The Rocky Mountains decrease in height toward the south, near the line between New Mexico and Colorado. Here is situated Raton Pass, an ancient Indian highway from the valley of the Arkansas to the Rio Grande. In the early half of the last century this trail was much used by the caravans of traders and came to be known as the Santa Fe trail. [Illustration: FIG. 93.--ON THE CONTINENTAL DIVIDE IN SOUTHWESTERN WYOMING] In the early days of the American occupation of California, the Santa Fe trail became an important route to the Pacific. From the Mexican town of Santa Fe it led down the valley of the Rio Grande, following the old road to Mexico, and then turned west across the broad plateau of the continental divide, not far from the present course of the Southern Pacific Railroad. Passing Tucson, the road kept near the course of the Gila River to Fort Yuma, and then led over the Colorado Desert to Los Angeles. This path avoided all the high mountains, but much of it lay across deserts, where the heat and scarcity of water made it an impracticable route for the emigrants. One not acquainted with the physical geography of the West might wonder why the gold-seekers on their way to California did not make use of the Missouri River, which, except for the Great Falls, was navigable for small boats to the very base of the Rocky Mountains. A partial explanation is found in the report of the hardships endured by the Lewis and Clark exploring expedition, and later by the Astor party, which went out to found a fur trading post at the mouth of the Columbia. It had been supposed that after once crossing the continental divide it would be an easy matter to embark upon some stream and float down to the Pacific Ocean. The parties referred to became lost in the defiles of the mountains, and when they finally reached the Snake River it was only to find that rapids and waterfalls continually obstructed navigation. Although there was in most places plenty of water upon this northern route, yet the mountains were impassable for wagons.