Various phenomena of plastic flow under shearing stress are studied at confining pressures up to $50,000 kg./cm.^2$ The shearing stress required to produce flow may-increase many fold beyond its normal value. There may be either "hardening" or "softening" phenomena while a steady state is being reached. In addition to the phenomena of flow, and often simultaneous with them, rupture in shear, followed by self-healing, frequently occurs. This rupture in shear may become more or less pronounced at high pressure. There are various time effects: some substances show very little variation in shearing force for a wide variation of speed of plastic flow, and others show very considerable variations. Self-welding is of almost universal occurrence. Permanent alterations of density are produced, with irreversible transitions to other modifications. Finally, there are various chemical effects of decomposition or synthesis.