80 L. Leigh Fermor — Laterites of French Guinea. 



the alumina, although less apparent, is none the less real, as Professor 

 Lacroix demonstrates by means of chemical analysis (compare 

 analyses Nos. 3, 7, 10 with Nos. 2, 6, 9, see Table I), and has followed 

 in detail under the microscope ; little by little the ferruginous 

 products, which, in the diabases and syenites, occupy the places of 

 the heavy silicates, are replaced by gibbsite, which, in addition, fills 

 in all the spaces in the rock. This mineral must be regarded, 

 therefore, as rather soluble in the underground waters, as is obvious 

 when one recalls cases of crystalline gibbsite in geodes and cracks in 

 laterites and ores from Guinea, India, Brazil, and New Caledonia. 

 The final result of the transport and recrystallization of gibbsite is 

 the formation of granular crystalline rocks of relatively coarse grain 

 composed almost entirely of gibbsite. 



Ferruginous Types. — In the case of laterites derived from peridotites 

 the zone of concretion, owing to the poverty of the original rock in 

 alumina, corresponds with the ferruginous crust. This crust shows 

 every variety of concretionary form of limonite and stilpnosiderite ; 

 and at the actual surface the limonite becomes more or less dehydrated 

 with appearance of red tints, indicating doubtless the formation of 

 hematite (see analysis No. 10, Table I, p. 79). 



Bauxitic Types. — In contrast to the zone of concretion of the 

 laterites derived from diabases and syenites, the characteristic of 

 the zone of concretion capping the kaolins and lateritic clays of 

 the zone of leaching of the mica-schists, granites, and gneisses, is that 

 the aluminium hydrate, which, little by little, replaces the aluminous 

 silicate, assumes at first the colloidal form (see analyses JSTos. 11, 12, 

 13, Table II). Although, in this case, gibbsite is often present, it is 

 always accompanied by an important and often predominant pro- 

 portion of colloidal hydrates, and Professor Lacroix maintains that 

 this crystalline mineral is a product of transformation of the colloidal 

 forms ; and by means of the evidence of thin sections he proves his 

 contention in the case of the pisolitic laterites. 



Pisolitic Laterites. — Speaking generally, one of the characteristics 

 of the crust or cuirass is the abundance of pisolitic forms. It has 

 long been held that pisolites in general have been formed in waters 

 in movement, and recently this idea has been used to explain the 

 formation of the pisolitic bauxites of Arkansas. But although 

 Professor Lacroix admits the legitimacy of such an opinion in the 

 case of the siliceous or calcareous pisolites of thermal springs, he 

 regards it as indefensible in the case of laterite. On the contrary, 

 the pisolites of laterite, like those found in clays formed by the 

 decalcification of limestone, have been formed in situ in a medium in 

 a state of rest, and in general have not suffered transport since their 

 formation ; their form, degree of regularity, and size depend on the 

 physical state of this medium. The nest condition for the formation 

 of pisolites is, according to Lacroix, the existence of a homogeneous 

 medium, enclosing few or no solid elements insusceptible of con- 

 cretionary re-arrangement, unless in small grains; this explains the 

 abundance of pisolites in the lateritic crusts of mica-schists, clays, 

 and alluvium. 



Professor Lacroix here (p. 342) refers to the section of my 



