Photomicrograph of folded schist.  The flat pink crystals are muscovite; the white, squarish crystals are quartz.  Note that the muscovite crystals define the layering, or FOLIATION, in this rock.  Since we know that the muscovite grew during metamorphism, we can infer that the foliation has been folded by a geologic event that post-dated the metamorphism.

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