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flaser rock (flaser gneiss) A rock displaying ovoidal megacrysts enclosed within a fine-grained, streaky, anisotropic groundmass. The groundmass is typical of a mylonite fabric produced by intense deformation. The megacrysts are regions of the original rock which have survived deformation. ‘Flaser’ is a German dialect word meaning ‘irregular vein’, or ‘knot’ (as in wood).