immense
mass of pliocene gravel, apparently non-auriferous, made up of pebbles
of granite, red and green jaspers, silicious slates, and other
metamorphic material. In the Santa Lucia range, near the Mission San
Antonio, placers have been worked to some extent, and gold has been
found in small quantities in several places.
The
miocene strata of the ranges in Santa Barbara and Ventura counties are
covered unconformably in places by nearly horizontal and slightly
disturbed pliocene beds. In various places south of the junction, near
Fort Tejon, of the Sierra Nevada and Coast Ranges, pliocene gravels
occur over small areas. At San Francisco canon these gravels have been
washed and more or less gold obtained at various times since 1841
according to some authorities, and since 1838 according to Father
Venegas.
Along
the San Gabriel range gold-washing has been carried on intermittently
with more or less profit. At the base of the Sierra de Santa Ana are
immense accumulations of gravel made up of the wash of disintegrated
tertiary strata.
Gold, Silver, and Copper Veins.—Veins of gold, silver, and copper have been reported at different localities along the Coast Ranges.
Eruptive Rocks.—A
belt of eruptive rocks, of which Mount St. Helena is the culminating
point, extends from near Napa to Clear Lake down to Suisun Bay, and
large areas in this region are covered by lava, obsidian, pumice, and
volcanic ashes. Especially in the vicinity of Clear Lake modern
volcanic formations abound, and hot springs, sulphur beds, and other
evidences of modern igneous action are common; but to the north of
Clear Lake no volcanic phenomena of the kind are known, and south of
San Francisco volcanic rocks are not found in any large quantities. Hot
and sulphur springs are, however, quite common in the Coast Ranges.
