Of the known, naturally occurring elements, the major proportion are metals.

They form about 25% of the earth's crust by weight. The most common is aluminium (8%), followed by iron (5%). Others such as chromium (0.02%), copper (0.007%) and silver (0.00001%) are much more scarce. All metals, with

the exception of gold, are found chemically combined with other elements in the form of oxides and sulphates. These ores are not distributed evenly. Substantial deposits tend generally to be found in localised areas.

Exploitation by open cast working or deep mining depends on commercial considerations. These include such matters as size of deposit, ease of location, scarcity and demand. The history of English tin mining (picture to left) illustrates how these circumstances continue to fluctuate.