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Hei Ling Chan. Porphyritic medium-grained granite, pegmatite and aplite are well exposed on the western andsouthern coasts of Hei Ling Chau. On the southwest coast (2216 1175), the granite is typically pink and white,mottled black, with pink alkali feldspar phenocrysts up to 40 mm long. The equigranular groundmass contains pinkalkali feldspar up to 8 mm and white plagioclase less than 6 mm. Quartz occurs as interstitial crystal aggregates upto 10 mm across. The rock also contains distinctive pegmatitic patches. At a locality (2241 1250) near the southerntip, there are boulders of granite with pegmatite that display perthitic exsolution in the alkali feldspar (Plate 21).About 100 m to the westnorthwest there is a 1 m-wide, subhorizontal aplite dyke with a pegmatitic core.The presence of dark aphanitic xenoliths, up to 0.5 m across, has been noted in medium-grained granite on the westcoast (2163 1242). Close to the jetty (2100 1305), there are exposures of strongly discoloured, finely-fractured andjointed, essentially fine-grained granite. This is megacrystic, and looks in part like medium-grained granite. Furthersouth, there is a swarm of feldsparphyric rhyolite dykes, between which there are slivers of medium-grained graniteup to 8 m wide.Kan Yi Chau. Porphyritic medium-grained granite is exposed on the west coast of Kau Yi Chau (Plate 22). Therock contains alkali feldspar megacrysts up to 30 mm long with abundant biotite crystals and crystal aggregatesfrom 1 to 10 mm. At one locality (2567 1612) in the southwest, the granite is intruded by fine veins of biotite-richgranite and sub-horizontal pegmatite veins from 0.1 to 0.2 m wide.Siu Kau Yi Chau. Exposures of porphyritic medium-grained granite on this island are limited, the dominant rocktype being feldsparphyric rhyolite dykes. Little of the granite country rock is exposed, but where seen, on thewestern coast, it is medium grained and porphyritic.Cheung Chau Equigranular medium-grained granite is well exposed on the coast of Cheung Chau, often displayingwell-developed honeycomb structure on weathering (Plate 54). The granite is light pink, weathers to a yellowishbrown, and is very uniform in texture and appearance over the whole island. Numerous east-, toeastnortheast-trending feldsparphyric rhyolite dykes cut the granite. They often have well-developed chilledmargins, for example, on the coast at Italian Beach (Pak Tso Wan) in southwest Cheung Chau (1999 0654), and onthe coast south of Fa Peng in southeast Cheung Chau (2208 0720). In addition, the granite is cut by many smallaplite (Plate 38) and basalt dykes, and by thin quartz veins. South of the cemetery at Kau Kung Tong (2043 0664),in southwestern Cheung Chau, there is an eastnortheast-trending zone of quartz veins alternating with aplite, up to20 m wide. Localized kaolinization, probably the result of weathering, can be seen in an isolated exposurenortheast of Cheung Kwai Estate (2082 0846).Shek Kwu Chau Equigranular medium-grained granite with a distinctive pinkish colour in fresh exposures can beseen over most of the island. Only rarely does the rock vary texturally. Mafic xenoliths, up to 0.15m across, occurin exposures on the northwest coast (1634 0666), along with biotite concentrations and small pegmatite patches(16460680).Soko Islands. Porphyritic medium-grained granite (Plate 23) outcrops on both main islands of the Soko Islandsgroup. It is cut by numerous, eastnortheast-trending, feldsparphyric rhyolite dykes.PetrographyA sample of medium-grained granite from Tai Long Wan (HK8710, 1800 0878) contains pinkalkali feldspar crystals up to 10 mm across, white plagioclase and grey quartz, up to 5 mm, andbiotite flakes and aggregates less than 3 mm. In thin section, the biotite is pleochroic,and variesfrom greenish-brown to green in plane-polarised light. Zircon commonly occurs as inclusions inbiotite grains, which may be partly altered to chlorite. Other accessory minerals includemagnetite, titanite and monazite.Age RelationsThe medium-grained granite in the district has not yet been isotopically dated. However, with theexception of Sha Chau, it is cut throughout the district by rhyolite dykes of the Lajitau dykeswarm. These dykes mostly predate volcanic rocks of the Lantau Formation which have a68