases, and above all of peridotites. 



In Professor Lacroix' opinion the formation of a continuous crust 

 depends on definite topographical conditions (horizontal plateaux or 

 gentle slope of the ground). 2 Thus the author regards as inadmissible 

 the hypothesis according to which the formation of the iron-ores of 

 Guinea is ascribed to the mechanical concentration of iron oxides by 

 running water ; 3 instead, in the cases described by Professor Lacroix, 

 the hydroxide of iron has superposed itself on the pre-existing 

 elements of the superficial crust : " il s'accumule de bas en haut." 



Corresponding to the three modes of alteration in the zone of 

 leaching, resulting, respectively, in the formation of gibbsitic 

 laterites, ferruginous laterites, and kaolins and lateritic clays, 

 Professor Lacroix finds three types of end-products resulting from 

 the completion of the process of lateritization in the zone of 

 concretion. These are gibbsitic, ferruginous, and bauxitic types, 

 respectively, of which the latter type in particular tends to 

 be pisolitic. 



Gibhsitic Types. — The readiness with which iron oxide takes part 

 in concretionary changes is manifest to the eye, but the mobility of 



1 Lacroix notices that this ascension of iron towards the surface was observed 

 long ago by Hislop (1857, Lacroix gives 1863), and later by Maclaren (1906), in 

 the laterite of India, and by Arsandaux in that of Western Africa and of the 

 Congo, and by many other authors. 



2 While this is doubtless true from the point of view of the actual result, 

 Professor Lacroix overlooks the circumstance that on steep slopes the products 

 of weathering are rapidly removed by erosion, so that lateritic products have no 

 chance of accumulating. 



3 Lacroix remarks that this theory has already been proposed by Holland 

 (Geol. Mag., 1903, p. 62) to explain the formation of the low-level laterites 

 of India, to which Holland attributes a detrital origin. Lacroix does not, 

 however, regard the two cases as comparable. 



