Ishi Tuff Member of the Tuscan Formation. Here named, is in western part of the Tuscan Formation along the northeastern side of the Sacramento Valley from Chico to Red Bluff, California. Consists of white to light-gray, fine-grained, pumiceous airfall tuff composed of rhyolitic to dacitic glass and distinctive bronze-colored mica flakes commonly reworked and contains lenses and partings of sandstone and siltstone. Grades laterally into tan micaceous siltstone. Conformably overlies a layer of volcanic conglomerate, 2 to 3 m thick, and locally, a layer of tan siltstone, 1 m thick, at the top of unit B of the Tuscan; conformably and sharply underlies unit C lahar. Thickness is 0.3 m. Age is Pliocene based on stratigraphic relations.

Source: US geologic names lexicon (USGS Bull. 1565, p. 141-142).