Franciscan exposed at the Permanente and Pacifica cement quarries also indicate a shallow-marine setting, with deposition on top of a seamount in the tropical Pacific Ocean and subsequent transport and accretion by the Pacific Plate onto the California continental margin. Thus, even though most of the Franciscan appears to have been deposited in a deep-water setting, it is a complex and diverse assemblage of rocks, and shallow-water settings, though not the norm, existed as well. Economic importance Although no significant accumulations of oil or gas have been found in the Franciscan, other opportunities have been exploited over the years. During the 19th century when gold mining was one of the main industries in California, cinnabar associated with serpentine in the Franciscan and Great Valley Group was mined for quicksilver (mercury) needed to process gold ore and gold-bearing gravels. Some of the more important mines were those at New Idria and New Almaden, the Sulphur Bank Mine at Clearlake Oaks, and the Knoxville Mine (cf. McLaughlin Mine) and others at Knoxville. The Franciscan also contains large bodies of limestone pure enough for making cement, and the Permanente Quarry near Cupertino, California is a giant open-pit mine in a body of Franciscan limestone that supplied most of the cement for building the Shasta Dam across the Sacramento River. The Rockaway Beach Quarry at Pacifica is another example of a major limestone quarry in the Franciscan. See also Accretionary prism Notes References Berkland, J. O., Raymond, L. A., Kramer, J. C., Moores, E. M., & O'Day, M. (1972). "What is Franciscan?". AAPG Bulletin, 56(12), pp. 2295a-2302. Hamilton, W. (1969). "Mesozoic California and the underflow of Pacific mantle". Geological Society of America Bulletin, 80(12), pp. 2409-2430. External links San Andreas Discussion - Tectonic Wedging Geology of the Golden Gate Headlands - National Park Service Geology of San Francisco and Vicinity The Geology of the Franciscan Complex in the Ward-Creek-Cazadero Area, Sonoma County, California by Rolfe C. Erickson Terranes Cretaceous California Jurassic California Upper Jurassic Series Geologic formations of California Geology of Monterey County, California Geology of San Francisco Geology of San Luis Obispo County, California Geology of San Mateo County, California Geology of Santa Barbara County, California Geology of Ventura County, California Metamorphic complexes Natural history of the California Coast Ranges Petrology Structural geology Geology of Marin County, California