52 H. I. JENSEN. 



(1) The occurrence of the same sulphide minerals as rock 

 constituents in the normal andesites of the district. 



(2) The fact that most of the included crystals consist 

 of felspar and felspar aggregates, (though green hornblende 

 and biotite are present as well). In the case of the meta- 

 somatic replacement of a glassy base we should expect 

 most of the inclusions to consist of ferro-magnesian minerals, 

 these normally consolidating first in the Lautoka andesites. 



(3) The thoroughly crystalline nature of the whole mass 

 and the mossy way in which the sulphide mineral fits into 

 the enveloping andesitic material which itself has under- 

 gone no alteration as we should expect through vapour 

 action. The specimen has an appearance suggesting 

 simultaneous crystallisation. 



In favour of the metasomatic replacement hypothesis it 

 might be argued: — 



(a) That the metasomatic replacement of the base of a 

 hypo-hyaline rock with copper sulphide could give rise to a 

 crystalline mass of copper ore iucluding microlites. 



(b) That in the normal rock the cupriferous mineral con- 

 solidated early, whereas in the ore it consolidated simul- 

 taneously with or after the felspar microlites. 



(c) That biotite occurs sparingly included in the ore, but 

 does not occur in the normal Fijian andesites. Biotite in 

 these rocks is however not necessarily a product of meta- 

 somatic change, as it is a mineral of common occurrence 

 in andesites, especially in hornblendic varieties. 



Against the second argument (b) it might be urged that 

 in the normal rock, the copper ore being present only in 

 minute quantities, the order of consolidation would depend 

 mainly on the laws of solubility, whereas in the larger 

 specimen, which represents according to the first hypothesis, 

 a portion of the magma supersaturated with metallic sul- 



