MLA
Earle, George W. Letter from George W. Earle (Charleston, South Carolina) to David Flavel Jamison, Asking to Resign When the State Troops Are Turned over to the Confederacy - April 20, 1861. 1861. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 4, Folder 7. https://digital.library.emory.edu/purl/2150gb5nm4-cor.
APA, 6th edition
Earle, G. W. (1861). Letter from George W. Earle (Charleston, South Carolina) to David Flavel Jamison, asking to resign when the State troops are turned over to the Confederacy - April 20, 1861 (Box 4, Folder 7) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/2150gb5nm4-cor.
Chicago
Earle, George W. “Letter from George W. Earle (Charleston, South Carolina) to David Flavel Jamison, Asking to Resign When the State Troops Are Turned over to the Confederacy - April 20, 1861.” Letters (correspondence). Civil War Collection, 1861. Box 4, Folder 7. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/2150gb5nm4-cor.