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A low-grade, regional metamorphic rock containing actinolite, epidote, and albite, and lacking a cleavage. Greenstones form by metamorphism of basic igneous rocks, the actinolite and epidote components being derived from the ferromagnesium minerals present in the original rock. Actinolite and epidote impart a green colour to the massive rock, hence the name ‘greenstone’. — A Dictionary of Geology and Earth Sciences (4 ed.), Oxford University Press