United States. National Park Service, Yard, R. S., & Story, I. F. (1931). The national parks portfolio (6th ed.). U.S. Govt. Print. Off..
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रUnited States. National Park Service, Robert Sterling Yard, और Isabelle F. Story. The National Parks Portfolio. 6th ed. Washington: U.S. Govt. Print. Off., 1931.
एमएलए (8वां संस्करण) प्रशस्ति पत्रUnited States. National Park Service, et al. The National Parks Portfolio. 6th ed. U.S. Govt. Print. Off., 1931.
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