The wonder with which the first western pathfinders discovered the varieties of terrain included in the Louisiana Purchase is consistently evoked in this description of four decades of exploration and settlement during the early 19th century. For instance, the seemingly limitless miles of what is now Arizona and Nevada, ""starkly beautiful in their terrifying way, (were) a farmer's idea of the other side of the moon."" Legend was that most land west of the Mississippi formed the Great American Desert. Frontiersmen gave up their earlier practice of building log cabins and singlehandedly setting large swatches of land; for western Indians were horse Indians, ""nomadic, predatory and nonagricultural,"" and old methods of Indian fighting were ineffective against them. These decades were mainly given over to penetrating the Rockies and establishing the ferociously competitive fur trade, which by 1846 would fade into the great exodus to California and then the Gold Rush. Mr. Rawling, beginning with the Lewis and Clark expedition (1804-1806), bases his study upon explorations along the area's five main rivers: the Missouri, Platte, Arkansas, Green and Columbia. While The Pathfinders does not replace Bernard De Voto's large trilogy covering the course of empire, he has produced a colorful work of wide general interest. His insight into Indian thought and mores is extraordinary.