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undertaken, and the speed with which minerals could be
put to use. Reliance shifted from the slow acquisition
of plentiful or renewable resources to the rapid
exploitation of non-renewable resources.^ Where once
productive activity had consisted of harvesting or
simple manufacture based on natural processes, it changed
to rely upon mining activities, upon true extraction.
Wood in houses came to be replaced by iron in skyscrapers.
Horsepower developed by fuel burning metal engines
replaced animal power. Even organic processes were
wrung for more and more produce as land was made to
yield two bushels of grain where it had yielded one.
Increases in production could occur at a much more rapid
pace and new materials could be drawn not only from the
land's surface, but from beneath the surface as well; it
was almost as if a whole new world had been discovered
and its resources made available for man's use.
But while the new kinds of production changed
the nature of materials problems, it did not do away
Labeling some resources as renewable and others
as not renewable is somewhat arbitrary and misleading. In
fact, a resource material may be thought of as being
renewable so long as the rate at which it becomes avail
able in usable form equals or exceeds its rate of
exhaustion. Until recently, for example, the rate at
which our forests were cut exceeded the rate at which they
were replanted. On the other hand, salt is deposited at a
rate that makes its exhaustion unlikely. It is therefore
a renewable resource. Most mineral concentrations are
renewed very slowly, however, at a rate figured in terms
of geological time insofar as iron, copper, and most
other metals are concerned.