137 Microscopic Petrography – The Younger Dacite Flows – The Redcloud Dacite

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

Microscopic Petrography

 

The Younger Dacite Flows

     The Redcloud Dacite

None of the dacite flows from the Northern Arc of Vents is richer in hornblende and tridymite or is more spherulitic than the thick flow forming the great V-shaped cliff of Redcloud. Lithoidal, cryptocrystalline material is much less abundant than in the dacites just described. By far the bulk of the Redcloud dacite is composed of clear, colorless glass, though much of the flow shows a fine banding of vitrophyric and pilotaxitic layers. The content of phenocrysts varies from almost nil to approximately 10 per cent. Hornblende locally forms as much as 5 per cent of the total volume. Hypersthene generally follows in the order 6f abundance; augite is always subordinate. Zoned phenocrysts of andesine-labradorite, up to 5 mm. in length, normally exceed the combined ferromagnesian minerals in volume.

 

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