Davenport, S.J.
Washington, DC: United States Department of the Interior, 1954
Buku Teks
Paul L. Magill, Francis R. Holden, Charles Ackley, editor; Frederick G. Sawyer, editorial consultant
McGraw-Hill, 1956
Buku Referensi
Cooper, C. David
Waveland Press, 1986
Buku Teks
Williamson, Samuel J.
Addison-Wesley, 1973
Buku Teks
Crawford, Martin
Tata McGraw-Hill, 1976
Buku Teks