part 4J NORITE OF SIERRA LEONE, 343 



section the younger norites, as compared with the older norite, 

 generally seem to be richer in liA^persthene, in iron-ore, and in 

 micrographic intergrowths o£ felspar and augite ; moreover, the 

 felspars of the younger norites are slightly more basic. 



(8) The field-relations of the older and younger norites are 

 •often ver}^ complex, owing to the irregular manner in which the 

 younger rocks broke through the older ; highly-corroded xenoliths 

 of the older norite are abundantly included in the younger norites, 

 iind there is much evidence to show that the older rock has been 

 incorporated on a large scale in the younger rocks. 



(9) The beerbachite intrusions are generallj^ small, and of 

 irregular form. The larger masses actively disintegrated and 

 assimilated the preceding intrusions of norite. The beerbachite 

 consists essentially of fine-grained granular labradorite and hyper- 

 ;sthene, with some augite and iron-ore. 



(10) The norite-aplite veins occur in the norite in the form 

 of fine threads consisting mainly of quartz and micropegmatite. 

 These threads are, however, only the relatively-acid terminations 

 •of wider veins which are sometimes seen to attain a thickness of as 

 much as 9 inches. The thick j^ortions of the veins are light in 

 colour, and possess a pale greenish tinge ; they are of medium 

 "texture, and consist chiefl}^ of acid soda-lime felspar, orthoclase, 

 quartz, and micropegmatite, with small quantities of pyroxene, 

 hornblende, biotite, and apatite. 



(11) The norite-aplite veins were succeeded by a series of more 

 •or less ophitic enstatifce-dolerite dykes, free from olivine and rich 

 "in interstitial acid felspars. In many respects, these dykes closely 

 resemble the well-known British quartz-dolerites ; they are, never- 

 theless, free from quartz and micropegmatite. 



(12) Several stages of differentiation may be distinguished in 

 the complex : for instance, deep-seated differentiation, differentia- 

 tion during intrusion, and differentiation in place. 



(13) The older norite was subjected to varying degrees of meta- 

 morphism by the younger members of the complex. A resultant 

 •effect of frequent occurrence in the norite was the recrystallization 

 of parts of the felspar and augite into a graphic intergrowth of 

 these two minerals. The intrusives more closely related to the 

 norite in composition caused active corrosion and recrystallization, 

 but did not set up new minerals ; the relatively acid intrusions, 

 however, such as the norite-aplite and the dolerite, converted the 

 -pyroxenes to hornblende and modified the felspars, besides effecting 

 other changes. 



(14) Iron-ores occur in the norite only as disseminated grains, 

 as small masses up to several inches in length, and as narrow 

 schlieren; they are highly titaniferous. Sulphides and other 

 economic minerals often associated with noritic intrusions are rare 

 ■or absent. 



(15) Laterite is developed on the rocks of the complex only to a 

 slight degree ; it occurs principally on ancient platforms carved 

 into the mass, and on parts of the complex characterized by 

 numerous joints. 



