450 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



flexion of the cleavage in bands of plicated strata towards a parallelism with the 

 gently dipping slopes of the anticlinal waves. This remarkable fact of an intimate 

 dependence of the cleavage upon the composition and mechanical texture of the 

 structure is, I conceive, of itself sufficient to refute the hypothesis somewhat in 

 favour at present, of the purely mechanical origin and nature of the cleavage- 

 producing force ; for we cannot conceive how a mechanical force, either of com- 

 pression or of tension, transmitted, as necessarily it must be, very equally through 

 parallel layers of coarse and fine materials, should have exerted no Assuring action 

 the moment it reached the surfaces of the coarser beds, and yet have been able to 

 cleave into thin parallel slaty laminae the whole body of the fine-grained argillace- 

 ous strata. One would more naturally suppose that the less firmly aggregated 

 softer mud rocks or shales would have been even less easily fissured by sharp 

 cleavage joints, than the more massive and better cemented grits. It is of impor- 

 tance to notice here, that subsequent disruption of the strata may change the 

 normal position or dip of the cleavage, after its formation, and give rise to some 

 of the apparent deviations from the general law of direction above enunciated. 



The Cleavage Susceptibility alternately greater and less in Parallel Planes. 



Cleavage is a susceptibility in rocks of a certain composition, and in a parti- 

 cular stage of metamorphism, to split in definite straight parallel planes. The 

 cohesive force is obviously at a minimum of intensity in the direction perpendi- 

 cular to these planes. In the other two rectilinear axes of the cube, one side of 

 which is coincident with the cleavage plane, the force of cohesion next in degree of 

 intensity is the horizontal one, or that in the direction of the strike of the cleav- 

 age, while the most intense cohesion of all is that in the direction of the cleav- 

 age dip. It is in this latter direction that the molecular forces of attraction en- 

 gendering incipient crystallization seem to have been most powerfully awakened 

 while the polarities have been feeblest in the lines perpendicular to the cleavage 

 planes ; but apart from these three directions and grades of corpuscular force, we 

 have indications, in any homogeneous mass of cleavage-traversed slate, or other 

 rock, of the presence of two grades of the minimum cohesion, constituting the 

 cleavability, disposed side by side in alternate parallel order ; in other words, where 

 the cleavage is fully developed, the rock will be found to contain certain nearly 

 equidistant closely contiguous planes of maximum cleavability, or, what is the 

 same thing, of minimum lateral cohesion — the material of each thin plate of the 

 slate cohering more strongly together than these adjacent plates cohere to each 

 other. The existence of such planes is indicated by the manner in which any 

 mass of very cleavable slate, long exposed to atmospheric agencies, invariably 

 breaks up ; as we may see in any naked outcrop. If the cohesion of the mass in 

 a direction perpendicular to the cleavage planes were equally strong in all parallel 

 planes that we can imagine pervading it, it is impossible to understand how any 



