Ballard, Robert D.
National Geographic Society, 1983
Buku Referensi
Hunt, Kellogg W.
Houghton Miffiln, 1967
Buku Teks SO
edited by Claude T. Bissell
Published in association with Carleton University by University of Toronto Pres, 1957
Buku Teks SO
edited by A.F. Scott
Macmillan, 1953
Buku Teks SO
editor, Clifton Fadiman
Doubleday, 1990
Buku Teks SO