Alcyone trachyte. Gray vitrophyre where moderately fresh, but much of rock is altered and mottled greenish gray. Mainly flows, but probably some green porphyritic varieties are intrusive. Thickness estimated at 2,800 feet, but may not be more than 1,000 feet. Locally separated from overlying Esperanza trachyte by 100 to 150 feet of sandy beds. Rests in places on 200 feet of coarse-grained breccia.
Named from Alcyone mine, in sec. 33, T. 18 N., R. 20 W., Oatman district, Mohave Co., northwestern AZ.

Source: US geologic names lexicon (USGS Bull. 896, p. 28); supplemental information from GNU records (USGS DDS-6; Denver GNULEX).

Name Alcyone Trachyte changed to Alcyone Formation. Unconformably overlies Oligocene? basaltic rocks locally or Proterozoic metasedimentary rocks regionally. Includes tuffs, flows, sedimentary tuff breccia, welded tuff, and landslide breccia. The lower and middle welded tuffs are iron-rich alkali quartz trachyte with low calcium and high potassium. The upper lava flows and plugs are subalkalic quartz latite and dacite. Welded tuffs and flows consist of 65 percent groundmass and 35 percent phenocrysts of sanidine, plagioclase, biotite, and minor pyroxene and quartz. Is 450 to 730 m thick. Shows local thickening in an inferred volcanic depression centered about 8 km northwest of Oatman, Mohave Co, AZ in the Plateau sedimentary province. Geologic maps; mapped along west side of report area west of town of Oatman. Underlies Esperanza Quartz Latite (redescribed) or Oatman Andesite. Erupted in a volcanic depression between 19 and about 23 Ma. Age modified from Tertiary to Miocene. Chemical analyses.

Source: GNU records (USGS DDS-6; Denver GNULEX).