L. Leigh Fermor — Laterites of French Guinea. 31 



containing AI2 3 . 3 H 2 in the crystalline state, and as bauxitic 

 [bauxitiques) those containing only colloidal hydrates. 1 



Then, considering the silicates of aluminium found in the laterites 

 of Guinea, Professor Lacroix finds that they exist sometimes as 

 crystalline kaolinite and sometimes as halloysite, the latter being either 

 more or less crystalline or completely colloidal. He regards it, 

 therefore, as impossible to unite all these products under the term 

 lithomarge as proposed by me (I.e., p. 511), because I have suggested 

 that lithomarge is the colloidal variety of kaolinite. This criticism 

 seems to me perfectly valid, but see the next paragraph; the author 

 then summarizes his conclusions in the following scheme of nomen- 

 clature, with which I agree in most respects, the mineralogical 

 precision introduced being a very acceptable improvement : — 



Divisions basees Subdivisions bashes 



sur la composition chimique. sur la composition mineralogique. 



T , , , ., Hydrates d'alumine. 



r Gibbsitiques. Cristallise (hydrargillite). 

 Bauxitiques. Colloidaux. 



Silicates d'alumine. 

 Kaoliniques. Cristallise' (kaolinite). 

 Argileuses. Cryptocrystallins ou 

 colloidaux. 



(Hydrates > 90, p. 100). 

 II. Laterites silicatees 



(Hydrates = 90 a 50, p. 100) 

 III. Kaolins, argiles lateritiques 



(Hydrates < 50, p. 100). 



Each of these types should be qualified by quartzose (quartzifere) 

 when it contains primary (ancien) quartz. 



It will be seen that Professor Lacroix uses the term argileuse 

 instead of the adjectival equivalent of halloysite, with which he 

 identifies the cryptocrystalline and colloidal silicates present. It is 

 interesting to note that other authors have referred the colloidal 

 silicate to the same substance. Thus Bauer 2 regards it as 

 a " Halloysite ahnlichen Aluminiumhydrosilikat" ; whilst Mr. E. S. 

 Simpson 3 speaks of " halloysite (amorphous Al 2 3 . 2 Si 2 . 2H 2 O) ". 

 Halloysite is an indefinite colloidal substance corresponding to the 

 kaolinite formula with additional water, and may itself prove to be 

 the colloidal form of kaolinite, with this additional water in a state 

 of adsorption or solid solution, like a portion of the water of zeolites. 

 Dana in his System of Mineralogy divides the old term lithomarge 

 between kaolinite and halloysite, retaining it for the firm and compact 

 form of kaolin. If, however, halloysite prove to be related to kaolinite 

 in the way suggested above, then obviously the term lithomarge will 

 be a very convenient one to use for cryptocrystalline and colloidal 

 clays in which the ratio Al 2 3 : Si 3 is 1:2 with a water content 

 varying from that of kaolinite to that of halloysite. I was probably 



1 This nomenclature appears at first sight to be heterogeneous, because 

 gibbsite is a mineral and bauxite a rock (according to Lacroix himself) ; but 

 this use of bauxitic must be taken as analogous to that of lateritic, and as 

 referring to the presence of bauxitic constituents. It is doubtful if it will be 

 possible thus to restrict the use of the term bauxite outside the realms of 

 mineralogy and petrology, because for economic purposes we have available no 

 other inclusive term, except aluminous laterite or aluminium- ore. 



2 N.J. Min. u. Pet., Festband, 1907, p. 87. 



3 "Laterite in Western Australia": Geol. Mag., 1912, p. 400. 



