F. Dixey — Later itization in Sierra Leone. 215 



surface of each mass and forms generally a white kaolinized shell, 

 which continually increases in thickness until it involves the whole 

 mass. The white mass gradually breaks up later and becomes 

 increasingly stained on the destruction of the iron-ores and ferro- 

 magnesian minerals ; it forms finally a great thickness of soft red- 

 brown lateritic clay, almost homogeneous in composition and texture. 

 The uppermost portion of this lateritic clay then forms locally a 

 hard crust of laterite up to several feet in thickness, due to the 

 growth and ultimate coalescence of numerous small ferruginous 

 concretions which gradually appear in it. In some places the con- 

 cretions give rise only to a coarse laterite gravel. 



A similar decomposition of the mica-schists, gneisses, and granites 

 of French Guinea has been described by Professor Lacroix.' He 

 includes the kaolinitic decomposition product and the lateritic 

 clay in his " zone of leaching " and the laterite crust in his " zone 

 of concretion ". The granites of Mozambique,''^ however, are rarely 

 kaolinized ; where they have undergone this mode of alteration, 

 the kaolin does not pass into lateritic clay and laterite, but is capped 

 by a thickness of damp, black soil. The alteration in this case is 

 ascribed to a heavy growth of vegetation. 



It can frequently be demonstrated in railway cuttings (PI. Ill, 

 Fig. 2) that a great thickness of laterite may be formed from alteration 

 in situ of crystalline rocks, for the forms of the original masses are 

 still in evidence, and so also are the quartz veins which pierced them ; 

 such masses are now represented only by faint patches or ghosts, 

 whereas the quartz veins stand out clear and unaltered, showing 

 their original form and foliation. There is a progressive change 

 from the unaltered rock to the laterite crust, the crystalline rocks 

 in this way presenting a marked contrast to the basic rocks (p. 212). 



The Pleistocene and Recent beds of the plain become lateritized 

 in a similar manner, and the resultant product is indistinguishable 

 from the laterite formed directly from the crystalline rocks. 



Lateritic iron-ores are formed locally where iron is abundant 

 in the country rock, but with the exception of those in the 

 Colony (as distinct from the Protectorate) (see p. 217), they are of 

 no importance. 



4. (i) Lateritization of Detrital Deposits. 



Detrital deposits are also affected by lateritization changes. 

 The phenomena are apparent in a series of stratified beds, which 

 cover the marine platform encircling the Colony. Other detrital 

 deposits affected are the surface accumulations in valleys and gaps, 

 and talus-slopes resting on the steep seaward flanks of the hills. 

 Phenomena similar to those described below are well exposed 

 along the cliffs of the BuUom Shore and in cuttings along that part 



1 Op. cit., pp. 36, 80. 



^ Holmes, op. cit., \). 536. 



