S6 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 2, 



any pressure arising within this bed. On the upper side of the slab 

 a specimen of Spirifer disjunctus, C E D, has been tiattened by an ob- 

 lique pressure moving from B towards A. On the lower side are two 



Fig. 15. 



^A, 



K 



.r/ 



^■- 



"Ni*' 





"TT 



.... 



- 



— 



^^ 



















B 



r" 









•— 







ir 



— 



— 



jr-^ 



"~ r 





""lo" — 



— 





r 







- 



- 





r 



"a 



~- 



- 



— 





r 



-4 



specimens of Orthis resvpinatus^ C F D, pressed in a contrary direction, 

 or from a towards b. But in both cases the pressure is such as would 

 be produced by an expansion of the mass in the direction of the dip 

 of the cleavage Y Y. Such an expansion would produce a pressure 

 on each surface equivalent to a force moving from the centre of the 

 bed, in the direction of the lines Y Y. It is here obvious that 

 there can be no difference in the distortion of the specimens which 

 come from the upper or the under side of a bed ; and it is impossible 

 to distinguish them. The same proof of the expansion of the mass 

 along the planes of cleavage may be found in beds of every thick- 

 ness. A bed of slate from Tintagel not the third of an inch thick 

 exhibits exactly the same phaenomena. 



The comparison of the two sides of the thin beds of shells which 

 separate the slate beds gives the same result : the shells are pressed 

 in what appear opposite directions, but which are in each case the 

 directions of a force pressing from the centre of the mass towards 

 each surface along the oblique planes of cleavage. Fig. 16 shows 



Fig. 16. 



the section of two slabs of slate enclosing a thin bed full of shells of 

 the Spirifer giganteus from Tintagel : one side of each shell is seen 

 in relief on the surface of the slabs, the other being hid : part of the 

 central bed has dropped away leaving a hollow space. A B, ah^ 

 represent the surface of the beds and Y Y the cleavage planes. The 

 two surfaces are covered with shells C D, of which those on the 

 upper surface are all pressed from B towards A, and on the lower 



