Volcano in San Diego County (part 1) The Throat
This is called Calavera Volcano. It is in Carlsbad California. Just across a valley from where I live currently. Great place to hike and check out a little history.
The plug you see contains a fine - grain Basalt which actually clogged the volcano's throat. After it's last eruption, the cone slowly eroded, leaving only the plug which is pictured.
Look closely and you can see people in the photos, which show the size of this plug. One of the people is my son, the other a friend of his.
They actually found a trail right up one of these veins, and climbed to the top of the rim.