216 F. Dixey — Later itization in Sierra Leone. 



of the railway running between Waterloo and Songo. The deposits 

 of the Bullom Shore range from coarse felspathic sands to almost 

 pure clays. Lateritization is most pronounced in the sands and 

 diminishes in the more argillaceous beds until it is practically 

 non-existent in pure clay. The probable explanation is that the 

 sands allow surface waters to percolate through them more freely, 

 and also that they contain a much higher proportion of iron in the 

 form of grains of magnetite. 



The typical laterite exposed along the clifis and in stream beds 

 is a red-brown to bright red scoriaceous rock, moderately hard, and 

 in the hand specimen coarsely porous ; the diameter of the pores 

 is about 0'4 in. The texture of the walls surrounding the pores is 

 usually compact, the surfaces being smooth or rough according to 

 increasing size and number of the sand grains. The laterite locally 

 contains small masses, lenticles, and streaks, rarely exceeding 2 feet 

 in length, of loosely compacted yellow sand, mixtures of sand 

 and magnetite grains cemented into a purple mass, and of clay con- 

 taining anything from mere iron staining to a very high proportion 

 of haematite and limonite. When a small lump of the laterite is 

 exposed to the weather for a long time it acquires a smooth lustrous 

 surface, dark-brown to black in colour. The pores in the laterite 

 often remain of uniform character to a depth of many yards ; beyond 

 this depth the porous mass passes into a red compact sand pierced 

 by irregular streaks and pipes cemented by iron oxides. 



This porous laterite forms a hard crust about 10 feet thick at 

 the top of the Pleistocene deposits of the j)latform ; it is exposed 

 along the ujjper part of the cliffs and as huge fallen blocks at 

 their foot. 



Origin of the Scoriaceous Laterite. — Lateritization depends largely 

 upon the amount of iron originally contained in the beds subject 

 to the alteration, and also upon the ease with which water can 

 percolate through them. Just as the modern marine sands vary 

 considerably in their degree of fineness and the proportion of iron 

 and silt they contain, so probably did the older deposits now up- 

 raised to form the Pleistocene platform. 



The sandy deposits offer the best means of observing the changes 

 involved, which were as follows : — 



Owing to the breaking down and hydration of the iron by the 

 combined effects of waters and varying temperatures, pale yellow 

 spots were j)roduced in the beds. The material forming the spots 

 hardened into little lumps about the size of a pea ; as these lumps 

 grew larger their centres became brown, and then red, possibly as 

 the result of increasing dehydration. Consequently, at any later stage, 

 they possessed concentric colour-zones in red, brown, and yellow. 

 In time the lumps, or concretions, coalesced into irregular nodular 

 masses. These masses finally formed a thick crust and arrested 

 the concretionary processes by causing the waters to flow in definite 

 channels through or over the surface of the crust, instead of 



