SUPPLEMENTARY NOTE A. 109 



In the matter of slaty cleavage, Sorby (who, it is well known, 

 ably supported the view originally brought out by Daniel Sharpe 

 many years before my Report was published) has of late 

 returned to the subject. In his Anniversary Address, as 

 President of the Geological Society, he makes known some 

 microscopic observations in connexion with the development of 

 slaty cleavage. It would appear " that, though in some cases " 

 his " original explanation of very perfect cleavage may be true " 

 (that is, due to pressure alone, aad independent of any pre- 

 existing divisional structure), he now recognizes "fine lamination 

 in the plane of deposition" and " very close joints," as noteworthy 

 agents, and affording an ee explanation which removes a very seri- 

 ous difficulty in completely explaining the mechanical origin of 

 slaty cleavage in rocks which have yielded to pressure as imper- 

 fectly plastic substances "*. 



Do not these observations manifest a decided leaning in the 

 direction of the theory which, for several years, I have been 

 advocating ? viz. that slaty cleavage, instead of being simply 

 the result of pressure, is the outcome of pressure exerted against 

 preexisting divisional planes of jointing and bedding. 



Reverting to the disturbances and accompanying igneous 

 upbursts which have followed zones of jointing, the least 

 reflection will make it clear that they would powerfully affect 

 rocks possessed of this divisional structure, assumably com- 

 pressing them at right angles to the course of an axis of 

 disturbance, and bringing the joint-planes into immediate con- 

 tact, thus developing slaty cleavage. The same agencies, besides 

 producing enormous dislocations or faults, must have, by their 

 transgressive action, flexured rocks into mountain chains and 

 intervening valleys, also into parallelism with an axis of dis- 

 turbance. 



Although agencies of the kind must have often obliterated 

 jointing, I assume that cleavage-planes represent it also that 

 the strike of these planes indicates the course or direction pur- 

 sued by the obliterated jointing, and consequently the system 

 to which this divisional structure belonged. 



There are two systems of the kind one meridional, and the 

 other equatorial depending on the maximum frequency of the 

 joints within certain points of the compass. The first, especially, 

 * Quart. Journ. Geol. Soc, vol. xxxvi, pp. 72-74 (1880). 



