part 4] is^ORiTE of siehea leone. 329 



The presence of ilmenite can occasionally be detected bv charac- 

 teristic cleavage and also by white patches and highly refracting 

 borders of leucoxene ; generall}'', however, the iron-ore is too fresh 

 to show these decomposition-products. Narrow borders of p3a'0xene 

 .also occur, and scraps of biotite are often associated with the ore. 



Although iron-ore occurs so persistently in the norite, and may 

 -even occur locally as numerous ' schlieren ' up to 3 inches in thick- 

 ness and as small masses several inches long, there are unfortunately 

 110 indications that the ore is anywhere sufficiently concentrated to 

 be worth working for industrial purposes. 



Hornblende. — Grreen and brown varieties of this mineral occur 

 in the aplite-veins, frequently intergrown vs'itli biotite. It is 

 tibsent from the norite, except where produced by contact-meta- 

 morphism (see below, p. 341). 



Biotite. — This mineral normally occurs in the norite only as 

 minute brown flakes associated with the iron-ores ; it is fairly 

 common in the aplite-veins and in the dolerite, and frequently 

 iirises in the norite as a result of contact-metamorphism. 



Apatite. — As in the St. David's Head rocks,^ this mineral 

 ■occurs most commonly in the more acid members of the complex. 

 It forms countless minute needles and numerous grains and prisms 

 in the aplite-veins, and occurs abundantly also as small needles in 

 the interstitial matter of the dolerites. 



Zircon occurs sparingly in the complex in the form of minute 

 prisms, and r utile has been found in concentrates of river- 

 o-ravels. 



.» 



(4) Iiitergrowtlis oi^ the Common Minerals, and 

 Crystallization of the Norite-jMagma. 



One of the most interesting features of the Sierra- Leone norite 

 is the number of binary and ternary intergrowths occurring 

 between the principal minerals. 



Prof. J. H. L. Vogt,2 in an investigation into the sequence and 

 process of crj^stallization in gabbroidal rocks, noted in the 

 labradorite-norite of the Lofoten Islands a ternary S3^stem which 

 is closely paralleled in the Sierra-Leone norite. The labradorite- 

 norite referred to was 'chemically and mineralogically on the 

 boundary between labradorite-rock and norite'; it was jDorphja-itic, 

 with xenocrysts of labradorite. Vogt says (^op. cit. pp. 91, 95) : — 



' We may distinguish between the following three stages of crystallization : — 



[A.] (1) Plagioclase alone (phenocrysts) ; 



(2) then plagioclase and magnetite contemporaneously ; 



(3) plagioclase, magnetite, and pyroxene-minerals, besides biotite, 



contemporaneously. 



1 J. V. Elsden, ' The St. David's Head Rock Series ' Q. J. G. S. vol. Ixiv 

 (1908) p. 289. 2 j5jYZ. vol. Ixv (1909) p. 81. 



