Irving based this account on the manuscript of Benjamin Louis Eulalie de Bonneville. A French-born America soldier, Bonneville obtained leave from the army in 1832, and with a party of about 100 men embarked on a three-year involvement in the fur trade. Some historians believe that the expedition was actually a quasi-governmental spying venture on the Spanish in California. The first of the maps, "A Map of the Sources of the Colorado and Big Salt Lake..." is called by Wheat "an excellent map, by far the best yet published of this region...." The second is titled "Map of the Territory West of the Rocky Mountains." BAL 10151; Graff 2160; Howes I85; Sabin 35195; Smith 5046; Streeter 2092; Wagner-Camp 67:3; Wheat Transmississippi 423 & 424.