Vol. 65.] AND MAGS ESI AN ROCKS OF NEW ZEALAND. 377 



Turning to the nephrite of New Zealand, my conclusions are 

 that its formation has been due to more than one type of chemical 

 and mineralogical change. Of the four following modes of origin 

 advanced, there is direct petrographical evidence of the first three, 

 and a strong presumption of the fourth. 



(1) Uralitization of pyroxenes. — The process, as observed 

 microscopically, has already been described, and it is further to be 

 noted that this general process is the commonest mode of deep- 

 seated alteration of the pyroxenes throughout these magnesian rocks. 

 This mode of origin of the New Zealand nephrite has been described 

 in some detail by Dr. Dieseldorff [5, pp. 334 et seqq.~\. 



(2) Contact-action. — To this agency has been due the fibrous 

 amphibole described from the serpentine of the Duu Mountain 

 district, adjoining the limestone-contact. 



(3) Direct change of olivine into nephrite. — This 

 change, described above, has not, so far as I am aware, been 

 recorded hitherto ; but there is no doubt that, in the specimens 

 examined, it is seen in progress. It may be noted that, unlike the 

 uralitization of pyroxenes, it involves a change from orthosilicate to 

 metasilicate. 



(4) Deep-seated metamorphism of serpentine-talc- 

 carbonate rocks or of their prototypes. — The evidence for 

 the action of this process in the formation of the New Zealand 

 nephrite is largely indirect. It is the cause assigned by Dr. Kal- 

 kowsky for the formation of the nephrite of Liguria [17, p. 375], 

 and it is significant that the nephrite found in situ in the Griffin 

 Range (Westland) is practically confined to masses of serpentine- 

 talc rock with calcite and dolomite. This association strongly 

 suggests a causal connexion, and it is readily conceivable that the 

 metamorphic processes which resulted in the development of this 

 type of rock would give rise to the fibrous amphibole nephrite. It 

 seems, then, very probable that this general process, which may 

 have involved, indeed, one or more of those enumerated above, has 

 been operative in the formation of the New Zealand nephrite. 



The foregoing processes are, however, all of a chemical nature, 

 and involve nothing more than the production of an aggregate of 

 fine amphibole fibres. Over and above these changes, the factor 

 essential to the production of true nephrite is the mechanical one 

 of intense pressure and movement, superimposed on the production 

 of the fibrous amphibole. Thus true nephrite shows under the 

 microscope a foliated or felted structure, to which it owes its very 

 superior hardness or toughness, and which is certainly the result of 

 great pressure and movement. Further, the nephrite is found only 

 in those localities where, other things being equal, the matrix shows 



