364 MR. A. M. EINLAYSON ON THE NEPHRITE [Aug. IQOQ, 



Summary. — From the available evidence, it appears that the 

 Anita Bay rocks are an ultrabasic group, probably intrusive into- 

 the surrounding rocks and differentiated during their intrusion. 

 From analogy with the other occurrences, the talc-rock is probably 

 a hydrated pbase of the intrusion. Subsequent dynamic processes 

 of considerable intensity account for the cataclastic structure of 

 the dunite, and, so far as can be judged, for the formation of the 

 bowenite. 



General Remarks on the Magnesian Rocks. 



(1) Original nature of the rocks. — The rocks which have 

 here been described and discussed consist of a series ranging from 

 diorjtes to dunites, and showing the greatest development near the 

 latter end of the series. In most of the separate occurrences- 

 examined and described, the series appears to change progressively 

 in basicity from one side to the other of the intrusion, and this- 

 seems to hold also for the prototypes of the serpentine-talc rocks. 



Of the nature of the felspars little can be said, owing to their 

 alteration (in almost every case) into saussurite. They occur,, 

 moreover, but rarely, the bulk of the rocks being peridotites. 

 The olivine is generally near to the forsterite type. The original 

 ferromagnesian minerals are characteristically pyroxenes, any am- 

 phiboles present being uralitic. The pyroxenes comprise enstatite, 

 diallage, and diopside. Of these, enstatite appears characteristic 

 of the less basic peridotites ; diallage, which is the predominant 

 species, of the intermediate types ; and diopside, in subordinate 

 quantity, of the more basic. Finally, the spinels, which are the 

 characteristic accessories, show considerable varietj', ranging from 

 picotite in the less basic types to chromite in the olivine-rocks. 



(2) Relation to the surrounding rocks. — In at least 

 four localities the magnesian rocks are seen to be clearly intrusive 

 into the sedimentaries, and it seems reasonable to infer that all the 

 rocks of this series are intrusive. 



(3) Correlation. — The argument for the contemporaneity of 

 these different occurrences rests on their significant linear distri- 

 bution and their remarkable petrographic similarity. It is con- 

 sidered almost certain that the members of the series extending 

 from D'Urville Island in the north to Western Otago in the south 

 were intruded, if not contemporaneously, at least during some one 

 definite phase or period in the geological development of the island. 

 Whether the occurrences at Milford Sound, Farapara, and Gibbston 

 are to be grouped with the main belt, cannot be determined with- 

 out further investigation, if indeed this association be ever capable 

 of exact proof or disproof. 



In short, it is highly probable that all the exposures which are 

 included in this petrographic province were intruded during some 

 definite stage in the process of formation of the South Island. The 



