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THE ENVIRONMENT
as a site where a rare tree or a rare species of butterfly can be found. Up to Decem- ber 31, 1977, a total of 11 of these sites had been identified for future conservation action.
Topography and Geology
Hong Kong lies on the edge of an eroded mountain chain that extends along the south coast of China and is largely composed of folded and metamorphosed volcanic and sedimentary rocks with younger intrusions of granitic rocks mostly of the Juras- sic period. The oldest sedimentary rocks found in Hong Kong are those of the Tolo Harbour Formation. This formation is exposed at Ma Shi Chau and contains fossils dated as most probably Permian in age.
However, its stratigraphic relationships are somewhat uncertain. The formation of minerals associated with the intrusion of the granitic rocks has been of limited economic benefit to Hong Kong. Lead, zinc, tungsten, beryl and graphite have been mined intermittently, but only in small quantities.
Because of the hilly terrain, agricultural land is restricted. The most important area is the flat alluvium around Yuen Long in the Deep Bay area. Outside the alluvial areas, soil cover is usually thin - sometimes no more than five or 7 centimetres. In general, the natural residual soils are acidic and of low fertility, needing the addition of lime, potash and superphosphates. However, given intensive labour input, water supply rather than soil condition tends to be the controlling factor in farming. The predominantly crystalline character of the rock formations unfortunately makes them unsuitable for underground storage, bringing about the necessity to concentrate on the collection of surface run-off for all water supplies. The highly-variable rainfall of the area has led to periodic water shortages. Most of Hong Kong's surface water supply has now been captured through the construction of catchments and reservoirs. Following completion of the High Island reservoir scheme, desalination processes on a large scale may become necessary.
Hong Kong lies in the double-cropping rice zone of East Asia, but more profitable vegetable crops have increasingly displaced rice during the past 25 years. Fish ponds also are an important form of rural land use.
Climate
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Hong Kong lies within the tropics but, unlike many other tropical places, it experiences distinct seasonal changes in weather. The winter months are characterised by frequent outbreaks of cold and often dry air originating from the Asian continental anti- cyclone. It is not uncommon during January, February and March to have tem- peratures below 10 degrees Celsius, although the mean temperatures from December to March are around 15 to 17°C. During outbreaks of cold air, the winds often become strong from the north or east. The prevailing wind during winter, and for most of the year, is a moderate easterly.
In summer, the weather is tropical: hot and humid with occasional showers or thunderstorms. Winds are often light and variable, although the south-west monsoon is the prevailing wind affecting South-east Asia. Afternoon temperatures frequently exceed 32°C between June and September with mean temperatures of 27 to 28°C. Tropical cyclones are most common between July and September. In an average year,