form in it. The prisms are of considerable size, as much as 1 mm. long and 0·18 mm. in diameter. The prisms are not numerous in the olivine or ilmenite, but are abundant in the younger constituents. Ilmenite in grains of 1·5 mm. in [diameter is rather frequent, and displays its usual structure. Olivine is fairly common in crystals as much as 6 mm. in diameter, and is somewhat serpentinized. The augite is a titaniferous

Geological Structures of Part of East Otago.

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 nepheline is abundant, For the most part its crystallization was finished before that of the feldspar. It is largely altered to natrolite. The feldspar 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, allotriomorphic 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 feldspar, 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 0·25 mm. in diameter. The augite is in moderate to small crystals, sinking to the dimensions of microlites, 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 allotriomorphic 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 Hcl.

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 neighbouring 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 allotriomorphic nepheline plates to types of a dense nature in which the nepheline is extremely hard to identify.