as exhibited in the Devonian Limestones of Devonshh-e. 35 



extremely irregular -ivhen magnified sufficiently to show their real 

 nature, and so entirely difi'erent from crystalline cleavage planes 

 (which are straight or consist of a combination of straight lines, 

 independent of such particles), that to compare them together 

 and to call slaty cleavage crystalline cleavage, is in my opinion 

 most inaccurate and inappropriate. 



But another way in which slaty cleavage might be due to 

 crystallization is that there were many small crystals formed, 

 with such a crystalline polarity that there was a general line of 

 weakness produced in the rock ; and, since this view of the sub- 

 ject is, a priori, not at all improbable, I shall examine the facts 

 to be observed in cleaved limestones, to see whether or no there 

 is any such polarity to be found in them. The analyses given 

 above show that various chemical changes have occuiTed in some 

 of them. In many, small perfect crystals have been formed, 

 either of calcareous spar, dolomite, or brown-spar. If then there 

 was any tendency to crystalline polarity, we should expect that 

 their axes would have some relation to the dii'ection of the clea- 

 vage j but, instead of this being the case, they are inclined pro- 

 miscuously in all positions. For instance, in Stonehouse No. 3, 

 in a section cut perpendicular to the cleavage of the rocks in the 

 immediate vicinity, the rhombs of dolomite are arranged in such 

 a promiscuous manner that there is no line of weakness and no 

 slaty cleavage. The same is the case in Stonehouse No. 2 ; and, 

 indeed, when thus crystalline on account of chemical changes, so 

 that the mechanical structure is obliterated, there is no cleavage 

 whatever ; and, instead of giving rise to it, we may clearly see 

 that it has a most effective contrary action. Perhaps it may be 

 thought that the change took place after the cleavage was pro- 

 duced ; but this is disproved by the fact, that in contiguous 

 specimens the dolomitic crystals have been broken up, and com- 

 pressed by the mechanical actions that, according to my view of 

 the subject, developed the cleavage. Again, in Paignton No. 1, 

 the crystals of dolomite and brown-spar are seen to have no rela- 

 tion to the cleavage, and are neither fractured nor altered, and 

 appear to me to have been produced afterwards, I never saw a 

 cleaved limestone of which it might be said that the crystals 

 were developed at the same time as the cleavage, so as to give 

 rise to it ; and I therefore conclude that the only relation 

 between the crystals and the cleavage is that, when they were 

 formed before it, they were effected by the mechanical compres- 

 sion in the same manner as organic fragments, but that they 

 had very little, if any other influence in producing it. If they 

 had, we should expect to find that the greater the amount of 

 crystalline change, the more perfect would have been the deve- 

 lopment of the cleavage, whereas the very reverse is the case. 



D 2 



