14(3 Henry Clifton Sorby on the 



as the nature of the beds changed suddenly or gradually, as is really 

 found to be the case. 



The inequalities at the junctions of different kinds of beds, and 

 the peculiar wrinkling of their surface, agree perfectly with this 

 mechanical theory. I have examined sections cut in the plane of 

 bedding perpendicular to the cleavage, and find that the arrange- 

 ment of the particles corresponds to the wrinkles, and is just such 

 as would necessarily occur if there had been an irregular giving way 

 of the rock so as to form them. 



If the direction of the cleavage be examined in the various parts 

 of the case figured in this memoir, I cannot conceive how they could 

 possibly be explained, except by such a theory as I am now advo- 

 cating. In the coarser-grained sandy bed it coincides with the axes 

 of all the contortions, and is in the line of greatest elongation of the 

 thickness of the bed, and perpendicular to the line of pressure. It 

 is arranged in fan-shape in all the contortions, as though they had 

 been squeezed together after the sandy bed had suffered as much 

 compression as it admitted of. The cleavage in the fine-grained beds 

 at some distance from the contorted one, is perpendicular to the 

 line of squeezing, as indicated by its puckering up, and the increase 

 and diminution of its thickness, in passing round the contortions ; 

 but when approaching their rounded ends, though the cleavage 

 passes straight forward in the line of their axes, it spreads out 

 on each side, and curves down into the sharp-ended spaces be- 

 tween them, in just such a manner as would necessarily occur if tho 

 coarser-grained bed had been less compressed than the other. It 

 would also follow, that the above-mentioned fan-shaped arrangement 

 would be of greatest amount in such beds as offered much resistance 

 to change of dimensions, whereas in fine-cleaved slates it would be 

 very small, or even not occur at all ; and such is the fact observed 

 in the rocks themselves. It would also necessarily follow from this 

 theory, that the strike of the cleavage would usually coincide with 

 the general strike of the beds, and be parallel to the main axis of 

 elevation of the district, as has been found to be so commonly the 

 case. The dip of the cleavage planes over any extensive district 

 would likewise be as has been observed. The structure of the so- 

 called double-cleaved slate admits of most easy explanation, as do 

 a number of other facts connected with the subject ; and, so far as I 

 am aware, there are none which cannot be explained by this theory, 

 or by suppositions most perfectly reconcilable with it. 



It may perhaps be objected that the cleavage of slate is too regular 

 and parallel in its range over a given district, to agree with the sup- 

 position of its being due to the cause I have suggested ; but I think 

 there is abundance of evidence to shew that such a physical change 

 of dimensions has really occurred with the kind of regularity observed 

 in respect to the cleavage planes. Such metamorphic schists as 

 those of the north-east of Anglesea, have a peculiar linear graining 



