Columbia subject headings:
Fossil fuels--Colorado--History--19th century.
Fossil fuels--Colorado--History--20th century.
Harvard subject headings:
Fossil fuels--Colorado--History--19th century.
Fossil fuels--Colorado--History--20th century.
Princeton subject headings:
Fossil fuels--Colorado--History--19th century.
Fossil fuels--Colorado--History--20th century.