xxi MAGMATIC PARAMORPHISM 307 



greenish and had an extinction angle of 40. In the Vanua Levu 

 rocks, however, there is a mixture in these dark margins of both 

 monoclinic and rhombic pyroxene ; and the process may be 

 observed in all stages as it advances into the interior of the crystal, 

 until a dark pseudomorph or paramorph of pyroxene and 

 magnetite results. When the magnetite prevails, the pseudomorph 

 may ultimately form a black patch in which the process is 

 obscured. But when, as is generally the case, the pyroxenes 

 are more frequent, it occurs as a dark grey mass. 



Finally follows the dispersion of the pseudomorph, which 

 first becomes a loosely arranged aggregate of the two pyroxenes 

 and magnetite, and then breaks down, and at length is only 

 represented by small pale patches of its original constituents. 

 These patches are easily recognisable, and in not a few rock- 

 sections offer the only indication of the previously existing 

 hornblende phenocryst. There can be little doubt that this is 

 the source of much at least of the often abundant pyroxenes of 

 the groundmass, which are usually most frequent in the vicinity of 

 the patches. 



In the earliest stage when the dark border alone exists, it is 

 not easy to distinguish the one pyroxene from the other, the 

 granules and prisms being colourless and very minute, less than 

 QI in size. But in a far advanced stage of the paramorphism the 

 granules and prisms become sometimes much larger, the first 

 attaining a breadth of '04 and '05 mm. and the last a length of "15 

 mm. Finally the interior of the paramorph is seen to be more or 

 less completely composed of very pale brown augite and pale 

 yellow rhombic pyroxene in coarse grains and prisms, the first 

 distinguished by its oblique extinction of 30 to 35, the last 

 recognised by its straight extinction and feeble though distinct 

 pleochroism. 



Although as a rule the paramorph becomes dispersed and its 

 pyroxene constituents are added to the groundmass, it sometimes 

 exhibits a change of another character. In this case the outer 

 portion is alone dispersed, whilst the growth of a single large 

 crystal of pyroxene proceeds within the mass. In a later stage, 

 when the dispersion of the outer part is complete, we have a fresh- 

 looking pyroxene phenocryst with unformed edges, on the borders 

 of which little granules and prisms of pyroxene may be seen 

 arranging themselves, as if the crystal-building was still in progress, 

 or rather as if it had been interrupted and left unfinished by the 

 too rapid dispersion of the outer portions of the paramorph. 



X 2 



