Levin, H. L. (1999). The Earth through time (6th ed.). Fort Worth : Saunders College Pub.
Cita Chicago (17th ed.)Levin, Harold L. The Earth Through Time. 6th ed. Fort Worth : Saunders College Pub, 1999.
Cita MLA (9th ed.)Levin, Harold L. The Earth Through Time. 6th ed. Fort Worth : Saunders College Pub, 1999.
Atenció: Aquestes cites poden no estar 100% correctes.