Vol. 65.] AND MAGNESIAN ROCKS OF NEW ZEALAND. 365' 



structural significance of the magnesian belt (see p. 366) is too 

 evident to be overlooked in this connexion. 



(4) Age of the rocks. — Under this head little can be said,, 

 until the geological structure of the island is better understood and 

 more fully worked out. The peridotites are intrusive into rocks 

 which vary in age from Ordovician to Triassic or later, and it 

 is probable that they are all of post-Triassic date. They have 

 been, however, especially in those districts which have been most 

 affected by mountain-forming agents, profoundly metamorphosed by 

 regional processes. The great period of folding and of mountain- 

 formation in New Zealand is believed to have occurred towards the 

 end of the Jurassic or in the early Cretaceous Era, during and 

 following the intrusion of the granitic belt along the western flank 

 of the Alps. The magnesian rocks, therefore, appear to have been 

 intruded during the Jurassic-Cretaceous period of folding and eleva- 

 tion ; but their relation, in point of age, to the belt, of granites which 

 runs for some distance parallel to them, still awaits elucidation. 



(5) Hydrothermal action. — The effects of contact-meta- 

 morphism have been already noticed. It remains to emphasize 

 the hydrothermal action that has accompanied the intrusives in 

 certain of the localities. It is noteworthy that the most highly- 

 serpentinized occurrences are associated closely with evidences of 

 considerable solfataric action. Thus the sulphide zone at the 

 Dun Mountain is perfectly serpentinized, and the serpentines of 

 the Hokitika area are likewise associated with solfataric effects. 

 Again, where such action has been wanting, as round the dunite of 

 Kelson and at Milford Sound, serpentinization is absent, although 

 the rocks have often been much crushed by pressure and movement. 

 The study of the processes of serpentinization throughout these 

 rocks strongly suggests that hydrothermal action, during and 

 following the intrusions, has been a potent factor of serpen- 

 tinization. The uprising vapours and solutions, being entangled 

 in the solidifying and cooling rocks, must have been active in the 

 silicifi cation and hydration of olivine and the consequent wide- 

 spread formation of serpentine. 



(6) Subsequent metamorphism.— Later alterations com- 

 prise mechanical effects on the rock-masses, and alteration of the 

 constituent minerals. Mechanical effects due to pressure and 

 rock-folding are seen in the shearing and foliation of serpentine- 

 and talc-rocks, and in the cataclastic structure of some types. 



As regards the alterations of minerals, deep-seated processes have 

 effected the change of plagioclase into saussurite, of pyroxenes into 

 uralite and into antigorite, and, apparently, of talc into bowenite. 

 The production of antigorite and bowenite is essentially an effect 

 of great pressure. Whether the pyroxenes change to uralite or to 

 antigorite must obviously depend on the availability of the required 



