Felsite
“Fine-grained and light-colored volcanic rock”
Felsite is a general field term for any light-colored, fine-grained volcanic or hypabyssal rock whose individual minerals are too small to identify without a microscope. It's a practical name geologists use when they encounter a pale volcanic rock in the field before detailed lab analysis can determine the exact rock type.
quartz, alkali feldspar, plagioclase, minor biotite or hornblende (too fine to identify in hand sample)
Snowdonia (Wales), Lake District (England), Yellowstone (USA), Harz Mountains (Germany), worldwide in volcanic terranes
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Felsite is one of geology's most honest rock names — it essentially means 'I can tell it's felsic, but the crystals are too tiny for me to say exactly what it is without a microscope.'
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