The fourth volume in this series consists of eleven chapters. The first five deal with more theoretical aspects of the kinetics and mechanisms of meta­ morphic reactions, and the next six consider the interdependence of defor­ mation and metamorphism. All papers deal with natural processes that inter­ act on various time scales and with different degrees of mass and heat transfer. Consequently, many fundamental axioms of metamorphic petrol­ ogy and structural geology are questioned both for their accuracy and their usefulness. In raising such questions, most contributors have pointed to ways in which the answers could be forthcoming from appropriate experi­ mental studies or observations on natural materials. In their discussion of how order/disorder can influence mineral assem­ blages, Carpenter and Putnis emphasize that metastable crystal growth is common in metamorphic systems and state' 'there may be some reluctance (among many earth scientists) to accept that significant departures from equilibrium could occur." On the basis of presented evidence, they question whether reactions ever occur close to an equilibrium boundary. The neces­ sity for pressure or temperature overstepping is also required by nucleation­ rate theory. In any case, the degree of order is severely influenced by these kinetic effects in igneous, sedimentary, and metamorphic environments.