In 1934 the writer presented a theory of salt-dome formation and illustrated it with a model which indicated: (1) that the motive force causing salt uplift in simple domes is essentially the gravitational force resulting from the fact that the density of the salt is less than that of the surrounding sediments and (2) that both salt and sediments behave essentially as highly viscous fluids. The present paper considers the experimental and theoretical work, largely by others, carried out since that time, which has a bearing on this fluid-mechanical theory.

In 1937 Hubbert considered fundamental dimensional criteria which should be applied to scale models and derived the numerical relations between the physical constants of a model and its prototype in nature which should be fulfilled to give true dynamic similarity. Dobrin in 1941 determined physical constants of a fluid salt-dome model, applied Hubbert's analysis, and established the fact that the model fulfilled the dimensional criteria. By measurement of flow rates and viscosities in a model, he determined the equivalent viscosity for the sediments and arrived at a value which is in reasonable accord with determinations by other means. The experiments by Griggs and the thermodynamic development of the physics of stressed solids by Goranson in recent years have interpreted the physica properties of rock material, in terms of long-time stresses. This work has clarified and evaluated certain fundamental properties such as "strength," "plasticity," et cetera, that are directly applicable to the fluid-mechanical postulate of salt-dome formation. Finally, recent extensive geophysical work and drilling around salt domes have revealed the existence, in many places, of rim synclines which are a natural consequence of the fluid-mechanical theory and which were relatively unknown or unrecognized as such at the time of the earlier paper. All of this work seems to confirm the general hypothesis that salt-dome formation is largely a fluid-mechanical process.

Some further model experiments by the writer have been made in an attempt to simulate dome formation where the salt flow is modified by strong layers in the sediments penetrated by the dome. The salt forms thus indicated are shown to be qualitatively similar to reasonable deductions from geophysical and drilling data over a particular dome which probably formed under such circumstances.