The inexact geometric fits of the conceptual models of Couples and Stearns (this volume) are due largely to idealizations required by the theoretical analysis. The primary reasons for the mismatch are the assumptions of continuity and isotropy. Real rocks are not adequately described by these properties. Additional considerations of the state of stress-that is, stability index and principal-stress reorientation- are interesting in themselves but do not seem significantly to affect the construction of models using the theoretical solutions. On the basis of information currently available, the mechanical models of Couples and Stearns appear reasonably sound.