In the 1870s the Silverton district was very remote, and difficulties with transportation retarded activity there. In 1882, however, a narrow-gauge railroad was built connecting Silverton with Durango, and the problem of transporting ore out of the isolated mountain valley was simplified. The railway still exists; a train makes daily passenger runs during the summer—the only remaining operating narrow-gauge line in the United States. The track follows the Animas River canyon, whose cliffs and crags are dotted with long-abandoned mines, prospect holes, and mine buildings, monuments to the tenacity and determination of the men who mined here. Production in this district was more than $22,000,000 in gold and $20,000,000 in silver between 1874 and 1923. New activity is evident here, as in other silver-rich areas of Colorado, because of recent demand for silver, lead, and zinc. [Illustration: Silverton lies in a remote mountain valley in the San Juan Mountains. Silver, gold, lead, and zinc have been mined here since 1874. Storm Peak, composed of Tertiary volcanic rocks, forms the backdrop; the narrow-gauge railroad track is visible in the foreground. (Jack Rathbone photo)] Ouray Ouray was settled in 1875, when gold and silver deposits were found near Mount Sneffels. Since 1877, mines in Ouray County have produced over $35,000,000 in gold and $32,000,000 in silver. The district is still quite active: in 1965, mines in this area produced more than $9,000,000 in gold, silver, copper, lead, and zinc, about a third of total Colorado production of these metals for that year. [Illustration: A few miles south of Ouray, along Uncompahgre Gorge, an old mine clings to the slope below the Million Dollar Highway (U. S. 550). Abrams Mountain rises in the background. The Precambrian Uncompahgre Quartzite outcrops up to about the road level; Miocene Sunshine Peak Rhyolite caps the peak. (Jack Rathbone photo)] A mile north of Ouray a prominent intrusive stock marks the center of mining activity closest to Ouray. The richest deposits of the Ouray area, however, lie about five miles southwest, near Mount Sneffels and Red Mountain Creek. There, several large mines, including the famous Camp Bird mine, have operated for many years, extracting ore from hundreds of veins that underly the surface. Some of these veins are two to four miles long. They are in Tertiary volcanic rocks of the San Juan Formation. Quartz and calcite are the common gangue (non-economic) minerals, and pyrite, sphalerite, galena, and chalcopyrite are the most abundant ores. Most of the silver is in the galena; gold occurs in streaks and nodules associated with quartz.