756 A TREATISE ON METAMORPHISM. 



of the process there would be a tendency to retard the growth of the min- 

 eral particles which had formed at an earlier stage. Another effect of 

 rotation would be to decrease the regularity of the similar arrangement of 

 the mineral particles as compared with those produced during nbnrotational 

 distortion; for at any stage the new mineral particles would form in 

 positions somewhat different from those of the particles formed at an earlier 

 stage. I suspect that in rotation with reference to the forces during devel- 

 opment lies the principal cause of imperfect arrangement of the cleavage- 

 making mineral particles of the slates, schists, and gneisses. This point is 

 more fully developed by Leith." 



While in the slates, schists, and gneisses many mineral particles are 

 able to maintain themselves, a vastly greater number are probably destroyed 

 by the process of recrystallization. This is certainly true in those very 

 numerous instances in which the average size of the mineral particles of 

 the metamorphosed rocks is much greater than the average size of the 

 particles in the unaltered rocks. It has been pointed out (pp. 674-675) 

 that in many instances hundreds or thousands or even a million or more 

 of particles may be required to produce a single individual. From such 

 cases there are all gradations to those where the particles of the recrys- 

 tallized rocks are not larger than those of the unaltered rocks, and through 

 these to those in which the particles of the recrystallized rocks are smaller 

 than those of the unaltered rocks. In proportion as the particles of the 

 original rocks are large, an increasing proportion of them, or of parts of 

 many of them, is likely to be preserved in the recrystallized rock. In 

 proportion as the particles of the unaltered rock are small, a decreasing 

 proportion of them is likely to be preserved in the recrystallized rock, 

 although in the latter case the absolute number of mineral particles preserved 

 and enlarged may be as great as or greater than in the former case. 



On pages 751-753 the modifications of the forms of the original particles 

 have been considered. Substantially the same principles apply to newly 

 formed mineral particles. With or without a nucleus, a mineral particle 

 begins to grow. Because of the differential stresses the molecules which 

 first segregate to form minerals that have strong crystal habit orient them- 

 selves in such positions that their shortest axes are in the direction of maxi- 

 mum pressure, their longest axes are in the direction of minimum pressure, 



«Leith, C. K., Rock cleavage. 



