Rev. 0. Fisher — On Cleavage and Distortion. 



405 



Dr. Hangliton's 0, and may be observed ; but he has taken the 

 bedding as retaining its original position. 



Horizontal 



The chief difSculty that will be felt in accepting the theory now 

 proposed, that cleavage is due to an internal movement of the rocks 

 rendered necessary by the disturbed region having been left after 

 elevation in a position too lofty for equilibrium, will probably arise 

 from the great amount of the vertical movement necessary to give 

 a sufficient amount of shear. But it must be recollected that an 

 equally great amount of movement is implicitly assumed in the theory 

 wliich attributes the cleavage to the identical compression which 

 crumpled the rocks. For, whatever be the cause, the observed 

 distortion implies a corresponding shear. In mitigation of this 

 difficulty it may be noticed that the shear is relative, and, if great 

 at one place in one direction, may be compensated (on the average) 

 by one equally great in the opposite direction no very long way off. 

 Such changes would be betrayed by changes in the direction of dip 

 of cleavage, which in fact takes place pretty frequently. Again, the 

 great relative shifts, which are known to take place in certain cases 

 by faulting, may be equally expected to occur by shearing in cases 

 where faulting is not possible. Subsequent disturbances are to be 

 expected, for Jukes says that, " when running sections in North 

 Wales, he was occasionally struck by the fact of a sudden change in 

 the strike and dip of the cleavage occurring immediately after 

 crossing a fault. It seemed to indicate that the cleavage plains had 

 been affected by the faults, and no longer lay in their original positions 

 with respect to the horizon and compass bearings." ^ 



To express the ratios of the axes of the ellipsoid of distortion i 

 let them be a, b, c. Then b will be the radius of the sphere. 

 Suppose the sphere to be compressed into a spheroid, and that its 

 polar axis becomes mb. Then m will not differ much from unity. 

 We have already estimated it at 0-97 in a particular case. It may 



1 Manual of Geology, p. 271, ed. 1862. 



