Experiments by Nettleton and others to simulate salt dome formation by means of superposed viscous liquids have clearly demonstrated that gravitational instability provides a physically sufficient explanation of the origin of these structures. In the hydrodynamic theory of stability of a layered sequence, exceedingly high viscous parameters must be used. Very few reliable direct measurements of the viscosity of rocks, obtained in the laboratory under realistic conditions of strain rate, pressure, and temperature, are available. The reason is that creep is a complex phenomenon, part of which is of a transient nature. Most creep rates measured for rocks and cited in the literature refer to transient creep and are probably of little value with regard to a determination of ro k viscosity. Only the steady-state creep behavior

End_Page 354------------------------------

allows such a determination. It is encouraging that a survey of those experiments in which steady-state creep of salt and other rocks was measured reveals viscosity parameters which, when used in hydrodynamic theory, explain in a satisfactory manner the geometry and growth rates of salt structures observed in many areas of the world.

End_of_Article - Last_Page 355------------