PART I. CHx\PTER X. 



139 



Slaty Cleavage. 



between two joints has no tendency to cleave in a direction 

 parallel to the planes of the joints, whereas a rock is capable 

 of indefinite subdivision in the direction of its slaty cleavage. 

 In some cases where the strata are curved, the planes of cleav- 

 age are still perfectly parallel. This has been observed in the 

 slate rocks of part of Wales (See Fig. 125.), which consist of a 



Fig. 125. 



Parallel planes of cleavage intersecting curved strata. (Sedgwick.) 



hard greenish slate. The true bedding is there indicated by a 

 number of parallel stripes, some of a lighter and some of a 

 darker colour than the general mass. Such stripes are found to 

 be parallel to the true planes of stratification, wherever these 

 are manifested by ripple-mark, or by beds containing peculiar 

 organic remains. Some of the contorted strata are of a coarse 

 mechanical structure, alternating with fine-grained crystalline 

 chloritic slates, in which case the same slaty cleavage extends 

 through the coarser and finer beds, though it is brought out in 

 greater perfection in proportion as the materials of the rock are 

 fine and homogeneous. It is only when these are very coarse 

 that the cleavage planes entirely vanish. These planes are usu- 

 ally inclined at a very considerable angle to the planes of the 

 strata. In the Welsh chains, for example, the average angle is 

 as much as from 30° to 40°. Sometimes the cleavage planes 

 dip towards the same point of the compass as those of stratifi- 

 cation, but more frequently to opposite points. It may be stated 

 ^s a general rule, that when beds of coarser materials alternate 

 with those composed of finer particles, the slaty cleavage is 

 either entirely confined to the fine-grained rock, or is very im- 

 perfectly exhibited in that of coarser texture. This rule holds, 

 whether the cleavage is parallel to the planes of stratification 

 or not. 



In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, 

 enormous masses of limestone are cut through so regularly by 

 nearly vertical partings, and these are often so much more con- 

 spicuous than the seams of stratification, that an unexperienced 

 observer will almost inevitably confound them, and suppose the 

 strata to be perpendicular in places where in fact they are almost 

 horizontal.* 



* Introduction to Geology, chap. iv. 



