We made a brief reference to metamorphism of meteorites in Chapter 6. Metamorphic rocks constitute an important part of the Earth’s crust. During the last two decades very significant progress has been made in the experimental techniques for the study of rock systems. Particularly notable in this regard is the development of high-pressure techniques, the electron-microprobe analyzer for chemical analysis, and the sophisticated methods of studying solid-gas equilibria with controls of fugacities of O2, CO2, and H2O. Theoretical work has been concerned with the method of graphical representation of mineral assemblages, application of Schreinemaker’s method of analysis of phase equilibrium, the nature and composition of crystalline and fluid solutions, and the treatment of open and closed systems during metamorphism. As a result of the various experimental and theoretical studies (see reviews by Hewitt and Wones, 1971; Winkler, 1974), our understanding of terrestrial metamorphism has improved tremendously over the last decade.