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Andesite porphyry dome of Tertiary age. Zeto point basalt porphyry, from the Sand Hill Quarry, about half a mile north of the breakwater at the entrance to Sweeper Cove. Hornblende andesite porphyry. A medium light-gray (N7) porphyritic rock, about one-fourth phenocrysts. Phenocrysts are predominately plagioclase, ranging in size from 5 to 0.1 mm. The median composition of the plagioclase is about An55. Many of the phenocrysts show reverse zoning to a composition of about An73, succeeded by normal zoning to about An50. Hornblende is the commonest of the colored constituents; the prisms show outer rims of augite, which are due to reaction with the magma. The single large flake of biotite has a reaction rim of magnetite and is embedded in green hornblende. One round grain of quartz has the appearance of a xenocryst. The groundmass consists of plagioclase, averaging about An43, quartz, and orthoclase. Some calcite is in part interstitial to clear, unaltered plagioclase crystals of the groundmass, but has developed a reaction border of epidote against augite. |