1846.] 



SHARPE ON SLATY CLEAVAGE. 



85 



are flattened and drawn out considerably in one direction, without 

 the crumpling up or folding under of any part of the shell; as 

 in fig. 14- representing a specimen of Spirifer giganteus restored 

 where the hinge had been broken off. The pressure of the rock in 



Fig. 14. 



A 



JD 



3 if a noqii 

 foB ''ig h£iu(y\ 



£¥■ 





the direction perpendicular to the cleavage has caused the flattening 

 of the shell, and there has been an expansion of nearly 50 per cent, 

 in another direction along the plane of cleavage. Not having seen 

 this or any similar specimen in situ, I could not learn the direction 

 of the line of elongation, but there can be little doubt that it is par- 

 allel to the dip of the cleavage planes ; for in every other case the 

 elongating force has acted in a direction parallel to their dip, not- 

 withstanding variations in the position of the bedding. 



Having examined all the principal cases of distortion in detail, we 

 may take a broader view of the subject and consider them with re- 

 ference to the position of the shells in the bedding. The beds of: 

 slate vary much in thickness and are frequently separated by thin 

 layers of shells, some of which are impressed on each of their surfaces. 

 When we compare the fossils on the opposite sides of a bed of slate, 

 we find them distorted along lines of which the direction is parallel; 

 but if there has been any oblique pressure, it has acted in opposite 

 directions on the two sides of the bed. This will be better under- 

 stood from fig. 15, which represents the section of part of a thin bed 

 of slate at South Petherwin, of which the lines A B, a 5, are the upper 

 and lower surfaces, each of which bears in relief flattened and dis- 

 torted casts of bivalve shells C D, of which one side is seen, the other 

 being buried in the slate; the cleavage Y Y is very oblique. Be- 

 fore the slab was removed it rested of course upon a bed of rock 

 below, and was covered bv another above which otFered resistance to 



