Origin of Slaty Cleavage. 139 



The difference in thickness of the beds in different parts of the 

 contortions, and the doubling of the beds, which are necessarily re- 

 lated to one another, give rise to what may be called an axis for 

 each contortion ; which, from the nature of the case, must lie in the 

 line of greatest thickening of the beds, and therefore shews the direc- 

 tion of the greatest elongation of the mass of deposit, and is usually 

 perpendicular to that of maximum pressure. Now I find that, 

 though contiguous contortions may have their axes inclined at very 

 various angles, even within a distance of not many yards varying by 

 a right angle, yet the dip of the cleavage invariably agrees with 

 them ; that is to say, it does not pass through them dipping at a regular 

 angle, as would most probably be the case if it was not due to a 

 mechanical cause, but, in each part, coincides with the line of great- 

 est elongation. In the example figured, the axes of the various con- 

 tortions are nearly parallel ; but it will be seen that the cleavage 

 coincides with them. 



In districts where the cleavage dips at a high angle, the contor- 

 tions have also their axes similarly inclined ; whereas, when it is 

 nearly horizontal, so also are their axes. 



In slaty rocks of very mixed structure, — as for instance some in 

 the north of Devonshire, — the greatly contorted beds are those which 

 have only an indistinct or imperfect cleavage, and are of such a 

 nature as not to have so readily undergone a change of dimensions 

 as beds above and below them. I have frequently seen cases where 

 such beds are contorted, so as to indicate a very great amount of lateral 

 pressure and change of dimensions, whilst the finer beds just above 

 and below them are most distinctly seen not to have been contorted 

 at all. The case figured illustrates this in a very satisfactory man- 

 ner. It would seem that a sandy bed had been forced into sharply 

 curved contortions, and its dimensions altered in different parts by 

 the pressure, as previously mentioned. The distance from the lower 

 ends of the two principal contortions was, in a direct line, nine 

 inches, whereas, measured in the line of the bed, it was thirty-eight ; 

 and therefore these two points must at first have been about that 

 distance apart, but were forced towards one another, so as to be now 

 at a distance of only one-fourth that amount. Above and below the 

 contorted sandy portion, the beds of fine-grained shaly slate are 

 somewhat disturbed, but in a distance of a few feet a *e not at all 

 so ; the thin bands of more sandy deposit being, as usual, only 

 broken up into small detached portions, which appear as spots in a 

 section perpendicular to cleavage in the line of dip, but as bands in 

 its plane. This is only shewn in the upper side of the contorted 

 bed, but it was the same below it. Hence it appears to be proved, 

 as clearly as possible, that the finer beds have been squeezed to 

 about one fourth of their original thickness, partly no doubt by ab- 

 solute forcing together of their ultimate particles, but also by elon- 

 gation in the line of dip of cleavage ; the general direction of which 



