mylonite

Metamorphic rock composed mainly of feldspar, quartz, and micas and comprising fine-grained highly-deformed layers alternating with layers of less-deformed relict grains often ovoidal in shape. The varied degree of deformation among bands produces a typically streaky appearance. They are found in areas in which rock layers have been deformed by sliding past one another parallel to their boundaries (shear strain).

Once thought to be solely the result of brittle, grinding deformation (cataclasis), it is now recognized that microscopic bending followed by recrystallization has allowed portions of these rocks to deform ductily (see ductile material). Their unique texture can be used to reconstruct directions of movement that occurred during their formation.