Album caption and index card: Photomicrograph of rhyolite porphyry of group 7 from dike 1 mile north of Argentine Pass. The phenocrysts have borders of fine-grained oriented quartz in the groundmass. o, sericitized orthoclase; q, quartz. Crossed nicols. Boulder County, Colorado.
Notes: Published as figure XIII-C in U.S.Geological Survey. Professional Paper 223. 1950.