During recent years the question of the conditions of stress and strain in the earth's crust has received a very considerable amount of attention. Perhaps one of the most fruitful lines of inquiry has been that which has aimed at determining the connexion between the varying densities of the crustal rocks and the different degrees of relief of the surface of the earth. It soon became apparent that in mountainous or elevated regions the deep-seated rocks were of less density than those beneath lowlands and seas, and it became clear that the elevated regions were sustained by the buoyancy of the lighter rocks beneath them. The theory that the earth's crust was in isostatic equilibrium thus arose.