34 L. Leigh Fermor — Later ites of French Guinea. 



stilpnosiderite of Ullmann (1814) for the colloidal form of the same 

 mineral. Details are given of the microscopic characteristics of hoth 

 these minerals. The red tints often seen in the ferruginous cuirass of 

 laterites are regarded as undoubtedly due to the dissociation of the 

 yellow hydrate under the influence of the sun's rays. But Professor 

 Lacroix regards the product as a mixture of hematite with the initial 

 limonite or stilpnosiderite, and not as a special hydrate of formula 

 2 Fe 2 O3 . H2 0, to which the name hydrohematite or turgite has heen 

 attached. He has never detected the presence of goethite in the 

 laterites of Guinea, although it has heen reported as existing in some 

 of the laterites of that country. 



The silicates of aluminium exist in two forms, crystalline (Jcaolinite) 

 and colloidal, and of the latter the author detects two varieties, one 

 of them cryptocrystalline, which is regarded as halloysite, and the 

 other isotropic, which is supposed to possess a composition near to 

 that of balloysite. From the fact that hydrochloric acid never 

 breaks up more than a very small portion of the clays, he judges that 

 there cannot be present any important quantity of silicates more basic 

 than halloysite, such as allophane. Attempts made in Professor 

 Lacroix's laboratory to repeat the distinction of silicates from hydrates 

 by staining, as announced by Dittler and Doelter, gave such irregular 

 results that they were discontinued. 



It has not proved possible to detect mineralogically the presence of 

 titanium compounds in laterite. But it is supposed that the portion of 

 the titania of laterites that is soluble in hydrochloric acid exists as 

 orthotitanic acid, Ti 2 . 2 H 2 0, whilst the idea of Arsandaux that the 

 portion of the titania of laterites that is soluble only in sulphuric 

 acid, which is the greater part, exists as metatitanic acid, Ti 2 . H 2 0, 

 is accepted as probable. For these titanic hydrates Lacroix proposes 

 the name doelterites, in honour of C. Doelter, who has done so much 

 work on colloid minerals. 



It is supposed that the small quantities of chromic oxide detected 

 in some laterites exist as a hydrate of Cr 2 03, probably in the 

 colloidal form. The only manganiferous product in the Guinea 

 laterites consists of a mixture of stilpnosiderite and psilomelane, and 

 was found in Los Isles. 



IV. The Processes and Peodtjcts oe Lateritization. 



In chapter v of his memoir, the author first states the chemical 

 result of the formation of laterite, namely, the elimination of the 

 alkalis, alkaline earths, and silica of the original rock, and the 

 persistence of hydrated oxides of aluminium and iron with a little 

 titanic acid, noticing that in different cases the rock is formed in 

 different ways, which lead to important miner alogical and structural 

 variations in the final product. 



Starting from the intact rock Professor Lacroix has been led to 

 distinguish two superposed zones, the zone de depart, which we may 

 translate as the zone of leaching, and the zone de concretion. Between 

 the two zones there is perfect continuity, their separation being 

 arbitrary, and necessary only for the sake of a clearer exposition of 

 the problem ; the point chosen is that where the original structure of 



