186 Reports & Proceedings — Qeological Society of London. 



Upfield Green (1889), C. 0. Trechmarin (1882), A. N. Leeds (1893), 

 R. Boyle (1911), A. M. Pinlayson (1909), and others. 



The President went on to discuss the present position and outlook 

 of the study of metamorphism. The rapid development of physical 

 chemistry and the successful application of experimental methods 

 to petrological questions have greatly changed the situation during 

 recent years, and for the first time it seems possible to approach the 

 subject of metamorphism systematically from the genetic standpoint. 

 For the geologist this implies the critical study, not only of the 

 great tracts of crystalline schists and gneisses, but equally of meta- 

 morphic aureoles, of pneumatolysis and other contact-e:ffects, and of 

 the phenomena, mechanical and mineralogical, related to faults and 

 overthrusts. It implies, moreover, the recognition that these are 

 all parts of one general problem, that of the reconstruction of rocks 

 under varying conditions of temperature and stress. In practice, 

 this problem is complicated by the fact that perfect adjustment of 

 chemical equilibrium cannot be assumed, either in the rocks prior to 

 metamorphism, or during the process of metamorphism itself. 



Some consideration was devoted to the solvents which play an 

 essential part in metamorphism and to the limits of migration of 

 dissolved material within a rock-mass. The Address proceeded to 

 the discussion of what is the most fundamental characteristic of 

 metamorphism : namely, that recrystallization takes place in a solid 

 environment, and so may be profoundly affected by the existence of 

 shearing stress. Stress of this type, on the one hand, arises from 

 the crystal growth itself, and on the other hand is called into play 

 by external forces. The automatic adjustment of the internally 

 created stress to neutralize that provoked from without affords the 

 key to all structures of the nature of foliation. The mineralogical 

 peculiarities characteristic of the crystalline schists must find their 

 explanation in kindred considerations; for it can be shown that the 

 chemistry of bodies under shearing stress differs in important respects 

 from the chemistry of unstressed bodies. The result is seen in 

 the appearance of a certain class of "stress-minerals" where the 

 dynamic element has figured largely in metamorphism, while in the 

 same circumstances the formation of minerals of another class seems 

 to have been inhibited. But, while some of the general principles 

 governing the effects can be formulated, the explanation of these 

 lines of the observed associations of minerals is a task for the future. 

 It may be that many of the particular problems involved will in 

 time be brought within the scope of laboratory experiment. 



The conditions governing metamorphism are temperature and 

 shearing stress, with uniform pressure as a factor of less general 

 importance. If the erogenic forces are sufficient to maintain shearing 

 stress everywhere at its maximum, the stress itself becomes a 

 function of temperature, since this determines the elastic limit, and 

 the principal conditions of metamorphism come to depend upon a 

 single variable. This degree of simplification, however, is not to be 

 expected universally. One disturbing factor is the local rise of tem- 

 perature sometimes caused by the mechanical generation of heat in 

 the crushing of rock-masses. 



