WE have demonstrated striking similarities in elastic wave propagation through rocks and single crystals in recent experiments. In particular, we find that two shear waves with displacements perpendicular to one another and with differing velocities are often propagated through rocks. This is illustrated in Fig. 1 at 5 kb pressure for samples of slate and dunite; an ultrasonic technique described by Birch1 was used. One MHz AC-cut quartz transducer generated and received the shear waves, and an electrical pulse of 50 V was used to excite the transducers. The rock samples were cylinders 2.54 cm in diameter and approximately 6 cm long.