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A typical, finer-grained sample of basalt (HK9237, 1156 1827) from the peninsula south of KwoLo Wan'is a dark green, very fine grained, and speckled with mafic phenocrysts. Thephenocrysts, seen in thin section, are composed of secondary chlorite and calcite, and thegroundmass contains abundant hornblende, with plagioclase and epidote.Laniprophyre dykes in hand specimen can display prominent, dark green, mafic phenocrysts, andoverall are mafic-rich relative to the basaltic dykes. A thin section (HK10859, 1117 1986) of therock contains abundant augite and hornblende, with some plagioclase which is mostly less than 1mm long (Plate 37). Chlorite replacement of lath-like mafics is common, and there are nofeldspar phenocrysts. The composition of the lamprophyre is probably vogesite.Age RelationsAllen & Stephens (1971) considered that the basalt dykes were emplaced during separate UpperCretaceous and Tertiary phases. They cut the granite (Late Jurassic-Early Cretaceous) countryrock, but are themselves cut by small, northwest-trending faults. The lamprophyre dykes also cutrhyolite dykes and granite, and are considered to be of a similar age to the basalt dykes. Atone locality (1051 1920), north of the test embankment, a lamprophyre is cut by a quartz veinabout 0.25 m wide.Aplite and PegmatiteDistribution andLithologySmall icnpersistent dykes of aplite are occasionally found in the granites. For example, they occurin small groups at a number of places around the coast of Chek Lap Kok, where the dykes areleucocratic, with a markedly equigranular grain size of around 0.5 mm. They can be from 25 mmto 5 m wide, but are commonly around 0.25 m, The aplite and granite are closely related in ageand genesis. Similarly, apiite dykes occur in medium-grained granite on Cheung Chau (Plate 38).Pegmatite, or very coarse-grained granite, with crystals over 20 mm across, is rare, and generallyoccurs as pods or lenses too small to be shown on the published map of the district. However, anirregular, roughly east-west trending pegmatite is exposed west of Fu Tei Wan (1075 1824).Pegmatite is only found within granite, and formed at a late stage during the cooling of theplutons.Quartz veinsQuartz veins occur in rocks of all types and all ages in the district, but were clearly emplacedduring several phases. A vein chronology has not been established in the district, and only themost substantial quartz veins are shown on the published maps,DetailsChek Lap Kok (prior to development). Quartz veins are widely distributed throughout the island, and mostlytrend east to eastnortheast They cut the granite country rock and most minor intrusions, and are generally narrow,ranging from stringers a few millimetres wide, to veins up to 03 m wide. Quartz veins are especially common inand around the kaolinized granite east of the test embankment. The quartz veins commonly form upstanding ribs inthe deeply weathered granite. The thin veins tend to separate along cracks on joint faces to produce mosaic-likesheets of quartz. Soil creep bends and displaces these sheets to form debris trails downslope,As well as discreteveins, there are places where the granite country rock has been silicified. To the south of the test embankment, in acoastal exposure (1072 1835), the fine-grained granite country rock has been finely veined and silicified in a zone,at least 1 m wide, which trends southsoutheastPetrographically, the veins typically comprise massive white crystalline quartz. Euhedxal crystals are rare, althoughcrystal aggregates up to 0.2 m across occur, with individual crystals up to 45 mm across (HK9555,1091 1912).90