MLA
Pierce, George F. (George Foster), 1811-1884. Letter from George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - January 21, [no year]. January 21, year unknown. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/163stqjr6w-cor.
APA, 6th edition
Pierce, George F. (George Foster), 1811-1884. (January 21, year unknown)[Letter from George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - January 21, [no year]]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/163stqjr6w-cor.
Chicago
Pierce, George F. (George Foster), 1811-1884, Letter from George F. Pierce (Sparta, Georgia) to Robert Paine (Aberdeen, Mississippi) - January 21, [no year], January 21, year unknown, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/163stqjr6w-cor.