During the War between the States, Hayden practiced medicine for the only time in his career, serving with the Army as a surgeon. Following the War he received his first formal degree in geology when he was elected Professor of Geology and Mineralogy at the University of Pennsylvania in 1865, a post he held mainly in absentia for 7 years. For the next several years, much of his time was spent studying and reporting on the geology of the Nebraska Territory and Rocky Mountain Region. In 1869 Hayden’s activities became officially organized under the Department of the Interior, as the United States Geological and Geographical Survey of the Territories. In that same year he completed a highly successful expedition through the western mountains from Denver to Santa Fe. This expedition set the pattern for those to follow, for his team studied not only the geology, but virtually all natural phenomena which they encountered, including wildlife, water resources, and mineral deposits. [Illustration: Dr. Hayden in Union Officer’s uniform during the Civil War. This was the only time Hayden actually practiced medicine.] Hayden’s historic expedition into the Yellowstone area in 1871, was preceded by two expeditions which fired the imagination of those interested in that largely unknown region. The Folsom-Cook group penetrated the Yellowstone Country in 1869, followed by the Washburn-Langford-Doane Expedition in 1870. Lieutenant Gustavus C. Doane, who served as the leader of the military escort for this latter expedition as well as for the later Hayden Survey, filed a detailed report which was published as a Congressional document and became a landmark of the Yellowstone story. The following is taken from his report: [Illustration: Hayden, mounted here on his horse “Patsy”, maintained a tenuous link with his professor’s chair at Pennsylvania by frequently wearing a frayed dress coat.]