part 4] NORITE OF SIEEEA LEONE. 305 



norite-complex itself, while probably later than pre-Cambrian, 

 is nevertheless of very great age ^ : — 



(1) The complex as a whole is unfoliated, although half -enclosed by highly- 

 foliated crystalline rocks. 



(2) The complex is elongated parallel to 



(a) The foliation of the crystalline rocks ^ ; 



(b) to the general trend of the coastline ; since, however, there is 



good reason to suppose that the present coastline was deter- 

 mined by a series of important more or less parallel faults, it 

 is probable that the complex originated as an intrusion along 

 a considerable fault running parallel with the foliation of the 

 crystalline rocks. 



(3) In its great size, its highly-magnesian character (resulting in the 

 frequent derelopment of rhombic pyroxenes), and the extraordinary freshness 

 of its constituent minerals, it presents a close analogy with the Charnockite 

 Series of India,-^ certain phases of the anorthosites of Canada,"* the Scandi- 

 navian norites, and other well-known intrusives all considered to be of pre- 

 Cambrian age. 



(4) The enormous amount of erosion undergone by the norite and the rocks 

 into which it intruded must have required a very long period of geological 

 time in which to take place. For instance, the complex, which despite 

 erosion is even now exposed to a depth of 3000 feet, m_ust have been covered 

 at the time of its intrusion by a thickness of many thousands of feet of 

 sediments or other rocks, all of which have now completely disappeared. 

 Also, within a radius of 70 miles on the landward side of the complex the only 

 rocks so far observed to rise above the low-lying Pleistocene sediments are 

 the crystaUine schists. These schists, which rarely attain a height of more 

 than 700 feet above sea-level, were invaded by gneissose granites such as now, 

 on account of their superior resisting powers, make up the high land forming 

 the north-eastern part of the Protectorate. But even these hard g-ranites, 

 which have been carved into a series of plateaux one above the other, do not 

 in general attain a height of more than 2000 feet above sea-level anywhere 

 within 130 miles of the isolated complex. Finally, certain of the extensive 

 platforms carved into the ancient crystalKne masses have their counterparts 

 on the norite-complex (see above, p. 301) ; one of these platforms of the 

 Protectorate, standing at a height of about 1200 feet, supports a thickness of 

 nearly 700 feet of (?) early Palasozoic sediments, which even now are prac- 

 tically horizontal.' Thus it is seen that the erosion of the complex and the 

 surrounding rocks must have been very great indeed. 



It may be noticed at this point that Giirich, in the paper cited above (p. 299), 

 concluded that the norite was considerably younger than the pre-Cambrian. 

 This conclusion, however, was based on several errors of observation : for 

 instance, he failed to distinguish the laterite formed by alteration of the 

 Pleistocene sediments from that derived from the norite by lateritization in 

 place (see below, p. 321), and, further, he regarded both of them as volcanic 

 rocks or tiiffs. He was, nevertheless, fairly well acquainted with laterite, as 

 we know it, from many parts of the West Coast. In consequence of this 

 confusion he considered the norite to be a laccolitic intrusion into the ' tuffs,' 



^ It is proposed to give additional evidence as to the great age of the 

 complex, based upon certain important relations existing between the norite 

 and the large basic intrusions of French Gruinea (referred to below) in a. 

 subsequent paper on the geology of Sierra Leone. 



2 F. Dixey, Eep. Geol. Surv. Sierra Leone, 1919, p. 8 ; also ibid. 1920. 



3 T. H. Plolland, Mem. Geol. Surv. India, vol. xxviii (1900) p. 119. 

 ■* A. Lawson, Amer. Geologist, vol. vii (1891) p. 153. 



^ F. Dixey, Hep. Geol. Surv. Sierra Leone, 1920. 



