MLA

Schonborn, Antoin. Red Buttes of Powder River. 0AD. https://collections.library.yale.edu/catalog/2112402.

APA, 6th edition

Schonborn, A. (0 C.E.). Red buttes of Powder River [Illustrations, Watercolors (Paintings)]. https://collections.library.yale.edu/catalog/2112402.