306 Transactions. 



variety, with pronounced zonal and hour-glass structure, and with the 

 usual pleochroism, and occasionally shows the steel-blue birefringence 

 due to the high dispersion of the optic axes. A narrow margin is 

 usually dark green. The mineral is completely idiomorphic. The nephe- 

 line is abundant, For the most part its crystallization was finished 

 before that of the feldspar. It is largely altered to natrolite. The feld- 

 spar is much twinned on the albite and pericline laws, and has in 

 many places the appearance of microline. The extinction-angle, however, 

 proves it to be andesine. It was the last of the larger crystals to 

 form. There is here and there a small quantity of groundmass. It 

 consists of idiomorphic and often bent crystals of feldspar, probably 

 andesine, ailotriomorphic aegerine, nepheline converted into natrolite, and 

 much apatite. Of these minerals, the nepheline was the last to form. 

 In some of the finer-grained specimens the idiomorphism of the augite is 

 less pronounced, and occasionally shows ophitic structure with the feld- 

 spar, which is then distinctly anterior in crystallization to the nepheline. 



The basaltoid forms of the rock are well exemplified by a large dyke 

 at Clarendon (analysis B). Here apatite is much less noticeable. The 

 ilmenite and olivine are not more than 025 mm. in diameter. The 

 augite is in moderate to small crystals, sinking to the dimensions of 

 miciolites, but always idiomorphic. Feldspar is not abundant, and is 

 always in the form of microlites. Nepheline is quite abundant, and is 

 in the form of ailotriomorphic plates enclosing numerous crystals of olivine, 

 augite, and ilmenite. Lava-flows that cover a considerable area of the 

 country to the west of the dyke are also somewhat similar, but are of 

 much finer grain (analysis C). The irregular plates of nepheline are in 

 these rocks extremely small and hard to distinguish except by micro- 

 chemical methods. This type of rock has previously been described by 

 Andrew, who, however, failed to distinguish the nepheline, though he 

 recognized that much of the rock was soluble in dilute HC1. 



The type from Omimi is particularly interesting from the point of 

 view of structure (analysis D). The apatite and ilmenite have the same 

 features as before. The olivine, however, is in extremely small needles, 

 sometimes 1 cm. long, but only 0-08 mm. wide. The direction of neigh- 

 bouring crystals is in remarkably parallel lines in longitudinal as well as 

 transverse section. They are similarly oriented over a considerable area. 

 The phenocrysts of augite have pleochroism, zonal and hour-glass structure, 

 as in the Waihola type. A similar appearance of lattice structure in the 

 feldspar is also very noticeable. The nepheline is wanting in crystallo- 

 graphic boundaries, and is usually intergrown in complete micrographic 

 fashion with augite. In some instances at least this augite is in optical 

 continuity with the large crystals. This micrographic intergrowth is 

 sometimes found in the groundmass in an extremely minute scale, and 

 constitutes its dominant feature. The augite is sometimes slightly green 

 in its smaller members. There are minute crystals of feldspar and apatite 

 crystals in the groundmass. The intergrowth is of the same nature as 

 that found in the Lobauer Berg type, but is much more complete, and 

 is shown on a finer scale than in the German type. 



As at Waihola, the rock is associated with basaltoid types, the exact 

 distribution of which and their relation to the coarser type of rock has 

 not yet been fully made out by Mr. Smith. Here, however, there appears 

 to be a complete series through types with large ailotriomorphic nephe- 

 line plates to types of a dense nature in which the nepheline is extremely 

 hard to identify. 



