Earth resources.

Reliable supplies of metals have historically been the keys to industrial and technological development. But many metals are subject to the possible exhaustion of traditional kinds of deposits. A continued supply of such metals, which include tin, tungsten, silver, lead, zinc, and many others, will require their recovery from common rocks, in which they are found in solid solution in common silicate minerals. Recovery from unconventional sources will be so energy intensive that we may eventually have to stop mining such metals. The greatest challenge facing the U.S. Geological Survey in its second century will be the problem of resource limitations.