184<6.] SHARPE ON SLATY CLEAVAGE. 87 



surface in the opposite direction from a towards 5, the pressure 

 having been towards each surface along the cleavage planes Y Y. 



Similar instances will be found in every bed of fossiliferous slate ; 

 and they prove that the mass forming each bed of slate has been 

 expanded in the direction of the dip of the cleavage. A motion of 

 the whole mass upwards in the direction of the cleavage would 

 produce a compression of the fossils in one direction only : but a 

 pressure acting equally but in contrary directions on the opposite 

 sides of the same bed, or in other words, a force pressing from the 

 centre of a bed towards each of its surfaces, proves that an expan- 

 sion of the mass of each bed has taken place, which, as already stated, 

 may have been caused by pressure in another direction. 



From this examination of distorted fossils under various circum- 

 stances, we may conclude that their present forms rriay be accounted 

 for, by supposing that the roci^s in which they are imbedded have 

 undergone compression in a direction perpendicular to the planes of 

 cleavage, and a corresponding expansion in the direction of the dip 

 of the cleavage. It is true that my proofs have been drawn from a 

 limited district ; but similar evidence is found in all the fossiliferous 

 slaty rocks with which I am acquainted, including beds of different 

 ages in Cornwall, Devonshire, Cumberland and North Wales. And if 

 we find certain changes connected with their slaty character to have 

 taken place in all fossiliferous slate rocks, it may be inferred that the 

 same have taken place in all slate rocks; though, in the absence of 

 organic remains, we have not that evidence of the changes which 

 the distortion of the fossils has afforded. Therefore it may be as- 

 serted as probable, that all rocks affected by that peculiar fissile 

 character which we usually call slaty cleavage have undergone — 



1st. A compression of their mass in a direction everywhere per- 

 pendicular to the planes of cleavage. 



2nd. An expansion of their mass along the planes of cleavage in 

 the direction of a line at right angles to the line of incidence of the 

 planes of bedding and cleavage ; or in other words, in the direction 

 of the dip of the cleavage. 



No proof has been found that the rock has suffered any change in 

 the direction of the strike of the cleavage planes. We must there- 

 fore presume that the masses of rock have not been altered in that 

 direction. 



Symmetry in the arrangement of the planes of cleavage over large 

 areas, arid connection between the direction of the cleavage and the 

 position of the beds. 



Every one who has attended to the geology of any slate district is 

 familiar with the frequent agreement in the strike of the cleavage 

 and bedding of the slates ; this is so remarkable that it has every- 

 where attracted attention. But unfortunately few geologists have 

 troubled themselves with the dip of the planes of cleavage; even if 

 its direction is loosely mentioned, the exact angles are rarely given. 



