=Transcontinental Trails.=--Following the purchase of the Louisiana territory there was, of course, an extension of settlement to the prairies beyond the Missouri. The State of Missouri was early occupied and became a state in 1821, but it was many years later before other portions of the Louisiana Purchase were sufficiently settled to become territories.[32] The settlement of these lands, together with the opening up of Oregon and later California with its great gold rush, created a demand for transcontinental roads. The mountain ranges were searched for passes, possibly not so much for the purposes of settlement as means for going to and coming from fur trading posts which large companies established throughout the whole Rocky Mountain region. St. Louis became the greatest fur center in the world, a position which she probably holds still.[33] Provost, leader of a detachment of the Rocky Mountain Fur Company (Wm. H. Ashley, of Virginia, founder), found the South Pass by way of the Sweetwater branch of the North Fork of the Platte River, 1823. This pass held preëminence as a crossing through the Rockies to the great interior basin and to the Pacific coast. Already has been mentioned the crossing of Lewis and Clark in the North. Bridger discovered the pass in Southern Wyoming bearing his name, about 1824. This defile though wide enough for an army to pass through seems narrow because of its lateral walls of red granite and metamorphic sandstone extending almost perpendicularly from 1000 to 25,000 feet. The overland mail route prior to the building of the Union Pacific Railroad was through this pass. Jedediah Smith, who succeeded Ashley as head of the Rocky Mountain Fur Company, explored practically all the region from Great Salt Lake to the Pacific, and from San Diego to the upper Columbia River in Canada. To him is the world indebted for its first knowledge of much of the vast region west of Salt Lake as by other active members of this company was revealed the sources of the Platte, the Yellowstone, the Green and the Snake Rivers, and possible routes through the almost impassable mountains drained by them. New England was especially interested in the Oregon country and through men from there the Humboldt River route was discovered. During this same period there were being opened up trade and trade routes with the Spanish possessions farther south. In 1822 a wagon train was taken from Missouri to Santa Fé by a man named Beckwith to trade for horses and mules, and trap along the way. For years St. Louis was headquarters for many overland traders to these regions, taking to them cloths and other manufactured goods and bringing back furs, silver, mules, and horses. [Illustration: TRANSCONTINENTAL TRAILS IN THE UNITED STATES]