L. Leigh Fermor — Laterites of French Guinea. 33 



decomposition of the nepheline-syenites, gabbros, diabases, and 

 peridotites (pp. 271-301); (ii) The products of decomposition of the 

 mica-schists, gneisses, and granites (pp. 802-317); (iii) The laterites 

 d? alluvions or lateritoids (pp. 318-324) ; (iv) The minerals of laterites. 

 It will be convenient to notice briefly the contents of chapter iv, 

 and then to give the substance of chapter v in some detail, which 

 summarizes admirably the subject-matter of chapters i to iii, and also 

 gives the author's ideas as to the conditions of formation of laterite. 



III. The Minerals of Laterite. 



From the mineralogical point of view the most remarkable feature 

 of lateritization is that practically all the minerals so produced are in 

 a state of hydration, either in a colloidal phase (hydrogel) or in a 

 crystalline phase, and most often with the two together. In certain 

 cases it can be shown that crystallization takes place at the expense 

 of the hydrogels, but, as related later, this is not always the case for 

 aluminium hydrate. 



Full details are given (p. 326) of the microscopic characters of the 

 crystalline form of aluminium hydrate, namely, hydrargillite or gibbsite, 

 which is the only crystalline aluminium hydrate found by Professor 

 Lacroix in the laterites of Guinea, there being no evidence for the 

 existence of diaspore. 



The history of the colloidal hydrates of aluminium in laterites cannot 

 be divorced from that of the original bauxite, which is only a laterite 

 of a former period found in regions no longer tropical. As long ago 

 as 1901 Professor Lacroix 1 showed that the French bauxites are 

 without exception formed of colloidal hydrates, existing alone or in 

 association with colloidal silicates, demonstrating that bauxite cannot 

 be regarded as a mineral, because it does not correspond with the 

 supposed specific formula of Al 2 3 . 2 H2 ; thus in France it has 

 a composition approaching closer to A1 2 03.H 2 than to any other, 

 whilst in other regions, as in Arkansas, the composition approaches 

 that of Al 2 O3 . 3 H2 0- He concludes, therefore, that bauxite must 

 be regarded as a rock composed of various colloidal aluminium 

 hydrates mixed, according to the case, with ferric hydrates, clay, etc. 

 Passing in review the names proposed by various authors for these 

 colloidal, hydrates, he rejects the a-kliachite and ft-lcliachite of Cornu, 2 

 the sporogelite of M. Kispatic, 3 and Dittler & Doelter's use of 

 bauxite, and their proposed bauxitite,* adopting the term alumogel 

 suggested by M. Pauls. 5 Judging from both his own work and that 

 of M. Arsandaux, Professor Lacroix accepts the existence of one 

 colloidal hydrate with one molecule of water (France and Guinea), 

 and of another with three molecules of water (Guinea and Arkansas). 



Dealing with the ferric hydrates, he uses the term limonite for the 

 crystalline form of 2 Fe 2 O3 . 3 H 2 0, and revives the old name 



1 Mineral, de la France et de ses colonies, iii, p. 342. 



2 Zeitsch. Chem. industr. Kolloide, iv, p. 90, 1909 (Lacroix). 



3 Neues Jahrb., Beil. Bd. xxxiv, p. 518, 1912. 



4 Centralblatt f. Mm., 1912, pp. 19 and 104 ; and 1913, p. 193. 



5 Zeitsch. prakt. Geol., xxi, p. 545, 1913 (Lacroix). 

 DECADE VI. — VOL. II. — NO. I. 3 



