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These beds are extensive in the west of the district, but are less common in the north. Outcrops ofsiltstone usually form prominent ridges and positive features on the hillside, as they are moreresistant to erosion than the adjacent lavas and tuffs.Rhyolite and Banded Rhyolite LavaDetailsNei Lak Shan. This is the type area of the Lantau Formation; from the summit of the mountain to the south(towards Po Lin Monastery) and southwest, there are exposures and corestones of banded, porphyritic, rhyolitelava. The rock is grey, with prominent white feldspar crystals and rust-coloured quartz crystals up to 5 mm. Thebands, which are 5 to 20 mm thick, are uniform and planar, but with diffuse boundaries. They are best seen onweathered surfaces (Plate 9) where harder bands form upstanding ribs. In fresh exposures, compositional variationrelated to the ribs is unclear, but the harder bands are sometimes a slightly darker grey.Banded lava, or uniform grey porphyritic rhyolite lava, crops out over a wide area around Nei Lak Shan, and isoccasionally interbedded with thin tufiite and siltstone layers. Typical lava occurs on the hills south of Ngong Ping,northwest and northeast of Nei Lak Shan. Overall, banding is subhorizontal, albeit with local variations. Forexample, on the summit of Nei Lak Shan (0888 1384) and 500m southsoutheast, the lava displays roughlyhorizontal banding. Further south (0911 1307), it may dip up to 30° to the northwest, and between 400 and 500 msouthwest of the summit, it dips 18° to the southwest.West of Nei Lak Shan, the succession is pervasively altered and metamorphosed, but primary banding in the lavasis often preserved. Contorted, or folded banding is well displayed near the crematorium (Plate 10, 0781 1324) eastof Ngong Ping. There, across a distance of 50 m, the dip of the banding varies from 67° to the south, to 61° to thenorth. This folding is interpreted as primary, resulting from flow irregularities during emplacement of the lava, andnot due to post-consolidation tectonic processes.About 1 km north of Nei Lak Shan, there is a zone of autobrecciation in the rhyolite lava which liesstratigraphically below an impersistent pyroclastic breccia (Plate 11). The autobrecciated lava (0898 1474) has apitted weathering surface on corestones, and occurs with evenly-banded lava and finely-banded lava. The bandedlava (0925 1494) crops out below the autobrecciated zone, and has feldspar crystals up to 8 mm in length, andbands less than 10 mm wide.Sham Shek Tsuen. Exposures of banded lava occur hi a stream (0718 1518) about 80 m from the faulted contactwith Carboniferous strata. The banding dips 50-60° to the southeast, close to the subvertical reverse fault. Adjacentto a narrow tuffaceous bed, there are exposures of a contorted and autobrecciated, finely-laminated lava, containingblocks up to 0.6 m across. Planar and folded flow-laminae in the rhyolite lava are less than 1 mm thick, andfissures perpendicular to this flow-banding pervade the rock. Overall, these features indicate a much more viscouslava than the coarsely porphyritic, planar-banded lavas at the type area on Nei Lak Shan.xai O. Lavas exposed along the south of Tai O contain Hthic fragments up to 20 mm, but lack the characteristicbanding of the formation. However, the presence of glomeroporphyritic crystal aggregates and shattered, but onlyslightly disaggregated crystals, set in an originally vitric matrix, suggest an effusive, rather than a pyroclastic,origin. The lithic fragments in the lava near the jetty (0289 1271) on west Tai O are of black, aphanitic lava, whichresembles the vitric matrix to the phenocrysts. ?^ySouthwest Lantau Island. South of the summit (0335 0900) of Tai Horn Shan, small exposures of finely banded,locally autobrecciated lava lie stratigraphically above a turfite and tuffaceous siltstone member. Brecciated rhyolitelava, also appears overlie a fossiliferous mudstone bed on the ridge (0273 0816) southwest of Sham Hang Lek,although the contact is not exposed. The breccia is interpreted as autobreccia formed at the base of a rhyoiite lavaflow, which is uniform and coarsely porphyritic elsewhere in the area. North of Sham Hang Lek (0313 0890), finebanding in lava dips southwest at 18°.Cheung Shan. At Cheung Shan (0530 1345), fine banding, 10-50 mm wide, is well developed in lava. The bandingundulates gently, but generally dips 20 to 50° to the south. About 300 m to the east, on the summit of Cheung Shan,relict banding dips steeply to the southeast, and on Fan Shui Au, banded rhyolite lava dips consistently to the southat 50° to 80° Below the Pak Kok Member, along the coast north of Pak Kok, banding in lavas is again welldeveloped. It strikes roughly eastnortheast and dips variably 18 to 82°, but folds and angular unconformities alsooccur locally (0586 0886).Shek Pik On the east coast (0753 0868) of Tung Wan, a finely banded, flow-folded rhyolite lava about 5 m thickand dipping steeply south, occurs within a sequence of uniform porphyritic rhyolite lavas, To the sou% the lava44