L. Leigh Fermor — Laterites of French Guinea. 81 



classification of laterites instituted for lake laterites, a name 

 suggested for any of the pisolitic laterites that can be proved to 

 have been formed by deposition from solution in bodies of water 

 on the analogy of the Swedish lake ores of iron. Professor Lacroix 

 has not seen personally in Guinea any pisolitic rocks, formed 

 necessarily by a process of this nature, and remarks that the 

 conditions realized in the temporary pools of water on the surface 

 of the bowals 1 are sufficient to explain all the facts observed. It 

 should he noted that I did not suggest that pisolitic laterites in 

 general would prove to be lake laterites, but proposed this division 

 to allow for eventualities, suggesting only that two particular cases 

 of pisolitic limonite might prove to belong to this division. In view 

 of Lacroix' account of the pisolitic laterites of Guinea, I am quite 

 prepared for the future to show that true lake laterites are a very 

 rare type. Obviously, however, there must be some relationship 

 between laterites deposited chemically in lakes, and those formed 

 in small pools on the surface of already existing laterites. 



Continuing, Professor Lacroix remarks that ferruginous pisolites 

 are found in the cuirass of all the lateritic types, colloidal hydroxide 

 of iron being a ubiquitous product ; but the same is not the case for 

 very aluminous pisolites. In Guinea they are not found in the 

 lateritic cuirass of the nepheline-syenites, and they are rare in that 

 of the basic rocks ; but on the contrary they are common in the 

 cuirass of the mica-schists, granites, and clays, not only on account 

 of the conditions of the medium already referred to, but also for 

 another reason. 



In the gibbsitic laterites the aluminous hydrosol circulates in the 

 zone of concretion in contact with gibbsite formed from the 

 commencement of alteration and disseminated everywhere, and 

 the crystals of this mineral serve as nuclei, determining an almost 

 complete crystallization. But, in the alteration of mica-schists, the 

 whole of the alumina is liberated from its silicated compounds in the 

 form of a gel, and exists in part in this condition in the cuirass ; it 

 can thus easily rearrange itself into pisolites, as long as crystallization 

 has not set in. Whenever, as at Fatova and Siguiri, gibbsite is seen 

 in these pisolites, the irregularity of its disposition, and the absence 

 of ' regular fibro -lamellar structure starting from a centre, leave no 

 doubt as to its secondary origin. 



Judging from the examples given in this memoir, it might be 

 thought that the persistence of the colloidal state bore some relation 

 to the composition of the aluminium hydrate, the hydrate with one 

 molecule appearing to be abundant in the bauxitic laterites, and that 

 with three molecules in the laterites formed from the diabases and 

 syenites. The crystallization of gibbsite in the pisolites would then 

 be, as M. Arsandaux thinks, the result of hydration. 2 But the 



1 Name in the Foula language (p. 273) for vast horizontal or undulating 

 tracts in Guinea, either quite bare, or covered with a meagre vegetation of 

 grasses, Cyperus, and semi-aquatic plants. 



2 Lacroix notes that Maclaren (Geol. Mag., 1906, p. 546) supposes that in 

 India gibbsite results from hydration, contrarily to the opinion of Holland, who 

 regards laterite as characterized by a dehydration (Geol. Mag., 1903, p. 65). 

 Judging from Lacroix' researches it will be seen that there is probably little 



DECADE VI. — VOL. U. — NO. II. 6 



