Glass Buttes complex lies at the northern margin of the Basin and Range province in central Oregon and is cut by the northwest-trending Brothers fault zone. An older acrystalline volcanic sequence of high-silica rhyolites (>75% SiO2) forms a broad platform composed of domes and flows with minor pyroclastic deposits. The high-silica rhyolite sequence is divided on the basis of texture into 1) zoned flows and domes, 2) obsidian flows, 3) felsite flows, and 4) biotite-phyric flows and domes.