The volcanic rocks of the Harney Basin comprise a bimodal
assemblage of rhyolites (SiO₂ greater than 72%) and basalts (SiO₂
less than 52%). Tholeiitic basalts account for 15 rocks analyzed and
alkalic basalts for three. Most basalts contain phenocrysts of olivine
and plagioclase. Pyroxene phenocrysts were not found. Rhyolite
flows...