Não informado. (2004). Information Circular. Department of Natural Resources, Division of Geological Survey.
Chicago-referens (17:e uppl.)Não informado. Information Circular. Columbus, Ohio: Department of Natural Resources, Division of Geological Survey, 2004.
MLA-referens (8:e uppl.)Não informado. Information Circular. Department of Natural Resources, Division of Geological Survey, 2004.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.