Origin of Slaty Cleavage. 147 



on the surface of their beds, but no true cleavage. This linear grain- 

 ing is due to small puckerings of the beds, and may be called " plica- 

 tions of the first order." They are not parallel to other sets of 

 plications which have occurred after their formation. I have care- 

 fully examined their direction over a considerable area, and laid them 

 down on a map, and find that they trend parallel, or turn gradually 

 about, in precisely the same manner as the strike of the cleavage 

 planes in slate rocks. Similar facts have been often observed with 

 respect to larger contortions. There can be no doubt of the mecha- 

 nical origin of both these kinds of plications, and hence we have 

 evidence to shew that wide districts have been compressed laterally 

 in just such a manner as would produce a similar arrangement of the 

 strike of the cleavage planes in rocks of such a character as have 

 had cleavage developed, when they have suffered similar compression 

 under somewhat different circumstances. It has also been urged 

 against this theory, that if masses of rock of different kinds had been 

 compressed, they would not have given way uniformly. This, how- 

 ever, must have arisen from some misapprehension of the real ar- 

 rangement of the cleavage in such rocks ; for, as I have shewn, the 

 facts prove that they have not given way uniformly, and this very 

 circumstance explains many of its irregularities. 



Perhaps it may be said, How is it possible that hard rocks could 

 have had their dimensions changed to the extent described? To 

 this I would reply, If the rocks be examined, it will be seen that it 

 really has occurred, and I would suggest that solidity is but a com- 

 parative property, and that the intensity of the forces in action during 

 the elevation of a range of mountains, could gradually change the 

 dimensions of rocks ; for it is well known that many hard and brittle 

 substances will admit of such movements, as for instance the ice of 

 glaciers, and hard and brittle pitch. 



I would now ask, How is it possible to reconcile all the mechani- 

 cal facts I have described, which are so clearly related to the clea- 

 vage, with the supposition of its being due to electrical action, or any 

 other non-mechanical cause ? If I be not greatly deceived, they all 

 form a most complete whole, if viewed in the light I have placed 

 them ; whereas, so far as I can see, they are quite incomprehensible 

 on the latter supposition ; nor, so far as I can learn, have its most 

 zealous supporters ever given any satisfactory reason for the manner 

 of distribution of the cleavage planes, even assuming them to be as 

 regular and uniform as some authors appear to describe them. Mr 

 Sharpe's theory, of course, only differs from mine in his assuming 

 that the particles have been really compressed ; whereas I am per- 

 suaded, that in general they have only suffered a change of position. 

 This, however, no doubt resulted from the different method of research 

 I have adopted. It would however, cause me to extend this com- 

 munication to too great a length, to enter fully into all these questions, 

 or describe many other facts 1 have observed connected with the sub- 



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