3o8 A NATURALIST IN THE PACIFIC chap. 



It will be gathered from the above that the source of the 

 pyroxene of the groundmass is to be found in the magmatic 

 paramorphism of the porphyritic hornblende. The hornblende is 

 dark-brown, markedly pleochroic, and extinctions up to 15° are 

 given in prismatic sections. It is well known that the conversion 

 of a hornblende crystal into an aggregate of pyroxene prisms and 

 magnetite was long since experimentally effected by Doelter and 

 Hussak by immersing the hornblende in molten basalt, andesite, 

 &C.1 I would imagine that the transformation of the hornblende 

 and the dispersion of the paramorph occurred under two con- 

 ditions ; in the first case whilst the " flow " was still in motion 

 when the resulting pyroxene would be mixed up in the 

 magma ; in the second case after movement had ceased, but before 

 consolidation of the groundmass, when a paramorph or pseudo- 

 morph would be formed. 



Oligoclase-Trachytes 



The term " trachyte " is here applied in a general sense to a 

 group of light-grey intrusive acid rocks, having a specific gravity 

 when compact of 2-4 to 2-45 and showing phenocrysts of glassy 

 felspar, but not of quartz. These rocks, which are especially 

 characteristic of the districts around Tawaki and Mount Thuku 

 and of the Wainikoro sea-border, are often open-textured and 

 sometimes a little vesicular, whilst several of them exhibit some 

 degree of alteration in the groundmass. In all cases they appear 

 to be intrusions rather than surface-flows ; and at times they 

 display a columnar structure.^ 



The difference between the oligoclase-trachytes in various 

 localities appears to be mainly concerned with the varying degrees 

 of crystallisation. There are two principal varieties. In the most 

 crystalline type there are small phenocrysts of glassy felspar and 

 a few of pale augite, the angle of extinction of the last being 

 over 30 degrees. The felspar phenocrysts, which contain but few 

 inclusions and have sharp rectilinear outlines, in most cases show 

 zoning and give lamellar extinction of 5° to 12° indicating oligo- 

 clase ; but some of them have the tabular untwinned or simple 

 twinned form of sanidine. The groundmass is in the main 

 composed of minute felspar-lathes, less than 'i mm. in length, 

 arranged in a dense plexus, and giving nearly straight extinctions. 



1 Neues Jahrb.fur Mineralogie, 1884. 



^ For their mode of occurrence, see pp. 215, 220, 230-233. 



