The question of the supposed isostatic adjustment of the earth's crust to increase or decrease of load has recently come prominently to the fore in connexion with the Wegener hypothesis and Joly's theory of geological transgression and revolution. The principle itself is fairly well established by several independent lines of evidence, but there is room for clearer thinking as to the mechanism attending its operation. Very varied opinions are for instance held as to therapidity and completeness of the adjustment, and unjustifiable assumptions are made with regard to the displacement of magma beneath depressed areas. It is very necessary to keep an open mind on these questions and to search the available evidence for means of defining the nature of the attendant sub-crustal movements.