part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxvii 



set free and dissipated in the complete chemical breakdown of 

 a crystalline rock is very considerable, probably greater than the 

 latent heat of fusion. Pure thermal and pure dynamic meta- 

 morphism thus present a strong contrast, in that the reactions 

 are endothermic in the former and exothermic in the latter. 

 Although we cannot reason strictly to the more general case, in 

 which both thermal and dynamic factors enter, it is clear that 

 a certain compensation may be assumed. The energy-change 

 under the complex conditions will be either positive or negative, 

 and often of small magnitude. Here we may perceive a kind of 

 economy in nature, which helps to explain the vast extension 

 sometimes attained by metamorphism of this type. 



This consideration is to be borne in mind in connexion with 

 the so-called ' metamorphic cycle ' — a conception which has come 

 down to us from the davs of Lvell. If we be^in, for clearness, 

 with crystalline igneous rocks, the ideal cycle consists, in the 

 language of Van Hise, of a phase of catamorphism followed by 

 one of anamorphism. The large quantity of energy liberated 

 during the former phase passes for the most part into a form in 

 which it is irrecoverable, and, if the complementary phase is to 

 close the cycle by restoring the original igneous rocks, very heavy 

 drafts must be made upon the Earth's internal store of energy. 

 We see, how T ever, that no such extravagant demand is involved in 

 the extensive conversion of sediments to crystalline schists by the 

 joint agency of heat and mechanical stress. We may reasonably 

 adopt the conclusion of Leith and Mead that the cycle is in 

 general an incomplete one ; and that, in so far as the conditions 

 for regional metamorphism have been recurrent in geological time, 

 there has been an increasing accumulation of metamorphic at the 

 expense of igneous rocks. This seems to accord with geological 

 evidence. After taking account of the very interesting phenomena 

 described in particular districts by Sederholm and others, I can 

 rind little support for the belief that the subcrustal fusion which 

 should close the hypothetical cycle is in general realized. 



One admonition brought home to us by the recent work of 

 chemists is, not merely the slowness of many of the operations of 

 Nature — an idea to which the geological mind is well habituated,— 

 but their incompleteness, from the point of view of the 

 establishment of chemical equilibrium. We know, for instance, 

 that many common rock-forming minerals may persist under 



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