part 4] IfOEITE OF SIEEEA LEONE. 307 



Leone norite is poorer in alkalies, generally in silica, and richer in 

 magnesia and lime., 



Not the least interesting feature concerning the study of the 

 Sierra-Leone rocks of both the Colony and the Protectorate is that 

 they tend to confirm Lacroix's statement as to the widespread 

 occurrence in West Africa of rocks rich in magnesium. This 

 richness in magnesium, which generally gives rise to more or less 

 abundant hypersthene,i leads to the conception of a West 

 -African Magnesian Province. ^ 



II. Field Obseevatioxs. 



(1) Tlie Older or Normal Norite. 



The oldest and most important member of the series is a norite 

 of medimn texture, which in succeeding paragraphs Avill be defined 

 as the normal norite. It varies but little in composition, 

 texture, and macroscopic characters from one end of the Colony 

 to the other. It consists essentially of a plagioclase (labradorite), 

 augite, and a rhombic pyroxene, with varying proportions of 

 olivine ; iron-ore and apatite occur in small amount, and are 

 unevenly distributed. An analysis of a single specimen of this 

 rock has given the following result : — 



Per cent. 



Silica 48-25 



Alumina 21'54 



Ferric Oxide 3-59 



Ferrous Oxide 3"60 



Manganous Oxide 0*45 



Lime 9*73 



Magnesia 6*30 



Potash 1-60 



Soda 1-76 



Phosphoric acid O'OIS 



Titanic acid 0*26 



Water above 105° C 2-90 



Total 99-99 



Prof. S. J. Shand has classified the rock as a leucocratic olivine- 

 Tiorite (^g — to l^ — subnorite).^ The norite is tough and commonly 

 grey in colour; with increasing iron content it becomes denser, 

 harder, and splintery, gives a strong metallic ring when struck 

 with the hammer, and weathers with a black lustrous surface. 



^ The formation of hypersthene will depend chiefly on the relative amounts 

 in the magma of both alumina and lime ; hypersthene would be unlikely to 

 form until these oxides had been taken up by felspars and augite. A mono- 

 clinic magnesia-pyroxene may accompany and even replace hypersthene in 

 the highly magnesian rocks : see A. Lacroix, Nouv. Arch. Mus. Hist. Nat. 

 Paris, ser. 5, vol. iii (1911) p. 117, and also J. V. Elsden, 'The St. David's 

 Head Rock Series ' Q. J. G. S. vol. Ixiv (1908) p. 288. 



^ See F. Dixey, 'The Magnesian Group of Igneous Rocks' Geol. Mag. 

 1921, p. 485. 



•^ ' The Norite of the Sierra Leone ' Geol. Mag. 1918, p. 23. 



