Ch. xxxyl] jointed structuee and cleavage. 60] 



mass. Siicli stripes are found to be parallel to the true planes of strati- 

 fication, wherever these are manifested by ripple-mark, or by beds con 

 taining 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 w>hen these are very coarse that the cleavage i)lanes entirely 

 vanish. 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 com^pass as those of stratification, but 

 more frequently to opposite points. It may be stated as 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 imperfectly exhibited in that of coarser texture. 

 This rule holds, whether the cleavage is parallel to the planes of stratifi- 

 cation or not."^^ 



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

 in straight and well-determined lines. They afibrd to the quarryman, 

 as Sh' R. Murchison observes, when speaking of the phenomena, as ex- 

 hibited in Shropshire and the neighboring counties, the greatest aid in 

 the extraction of blocks of stone ; and, if a sufficient number cross each 

 other, the whole mass of rock is split into symmetrical blocks. The 

 faces of the joints are for the most part smoother and more regular than 

 the surfaces of true strata. The joints are straight-cut chinks, often 

 slightly open, often passing, not only through layers of successive depo- 

 sition, but also through balls of limestone or other matter which have 

 been formed by concretionary action, since the original accumulation of 

 the sti'ata. Such joints, therefore, must often have resulted from one of 

 the last changes superinduced upon sedimentary deposits.f 



In the annexed diagram (fig. 707), the flat surfaces of rock a, b, c, 

 represent exposed faces of joints, to which the walls of other joints, j j, 

 are parallel ; s s are the lines of stratification ; d d are lines of slaty 

 cleavage, which intersect the rock at a considerable angle to the planes 

 of stratification. 



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

 masses of limestone are cut through so regularly by nearly vertical part- 

 ings, and these joints are often so much more conspicuous than the seams 

 of stratification, that an inexperienced observer will almost inevitably 

 confound them, and suppose the strata to be perpendicular in places 

 where, in fact, they are ahnost horizon tal.J 



Now such joints are supposed to be analogous to the partings which 



* Geol. Trans. 2d series, vol, iii. p. 461. 



f Silurian System, p. 246. 



X Introduction to Geology, chap. iv. 



