S22 DE. r. DiXEY ox THE [vol. Ixxviii, 



lumps of magnetite, it follows that tlie detrital accumnlatioiis 

 should contain a large quantity of iron, a quantity v.hich decreases 

 as the distance from the mountains increases. Investigation 

 confirms this, and in one locality on the north side a considerable 

 deposit of iron-ore has been found along the inner margin of the 

 plain. Most of this iron occurs now in the form of strings and 

 small masses of haematite runnino- throusrh the laterite. which is 

 itself in places sufficiently rich to rank as an ore. Elsewhere in 

 the beds constitutino- the plain there is always quite enough iron- 

 oxide to form a thick hard crust of laterite. 



(Sb) Otlier Economic ^lineral?. 



Economic minei-als other than iron-ores seem to be developed 

 only to a very slight extent in the norite, and this is probably due 

 to the absence of basic and marginal phases to the intrusion. 

 An analysis published in 1910 indicated 3-28 per cent, of chromic 

 oxide in a specimen of iron-ore taken from 'Bathurst 3Iountain." ^ 

 The present investigation, however, has not furnished any indica- 

 tion of this amount, either in the field or in thin section, and 

 recent analyses carried out in connexion ^ith the work do not 

 show in any one case more than 0*1 percent, of the oxide; in 

 most cases it is altogether absent. Xickel-oxide also has been 

 shown by one of these analyses to be present up to 0-1 per cent. 

 Sulphides are rejDresented only by grains of pyrites which are seen 

 but rarely in thin sections of the rock. A series of concentrates of 

 stream- gravels has yielded nothing of interest, other than small 

 quantities of rutile.- Bauxite containing over 51 j^er cent, of 

 alumina occurs in considerable Cjuantity, as a decomposition-product 

 of the norite.-^ 



III. Peteo&eathical Xotes. 

 (I) The Xorite-Aplite. 



An examination of a series of thin sections has shown that the 

 veins of norite-aplite become increasingly acid in composition 

 towards their extremities. 



A specimen from a vein of medium texture, 9 inches wide, was 

 found to consist chiefly of acid plagioclase, with much micropeg- 

 matite and a little quartz and orthoclase. In addition, it contained 

 small amounts of enstatite, brown hornblende, brown biotite, 

 magnetite, and apatite (the last-named as clouds of minute parallel 

 needles in the felspar, and as fairly numerous grains and prisms). 

 The plagioclase, from consideration of the refi-active index and 



^ ' Iron-Ore Eesources of tlie World ' Stockholm, vol. ii (1910) p. 1029. 



- Xote the occurrence of rutile in certaui anorthosites of Quebec : 

 J. P. Idclings. ' Igneous Eocks ' vol. ii (1913) p. 358. 



^ For an analvsis of bauxite, see F. Dixev. Eep. Geol. Surv. Sierra Leone. 

 1921, p. 18. ' 



