Some mines are superior, others medium, others marginal which it barely pays to work.

There is, therefore, a rise and fall of the margin of gold production with changes in prices and changes in the cost of production.

Large new deposits of gold are discovered from time to time and new methods of extracting gold are invented.

If, when it barely pays to work a mine, such changes occur, gold becomes worth less, and the poorer mines eventually must go out of use.

As gold rises in value some abandoned mines again come into use.