On the Wasaki Peninsula and in an area extending southwards around North and South Ruri are some 70 plugs which are predominantly phonolite but with two trending towards nephelinite. They range in diameter from 50 to 700 m, are circular or elliptical in plan and form prominent hills up to 100 m high. Ijolite xenoliths occur in many of the plugs west of the Ruri Hills and basement xenoliths, including fenitized granite, have also been found. The phonolite contains phenocrysts of sanidine, nepheline and pyroxene zoned outwards from augite to aegirine in a glassy to very fine-grained matrix of the same minerals with natrolite and analcime in some rocks. Two plugs consist of phlogopite phonolitic nephelinite in which feldspar is less abundant, the matrix is coarser and of euhedral nepheline and pyroxene and characteristic phlogopite. They contain rare xenoliths of phlogopite pyroxenite.
LE BAS, M.J. 1977. Carbonatite-nephelinite volcanism: an African case history. John Wiley, London. 347 pp.