So scattered and ramified were the enterprises and expeditions of the traders with the Indians that the frontier which they established was at best both shifting and unstable. Following far in the wake of these advance agents of the civilization which they so often disgraced, came the cattle-herder or rancher, who took advantage of the extensive pastures and ranges along the uplands and foot-hills to raise immense herds of cattle. Thus was formed what might be called a rancher's frontier, thrust out in advance of the ordinary farming settlements and serving as the first serious barrier against the Indian invasion. The westward movement of population is in this respect a direct advance from the coast. Years before the influx into the Old Southwest of the tides of settlement from the northeast, the more adventurous struck straight westward in the wake of the fur-trader, and here and there erected the cattle-ranges beyond the farming frontier of the piedmont region. The wild horses and cattle which roamed at will through the upland barrens and pea-vine pastures were herded in and driven for sale to the city markets of the East. The farming frontier of the piedmont plateau constituted the real backbone of western settlement. The pioneering farmers, with the adventurous instincts of the hunter and the explorer, plunged deeper and ever deeper into the wilderness, lured on by the prospect of free and still richer lands in the dim interior. Settlements quickly sprang up in the neighborhood of military posts or rude forts established to serve as safeguards against hostile attack; and trade soon flourished between these settlements and the eastern centers, following the trails of the trader and the more beaten paths of emigration. The bolder settlers who ventured farthest to the westward were held in communication with the East through their dependence upon salt and other necessities of life; and the search for salt-springs in the virgin wilderness was an inevitable consequence of the desire of the pioneer to shake off his dependence upon the coast.