A quantitative investigation is made of the influence of a strong elastic lithosphere on departures from isostasy. Isostasy is shown to be a special case of a more general principle that may be called the principle of isobaric equilibrium. This principle which expresses the condition for vertical equilibrium is formulated quantitatively both in terms of the resultant vertical stresses in the lithosphere and in terms of measurable gravity anomalies. Employing the principle, a stress function may be determined that permits an estimate of the fraction of the uniform superposed load carried by the lithosphere. By applying this method, it is found that for the central region of a block to be in substantial isostatic adjustment (96 per cent.), the linear dimension of the load must approximate 330 Km., a value more than six times that often assumed. The basic constant characterizing the elastic deformations of the lithosphere is determined by appeal to geological data from deltas and regions near a thrust fault. The values so obtained suggest a value of 5Β°4-15 Km. for the effective thickness of the lithosphere. The deformed figures of the lithosphere for certain special types of load have been worked out together with the stress function that measures both the gravity anomaly and the resultant vertical stress. * The opinions or assertions contained herein are the private ones of the writer and are not to be construed as official or reflecting the views of the Navy Department or the naval service at large. 47 48 Ross GUNN.