The Central City-Idaho Springs area was the principal metal mining region in the state until the late 1880s. In 1858, rich placer deposits were discovered in gravels and river terraces along both forks of Clear Creek. Exploration upstream led to discoveries of rich oxidized quartz veins at Central City, Black Hawk, and Idaho Springs. These veins, which generally trend northeast-southwest, extend through the mountains in a zone about six miles long and three miles wide between the two forks of Clear Creek. The ores filled a multitude of cracks and fissures in the Precambrian bedrock. The veins are usually less than five feet thick, and are almost vertical and often clustered in zones up to thirty feet wide. The position of one of the vein systems may be seen clearly between Black Hawk and Central City—the ore-bearing rock has been mined out, but a series of collapsed tunnels marks the line where the veins crossed the valley. A monument here commemorates the discovery of Gregory Gulch, one of the richest localities in the state. Several rich veins were mined in both directions—southwest from Central City and northeast from Idaho Springs—until the mines met. The Argo tunnel, marked by dilapidated buildings and extensive dumps on the north side of Idaho Springs, connected the two districts; it was completed in 1904. The “Patch,” a deep crater-like hole on Quartz Hill, about one mile southwest of Central City, is an intriguing feature in this area. It was produced by glory-holing, a mining technique in which a deep tunnel is deliberately caved by blasting, so that ores above the tunnel can be removed. This glory hole was dynamited below an irregular mass of highly broken rock where many ore-rich veins converged. After the caving, ores were taken out through the remaining part of the tunnel. The principal ore minerals of Central City and Idaho Springs are native gold, pyrite, sphalerite, galena, chalcopyrite, and tennantite. Prospecting for uranium was carried out during the 1950s but no uranium was ever mined here. The area has produced almost $200,000,000 worth of gold, silver, lead, zinc, and copper. A few mines still operate seasonally or on a small scale, but tourists, many of them riding Jeeps across the mountainous terrain to visit mines and ghost towns, are often more visibly active than the mines. Georgetown, Empire, and Silver Plume