Figure F21. Photomicrograph of the base of basement Unit 4 (Sample 183-1137A-33R-1 [Piece 1E, 121-124 cm]) showing the contact between highly oxidized, feldspar phyric basalt and the baked top of the crystal-lithic volcanic sandstone/siltstone (basement Unit 5). Note the abundance of angular (broken?) feldspar crystals in the sediment, probably of volcanic origin. Field of view = 2.75 mm (plane-polarized light).