part 4] NOEITE OF SIEERA LEONE. 817 



those of Sierra Leone ; for instance, they apparently cut only the 

 more acid members of the series, and they are free from ferro- 

 magnesian constituents and micropegmatite. Moreover, they are 

 considered to be of contemporaneous origin, whereas those of 

 Sierra Leone are distinctly younger than the enclosing rock. 



(5) The Dolerites. 



The injection of a number of dolerite-dykes, seen in different 

 localities to cut all the earlier intrusions, marked the last phase in 

 the history of the complex. It is quite possible, however, since in 

 the field the dolerite is distinguished with difficulty from certain 

 ■of the later intrusions of norite, that a few of the dolerite-dykes 

 were intruded considerably in advance of the main series. ^ The 

 dykes not infrequently made their way along the same channels as 

 the preceding aplite-veins ; this procedure is indicated chiefly by 

 the presence of patches of aplite still adhering to their original 

 walls, and also by a whitish alteration-product of the walls due to 

 ii contact-effect characteristic of the aplite. Sometimes, however, 

 the invading magma forced its way between an aplite-vein and its 

 wall-rocks, resulting in the inclusion of long irregular lenticles of 

 aplite parallel to the margins of the dolerite. An excellent ex- 

 ample of this may be seen on the foreshore a short distance north 

 of York ; here, included in a dolerite-dyke about a foot wide, is a 

 highly-corroded aplite-xenolith 4 feet long and about 6 inches in 

 maximum width. Several smaller xenoliths accompany the bigger 

 •one. 



The dykes show interesting affinities to both the norite and the 

 norite-aplite. To the norite they are related by the presence of a 

 rhombic pyroxene which accompanies the augite : in the smaller 

 dykes and in the marginal phases of the larger dykes, the rhombic 

 pyroxene is enstatite ; but in the central portion of the larger dykes 

 the enstatite is replaced by a form approaching the common 

 hypersthene of the norite [C. 121].^ In this connexion it may be 

 pointed out that, with regard to certain basic rocks near St. David's, 

 Dr. J. V. Elsden ^ considered the thin intrusions of enstatite-diabase 

 to differ from neighbouring quartz-norites only in ophitic tendency. 

 To the norite-aplite, on the other hand, the dolerite-dj'^kes are related 

 by the presence of interstitial orthoclase and acid plagioclase, often 

 considerable in amount, together with enstatite, some biotite, and 

 innumerable needles of apatite. Thus it would appear, particularly 

 in view of the field relations, that the aplite-veins were the fore- 

 runners of the dolerite, rather than the successors of the norite. 



^ In one locality, near York, coarse dolerite is traversed by a dolerite-dyke 

 possessing chilled margins. 



2 Throughout this paper the numbers in square brackets refer to thin 

 sections of the ' Colony ' rocks. 



•^ ' On the Igneous Rocks occurring between St. David's Head & Strumble 

 Head (Pembrokeshire) ' Q. J. G. S. vol. Ixi (1905) p. 591. 



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