46 A TREATISE ON METAMORPHISM. 



changes may take place without addition of material, or added material 

 mav be derived from some other particle or particles near or remote. 

 Material subtracted from any given mineral particle may be added to 

 another mineral particle at a greater or less distance. Illustrating the above 

 are the alterations of feldspar into muscovite and quartz, and of olivine 

 into serpentine, magnesite, magnetite, and quartz. Chemical action is in 

 most cases accomplished through solutions. Therefore its detailed discus- 

 sion is considered in connection with the agents of metamorphism, gaseous 

 solutions, and aqueous solutions. (See Chapter III.) 



GRAVITY. 



Gravity is now the great dominating force of the universe. Indeed, 

 it is a main original source of energy. Certainly it is the source of energy 

 which has largely controlled the development of the solar system, including 

 the sun and all the planets and satellites. The transformations of gravity 

 into chemical energy, heat, light, and other forms of energy are important 

 factors in the development of the solar system, including the earth. More- 

 over, gravity still remains as the great dominating force which controls 

 earth movements," both vertical and horizontal, and also the circulation of 

 the water, both overground and underground. By earth movements are 

 meant all movements of the solids or rocks of the earth not in solution. In 

 this broad sense the movement of glaciers is an earth movement. 



The direct work of gravity in metamorphism may be considered under 

 two headings — mechanical action and water action. 



MECHANICAL ACTION. 



Rocks may be stressed within the elastic limit, or the stress may 

 extend beyond the resisting power of the material. In either case the 

 rocks are strained. Strain may occur with or without chemical action. 

 Strain is always accompanied by some transfer of energy into heat. 

 When the rocks are strained the molecules are moved with reference to 

 one another. If the strain be within the elastic limit and chemical chaug-e 

 does not take place, the molecules are only slightly farther apart or closer 

 together, and when the stress is removed they may return to their original 



a Van Hise, C. R., Earth movements: Trans. Wisconsin Acad. Sci.., Arts, and Letters, vol. 11, 

 L898, pp. 512-514. 



