During the centuries from Cabeza de Vaca's 1534 journey across Texas, New Mexico, and the entire Mexican mainland to the day when Frederick Jackson Turner could report to the 1893 meeting of the American Historical Society that the frontier line of settlement no longer existed, Adams' hard-in-the-saddle chronicle crosses the Great Plains many times, a plains defined as the area from North Dakota south to Texas and west to the Rockies. Each significant trek is described in the utmost detail, but the major exploration is credited to Lewis and Clark, whose tale is efficiently retold. Their Spanish predecessors had neither the resources nor the inclination to conquer the Plains, and the explorations of the Hudson Bay Company and the French fur traders along the 49th parallel remained limited by their immediate aims; Lewis and Clark, however, were not responsible for returning with a profit, and enjoyed full national support for their mapping and collecting. The book suffers from a preoccupation with Indian encounters, which undoubtedly held great concern for the explorers themselves; but the reader meets every tribe in the West without learning about the Indians' existence outside the clashes. After the years of exploration come the cowboys, buffalo hunters, gold strikers, and railroad builders, but there is little discussion of the actual settlers and their pattern of expansion. Preeminently a big, unreflective adventure survey (to include 32 photographs) by the author of biographies of John James Audubon, Geronimo, and Sitting Bull.