Organized in 1850 as one of the original 27 California counties, Mariposa was the largest county in geographical terms, with an area of about 30,000 square miles covering one-fifth of the state. It included most of present-day Mono and Inyo counties, all of present-day Merced, Madera, Fresno, Kings, Tulare, and Kern counties, as well as small portions of San Benito, Los Angeles, and San Bernardino counties. The area consisted of a half-dozen mining towns, several bar camps, and one awarded land grant: explorer John Fremont’s Rancho Las Mariposas. The town of Mariposa was laid out on this acreage and became the county seat in 1851. The county took its name from Mariposa Creek, named by Spanish explorers in 1806 when they discovered a great cluster of butterflies (“mariposas” in Spanish and Portuguese) in the foothills of the Sierra Nevada. Each year, the first weekend in May, residents mark the annual arrival of migrating Monarch butterflies with a festival and parade. Tourism is the county’s primary economic source—the county’s eastern section is the central portion of Yosemite National Park—and many aspects of the area’s mining history are depicted in exhibits at the Mariposa History Museum and the California Mining and Mineral Museum. Two small gold mines, the Mockingbird and the Colorado Quartz, intermittently produce world-class specimens of crystalline gold for mineral collectors. The county seat remains at Mariposa. As of the 2010 census, the population was 18,251.