Drive along the public roads and see if it is possible to get pieces of diatomaceous earth. The mine itself is private (no trespassing).
Diatomite Mines (Diatomite Pit North), Quincy, Grant County, Washington, USA(mindat.org) Diatomite - middle Miocene (∼15 Ma) freshwater lacustrine deposit located between flows of the Columbia River Basalt Group.
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/places/mines/diatomite-mines/diatomite-mines.md