F. Dixey — Later itization in Sierra Leone. 213 



proceeds rapidly along certain bands, which when dipping at a high 

 angle are altered to a considerable depth. A number of little coves 

 and a series of parallel reefs running out obliquely from the shore 

 owe their origin to the removal of such weathered bands by wave 

 action. Examples can be seen about Kent and near Aberdeen 

 Creek. 



It is possible to trace in the hand specimen the change from a 

 core of unaltered norite to the outer zone of red-brown laterite 

 ('"' gibbsitic laterite " of Pi'ofessor Lacroix '). The grey core is 

 enclosed by a pale cream-coloured halo with brown pseudomorphs 

 of the pyroxenes. This pale halo gives ])lace gradually through 

 yellow and then brown to the red-brown outer zone ; the halo and the 

 outer zone correspond respectively to Professor Lacroix's " zone 

 of leaching " and " zone of concretion ".'^ The transition from core 

 to halo is a rapid one and takes place within 0'2 in. The colours 

 of the halo and succeeding zones are due to a whitish .alteration- 

 product of the felspars, which becomes stained to an increasing 

 depth as the ferromagnesian minerals decom])ose. As the alteration 

 progresses the individual crystals gradually become unrecognizable 

 and the mass itself becomes crowded with small concretions and 

 pierced by narrow ferruginous veins, until finally it forms coarsely 

 porous red laterite. This secondary laterite bears close resemblance 

 to that formed from detrital deposits.^ 



As regards individual minerals /e/sj^ar is the first to be affected. 

 It passes into a white opaque alteration product, which spreads 

 along the cleavage cracks, and for a time leaves flakes of the mineral 

 fresh and unaltered. Then a clear white recrystallization sets up 

 along the old cleavage cracks and the remaining parts of the crystal 

 dissolve out, resulting in a framework of parallel ])lates studded with 

 minute colourless crystals (probably gibbsite). Later iron staining 

 commences, and the pseudomorph gradually breaks up. Diallage 

 changes in colour from almost black to brown at a very early stage, 

 and with the rhombic pyroxene causes a mottling of the light- 

 coloured halo ; it is then gradually pseudomorj^hed in parallel 

 brown scales, exceedingly fine, and possessing an earthy lustre. 

 The rhombic pyroxene develops a close, plaity structure, and retains 

 its bronzy lustre to a late stage. The iron-ore is recognizable, and 

 apparently unaltered long after all the other minerals have 

 disappeared. Olivine is not recognizalile after late'ritization has 

 commenced. 



' Op. cit., p. 35. 



•^ Op. cit., p. 34. 



^ Cf. Arsandaux : " Toutes les formations lateiitiques niontrent deux 

 zones ; (i) La premiere zone, contigiie a la roche oriainelle en rejiroduit exacte- 

 ment la structure ; elle en constitue une veritable pseudomorphose ; les 

 felspaths de la roche fraiche sont rojnplaces j^ar une matiere blanche ; les 

 elements colores par du sesquioxyde de for. (ii) La seconde zone, superposee 

 a la premiere, est depourvue de tout ordonnancement de ses elements con« 

 stitutifs " {Handbuch der regionalen Geologic, vii, 6«, p. 61). 



