Columbia subject headings:
Copper industry and trade--United States.
Lead industry and trade--United States.
Tariff--United States.
Zinc industry and trade--United States.
Harvard subject headings:
Copper industry and trade--United States.
Lead industry and trade--United States.
Tariff--United States.
Zinc industry and trade--United States.
Princeton subject headings:
Copper industry and trade--United States.
Lead industry and trade--United States.
Tariff--United States.
Zinc industry and trade--United States.