The Napoleon mine at Copperopolis in Calaveras County opened in 1860, and was so productive that it ignited a boom in other copper-mining properties from 1862 to 1866. The boom stimulated development of copper mines along the "Foothill copper belt", a 250-mile long zone of copper deposits in the Sierra Nevada foothills, running from Butte County in the northwest to Fresno County in the southeast. Production nearly ceased after 1868 when the shallow oxidized ore was exhausted, and the deeper sulfide ores were found to be poorer in gold and silver. The Foothills belt yielded 91 thousand tonnes of copper and 23 thousand tonnes of zinc.