30 L. Leigh Fermor — Laterites of French Guinea. 



Professor Lacroix next prints in full the tabular classification for 

 laterites and some other surface rocks drawn up by me in 191 1. 1 

 Putting aside for the time my terms lake laterite, lateritoid, and 

 lateritite, he accepts my classification for the group " des laterites 

 proprement dites", with certain reservations necessitated by the 

 characters of the rocks studied. 



In the first place he joins together my divisions I (lithomarge, clay, 

 soils, etc.) and II (lateritic rocks) in order to avoid too much detail 2 ; 

 and with reference to quartzose laterites he thinks it more logical to 

 base the divisions, not on the composition of the rock taken as a whole, 

 but " sur ses elements neogenes " only. He therefore subdivides 

 laterites carrying quartz, not according to the total tenor in lateritic 

 constituents, but according to their richness in these latter after 

 deducting quartz of primordial origin. 3 



Professor Lacroix then brings forward a mueh more important 

 modification or rather improvement of my classification, which is 

 based on chemical and not on mineralogical composition. He writes 

 (p. 261)- 



"Pour rester dans les principes gen^raux de la p^trographie, il est done 

 indispensable de tenir compte de cette composition mineralogique, et le but 

 principal de ce m&moire [italics mine] est precisement d'apporter des 

 documents nouveaux a ce point de vue. En consequence, il faut faire 

 intervenir dans la classification la notion de l'^tat, cristallin ou colloidal, dans 

 lequel se trouvent les constituants neogenes (hydrates et silicates)." 



Professor Lacroix considers the physical state of the hydroxide of 

 iron as of no importance from the point of view of classification, 

 because the two phases limonite (crystalline) and stilpnosiderite 

 (colloidal) are rarely completely separated. The hydrates of 

 aluminium, on the other hand, occur either as gibbsite or in the 

 colloidal state. 4 



In my paper already cited (I.e., p. 561) I advanced reasons for 

 avoiding the custom of several authors in regarding laterite and 

 bauxite as synonymous terms, and suggested the necessity of 

 restricting the term bauxite to the varieties of laterite sufficiently rich 

 in Al 2 3 to be used as ores of aluminium. Even this restriction is 

 not sufficient for Professor Lacroix (p. 262) — 



' ' Puisque la bauxite — au moins celle qui a servi de type et qui constitue 

 tous les gisements francais — est caracterisee, ainsi que je l'ai montre depuis 

 longtemps, 5 par l'absence complete de produits cristallises." 



Consequently he qualifies as gibbsitic 6 (gibbsitique) those laterites 



1 "What is Laterite ? " : Geol. Mag., N.S., Dee. V, Vol. VIII, p. 514, 1911. 



2 There seems to be no good reason why we should exact less precision in 

 naming products with less than 50 per cent of lateritic constituents than with 

 products containing more than 50 per cent. Indeed, greater possibilities of 

 variation amongst the former group necessitate greater precision rather than less. 



3 The undesirability of this course is indicated by consideration of the case 

 of laUrite d'alluvions noticed in Section IV. 



4 This can be regarded only as a general statement, for examples of the 

 crystalline and colloid forms occurring together are given later in the memoir. 



5 Min. de la France et de ses colonies, iii, p. 342, 1901. 



6 Throughout this memoir the author uses the term hydrargillite rather 

 than gibbsite, but in the adjectival form he prefers gibbsitic as being less 

 cumbrous. 



