![20.2 Black rhyolite obsidian. Locality unknown. Locally pumiceous white glass is surrounded by the dominant black volcanic glass. One glassy round quartz (?) phenocryst is present in the upper left and one granular alkali feldspar (?) clot forms a white patch in the lower middle of the sample. Note the folded layers. Sample courtesy of Sonoma State University Geology Department. [Scale is standard GSA scale with cm divisions.]](../20.2.JPG)
20.2 Black rhyolite obsidian. Locality unknown. Locally pumiceous white glass is surrounded by the dominant black volcanic glass. One glassy round quartz (?) phenocryst is present in the upper left and one granular alkali feldspar (?) clot forms a white patch in the lower middle of the sample. Note the folded layers. Sample courtesy of Sonoma State University Geology Department. [Scale is standard GSA scale with cm divisions.]