Sichuan is the biggest province of the People's Republic of China, with a broad area of over 560 thousand square km and a population of over 90 million people. The central portion of it is a big basin about 200 thousand square km in area, surrounded by many mountains with average altitude between 1,000 and 3,000 m above sea level. About 90 per cent of the population of the province lives in this area. The west portion of the province, called West Sichuan Plateau with average altitude of over 3,000 m, is relatively unpopulated. According to a census of a district of Zigong in 1973, 39.2 per cent of the population was below age 15 and 4.3 per cent above age 65. [In Britain (1975) the corresponding figures were 25 per cent and 14 per cent—Editor].