Petrography of Limestone Although macrofossils were not detected, the hard grey limestone (7751, 7754–56) possesses an internal structure consisting of prismatic shell fragments surrounded by plentiful isolated prisms of the completely shattered shell (Fig. 3). This structure appears comparable with that described by J. M. Bell, E. C. Clarke and P. Marshall (1911, p. 18) and C. T. Trechmann (1917) from thin sections of Maitai limestone. “It [thin section of Maitai limestone] contains many shell fragments with corroded edges and the surrounding matrix is also largely made up of isolated prisms of the decomposed shell” (Trechmann, op. cit., p. 56). This similarity was confirmed when thin sections of Maitai limestone from Wairoa gorge were examined by the writer. The prismatic shell fragments are almost certainly derived from Maitaia (Marwick. 1935, p. 295) = Aphanaia of Trechmann (loc. cit.). Petrography of Spilites, Albite Dolerites and Keratophyres (i) Structure. Both porphyritic and non-porphyritic types occur in the dark green spilites (7920–7967, 7912). In the former, phenocrysts of augite and feldspar (1–2 mm.) are set in a pilotaxitic base of augite prisms (0·2 mm.), feldspar laths (0·6 mm.), iron ore and chloritic minerals (Fig. 5). Larger plagioclase and augite phenocrysts (2–3 mm.) are present in rocks (7931, 7946) transitional to dolerites. The non-porphyritic types consist of a pilotaxitic mosaie only (7935, 7941, 7944). A glassy phase, developed as selvedges in the pillows, is shown in two cases (7910, 7913) where abundant phenocrysts of feldspar and augite are set in a black glassy groundmass, in which plentiful feldspar microlites and pyroxene crystallites can be distinguished, and from which plentiful iron ore granules have crystallised. Variolitic structure is occasionally developed in rocks (7922. Fig. 4–No. 7842. Volcanic tuff composed mainly of igneous fragments, iron ore, pyroxene and hornblende. X 17. Fig. 5–No. 7912. Spilite showing augite and albite phenocrysts set in an altered pilotaxitic ground mass. X 28,