302 DE. r. DTXET ox THE ^vol. Ixxviii. 



smaller platforms can be recognized at intermediate and at 

 lower levels ; those at the lower levels correspond in height ta 

 low liat-topped hills and ridges that ocenr in neighbouring parta 

 of the Protectorate. 



Erosion of another type has given the stock a deeply-incised 

 draina^e-svstem which is remai-kable for its regularity and sim^ 

 plicity (see map. fig. 1, p. 300). The principal watersheds are the 

 three parallel ridges Avhich ti'averse the Colony from end to end^ 

 and two groups of spurs which spread fan-wise from each end of 

 the mountain-mass ; the ridg-es throw off at ri^ht ano-les a 

 number of short spurs which, on the northern side at least, are of 

 very regular form. This arrangement is probably due to the fact 

 that the master-joints of the stock are parallel and transverse ta 

 its length in the iniddle portion, but radial at the ends ; and. as the 

 original covering was denuded, the young sti'eams discovered these 

 lines of weakness and developed along them. Many bold cliffs, 

 and joint-faces rising out of the dense bush extend along the sides. 

 of the main and the tributary valleys. 



(3) Size ar.d Form of the Occurrence. 



The Sierra -Leone norite is remarkable in bein^ the largest 

 intrusion of norite of which I have been able to ti'ace any record. 

 Elsewhere, however, norite is well known to occur in intimate 

 association with other rocks which form intrusions of enormous, 

 size. The norite of Sierra Leone is further remarkable in that it 

 occurs as a stock which, regarded as a basic stock, is of veiy con- 

 siderable size. Most of the known gTeater basic and ultrabasie 

 masses, with the exception of certain anorthosites, occur not as 

 stocks, but rather in the form of enormous laccolites and sills :: 

 as, for example, the Busliveldt,^ Insizwa,- and Sudbury ^ intrusions. 

 Both as a basic mass and as a norite-mass, the SieiTa-Leone norite 

 is far more considerable than any of the British occurrences.^ 



It is worthy of notice that, so far as can be observed, the Sierra- 

 Leone norite as a whole shows neither a marginal nor a sti*atiform 

 structure ; and further, that it is isolated from all other igneous, 

 rocks, apart from the dolerite-dykes and the small veins of norite- 

 aplite which traverse it. In these respects it differs considerably 

 from the important basic masses ah'eady mentioned : the Bushveldt,. 

 Insizwa. Sudbury, and many other great laccolites and sills exhibit 

 a stratiform armngement ascribed to the control of differentiatic-n 



^ G. A. r. Molen^-aaff. ' Geologie de la Eepublique Sud-Africaine du Trans- 

 vaal ' Bull. Soc. Geol. France, ser. 4, toI. i (1901) p. 62. 



- A. L. Du Toit, 'Eeport on the Copper-Xickel Deposits of tlie Insizwa, cfcc." 

 loth Ann. Eep. Geol. Comm. Cape of Good Hope (1910) p. 111. 



^ A. E. Barlow. ' Xickel & Cojjper Deposits of the Sudbury Mioiiig- District" 

 Ann. Eep. Geol. Snrv. Canada, vol. xiv, pt. H (1904). 



■* Xote, however, that certain norites recently described from the neigh- 

 bonrhood of Himtly in Aberdeenshire attain a considerable size — teste H. H. 

 Eead. in' The Contaminated Magmas of Aberdeenshire ' (paper read to Sect. C., 

 Brit. Assoc. Edinburgh. 1921 ^ 



