I visited Sibley Volcanic Reserve on Sunday and was transfixed by this:
The light-colored blobs are amygdules, or fossil bubbles. Many of the lava flows that issued from a small volcano here about 10 million years ago were full of gas bubbles. Later those filled with minerals, and today the amygdules are weathering out. The minerals involved include quartz, its noncrystalline variety chalcedony, and various zeolite minerals. Read a little more about them on my About.com site. Amygdules were named by a Berkeley geology professor, Joseph Le Conte, in 1878. He surely saw them in these very hills.