Vol.65.] NEPHRITE AND MAGNESIAN K0CKS OF NEW ZEALAND. 357 



passes gradually into a serpentinized dunite ; but fresh dunite does 

 not appear in this section, which is closed by the tough red slates 

 and breccias of the Te Anau Series. 



Section 2 (Dun Mountain tramway). — This tramway, 

 which was built many years ago to convey into Nelson the chrome- 

 ore from the workings on the Dun Mountain, gives an excellent 

 section (fig. 3, p. 256). The slates pass down into a fairly broad 

 band of limestones, which pass gradually, as before, into the garnet- 

 pyroxene rock of the contact-zone, carrying occasional included 

 masses of baked and indurated slates. This is succeeded by the 

 wide serpentine belt, with irregular inclusions and veins (in its 

 western portion) of garnet-rock and of wollastonite. After about 

 2 miles, the serpentine is succeeded by a coarse pyroxenite, with 

 associated saussurite-gabbro and garnet-gabbro. Chromite occurs 

 ■segregated conspicuously in the neighbourhood of the pyroxenite, 

 where it was mined to some extent in former years. Then follow 

 serpentines and pyroxenites bordering the olivine-chromite rock or 

 dunite. This latter occupies an apparently isolated area of about 

 half a square mile on the summit of the Dun Mountain, where it 

 weathers with the formation of a thick reddish crust ; but coarse 

 scree-deposits give it the appearance of having a much more 

 extensive outcrop. Towards the north-west it sometimes passes 

 by degrees into serpentine, and at other times it is succeeded by 

 pyroxenite with scarcely any marginal variation. The dunite is 

 bordered on the south-east by the Te Anau Slates. 



Davis believed that the dunite was intruded later than the mass 

 •of the serpentines [7, p. 117], but the evidence adduced by him in 

 support of this is inconclusive. On the other hand, the .gradual 

 passage which can be traced in places from fresh dunite into 

 serpentine is strong evidence that the whole series forms a con- 

 temporaneous differentiated complex. 



Petrography of the Intrusive Rocks. 



Dunite (PI. XV, fig. 2). — This rock has been described by 

 previous workers [14, pp. 154-55]. The associated chromite occurs 

 as segregations of fairly pure ore, chiefly along the margins between 

 the dunite or serpentine and the pyroxenite. Microscopic examina- 

 tion shows that chromite was the first constituent to separate out 

 during rock-differentiation. It is worthy of note that this mineral 

 also occurs, in places, embedded in the garnet-rock of the contact- 

 zone. 



Serpentine. — The massive serpentines show all stages in the 

 alteration of olivine, though generally the rocks are serpentinized 

 and veined intricately with secondary chrysotile. The more 

 foliated serpentines show microscopically the finely laminated 



Q. J. G. S. No. 259. 2 b 



