Near Boulder Dam occur camptonite dikes of possible Quaternary age characterized by amphibole (kaersutite) phenocrysts of exceptional size and unusual distribution—a distribution which suggests that these phenocrysts have formed essentially in place. Rough calculations suggest that these crystals grew to four-inch size in less than 25 days. Serpen-tinization of olivine, formation and size of amygdules, and growth of amphibole phenocrysts are related to distribution of volatiles in the dike magma. Chemical analyses of the amphibole and of the chilled margin of a dike are presented and discussed.

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