Ixvi pkoceedtngs or the geological society, [vol. lxxiv r 



may serve as a model metamorphic region ; while the study of it 

 in this aspect is rendered possible by the admirable detailed 

 survey of the ground which is now well advanced towards 

 completion. 



It is essential to what I have styled the genetic point of 

 view, that a discussion of the transformations suffered by rock- 

 masses compels regard to the proximate causes of these trans- 

 formations. Metamorphism is the response to changed conditions 

 in respect of temperature and stress, and a logical treatment of 

 the subject should proceed with direct reference to these agents 

 of metamorphism, the thermal and the dynamic. One main 

 difficulty, on the physico-chemical side, arises from the interaction 

 of these two factors, which makes the conditions controlling 

 metamorphism a function of two seemingly independent variables. 

 The general case, however, must clearly include the particular 

 cases in which one of the factors is predominant and the other 

 negligible ; and it is in accordance with scientific method that 

 these two special cases of pure thermal metamorphism and pure 

 ctpiainic metamorphisni should be exhaustively studied as pre- 

 liminaiy to attacking the general problem. It might seem 

 needless to vindicate a proposition so elementary, had it not 

 been habitually obscured by the practice of approaching the 

 subject from the end rather than from the beginning. In almost 

 all discussion of the crystalline schists and gneisses, what is 

 commonly termed ' local ' or ' contact ' metamorphism has been 

 explicitly ruled out. It is doubtless true that pure thermal 

 metamorphism can never be developed on a very extended scale, 

 since, when any large tract of rocks suffers rise of temperature, 

 expansion necessarily sets up internal stresses, and the dynamic 

 factor is automatically brought into play. But, in a consideration 

 of causes and effects, the question of extension is not primarily 

 relevant, and this divorce between so-called ' regional ' and ' local ' 

 metamorphism is seen as a gratuitous narrowing of the field of 

 view. 



A like remark applies to pure dynamic metamorphism at low 

 temperatures. This too is ty])ically localized, especially near 

 faults and overthrusts in the shallower levels of the Earth's crust. 

 The chemical changes here are of the nature of degradation, 

 and indeed the products are largely the same as the constituent. > 

 of residual clays resulting from Aveathering. The amount of heat 



