Ch. XXXVI.] 



METAMORPHIC ROCKS. 



W 



are four distinct forms of structure exhibited in rocks, namely, strati- 

 fication, joints, slaty cleavage, and foliation ; and all these must have 

 different names, even though there be cases where it is impossible, 

 after carefully studying the appearances, to decide upon the class to 

 which they belong. 



Professor Sedgwick, whose essay " On the Structure of large Min- 

 eral Masses " first cleared the way towards a better understanding of 

 this difficult subject, observes, that joints are distinguishable from 

 lines of slaty cleavage in this, that the rock intervening 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 cases where the strata are 

 curved, the planes of cleavage are still perfectly parallel. This has 

 been observed in the slate rocks of part of "Wales (see fig. 75S), 



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



which consist of a hard greenish slate. The true bedding is there 

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

 of a darker color 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 propor- 

 tion as the materials of the rock are fine and homogeneous. It is 

 only when these are very coarse that the cleavage planes entirely van- 

 ish. These planes are usually inclined at a very considerable angle to 

 the planes of the strata. In the Welsh hills, 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 stratification, 

 but more frequently to opposite points. It may be stated as a gen- 

 eral rule, that when beds of coarser materials alternate with those 

 composed of finer particles, the slaty cleavage is either entirely con- 

 fined to the fine-grained rock, or is very imperfectly exhibited in that 

 of coarser texture. This rule holds, whether the cleavage is parallel 

 to the planes of stratification or not.* 



In regard to joints, they are natural fissures which often traverse 

 rocks in straight and well-determined lines. They afford to the 



* Geo!. Trans., Second Series, vol. in. p. 461. 



