1846.3 SHARPE ON SLATY CLEAVAGE. 97 



ton limestone quarry, where the vertical line of cleavage passes, the 

 beds are most violently contorted. Beyond this line is low ground 

 at Barnstaple, in the band between the two lines of vertical cleavage. 

 From Pilton to Ilfracombe, with cleavage highly inclined, the strata 

 are elevated in high hills, and at Linton, where the inclination of the ; 

 cleavage is only "35°, the beds seldom dip more than 5°. So again 

 on the S. coast of Devonshire, disturbed and elevated strata occur in 

 company with a highly irclined cleavage. These observations are ■> 

 less complete than those relating to Carnarvonshire, but the theoreti- ' 

 cal conclusions to be derived from them are the same. 



The regularity of the direction of the cleavage is not at all broken 

 in the neighbourhood of the granite of these counties, from which it (a 

 is to be inferred that the granitic eruptions had taken place and be- 

 come solid before the cleavage was produced : indeed some remarks u 

 of Sir H. T. De la Beche's lead me to suppose that the cleavage is (i 

 continued through the granite*. 



Application of the laws of compression and expansion of slaty rocks 

 to an area over which the position of the cleavage planes has been 

 ascertained. 



It has been shown in the first part of this paper, that there was 

 reason to believe that all slaty rocks had undergone a compression of 

 their mass in a direction perpendicular to the planes of cleavage. 

 Now that we have discovered the system upon which the cleavage 

 planes are arranged, we may judge of the direction of the pressure 

 which compressed the slates, and endeavour by that means to find 

 out its cause. I have endeavoured to prove, that though each plane '^ 

 of cleavage runs on for great distances in a uniform direction, the *; 

 planes are so arranged side by side as to make it probable that they ' 

 are not true planes, but rather portions of great curves having a ' 

 common axis and bounded by vertical lines. 



Assuming these positions to be correct, it follows that the pressure 

 must have radiated either from a point on the common axis, or frona , ' 

 a curved surface of which the outline must be similar to the curve '' 

 into which the cleavage can be resolved. The first case is too iniT,'^^ 

 probable to be worth discussing ; the second will be made more intel- , 

 ligible by a diagram representing a section of the supposed area. The "l' 



black lines represent the dips of the cleavage at the various parts of- 

 the surface, and the dotted lines point out the direction of the pres- 



* Report on Cornwall, &c., p. 163. 

 VOL. III. PART I. H 



