Yol. 65.] AND MAGNESIAN ROCKS OF NEW ZEALAND. 363 



most specimens show little or no serpentine- Slides cut from 

 specimens which contain bowenite-veins, however, show a progressive 

 destruction of talc (PI. XVI, fig. 3), which is replaced by carbonate 

 until only residual cores of talc remain, while serpentine appears 

 in irregular veins and stringers showing the characteristic struc- 

 ture of bowenite already described. Finally, the bowenite itself 

 contains, as has been said, residual flakes of talc and patches of 

 magnesite. 



The evidence appears to show that the presence of the bowenite- 

 veins is associated with intense movement, pressure, and crushing 

 of the rock. If we suppose the talc to be converted to bowenite 

 by such processes under deep-seated conditions, it is evident that 

 some magnesia is left, which presumably forms magnesite by sub- 

 sequent carbonation under conditions of atmospheric weathering. 



The following analyses illustrate the process of alteration, No. I 

 being the theoretical composition of talc and Nos. II, III, and IY 

 being analyses of the talc-rock showing progressive stages of 

 alteration to bowenite and magnesite. 



I. II. III. IV. 



H 2 4-80 5-24 5-46 6-86 



Si0 2 63-50 5645 48-41 36-41 



MgO 31-70 31-22 33-05 38"61 



FeO — 2-71 1-46 2-15 



Cr 2 3 — 0-56 0-31 0-45 



C0 2 — 4-70 12-05 1511 



Totals ... 100-00 100-58 100-74 99"59 



These analyses show, as regards the chief constituents, an in- 

 crease in H 2 0, MgO, and CO.,, and a corresponding decrease in Si0 2 , 

 which agrees with the microscopic evidence. 



Origin of the bowenite. — We have seen that the bowenite 

 and its matrix have both been subject to intense pressure, probably 

 concentrated in those places where bowenite-veins now occur, while 

 the analyses show the formation of magnesium carbonate at the 

 expense of the talc. We know, further, that the change from talc 

 to serpentine is one that can take place only under fairly deep- 

 seated conditions of metamorphism [22, p. 351], talc being, under 

 superficial conditions, practically an end-product. The formation 

 of the bowenite appears therefore to be due to dynamic effects, 

 while the magnesite has doubtless been formed subsequently, under 

 more superficial conditions, from talc and from excess of magnesia 

 left during the serpentinization. Further than this it is difficult to 

 go, without more detailed field-study of the relations of the different 

 rock- types. 



