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dipping northwest at 46°, is present in a deeply weathered tuff in exposures (0480 1380) near the footpath to TaiO.Tai O. Eutaxite boulders are common in debris deposits on the hills east of Tai O, together with less commonboulders of vitric tuff and quartzitic sandstone. Exposures (0448 1340) of highly weathered vitric tuff can be seenabove Tai 0 cemetery. The weathered rock is white and contains small, broken to euhedral quartz crystals. Nearby,there are boulders of eutaxite which are more typical of the member.At San Tsuen, south of Tai O, a north-trending ridge is composed of quartz ash-bearing tuff with a fine fabric ofrelict fiamme. Near the Service Reservoir (0447 0396), the relict fiamme dip westsouthwest at 38°. Further west,altered and mylonitized tuffs of the Shing Mun Formation are exposed across a northwest-trending fault.Tsim Fung Shan - Tai Horn Shan. The member forms a narrow, laterally discontinuous outcrop which extendsfrom Tsim Fung Shan (041 114), south through Nga Ying Sham, Shui Lo Cho and Man Cheung Po, to Tai HornShan (033 093). Most of the exposures on the northwest flanks of Tsim Fung Shan consist of crystal ash-bearingvitric tuff. At several localities, well-developed eutaxitic fiamme, 10 to 30 mm long, are preserved in the tuff Noclear base to the member has been defined, although tuff and tuffaceous sandstone to the west are probably part ofthe Shing Mun Formation. The member is overlain by lavas, tuffaceous sandstones and tuffites of the LantauFormation (undivided). West of Man Cheung Po, the member crops out below a fossiliferous siltstone sequence.The eutaxitic fabric is occasionally visible m the quartz and feldspar vitric tuffs. The pyroclasts are generally lessthan 2 mm across.Southwest of Man Cheung Po, towards Tai Horn Shan, a faint streaky fabric is often visible, although itsorientation is rarely clear. The relict fiamme are, atypically for the member, generally less than 20 mm long, andthe euhedral to subhedral quartz crystals rarely exceed 2 mm. The member lies below siltstone of the LantauFormation, and above lapilli-ash tuff of the Shing Mun Formation. However, neither contact is exposed, and thestratigraphic relationships can only be inferred.Cheung Sha. The only unaltered exposures of the member occur in a narrow outcrop in south Lantau Island whichdominantly comprises eutaxite, with minor, coarse ash-bearing vitric tuff. The eutaxitic fabric is well displayed inan old quarry (1214 1065) adjacent to the catchwater. White fiamme, which are coarsely porphyritic, and havesharp, ragged edges, dominate the grey tuff. They define a fabric which dips northwards at 70°, and similarly steep,northerly dips also occur at several localities along the Tung Chung Road. Stratigraphically above the eutaxite, thetuffite and lava belong to the Lantau Formation, whereas below, the tuffs are typical of the Shing Mun Formation,but contact relationships have not been seen. East of the Tung Chung Road, the member has not been seen, but tothe west, boulders confirm its presence on the hillside above Tong Fuk.Volcanic EnvironmentThe dominant eutaxitic welding fabric indicates that the tuff was deposited from a hot, massiveash-flow eruption. Its eruption marked the onset of eruptions from a large magma chambersituated beneath Lantau. Older pyroclastic flow deposits contain a noticeable lithic component,but the eutaxite is essentially monomict, and its fiamme are compositionally related to theensuing, lava-dominated succession. These early eruptions of the Lantau Formation werefollowed by a period of quiescence, during which sediments accumulated in local depressions.Pak Kok MemberThe Pak Kok Member is defined for the first time in this memoir. It includes tuffs, tuffites,siltstones and mudstones which crop out in southwest Lantau Island, Whereas most epiclastic, ortuffaceous., horizons in the Lantau Formation are lithologically homogeneous and, or, generallythin, this member is heterogeneous and thick. At its type locality, Pak Kok (065 081) on thesouthwest coast of Lantau Island, the member is at least 300m thick, and includeshydrothermally altered, pyroclastic and epiclastic deposits including ash-bearing vitric tuff,pebbly tuffite, and siltstone. The member also crops out at Shek Pik; and between Kwun YamShan and Kung Shan, and may also include relatively thin and discontinuous tuffites andsiltstones that crop out further northeast, towards Ngong Ping.55