About ten miles south of Ouray, along the “Million Dollar Highway” (U. S. 550), the Red Mountain district lies on the northwest edge of the Silverton volcanic cauldron. It contains a number of small pipelike bodies very rich in silver-copper and silver-lead ores. Following the mid-Tertiary volcanism and ore intrusion, surface rocks in this area were intensely oxidized: resulting iron oxides now form the gaudy reds and yellows of Red Mountain and the slopes near Ironton. This alteration, as well as the fact that much of the area is covered with fallen rock, stream gravels, or glacial deposits, compounds difficulties of locating the small though high-grade ore deposits. The Idarado Mine, on the east side of U. S. highway 550 near Red Mountain, used to produce ores from nearby volcanic pipes; now it produces from veins some distance to the northwest. The area is honeycombed with tunnels and shafts. Aspen Silver was found at Castle Creek and on Aspen Mountain in 1879. A group of prospectors from Leadville, apparently after examining maps of the Geological and Geographical Atlas of Colorado published in 1877, explored along the line of Paleozoic limestones encircling the Sawatch Range. As they had hoped, they found ores similar to those at Leadville in rocks of the same age. Mining began at Aspen in 1880. Here, as at Leadville, intrusion of granite porphyry into or near the Leadville Limestone had broken and deformed the layers, and ores were deposited in fissures and as replacements during cooling of the intrusions. The intricacy of faulting which controls the ore pockets in the limestone is well shown on the map of Aspen Mountain in Chapter II. Glaciation occurred in this area, and glacial deposits cover most of the ore bodies and outcrops so that little bedrock is exposed. Mapping was accomplished by extrapolating to the surface the bedrock patterns shown in mine pits, shafts, and tunnels. Aspen produced some of the richest silver ores in the world, and thrived as a boom town for most of two decades. In 1888 the value of ores produced reached over $7,000,000; the next year it topped $10,000,000. After the silver crash of 1893 production declined rapidly; the last mines were closed in the 1920s. Total production of silver, lead, zinc, and copper reached about $100,000,000. There was virtually no gold in the ores at Aspen. Creede [Illustration: Creede and its mines are located in an area of Tertiary rhyolite and dacite, light-colored volcanic rocks.]