MLA

Marsh, Othniel Charles, 1831-1899. Red Cloud Correspondence. Box 52, folder 21. 1875 August 16-31. https://collections.library.yale.edu/catalog/32510319.

APA, 6th edition

Marsh, Othniel Charles, 1831-1899. (["Box 52, folder 21"])(1875 August 16-31)[Red Cloud Correspondence]. https://collections.library.yale.edu/catalog/32510319.