Are viscosity ratios of rocks measurable from cleavage refraction?

In this Questions contribution, I consider some of the accepted methods of measuring viscosity ratios in rocks, and pose a new one. For a wide range o f strain and orientations, it can be shown that the bedding normal is immea surably close to the XY plane, and this has applications to the relationshi p between cleavage and strain. I therefore propose that with some limitatio ns, cleavage refraction can provide a measure of effective viscosity ratios in layered rocks. Examples show that cleavage refraction across competence contrasts yields surprisingly small viscosity ratios. This method might al so provide a way of distinguishing Newtonian from non-Newtonian behaviour o f rocks over time and space. (C) 1999 Elsevier Science Ltd. All rights rese rved.