Plate 10f.: Folding of opal-CT cherts by brittle brecciation generally becomes obscured with continued diagenesis. Here, post-deformational transformation of opal-CT (white) to quartz (black) follows preferred horizons, cross-cutting the individual chert fragments and obliterating most macroscopic evidence of the brittle deformation. This chert would look ductilely deformed after quartzification has gone to completion.