part 4] 



NOEITE OF SIERRA LEONE. 



319 



The mineraloo:ical differences between the various members of 



the norite-complex may be smnmarized in tabular form 

 follows : — 



as 





6 



w 





s s 









(£ 



ni 





a3 







ce 



.2 ^ 



•S ^. 





CO 







. 



ce 







o 



O Q, 



^ 2 



PI 



f-l 



o 



^ 



r— 1 





o 



o 



■43 





ha 





_> 



o 



1 



o 



-4-2 

 ft 



j:3 







^ 



P^ 



^ 



O 



f-l 

 1— 1 



W 



w 



^ 



o 



O" 



Norite 



Beerbachite.. 



— 



— 







— 



! 

 1 







— 



— 



— 



— 





— 







Norite-Aplite 



— 



— 



— 











— . 



«=- 



Dolerite 



— 



~ 



1 









1 

 1 



— 





The relative frequency of occurrence of tlie minerals is represented by the 

 magnitude of the dashes as follows, in descending order : — mhb — — 



(6) Differentiation and Assimilation. 



Several stages of differentiation may be distinguished in the 

 Sierra-Leone rocks : — 



(i) Deep-seated differentiation. — The source from M^hich 

 the complex was derived underwent differentiation into a series of 

 cognate sub-magmas which, on the w^hole, became intruded in the 

 usual order of decreasing basicity : norites proper, rich in olivine, 

 were the first to arrive, and they were followed in turn b}^ norite- 

 pegmatite and beerbachite. Then came the aplite-veins, very small 

 in total volume, and finally a series of enstatite-dolerite dykes 

 free from olivine and often rich in interstitial acid felspars. 



(ii) Differentiation during intrusion. — The cooling of 

 the normal norite-magma on intrusion led to a partial separation of 

 the felspathic and ferromagnesian constituents in the liquid state. 

 This heterogeneous product was drawn out, as intrusion proceeded, 

 into bands which were fixed later by crystallization. The resultant 

 'banding was sometimes accentuated by small quantities of the 

 -coarse norite-magma, which Avere caught up by the normal norite- 

 inagma and interbanded with it. 



(iii) Differentiation in place. — This phase led to the 

 'Segregation of magnetite from the remaining constituents, and 

 ^produced the streaks of this mineral within and parallel to the 

 -margins of certain bands. Locally, the ferromagnesian constituents 

 iseparated (©ut in a similar manner, and formed small patches. 



