I'm not sure Omaha2
I got the feeling it was ash deposits that settled in a low area and silicified like sedimentary rock and the ash was created by a volcano. I decided all of that just because the guy I brought it from called it silicified ash and that's what I come up with in my mind as to why it's called that. So! don't take it to the bank or nuthin.
I'll find out for sure in April at the SWLA knap in.