engle, Eloise
The John Day Company, 1963
Buku Teks SO
edited by Gerard J. Mangone
Van Nostrand Reinhold, 1983
Buku Teks SO
Artikel Jurnal
Departhemen of Geography University of Waterloo, 2005
Buku Teks SO
Handy, Willowdean C.
Australian National University Press,, 1973
Buku Teks SO