Rev. 0. Fisher — The Cause of Slaty Cleavage. 177 



may in the circumstances have taken place through shearing. Of 

 course this would not exclude faults formed antecedent to the 

 cleavage, and which may have accompanied the elevatory movements. 



But to return to the unfavourable instance which Mr. Harker has 

 supposed. At the central portion there will be no shear, and there- 

 fore no cleavage. Where, on either side of this, the shear just com- 

 mences, it would be too slight to induce perceptible cleavage. There 

 the undeveloped cleavage would be at right angles to the unde- 

 veloped cleavage on the opposite side of the crest. But at distances 

 from the centre, at which the shear, and therefore the cleavage, be- 

 came pronounced, the two dips, though in opposite directions, would 

 be steeper than to be at right angles to one another — as Mr. Harker 

 has represented them. Neither, though the dip of cleavage is usually 

 high, is it by any means always so.^ But take the instance suggested 

 as a difficulty; that is, suppose the shear to be vertical, and of 

 such an amount as would cause the cleavage to be inclined at about 

 22° to it. Then the dip would be 68° — a by no means low angle. 



The early observers of cleavage came to the conclusion that it 

 was a feature produced by some very extensive cause, operating 

 after the rocks had undergone great displacement. This is clearly 

 at variance with the theory, that the pressure which caused the 

 displacement caused also the cleavage. Mr. Harker makes no 

 allusion to this point; nor to the particular instance, in which it 

 seems to be illustrated, viz. by the section which I gave from the 

 cliff at Hope's Nose. I am not aware of the exact evidence which 

 led Sedgwick and the rest to this conclusion ; but it seems to me 

 to follow of course from the fact, that dip of cleavage is nearly 

 independent of bedding. For fix the thoughts upon a material 

 sphere within the rock undergoing contortion. If the pressure 

 which causes the contortion also produces cleavage, this sphere 

 will have been distorted into a flattened ellipsoid. If, at some later 

 stage of the process, the stratum of which it forms a part be further 

 rotated into a new position, the ellipsoid will be rotated with it, so 

 that, if the formation of the cleavage preceded the contortion, the 

 parallelism of the cleavage planes throughout a district would be 

 subsequently destroyed. The same would be the case, though to 

 a less degree, if the contortion and supposed cleavage-causing com- 

 pression went on together. Neither must it be forgotten that the 

 formation of cleavage according to the compression theory, and 

 contortion, are two distinct modes of satisfying compression, and that 

 the sum of the two would be constant; so that with the more of 

 one, there would be less of the other. I cannot help thinking that, 

 if cleavage had been induced by the same exertion of pressure which 

 contorted the rocks, instead of the cleavage ranging in nearl}^ 

 pai'allel planes over considerable belts of country, it would lie in 

 undulations more or less normal to the curvature of the folds, and 

 not in planes independent of them. 



1 Jukes's Manual, p. 271, ed. 1862. 



DECADE III. — VOL. I.— NO. IV. 



12 



