F. Dixey — Lateritization in Sierra Leone. 211 



AVES. 



Bird remains are represented by only a few imperfect bones, 

 but these are worth mentioning, some being those of passerines ; 

 one the proximal portion of a right humerus of Tiirdus 1 miisiciis. 

 So far as I am aware remains of passerine birds have not been 

 previously obtained from Maltese deposits. 



My best thanks are due to Dr. Smith Woodward, F.E.S., for 

 allowing me to examine this interesting little collection, and to 

 Mr. M. A. C. Hinton for valued help in their determination. 



Notes on Lateritization in Sierra Leone, 



By F. Dixey, M.Sc, F.G.S., Government Geologist, Sierra Leone. 



{Published by permissiori of H.E. the Governor of Sierra Leone.) 



(PLATE III.) 



Contents. 



1. Introduction. 



2. (i) Lateritization of the Norite. 



(ii) Alteration of Norite rich in Iron. 



3. Lateritization of Granitic Rocks. 



4. (i) Lateritization of Detrital Dejoosits. 

 (ii) Lateritic Iron-ores. 



5. Lateritization as a Factor in Cave Production. 



6. Summary. 



I. Inteoduction. 

 TN the course of the Geological Survey of Sierra Leone in 1918- 

 -L 1919 certain features of interest were observed in the lateritization 

 of the norite ^ of the Colony and of the Pleistocene deposits which 

 form the coastal plain surrounding the norite mass. These features 

 form the basis of the following notes. 



Laterite is a rock of variable composition, consisting of a mixture 

 of hydrated oxides of iron, aluminium, titanium, and, rarely, 

 manganese, and formed by the sujoerficial decomposition of certain 

 rocks ; it is a residual accumulation resulting from the removal 

 in solution from the rocks affected of combined silica, lime, magnesia, 

 soda, and potash. It may be formed by the alteration in situ of 

 original rocks or by the alteration of detrital dejDosits ; the first of 

 these two products is termed high-level or residual laterite, and the 

 second, low-level or detrital laterite, or, as suggested by Dr. L. L. 

 Fermor, " lateritite." ^ The detrital laterite is made up of the 

 detritus of other forms of laterite, mixed with the quartzose and 

 argillaceous detritus of granitic and other rocks ; the whole is 

 cemented together by the action of water on the lateritic constituents. 



' The present author hopes to publish an account of this norite at an 

 earlj;^ date. 



2 Fermor, " What is Laterite ? " : Geol. Mag., 1911, p. 507. 



