MECHANICAL ACTION. 47 



positions, or nearly so. If under the stress chemical interchange also takes 

 place between the molecules, when the stress is removed the body may still 

 return to nearly its original form. But if the strain extends beyond the 

 elastic limit the form of the body is notably changed, as when a piece of 

 wrought iron or steel is drawn out or when a piece of cast iron is crushed. 

 Mechanical action may therefore be considered as molecular or mass. 

 By molecular mechanical action is meant differential movements of the 

 molecules. By mass mechanical action is meant differential movements of 

 large masses of the rocks. Molecular movement also frequently involves 

 differential movements of the atoms. Metamorphism by molecular move- 

 ment has generally been called static metamorphism. But molecular 

 mechanical action is always accompanied in some degree by mass 

 mechanical action, .though this process may be subordinate. The term 

 "dynamic metamorphism" has usually been restricted to alterations in con- 

 nection with mass deformation. But mass mechanical action is always 

 accompanied by molecular mechanical action as an important and essential 

 concomitant, although this invariable relation has not always been recog- 

 nized. Further, as mass movement becomes important molecular move- 

 ment, instead of becoming less important, is likely to be of even greater 

 consequence. There is therefore gradation between molecular mechanical 

 action and mass dynamic action. 



MOLECULAR MECHANICAL ACTION. 



Molecular mechanical action involves various degrees of movements. 



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 cal- 

 cite — the movement of the molecules may not involve more than a rear- 

 rangement of those which are adjacent. In the case of substances strained 

 within the elastic limit, the molecules are simply pressed slightly closer 

 together or pulled slightly farther apart, and yet these very slight adjust- 

 ments may have a profound effect upon the physical properties of the 

 materials. For instance, amorphous glass when strained but slightly and 

 well within its elastic limit becomes an anisotropic substance. Leucite 

 crystallizes in the isometric system at high temperatures. As the mineral 

 cools it passes at once into an anisotropic form. The transformation from 



