Electrical (resistivity) anisotropy is found to vary directly and closely as thermaland hydrodynamic anisotropy in clays. This effect is strongest in clays deposited where hydrodynamic forces are strong, i.e., in shallow water. This is the environment of deposition for arenites, however. Therefore, anisotropy in clays may indicate the presence of oil or gas reservoirs. The behavior of the parameter depends, in the main, upon data obtained by the long lateral sonde. —M. E. Burgunker