754 A TREATISE ON METAMORPHISM. 



Therefore a given portion of a definite mineral particle in a slate, 

 schist, or gneiss may not have been recrystallized at all, or, on the other 

 hand, may have recrystallized several or many times. It is believed that 

 ordinarily the recrystallization is far advanced or complete for all parts of 

 a typical schist, although this is far from the case in the semicrystalline 

 schists or imperfectly schistose rocks. Where deformation is mainly accom- 

 plished by recrystallization the process may be called recrystallization 

 flowage or chemical flowage. 



Of course, during this rearrangement it is not supposed that the 

 identical molecules which are taken from the more severely stressed parts 

 of a grain are necessarily deposited at the places of less stress upon the 

 same grain. Undoubtedly there is great interchange of material between 

 the particles by means of the solutions. It is, however, thought probable 

 that in many cases of deep-seated deformation, where the passage of solu- 

 tions is difficult and slow, much of the identical material which is taken 

 from a grain at one place is added to it at another place. 



When new individuals are produced in any way, as by granulation, or 

 by deposition as new mineral particles, perhaps as different species from 

 any originally in the rock, they are subject to the same laws as the original 

 mineral particles. Many have a tendency to form with dimensional orienta- 

 tion, which usually carries with it similar crystallographic orientation. How- 

 ever, it is only rarely that the orientation of the particles of a given mineral 

 approximates exactness. One mineral — for instance, mica — may be well 

 oriented, whereas such minerals as quartz or calcite may not be oriented. 



In proportion as the minerals readily respond to the forces of recrys- 

 tallization, or are mobile, they do not gain or retain a regular arrangement. 

 After mass movement has ceased the temperature may be high enough 

 and the heat be held long enough for the solutions to completely recrystal- 

 lize the minerals under mass-static conditions, and therefore orientation 

 may be lost. In proportion as minerals do not readily recrystallize or 

 stubbornly resist the force of recrystallization, the minerals once oriented 

 retain their regularity of arrangement. 



The most mobile of the important minerals is calcite. As explained 

 by Adams and Nicolson, a the mobility of this mineral is doubtless partly 



f < Adams, F. D., and Nicolson, J. T., An experimental investigation into the flow of marble: 

 Philos. Trans. Royal Soc. London, series a, vol. 195, 1901, pp. 363-401. 



