[jdavis703]

I think you're thinking of Mount Diablo. It's not quite volcanic anymore though (however when California was just the ocean it was), but is instead caused by uplift to the earth's plates: https://en.wikipedia.org/wiki/Mount_Diablo#Geology.

[jmalicki]

Or possibly Sibley Volcanic Regional Preserve http://www.ebparks.org/parks/sibley.htm , which is closer (though not north of) Berkeley.