When southern Arizona became a United States possession by virtue of the Gadsden Purchase of 1853, American prospectors and investment started mining silver deposits previously known to the Spanish and Mexicans in present Santa Cruz and Pima counties. The Santa Rita mine in the Santa Rita Mountains and the Heintzelman mine in the Cerro Colorado Mountains both started in 1856, the Mowry mine in the Patagonia Mountains started in 1857, and the Salero mine in 1858.