34 Mr. H. C. Sorby on Slaty Cleavage, 



to itj and are nearly equiaxed. In some thin-bedded limestones 

 thev have their longer axes in the line of stratiticatiou. The 

 organic clay occurring in the most highly cleaved Y\<r. 6. 

 limestone of Kiugskerswell has, however, a structure 

 as shown in fig. 6, magnified 200 linear, where the 

 crystalline granules have a very unsymmetrical cha- 

 racter, having their axes in the plane of cleavage very 

 much longer than perpendicular to it, as though the 

 compression indicated by the joints of the encrinites 

 and larger crystals had afi"ected the smallest, consti- 

 tuting the ultimate structure of the rock. I think no 

 one could compare figs. 5 and 6 without perceiving 

 that this compression would produce a line of weak- 

 ness. In fig. 5 it will be seen that there is no parti- 

 cular direction along which the rock could break with- 

 out fracturing the crystals; whereas in fig. 6 there is clearly one 

 along which a fracture could extend by passing amongst them, 

 whilst in that perpendicular to it such could not be the case 

 without fracturing very many. 



Taking then all the above facts into consideration, I do not 

 see how we can arrive at any other conclusion than that the 

 cleaved limestones have been very considerably compressed ; and 

 that this compression has so changed the position and form of 

 the particles of which they are composed, that such a structural 

 weakness has been produced in a plane perpendicular to the 

 pressure, that they may be split along it iu the manner charac- 

 teristic of slaty cleavage. 



Having now described facts, which to my own mind cany 

 complete conviction that slaty cleavage is the result of mecha- 

 nical action, I proceed to examine what evidence there is of its 

 being due to crystalline forces. If such were the case, there are 

 three ways in which it might be produced. In the first place, it 

 might be supposed that the rock was analogous to a simple, 

 large crystal, and that the slaty cleavage was similar to crystal- 

 line cleavage planes. However, I am persuaded that no one who 

 had examined a thin section of a cleaved rock with a microscope, 

 so as to see what the structure I'eally is, would for one moment 

 advocate this view of the subject ; for there is such a complete 

 difiierence between them that it would in my opinion be nearly as 

 easy to prove that stratification was the result of crystallization. 

 If any geologist does really believe in such a cause, I can only 

 attribute it to his supposing that cleavage planes are perfect 

 planes ; whereas the microscope shows most clearly that this is 

 not at all the case. Even in the most perfect slates they are 

 merely fractures passing amongst the particles, like the dotted 

 lines in fig. 2, so as to have surfaces, smooth and level enough 

 to the naked eye, because the grains are so minute, but seen to be 



