Sd'o UK. f. DIXLY ox THE [yoI. IxXVlii, 



and. where certain bands of the norite •were deeply later Ltized at their outcrop 

 while neighboiu'ing bands were practically unaltered, he concluded that the 

 norite had locally forced its waj' along the bedding of the ' tuffs ' in the form 

 of small sills. Finally, since the crystalline schists of pre-Cambrian age 

 would probably be steeply inclined if at all present in the country, whereas 

 the ' tuffs ' were horizontally bedded and were presumably • younger " than 

 the schists, therefore the norite itself, ' younger ' than the " tuffs.' must neces- 

 sarily be much later than pre-Cambrian in age. Giirich considered these 

 conclusions to be supported by analogy with the conditions prevailing in 

 the Los Islands. 



(6j Relation to other AVest African Basic Rocks, and 

 also to a West African ]^Iagiiesian Province. 



Two great petrograpliic groups, both made up of rocks rich in 

 hypersthene, are known ^ to exist in West Africa; thev consist 

 respectively of a series of peridotites, gabbros, and diabases 

 occurring principally in French Guinea, and of a series ranging 

 from norites to hypersthene-granites, which has been compared 

 by Prof. Lacroix with the Charnockite Series of India. The 

 peridotites of the first series approach wehrlite in composition, 

 and consist mainlv of olivine with a little diallasre. The diabases 

 include types rich in olivine, hypersthene, and a monoclinic 

 magnesian pyroxene respectively. The gabbros form the most 

 important group, and they occur in large masses : they consist of 

 felspars ranging from labradorite to bytownite, diallage, and small 

 quantities of a magnesian pyroxene (pigeonite) ; hypersthene and 

 olivine occur in variable amounts. The felspars are fi'ee from 

 zoning, and show a tendency to parallelism ; the structure 

 approaches an ophitic character. These gabbros show varieties 

 approaching towards norite and ti-octolite respectively ; but, so far, 

 actual examples of these types are not known in French Gruinea. 

 From the limited information available at the time when he wrote, 

 Prof. Lacroix considered that the rocks around Freetown were 

 similar to these gabbros ; and it is interesting to note that the 

 present investigation not only confirms this similarity, but goes to 

 show that the Sierra-Leone norite is probably part of the same 

 srreat basic series. 



The other great series, analogous to the Charnockite Series, was 

 considered by Lacroix to occur almost exclusively on the Ivory 

 Coast ; but from recent Survey work in the Sierra-Leone Protec- 

 torate it appears that rocks similar to these are well developed 

 among the old crystalline schists in this area also.- The Sierra- 

 Leone norite, however, differs considerably from this series, notably 

 in the absence of granular structure, in the absence of quaiiiz, and 

 in th^ abundance of olivine and augite. These mineralogical difi'er- 

 <enees are expressed chemically to some extent in that the Sierra- 



'^ A. Lacroix, ' Les Syenites Xepheliniques de I'Archipel de Los ' Xouv. 

 Arch. Mus. Hist. Xat. Paris, ser. 5, vol. iii (1911) p. 108: id. ' Sm- I'Existence 

 a la Cote d'lvoixe d'une Serie Petrographique comparable a celle de la 

 Charnockite " C.E. Acad. Sci. Paris, rol. cl (1910) p. 18. 



- F. Dixey, "Reps. Geol. Sury. Sierra Leore, 1920-21. 



