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2. Olivine-Norite. 



Olivine-Norite : Rosenbusch. 



Hyperite : TiJrnebohm. 



Sp. Gr. 2-90. 



The rocks of deep seated or plutonic origin, of a coarse 

 grained granitoid texture, composed of a basic plagioclase 

 felspar, and the laminated augite called diallage, with or 

 without olivine, are known as gabbros. In addition to, or in 

 place of, the diallage, common augite is sometimes met with, 

 without, however, affecting the name of the rock. Further, 

 the monoclinic pyroxene is occasionally replaced by a 

 rhombic one, and when this happens we have norite. At the 

 same time gabbros exist, in which both the rhombic and 

 monoclinic forms of pyroxene occur, and this tends to show 

 that the line separating gabbros and norites is not a sharp 

 one. Still it is the habit of modern petrographers to reserve 

 the name norite for those gabbros in which the pyroxene is 

 solely or mostly rhombic. Olivine-norite is the hyperite or 

 hypersthenite of Norwegian geologists. It has also been 

 recorded from Finland, France, Austria, Canada, United 

 States, Sumatra, and Madagascar ; always associated with 

 gabbros. It does not appear to occur in Britain. 



The macroscopical appearance of this rock is perfectly 

 fresh ; it shows a somewhat compact but coarse granitoid 

 structure from the closely embedded plates of pyroxene. 

 In colour it is a mottled dark greenish black. The pyroxene 

 is the darkest coloured constituent, it being almost black, 

 with a glimmering surface; the interspaces which are 

 occupied by the felspar and olivine are of much finer texture, 

 rather dull and pale green in colour. It is found forming a 

 mass of considerable extent associated with the serpentine 

 and augite-syenite of the district. 



Microscopical Appearance. — Its texture is granitic, that 

 is to say, the free crystallisation of the constituent minerals 

 has been interfered with mutually, and the forms (with the 

 exception of the olivine) are those of broad irregular plates. 

 The component minerals in order of importance are bronzite, 

 plagioclase felspar, olivine, a little monoclinic pyroxene, 

 and titaniferous iron oxide in grains. 



Rhombic Pyroxene. — The irregular ophitic plates of this 

 mineral have no tendency to idiomorphism. It is light 

 brown to bronze in colour with slight pleochroism where 

 clearest, markedly pleochroic where the lustre is most 

 bronzy. The striation resembles that of diallage, but the 

 straight extinction and pleochroism distinguish it. The 

 interference colours between crossed nicols are of a lower order 

 than those of monoclinic pyroxene. Enstatite proper is 



