135 Microscopic Petrography – The Younger Dacite Flows – The Grouse Hill 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 Grouse Hill Dacite

Patton termed the Grouse Hill lava an andesitic dacite. Chemical analysis (no. 22) shows clearly, however, that the lava is an acid dacite almost identical with the flow of Llao Rock. The analyzed sample is representative of the dark-gray and black glassy phase of the flow, and has the following content: clear glass, with swarms of subparallel trichites and belonites, presumed to be oligoclase and augite, 73 per cent; corroded and glass-charged, zoned phenocrysts of andesine-labradorite, 20 per cent; phenocrysts of brownish-green hornblende, 2 per cent; augite, 2 per cent, and hypersthene, I per cent; granular ore, 2 per cent.

The duller, lithoidal parts of the flow contain little or no glass and have a groundmass composed largely of microlithic oligoclase laths and interstitial cryptofelsite.

 

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