10 G. F. Becker — Current Theories of Slaty Cleavage. 



normal to the plane of cleavage assigned by my theory and 

 marked by broken lines, but it is at an angle of 29° to the minor 

 axis of the strain ellipsoid. The crennlation of a bed merely 

 shows that the axis of folding lies between the minor axis of 

 the strain ellipsoid and the direction of unaltered length ; it 

 does not even tend to prove the actual position of the strain 

 ellipsoid. On any theory of slaty cleavage it is easy to con- 

 struct crenulated beds at any of a wide range of inclinations to 



the cleavage, and therefore no one angle has any valuable 

 significance. 



Mr. Leith, furthermore, advances the view that intrusions of 

 great masses of igneous rocks are known to compress adjacent 

 rocks in directions normal to the periphery of the intrusive 

 mass, and that cleavage is developed in the surrounding rocks 

 parallel to the periphery of the intrusive masses. Now, when 

 batholiths invade a region, they unquestionably produce an 

 outward pressure which is commonly manifested by fractures 

 and apophyses in the surrounding rock. The strains set up 

 must be of an enormously complicated kind and the outlines 

 of the batholith itself usually show great irregularity. That 

 anyone should be able adequately to analyze these strains so as 

 to determine the principal axes, or even to establish with any 

 fair degree of approximation the parallelism of the schistosity 

 to the outlines of the batholith, is to me quite inconceivable. 



Mr. Leith's last argument on this subject is that crystals and 

 pebbles included in schistose rocks are frequently fractured or 

 sliced, and that this slicing does not take place parallel to the 

 schistosity, but at a considerable angle to it. From this dis- 

 crepancy, he argues that the cleavage has a different origin 

 from the slicing, and that while the slicing occurs at an angle 

 to the direction of greatest negative normal stress, the schistos- 

 ity is perpendicular to it. Now, if an included pebble or crystal 

 had precisely the same properties as the surrounding mass, it 

 would of course yield like the surrounding mass, and would show 

 the same schistosity and nothing more. On the other hand, if 



