Natural salt deposits contain small brine inclusions that can be set into motion by a temperature gradient arising from storage of nuclear wastes in the salt. Inclusions totally filled with liquid move up the temperature gradient, but cavities that are filled partly with liquid and partly by an insoluble gas move in the opposite direction. The velocities of these gas-liquid inclusions are calculated from a model that includes heat transport in the gas-liquid-solid composite medium, vapor transport of water in the gas bubble, and molecular and thermal diffusion of salt in the liquid phase as the principal mechanisms causing cavity motion. An analytical expression for the inclusion velocity is obtainable by approximating the cubical cavity in the solid as a spherical hole containing a central gas bubble and an annular shell of liquid. The theory predicts a change in the migration direction at a critical volume fraction gas in the cavity. For NaCl, the theory gives the velocities of migration down the temperature gradient which are in satisfactory agreement with experimental data.