322 Transact ion .t. 



chroism is usually accepted as sufficiently distinctive. After hornblende, 

 hypersthene is the chief ferro-magnesian constituent in this rock. It 

 occurs in rather elongated crystals, some of which show cross-fractures, 

 and in smaller rounded grains. 



Magnetite is fairly abundant, in irregular masses, moulded on the 

 other minerals. Thomson* thinks the iron-ore is probably ilmenite. 

 He gives no reason, however, for thinking it to be ilmenite rather than 

 magnetite. Hamiltonf noticed considerable disturbance of the magnetic 

 needle while he was in this district, but, as ilmenite also affects the 

 magnetic needle, Hamilton's observations do not point conclusively to 

 magnetite, though they are certainly significant. 



Order of Crystallization. — Observation of the form and arrangement 

 of the minerals in this rock does not support the theory suggested by 

 Rosenbusch as to the normal order of crystallization. According to his 

 theory, magnetite should have been the first mineral to crystallize, and 

 in sections there should be at least some well-shaped crystals idiomorphic 

 towards the other constituents. Next in order the ferro-magnesian 

 minerals should have separated out, forming crystals idiomorphic towards 

 the feldspar, the last mineral to crystallize. 



The actual sequence of events, however, seems to have been as follows : 

 First a small amount of feldspar crystallized out, for crystals of this 

 mineral are enclosed in both magnetite and hornblende ; then the 

 remaining feldspar and the ferro-magnesian minerals crystallized out, 

 and sometimes the latter constituents show an approach to idiomorphism, 

 while sometimes the opposite is the case ; finally magnetite separated from 

 the magma, and formed interstitially. 



In accordance with recent theories of the dependence of structure on 

 eutectic relations,! the explanation would be as follows: Feldspar was 

 originally in excess, and an amount crystallized out sufficient to reduce 

 the ratio of feldspar to ferro-magnesian minerals to a eutectic propor- 

 tion. These minerals then crystallized together until a eutectic propor- 

 tion between them and the magnetite was arrived at, when all crystallized 

 together, but since a large percentage of the rock was already in a 

 crystalline state the magetite had little chance of becoming idiomorphic. 



The Origin of the Hornblende. — The Rev. H. Baron in conversation 

 with Captain Hutton§ long ago expressed the opinion that all the horn- 

 blende in this rock is of secondary origin. This opinion receives support 

 in the fact that very many of the pyroxene crystals are bordered by a 

 rim of hornblende, which first appears as a narrow fringe, pale green 

 in colour, and rather faintly pleochroic. This fringe increases in width 

 at the expense of the pyroxene, and as it widens it increases in depth 

 of colour and in intensity of pleochroism. Finally the pyroxene is 

 entirely replaced by amphibole. The examination of a very few sections 

 shows this change in all its stages. These observations show definitely 

 that at least a large amount of the hornblende is secondary. 



* Thomson, J. A., " Notes on some Rocks from Parapara, Bluff Hill, and Waikawa," 

 Trans. N.Z. Inst,, vol. 42 (1910), p. 33. 



f Hamilton, " Notes on the Geology of the Bluff District," Trans. N.Z. Inst., vol. 19 

 (1886', p. 452. 



% Vogt, J. H. L., l> Physikalische-chemische Gesetze der Kristallisation folge in 

 Eruptivgesteine," Isch. min. u. petr. Mitt. 24, p. 437, 1905. 



§ Hutton, P. W., " Corrections of the Names of some New Zealand Rocks," Trans. 

 N.Z. Inst., vol. 31 (1899), p. 484. 



