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/mindat.org.md
Why did you let me survive the shame of my defeat? To defeat an enemy yet let him live may seem kind, but it’s cruel! General Hyoe Tadokoro, *The Hidden Fortress