| Text Description: |
Rhyolite Domes. Four conspicuously light-colored rhyolite domes (A, B, C, D) along southeast flank of Mount Drum, roughly coincident with arc of biotite-hornblende dacite domes (Qldd), appear to represent climax of activity of first cycle of Mount Drum volcanism. Domes are all deeply dissected by erosion and, with exception of dome D, expose light-colored cores cut by numerous dark-colored dacite dikes (Qdd). Rocks are light gray, pink, and buff, and weakly porphyritic with small (2 mm) phenocrysts of relatively abundant plagioclase, common to rare biotite that is generally oxidized, and very rare hornblende and resorbed quartz in a felsitic vapor-phase crystallized or spherulitic groundmass that is locally pilotaxitic. |