Linear strain of a rock smaple as a function of hydrostatic pressure can be measured with a precision of 2¿10-6. Such high-precision data for three orthogonal directions allow calculation of the distribution function for the porosity due to cracks closing completely at a given pressure. Such data for at least six independent directions yield the zero-pressur strain tensor due to cracks closing completely at a given pressure. The principal values and axes of this tensor distribution function provide information about the orientation of cracks as a function of closure pressure. In this manuscript we first develop the mathematical basis for the technique and then illustrat it with differential strain data for two samples, the Westerly (Rhode Island) granite and the Twin Sisters (Washington) dunite. Strain tensor calculations reveal that each of these samples has a different type of anisotropic crack distribution.