MLA
Allen, R. Y. Letter from R. Y. Allen (San Francisco, California) to Robert Paine - August 21, 1849. 1849. Pitts Theology Library, Box 1, Folder 1. https://digital.library.emory.edu/purl/69673n5vht-cor.
APA, 6th edition
Allen, R. Y. (1849). Letter from R. Y. Allen (San Francisco, California) to Robert Paine - August 21, 1849 (Box 1, Folder 1) [Letters (correspondence)]. Pitts Theology Library. https://digital.library.emory.edu/purl/69673n5vht-cor.
Chicago
Allen, R. Y. “Letter from R. Y. Allen (San Francisco, California) to Robert Paine - August 21, 1849.” Letters (correspondence). Robert Paine Papers, 1828-1882, 1849. Box 1, Folder 1. Pitts Theology Library. https://digital.library.emory.edu/purl/69673n5vht-cor.