138 



LYELL'S ELEMENTS OF GEOLOGY. 



Jointed Structure Cleavage and Joints. 



and having been superinduced by causes absolutely independent 

 of gravitation. All these different structures must have different 

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

 after carefully studying the appearances, to decide upon the class 

 to which they belong.*' 



Joints, — Now in regard to the second of these forms of 

 structure or joints, they are natural fissures which often tra- 

 verse rocks in straight and well determined lines. They afford 

 to the quarryman, as Mr. Murchison observes, when speaking 

 of the phenomena, as exhibited in Shropshire and the neigh- 

 bouring 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. f 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 deposition, but also through balls of limestone or 

 other matter which have been formed by concretionary action, 

 since the original accumulation of the strata. Such joints, 

 therefore, must often have resulted from one of the last changes 

 -superinduced upon sedimentary deposits.:}: 



In the annexed diagram the flat surfaces of rock A, B, C, re- 

 present exposed faces of joints, to which the walls of other 

 joints, J J, are parallel. S S are the lines of stratification ; C C 

 are lines of slaty cleavage, which intersect the rock at a con- 

 siderable angle to the planes of stratification. 



Fig. 124. 



Stratification, joints, and cleavage. 



Joints according to Professor Sedgwick are distinguishable 

 from lines of slaty cleavage in this, that thoTock intervening 



* Geol. Trans., Second Series, vol. iii. p. 480. 



t The Silurian System of Rocks, as developed in Salop, Hereford, &c., p. 245, 

 t Ibid. p. 246. 



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