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Volcanic EnvironmentEuhedral crystals, and crystals that have shattered in situ, indicate a fluid medium (lava),whereas the larger broken crystals are consistent with a fragmental origin (pyroclastic). Branney& Kokelaar (1992) argue that there is a continuum from fountain-fed lava flows to lava-likeignimbrite. They propose that there is a transition during emplacement of ignimbrites, fromparticulate to non-particulate flow. This could account for the presence of both lava-like andtypical ignimbrite textures in the Lantau Formation. Hence, as parts of the formation displaytextures indistinguishable from lavas, they can be referred to as such. Alternatively, the bandedlithologies could be interpreted as high-grade ignimbrites, displaying complex lithofacies changesin individual flow units from autobrecciated through to dominantly pyroclastic textures. Flowunits can be inferred by interpreting intercalated sedimentary and tuffaceous rocks as havingformed during periods of relative volcanic quiescence.The volcanic environment varied from high temperature lava fountains, possibly extruding themolten rock in pulses, to slightly lower temperature ash columns that collapsed and coalescedclose to the vent, forming welded flows. Quiescent phases between the eruptions were sometimeslong enough for sediments to be deposited. The abundant flora in some of these intercalationspoints to swampy basins on the flanks of the volcanic edifice. Mass flows, including primarylahars, and involving redeposited tuffaceous material, was probably common on the sides of thevolcano and caused rapid filling of small basins and hollows.Sedimentary and Tuffaceous RocksDetailsNei Lak Shan. Tuffaceous sandstone and tuffite form an erosion-resistant feature on the northern slopes of Nei LakShan. The member also includes porphyritic rhyolite lava, with well-developed banding, and fine ash crystal tuff,and lies above a thin pyroclastic breccia. The tuffaceous member is less than 70 m thick, and appears to be laterallyimpersistent The pyroclastic breccia may be as little as 10 m thick, but has not been traced laterally.Ngong Ping. The prominent ridges west of Ngong Ping are primarily composed of tufflte, but arc locallyinterbedded with fossiliferous mudstone. Tuffite, tuffaceous siltstone, sandstone and rare mudstones weretemporarily exposed along the Ngong Ping Road. Close to the Tai O Road (0698 1194), two 0.1 to 0.5 m thick,subhorizontal ash bands can be seen in a cut slope, separated by 5 m of tuffite.The ridge, about 400 m southwest of the statue of the Buddha, is composed of light grey siltstone and tuffaceoussiltstone. Bedding is only locally displayed. The member dips southwest at 11 to 40°, and is probably less than 50m thick. It thins rapidly to both north and southeast. Nau & Wu (1991) recorded at least 20 m of sandstone,siltstone and mudstone, in part tuffaceous, dipping east to southeast at 17 to 23° (0802 1254), and containing somewell-preserved plant fossils. These beds were also noted by Lee et al. (1990).Sham Wat Close to the weir (0655 1327) at the start of the catchwater, a sequence, dipping 10° south, comprisesgrey, finely laminated, cherty tuffaceous siltstone and mudstone, and contains fining-upward, graded units, 1 to 4mm thick. About 300 m to the northwest (0654 1349), close to the Sham Wat Road, finely laminated mudstonecomprising graded units, 0.5 to 3 mm thick (HK 9216), is overlain by a massive, light grey, cherty tuffaceoussiltstone. Bedding dips 24° to the south, varying nearby to 28° to the north. Some of the rocks are brecciated (06291389), but sedimentary laminae, dipping at up to 44°, can still be discerned.On the hillside east of Sham Wat, a tuffite unit, less than 50 m thick, lies below autobrecciated rhyolite lava, Ittypically comprises grey, porcellanous, tuffaceous siltstone, but varies from tuffaceous sandstone to vitric tuff.Cheung Shan. Sedimentary rocks crop out extensively south of the summit of Cheung Shan, Near the summit(0587 1341), finely laminated siltstone dips south at 12°. Most exposures nearby are greenish-grey or light grey,bedded or finely laminated, typically cherty, and vary from mudstone to tuffaceous sandstone. Approximately 400m south of the summit (0559 1305) fragments of mudstone, up to 50 mm long, are embedded in a tuffaceousmatrix. This intraclast texture was probably formed by debris flow or slumping,Man Cheung Po. Fossiliferous siltstone has been excavated in pits and trenches at the campsite (0428 1018). Therock is typically weathered to a brownish-white colour, and contains small fern-like leaf impressions. Nearby50