PHENOMENA OF CLEAVAGE. 



European houses. The edges of these plates may be traced with care 

 on the vertical surface of the joint J, and the sloping surface of the 

 bed B, and are represented in the figure by 

 fine lines. 



It will be observed that these lines do 

 not cross the bed marked g (fig. 36). This is 

 supposed to be a hard grit or conglomerate, 

 and such rocks are sometimes only in a slight 

 degree affected by the cleavage, which, how- 

 ever, is perfect above and below them in 

 fine-grained and more argillaceous strata. 

 Certain small joints, however, and numerous 

 cleavage planes, often cross sandstone beds, 

 and then the cleavage and joint planes in 

 these beds are not parallel to the general 

 cleavage, but meet the surfaces of stratifica- 

 tion, as in this figure, at angles more nearly 

 approaching to a right angle. At I the 

 cleavage crosses nodular limestone or ironstone, and in these irregular 

 layers becomes irregular, curved, and confused. 



On the surfaces of stratification the cleavage structure is frequently 

 traced in narrow interrupted hollows and ridges ; these surfaces have 

 in fact been folded, or plaited, or puckered by the force which occa- 

 sioned the cleavage ; and the little folds thus occasioned are traceable 

 across shells, trilobites, &c., which are thus more or less distorted in 

 figure. 



On a careful scrutiny of these shells and trilobites, we find that 

 they have been compressed or elongated in one direction, so that a 

 semicircular shell (Orthis), whose hinge-line lies parallel to the cleav- 

 age edges on B, is found to be altered to the figure a ; another, whose 

 hinge-line lies across these edges, assumes the shape of 6 ; and a third, 

 whose hinge-line is oblique to the edges of cleavage, becomes distorted 

 c. To make this more clear, the letters B B are represented as having 

 idergone the compression in question. 



When, as sometimes happens, there are on these surfaces shells and 

 jbbles, too solid and firmly compacted to yield to the cleavage force, 

 are not altered in figure, but the cleavage laminae in the mass 

 )und them are changed a little in 

 lirection. Some alteration of direction 

 iquently occurs when the cleavage is 

 sing from one bed to another of a 

 iifferent degree of solidity. And in 

 >me tracts of country (e.g., the district 

 )f Cork), the cleavage planes commonly 

 not parallel in contiguous beds, of 

 mlike quality, but appear as in fig. 37. 



le cleavage plane is most oblique to the bedding in the softest and 

 lost argillaceous strata. 



One general relation appears between the stratification and the 



37- 



