Yol. 65.] AND MAGNESIAN ROCKS OF NEW ZEALAND. 375 



olivine cores are perfectly fresh, without any sign of serpentinization. 

 In many places, the fibres, which are colourless and have a maximum 

 extinction-angle of 18°, appear to be working their way towards the 

 centre of the grains of olivine, replacing it as they grow. Finally, 

 the olivine is more or less completely replaced, a few small residual 

 grains being left in a dense mesh of fibres. The latter as a rule 

 have no dominant direction, but interlace and cross one another at 

 all angles. At other times the olivine is crossed by a series of more 

 or less parallel cracks, which are occupied by fine amphibole fibres 

 disposed across the cracks. The structure is altogether distinct 

 from the serpentinization of olivine, and cannot be mistaken for it. 

 The other constituent of the rock is pyroxene (probably enstatite) 

 in fairly coarse irregular plates, completely converted into a dull 

 brown bastite, with a fringe of opaque iron oxides. There is no 

 trace of uralitization of the pyroxene, and there is little doubt that 

 the alteration-processes observed in this rock are two : namely, 

 serpentinization of the pyroxene, and nephritization, if one may use 

 the term, of the olivine. The rock has, however, not undergone 

 the shearing and stress which would be necessary to transform it 

 into true nephrite. 



An analysis of the specimen yielded the following result : — 



Si0 2 43-00 



AL0 3 2-35 



Fe 2 3 209 



FeO 4-68 



CaO 10-65 



MgO 32-24 



K 2 024 



Na 2 0-31 



H 2 407 



Total 99-133 



The analysis thus gives evidence of exactly those changes which 

 are observed under the microscope. They may be illustrated by 

 the following equations : — 



(1) 3Mg 2 SiO 4 +20aO+5SiO 2 = 2CaMg 3 (Si0 3 ) 4 



Olivine (forsterite). Amphibole- 



(2) 3MgSi0 3 + 2H 2 = H 4 Mg 3 Si 2 9 +Si0 2 



Enstatite. Serpentine. 



Since the serpentinization of the enstatite involves loss of silica 

 it is possibly this silica which is added to the olivine along with 

 lime to effect the change of olivine into amphibole. The lime has 

 been derived, either from some other constituent of the rock, or 

 from a neighbouring source. The change is a most peculiar one,. 



