1846.] 



SHARPE ON SLATY CLEAVAGE. 



81 



by the lines Z Z. In the side view of the same shell, fig. 5, we see 

 that it has been greatly expanded in the direction corresponding to 

 the dip of the cleavage planes. 



Fig. 5. 



From these and similar cases, we learn that the shells have been 

 compressed by a force acting in a direction perpendicular to the 

 planes of cleavage, and that the compression of the mass between the 

 cleavage planes has been counterbalanced by its expansion in a di- 

 rection corresponding to the dip of the cleavage. 



Where the cleavage intersects the bedding at an angle of 10° or 

 15°, the present forms of the shells are very various and remarkable, 

 yet the changes may be traced to the operation of the same causes 

 under different circumstances, of which the oblique direction in which 

 the pressure has acted on the shells appears to have had the most 

 important share in modifying their form. The distortion in these 

 cases is much greater than in those already described, and it is of a 

 more complicated character. In a large majority of cases, in which 

 the shell is of an arched form, the changes consist in a great length- 

 ening of one side of the shell, and a comparative shortening of the 

 other side, which is frequently crumpled up and squeezed under the 

 middle of the shell : this will be best understood from the drawings. 

 Fig. 6 represents a specimen 

 of Spirifer disjunctus from 

 South Petherwin, lying in an 

 oblique direction towards the 

 cleavage planes : the lines Z Z 

 show the strike of the cleavage 

 across the bed, and Y Y the - 

 direction of the dip of the 

 cleavage. The specimen is a 

 good deal flattened, and looks 

 as if it had been pressed to- 

 wards the upper line Z Z by 

 a force moving in the di- 

 rection of Y Y, and pressing 

 downwards and forwards at 

 the same time; the friction 

 or the resistance of the mass 

 in which it was imbedded re- 

 tarding the advancement of 



VOL. in. PART I. 



Fig, 6. 



nV 



ir 



