212 F. Dixey — Lateritization in Sierra Leone. 



2. (i) Lateritization of the Norite. (PL III, Fig. 1.) 



The country rock of the Colony is a medium-grained norite, 

 associated with which is a relatively small development of a coarse 

 norite and an aplitic variety ; the medium-grained type offers the 

 best means of following the process of lateritization. 



In consequence of rapid erosion induced by high relief, laterite 

 formed in situ has been unable to accumulate to any considerable 

 depth except in special circumstances. On valley-sides, hill-crests, 

 and cols, the laterite is rarely more than a few inches deep. On 

 rounded hills and gentle slopes the country rock crops out in a 

 characteristic manner as numerous large boulders and crags, or 

 even low isolated rounded surfaces. The outstanding character 

 of these various masses is due in all cases to lateritization, which 

 progressed along master joints. The lateritized portion was more 

 readily removed by erosive agencies, and the masses of unaltered 

 rock were left standing in relief. The masses of rock exposed in 

 this way exfoliate in thin shells and gradually assume a rounded 

 form. Their exposed surfaces are either fresh or covered with only 

 a thin skin of laterite. AVhere the rock is closely jointed, lateritization 

 proceeds uniformly throughout and extends to a depth of several 

 yards. Examples of this occur over part of the gentle slope ' on 

 which the W.A.R. camp stands, near Wilberforce, and in the valley 

 of the Congo south-west of Freetown. The depth of lateritization 

 was so great on the right bank of the Congo at a height of about 

 500 feet O.D. that a ]n'oject for building a small dam across the 

 valley ended in failure, because it would have been necessary for 

 the dam to rest everywhere upon solid rock, and the cost of this was 

 prohibitive. 



A clearing was made up the steep valley sides just above a narrow 

 gorge ; on the left bank the norite was exposed under a thin cover 

 of laterite, but on the right an excavation cut through 12 feet of 

 laterite before exposing the norite, and even when extended 

 laterally met only the altered rock. The alteration had proceeded 

 along joints until only cores of unaltered rock were left ; the cores 

 were in the form of slabs because of the platy jointing prevailing 

 in this place. The slabs or cores were surrounded by more or less 

 concentric layers of red laterite, which became more irregular in 

 shape outwards. Sometimes all that remained of the original mass 

 was a red " ghost " with concentric lamination and an irregular 

 outline. 



In most localities in the Colony the core of a lateritized mass 

 tends to become continually more rounded in form as it diminishes 

 in size. But where the norite is banded lateritization sometimes 



^ Cf. Professor Lacroix, " The formation of a continuous crust depends on 

 definite topographical conditions (horizontal plateau or gentle slope of ground).' ' 

 Termor, " The Work of Professor Lacroix on the Laterites of French Uuinea " : 

 Oeol. Mag., 191.'i. 



