386 



REPORT — 1856, 



Fig. 27. 



§ 7. Cleavage accompa7iied by change of dimensions in Rochs. 



In rocks subject to cleavage, the parts of the mass have undergone some 

 change of place ; and the whole mass has suffered compression in one di- 

 rection. This will be evident from the following facts : — 



(a.) Surfaces of stratification are frequently undulated and wrinkled by 

 edges of cleavage. 



Thus in Diagram 27, let S be a surface of stra- 

 tification, K a plane of cleavage, and J a vertical 

 joint. The cleavage edges are often traced on the 

 bed S by undulated, interrupted ridges and hollows, 

 which appear in no other surfaces, and suggested to 

 me the idea of a " creeping movement among the 

 particles of the rock, along the plane of cleavage, the 

 effect of which was to roll them forward, in a direc- 

 tion always uniform, over the same tract of country." 

 In this expression the term ' creep ' is borrowed from 

 experience in collieries, where argillaceous strata are 

 frequently thrown into undulations which slowly 

 propagate themselves under continued pressure. 



These undulations are often formed on a plane highly inclined to the axis 

 of pressure, as in the case of slaty cleavage. The interrupted character of 

 the ridges and furrows on the plane of the strata arises sometimes from the 

 unequal yielding power of the materials. 



(6.) These undulations are really due to pressure of some kind, and affect 

 the figure of shells and other flexible and compressible objects on the sur- 

 faces of the strata, so tliat in the direction of the dip of the strata these objects 

 are often much shortened in dimension. 



Thus a thin object originally circular, fig. 28 (as Orbicula), becomes short- 

 ened to an elliptical figure, fig. 29, on the plane, and arched, as fig. 30, in the 

 section. Thus it is certain that the effect of cleavage is to cause relative 

 " motion among the parts of stratified rocks," such as would be produced 

 by a compression in the direction perpendicular to cleavage. 



Fig. 28. Fig. 29. 



Fig. 30. 



I am not aware of any observations on record regarding these curious 

 pheenomena of change of place in the parts of a slaty mass prior to 1843, 

 when I communicated them with other facts to the British Association at 

 Cork*. One of the points then much insisted on was the fact of the move- 



• " On certain movements in the parts of Stratified Rocks," Reports of Brit. Assoc. 

 1843, p. 61. 



