Rhyolite porphyry

Rhyolite porphyry.

Extrusive igneous rock
This stream-rounded cobble is a rhyolite porphyry. It contains small phinocrysts of quartz and zoned feldspars. This sample came from a gravel bar along Tres Pinos Creek in San Benito County, CA.
https://gotbooks.miracosta.edu/rocks/igneous/36.html 2/2/2021