Origin of Slaty Cleavage. 145 



It is a fact well worthy of remark, that, on each side of the larger 

 rounded grains of mica, in the line of cleavage, in well-cleaved slates, 

 the particles are arranged evenly at all angles, over small trian- 

 gular spaces, having their bases towards the grain. This is just the 

 part which would be protected from change of dimensions by its pre- 

 sence ; and this fact is therefore very good evidence of the slate having 

 had originally such a structure as would be changed into its present, 

 if its dimensions had been altered in the manner and to the extent in- 

 dicated by the breaking up of other rounded grains of mica seen in 

 the same thin section. 



What I therefore contend is, that there is abundance of proof that 

 slate rocks have suffered such a change of dimensions, as would ne- 

 cessarily alter the arrangement of their ultimate particles from what 

 is found in rocks not having cleavage to that in those which have, 

 and hence develop a line of structural weakness in the direction in 

 which it really does occur. 



Some slates have a very poor cleavage, although their mineral 

 composition is similar to that of such as often have a most perfect. 

 In these the green spots indicate a comparatively small change of 

 dimensions ; and in others having no cleavage, the contortions and 

 spots shew that little or none has occurred. Whence it should ap- 

 pear that the perfection of cleavage depends both upon the ultimate 

 mineral composition, and the amount of change of dimensions of the 

 rock. 



When slates are composed of alternating beds of different charac- 

 ter, the cleavage almost always does not pass straight through them, 

 but lies nearer to the plane of bedding in the finer-grained and more 

 perfectly cleaved varieties. When the cleavage cuts the beds at a mode- 

 rate angle, this difference is often very considerable ; but where the bed- 

 ding is perpendicular or parallel to it, there is little or no variation. 

 When the change in mineral structure of the beds is sudden, the in- 

 clination of their respective cleavages is sharp and angular ; but if 

 it be gradual, it passes from one to the other in a curve. These facts 

 are most easily explained by this theory. When such a mass of 

 rocks was compressed, certain beds would yield much more readily 

 than others, both to absolute compression and elongation. In such 

 contortions of coarse-grained beds interstratified with fine, as that 

 figured, the fact of them being so whilst the fine are not, and the 

 spreading-out arrangement of the cleavage planes in the finer, at the 

 vertices of the contortions of the coarser, as shewn in the figure, 

 prove that they did not admit of so much absolute compression as 

 the fine. In uncontorted alternating beds of such characters the 

 amount of elongation in the line of dip could not vary, and, there- 

 fore, it would necessarily follow that the more compressible would 

 be more compressed in the plane of bedding than the others. Hence, 

 the line of cleavage would lie more towards that plane in the fine 

 than in the coarser, the junction being angular or curved, according 



VOL. LV. NO. CIX. — JULY 1853. K 



