Riddled with footnotes, weighted with documentation, this monumental study by a history professor at the University of Texas is an account of 19th-century explorations, scientific, military and commercial, in the American West, and a philosophical interpretation of their influence on the development of this country. Giving as his objective ""to attempt the beginning of a new and more general assessment of the role"" and nature of Western exploration ""... and its consequences as an activity for civilization in general,"" the author divides his book into three parts, with the explorers playing different roles in different periods of time. Part I, ""Exploration and Imperialism,"" 1804-45, deals with early scientific, fur-trading and army expeditions (Lewis & Clark, Pike, Ashley, etc.); rival fur companies; mountain men and trader-explorers, such as Peter Skene Ogden, Jedediah Smith, etc. Part II, 1845-60, treats of ""the explorer as diplomat"" with the focus on Fremont, and also with the settlement of boundary disputes, ""the search for the iron trail,"" and the arrival of ""new mountain men"": topographical engineers, cartologists, geologists and archaeologists. Part III, 1860-1900, the longest and most valuable section, gives detailed accounts of late 19th-century government surveys and excellent brief biographies of the men who conducted them. In a book ""based on virtually every printed work relating to American exploration in the 19th-century"" there are, however, a few somewhat surprising omissions, among them the Whitmans, who established the first Indian mission school in the northwest -- missionary explorers; the successful but forgotten experimental journey of ""Uncle Sam's Camels"" from Texas to California in 1857; Edward Creighton, who in midwinter, 1860, alone and on horseback, surveyed much of the route later followed by the first telegraph line from the Missouri to the Pacific. These are minor flaws in a book of special scope and enormous reference value.