MLA

of Mines, Montana. State Bureau, and Metallurgy. Map of Central and Eastern Montana Showing Major Geologic Structures [Cartographic Material] / Montana State Bureau of Mines. The Bureau, 1921. https://collections.library.yale.edu/catalog/15821236.

APA, 6th edition

of Mines, M. S. B., & M. (1921). Map of central and eastern Montana showing major geologic structures [cartographic material] / Montana State Bureau of Mines. [Geological maps]. The Bureau, https://collections.library.yale.edu/catalog/15821236.