82 Van Hue — Metamorphum of Rocks and Hock Fhvmge. 



Metamokimiis.m from the Dynamic Point of View. 



Dynamic action is of two kinds — molecular dynamic action 

 and mass dynamic action. By molecular dynamic action is 

 meant interchange between the molecules. Metaniorphism by 

 such interchange lias generally been called static metamorph- 

 ism. Molecular dynamic action is always accompanied in 

 some degree by mass dynamic action. By mass dynamic action 

 is meant deformation of the body of the rocks. To alterations 

 in connection with such deformation the term dynamic meta- 

 morphism is usually restricted. Mass dynamic action is always 

 accompanied by molecular dynamic action. It is recognized 

 that there are all graduations between molecular dynamic ac- 

 tion and mass dynamic action. However, in many regions the 

 phenomena are produced mainly in connection with one or the 

 other. 



Molecular Dynamic Action. 



Molecular dynamic action involves various degrees of move- 

 ments. 



(1) Presumably the lesser movements are the cases of change 

 in crystalline form and of strain within the elastic limit. In 

 the change of a substance from one crystalline form to another 

 — as, for instance, of aragonite to calcite — the movement of the 

 molecules may not involve more than a redistribution or re-ar- 

 rangement of those which are adjacent. In the case of sub- 

 stances strained within the elastic limit the molecules are 

 simply pressed slightly closer together or pulled slightly 

 farther apart, and yet these very slight adjustments may have 

 a most profound effect upon the physical properties of the 

 materials. 



(2) In a second class of movements there is a rearrangement 

 of the chemical elements by which new compounds are pro- 

 duced from old compounds. Material may be added to or sub- 

 tracted from a given mineral or from glass, or either minerals 

 or glass may be altered into two or more other minerals, with 

 the simultaneous addition or subtraction of material. The 

 added material in any case may come from some other particle 

 not far distant. The material subtracted in any given case 

 may be added to another particle at a greater or less distance. 



In the majority of changes by molecular dynamic action, 

 under both (1) and (2), which come within our observation, 

 the chemical reactions usually result in a liberation of heat or 

 running down of energy, under the first part of van't HofE's law. 

 This may be illustrated by hydration, whicli is, perhaps, the 

 most characteristic change of molecular action, such minerals 

 as chlorite, kaolinite, zeolites and epidote forming abundantly. 



