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The contact between foliated sandstone and megacrystic" fine-grained granite is exposed at the northeastern end ofthe bay (0714 1612). The sandstone is also cut by a thin, eastnortheast-trending quartz syenite dyke. A skarndeposit has been recorded at this locality (Peng, 1991). It comprises a zone, about 5 m wide, which contains garnet,vesuvianite, diopside and epidote, with scattered magnetite.West Brother island (prior to development). The island is dominantly composed of silty sandstone, but alsocontains prominent beds of graphitic siltstone. Graphite-rich horizons are also present, and were mined in the1950s and 60s. These horizons are highly eroded and are not well exposed on the shoreline, but have been mappedunderground (Ruxton, 1957; Woods & Langford, 1991). Neither the base nor top of the succession were seen, butthe siltstone and sandstone sequence is at least 200 m thick.The west coast of the island is dominated by graphitic siltstone and graphite-rich horizons. Near the abandonedmines and associated buildings, the beach is dominantly composed of pebbles and cobbles of graphitic siltstone.Near the northern tip of the island, graphitic siltstone, is well bedded, dips northeast at 52° , and underliesbrown-weathering, silty sandstone. The east coast is dominated by this massive, silty sandstone, in which beddingis absent, or poorly defined. The finely interlaminated graphitic siltstone and siltstone include load structures whichconfirm that the sequence is the right way-up. Cross-lamination also occurs, but was not observed in situ. Puregraphite was only seen on spoil heaps beside the old mine workings (1456 2117). The graphite probably came fromthe axial zone of the northeast-plunging syncline which forms the island. The rock has lustrous foliation surfaces,and has probably been thermally metamorphosed.The northern and eastern parts of the island are dominated by banded and massive, silty sandstone, with locallyextensive mineral boxworks. Bedding is obscure in the east, but nearly vertical erosional hollows, strikingeastnortheast, probably lie along softer strata.East Brother island (prior to development) The island is dominated by red-weathering, finely laminated siltstoneand sandstone (Plate 6), massive siltstone and fine-grained sandstone. Thin quartzitic sandstone beds at thesouthern end of the island are less than 2 m thick. Quartzitic sandstone also occurs at the northern end, and maybe partly responsible for the hog-back appearance of the island. Well-defined bedding dips steeply on the westcoast of the island, and can also be seen occasionally on the east coast in the more massive siltstone and siltysandstone. The succession is at least 100 m thick, but no top or base has been observed.Plate 2 - Porphyritic Rhyolite Lava Overlying Sandstone (0459 1420) at San Chan. (The sandstonemay be part of the Mai Po Member (Carboniferous), but it is more likely that both the lavaand sandstone are Jurassic).32