ENG-1964 — Page 280

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GEOGRAPHY AND POPULATION

monocline which strikes north-east to south-west and is parallel in trend with the China coast. Its axis passes almost exactly through the centre of Hong Kong and is marked by a depression which is the Tolo Channel. The area consists of many rugged and irregular islands with deeply dissected peninsulas. The general appearance is that of an upland terrain which the sea has invaded. The uplands and mountains are eroded remnants of rock formations. Weathering is almost entirely caused by chemical action, helped by the alterna- tion of wet and dry seasons. As a result decay to a laterised rock mantle is common, often to depths of more than 100 feet.

The highest peaks and the most prominent ranges of hills are composed of either porphyries or volcanics. These are in contrast to the granite hills which generally occur at lower elevations but have well-etched peaks and sharp ridge lines. The plains are all recent alluvial deposits. Erosion benches can be found marking former sea levels up to 400 feet or more, which demonstrate the rise and fall of the whole region within recent geological times. Borings in the harbour have revealed submerged weathered rock surfaces overlain by peat deposits. The highest peaks, such as Lantau, Sunset and Tai Mo Shan, are all about 3,000 feet high and are composed of resistant, fine-grained crystalline rocks. By contrast the Kowloon Hills are composed of coarse-grained granite and have lower elevations, varying from 800 to 1,200 feet. The age of this granite has recently been determined by the Rubidium- Strontium method as approximately 134 million years. Thus it belongs to the Upper Jurrassic (Portlandian) period.

Only the soil of the flat agricultural alluvial districts around Yuen Long in the Deep Bay area has any depth. Elsewhere in the Colony the soil cover is usually thin, sometimes no more than two or three inches. In general the natural residual soils are acid and of low fertility, needing the addition of lime, potash and superphosphates. The predominating crystalline character of the rock formations makes them unsuitable as acquifers for under- ground storage and this makes it necessary to concentrate on the collection of surface water for water supplies. The highly variable and erratic rainfall regime of the area alone accounts for many of the water shortages. In 1963, for instance, the total rainfall was only 35.48 inches, compared with the yearly average of 85 inches. In order to husband and conserve water reserves it has been necessary