The Contact: Competent Rock

The Sierra Nevada of California is riddled with tunnels drilled by seekers of gold and silver, most densely where the contacts of geologic formations have created fractures with gold-laden quartz. "Competent" rock means that miners could advance the drift without structural support. Some tunnels stretch for many miles; others end abruptly. In the early days, people were willing to go in with a candle, an iron bit, and a hammer to try their luck no matter how hard the rock or how dangerous the work.