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Geyer was a German botanist who collected extensively over the Great Plains and Rocky Mountains. In 1843, he joined up with Sir William Drummond Stewart's expedition into northwestern Wyoming. He then split off from this group and joined Jesuits traveling to Father DeSmet's St Mary's mission (near Corvallis) in the Bitterroot Valley. Geyer entered Montana over the Targhee Pass, descended the Madison River, crossed the Beaverhead and Big Hole Valleys, and entered the Bitterroot Valley via Gibbons Pass. He left Montana following the Clark Fork River. In addition to collecting plants, Geyer's notes provide a detailed description of the vegetation in western Montana. Among his observations are quantitative references to giant ponderosa pine and western red cedar. Figure 9 shows the approximate route taken by Geyer while in Montana.

Geyer, C.A.  1895.  Notes on the vegetation and general character of the 
     Missouri and Oregon Territories made during a botanical journey from 
     the state of Missouri Across the south-pass of the Rocky Mountains to 
     the Pacific, during the years of 1843 and 1844.  London Journal of 
     Botany 4:479-492, 653-662; 5:22-41, 198-208, 285-310, 509-524.

Hooker, W.J. 1847-1856.  Catalogue of Mr. Geyer's collection of plants 
     gathered in the Upper Missouri, the Oregon Territory, and the 
     intervening portion of the Rocky Mountains.  London Jour. Bot 
     6:65-79; Hooker's Jour. Bot. 3:287-300; 5:257-265; 7:371-378; 8:16-19.

Thompson, L.S.  1985.  Montana's Explorers: The pioneer naturalists.  The 
     Montana Magazine, Helena.