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abundant very fine-grained lithic fragments. The rock also displays some secondary alteration tocalcite and sericite.Volcanic EnvironmentThe Sunset Peak Member is a localized, welded pyroclastic flow that formed during a late,relatively quiescent stage in the development of the volcanic pile. The member includes eutaxitesdeposited from ash flows, but is dominated by tuffs, with cognate lithic fragments, which wereerupted from small vents, or fissures. The volcanic pile is complex and has a varied stratigraphy.Mount Davis FormationThe Mount Davis Formation, which forms part of the Repulse Bay Volcanic Group (Table 3), isdefined for the first time in this memoir. It comprises the coarse ash crystal tuff of western HongKong Island formerly ascribed to the Tai Mo Shan Formation (Strange & Shaw, 1986). The typelocality is Mount Davis, on Hong Kong Island, immediately east of the Lantau district. Theformation, which includes eutaxitic tuffs and epiclastic beds, overlies the Ap Lei ChauFormation. There is no stratigraphic overlap between the Mount Davis and Lantau formations.On Sheet 10 (Silver Mine Bay), it was implied that the Lantau Formation is younger than theMount Davis Formation. Subsequently, however, Rb-Sr age-dating of the Ap Lei Chau andLantau formations by Darbyshire (1990, 1993) has suggested that the Mount Davis Formationmay be younger than the Lantau Formation. Thermally metamorphosed rocks of the MountDavis Formation are exposed, in the district, on Green Island (Strange & Shaw, 1986) and onKau Yi Chau to the west.DetailsKau Yi Chau. Lapilli-ash crystal tuff, with elongate, dark, lithic lapilli, is exposed along the east coast of theisland. The lapilli and mafic crystals define a prominent lineation. There may also be small, sharp-edgedporphyritic lapilli. Blocks of eutaxite on the beach are probably derived from a higher, but unexposed stratigraphiclevel within the formation. The eutaxite is black, with abundant crystals of quartz and feldspar up to 5 mm. Thefiamme are up to 300 mm by 50 mm, and the rock resembles eutaxite partings in the Tai Mo Shan Formation. Thetuffs are cut by quartzphyric and feldsparphyric rhyolite dykes. At the northern end of the beach (2620 1624), thereis an easterly-trending contact between lapilli-ash crystal tuff to the south, and a chilled, largely aphardtic dyke tothe north.58