part 4] NOEiTE or sierea leone. 321 



covered with a thin ferruginous skin, which possesses a dull 

 metallic lustre, is dark brown to black, and very hard. 



A more detailed account of the lateritization of the norite has 

 been given elsewhere.^ 



At Mount Aureol the lateritization of well-jointed norite has 

 given rise to a curious effect, in the form of a slightlj-raised and 

 resrular lattice-work on what is now the floor of an old bari-ack- 

 square. Originally the norite at this place was strongl}^ affected 

 by two series of joints which ran at right angles ; there was also a 

 third series of joints, running obliquely to the others, and less well 

 developed. The joints at right angles formed a large number of 

 small rectangles with sides measuring as much as 12 inches in 

 length. Lateritization first proceeded along the joints, and then 

 it attacked the intervening spaces ; the first-formed laterite was, 

 however, more resistant than the second, and this relative hardness 

 ultimately led to the formation of a raised rectangular pattern on 

 the floor of the square. 



(8 a) Iron-Ores in the Norite. 



The iron-ores that occur in the norite are magnetite, ilmenite, 

 and intermediate forms : octahedra of magnetite have been collected, 

 as also skeletal crystals of ilmenite ; but in general the ores are 

 not distinguishable in the field, and the term magnetite will be 

 used here to include them all. All forms of the norite include 

 magnetite as larger or smaller disseminated grains : it is developed 

 also as spongy growths, or as dense shapeless masses measuring up 

 to 3 inches in length, and even as streaks and seams several inches 

 thick, formed as the result of segregation in the banded norite. 

 Locally, the norite is very rich in such ores ; but the occurrences 

 are not individually large enough for commercial exploitation. 



A considerable quantity of iron-ore crops out along the fore- 

 shore west of Aberdeen Creek. The norite at this place is highly 

 ferruginous, and contains numerous seams of magnetite intergrown 

 Avith variable amounts of felspar ; nevertheless, almost all the iron 

 exposed is in the form of red oxide (hiematite) due to the alteration 

 of the magnetite in sifit, and to concentration of the iron with 

 progressive lateritization of the norite. The foreshore south of 

 York over a distance of some hundreds of yards consists of coarse 

 norite richly impregnated with magnetite as small segregations 

 and as narrov/ seams, and north of John Obey is a small exposure 

 of coarse norite showing magnetite-crystals (measuring as much 

 as 3 inches in length) intergrown with pyroxenes. 



The coastal plain which extends intermittently all around the- 

 Colony consists almost entirely of detritus washed down from 

 the mountains, and since the iron-ore (when weathered out from 

 the parent rock) is carried dow^n wards in the form of grains and 



^ See F. Dixey, ' Notes on Lateritization in Sierra Leone ' Geol. Mag-. 

 1920, p. 211 ; also W. M. Davis, ' Physiographic Relations of Laterite ' GeoL, 

 Mag. 1920, p. 429. 



