12 MR. E. H. CUNNINGHAM-CRAIG ON [Feb. I9O4.. 



geological parlance is called a shear. The angle at which the 

 surface lies to the force determines the magnitude of each com- 

 ponent. Thus, if a bed presents surfaces exactly normal to the 

 force, the shear-component is nil, and the only effect that can take 

 place is a compression of the bed which may cause a cleavage ; 

 while, if the bed lies at a lower angle to the force, the shear- 

 component is proportionately greater. Theoretically, I believe, the 

 shear-component should be greatest when the surface is inclined 

 at an angle of 45° to the force, but in actual fact we find that the 

 shearing is greatest in horizontal or almost horizontal folds. The 

 reason for this is not far to seek, as the flat-lying folds are 

 farthest from the main axis of folding, that is, they occur where the 

 greatest movement is possible. The effect of the folding now 

 becomes apparent : in tightly-packed vertical folding shear-move- 

 ments may be impossible, a cleavage of the finer bands of a rock 

 may be the only appreciable effect, and original structures may be 

 to a great extent preserved ; while, in a gently-inclined set of folds, 

 the shearing may be very great, original structures may be com- 

 pletely destroyed, and a high degree of schistosity induced parallel to 

 the original bedding. The greatest shearing-movements naturally 

 take place along those surfaces where there is the greatest difference 

 in physical properties between the beds on opposite sides of the 

 surface ; and thus the shearing is concentrated, so to speak, in the 

 finer and softer beds against harder and more massive beds which 

 may not be greatly affected, thus causing what Mr. Lamplugh has 

 described in the Geological Survey Memoir upon the Isle of Man 

 (1903) as ' grit-band metamorphism.' The shearing may have taken 

 place during the folding and the bending-over of the crests of the 

 folds away from the central axis of folding, but much of the shearing 

 has probably taken place at a slightly-later stage, while any sub- 

 sequent crust-movement may intensify the same effects. Should a 

 later movement, however, be checked by the presence of any massive 

 barrier, and should the ' load ' or weight of superincumbent material 

 be sufficient to prevent the development of a series of large folds, 

 puckering of the finer beds would be the result, and would be 

 shown by puckered folding, wrinkling of the finer and more elastic 

 bands, and finally by strain-slip cleavage in the laminae. 



III. Constructive Metamorphism. 



The effects of constructive metamorphism upon a rock depend on 

 (1) its chemical composition, which determines the possibility of 

 the formation of new minerals : and (2) the extent to which the 

 rock has been, or is being, affected by dynamic metamorphism. 

 It seems that the shearing and crushing processes, by mingling 

 more intimately the material from which the rock is built, afford 

 more favourable conditions for the segregation necessary in building 

 up a highly-crystalline rock under constructive metamorphism. 

 Thus, where original structures have been most completely broken 

 down, the constructive metamorphism can exert greater effect upon 



