Porphyritic phonolite (Shonkin Sag Laccolith, Early Eocene, 50-53 Ma; Buck Ranch, Highwood Mountains, Montana, USA) 1

James St. John
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Porphyritic phonolite (Shonkin Sag Laccolith, Early Eocene, 50-53 Ma; Buck Ranch, Highwood Mountains, Montana, USA) 1