144 Henry Clifton Sorby on the 



that their arrangement should be altered from that found in rocks 

 not having cleavage to that in those having it ; which explanation 

 must of course be such as would agree with every other fact con- 

 nected with the subject. 



Now I trust I have already shewn that there is abundance of 

 evidence to prove that rocks having slaty cleavage have been greatly 

 compressed in a line perpendicular to cleavage, and elongated to a 

 certain extent in the line of its dip. Taking for the amount of these 

 chmges those I have already mentioned for the slate of Penrhyn and 

 Llanberis, it is easy to calculate, mathematically what would be the 

 arrangement of the unequiaxed particles in such a rock as Water of 

 Ayr stone, if its dimensions were so changed. Supposing that A = 

 the angle of inclination of the longer axis of any unequiaxed particle 

 to the line along which the maximum elongation would occur, and 

 that a = this angle after it had taken place, we should have, per- 

 pendicular to cleavage in the line of dip, tan a =-^— ^ — ; in that 



of strike tan a = ; and, in the plane of cleavage, tan a — 



o'75 



. From these relations it necessarily follows that the particles 



would then be arranged in precisely such a manner as is seen to be 

 the case in such a rock having cleavage, the agreement being most 

 perfect in every particular, both in kind and amount, as seen in sec- 

 tions cut in each direction. 



Though such calculations may be fully relied on, yet, to satisfy 

 myself that they were correct, I have tested them by actual experi- 

 ment. Having mixed some scales of oxide of iron with soft pipe- 

 clay, in such a manner that they would be inclined evenly in all di- 

 rections, like the flakes of mica in Water of Ayr stone, I changed 

 its dimensions artificially to a similar extent to what has occurred 

 'in slate rocks. Having then dried and baked it, I rubbed it to a 

 perfect flat surface, in a direction perpendicular to pressure and in 

 the line of elongation, which would correspond to that of dip of 

 cleavage, and also, as it were, in its strike, and in the plane of 

 cleavage. The particles were then seen to have become arranged in 

 precisely the same manner as theory indicates that they would, and 

 as is the case in natural slate ; so much so, that, so far as their 

 arrangement is concerned, a drawing of one could not be distinguished 

 from that of the other. Moreover, it then admitted of easy fracture 

 into thin flat pieces in the plane corresponding to the cleavage of 

 slate, whereas it could not in that perpendicular to it. Even in 

 clay which has but few very unequiaxed particles, a most distinct 

 lamination is produced by changing its dimensions, as described 

 above, but it would not cleave perfectly, no more than will natural 

 slate of similar mineral composition, and moreover one cannot ob- 

 tain their firm, uniform structure. 



