Rev. 0. Fisher — On Faulting, Jointing and Cleavage. 269 



described by Phillips, the structure is easily observed, because the 

 included masses are quite large. In a specimen from the same 

 locality in the Woodwardian Museum at Cambridge, angular masses, 

 of what was perhaps originally hornblendic schist, are imbedded in 

 a coarse greenish ground-mass. They are of various sizes, up to an 

 inch and a half in length by three-quarters of an inch wide.^ Sharpe's 

 description of these slates, quoted by Phillips, exactly applies to this 

 one. " In all these slaty breccias the included masses are flatter 

 between the planes of cleavage than in any other direction. Their 

 flattest sides are always parallel to the cleavage planes — they are 

 usually rather longer on the line of dip of cleavage than along the 

 strike." In the specimen in the Woodwardian, the included fragments 

 now form small separate patches of a comparatively fine-grained slate 

 with fine striae running in the direction of the cleavage, so that the 

 contrast of surface shows very clearly the different effect of the same 

 action upon fine and coarse-grained rocks. These fragments are all 

 thin,. and none of them can with certainty be identified as passing 

 through on the two faces of the slate ; which is about three-eighths 

 of an inch thick. They appear to have retained their original 

 outlines, but to have been flattened to their present thinness by 

 pressure, and rearranged so that they lie with their longer dimen- 

 sions in the direction of the dip of the cleavage. 



The internal action which could produce all these result's is un- 

 doubtedly a pressure combined with a shear. A shear may be 

 defined as a movement, by which the material on the opposite sides 

 of any selected surface of shear are constrained to move with different 

 velocities, the velocity increasing more and more as we advance 

 further in one given direction. Fixing the attention on any one 

 surface, as if it were at rest, the effect v/ill be the same as if the 

 material was moving in opposite directions on the two sides of ifc. 

 The difference of action on the upper and under sides would tend 

 to turn long, or flat, particles round, until they lay nearly in the 

 direction of the surface ; while a similar difference of action arising 

 from unhomogeneity in the material would have the effect of placing 

 them symmetrically with respect to the direction of the shear. 

 These are the characteristics of a cleavage plane. A hard particle 

 would be anchored in the material either on one or the other side 

 of the surface, and would protect the material behind it, attaching it 

 to its train, and thus a kind of " craig and tail " on a minute scale 

 would be formed by it ; thus giving rise to the strias on the surfaces 

 of cleavage. These would consequently be scarcely observable in 

 slate formed out of very fine sediment, as is the case. The thinning 

 of the particles points to great compression, and the same compres- 

 sion it has been, which has kept the molecules in cohesive contact, 

 and enabled them to flow like a viscous substance, as in M. Tresca's 

 experiments. Such shearing, however, has no effect to cause the 

 .section of any merely geometrical solid figure by one of the shearing 

 surfaces to become elongated. Further investigation is perhaps de- 



^ Eeport of Britisli Association oa " Cleavage," 1856, p. 389. London, 1857. 



