128 Mr. Sorby on the Theory of the Origin of Slaty Cleavage. 



not see why those varieties of the limestone, which have been 

 changed into dolomite, should not be called metamorphic, because 

 they do not contain mica or such other minerals as occur in 

 rocks to which that term is usually applied. I would also par- 

 ticularly call attention to what I state in describing figs. 5 and 6, 

 where I show that the difference in the structure of non-cleaved 

 and cleaved, fine-grained organic clays, consisting, now, almost 

 entirely of minute crystals of calcareous spar with no mica, or 

 fragments of any other such material, can be explained on the 

 theory of mechanical compression and change in dimensions. 



I think this will be sufficient to prove that Dr. Tyndall has 

 unfortunately misunderstood my meaning, in concluding that I 

 considered slaty cleavage to be invariably due to the change in 

 the position of flakes of mica; and I much regret that this 

 should have occurred, for it has led him to describe a number 

 of most interesting experimental facts as if they disproved my 

 theory, whilst, on the contrary, I hail them with the greatest 

 pleasure as confirming it in the most remarkable manner. 



What I chiefly wish to impress on the attention of geologists 

 and physicists is, that slaty cleavage is due to mechanical causes ; 

 that cleaved rocks are compressed rocks; and that the com- 

 pression in general has not only changed the arrangement of the 

 unequiaxed particles of which they are composed, but in some 

 cases has also altered their form. There are scarcely any rocks 

 whose particles are not thus unequiaxed, and I must still main- 

 tain that, other circumstances being the same, those have the 

 best cleavage that are composed of particles whose length and 

 thickness differ most. 



The example of wax described by Dr. Tyndall (Phil. Mag. 

 July 1856, p. 44) is indeed most excellent. If a small quantity, 

 so melted on a piece of glass as to be sufficiently thin, be 

 examined when cold with a magnifying power of about 400 

 linear with polarized light, it is seen to be composed of prismatic 

 crystals usually about jjy^-Q to ^^^nru^^ ^^ ^^ ^^^^ ^^^S ^^^ 

 3TjVit *^ TuiiTU*^ broad, being almost invariably very unequi- 

 axed. These are arranged in imperfectly radiating groups, 

 and interlace promiscuously in all directions ; and this is just 

 such a structure as, according to my views, would, if it was 

 compressed, give rise to a cleavage of so perfect a kind that I 

 can readily believe that the presence of larger particles would 

 only impair it. Similar remarks would also apply in the case 

 of white-lead and graphite, as mentioned by Dr. Tyndall (ibid. 

 p. 46) ; for carbonate of lead is seen with the microscope to 

 consist of minute prismatic crystals, and graphite of thin scales. 



Though 1 do not consider mica a necessary element in the 

 production of cleavage, yet still I think it is very much more 



