Relict fracture cleavage in paraschists
To the commonly known criteria suggesting that metamorphosed rocks were originally sedimentary may be added relict fracture cleavage, when confined to certain thin beds. Thin beds in the Archean Vishnu series of the Grand Canyon possess cross structures at high angles to their bounding surfaces. These transverse planes presumably were developed in incompetent layers during crustal deformation. Later they became the loci of biotite crystallization and thus were preserved. Close folding of parts of the Vishnu series on Vishnu Creek indicated by reversals in attitude of fracture cleavages is corroborated by the testimony of drag folds.