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fine-grained sandstone and siltstone. These rocks are thought to have been regionallymetamorphosed, and hence are described as metasandstones and metasiltstones on the publishedmaps. However, the pre-fix meta- is omitted, for convenience, from the following descriptions.There are also occasional beds of massive to poorly-laminated, graphitic siltstone, up to severalmetres thick on West Brother island, which are pure enough to have justified commercialextraction of the graphite in the past.The member is probably more than 400m thick around Sham Wat. However, close to isoclinalfolding and additional faulting have probably repeated the sequence throughout the outcrop. OnWest Brother island, a graphite mine extended to - 90 mPD (Woods & Langford, 1991), yet didnot reach the bottom of the succession, which may therefore be over 300 m thick. Therelationship between the Mai Po Member and the adjacent marble of the Yuen Long Formationhas not been established from the boreholes sunk offshore to the east of East Brother island. Thecontact is, however, presumed to be largely conformable, although it may be locallyunconformable (Langford etal, 1989; Frost, 1992).Allen & Stephens (1971) assigned the strata of west Lantau Island and The Brothers islands tothe Tai 0 Formation, and believed that the top of the formation was marked by a tuff lyingconformably on sandstone. Ruxton (1960) had previously suggested there was an unconformityabove the sedimentary sequence, but he thought that this had previously been confirmed byUglow in the 1920s at a locality near Fan Lau (Jones, 1927, in Ruxton, 1960), but this localitylies outside the Carboniferous outcrop.The strongest evidence of discordance at the top of the formation is at Sham Wat Wan, where theMai Po Member is faulted against Jurassic eutaxite. Angular discordance is also evident from thedips of strata in Tai 0 cemetery, and on the headland west of San Chau. In all instances the dipsin the siltstones and sandstones differ from those of the faulted contact, as, also, docs the dip ofbanding, or eutaxitic fabric, in the volcanic sequence.DetailsTai O. The Lok Ma Chau Formation on the western end of Tai O island strikes mainly eastnortheast. Steep dips toboth north and south indicate that tight, to isoclinal folding may exist (e.g. at the northeastern tip of Tai O, near thesewage treatment works). Younging directions, determined from upwardly-fining units, consistently indicate thatthe strata are not inverted. Along the northern coast of Tai O the member comprises finely laminated siltstones, andfine-grained sandstones with abundant sedimentary structures. Lenticular bedding occurs in interbedded red to greysiltstone and fine sandstone near the western tip of the island (Plate 1, 1241 1319). Adjacent exposures show finelybedded sandstone and siltstone in upward-fining, graded units. These units dip and young to the southeast, whereasto the west (1226 1304), similar units of light grey to light red, fine-grained sandstone and siltstone, about 20 mmthick, young and dip to the northwest.The eastnortheast-trending ridge through the summit at Fu Shan (0318 1331) is dominantly composed of massive,fine-grained quartzitic sandstone and siltstone. Similar rocks, together with finely laminated sandstone, are foundto the east, on the island of Po Chue Tarn (039 134).Near the Sewage Treatment Works (0348 1361), siltstone and fine-grained sandstone dominate the sequence, andinclude some pebbly horizons. Rarely, the pebbles are up to 50 mm across, but more typically they range from 1 to4 mm, and are angular to sub-rounded.Tai O Cemeteiy. On the hillside above the cemetery, quartzitic sandstone and siltstone crop out close to eutaxiteand tuff of the Cheung Shan Member of the Lantau Formation. The contact is inferred to be a high-angle fault,trending north-south and downthrowing the younger volcanics to the south, against the Carboniferous strata, Astream (0440 1365) to the north exposes reddish-white, fine-grained quartzitic sandstone, reddish siltstone andmudstone. However, the hillside is dominated by slope debris comprising eutaxite and tuff boulders from thehillside to the east. '30