John Charles Fremont and his mapping of the wild country west of the Missouri are the subject of this large, wide-eyed, genuinely exciting book. Along with the personal qualities of ""an epic hero,"" Fremont possessed the advantages of first-rate training by the French explorer-scientist J. N. Nicollet and patronage from his father-in-law, the powerful Missouri Senator Thomas Hart Benton. As an officer in the US Topological Engineers Corps, he led ambitious expeditions to the West Coast and back in the 1840s despite the federal government's inclination to leave the Oregon Territory to the British. His band produced giant government reports on emigration routes, ecology, and geology--after living on mule meat during winter crossings of the Rockies, exploring the ghostly islands in the Great Salt Lake, and scalping Indians in reprisal raids (though Egan claims Fremont had a generally humane attitude toward them). Some of the most arduous adventures indicate a lapse of leadership on Fremont's part; but the book confines itself to stressing his impulsive dealings with the Army brass, who court-martialed him during an infight over the administration of a newly won California. Egan also gives only brief attention to Fremont's political career, his slander-ridden Presidential effort in 1856, and his personal opposition to slavery. Sustained through his postwar eclipse by a tough and beautiful wife, he died seeking a pension. The book succeeds in showing exploration as creative initiative rather than individual escapism; less academic than Nevins' Fremont the Explorer, it scores as grand storytelling.