Columbia subject headings:
Harvard subject headings:
Coal mines and mining--Germany--Ruhr (Region)--History.
Horses--Germany--Ruhr (Region)--History.
Mineral industries--Germany--Ruhr (Region)--History.
Pit pony--Germany--Ruhr (Region)--History.
Princeton subject headings:
Coal mines and mining--Germany--Ruhr (Region)--History.
Horses--Germany--Ruhr (Region)--History.
Mineral industries--Germany--Ruhr (Region)--History.
Pit pony--Germany--Ruhr (Region)--History.