220 F. Dixey — Later itization in Sierra Leone. 



i. The granitic rocks of the Protectorate are frequently lateritized 

 to a depth of 15 feet, and sometimes even to 30 feet. Nevertheless 

 laterite is only slightly developed in the inselberg mountain districts 

 of the northern part of the country. Lateritization of granitic 

 rocks proceeds as follows : the rocks decompose into a white 

 kaolinitic product, which passes into brown lateritic clay (" zone of 

 leaching ") ; this lateritic clay then develops into laterite (" zone 

 of concretion "). There is a progressive change from the unaltered 

 rock to the laterite crust, the granitic rocks in this respect presenting 

 a marked contrast to the basic rocks. 



5. Lateritization affects detrital deposits also, particularly the 

 Pleistocene sands and clays of the coastal plain. The phenomenon 

 is most pronounced in the felspathic sands and diminishes in the 

 more argillaceous beds until it is practically non-existent in pure 

 clay. The typical lateritite is a red-brown scoriaceous rock, 

 moderately hard, and in the hand specimen coarsely porous. It 

 forms a crust about 10 feet thick at the top of the Pleistocene 

 beds. This lateritite is indistinguishable from the laterite derived 

 from other rocks. 



6. The scoriaceous laterite (lateritite) was produced by the growth 

 and ultimate coalescence of numerous small ferruginous concretions, 

 followed by the removal of the soft material between the concretions. 



7. The laterite formed from detrital deposits passes locally into 

 lateritic iron-ore by increase in the percentage of iron oxide. Such 

 iron-ores are well developed along the inner margin of the coastal 

 plain encircling the colony, and occur principally in the neighbour- 

 hood of Devil Hole, near Waterloo. They were derived originally 

 from the titanomagnetite which enters largely into the composition 

 of the norite, and they are consequently very rich in titanium. 



8. Lateritization is frequently an important factor in the 

 production of caves and subterranean channels, owing to the ease 

 Avith which the unconsolidated material is removed from beneath 

 the thick hard crust of laterite. 



In conclusion, I have great pleasure in recording my indebtedness 

 to Professor A. H. Cox, of University College, Cardiff, who kindly 

 made many useful suggestions during the writing of these notes. 



EXPLANATION OF PLATE III. 



Fig. 1. — -Lateritized norite, in railway cutting, Wilberforce, Sierra Leone. 

 The photograph illustrates a late stage in the lateritization of two blocks 

 of norite. Each block is now represented o\i\y by a core of unaltered norite 

 enclosed in concentric shells of laterite. The original upper surface of 

 the right-hand block is indicated by the position of the hammer. 



Fig. 2. — Residual laterite derived from granite, railway cutting near Baoma, 

 Sierra Leone. The unaltered granite forms a ridge whicli traverses the 

 cutting obliquely from right to left. 



