MEANING OF ROCK FLOWAGE. 755 



due to movement along gliding planes. Quartz is also somewhat mobile. 

 Therefore these minerals in marble and in recrystallized quartzite frequently 

 lack regularity of arrangement. However, in some cases even calcite may 

 show well-developed dimensional and to some extent similar crystallographic 

 orientation. The usual almost complete lack of regular arrangement for 

 calcite is illustrated by most of the marbles from the Laurentian Mountains 

 to Alabama. The complete recrystallization of quartz to a coarse grano- 

 litic-textured rock, the individuals wholly lacking orientation, is illustrated 

 by the quartzites of the Wausau district of central Wisconsin. 



In the process of recrystallization of rocks it is not supposed that every 

 large mineral particle retains a nucleus for lateral growth; indeed, it is 

 certain that in a rock some large particles of a mineral may retain the 

 modified integrity above described, while other particles of the mineral 

 may be wholly destroyed. It is believed that orientation of the mineral 

 particles in reference to the varying stresses has an influence upon their 

 preservation. If the original particles happen to be in such positions that 

 they would develop as authigenic minerals under the differential stresses, 

 this is thought to be favorable to the preservation of their nuclei and to 

 growth. It is believed that in proportion as the particles vary from such 

 positions the mineral particles are likely to be destroyed. The effect of 

 position with reference to the principal stresses upon the persistence of a 

 given particle is probably great in proportion as the mineral has a tend- 

 ency to be influenced in its dimensional arrangement and crystallographic 

 orientation by the stress differences which exist during deformation. To 

 illustrate: The position of the axes in reference to the greatest pressure in 

 mica, which shows a marked tendency to parallel arrangement, would be a 

 more important factor in its preservation than in quartz, which only rarely 

 shows regular arrangement. a 



In the foregoing nonrotational distortion has been assumed. In case 

 the deformation includes a rotational element, during the recrystallization 

 each of the particles would be similarly rotated, as well as flattened or 

 recrystallized, or both; and consequently the direction of the maximum 

 compression, and therefore the most favorable direction of elongation of 

 the mineral particles, would change in reference to them. At any stage 



«Van Hise, C. E., Principles of North American pre-Cambrian geology: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 1, 1896, p. 635. 



