The national parks portfolio,
by Robert Sterling Yard, formerly editor, National Park Service.

APA Citation

United States. National Park Service., Story, I. F. (Isabelle Florence)., Yard, R. Sterling. (1931). The national parks portfolio. 6th ed. Washington: U.S. Govt. print. off..

MLA Citation

United States. National Park Service, Isabelle F. (Isabelle Florence) Story, and Robert Sterling Yard. The National Parks Portfolio. 6th ed. Washington: U.S. Govt. print. off., 1931.

Warning: These citations may not always be complete (especially for serials).