Rev. 0. Fisher — On Faulting, Jointing, and Cleavage. 271 



parallelism to this direction at intermediate points. In other words, 

 that it is vertical at synclinals and anticlinals, and slightly inclined 

 to the Yertical between these. 



Sharpe had remarked, "There are several circumstances which 

 tend to show that the centre of an elevated district may have 

 somewhat sunk down again after the completion of the original 

 elevation, and of the cleavage." ^ The concluding words make it 

 clear that he supposed the cleavage to have been established before 

 the sinking. That it was subsequent to the elevation, and accompany- 

 ing contortion and disruption of the strata, is evident, because 

 it has not itself been affected by the contortion. But if cleavage 

 was established after the elevation, and before the sinking, it is 

 difficult to imagine to what mechanical action it can have been due. 

 The suggestion now offered is that it was this very sinking, which 

 Sharpe appears to have suspected from independent evidence, that 

 originated the cleavage. We know that cleavage has been ac- 

 companied by pressure ; and it is clear that an elevated ridge, on 

 settling down, would be aifected by a powerful horizontal pressure 

 across it. But it is surely a mistake to suppose, that pressure across 

 what became cleavage planes, could by itself be sufficient to induce 

 cleavage. Some additional exhibition of force is requisite to produce 

 that g-Mas*- viscous shearing of the rock along the planes of cleavage, 

 which we have pointed out to be the immediate cause of slaty 

 structure. Let us then enquire whether a sinking and settling 

 downwards of the elevated and contorted tract would induce a 

 shearing of the rocks of which it was composed, and in the directions 

 required for the phenomena of cleavage. 



Fig. 2. 



N 



J^^XZl 



""^^"^"^"7^--^ 



\\R \t 



~~~^/^^^^^^ 



\ \ 



A 1 



W 1 



1 



H a6 s <^ ^^ 



First conceive the simplest case of a single crest at K. Let 

 H S Mhe the horizontal line within the elevated tract occupying 

 at the moment under consideration such a position that the down- 

 ward settlement would tend to compress the parts above it and 

 distend those below it. At L H and J^M, where the tract abuts on 

 the undisturbed crust, there is no tendency to sink, and it is obvious 

 that the greatest tendency in that direction will be along K S. 



Take Aa, Bb, two surfaces very near together. Then the condition 

 of their being surfaces of shear and cleavage will be that the velocity 

 during settlement along one of them shall be uniform and shall ditfer 

 from that along the other by the same difference everywhere, so that 

 it also is uniform. 



1 Journ. Geol. Soc. vol. iii. p. 98. 



