72 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



broken up, before being finally accumulated in the rocks in which 

 we now find it. 



Development of Slaty Cleavage. 



"Whilst now treating on the structure o£ the fine-grained slates, 

 it will be well to consider some facts connected with cleavage 

 which until quite recently have been a great puzzle to me. As 

 I pointed out in various papers published many years ago, the 

 structure of well-cleaved slates, like those of Penrhyn, may be 

 easily explained by supposing that the ultimate particles of mica 

 were originally inclined in a nearly uniform manner in all direc- 

 tions, and were subsequently rearranged by lateral pressure, so as 

 to lie more or less completely in the plane along which the rock 

 yielded as a plastic substance. There is so much independent 

 proof of pressure having acted in the direction and to the extent 

 necessary to explain all the various phenomena, and the mechani- 

 cal theory of cleavage is now so generally accepted, that it may 

 perhaps be thought superfluous to support it by further facts. 

 There was, however, always this difficulty, that I had never found 

 any uncleaved slate-rock having now the exact structure which I 

 assumed that the cleaved rocks had originally ; and in endeavouring 

 to gradually trace back the structure of slates having a very perfect 

 cleavage to those having none, I did not find, as I anticipated, a 

 gradual passage, but such an apparent break of continuity as to 

 lead me to conclude that there are two distinct tjrpes of cleavage — 

 one due to the arrangement of the ultimate atoms when the rock 

 yielded as a plastic substance, and the other due to very close joints 

 when it yielded by fractures like a partially rigid substance. More 

 careful and detailed examination of other specimens, however, has 

 convinced me that, though in some cases my original explanation of 

 very perfect cleavage may be true, yet very often the two kinds of 

 structure are only different stages of one process. Thus, for ex- 

 ample, some of the undisturbed parts of a very imperfectly cleaved 

 slate from Liskeard show very clearly that the original structure 

 was merely fine lamination in the plane of deposition, nearly all the 

 minute flakes of mica lying in the plane of stratification. These 

 beds are of such extreme thinness that occasionally they are only 

 y^L^ of an inch in thickness. In some parts they have been 

 squeezed by lateral compression into contortions similar to those 

 often seen on a large scale, only so small that they are invisible to 

 the naked eye. When magnified about 100 linear, the general 

 appearance is as shown by fig. 3. 



It is obvious, however, that such a rock could yield in this manner 

 only to a very moderate extent without the contortions breaking 

 and the detached portions moving one over the other, so as to give 

 rise to a system of approximately parallel planes of discontinuity, 

 or close joints, as shown by fig. 4, which also is magnified about 

 100 linear. These are similar to those which may be seen so well 

 in the mica-schist on the shore of Loch Lomond, not far south of 



