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atoll structure . In a metamorphic rock, porphyroblasts with hollow centers resembling atolls. The ring may be almost complete or consist of a chain of granules (Joplin, 1968).
atoll structure . In a metamorphic rock, porphyroblasts with hollow centers resembling atolls. The ring may be almost complete or consist of a chain of granules (Joplin, 1968).