Vol. 65.] AND MAGNESIAN KOCKS OF NEW ZEALAND. 359 



SiOo 42-45 



A1 2 3 23-27 



Fe;O s 7-1)1 



FeO 1-22 



CaO : 21-52 



MgO 1-45 



H 2 2-62 



Total 10044 



Serpentine-amphibole rock (PI. XV, fig. 5). — This type, 

 which also occurs in lenticles in association with the ores, is dark 

 greenish grey in the hand-specimen, and consists of a moderately 

 hard serpentine, veined by a massive grey mineral of considerable 

 hardness (6 to 7). Under the microscope this latter appears as 

 numerous branching and intersecting veins of a grey-brown, slightly 

 pleochroic mineral, which is resolved between crossed nicols 

 into a mass of tufted and sheaf-like fibres exhibiting bright inter- 

 ference-colours of the second order and a maximum extinction-angle 

 of 15°. A separation and analysis was made, with the following 

 result, indicating an amphibole : — 



Si0 2 48-20 



A1 2 3 1-35 



Fe 2 3 4-67 



FeO 10-96 



CaO 12-62 



MgO 19-58 



K„0 0-66 



H" 2 143 



Total 9947 



1 It is thus evident that we are dealing here with a fibrous amphi- 

 hole, which appears to have heen formed by interaction between 

 the limestone and the intrusive magnesian rocks. 



Sulphide ores. — These are chalcopyrite and pyrrhotite, which 

 occur as lenticular bodies along and near the contact-zone in the 

 serpentine. Mining operations have shown that chalcocite and 

 native copper occur as secondary ores above the groundwater-level, 

 occasionally in large bunches. Malachite and chrysocolla are 

 conspicuous near the surface. The origin of the primary sulphides 

 may be provisionally referred to contact-metamorphic action. 



Conclusions. — The magnesian rocks of the Dun Mountain 

 district consist of a differentiated series of peridotites intruded into 

 Mesozoic slates and limestones. Processes of hydration, which 

 appear to have been concentrated chiefly in the neighbourhood 

 of the belt of sulphides, and to have acted with diminishing 

 intensity towards the other side of the intrusion, have resulted in 

 the serpentinization of the olivine-rocks, with the exception of the 



2b2 



