760 A TREATISE ON METAMORPHISM. 



MEANING OF ROCK' CLEAVAGE. 



The conclusions of the foregoing pages show clearly the meaning of 

 rock cleavage. I have already held that this structure is largely accord- 

 ant with the cleavage planes of minerals the particles of which have 

 crystallographic orientations; that these particles are mainly authigenic, 

 and therefore that rock cleavage is a capacity to part largely due to the 

 actual cleavage of similarly oriented mineral particles." As the cleavage of 

 mineral particles has long been known to be a molecular structure, it 

 follows that the cleavage of rocks is also largely a molecular structure. I 

 have also explained that the similar crystallographic orientation is fre- 

 quently, indeed usually, accompanied by an arrangement of the mineral 

 particles with their longer diameters in the same plane as the cleavage, and 

 that this dimensional arrangement is a factor in rock cleavage, although 

 less important in most cases than that of the crystallographic orientation of 

 the mineral particles.'' Leith has shown that the dimensional arrangement 

 dependent upon mineral habit is the factor which controls the similar 

 crystallographic arrangement. 



Where similar dimensional and crystallographic orientation prevails 

 with a number of the important cleavable minerals, and a similar dimen- 

 sional arrangement without crystallographic orientation prevails with other 

 important minerals, cleavage is perfect. The ruptures take place by taking 

 advantage of the cleavage of the mineral particles or by partings between 

 their longer dimensions, and both these factors give easier rupturing- than 

 transverse to the cleavage. Since rock cleavage is fully discussed by Leith 

 in the publication referred to above, no detailed discussion is here made, 

 but it is mentioned in order to put this part of the subject in proper relation 

 to rock flowage. 



EFFECT OF ROCK FLOW ON TEXTURES AND STRUCTURES. 



As soon as rock flowage, combining mass and molecular movements,, 

 is inaugurated, the interior movements begin the destruction of the original 

 textures and structures. With comparatively little flowage the original 



"VanHise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. Eept. 

 U. S. Geol. Survey, pt. 1, 1896, pp. 633, 635. 

 &Van Hise, cit., p. 635. 

 * Leith, C. K., Rock cleavage. 



