Mystery Solved...

Brecciated Cherts, Channel Islands, CA

I collected these stones along the shore of Santa Cruz Island in California's Channel Islands last year. They were obviously silica-rich and took a nice polish, but were structurally all over the map: some were clearly banded like a conventional sedimentary rock, while others were fractured and chaotic.  I collected a dozen or so, but could not seem to find out what they were.

Then, recently, I struck up a conversation with a California geologist, who turns out to have done his PhD on the rocks of the Monterey Formation and immediately identified these as nice examples of brecciated cherts from the Monterey!   I still haven't figured out the exact mechanism of their formation, but they are pretty unusual in any case.