Porphyritic specimen. Examination in thin section reveals that the specimen is made up largely of the outer shells of fresh-looking phenocrystic and lathlike feldspars, with interiors replaced by isotropic glass. A little quartz and one or two small crystals of an altered ferromagnesian mineral, probably biotite, are also present. The feldspathic shells are thin, but each is fairly continuous, and both twinning and zoning are plainly visible. The appearance suggests an andesite or a fine-grained diorite as the original rock.
The Katmai magmatic province