36 L. Leigh Fermor — Laterites of French Guinea. 



certain points in these islands it lias locally pursued a totally different 

 course, with formation of a plastic clay (see analysis No. 4, Part II). 

 The association of these two different types of alteration is so intimate 

 that Professor Lacroix has no explanation to offer of their coexistence 

 at the same point (p. 281). It impels the question whether, in the 

 normal case, gihbsite, instead of being the first secondary product 

 formed from the felspars, may not itself have been derived from the 

 destruction of an aluminium silicate. Judging from the examination 

 of a considerable number of specimens in which the type of trans- 

 formation is always the same, Professor Lacroix answers this question 

 in the negative, thinking it likely that the decomposition has been 

 from the beginning a hydrolysis of the original silicates. 1 



Ferruginous Laterites. — Ferruginous laterites in the zone of leaching 

 result from the alteration of peridotites, owing to the poverty of these 

 rocks in aluminiferous silicates. The ferro-magnesian silicates de- 

 compose in the same manner as in the gabbros, furnishing colloidal 

 ferric products, containing some alumina (see analysis No. 9, Part II). 

 The rock is porous, ochreous, light, and friable. 



Kaolinic and Argillaceous Decomposition. — This, the third form of 

 alteration in the zone of leaching, characterizes the mica-schists, 

 gneisses, and granites. It is clearly distinguishable from the first 

 case, because, instead of an abrupt alteration without intermediate 

 phase, there is a progressive change, so gradual and extending to such 

 a depth that the unaltered rock is seldom seen. In the mica-schists 

 of Fatoya, borings pushed to a depth of 60 metres have not reached 

 the fresh rock, probably because the vertical disposition of the schists 

 has facilitated their alteration. Consequently, in contrast to the 

 rocks considered above, in which the original structure persists for 

 only a few metres above the original fresh rock, the initial structure 



1 Mr. E. S. Simpson (Geol. Mag., 1912, p. 401) has criticized adversely 

 my postulation (Geol. Mag., 1911, p. 459) of two distinct modes of 

 decomposition of rocks resulting respectively in the formation of clay and 

 laterite, apparently because in Western Australia primary laterite is always 

 found to overlie an almost pure pipeclay and this in turn a crystalline rock. 

 From the work of Lacroix it is seen that the felspar in a crystalline rock, such 

 as nepheline-syenite, may alter directly into gihbsite or directly into a hydrous 

 aluminium silicate. When the latter forms, a chemical change may cease at 

 this stage, as in Los Archipelago, or it may proceed further, as in the case 

 of the mica-schists noticed on a later page in this paper. With reference to 

 Mr. Simpson's comparison of lateritization with the formation of saline 

 efflorescences, Lacroix' work indicates a marked distinction. A saline 

 efflorescence is deposited on the surface of the ground ; the constituents of 

 a lateritic crust, on the other hand, are partly residual (although recrystallized) , 

 formed approximately in their present place from the once fresh primary rock, 

 at the time when the zone of leaching was at this level ; and have partly been 

 brought up from the zone of leaching and added to the residual portions : in 

 any case these constituents were deposited inside the crust and not on the 

 surface, except perhaps to a very minor degree. In the following paragraph 

 Mr. Simpson gives a curious definition of a replacement of a rock, namely, 

 " a deposit accumulating in the actual space originally occupied by the solid 

 rock which has yielded tbe materials wbich compose the laterite." Surelythe 

 essential feature of replacement is that the replacing mineral has been brought 

 from elsewhere, and has been deposited in the place of another mineral removed 

 in solution. 



