APA (7th ed.) Citation

Roberts, C. G. D. (1969). Earth's enigmas. Books for Libraries Press.

Chicago Style (17th ed.) Citation

Roberts, Charles G. D. Earth's Enigmas. Freeport, N.Y.: Books for Libraries Press, 1969.

MLA (9th ed.) Citation

Roberts, Charles G. D. Earth's Enigmas. Books for Libraries Press, 1969.