182 Dr. Tyndall on Slaty Cleavage. 



relation to this arrangement as those so often seen in many crystal- 

 line bodies do to that of their ultimate atoms. They appear, in 

 general, to have been mainly due to meteoric agencies, their position 

 having been determined by the structural weakness. In accounting, 

 then, for so-called slaty cleavage, it is only requisite to show how 

 such particles could have had their position so changed 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 connected with the 

 subject. 



" Now I trust I have already shown 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 

 changes 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 axes of any unequiaxed particle 

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

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

 dicular to cleavage in the line of dip, tan «= ; in that of strike, 



6 



tana= ^" ^ ; and in the plane of cleavage, tana= -^^. From 

 3'75 1*6 



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 sections 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 

 directions, 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 per- 

 fect 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." 



If the reader feel sufficient interest in the question, I would 

 invite him to turn to my paper in the last Number of this 



