214 F. Dixey — Lateritization in Sierra Leone 



(ii) Alteration of Norite rich in Iron. 



Lateritization proceeds very readily where the norite is rich in 

 disseminated magnetite (titanomagnetite), but it goes on slowly 

 if the proportion of iron be increased beyond a certain point. This 

 is illustrated on the foreshore west of Aberdeen Creek, where certain 

 inclined bands in the norite have been thoroughly lateritized for 

 many yards along their outcrop, and seams of magnetite in them, 

 up to 2 inches thick, run practically unaltered from norite into 

 laterite. Some of the seams contain a little felspar in parallel 

 arrangement with the magnetite. In such cases the magnetite is 

 often left as a spongy mass on the weathering of the felspar. Further 

 within the lateritic masses there occur more or less spherical cores 

 of iron-rich norite, enclosed in concentric shells ; such cores 

 apparently owe their unaltered state to a high iron content. 



In addition to the seams of magnetite in the normal norite, there 

 occur locally in the coarse norite irregular crystals and small spongy 

 masses of magnetite which also remain unaltered when the enclosing 

 rock is lateritized. Good examj)les of these masses, and of crystals 

 ranging up to 3 inches in length, can be seen in coarse norite exposed 

 on the foreshore immediately south of York. 



3. Lateritization op Granitic Rocks. 



The solid rocks of the Protectorate have been lateritized to a 

 considerable de^jth in many places. Where they rise above the 

 Pleistocene deposits of the plain they are frequently seen in railway 

 and road cuttings to be lateritized to a depth of 15 feet, and some- 

 times to 20 or even 30 feet (PI. Ill, Fig. 2). In the more hilly and 

 mountainous parts of the country, especially in the north, laterite 

 is not nearly so well developed. The mountain sides are often very 

 steep and bare, and uncovered by vegetation except for a few tufts 

 of grass, and even in the broad valleys oj)ening out on to the plain, 

 and on the northern part of the plain itself, the laterite occurs only 

 as patches of gravel. The conditions in this part of the country are 

 similar in many respects to those of the mountain districts of 

 Mozambique, concerning which Dr. A. Holmes states : ' '" It is 

 important to notice that the steep slopes of the inselberg peaks and 

 mountain blocks are always free from deposits of the lateritic 

 constituents. Solutions must certainly exist which on evaporation 

 would give rise to such deposits, but the mountain rains and night 

 dews suffice to carry them down to the debris-covered plateau 

 surface below, where they contribute to the formation of the lateritic 

 earths which so frequently sweep around the base of the hills." 



The lateritization of the granitic rocks proceeds as follows : the 

 rocks become broken down into small masses by weathering at the 

 surface and by decomposition along joints and the less resistant 

 bands ; the decomposition then proceeds inwards from the outer 



1 " The Lateritic Deposits of Mozambique " : Geol. M.i.G., 1914, p. 534. 



