The laboratory investigation on which this thesis is based has involved over 6 creep-years of experiments using potash rock from New Brunswick, Canada. The experimental programme consisted of two parts: incremented-stress creep tests designed to determine creep strain and creep strain rates, and a series of tests in which the sensitivity of creep as a function of stress history was explored. The duration of tests was between 7 and 298 days, with most tests lasting about three months. The experiments have demonstrated that potash deformation is highly time-dependent even at a stress as low as 2 MPa. In the absence of steady-state creep, the creep rate is never constant in being a function of time. The stress-dependence of the creep rate was established at a number of time intervals in the form of a power function. (Abstract shortened by UMI.)