United States. National Park Service & Yard, R. S. (1925). The National parks portfolio (4th ed.). Govt. Pr. Off..
शिकागो शैली (17वां संस्करण) प्रशस्ति पत्रUnited States. National Park Service और Robert Sterling Yard. The National Parks Portfolio. 4th ed. Washington: Govt. Pr. Off., 1925.
एमएलए (8वां संस्करण) प्रशस्ति पत्रUnited States. National Park Service और Robert Sterling Yard. The National Parks Portfolio. 4th ed. Govt. Pr. Off., 1925.
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