IUGS definition of mylonite: "Fault rock that is cohesive and characterized by a well-developed schistosity resulting from tectonic reduction of grain size, and commonly containing rounded prophyroclasts and lithic fragments of similar composition to minerals in the matrix. Fine-scale layering and an associated mineral or stretching lineation are commonly present. Brittle deformation of some minerals may be present, but deformation is commonly by crystal plasticity."