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alteration of volcanic and adjacent intrusive rocks. The fluids were derived from the granitoids ata late stage during, or after, their emplacement. The effects of a more general (regional)metamoiphism, dominantly of post- Jurassic age, can be recognized in a broad zone, trendingnortheast-southwest across the district. Within this zone, the intensities of dynamic andhydrothermal metamorphism vary considerably. Quartz veins, trending mostly northwestsoutheast,crystallized from fluids derived at a late stage from intrusions. They are commonlyassociated with hydrothermal alteration.Superficial ^deposits of Quaternary age form large, flgfcl5aiig^^s^slwre ? and constitute thesea-bed in most of Ae^offshore area. In hilly terrain, colluvium, including debris flow and otherslope debris deposits, mostly oFPleistocene age, commonly mantles side slopes and lines valleys.Small alluvial deposits occur in hilly areas, but alluvium is generally restricted to fans developeddownslope of the colluvial deposits. Pleistocene alluvium covers small areas around Mui Wo andTung Chung. Beach deposits of sand usually form in front of alluvial deposits, especially insheltered coastal bays. Pleistocene alluvial deposits also occur offshore, together with slopedebris deposits, within the Pleistocene Chek Lap Kok Formation, which generally comprisescomplex, interbedded sand, silt, clay and gravel, with some organic material. It is probably offluvial, estuarine and intertidal origin. It is generally overlain by the Sham Wat Formation. Itcomprises mainly silfr^ clayjwith some tMn_sands, and is thickest, and most readily recognisableseismically, in the west of the district. The base of the formation is channelised and its top islocally reworked. It is oy^lain^by the extensive Holocene marine deposits of the Hang HauFormation which comprises dark grey mud with shells, and some sand. In several places, landreclamation has disturbed, or obscured, the alluvial and marine deposits in the coastal fringe,especially around Chek Lap Kok, the northern coast and northern Tsing Yi. ,As a result of strong currents, rock is exposed in the channels around Ma Wan and northeast ofLantau Island. Elsewhere there are ajew rocky shoals such as at Pun Shan Shek and DouglasRock. The stronger currents which flow along the channel from The Brothers islands past MaWan to beyond Green Island, have also influenced the lithology and form of the Quaternarysediments. Coarse sandy sediments of the Hang Hau Formation are relatively extensive in thesechannels, and current velocities are high enough to form sand banks south of Tsing Yi and offnortheast Lantau Island. Parts of these sand banks have recently been dredged for fill material.Away from the channels, current strengths decrease and the sea floor generally comprises soft tovery soft mud of the Hang Hau Formation. East of Peng Chau and Hei Ling Chau, acousticturbidity on seismic records is extensive, which reflects the presence of biogenic gas within thethick mud of the Hang Hau Formation.v *A regolith, or mantle of weathered rock, occurs over most of the district. The effects ofweathering vary with underlying rock types and this is reflected in topographic relief Forexample, andesite, granodiorite and Palaeozoic sedimentary rocks are the most deeply weatheredand eroded rocks and form the lower ground. Granite terrain is characteristically hilly and litteredwith exhumed corestones, and finer grained granites generally form sharper relief than coarservarieties. The acidic volcanic rocks are resistant to deep weathering and erosion, and form rockyhills with boulder fields of large, cuboidal corestones.Although there are no active mineral workings in the district, lead and silver have been minednear Mui Wo. Tungsten and graphite have also been mined, and occurrences of lead, zinc, tin,fluorite and beryl have been recorded. There are many former small-scale workings for quartzand clay, which occur within intrusive and interleaved volcanic rocks. Although there are noaggregate quarries in the district, there are large borrow areas on Tsing Yi and smaller borrowsin northeast Lantau, as well as large offshore sand dredging for reclamation fill around the SokoIslands, Ma Wan, Tsing Yi and northeastern Lantau Island.25 ,