MLA
O'Neill, John A., approximately 1836-. Union general Henry Halleck. 1861?/1880?. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/593ttdz1fr-cor.
APA, 6th edition
O'Neill, John A., approximately 1836-. (1861?/1880?)[Union general Henry Halleck]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/593ttdz1fr-cor.
Chicago
O'Neill, John A., approximately 1836-, Union general Henry Halleck, 1861?/1880?, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/593ttdz1fr-cor.