PART L CHAPTER X. 



141 



Jointed Structure and Cleavage, 



inches at the under surface. More or less time is required for 

 the process, which I have since seen in all its different stages."* 

 Here again we find the columnar or jointed structure in a solid 

 mass, which had been subjected to great changes of temperature. 



It seems, therefore, that the fissures called joints may have 

 been the result of different causes, as of some modification of 

 crystalline action, or simple contraction during consolidation, or 

 during a change of temperature. And there are cases where 

 joints may have been due to mechanical violence, and the strain 

 exerted on strata during their upheaval, or when they have sunk 

 down below their former leveL Professor Phillips has suggested 

 that the previous existence of divisional planes may often have 

 determined, and must greatly have modified, the lines and points 

 of fracture caused in rocks by those forces to which they owe 

 their elevation or dislocations. These lines and points being 

 those of least resistance, cannot fail to have influenced the direc- 

 tion in which the solid mass would give way on the application 

 of external force. 



It has been observed by Mr. Murchison, that in referring both 

 joints and slaty cleavage to crystalline action, we are borne out 

 by a well-known analogy in which crystallization has in like 

 manner given rise to two distinct kinds of structure in the same 

 body. Thus for example, in a six-sided prism of quartz, the 

 planes of cleavage are distinct from those of the prism. It is 

 impossible to cleave the crystals parallel to the plane of the prism, 

 just as slaty rocks cannot be cleaved parallel to the joints, but 

 the quartz crystal, like the older schists, may be cleaved ad infi- 

 nitum in the direction of the cleavage planes. f 



I have already stated that extremely fine slates, like those of 

 the Niesen, near the Lake of Thun, in Switzerland, are perfectly 

 parallel to the planes of stratification, and are, therefore, proba- 

 bly due to successive aqueous deposition. Even when the slates 

 are oblique to the general planes of the strata, it by no means 

 follows as a matter of course, that they have been caused by 

 crystalline action, for they may be the result of that diagonal 

 lamination which I have before described (p. 32.) In this case, 

 however, there is usually much irregularity, whereas those clea- 

 vage planes oblique to the true stratification, which are referred 

 to a crystalline action, are often perfectly symmetrical, and ob- 

 serve a strict geometrical parallelism, even when the strata are 

 contorted, as already described (p. 139.) 



In regard to the origin of slaty cleavage, where it is uncon- 



* Journ. of Roy. Geograph. Soc, vol. v. p. 19. 

 t Silurian System of Rocks, (fee, p. 246. 



