Frémont's most important work, chronicling his seminal expeditions that revealed the paths and trails that were to be the highways by which the gold seekers would rush to California beginning in 1849, with his important large map of the West. The first portion of the work reprints Frémont's report of 1843, covering his 1842 expedition to the Rocky Mountains, the second portion records his expedition of 1843-1844, delineating the major sections of the route subsequently followed by thousands of Oregon immigrants. This Senate issue contains scientific data not present in the smaller House issue. Wheat describes the map at great length, and attaches great importance to it: "The year 1845, however, though otherwise somewhat cartographically barren, because of a single event is in fact one of the towering years in the story of Western Cartography. In that year John C. Frémont's report of his journey to Oregon and California in 1843-44 was published. This report and the Frémont (Preuss) map which accompanied it, changed the entire picture of the West, and made a lasting contribution to cartography...." Cowan p.223-4; Graff 1436; Howes F370; Wagner-Camp 115:1; Wheat Transmississippi Vol. II, pp.194-200, Map 497; Zamorano Eighty 39.