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Exposures of the eastern outcrop of quartz syenite are very poor, and the strongest evidence for outcrop form comesfrom the occurrence of a topographic low, rich vegetation and reddish-brown, saprolitic soil on the ridge west ofTung Chung Wan (097 157). To the east, towards Tung Chung, scattered exposures have been noted, and quartzsyenite has been recorded close to medium-grained granite at the base of the slope (1846 1539).On the rocky coast north of San Shek Wan (0714 1611), there is a 3 m-wide, porphyritic, quartz syenite dyke withabundant feldspar megacrysts, up to 6 mm across. The dyke trends eastnortheast, and has a finer-grained, sheared,or trachytic margin against foliated sedimentary rocks to the north.Hei Ling Chan. Quartz syenite crops out as dykes and small bodies on southern Hei Ling Chau, along the eastcoast of Hei Ling Chau, and on both east and west Sunshine Island (Chau Kung To). The largest bodies are on thesouthern ends of both islands, where they are up to 700 m long, and less than 200 m wide.On southern Hei Ling Chau, the quartz syenite trends northeast with an inferred contact against fine-grainedgranite. The contact is obscured by boulders, and no age relationships can be determined. However, on the southerntip of Hei Ling Chau (2266 1147), presumably close to the southern margin of the dyke-like intrusion,medium-grained granite and aplite are cut by fine-grained quartz syenite with a trachytoid texture.On the southwestern tip of Sunshine Island (2306 1325), the quartz syenite has a chilled margin about 50 mm wideagainst feldsparphyric rhyolite. There is a rapid decrease in the feldspar phenocryst content closer to the contact,whereas the older rhyolite appears unchanged at the contact.Small dykes of quartz syenite are common along the east coast of Hei Ling Chau. Some have slightly chilledmargins against the granite country rock. One dyke (2256 1239) is cut by a flow-banded quartzphyric rhyolite dyke,10m wide and trending northeast.Quartz syenite was only seen exposed on the southwest tip of the island, but its presence is inferred from theabundant, dominant corestones which have toppled onto the beach near the northwest tip of the island (237 138).There, the feldsparphyric rhyolite adjacent to the syenite has been strongly deformed. Feldspar megacrysts arestreaked and contorted in a zone over 1 m wide. This is believed to be the result of the younger syenite deformingthe hot, plastic dyke rocks.Abundant boulders of porphyritic quartz syenite disguise the generally deeply weathered outcrop in the southwest."However, in one tumbled block (233 132), the contact between quartz syenite and feldsparphyric rhyolite can beseen. The rhyolite appears to be partially assimilated, or absorbed, by the syenite, and hence is the older rock. Thenorthern margin of the quartz syenite is exposed in a deeply weathered coastal section. It has a chilled margin,about 50 mm wide, in which there is a rapid decrease in the number of feldspar phenocrysts. The older rhyolite hasa distinct margin. Boulders of rhyolite on the point to the south (230 132) suggest that the quartz syenite has acomplex,, dyke-like form, incorporating slivers of older rock.PetrographyThin sections of quartz syenite from across the district are texturally and mineralogically similar.For example HK8117 (1765 1368) from the South Lantau Road contains subhedrai alkalifeldspar phenocrysts 2 to 10 mm long, in a groundmass of alkali feldspar, plagioclase, quartz,mafic and opaque minerals. The groundmass, typically 0.1 to 0.5 mm, forms roughly half therock. Alkali feldspars comprise 62% of the mineral content, mostly sericitized plagioclase only15%, and quartz 15 to 25% (e.g. HK10035, 0170 0804). Quartz rarely occurs as phenocrysts orxenocrysts. Mafic minerals include biotite, usually chloritized (c.5% of the rock), and euhedralhornblende (