Comment on “The petrologic case for a dry lower crust” by Bruce W. D. Yardley and John W. Valley

Introduction I hope to speak for most of the electromagnetic (EM) induction community in thanking Yardley and Valley [ 1997] for their interest in clarifying the conditions of existence of fluids and other grain boundary phases which can enhance electrical conductivity of the lower crust. My comment is'somewhat a personal view of how fluids, semiconduction, and solid phases (particularly graphite) may affect deep crustal conductivity. While Yardley and Valley recognize that the seismic community has been divided over adoption of free fluids as an explanation for velocity anomalies, there should be no assumption that EM researchers are of one mind on the causes of deep conductivity. Our understanding has surpassed some of the early work relied upon by the authors, and I attempt to clarify and update developments in our field.