MLA
Merrick [last name unknown]. Letter from Merrick (Charleston, South Carolina), to "My dear Mamie" [mother], regarding the welfare of his brother George on Morris Island - February 2, 1861. February 2, 1861. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/129p2ngg2n-cor.
APA, 6th edition
Merrick [last name unknown]. (February 2, 1861)[Letter from Merrick (Charleston, South Carolina), to "My dear Mamie" [mother], regarding the welfare of his brother George on Morris Island - February 2, 1861]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/129p2ngg2n-cor.
Chicago
Merrick [last name unknown], Letter from Merrick (Charleston, South Carolina), to "My dear Mamie" [mother], regarding the welfare of his brother George on Morris Island - February 2, 1861, February 2, 1861, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/129p2ngg2n-cor.