318 DE. F. DIXET 0]N' THE [vol. lxXYm\ 



The dykes consist essentially of a more or less ophitic enstatite- 

 dolerite, free from olivine and containing variable amounts of 

 interstitial acid felspar (see Petrographical Notes, p. 324). The 

 specific gravity of the rock is 3 '08. Thus we have in the one- 

 area two series of basic intrusions, not differing greatly in age,, 

 the one (norite) characterized by the presence of olivine, and the 

 other (dolerite) free from this mineral ; this association recalls, 

 the well-known occurrence of both olivine- and quartz-dolerites. 

 among the Carboniferous intrusives of the Edinburgh district. ^ 



The Sierra-Leone dolerite-dykes are exposed in considerable num- 

 bers along the foreshore on the south- vvestern coast of the Colon}^ ; 

 elsewhere, with the exception of one cutting across the lower part 

 of the Charlotte Falls, they have not been observed, probably 

 because the rocks that may contain tliem are poorly exposed. 

 Most of them are of an irregularly branching habit, and single 

 exanrples can rarely be traced far enough to give a mean strike. 

 The average direction of a number of which the strike could be 

 measured, was north S5° west. They vary in width from a few 

 inches to nearly 20 feet ; the narrower are fine-grained and compact 

 with sharp chilled margins, whereas the wider are coarse and are 

 sometimes porphyritic with augite and felspar. Near York, one of 

 the coarser dykes contains numerous skeletal crj'stals of ilmenite, 

 and it cuts a series of fine aplitic veins in the norite. Others 

 are vesicular, either along their margins or centrally. 



In addition to the dykes several masses of coarse dolerite occur 

 in the norite ; their boundaries are ill-defined, and their exact 

 relation to the enclosing rocks difficult to determine. 



The Sierra -Leone dolerites closely resemble certain quartz- 

 dolerites which occur elsewhere. For instance, they are similar ii> 

 several respects, particularly the strong zoning of the felspars,^ to 

 the Carboniferous quartz-dolerites of Scotland ; but the pale 

 rhombic pyroxene of these Scottish rocks is identified as a form of 

 hypersthene. Despite the obvious resemblances, however, it should 

 be noted that both quartz and micropegmatite are apparently 

 absent from the Sierra-Leone dolerite, the interstitial matter of 

 which consists only of orthoclase and highly-acid plagioclase. 

 Like that of the Scottish quartz-dolerites and of most of the rocks- 

 mentioned below, this interstitial matter is riddled with long- 

 needles of apatite. Other similar rocks are the Ratho intrusion' 

 near Edinburgh, -^ the Whin Sill,^ certain intrusions of the Kilsyth- 

 Croy district (Dumbartonshire),^ and those of Arran.^ 



^ ' The Geology of the Neig-hbourhood of Edinburgh ' Mem, Geol. Sury. 

 1910, p. 289. 



^ Ihid. p. 305 & tdI. is, fig. 3. 



3 J. J. H. Teall,'"' British Petrography' 1888, p. 190 ; & J. S. Flett, 'The- 

 Geology of the Neighbourhood of Edinburgh ' Mem. Geol. Surv. 1910, p. 306. 



^ J. J. H. Teall, Q. J. G. S. vol. xl (1884) p. 640. 



5 G. W. Tyrrell. Geol. Mag. 1909, p. 305. 



6 A. Harker, ' The Geology of North Arran, &c.' Mem. Geol. Siu-y. 1903. 

 p. 111. 



