398 



Rev. 0. Fisher — On Cleavage and Distortion. 



modern term " shear " in connection with cleavage, still that mode 

 of action is necessarily implied in any mechanical theory. Thus 

 Phillips wrote of " a creeping movement of the particles of the rock 

 along the planes of cleavage, the effect of which was to roll them 



Fig. 1. 



Diagrammatic Section at Hope's Nose, near Torquay. 

 Thirty feet thickness seen. The same cleavage runs through the whole series in 

 spite of the unconformity, showing that it was not produced by the pressure 

 which caused the folds. 



forward in a direction always uniform over the same tract of 

 country." ^ And Sir John Herschell says that, " Cleavage does not 

 absolutely require violent force, but only intermolecular movement in 

 a given fixed direction." This is a shear, and he remarks upon 

 "the tendency of the particles to arrange themselves when in motion 

 all in one direction, according to the laws, not of pressure but of 

 friction, a distinction which is quite necessary to be borne in mind." '^ 

 But it is not quite clear from this passage, whether he supposed the 

 planes of cleavage and of shear to be the same or not. I assumed 

 in my former paper that this is the case. But a suggestion from Mr. 

 Harker has led me to re-consider this point, and to inquire whether 

 the greater diametral plane of the ellipsoid of distortion may not be 

 the cleavage plane. 



The excellent illustrations to Sharpe's paper have enabled me to 

 test this question, and I have little doubt that the above conclusion 

 is correct. 



Fig. 2. 



1 On certain Movements in the Parts of Stratified Eocks, Brit. Assoc. 1843. 

 Quoted by Sharpe, loc. cit. p. 76. 



2 Phil. Mag. vol. xii. p. 198. 



