310 DR. r. DiXET ON THE [vol. Ixxviii, 



(iv) Toke. — Both normal and coarse norite cnt by pegmatite- veins up to 



18 inches in width ; normal norite pierced by fine siliceous veins, 

 (v) Kent, Cape Shilling-, and Banana Islands. — Normal norite 

 variably ferruginous ; banding beautifully developed on the foreshore 

 near the church at Kent. 



(2 a) The Younger Norites and Norite-Pegmatite. 



The younger norites form two or more series of mtrusions 

 cutting tlie older or normal norite ; but they are collectively of 

 small bulk, as compared with the original intrusion. In compo- 

 sition they differ little from the normal norite, and are distinguished 

 principally by their texture, which is generally much coarser than 

 that of the older rocks. There are, however, several other 

 distinguishing features of great use in the field : when weathered, 

 tlie younger norites are dark gre}'^ in colour, and possess a curious 

 scaly surface. Moreover, since jointing is but slightly developed 

 in them, their outcrops form large hummocks or well-rounded 

 residual boulders. The older norite, on the other hand, becomes 

 dull-red on exposure and, being well jointed, weathers into 

 rectangular blocks with sharp edges and clean-cut surfaces. One 

 other important point of difference remains : the normal norite is 

 generally well-banded, whereas the younger norites are free from 

 banding, although the felspars in them are often more or less 

 parallel. (See PI. XVI, fig. 2.) 



When thin sections of the two rocks are compared, however, 

 'Certain other differences are apparent : the coarser or j^ounger 

 norite is seen in most cases to be richer in hypersthene, in iron- 

 oi-es, and in micrographic intergrow^ths of felspar and augite ; 

 whereas it is generally poorer in felspar and also in apatite. 

 Moreover, the felspars of the coarser rock are slightly more basic, 

 and the darker colour which they give to the hand-specimens is 

 seen to be due to innumerable opaque inclusions of ver}^ small 

 size. 



A typical specimen of the younger norite consists essentially of 

 grey sub-parallel felspars, dark-brown to black h^^'persthene, and a 

 little granular augite, commonly diallage. The felspars are 

 tabular in habit, and generally about 1 inch long. Olivine and 

 magnetite are usually common : the former as rounded grains and 

 sometimes as plates enclosing felspar-laths, and the latter both as 

 grains and as interstitial growths. Locally, the magnetite forms 

 lenticular streaks and abundant sniall masses ranging up to 2 inches 

 in length. A partial analysis of a single specimen of this rock 

 shows : — Silica 41 "16 per cent., alumina 11 T3, iron oxides 22 "30, 

 and magnesia 17'68. This norite also contains small brown and 

 black patches, similar in some respects to those in the older rock, 

 I)ut more numerous. The black patches are pyroxenes in some 

 cases, and spongy growths of magnetite in others. The brown 

 patches are of two types which differ considerabl}'' in origin. 

 Those of the first type are small and fairly evenly distributed, and 

 they each consist of a rounded augite- crystal, typically about 



