§ 1. Plastic deformation of solid matter under high confining pressures has been insufficiently studied. Jeffreys 1) devotes a few paragraphs to deformation of solid matter as a preface to his chapter on the isostasy problem. He distinguishes two properties of solid matter with regard to its behaviour to external forces: the Rigidity and the Strength. The rigidity being the resistance to elastic stresses, the strength the resistance to shearing forces. The strength has been surpassed when a differential force results in a permanent deformation. Therefore the strength equals the differential pressure necessary to effect shearing or plastic deformation.