U.S. Dept. of the Interior, Geological Survey, Exploration and Settlement 1850-1890, in: The National Atlas of the United States of America (Arch C. Gerlach, editor). Washington, D.C.: U.S. Dept. of the Interior, Geological Survey, 1970. Source: Perry-Castañeda Library Map Collection. Courtesy of the University of Texas Libraries, The University of Texas at Austin. http://www.lib.utexas.edu/maps/united_states/exploration_1850.jpg, public domain.