part 4] NORiTE or sieeea leois^e. 325* 



black inclusions ; these inclusions, which apparently consist of iron- 

 ore, may occur in parallel bands along certain twin-lainellse, or 

 may be scattered irregularl}^ throughout the crystal. They 

 generally lie parallel to two or more crystallographic planes in the 

 felspar. They are much more numerous in the felspar of the 

 younger norites ; this felspar, approaching bytownite in com- 

 position, is more basic than that of the normal norite. The felspar 

 of the beerbachite, on the other hand, is slightly more acid than^ 

 that of the normal norite. The plagioclase occurring in the aplite- 

 veins belongs to the acid end of the soda-lime series ; it exhibits' 

 minute twin-striation, and its variable composition is indicated b}^ 

 strong zoning. Finally, the plagioclase of the dolerite ranges in; 

 composition from basic labradorite to acid oligoclase ; the basic 

 portions of a crystal form a turbid central area, whereas the acid 

 parts make up a clear relatively'' narrow external zone. Porphyritic 

 plagioclases occur locally in some of the fmer-grained phases of the 

 norite ; the phenocrysts do not differ appreciably from the ground- 

 mass felspars in composition, and moreover, like the latter, they 

 contain occasional strings and clots of granular quartz. ^ 



Orthoclase occurs as a constituent of the aplite-veins, and it is 

 present also in the interstitial material of the dolerite. 



Localh^, the more basic felspars show small quantities of a 

 pale-brown alteration -product, which is developed in fibrous form. 

 transversely to cracks ; sometimes it appears as minute rhombs 

 [C 48]. It is of low refractive index and feeble birefringence. 



The felspar of the norite enters into an interesting series of 

 intergrowths with magnetite, augite, and olivine (see below, 

 p. 329). 



Pyroxenes, — ISText to the felspars the pyroxenes are the most 

 abundant minerals in the norite. They consist essentially of augite 

 and hypersthene ; but there occur also small quantities of a pale 

 augite, and of a colourless rhombic pyroxene. Enstatite occurs in 

 both the aplite and the dolerite. 



The augite of the norite is chiefly a smok3^-brown diallage ; 

 schillerization, as lines of minute grains and rods of iron -ore, is 

 sometimes strongly developed. 'Not infrequently the augite faintly 

 simulates the pleochroism and polarization-colours of the hyper- 

 sthene ; a similar feature has been noted in the augite of the 

 Kilsyth- Cro}^ intrusions by Mr. Gr. W. Tyrrell, who attributes it 

 to the presence of the hypersthene-molecule in the augite. ^ A 

 colourless to pale-green augite, probably diopside, occurs in the 

 aplite-veins. 



The hypersthene of the norite is associated with the augite in 

 variable projDortions ; generallj^, however, it is subordinate to the 

 augite. It is less common in the older than in the younger norite.. 



^ Quartz occurs similarly in the Charnockite Series : see A. Harker, 

 ' The Natural History of Igneous Eocks' 1909, p. 261. 



- ' Geology & Petrology of the Intrusions of the Kilsyth-Croy District 

 (Dumbartonshire) ' Geol. Mag. 1909, p. 306. 



